Segmented excavation construction method combined with soil-nailing wall support

Abstract

The application discloses a kind of segmented excavation construction methods combined with soil nailing wall support, belong to foundation pit support technical field, solve the technical problem that existing segmented excavation is difficult to control, and the safety risk caused by the sudden change of stress of supporting structure is caused by foundation pit deformation.It includes: dividing N excavation layers, setting each layer excavation depth and soil nail length;Layer by layer excavation, construction soil nail and shotcrete surface layer, and install displacement sensor and axial force sensor at the end of soil nail;After reaching the preset strength, read the cumulative horizontal displacement, current displacement rate and axial force increment;Compare with preset threshold respectively, if any is out of limit, then reduce the next layer excavation depth and increase the soil nail length, otherwise, construction according to original design parameters;Repeat until all excavation layers and support are completed.The application is mainly used for the segmented excavation construction of foundation pit in soft or sensitive stratum, can dynamically adjust the support parameters according to real-time monitoring data, effectively control the deformation of foundation pit, and improve the safety and adaptability of construction.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology. More specifically, this invention relates to a segmented excavation construction method combined with soil nailing wall support. Background Technology

[0002] In foundation pit engineering, soil nailing walls are a commonly used support structure, especially suitable for foundation pits with shallow excavation depths and relatively relaxed surrounding environments. The construction method of segmented excavation combined with soil nailing wall support typically involves excavating in layers according to a pre-designed excavation depth. First, the first layer of soil is excavated to the set depth. Then, soil nails are installed on the excavated surface of this layer, followed by a shotcrete surface layer. After the surface layer and the soil nail grout have reached a certain strength, the next layer of soil is excavated, and so on until the bottom of the foundation pit is reached. This layered, segmented excavation and layer-by-layer support process can control the deformation of the foundation pit sidewalls to a certain extent, ensuring temporary stability during construction.

[0003] In conventional design and construction processes, the excavation depth of each layer and the corresponding soil nail length are fixed values ​​predetermined based on the geological parameters, pit depth, and surrounding loads provided in the survey report, after stability calculations. During construction, on-site technicians excavate each layer to the depth specified in the design drawings and install soil nails according to the designed length. Theoretically, as long as the geological conditions are basically consistent with the survey report and construction is strictly carried out according to the design, the deformation of the pit and the stress on the soil nails should be within the expected and controllable range.

[0004] However, the following problems are often encountered in actual engineering. First, geological conditions have inherent uncertainties and variability. The stratigraphic parameters obtained through a limited number of boreholes and tests during the exploration phase are difficult to fully and accurately reflect the spatial distribution of soil layers throughout the entire excavation area. For example, at a location originally identified as stiff plastic clay, local soft plastic clay interlayers may be encountered after actual excavation, or loose areas may appear in what was originally relatively dense sand. This deviation between stratigraphic conditions and design assumptions will lead to inconsistencies between the earth pressure distribution and the interaction force between soil nails and the soil mass during actual excavation and the design calculation values. When the actual strata are weaker than the design assumptions, the same excavation depth may produce greater lateral displacement, and the tensile force borne by the soil nails will also increase significantly.

[0005] Secondly, over-excavation is difficult to completely avoid during construction. Although the design specifies the excavation depth for each layer, in actual operation, due to factors such as the precision of mechanical operation, errors in manual measurement, and local collapse or fall of soil, the actual excavation surface may be lower than the design elevation, resulting in over-excavation. Over-excavation means that a portion of the soil in that layer is removed, which is equivalent to increasing the exposed height of that layer. After over-excavation, the height of the unsupported foundation pit sidewalls exceeds the original design value, which will directly lead to an increase in lateral unloading and a more complete release of horizontal stress in the soil, thereby causing greater instantaneous displacement and subsequent creep deformation. At the same time, over-excavation will also cause the soil nails of the upper layer to bear additional tensile force because the lateral thrust of the soil above is increased due to over-excavation.

[0006] The consequences of these problems are that, in the absence of effective countermeasures, the deformation of the foundation pit may gradually accumulate and exceed the warning value, and the stress on the soil nails may also increase suddenly, even approaching or exceeding their pull-out bearing capacity. When the deformation is too large or the stress on the soil nails exceeds the material strength, it may lead to cracking of the shotcrete surface layer and damage at the connection between the soil nails and the surface layer, or even local or overall instability of the foundation pit, causing a safety accident. In existing conventional construction methods, the excavation depth and soil nail length of each layer are fixed parameters, and once the design is determined, they are rarely adjusted during construction. Even if on-site monitoring personnel discover that the displacement rate is too fast or the axial force is abnormal, they often can only rely on experience to temporarily decide to reduce the excavation depth of the next layer or increase the soil nail length, lacking quantitative adjustment basis related to the current monitoring data. Such experience-based adjustments are either too conservative, leading to unnecessary delays and material waste, or the adjustment range is insufficient, failing to effectively curb the continuous deterioration of deformation and stress.

[0007] For a long time, engineers have attempted to solve this problem, but have faced several difficulties. First, how to obtain key data reflecting the working status of the support structure in real time and conveniently. In the past, direct monitoring of soil nail axial force was rarely conducted during soil nailing wall construction. Axial force gauge installation requires certain conditions, and the accuracy and long-term stability of readings are challenging. While displacement monitoring is relatively easy, relying solely on displacement data makes it difficult to determine which layer of soil nails is experiencing a stress problem. Second, even if displacement and axial force data are obtained, how to establish a reasonable quantitative relationship between these data and subsequent adjustments to excavation depth and soil nail length? Because soil nailing walls are flexible support systems, the interaction between the soil and soil nails is highly complex, and there is a lack of simplified empirical formulas or adjustment coefficients suitable for rapid calculation on-site. Third, after adjusting the excavation depth and soil nail length, the stability check of the entire foundation pit needs to be considered simultaneously; improper adjustments may lead to new imbalances. Due to the lack of a systematic adjustment method, truly dynamic design and construction are often difficult to achieve in practice.

[0008] Therefore, in the segmented excavation construction combined with soil nailing wall support, how to reasonably adjust the subsequent excavation depth and soil nail length according to the actual response of the current construction stage while maintaining construction efficiency, so as to effectively control the deformation of the foundation pit and the sudden change in soil nail stress, is a long-standing technical problem that has not been effectively solved. Summary of the Invention

[0009] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0010] Another objective of this invention is to provide a segmented excavation construction method that combines soil nailing wall support. This method aims to solve the technical problem in existing construction where, due to uncertainties in geological conditions and over-excavation leading to excessively rapid horizontal displacement rate of the foundation pit or excessive increment of soil nail axial force, there is a lack of technical adjustment based on real-time monitoring data to quantitatively adjust the excavation depth and soil nail length of the next layer.

[0011] To achieve these objectives and other advantages of the present invention, a segmented excavation construction method combining soil nailing wall support is provided, comprising the following steps: Step 1: Divide the excavation pit into N layers according to the designed depth, and set the excavation depth h of the i-th layer. i The corresponding design length L of the soil nail i ; Step 2: Excavate the i-th layer of soil to the designed depth h. i Soil nails and shotcrete surface layer are constructed on the i-th excavation surface, and displacement sensors and axial force sensors are installed at the ends of at least one layer of soil nails that have been constructed. Step 3: After the i-th layer of shotcrete surface and soil nail grouting reaches the preset strength threshold, read the cumulative horizontal displacement S obtained by the displacement sensor. i and current displacement rate v i And read the axial force increment ΔF obtained from the axial force sensor. i ; Step 4: v i With the preset displacement rate threshold V th Compare ΔF i Compared with the preset axial force increment threshold ΔF th Compare; if v i ≥V th or ΔF i ≥ΔF th Then adjust the excavation depth h of the next layer. i+1 for h i+1 =h i+1 0 -δ, while adjusting the length L of the next layer of soil nails. i+1 For L i+1 =L i+1 0 +ΔL; ; ; where, δ is the adjustment amount of the excavation depth of the (i + 1)-th layer, with the unit of m; ΔL is the adjustment amount of the soil nail length of the (i + 1)-th layer, with the unit of m; k δ is the excavation depth adjustment coefficient, dimensionless; k L is the soil nail length adjustment coefficient, dimensionless; λ s is the formation sensitivity coefficient, taking 0.5 - 0.7 for hard plastic clay and dense sand; taking 1.0 - 1.2 for soft plastic clay and loose fill, dimensionless; Δh i is the actual over-excavation depth of the i-th layer, Δh i ≥0; Otherwise, carry out the excavation and support of the next layer according to the original design parameters h i+1 0 and L i+1 0 for the next layer excavation and support; Step Five: Let i = i + 1, and repeat Steps Two to Four until i = N to complete the construction of all excavation layers and corresponding supports.

[0012] Preferably, for the sectional excavation construction method combined with soil nail wall support, when the foundation pit adopts the composite support form of soil nails and prestressed anchor rods, if it is determined in Step Four that the parameters of the next layer need to be adjusted, the following sub-steps shall be executed: S41: Suspend the prestress application of the next layer of anchor rods; S42: First complete the construction of the soil nails and shotcrete surface layer of the next layer. After the strength of the grouting body of the soil nails in this layer reaches 70% of the design value, continuously monitor the axial force change curves of the soil nails in this layer and the completed soil nails in the adjacent upper layer for at least 24 hours, and record the initial axial force value F0 and the axial force change rate dF / dt; S43: Apply the prestress of the anchor rod by the m-level loading method. The number of loading levels m is determined according to the current axial force change rate dF / dt of the upper layer of soil nails according to the following rules: When dF / dt ≤ 0.10 kN / min, take m = 2 levels; When 0.10 < dF / dt ≤ 0.20 kN / min, take m = 3 levels; When dF / dt > 0.20 kN / min, take m = 4 levels; Divide the design prestress value into m equal parts, and apply one part for each level, that is, the prestress value applied for each level is 1 / m of the design prestress value; S44. Apply anchor prestress step by step. Immediately after each application, read the axial force change rate dF / dt of the adjacent upper soil nail. If dF / dt ≥ 0.20 kN / min after any single application and lasts for more than 10 minutes, stop tensioning and maintain the current prestress for 24 hours until dF / dt drops below 0.05 kN / min before continuing to the next level of loading. S45. After applying all m-level prestress, continuously monitor the axial force change rate dF / dt of the adjacent upper soil nail for at least 72 hours; if dF / dt ≥ 0.15 kN / min and shows an increasing trend, then perform secondary tensioning, with the tensioning value being 10 to 15% of the initial design prestress value.

