Construction methods for deep foundation pit excavation with graded slope in arid regions

Abstract

This invention discloses a method for graded slope excavation of deep foundation pits in arid regions, belonging to the field of geotechnical engineering technology. This method solves the problem in existing graded slope excavation methods that cannot differentiate support based on the risk of soil shrinkage and crack development, leading to local slope instability or wasted support resources. The key technical points are: conducting a site survey of the foundation pit soil to obtain natural water content, liquid limit, plastic limit, clay content, and crack characteristic parameters as quantitative indicators; dividing the slope into units based on these quantitative indicators and assigning crack development risk values; classifying each unit into high-risk and low-risk areas based on the crack development risk values; and excavating step-by-step from top to bottom according to the graded slope sequence, completing differentiated support construction for the corresponding area after each level is finished before proceeding to the next level. This method is mainly used for graded slope excavation of large foundation pits in arid regions with a risk of shrinkage and crack development.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering technology. More specifically, this invention relates to a method for graded slope excavation of deep foundation pits in arid regions. Background Technology

[0002] In deep foundation pit engineering in arid regions, graded slope excavation is a common construction method when site conditions permit. However, in arid regions, collapsible loess, expansive soil, and other special soil types experience shrinkage cracks on the slope surface due to continuous water loss under strong evaporation conditions. These cracks form an irregular network on the slope surface, damaging the integrity of the soil and creating a potential hazard for slope instability during subsequent excavation and unloading.

[0003] Current graded slope excavation methods, when addressing shrinkage cracks, typically employ uniform support parameters to reinforce the entire slope section, failing to differentiate between areas with varying degrees of crack development. This approach presents several problems: in areas with dense and interconnected cracks, uniform support often fails to provide sufficient resistance to prevent further crack propagation and penetration, easily leading to localized collapse; conversely, in sparsely cracked areas, uniform support may result in wasted support materials. This issue arises because existing methods lack the means to pre-assess the risk of crack development in different areas of the slope, and cannot precisely match support schemes based on varying risk levels.

[0004] In predicting the risk of crack development, existing technologies typically rely solely on empirical judgments based on basic physical properties of the soil, making it difficult to quantify the propagation behavior of shrinkage cracks under the combined effects of excavation unloading and moisture evaporation. Crack propagation is influenced not only by factors such as soil moisture content and clay content but also by the spatial distribution of the cracks themselves. However, current methods lack a systematic approach that combines crack geometric parameters with soil mechanical parameters to quantitatively calculate crack propagation length and connectivity through numerical simulation. This results in the inability to accurately assess the crack development risk in each unit of the slope, and a lack of quantifiable basis for subsequent risk zone delineation.

[0005] Regarding the differentiated design of support structures, existing methods lack clear zonal support construction schemes for different slope areas classified as high-risk and low-risk. High-risk areas require a structural form combining deep reinforcement and slope protection to resist potential sliding failure after crack penetration, but existing methods do not specify the component types, layout parameters, and connection methods for such structures. Low-risk areas require a structural form mainly based on shallow reinforcement, with resistance lower than that of the support structure in high-risk areas, but existing methods also do not provide specific construction details and construction sequence for such structures. These two issues together result in the current graded slope excavation construction method failing to achieve differentiated support design that balances safety and economy when facing slopes with developed shrinkage cracks in arid regions. Summary of the Invention

[0006] To achieve these objectives and other advantages of the present invention, according to one aspect of the present invention, a method for graded slope excavation of deep foundation pits in arid regions is provided, comprising the following steps: The soil at the site of the foundation pit is investigated to obtain quantitative indicators that can predict the risk of soil shrinkage crack development in arid areas. The quantitative indicators include the natural water content, liquid limit, plastic limit, clay content, and crack characteristic parameters that characterize the spatial distribution of cracks. Based on the quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a crack development risk value. Based on the crack development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk areas and low-risk areas. For the high-risk areas, a first support structure combining deep reinforcement and slope protection is adopted. The first support structure includes a vertical reinforcement inserted below the potential sliding surface and a horizontal reinforcement covering the slope surface. For the low-risk areas, a second support structure mainly based on shallow reinforcement is adopted. The resistance of the second support structure is lower than that of the first support structure. Following the graded slope sequence, earthwork excavation is carried out step by step from top to bottom. After each level of excavation is completed, the construction of the first and second support structures is completed according to the division of high-risk and low-risk areas corresponding to that level of slope, and then the next level of excavation is carried out until the bottom of the foundation pit is reached.