[0013] Preferably, the segmented excavation construction method combined with soil nailing wall support has a preset displacement rate threshold V. th and preset axial force increment threshold ΔF th Calculate using the following formula: ; ; Where γ is the soil unit weight, h i The current excavation depth is given by E, where E is the soil compression modulus, φ is the internal friction angle, and Δt is the current excavation depth. standard The standard monitoring cycle is given by d, where d is the diameter of the soil nail and τ is the standard value of the bond strength between the soil nail and the soil.

[0014] Preferably, in the segmented excavation construction method combined with soil nailing wall support, after calculating δ and ΔL according to the formula in step four, the following verification step is also performed: using the measured displacement data and axial force data of the first m layers to invert and obtain the equivalent soil shear strength parameter c. eq φ eq and elastic modulus E eq Where m≥2; input the parameters obtained from the inversion into the finite element numerical model, and assign 1.2 times and 0.8 times the design load coefficient to the (i+1)th layer respectively to perform displacement and axial force calculations; when the maximum displacement obtained from the calculation exceeds the preset control value or the maximum axial force exceeds the pull-out bearing capacity, multiply δ and ΔL by 1.2 times the amplification factor respectively; the inversion adopts the particle swarm optimization algorithm and is updated once every two layers of excavation.

[0015] Preferably, in the segmented excavation construction method combined with soil nailing wall support, step four involves v i With V th Comparison and ΔF i With ΔF th After comparison, the following destruction mode determination and differential adjustment steps are also performed: when v i ≥V th And ΔF i <0.6ΔFth When the condition is determined to be a precursor to surface layer bending deformation-dominated failure, the soil nail length adjustment ΔL for the next layer is increased by an additional 0.5L. i The excavation depth adjustment δ remains constant; when v i <0.5V th And ΔF i ≥ΔF th If the condition is determined to be a precursor to soil nail pull-out or bond slip-dominated failure, the adjustment amount δ for the next layer of excavation depth will be further reduced by 0.3h. i And multiply the soil nail length increment ΔL by a reinforcement factor of 1.3; when v i ≥V th And ΔF i ≥ΔF th When the condition is determined to be a precursor to a composite instability-type failure, δ and ΔL are doubled, the excavation of the (i+1)th layer is suspended, and two rows of full-length grouting anchor pipes are added to the i-th layer.

[0016] Preferably, in the segmented excavation construction method combined with soil nailing wall support, step three involves reading the current displacement rate v. i Then, the second derivative of the displacement rate, a, was calculated. i =d 2 S / dt 2 ;when a i >0 and a within three consecutive monitoring periods i When it is monotonically increasing, even if the current v i <V th The comparison condition in step four is also replaced with the virtual displacement rate v. i′ =v i ×(1+α·T), where α is the acceleration weighting coefficient and T is the ratio of the acceleration duration to the standard period; the formula for calculating T is T=(t current ‒t start ) / Δt standard , t current For the current moment (i.e., to calculate the second derivative of the displacement rate a) i The corresponding monitoring time is usually the time point of the latest displacement reading. start The starting time (referring to the first time the displacement rate v is detected) i The starting point of the sustained acceleration trend (i.e., the beginning of the first cycle in three consecutive monitoring periods), Δt standard The standard monitoring cycle is 30 minutes; when v i′ ≥V th At that time, the parameter adjustment in step four is triggered.

[0017] Preferably, in the segmented excavation construction method combined with soil nailing wall support, in step one, each excavation layer is divided into a corner zone, a long side mid-section zone, and a short side mid-section zone according to the plan shape of the foundation pit, and a displacement rate threshold V is set for each zone. th,corner V th,long and V th,short V th,corner =0.6×V th,long V th,short =0.9×V th,long Each area is independently equipped with a corresponding displacement sensor and axial force sensor; in step four, it is determined whether parameter adjustment is triggered in each area. When any area is triggered, only the next layer of support corresponding to that area is locally reinforced, and the local reinforcement is to increase the length of the soil nail in that area by 0.3L. i The horizontal spacing of the soil nails was increased to 0.7 times the original spacing, while the original design parameters were maintained in the remaining areas.

[0018] Preferably, in the segmented excavation construction method combined with soil nailing wall support, the soil sensitivity coefficient λ in step four... s The dynamic update is based on the cumulative ratio of the measured displacement of the excavated layer to the theoretical design displacement, and the update formula is λ. s =λ s0 ×(ΣS i,实测 / ΣS i,理论 ) β , where λ s0 β is the initial sensitivity coefficient based on the preliminary values ​​obtained from the geological exploration report, and β is the correction exponent with a value range of 0.8 to 1.2; when λ s When two consecutive layers have a displacement rate greater than 1.3, the preset displacement rate threshold V in step four is adjusted. th and preset axial force increment threshold ΔF th Divide by λ respectively s Perform downward correction; when λ s If two consecutive layers have a depth less than 0.7, the design excavation depth h for subsequent layers will be increased. i 0 Increased by 5%.

[0019] The present invention has at least the following beneficial effects: 1. This invention provides a segmented excavation construction method combined with soil nailing wall support. By reading the cumulative horizontal displacement, current displacement rate, and axial force increment at the ends of the soil nails after each excavation layer is completed, and comparing them with preset thresholds, dynamic assessment of pit deformation and support stress is achieved. When any monitoring index exceeds the limit, the excavation depth and soil nail length of the next layer are immediately adjusted according to a formula. The adjustment amount comprehensively considers the over-excavation depth and the stratum sensitivity coefficient, enabling the support parameters to adapt to the actual changes in stratum conditions. This method transforms traditional fixed-parameter construction into dynamic construction driven by monitoring feedback, effectively avoiding uncontrolled deformation or sudden increases in axial force caused by stratum uncertainty and over-excavation, significantly improving the safety and adaptability of segmented pit excavation, and is especially suitable for soft or sensitive strata.

[0020] 2. In the case of a composite support system combining soil nails and prestressed anchors, this invention implements step-by-step control of the anchor application process after parameter adjustment: Prestressing application is paused, the next layer of soil nails and surface layer construction is completed first, and then the loading level is determined based on the axial force change rate of the adjacent upper soil nails, with prestress applied level by level. The axial force change rate is monitored after each loading level; if it continues to exceed the limit, tensioning is paused until the axial force decreases. After applying all prestress, the axial force change trend is monitored again, and secondary tensioning is implemented if necessary. This process helps mitigate the adverse effects of sudden increases in prestress on the soil nails and surface layer, promotes synergistic stress distribution within the composite support system, and provides a relatively reliable operating method for composite support construction encountering parameter adjustment conditions.

[0021] 3. This invention provides calculation formulas for preset displacement rate thresholds and preset axial force increment thresholds. These thresholds no longer rely solely on experience but are quantitatively calculated based on physical and mechanical parameters such as soil weight, compression modulus, internal friction angle, soil nail diameter, and bond strength. The formulas reflect the theoretical relationship between soil deformation and soil nail stress at different excavation depths and consider the influence of standard monitoring cycles. Using these formulas to calculate thresholds helps to objectively determine whether the current construction status is within a controllable range, and the thresholds can be updated as the parameters of the excavated layers change. This approach allows the control standards for each layer to better match the current working conditions, contributing to improved scientific rigor and rationality of dynamic adjustments.

[0022] 4. After calculating the adjustment amounts for excavation depth and soil nail length, this invention adds inversion analysis and numerical simulation verification steps. Using measured displacement and axial force data of the completed soil layers, the equivalent soil shear strength parameters and elastic modulus are obtained through particle swarm optimization, enabling the numerical model to reflect the actual mechanical response of the current site soil layers. The inversion parameters are input into the finite element model, and displacement and axial force are calculated for the next layer by assigning both larger and smaller design load coefficients. If the calculation results exceed the control values, the adjustment coefficients are increased. This verification mechanism helps determine the rationality of the adjustment amounts, reducing potential over- or under-adjustments from empirical adjustments. Parameters are updated every two excavation layers, achieving a balance between computational efficiency and model accuracy.

[0023] 5. This invention identifies three precursory failure modes based on different combinations of displacement rate, axial force increment, and their respective thresholds: surface bending deformation-dominated, soil nail pull-out or bond-slip-dominated, and combined instability. Differentiated adjustment measures are then implemented for each mode: for surface bending-dominated failure, the soil nail length is increased; for pull-out-dominated failure, the excavation depth is further reduced and the soil nail length is increased; for combined instability, the adjustment amount is doubled and a full-length grouting anchor pipe is added. This differentiated adjustment strategy based on failure mode helps avoid the blindness that may result from a single adjustment method, enabling corresponding reinforcement for different mechanisms and providing a differentiated technical approach for controlling foundation pit deformation and sudden stress changes under different instability types.

[0024] 6. Based on monitoring the displacement rate, this invention further calculates the second derivative of the displacement rate to identify acceleration trends. When the second derivative is positive and monotonically increases over three consecutive monitoring periods, a virtual displacement rate is constructed even if the current displacement rate has not yet reached the threshold, incorporating acceleration and its duration into the judgment criteria. The calculation formula for the virtual displacement rate introduces an acceleration weighting coefficient and a duration ratio, which can amplify the potential impact of acceleration trends on displacement development. This approach helps to detect signs of accelerated deformation in the foundation pit earlier, compensating for the insensitivity of relying solely on rate thresholds to trend changes, and providing construction personnel with an auxiliary early warning method to adjust parameters before displacement exceeds limits.

[0025] 7. This invention considers the influence of the excavation pit's planar shape on deformation distribution, dividing each excavation layer into a corner zone, a long-side mid-section zone, and a short-side mid-section zone, and setting differentiated displacement rate thresholds for each, with a lower threshold for the corner zone. Sensors are independently installed in each zone and independently determine whether parameter adjustments are triggered. When a zone exceeds its limit, only the support area corresponding to the next layer below that zone is locally reinforced (by increasing the length of soil nails and densifying the horizontal spacing), while the remaining zones maintain their original design parameters. This strategy of zoned control and localized reinforcement helps avoid the increased material costs and extended construction period that might result from uniform reinforcement across the entire cross-section. It enables relatively precise support for weak areas, ensuring the safety of sensitive areas while maintaining overall construction efficiency.

[0026] 8. This invention achieves dynamic updating of the formation sensitivity coefficient. Based on the ratio of the measured cumulative displacement of the excavated layers to the theoretical design displacement, the initial sensitivity coefficient is exponentially corrected, making the adjustment calculations in subsequent excavations closer to the actual formation response. When the sensitivity coefficient is too high for two consecutive layers, the displacement rate threshold and axial force increment threshold can be lowered, adopting stricter control standards; when the sensitivity coefficient is too low for two consecutive layers, the design excavation depth of subsequent layers can be appropriately increased, improving construction efficiency while ensuring safety. This adaptive parameter update mechanism allows the control strategy to be continuously adjusted based on measured feedback during construction, facilitating a closed-loop interaction between dynamic design and construction.

[0027] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0029] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0030] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0031] This invention provides a segmented excavation construction method combining soil nailing wall support, which includes the following steps: Step 1: Divide the excavation pit into N layers according to the designed depth, and set the excavation depth h of the i-th layer. i The corresponding design length L of the soil nail i ; Step 2: Excavate the i-th layer of soil to the designed depth h.i Soil nails and shotcrete surface layer are constructed on the i-th excavation surface, and displacement sensors and axial force sensors are installed at the ends of at least one layer of soil nails that have been constructed. Step 3: After the i-th layer of shotcrete surface and soil nail grouting reaches the preset strength threshold, read the cumulative horizontal displacement S obtained by the displacement sensor. i and current displacement rate v i And read the axial force increment ΔF obtained from the axial force sensor. i ; Step 4: v i With the preset displacement rate threshold V th Compare ΔF i Compared with the preset axial force increment threshold ΔF th Compare; if v i ≥V th or ΔF i ≥ΔF th Then adjust the excavation depth h of the next layer. i+1 for h i+1 =h i+1 0 -δ, while adjusting the length L of the next layer of soil nails. i+1 For L i+1 =L i+1 0 +ΔL; ; ; Where δ is the adjustment amount for the excavation depth of the (i+1)th layer, in meters; ΔL is the adjustment amount for the length of the soil nails in the (i+1)th layer, in meters; k δ k is a dimensionless adjustment factor for excavation depth. L λ is the soil nail length adjustment factor, dimensionless; s The formation sensitivity coefficient is 0.5–0.7 for stiff plastic clay and dense sand, and 1.0–1.2 for soft plastic clay and loose fill, dimensionless; Δh i Let Δh be the actual over-excavation depth of the i-th layer. i ≥0; Otherwise, according to the original design parameter h i+1 0 and L i+1 0 Proceed with the next layer of excavation and support; Step 5: Let i = i + 1, and repeat steps 2 to 4 until i = N, completing all excavation layers and corresponding support construction.

[0032] The technical challenge addressed by this implementation method is that when using soil nailing walls for segmented excavation, actual geological conditions often deviate from the survey and design, and over-excavation is difficult to completely avoid during construction. Existing methods rely solely on fixed design parameters for layered construction, lacking a means to quantitatively adjust subsequent excavation depths and soil nail lengths based on real-time monitoring data. This can easily lead to excessively rapid horizontal displacement rates in the foundation pit or sudden increases in soil nail axial force, thereby posing safety risks. Compared to the closest existing technology, which involves layered excavation strictly following design drawings and only making temporary adjustments based on experience after obvious dangers occur, the core difference of this method lies in introducing a dual-index threshold comparison based on displacement rate and axial force increment. When either index exceeds the limit, the excavation depth and soil nail length of the next layer are adjusted according to a quantitative formula that includes over-excavation depth and geological sensitivity coefficient, achieving a shift from qualitative experience-based judgment to quantitative feedback control.

[0033] In step one, the excavation pit is divided into N excavation layers according to the total designed depth, and the design value h of the excavation depth of the i-th layer is set. i and the corresponding design value L of the soil nail length i Here are the explanations of the terminology: An excavation layer refers to an independent construction unit divided from top to bottom, and the vertical thickness of each unit is the excavation depth of that layer. A soil nail is a passive tension support member formed by drilling holes at a certain angle on the excavation surface, inserting reinforcing bars, and grouting. Its length is defined as the straight-line distance from the head of the soil nail, i.e., the shotcrete surface layer, to the bottom of the hole. When dividing excavation layers, the thickness of each layer should be determined comprehensively based on the geological conditions, the depth of the foundation pit, and the capabilities of the construction machinery. Typically, the thickness of each layer is controlled between 1.5 meters and 2.5 meters to ensure the feasibility of soil nailing construction and the early strength development of the shotcrete surface layer.

[0034] In step two, the i-th layer of soil is excavated to the designed depth h. i Soil nails and shotcrete surface layers are constructed on the i-th excavation surface, and displacement and axial force sensors are installed at the ends of at least one layer of soil nails that have been constructed. The specific details are as follows: Excavation is carried out using excavating machinery at the design elevation. After excavation, the slope is manually trimmed. Then, holes are drilled at the designed spacing and angle, soil nail reinforcement is placed, and pressure grouting is performed. After grouting is completed, a steel mesh is hung on the slope and shotcrete is applied to form the surface layer. At the ends of at least one layer of soil nails that have been constructed, typically the current layer and one or two layers above it are selected. Displacement and axial force sensors are pre-installed securely between the pad and lock nut on the soil nail head. The signal cables of all sensors should be led along the slope to the automatic data acquisition unit at the top of the pit, and waterproof and mechanical damage-resistant protection measures should be taken. The displacement sensor is used to continuously collect the cumulative horizontal movement distance of the soil nail head relative to the stable reference point outside the pit, and the axial force sensor is used to collect the change in tensile force borne by the soil nail rod.

[0035] In step three, after the i-th layer of shotcrete surface and soil nail grouting reaches the preset strength threshold, the cumulative horizontal displacement S obtained by the displacement sensor is read. i and current displacement rate v i And read the axial force increment ΔF obtained from the axial force sensor. i The preset strength threshold is typically set at 70% of the design strength of the shotcrete surface layer and 75% of the design strength of the soil nail grout. Under these conditions, the surface layer and soil nails possess preliminary bearing capacity, reflecting the true stress state after excavation. Displacement rate v i This refers to the cumulative increase in horizontal displacement per unit time. It can be achieved by continuously recording the displacement over time using an automatic data acquisition instrument, and taking the average rate within the most recent standard monitoring period. Axial force increment ΔF i It refers to the difference between the currently read axial force value and the baseline axial force value read after the previous layer of excavation is completed. It is used to measure the degree of influence of the current layer of excavation on the additional tensile force of the already constructed soil nails.

[0036] In step four, the current displacement rate v i With the preset displacement rate threshold V th Compare the axial force increment ΔF i Compared with the preset axial force increment threshold ΔF th Compare. If v i ≥V th or ΔF i ≥ΔF th Then adjust the excavation depth h of the next layer. {i+1} The original design value h i+1 0 Subtract the adjustment amount δ, and simultaneously adjust the length L of the next layer of soil nails. i+1 Original design value L i+1 0 Increase the adjustment amount ΔL. The adjustment amounts δ and ΔL are calculated using the following quantitative formulas: , where k δ λ is the excavation depth adjustment factor. s Δh is the formation sensitivity coefficient. i The actual over-excavation depth of the i-th layer; , where k L This is the soil nail length adjustment factor. Overcut depth Δh i Defined as the vertical distance between the actual excavated bottom surface and the designed excavated bottom surface, it is obtained by averaging the values ​​of three or more typical cross-sections after each layer of excavation is completed, and this value is not less than zero. (Sediment sensitivity coefficient λ) s This is a dimensionless parameter used to characterize the sensitivity of a specific soil mass to displacement and axial force responses under excavation and unloading conditions. It is taken as 0.5 to 0.7 for stiff plastic clay and dense sand, and 1.0 to 1.2 for soft plastic clay and loose fill. If the current displacement rate v...i Less than the displacement rate threshold V th And the axial force increment ΔF i It is also less than the axial force increment threshold ΔF th If no adjustment is needed, simply follow the original design parameters h of the next layer. i+1 and L i+1 0 Construction proceeded. Through the above comparisons and adjustments, a closed-loop control system was achieved, from monitoring data to parameter modification: when the displacement rate was too fast or the axial force suddenly increased, the excavation depth of the next layer was quantitatively reduced and the soil nail length was increased to suppress further deformation and continued deterioration of stress.

[0037] In step five, let i = i + 1, and repeat steps two through four until i = N, at which point all excavation layers and corresponding support construction are completed. After each layer is completed, the same monitoring, comparison, and adjustments are performed as needed, so that the support parameters of subsequent layers can be adaptively updated layer by layer according to the actual ground conditions, thereby maintaining a controllable deformation level and stress state throughout the entire foundation pit excavation process.

[0038] By implementing the construction method described above, step by step from one to five, the following technical effects can be achieved. Since the displacement rate and axial force increment are read after each step meets the strength requirements and quantitatively compared with preset thresholds, it is possible to promptly identify accelerated deformation and sudden stress changes caused by unfavorable geological conditions or over-excavation. When any monitoring indicator exceeds the limit, a dedicated adjustment formula incorporating the geological sensitivity coefficient and the actual over-excavation depth quantitatively calculates the reduction in the excavation depth of the next layer and the increase in the length of the soil nails for the next layer, avoiding the drawbacks of traditional methods that rely entirely on subjective judgment and arbitrary adjustments based on operator experience. This adjustment mechanism ensures that the adjustment range matches the level of risk, preventing both insufficient adjustment leading to uncontrolled safety risks and excessive adjustment causing unnecessary delays and material waste. Simultaneously, by directly incorporating the actual over-excavation depth into the adjustment calculation formula, localized over-excavation and its adverse effects, which are difficult to completely avoid during construction, are systematically compensated. This is achieved by proactively reducing the excavation depth of the next layer and increasing the length of the soil nails to offset the additional deformation and stress caused by the over-excavation of the current layer. The entire construction process formed a closed-loop control system of layer-by-layer monitoring, comparison, calculation, adjustment, and re-monitoring. After all N layers of excavation and support were completed, the cumulative horizontal displacement was effectively controlled within the design allowable range, the displacement rate was always below the safety threshold, and the soil nail axial force increment was also prevented from continuous sudden increases, thus significantly improving the safety, adaptability, and controllability of the segmented excavation construction of the foundation pit.