[0007] Preferably, the fracture characteristic parameters characterizing the spatial distribution morphology of the fractures include the average fracture length, the average fracture aperture, and the number of fractures per unit area.

[0008] Preferably, based on the aforementioned quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a fracture development risk value, specifically as follows: Based on the natural water content, liquid limit, plastic limit, and clay content of the soil, the matric suction and shear strength parameters of each unit are calculated using unsaturated soil mechanics formulas. The shear strength parameters include effective cohesion c', effective internal friction angle φ', and suction friction angle φ. b ; Using the average crack length, average crack opening, and number of cracks per unit area as statistical basis, a three-dimensional discrete crack network conforming to the statistical distribution is generated by a random simulation method, and the three-dimensional discrete crack network is embedded into the slope numerical model. Numerical simulation was performed on the entire process of graded slope excavation. In each numerical simulation time step simulating excavation unloading and water evaporation, the matrix suction was applied as an external force to the fracture surface, and the shear strength parameter was assigned to the fracture surface contact unit. The stress components around the crack tip in each element are extracted, the equivalent stress intensity factor at the crack tip is calculated, and the normal stress and shear stress on the crack surface are calculated using the contact elements. The calculation process for the normal stress and shear stress of the contact elements on the crack surface involves extracting the stress tensor at the integration point of the contact elements on the crack surface from the stress field of the current numerical simulation time step; projecting the stress tensor onto a local coordinate system defined by the normal and tangential directions of the crack surface to obtain the normal stress σ on the crack surface. n and shear stress τ; Based on the shear strength parameters, the normal stress, and the suction friction angle, the shear strength τ of the crack surface is calculated. f =c'+(σ n +ψ·tanφ b )·tanφ', where σ n Let K be the normal stress on the fracture surface and ψ be the matrix suction; when the equivalent stress intensity factor K at the fracture tip... eq Greater than the soil fracture toughness K IC Furthermore, the shear stress τ on the crack surface reaches the shear strength τ. f When, i.e., τ≥τ f The crack is determined to have expanded in the current time step; the expansion step is advanced along the crack angle direction determined based on the maximum circumferential stress criterion, and the maximum length of the expanded crack is recorded as the crack expansion length value of the element; at the same time, the connection between each crack is tracked, and the ratio of the crack connection area to the total area of ​​the element is taken as the crack connectivity rate value. The fracture propagation length value and the fracture connectivity value are used together as the fracture development risk value.

[0009] Preferably, the calculation process for the stress intensity factor at the crack tip of each unit during excavation unloading and moisture evaporation is as follows: At each numerical simulation time step of each level of excavation and water evaporation, for each element in the slope numerical model containing a crack tip, based on the stress field and displacement field of the current time step, the stress components around the crack tip are extracted, including the normal stress σ. x σ y σ z and shear stress τ xy τ yz τ zx ; Centered on the crack tip, a closed integral contour is selected. The radius of the integral contour is 0.5 to 2 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. Along the integral contour, the M-integral is calculated using the interaction integration method with preset Type I and Type II fracture auxiliary fields; the M-integral is then separated into Type I stress intensity factors K corresponding to the opening type. I and the Type II stress intensity factor K corresponding to the slip-out type II ; Based on the maximum circumferential stress criterion, K I and K II Synthesized into equivalent stress intensity factor K eq K eq K is determined by the following formula: eq =cos(θ c / 2)[K I ×cos 2 (θ c / 2)-1.5K II ×sin(θ c )], where θ c The crack angle is K. I ×sin(θ c )+K II ×(3cos(θ c )-1)=0; The equivalent stress intensity factor K eq This serves as the stress intensity factor at the crack tip during the numerical simulation time step.

[0010] Preferably, the method for calculating the fracture toughness of the soil is as follows: during the on-site investigation, samples of the undisturbed soil at the site of the foundation pit are taken and subjected to an indoor Brazilian splitting test to determine the tensile strength σ of the soil. t The measured tensile strength σ t Substituting the pre-defined linear empirical relation K IC =α×σ t The soil fracture toughness K was calculated. IC The coefficient α ranges from 0.30 to 0.40.