[0039] In another embodiment, in the segmented excavation construction method combined with soil nailing wall support, when the foundation pit adopts a composite support form of soil nails and prestressed anchors, if it is determined in step four that the parameters of the next layer need to be adjusted, the following sub-steps are executed: S41. Pause the prestress application of the next layer of anchor bolts. S42. First, complete the construction of the next layer of soil nails and the shotcrete surface layer. After the strength of the grout body of the soil nails in this layer reaches 70% of the design value, continuously monitor the axial force change curves of the soil nails in this layer and the adjacent completed soil nails in the upper layer for at least 24 hours, and record the initial axial force value F0 and the axial force change rate dF / dt. S43. Apply the prestress of the anchor bolts in the m-level loading mode. The number of loading levels m is determined according to the current axial force change rate dF / dt of the upper layer of soil nails according to the following rules: When dF / dt ≤ 0.10 kN / min, take m = 2 levels; When 0.10 < dF / dt ≤ 0.20 kN / min, take m = 3 levels; When dF / dt > 0.20 kN / min, take m = 4 levels; Divide the design prestress value into m equal parts, and apply one part for each level, that is, the prestress value applied for each level is 1 / m of the design prestress value. S44. Gradually apply the prestress of the anchor bolts. Immediately read the axial force change rate dF / dt of the adjacent upper layer of soil nails after each level is applied; if dF / dt ≥ 0.20 kN / min and lasts for more than 10 minutes after any single level is applied, pause the tensioning and maintain the current prestress for 24 hours until dF / dt drops below 0.05 kN / min, and then continue the next level of loading. [[ID=I6]] S45. After applying all m levels of prestress, continuously monitor the axial force change rate dF / dt of the adjacent upper layer of soil nails for at least 72 hours; if dF / dt ≥ 0.15 kN / min and shows an increasing trend, perform secondary supplementary tensioning, and the supplementary tensioning value is 10 - 15% of the initial design prestress value.

[0040] Based on the above steps 1 to 5, this embodiment further makes a specific description for the case where the foundation pit adopts the composite support form of soil nails and prestressed anchor bolts. When soil nails and prestressed anchor bolts are simultaneously set on the side wall of the foundation pit, the application of the prestress of the anchor bolts will have a direct impact on the axial force of the adjacent soil nails. If applied improperly, it is easy to cause a sudden increase in the axial force of the soil nails or even pull-out failure. Therefore, in step 4, if it is determined that the parameters of the next layer need to be adjusted, that is, the current displacement rate v i is greater than or equal to the preset displacement rate threshold V th , or the axial force increment ΔF i is greater than or equal to the preset axial force increment threshold ΔF th , instead of directly applying the prestress of the anchor bolts in the conventional manner, the following sub-steps are executed: First, execute sub-step S41 to pause the prestressing application of the next layer of anchors. That is, when monitoring data indicates that the excavation depth of the next layer needs to be reduced and the soil nail length increased, the prestressed anchors to be installed in that layer will not be tensioned temporarily to avoid applying prestress before the soil nails have fully played their role, which would result in additional loads.

[0041] Next, proceed to sub-step S42. First, complete the construction of the soil nailing and shotcrete surface layer for the next layer, i.e., layer i+1. After the grout strength of the soil nails in this layer reaches 70% of the design value, continuously monitor the axial force variation curves of the soil nails in this layer and the completed soil nails in the adjacent upper layer for at least 24 hours. Record the initial axial force value F_0 of the adjacent upper layer soil nails in the current state, and calculate the axial force variation rate dF / dt based on the continuous monitoring data. The axial force variation rate dF / dt is defined as the increase in the measured axial force value per unit time, usually expressed in kilonewtons per minute. Its value reflects the rate of increase of the tensile force on the soil nails under the influence of excavation and support of adjacent soil layers.

[0042] Then, sub-step S43 is executed to apply anchor prestress using a graded loading method. The number of loading stages, m, is determined according to the following rules based on the current rate of change of axial force dF / dt of the adjacent upper soil nails: when dF / dt does not exceed 0.10 kN / min, m equals stage 2; when dF / dt is greater than 0.10 kN / min but does not exceed 0.20 kN / min, m equals stage 3; when dF / dt exceeds 0.20 kN / min, m equals stage 4. The total design prestress value is divided into m equal parts, and the prestress value applied at each stage is one-mth of the total design prestress value. The purpose of graded loading is to coordinate the axial force response of the soil nails with the increase of anchor prestress, and to avoid abrupt changes in the axial force of adjacent soil nails caused by excessive single-stage loading.

[0043] Next, sub-step S44 is executed to apply anchor prestress in stages. After each stage of prestressing is applied, the rate of change of axial force dF / dt of the adjacent upper soil nail at the current moment is immediately read. If, after a certain stage is applied, dF / dt is greater than or equal to 0.20 kN / min and this rate lasts for more than 10 minutes, tensioning of subsequent stages is suspended, and the current prestress value is maintained for 24 hours. Only after continuous monitoring shows that dF / dt has fallen below 0.05 kN / min can the next stage of prestressing be applied. The purpose of this rule is that when a certain stage of prestressing causes the axial force of the adjacent soil nail to increase too rapidly and for a long period of time, it indicates that the bond between the soil nail and the soil may be approaching a critical state. Suspending tensioning and maintaining the current prestress allows the soil stress to redistribute. Loading can be resumed only after the rate of axial force increase has significantly decreased, thereby avoiding soil nail pull-out failure caused by continuous rapid tensioning.

[0044] Finally, execute sub-step S45. After all m-level prestressing has been applied, continuously monitor the axial force change rate dF / dt of the adjacent upper soil nails for no less than 72 hours. If dF / dt is found to be greater than or equal to 0.15 kN / min and shows a continuous increasing trend during this period, perform secondary tensioning. The prestress applied during secondary tensioning is 10% to 15% of the initial design prestress value. The purpose of secondary tensioning is to restore the active restraint capacity of the support system by appropriately tensioning the anchor bolts when soil creep or prestress loss leads to a decrease in anchoring force, thus preventing further deformation due to prestress attenuation.

[0045] By implementing the sub-steps S41 to S45 above, in cases of composite support where adjustments to the next layer of parameters are required, the adverse effects on the axial force of adjacent soil nails during the application of anchor prestress can be effectively controlled. Specifically, pausing prestress application and prioritizing the completion of soil nail and surface layer construction ensures that the soil nails first form bearing capacity; determining the number of loading stages based on the axial force change rate of adjacent upper-layer soil nails ensures that the loading scheme matches the current stress sensitivity of the soil nails; real-time monitoring of the axial force change rate and setting pause conditions during the step-by-step loading process avoids instantaneous over-limit of soil nail axial force caused by excessive single-stage prestress; long-term monitoring and secondary tensioning after all applications are completed compensate for the long-term effects caused by creep and prestress loss. Therefore, this implementation method is suitable for complex support conditions where soil nails and prestressed anchors work together. It can further ensure the safety of the anchor prestress application process in the composite support system by dynamically adjusting the excavation depth and soil nail length, and reduce the risk of sudden increase in soil nail axial force caused by sudden increase in prestress.

[0046] In another embodiment, the segmented excavation construction method combined with soil nailing wall support presets a displacement rate threshold V. th and preset axial force increment threshold ΔF th Calculate using the following formula: ; ; Where γ is the soil unit weight, h i The current excavation depth is given by E, where E is the soil compression modulus, φ is the internal friction angle, and Δt is the current excavation depth. standard The standard monitoring period is d, the diameter of the soil nail is τ, and the standard value of the bond strength between the soil nail and the soil is τ.

[0047] This embodiment targets a predetermined preset displacement rate threshold V. th and preset axial force increment threshold ΔF th The calculation method will be explained in detail. In step four, the current displacement rate v needs to be... i With the preset displacement rate threshold V th Compare the current axial force increment ΔF iCompared with the preset axial force increment threshold ΔF th Comparison, therefore V th and ΔF th The value of directly affects the triggering timing of parameter adjustment. This implementation method uses the following formula to calculate these two thresholds.

[0048] Preset displacement rate threshold V th The calculation formula is: Where γ is the soil weight, expressed in kilonewtons per cubic meter, representing the weight of a unit volume of soil; h i Δt represents the current excavation depth in meters, i.e., the vertical distance from the original ground surface to the bottom of the current layer after the i-th layer is excavated; φ represents the internal friction angle of the soil in degrees, reflecting the magnitude of the friction component in the soil's shear strength, and its tangent tanφ is dimensionless; E represents the soil's compression modulus in kilopascals, indicating the ratio of vertical stress to vertical strain under lateral confinement conditions; Δt standard The standard monitoring cycle is measured in minutes, typically 30 minutes, representing the time interval between two consecutive displacement readings.

[0049] Preset axial force increment threshold ΔF th The calculation formula is: Where d is the diameter of the soil nail in meters; τ is the standard value of the bond strength between the soil nail and the soil in kilonewtons per meter, defined here as the ultimate bond resistance provided per unit length of the soil nail. It should be noted that the value of τ should be determined based on the specific type of soil and grouting process. When the soil nail hole is of good quality and the grouting is complete, τ can be taken as a higher value; conversely, when the soil layer is loose or the grouting is insufficient, τ should be taken as a lower value.

[0050] In practical applications, V needs to be recalculated based on the parameters of each excavation layer. th and ΔF th For example, as the excavation depth h... i Increase layer by layer, γ×h i Gradually increasing, leading to V th The allowable displacement rate threshold for deeper excavation is appropriately relaxed, which aligns with the principle that soil at deeper depths experiences higher confining pressure and is therefore less prone to rapid deformation. Meanwhile, the soil's compression modulus E and internal friction angle φ are provided in the site investigation report and can be corrected based on measured data during construction. Standard monitoring period Δt standard Based on the sampling frequency setting of the monitoring system, it is usually 30 minutes. If the monitoring frequency is higher, it can be shortened appropriately, and vice versa. However, all comparisons of all periods must be unified to the same time reference. Axial force increment threshold ΔF th Primarily dependent on the soil nail diameter and bond strength, for the same site, d and τ are essentially fixed, therefore ΔF thThe design value of soil nail diameter usually remains constant across different excavation layers; however, when the design value of soil nail diameter varies with depth, ΔF needs to be adjusted accordingly. th The value of .

[0051] By using the above formula to calculate the preset threshold, the control indicators that were originally vaguely determined based on engineering experience are transformed into quantitative expressions directly related to soil physical and mechanical parameters, excavation depth, and soil nail geometry. This gives the displacement rate threshold and axial force increment threshold for each layer a clear theoretical basis and physical meaning. Furthermore, since the parameters in the formula, such as soil weight, compression modulus, internal friction angle, and bond strength, can all be obtained through conventional geotechnical tests or in-situ field tests, it has strong operability and repeatability. Compared with existing methods that use fixed values ​​or rely solely on experience to estimate warning values, the threshold calculation formula provided in this embodiment can adapt to changes in different sites, depths, and soil nail specifications, improving the matching degree between monitoring indicators and the actual engineering conditions. This makes the comparison and judgment in step four more scientific and reasonable, avoiding over-adjustment or under-adjustment caused by improper threshold setting.

[0052] In another embodiment, in the segmented excavation construction method combined with soil nailing wall support, after calculating δ and ΔL according to the formula in step four, the following verification step is also performed: using the measured displacement data and axial force data of the first m layers to invert and obtain the equivalent soil shear strength parameter c. eq φ eq and elastic modulus E eq Where m≥2; input the parameters obtained from the inversion into the finite element numerical model, and assign 1.2 times and 0.8 times the design load coefficient to the (i+1)th layer respectively to perform displacement and axial force calculations; when the maximum displacement obtained from the calculation exceeds the preset control value or the maximum axial force exceeds the pull-out bearing capacity, multiply δ and ΔL by 1.2 times the amplification factor respectively; the inversion adopts the particle swarm optimization algorithm and is updated once every two layers of excavation.

[0053] This implementation adds verification and correction steps for the adjustment amounts δ and ΔL. Specifically, after calculating the adjustment amount δ for the next layer of excavation depth and the adjustment amount ΔL for the next layer of soil nail length, these values ​​are not directly used as the final construction parameters. Instead, the following verification process is performed.

[0054] First, using the measured displacement and axial force data of the first m layers of completed construction, the equivalent soil shear strength parameters and elastic modulus are obtained through inversion analysis, where m is an integer greater than or equal to 2. The inversion employs a particle swarm optimization algorithm, a swarm intelligence optimization method that iteratively searches for the optimal solution in a given parameter search space by simulating the individual cooperation and information sharing mechanism in bird flock foraging behavior. The specific process is as follows: The parameter to be inverted is set as the equivalent cohesion c. eq Equivalent internal friction angle φeq and equivalent elastic modulus E eq Where c eq The dimensionless value of φ is kilopascal, representing the shear strength of soil when the normal stress is zero; eq The dimension of E is degrees, and its tangent value reflects the proportion of shear strength increasing with normal stress; eq The dimension of is kilopascal, representing the ratio of soil stress to strain. Using the measured horizontal displacement and soil nail axial force values ​​at each measuring point in the first m layers as target values, each set of parameters generated by the particle swarm optimization algorithm is input into the soil nail wall numerical calculation model to calculate the theoretical displacement and theoretical axial force at the corresponding locations. The sum of the squares of the errors between the theoretical and measured values ​​is used as the fitness function. The particle swarm optimization algorithm continuously updates the position and velocity of each particle, eventually converging to the parameter combination that minimizes the fitness function, i.e., the equivalent parameter c. eq φ eq and E eq The measured data required for the inversion include: the cumulative horizontal displacement value recorded by the displacement sensor at the end of each soil nail after each of the first m layers of excavation, and the axial force value of the soil nail recorded by the axial force sensor at the same time. After every two layers of excavation are completed, the value of m is increased by 2, and the parameter inversion is performed again to ensure that the equivalent parameters used in subsequent verification always reflect the actual mechanical response of the currently excavated soil layer.

[0055] Obtain the equivalent parameter c eq φ eq and E eqNext, these parameters are input into a pre-established finite element numerical model. This model is built according to the actual geometric dimensions of the foundation pit, layer thickness, soil nail spacing and length, and shotcrete surface layer thickness. The model boundary conditions are set as bottom-level fixed constraints and side-level horizontal displacement restrictions. The design loads applied to the model include the soil's self-weight, ground surcharge, and possible loads from surrounding buildings. For the (i+1)th layer, the next layer to be constructed, two design load cases are assigned: the first case multiplies the design load by a magnification factor of 1.2, and the second case multiplies the design load by a reduction factor of 0.8. Finite element calculations are performed under both cases to extract the maximum horizontal displacement of the foundation pit sidewall and the maximum axial force of the soil nails in that layer after excavation of the (i+1)th layer. The calculated maximum displacement is compared with a pre-determined preset displacement control value, and the calculated maximum axial force is compared with the design value of the soil nail pull-out bearing capacity. If the calculated maximum displacement exceeds the preset control value under the 1.2 times design load condition, or the calculated maximum axial force exceeds the pull-out bearing capacity under the 0.8 times design load condition, it indicates that the safety margin is insufficient for construction based on the current δ and ΔL, and there is a risk of excessive deformation or soil nail pull-out. In this case, multiply the δ and ΔL calculated in step four by a magnification factor of 1.2, i.e., use 1.2δ as the final adjustment amount for the next layer of excavation depth, and use 1.2ΔL as the final adjustment amount for the next layer of soil nail length. If the calculation results under both conditions do not exceed the corresponding control values, then keep the original δ and ΔL unchanged.

[0056] This verification step utilizes measured data from completed soil layers to invert and obtain equivalent soil parameters. This allows the finite element model to reflect the true mechanical behavior of the current site soil layers, rather than relying on initial parameters provided in the survey report, thereby improving the accuracy of numerical predictions in subsequent excavation stages. The particle swarm optimization algorithm is used for inversion, enabling efficient searching for the global optimum under multiple coupled parameters and avoiding getting trapped in local minima. Equivalent parameters are updated every two excavation layers, achieving a balance between computational accuracy and workload. This ensures timely correction of model parameters as the soil strata change, while avoiding the computational burden of inverting for each layer. Trial calculations are performed on the next layer with design load factors of 1.2 and 0.8 times, essentially conducting an envelope analysis to examine the response range of the support structure under load uncertainty. When the trial calculation results exceed the control limits, δ and ΔL are multiplied by a magnification factor of 1.2, equivalent to adding a 20% safety reserve on top of the original quantitative adjustment. This additional magnification is particularly important when the soil conditions are particularly weak or the load variation is large. Through the above verification and correction process, this implementation method ensures that the adjustment amounts δ and ΔL calculated in step four not only originate from the measured over-excavation and stratum sensitivity of the current layer, but also undergo dual verification based on inversion parameters and load envelope numerical simulation, thereby enhancing the rationality and reliability of the adjustment amounts and further reducing the construction safety risks caused by insufficient adjustment.

[0057] In another embodiment, the segmented excavation construction method combined with soil nailing wall support, in step four, v i With V th Comparison and ΔF i With ΔF th After comparison, the following destruction mode determination and differential adjustment steps are also performed: when v i ≥V th And ΔF i <0.6ΔF th When the condition is determined to be a precursor to surface layer bending deformation-dominated failure, the soil nail length adjustment ΔL for the next layer is increased by an additional 0.5L. i The excavation depth adjustment δ remains constant; when v i <0.5V th And ΔF i ≥ΔF th If the condition is determined to be a precursor to soil nail pull-out or bond slip-dominated failure, the adjustment amount δ for the next layer of excavation depth will be further reduced by 0.3h. i And multiply the soil nail length increment ΔL by a reinforcement factor of 1.3; when v i ≥V th And ΔF i ≥ΔF thWhen the condition is determined to be a precursor to a composite instability-type failure, δ and ΔL are doubled, the excavation of the (i+1)th layer is suspended, and two rows of full-length grouting anchor pipes are added to the i-th layer.

[0058] In step four of this embodiment, when the current displacement rate v is... i With the preset displacement rate threshold V th Compare and adjust the axial force increment ΔF i Compared with the preset axial force increment threshold ΔF th Following the comparison, a further step of determining the failure mode and making differentiated adjustments was added. Since foundation pit instability may originate from different mechanical mechanisms, and different instability precursors should be reinforced with different strategies, this implementation method classifies the failure precursors into three types based on the comparison results and implements targeted adjustment measures for each.

[0059] The first scenario: when v appears i ≥V th And ΔF i <0.6ΔF th When the current displacement rate has reached or exceeded the threshold, but the axial force increment is less than 0.6 times the threshold, it is judged as a precursor to surface layer bending deformation-dominated failure. In this mode, the horizontal displacement of the pit sidewall develops rapidly, but the increase in tensile force on the soil nails is not significant, indicating that the deformation mainly originates from the bending and deflection of the shotcrete surface layer itself rather than the pull-out or bond failure of the soil nails. Surface layer bending deformation is usually caused by insufficient surface layer thickness, weak reinforcement mesh, or high lateral soil pressure, leading to bulging deformation of the surface layer into the pit. This deformation pulls the soil nail head inward but has not yet caused bond failure along the entire length of the soil nail. For this mode, the following adjustment measures are taken: the adjustment amount ΔL for the next layer of soil nails is increased by an additional 0.5L. i L i Let be the design length of the soil nail for the i-th layer; simultaneously, the excavation depth adjustment δ remains unchanged from its original value calculated according to the formula. Increase the soil nail length by 0.5L. i The purpose is to extend the soil nail so that its anchoring section penetrates deeper into a more stable soil layer, thereby enhancing the soil nail's ability to restrain the surface layer and suppressing the development of bending deformation of the surface layer. The reason for keeping the excavation depth adjustment unchanged is that the axial force increment has not exceeded the limit, and there is no need to further reduce the excavation depth.