[0011] Preferably, the calculation process for the fracture propagation length value and the fracture connectivity value is as follows: At each time step of the numerical simulation simulating the excavation unloading and moisture evaporation process, the equivalent stress intensity factor K at the crack tip calculated at the current time step is used. eq With soil fracture toughness K IC Compare the shear stress τ on the crack surface with the shear strength τ. f Compare; When K eq >K IC And τ≥τ f When the crack propagates at the current time step, the crack angle θ is determined based on the maximum circumferential stress criterion. c Determine the expansion direction, and advance the crack tip forward along this direction by an expansion step Δa. The expansion step Δa is 0.1 to 0.3 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. When dividing the slope elements, the characteristic length of the element is 0.06m to 0.2m. Update the fracture geometry information by discretizing each fracture surface in the three-dimensional discrete fracture network into a set of fracture polygons, extracting the skeleton lines of each fracture polygon as its geometric segments, and recording the total length of the expanded fracture as the fracture expansion length value L of the unit. i ; Tracing the interconnections between each fracture, and detecting the shortest distance between any two fracture polygons in the updated fracture network, if this distance is less than the critical distance d... c The critical distance d c If the value is between 0.01 and 0.05 times the average length of the crack, it is determined that a through connection has occurred at this location, and the two cracks are merged into a single crack polygon. Calculate the ratio of the fracture connectivity area to the total area of ​​the element, and then calculate the fracture connectivity value C of the element using the following formula. i =A crack,i / A i ;where A crack,i Let A be the union area of ​​the areas of all crack polygons within the i-th cell on the projection plane. i The total area of ​​the i-th unit; The crack propagation length value L i and the fracture connectivity value C i Together, they serve as the risk value for the development of the aforementioned cracks.

[0012] Preferably, based on the crack development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk areas and low-risk areas, specifically: Set the crack propagation length threshold L th and the gap connectivity threshold C th The crack propagation length L of the i-th element is... i With L th Compare the values ​​and assign the fracture connectivity value C of the unit to the comparison. i With C th Compare; When L i >L th And C i >C th At that time, the unit was designated as a high-risk area; When L i ≤L th Or C i ≤C th At that time, the unit was designated as a low-risk area; The L th The value range is from 0.3m to 1.0m, and the value of C is... th The value range is from 0.2 to 0.5.

[0013] Preferably, the high-risk area employs a first support structure that combines deep reinforcement with slope protection, specifically: The vertical reinforcement is a micropile, which is a steel pipe grouting micropile. The diameter of the steel pipe ranges from 100mm to 200mm, and the water-cement ratio of the grout ranges from 0.45 to 0.55. The micropile is arranged at a spacing of 1.5m to 3.0m along the slope direction. The pile length is set to exceed the slope height by 1.0m to 2.0m. The depth of the pile body inserted below the potential sliding surface is not less than 0.4 times the total length of the pile body. The horizontal reinforcement is a high-density polyethylene geogrid with a tensile strength of not less than 80kN / m and a mesh size range of 100mm×100mm to 150mm×150mm. It is laid along the entire length of the slope, and the overlap width between two adjacent geogrids is not less than 300mm. The top of the micropile is fixed to the geogrid by a connector, which is a U-shaped steel bar with a diameter of 12mm to 16mm. The U-shaped steel bar surrounds the top of the micropile and hooks the transverse ribs of the geogrid.

[0014] Preferably, a second support structure mainly consisting of shallow reinforcement is used for the low-risk area, specifically: The second support structure includes soil nails and a steel mesh shotcrete surface layer, wherein the soil nails are full-length bonded anchors, the length of the soil nails is 0.4 to 0.7 times the height of the slope level, and the spacing between the soil nails in the horizontal and vertical directions is 1.2m to 2.0m. The thickness of the steel mesh shotcrete surface layer ranges from 50mm to 80mm, the concrete strength grade is not less than C20, the diameter of the steel bars in the steel mesh ranges from 6mm to 10mm, and the mesh size ranges from 200mm×200mm to 300mm×300mm. After the soil nailing is completed, the steel mesh is laid on the slope and shotcrete is sprayed to form the steel mesh shotcrete surface layer.