[0060] The second scenario: when v appears i <0.5V th And ΔF i ≥ΔF thWhen the current displacement rate is less than 0.5 times the displacement rate threshold, but the axial force increment has reached or exceeded the axial force increment threshold, it is judged as a precursor to soil nail pull-out or bond-slip-dominated failure. In this mode, although the overall displacement of the pit sidewall is not large, the soil nail stress is close to or exceeds its pull-out bearing capacity, indicating that slippage or local pull-out has occurred at the bond interface between the soil nail and the soil. This situation often occurs in soils with insufficient soil nail length, incomplete grouting, or soil layers with low bond strength such as soft plastic clay or loose fill. If not intervened in time, the soil nail will gradually be pulled out of the soil, leading to sudden failure of the support system. For this mode, the following adjustment measures are taken: the adjustment amount δ for the next layer of excavation depth is further reduced by 0.3h. i This means subtracting 0.3 times the design excavation depth of the i-th layer from the δ value calculated by the formula, thus reducing the actual excavation depth of the next layer by a greater margin than the original design value. Simultaneously, the soil nail length increment ΔL is multiplied by a reinforcement factor of 1.3, resulting in an additional 30% increase in ΔL. This additional reduction in excavation depth lowers the additional earth pressure on the soil nails after the next layer of excavation is completed, while the increased reinforcement factor directly increases the anchorage length of the soil nails, improving pull-out bearing capacity and thus inhibiting further development of bond slip.

[0061] The third scenario: when v appears i ≥V th And ΔF i ≥ΔF th When both the displacement rate and the axial force increment reach or exceed their respective thresholds, it is considered a precursor to composite instability failure. This mode indicates that the foundation pit sidewall is simultaneously facing the dual crisis of rapid deformation and excessive soil nail stress, often signifying that the overall stability is nearing its limit and may develop into overall slippage or collapse. To address this mode, more stringent adjustment measures are implemented: the calculated excavation depth adjustment δ is doubled, i.e., 2δ is used as the actual adjustment; simultaneously, the soil nail length adjustment ΔL is also doubled, i.e., 2ΔL is used. Furthermore, excavation work on the next layer, i.e., the (i+1)th layer, is suspended, and further excavation is prohibited. Two rows of full-length grouting anchor pipes are added to the current i-th layer. Full-length grouting anchor pipes are reinforcing components formed by drilling holes at certain intervals on the slope surface, inserting steel pipes, and performing full-length pressure grouting. Their length is usually equivalent to or slightly longer than the soil nail length of the i-th layer. After grouting, they form a high-strength bond with the soil, significantly enhancing the overall shear resistance of the soil in that layer. The purpose of adding two rows of full-length grouting anchor pipes is to immediately reinforce the currently excavated layer in an emergency situation where both displacement and axial force exceed the limits, preventing further deterioration of deformation and stress. Excavation can only continue downward after the grouting anchor pipes have reached the required strength.

[0062] The aforementioned differentiated adjustments based on failure modes avoid the blind application of the same set of adjustment parameters regardless of the specific instability mechanism. When surface bending deformation is dominant, increasing the length of soil nails strengthens the constraint on the surface layer without excessively reducing the excavation depth, thus maintaining construction progress. When soil nail pull-out is dominant, the focus is on reducing the excavation depth and enhancing the anchoring capacity of the soil nails, rather than simply increasing the length of the soil nails. For complex instability, the strongest comprehensive intervention measures are adopted, including doubling the adjustment amount, suspending excavation, and adding full-length grouting anchor pipes. This graded and categorized approach ensures that the adjustment of support parameters matches the actual instability mechanism, preventing safety accidents caused by insufficient reinforcement and avoiding material waste and project delays caused by over-reinforcement. Furthermore, by clearly defining the threshold conditions for the three modes, on-site technicians can quickly determine the current type of failure precursor based on measured data and execute construction according to the prescribed adjustment amounts, improving the scientific nature and operability of construction decisions.

[0063] In another embodiment, the segmented excavation construction method combined with soil nailing wall support, in step three, reads the current displacement rate v. i Then, the second derivative of the displacement rate, a, was calculated. i =d 2 S / dt 2 ;when a i >0 and a within three consecutive monitoring periods i When it is monotonically increasing, even if the current v i <V th The comparison condition in step four is also replaced with the virtual displacement rate v. i′ =v i ×(1+α·T), where α is the acceleration weighting coefficient and T is the ratio of the acceleration duration to the standard period; the formula for calculating T is T=(t current ‒t start ) / Δt standard , t current For the current moment (i.e., to calculate the second derivative of the displacement rate a) i The corresponding monitoring time is usually the time point of the latest displacement reading. start The starting time (referring to the first time the displacement rate v is detected) i The starting point of the sustained acceleration trend (i.e., the beginning of the first cycle in three consecutive monitoring periods), Δt standard The standard monitoring cycle is 30 minutes; when v i′ ≥V th At that time, the parameter adjustment in step four is triggered.

[0064] In this embodiment, the current displacement rate v is read in step three. iSubsequently, the calculation of the second derivative of the displacement rate and the construction of a virtual displacement rate were further added. Simply comparing the displacement rate with a preset threshold often only reflects the magnitude of the current deformation rate, but fails to capture the trend of accelerating displacement. When the displacement rate, although not yet reaching the threshold, shows a continuously positive and increasing acceleration, it indicates that the deformation is evolving from a stable state to an unstable state. If adjustment measures are only taken after the rate itself exceeds the limit, the optimal intervention opportunity may have been missed. Therefore, this implementation introduces the second derivative of the displacement rate to identify the accelerating displacement trend and construct a virtual displacement rate as a basis for early warning.

[0065] Specifically, in step three, the current displacement rate v is read. i Next, the second derivative of the displacement rate, a, is calculated. i Displacement rate v i Defined as the first derivative of the cumulative horizontal displacement S with respect to time t, i.e., v i =dS / dt, where the unit is millimeters per minute or millimeters per hour. The second derivative of the displacement rate, a. i It is then defined as the derivative of displacement rate with respect to time, i.e., a i =d 2 S / dt 2 =dv i / dt, its unit is millimeters per square minute or millimeters per square hour. i The physical meaning is the acceleration of the displacement rate, when a i A value greater than zero indicates that the displacement rate is increasing, i.e., the deformation is accelerating; when a i A value less than zero indicates that the displacement rate is decreasing, meaning the deformation is tending to converge; when a i A value equal to zero indicates that the displacement rate remains constant. In engineering monitoring, a i The average acceleration can be approximated by using the difference method based on displacement rate data from three or more consecutive monitoring cycles: the difference in rate between two adjacent monitoring cycles is divided by the time interval to obtain the average acceleration within that time period.

[0066] After calculating a i Next, determine its condition: when a i Greater than zero, and within three consecutive monitoring periods a i When the value of exhibits a monotonically increasing trend, that is, when 'a' is calculated in each cycle... i All are greater than a in the previous period i This indicates that the degree of displacement acceleration is continuously increasing. At this point, even if the displacement rate v at the current moment... i Still less than the preset displacement rate threshold V th It doesn't directly follow the original v. i Instead of making comparisons, a virtual displacement rate v is constructed.i′ Its calculation formula is: v i′ =v i ×(1+α·T). Where α is the acceleration weighting coefficient, dimensionless, and its value reflects the sensitivity of acceleration to displacement development. Based on engineering experience, it is usually taken between 0.5 and 1.5. The larger α is, the more significant the amplification effect of acceleration on the virtual velocity. T is the ratio of the acceleration duration to the standard monitoring cycle, dimensionless, and its calculation formula is T=(t current -t start ) / Δt standard In the formula, t current For the current moment, i.e., to calculate the second derivative of the displacement rate a i The corresponding monitoring time is usually the time point of the latest displacement reading, in minutes; t start The starting time is the point in time when the displacement rate v_i first begins to show a sustained acceleration trend, specifically the start time of the first cycle in three consecutive monitoring cycles, measured in minutes; Δt standard The standard monitoring period is limited to 30 minutes, with the unit being minutes. Therefore, T represents the time span from the start of the acceleration trend to the current moment, which is equivalent to how many standard monitoring periods. The larger T is, the longer the acceleration state lasts.

[0067] Whenever the calculated virtual displacement rate v i′ Greater than or equal to the preset displacement rate threshold V th When this happens, the parameter adjustment process in step four is triggered, including reducing the excavation depth of the next layer and increasing the length of the soil nails in the next layer. Conversely, if v i′ Still less than V th If the condition is not met, adjustments will not be triggered, and normal construction will continue.

[0068] This implementation method, by calculating the second derivative of the displacement rate and monitoring its continuous monotonically increasing trend, can promptly detect early signs of accelerated deformation without waiting for the displacement rate itself to reach a warning value. Traditional rate threshold judgment methods are essentially based on the magnitude of the rate, which introduces a time lag as the rate gradually accelerates from a small value to an excessive value. However, the virtual displacement rate constructed in this implementation method considers both the magnitude and duration of acceleration. This means that even if the current rate has not yet exceeded the limit, as long as the acceleration remains positive and continuously increases, the virtual rate may reach or exceed the threshold in advance, triggering parameter adjustments. This early warning mechanism provides on-site construction personnel with more response time, allowing them to take preventative measures such as reducing excavation depth or increasing soil nail length before the deformation significantly exceeds the limit, avoiding a reactive approach after the deformation rate has already significantly exceeded the standard. Simultaneously, using three consecutive monitoring cycles, a... iUsing monotonically increasing as a judgment condition can effectively eliminate false acceleration signals caused by single measurement errors or random fluctuations, thus improving the reliability of the judgment.

[0069] In another embodiment, the segmented excavation construction method combined with soil nailing wall support, in step one, divides each excavation layer into a corner zone, a long side mid-section zone, and a short side mid-section zone according to the plan shape of the foundation pit, and sets a displacement rate threshold V for each zone. th,corner V th,long and V th,short V th,corner =0.6×V th,long V th,short =0.9×V th,long Each area is independently equipped with a corresponding displacement sensor and axial force sensor; in step four, it is determined whether parameter adjustment is triggered in each area. When any area is triggered, only the next layer of support corresponding to that area is locally reinforced, and the local reinforcement is to increase the length of the soil nail in that area by 0.3L. i The horizontal spacing of the soil nails was increased to 0.7 times the original spacing, while the original design parameters were maintained in the remaining areas.

[0070] This implementation further refines the consideration of the excavation pit's planar shape in step one. Typically, the deformation distribution of an excavation pit on a plane is not uniform. Due to the mutual constraint of the soil on both sides, the lateral displacement of the corner area is often smaller than that of the middle area of ​​the long side, while the deformation of the middle area of ​​the short side falls between the two. Applying a uniform displacement rate threshold to the entire excavation pit could lead to an unreasonable situation where the corner area is overly strictly controlled while the middle area of ​​the long side is under-controlled. Therefore, in step one, each excavation layer is divided into a corner area, a middle area of ​​the long side, and a middle area of ​​the short side based on the excavation pit's planar shape. The corner area refers to the corner where two adjacent sides meet in the excavation pit's planar shape, typically taken from the corner vertex extending approximately one-fifth of the side length to each side. The middle area of ​​the long side refers to the middle portion of the longest side of the excavation pit, excluding the corner area. The middle area of ​​the short side refers to the middle portion of the shortest side of the excavation pit, also excluding the corner area.

[0071] Set a displacement rate threshold for each of the three regions mentioned above. Set the displacement rate threshold V. th,long As a benchmark value, this value can be determined based on the formula provided above or regional experience. Displacement rate threshold V in the corner zone. th,corner Take V as 0.6 times. th,long V th,corner =0.6×V th,long The displacement rate threshold V in the middle region of the short side. th,short Take V as 0.9 times. th,long V th,short =0.9×V th,longThe physical reason for the lower threshold in the corner area is that the soil at the corner is constrained on both sides, resulting in lower deformation capacity. Furthermore, excessive displacement at this point would adversely affect the adjacent slopes on both sides, thus requiring stricter control standards. The middle area of ​​the long side has the weakest constraint, making deformation most likely, and thus has the highest threshold, allowing for a relatively large displacement rate. The middle area of ​​the short side falls between these two, and a midpoint is taken.

[0072] Displacement and axial force sensors are installed independently in each region. Specifically, displacement and axial force sensors are installed at the ends of several soil nails in the corner region, the same sensors are installed at the ends of several soil nails in the middle region of the long side, and the same sensors are installed at the ends of several soil nails in the middle region of the short side. Sensor data in each region is collected and recorded independently without interference. The sensor density in each region should be sufficient to represent the typical deformation and stress state of that region. The sensors are typically denser at corners, as stress concentration is more pronounced in these areas.

[0073] In step four, it is determined whether the monitoring data for each region triggers parameter adjustments. Specifically, for the corner region, its current displacement rate is compared with the threshold V for that region. th,corner Compare the axial force increment with the preset axial force increment threshold ΔF. th Compare; for the middle region of the long side, compare its current displacement rate with V. th,long Compare; for the middle region of the short side, compare its current displacement rate with V. th,short Comparison. Any region showing v i ≥ V corresponding to this region th or ΔF i ≥ΔF th When an adjustment is triggered, it is determined that the area requires adjustment. However, the adjustment is not carried out uniformly across the entire foundation pit; instead, it is only implemented locally, strengthening the next layer of support corresponding to the area that triggered the adjustment. The specific measures for local reinforcement are: extending the length of the next layer of soil nails in this area to the original design length L. i+1 0 Increase the current soil nail length L by 0.3 times based on the existing soil nail length. i That is, the increase in soil nail length is 0.3L. i Simultaneously, the horizontal spacing of the next layer of soil nails in this area is increased to 0.7 times the original design spacing, meaning the horizontal distance between two adjacent rows of soil nails is reduced to seven-tenths of its original value. Except for the areas that trigger the adjustment, the remaining areas maintain their original design parameters and are not reinforced in any way.

[0074] For example, if monitoring data in the middle section of the long side triggers an adjustment, only the soil nails in the next layer located in the middle section of the long side will be lengthened and densified. The soil nails in the corner and middle sections of the short side will still be constructed according to the original design length and spacing. If both the corner and middle sections of the long side trigger adjustments simultaneously, each area will be locally reinforced independently. This method of independent judgment and local reinforcement avoids the material waste and construction delays caused by changing the entire next layer of the foundation pit to a reinforcement scheme.

[0075] This implementation method sets differentiated displacement rate thresholds based on the planar shape of the foundation pit, ensuring that the control standards for each area match its actual deformation potential. Stricter thresholds are used in corner areas to guarantee overall stability, while more lenient thresholds are used in long-side areas to avoid unnecessary frequent adjustments. Secondly, each area has independently installed sensors and makes independent judgments, accurately identifying the location of weak areas and preventing overreactions that trigger comprehensive adjustments to the entire foundation pit due to anomalies in individual areas. Thirdly, when an area requires adjustment, only the corresponding next-layer support is locally reinforced. Specific measures include increasing the soil nail length by 0.3 times and increasing the horizontal spacing by 0.7 times. Both measures have clear quantitative indicators, facilitating on-site implementation. This targeted reinforcement increases support capacity at the most needed locations without affecting normal construction in other areas, thus maximizing construction efficiency while ensuring construction safety. Finally, since the reinforcement is limited to one or more localized areas, changes in the length and spacing of the reinforced soil nails do not significantly increase the overall project volume and cost, demonstrating good economic efficiency.

[0076] In another embodiment, the segmented excavation construction method combined with soil nailing wall support, the stratum sensitivity coefficient λ in step four... s The dynamic update is based on the cumulative ratio of the measured displacement of the excavated layer to the theoretical design displacement, and the update formula is λ. s =λ s0 ×(ΣS i,实测 / ΣS i,理论 ) β , where λ s0 β is the initial sensitivity coefficient based on the preliminary values ​​obtained from the geological exploration report, and β is the correction exponent with a value range of 0.8 to 1.2; when λ s When two consecutive layers have a displacement rate greater than 1.3, the preset displacement rate threshold V in step four is adjusted. th and preset axial force increment threshold ΔF th Divide by λ respectively s Perform downward correction; when λ s If two consecutive layers have a depth less than 0.7, the design excavation depth h for subsequent layers will be increased. i 0 Increased by 5%.

[0077] Based on step four, this implementation method further achieves the formation sensitivity coefficient λ_ s The dynamic update. In the formulas δ and ΔL in step four, λ s The initial values ​​for λ are determined based on the soil type given in the geological survey report: 0.5 to 0.7 for stiff plastic clay and dense sand, and 1.0 to 1.2 for soft plastic clay and loose fill. However, the soil type descriptions provided in the geological survey report are often qualitative, and the distribution of soil layers at different locations within the same site may be uneven. Therefore, the initial values ​​for λ... s This may not accurately reflect the actual excavation response. Therefore, this implementation method uses the cumulative ratio of the measured displacement of the excavated layer to the theoretical design displacement to adjust λ. s Dynamic updates are performed to ensure that subsequent adjustment calculations better reflect the actual stratigraphic conditions that have been revealed.

[0078] The specific update formula is: λ s =λ s0 ×(ΣS i,实测 / ΣS i,理论 ) β Where λ s0 ΣS represents the initial sensitivity coefficient based on preliminary values ​​obtained from the geological survey report; it is dimensionless. i,实测 ΣS represents the sum of the cumulative measured horizontal displacements from layer 1 to the latest completed layer, totaling i layers, in millimeters; i,理论 This represents the sum of the cumulative horizontal displacements of each floor calculated theoretically under the original design parameters, in millimeters; β is the correction exponent, dimensionless, ranging from 0.8 to 1.2. The physical meaning of this formula is: when the actual cumulative displacement is greater than the theoretically calculated value, the ratio is greater than 1, and λ... s Greater than λ s0 This indicates that the formation is more sensitive than expected, and the subsequent adjustment should be increased accordingly; conversely, when the actual cumulative displacement is less than the theoretically calculated value, λ s Less than λ s0 This indicates that the formation is less sensitive than expected, and subsequent adjustments can be appropriately reduced. The exponent β is used to control the sensitivity of the correction; the closer β is to 1.2, the larger the correction magnitude; the closer β is to 0.8, the gentler the correction magnitude. 1.0 is typically used as the default value. In actual implementation, after each layer of excavation is completed and the measured cumulative horizontal displacement of that layer is obtained, the cumulative ratio is recalculated and λ is updated. s This is used for parameter adjustment calculations in the next layer. Additionally, this implementation also sets up a λ-based... s Threshold correction for the value range and design rules for adjusting excavation depth.

[0079] First amendment rule: When λ sWhen two consecutive layers have a displacement rate greater than 1.3, it indicates that the actual sensitivity of the formation is consistently significantly higher than the initial assessment. In this case, if the original preset displacement rate threshold V is still used... th and preset axial force increment threshold ΔF th This is equivalent to using an overly lenient standard to measure the actual response, which can easily lead to untimely adjustment triggering. Therefore, the preset displacement rate threshold V in step four is... th and preset axial force increment threshold ΔF th Divide by λ respectively s Perform a downward correction. The corrected displacement rate threshold is the original V. th Divide by the current λ s The corrected threshold for axial force increment is the original ΔF. th Divide by the current λ s Because of λ s When the value is greater than 1.3, the division operation reduces both thresholds, resulting in stricter control standards: smaller displacement rates and smaller axial force increments can trigger parameter adjustments. This correction facilitates early intervention in identified sensitive formations, avoiding reactive responses after deformation and stress have developed to a significant extent.

[0080] Second amendment rule: When λ s When two consecutive layers have a depth of less than 0.7, it indicates that the actual sensitivity of the strata is significantly lower than initially assessed, meaning the soil is harder or denser than expected, resulting in smaller displacements and axial forces at the same excavation depth. In this case, construction efficiency can be appropriately increased. Therefore, the designed excavation depth h for subsequent layers should be increased. i 0 This means the excavation thickness for each layer specified in the original design documents will be increased by 5%. The adjustment method is as follows: for each layer that has not yet been constructed, the original design depth will be multiplied by 1.05 to obtain the new design excavation depth value. It should be noted that this increase refers to the design excavation depth h. i 0 The adjustment amount δ is not the actual amount. Increasing the design excavation depth means that each layer can be excavated 5% thicker than originally planned, thereby reducing the total number of excavation layers or speeding up the construction progress. However, the adjustment amount δ in step four is still calculated according to the formula and is based on the increased h. i 0 Adjustments need to be made. Additionally, when λ... s When the value is restored to between 0.7 and 1.3, the threshold and design excavation depth should be restored to their original settings and no further corrections should be made.

[0081] This implementation dynamically updates the formation sensitivity coefficient by accumulating the ratio of measured displacement to theoretical displacement layer by layer, thus ensuring that the λ value used in subsequent construction is consistent with the actual displacement. sInstead of a fixed initial value, the value continuously approximates the actual geological mechanical response as the excavation process progresses. This adaptive mechanism effectively compensates for the discrepancy between geological survey data and actual conditions, improving the accuracy of the adjustment calculation formula. Simultaneously, when λ... s When λ remains consistently high, actively tightening the displacement rate threshold and axial force increment threshold is equivalent to automatically raising the safety alert level in sensitive formations; when λ s When the threshold remains consistently low, appropriately increasing the design excavation depth of subsequent layers is a reasonable way to improve construction efficiency while ensuring safety. The two rules of downward threshold correction and upward design excavation depth correction form a symmetrical adaptive control strategy, enabling the entire construction method to automatically adjust the control strictness and construction progress based on measured feedback, thereby enhancing the method's adaptability and economy to different geological conditions.

[0082] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0083] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A segmented excavation construction method combining soil nailing wall support, characterized in that, It includes the following steps: Step 1: Divide the excavation pit into N layers according to the designed depth, and set the excavation depth h of the i-th layer. i The corresponding design length L of the soil nail i ; Step 2: Excavate the i-th layer of soil to the designed depth h. i Soil nails and shotcrete surface layer are constructed on the i-th excavation surface, and displacement sensors and axial force sensors are installed at the ends of at least one layer of soil nails that have been constructed. Step 3: After the i-th layer of shotcrete surface and soil nail grouting reaches the preset strength threshold, read the cumulative horizontal displacement S obtained by the displacement sensor. i and current displacement rate v i And read the axial force increment ΔF obtained from the axial force sensor. i ; Step 4: v i With the preset displacement rate threshold V th Compare ΔF i Compared with the preset axial force increment threshold ΔF th Compare; if v i ≥V th or ΔF i ≥ΔF th Then adjust the excavation depth h of the next layer. i+1 for h i+1 =h i+1 0 -δ, while adjusting the length L of the next layer of soil nails. i+1 For L i+1 =L i+1 0 +ΔL; ； ； Where δ is the adjustment amount for the excavation depth of the (i+1)th layer, in meters; ΔL is the adjustment amount for the length of the soil nails in the (i+1)th layer, in meters; k δ k is a dimensionless adjustment factor for excavation depth. L λ is the soil nail length adjustment factor, dimensionless; s The formation sensitivity coefficient is 0.5–0.7 for stiff plastic clay and dense sand, and 1.0–1.2 for soft plastic clay and loose fill, dimensionless; Δh i Let Δh be the actual over-excavation depth of the i-th layer. i ≥0; Otherwise, according to the original design parameter h i+1 0 and L i+1 0 Proceed with the next layer of excavation and support; Step 5: Let \(i = i + 1\), and repeat Steps 2 to 4 until \(i = N\) to complete the construction of all excavation layers and corresponding support.

2. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, When the composite support form of soil nails and prestressed anchor rods is adopted for the foundation pit, if it is determined in Step 4 that the parameters of the next layer need to be adjusted, the following sub-steps shall be executed: S41: Suspend the prestress application of the anchor rods in the next layer. S42: First complete the construction of the soil nails and shotcrete surface layer in the next layer. After the strength of the grouting body of the soil nails in this layer reaches 70% of the design value, continuously monitor the axial force change curves of the soil nails in this layer and the completed soil nails in the adjacent upper layer for at least 24 hours, and record the initial axial force value \(F_0\) and the axial force change rate \(dF / dt\). S43: Apply the prestress of the anchor rods in the \(m\)-level loading mode. The number of loading levels \(m\) is determined according to the current axial force change rate \(dF / dt\) of the upper-layer soil nails according to the following rules: When \(dF / dt\leq0.10\ kN / min\), take \(m = 2\) levels; When \(0.10 < dF / dt\leq0.20\ kN / min\), take \(m = 3\) levels; When \(dF / dt>0.20\ kN / min\), take \(m = 4\) levels; Divide the design prestress value into \(m\) equal parts, and apply one part for each level, that is, the prestress value applied for each level is \(1 / m\) of the design prestress value. S44: Gradually apply the prestress of the anchor rods, and immediately read the axial force change rate \(dF / dt\) of the soil nails in the adjacent upper layer after each application; if \(dF / dt\geq0.20\ kN / min\) and lasts for more than 10 minutes after any single-level application, suspend the tensioning and maintain the current prestress for 24 hours until \(dF / dt\) drops below \(0.05\ kN / min\), and then continue the next-level loading. S45: After applying all \(m\) levels of prestress, continuously monitor the axial force change rate \(dF / dt\) of the soil nails in the adjacent upper layer for at least 72 hours; if \(dF / dt\geq0.15\ kN / min\) and shows an increasing trend, perform secondary supplementary tensioning, and the supplementary tensioning value is 10 - 15% of the initial design prestress value.

3. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, Preset displacement rate threshold V th and preset axial force increment threshold ΔF th Calculate using the following formula: ； ； Where γ is the soil unit weight, h i The current excavation depth is given by E, where E is the soil compression modulus, φ is the internal friction angle, and Δt is the current excavation depth. standard The standard monitoring period is d, the diameter of the soil nail is τ, and the standard value of the bond strength between the soil nail and the soil is τ.

4. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, After calculating δ and ΔL according to the formula in step four, the following verification steps are also performed: the equivalent soil shear strength parameter c is obtained by inverting the measured displacement data and axial force data of the first m layers. eq φ eq and elastic modulus E eq Where m≥2; input the parameters obtained from the inversion into the finite element numerical model, and assign 1.2 times and 0.8 times the design load coefficient to the (i+1)th layer respectively to perform displacement and axial force calculations; when the maximum displacement obtained from the calculation exceeds the preset control value or the maximum axial force exceeds the pull-out bearing capacity, multiply δ and ΔL by 1.2 times the amplification factor respectively; the inversion adopts the particle swarm optimization algorithm and is updated once every two layers of excavation.

5. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, In step four, v i With V th Comparison and ΔF i With ΔF th After comparison, the following destruction mode determination and differential adjustment steps are also performed: when v i ≥V th And ΔF i <0.6ΔF th When the condition is determined to be a precursor to surface layer bending deformation-dominated failure, the soil nail length adjustment ΔL for the next layer is increased by an additional 0.5L. i The excavation depth adjustment δ remains constant; when v i <0.5V th And ΔF i ≥ΔF th If the condition is determined to be a precursor to soil nail pull-out or bond slip-dominated failure, the adjustment amount δ for the next layer of excavation depth will be further reduced by 0.3h. i And multiply the soil nail length increment ΔL by a reinforcement factor of 1.3; when v i ≥V th And ΔF i ≥ΔF th When the condition is determined to be a precursor to a composite instability-type failure, δ and ΔL are doubled, the excavation of the (i+1)th layer is suspended, and two rows of full-length grouting anchor pipes are added to the i-th layer.

6. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, In step three, the current displacement rate v is read. i Then, the second derivative of the displacement rate, a, was calculated. i =d 2 S / dt 2 ;when a i >0 and a within three consecutive monitoring periods i When it is monotonically increasing, even if the current v i <V th The comparison condition in step four is also replaced with the virtual displacement rate v. i′ =v i ×(1+α·T), where α is the acceleration weighting coefficient and T is the ratio of the acceleration duration to the standard period; The formula for calculating T is T = (t current ‒t start ) / Δt standard , t current For the current moment (i.e., to calculate the second derivative of the displacement rate a) i The corresponding monitoring time is usually the time point of the latest displacement reading. start The starting time (referring to the first time the displacement rate v is detected) i The starting point of the sustained acceleration trend (i.e., the beginning of the first cycle in three consecutive monitoring periods), Δt standard The standard monitoring cycle is 30 minutes; when v i′ ≥V th At that time, the parameter adjustment in step four is triggered.

7. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, In step one, each excavation layer is divided into a corner zone, a long-side mid-section zone, and a short-side mid-section zone according to the plan shape of the foundation pit, and a displacement rate threshold V is set for each zone. th,corner V th,long and V th,short V th,corner =0.6×V th,long V th,short =0.9×V th,long Each area is independently equipped with a corresponding displacement sensor and axial force sensor; in step four, it is determined whether parameter adjustment is triggered in each area. When any area is triggered, only the next layer of support corresponding to that area is locally reinforced, and the local reinforcement is to increase the length of the soil nail in that area by 0.3L. i The horizontal spacing of the soil nails was increased to 0.7 times the original spacing, while the original design parameters were maintained in the remaining areas.

8. The segmented excavation construction method combined with soil nailing wall support as described in claim 1, characterized in that, The formation sensitivity coefficient λ in step four s The dynamic update is based on the cumulative ratio of the measured displacement of the excavated layer to the theoretical design displacement, and the update formula is λ. s =λ s0 ×(ΣS i,实测 / ΣS i,理论 ) β , where λ s0 β is the initial sensitivity coefficient based on the preliminary values ​​obtained from the geological exploration report, and β is the correction exponent with a value range of 0.8 to 1.2; when λ s When two consecutive layers have a displacement rate greater than 1.3, the preset displacement rate threshold V in step four is adjusted. th and preset axial force increment threshold ΔF th Divide by λ respectively s Perform downward correction; when λ s If two consecutive layers have a depth less than 0.7, the design excavation depth h for subsequent layers will be increased. i 0 Increased by 5%.

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