[0015] This invention offers at least the following advantages: The graded slope excavation method for deep foundation pits in arid regions, as described in this invention, establishes the prediction of drying shrinkage crack development risk based on quantifiable indicators by acquiring the natural water content, liquid limit, plastic limit, clay content, and crack characteristic parameters of the soil, thus avoiding the uncertainty caused by relying solely on experience. By using the average crack length, average crack aperture, and number of cracks per unit area as specific components of the crack characteristic parameters, the data types required for field investigation are clearly defined. Operators can directly obtain the inputs needed for modeling through measurement, reducing the ambiguity in the implementation of the method. Based on the physical properties of the soil and the statistical parameters of cracks, the combination of unsaturated soil mechanics calculations and three-dimensional discrete crack network modeling allows the crack development risk value of each unit of the slope to be quantitatively expressed in the form of crack propagation length and crack connectivity, providing a numerical basis for subsequent risk classification. The calculation process of the equivalent stress intensity factor at the crack tip is clearly given, including stress component extraction, integral contour selection, interaction integration, and synthesis steps based on the maximum circumferential stress criterion, making the fracture mechanics criterion operable in soil drying shrinkage crack simulation. The tensile strength was determined using the Brazilian splitting test, and the soil fracture toughness was converted using a linear empirical relationship. This provides a simple and easy-to-use method for obtaining material parameters for crack propagation criteria, avoiding reliance on complex direct fracture toughness testing equipment. The propagation simulation details the specific methods for propagation step size, crack skeleton line extraction, polygon discretization, shortest distance detection, and connected area union calculation, ensuring that crack propagation length and connectivity values ​​can be stably tracked and output, guaranteeing the consistency of risk value calculation results. Crack propagation length and connectivity are compared with preset thresholds, providing clear regional division rules and threshold value ranges, thus providing a unified operational standard for determining high-risk and low-risk areas. For high-risk areas, a support structure combining grouted steel pipe micropiles and geogrids is used, with specific limits on pile diameter, spacing, pile length, geogrid strength, mesh size, and pile top connection structure, ensuring that deep reinforcement and slope protection in this area form a synergistic load-bearing whole. For low-risk areas, a support structure combining full-length bonded anchor bolts and shotcrete surface layer with steel mesh is adopted, and the anchor bolt length, spacing, surface layer thickness and steel mesh parameters are limited, so that the shallow reinforcement scheme in this area can control the amount of support work while ensuring basic stability.

[0016] 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. Attached Figure Description

[0017] Figure 1 This is a flowchart of the graded slope excavation construction method for deep foundation pits in arid regions as described in this invention. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can implement it based on the description.

[0019] 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.

[0020] 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.

[0021] like Figure 1 As shown, this invention discloses a method for graded slope excavation of deep foundation pits in arid regions, comprising the following steps: The soil at the site of the foundation pit is investigated to obtain quantitative indicators that can predict the risk of soil shrinkage crack development in arid areas. The quantitative indicators include the natural water content, liquid limit, plastic limit, clay content, and crack characteristic parameters that characterize the spatial distribution of cracks. Based on the quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a crack development risk value. Based on the crack development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk areas and low-risk areas. For the high-risk areas, a first support structure combining deep reinforcement and slope protection is adopted. The first support structure includes a vertical reinforcement inserted below the potential sliding surface and a horizontal reinforcement covering the slope surface. For the low-risk areas, a second support structure mainly based on shallow reinforcement is adopted. The resistance of the second support structure is lower than that of the first support structure. Following the graded slope sequence, earthwork excavation is carried out step by step from top to bottom. After each level of excavation is completed, the construction of the first and second support structures is completed according to the division of high-risk and low-risk areas corresponding to that level of slope, and then the next level of excavation is carried out until the bottom of the foundation pit is reached.

[0022] In this technical solution, during the on-site investigation and data acquisition phase, a geological survey is conducted at the site of the foundation pit to obtain the physical and mechanical parameters of the soil and information on the distribution of cracks. Equipment that can be used during the investigation includes portable moisture content meters, combined liquid and plastic limit meters, and crack measurement systems combining laser rangefinders or digital cameras with image processing software. Through on-site sampling and laboratory tests, the natural moisture content, liquid limit, plastic limit, and clay content of the soil can be determined. To obtain the spatial distribution morphology of cracks, representative statistical areas can be selected on the slope surface, and the average length, average aperture, and number of cracks per unit area can be manually measured using the line method or window method. Alternatively, a drone equipped with a high-definition camera can be used to conduct photogrammetry of the slope surface, and the acquired images can be imported into image recognition software to automatically extract the crack network and statistically analyze the aforementioned crack characteristic parameters. These quantitative indicators collectively constitute the data basis for predicting the risk of drying shrinkage crack development.

[0023] During the risk assessment and regional delineation phase, the acquired quantitative indicators are input into the slope numerical analysis model. The model can be built using general-purpose finite element software or discrete element software platforms, running on ordinary engineering computers or workstations. Based on the soil's natural water content, liquid limit, plastic limit, and clay content, the Van Genuchten model can be used to describe the soil-water characteristic curve, and the Fredlund dual-stress variable strength theory is employed to calculate the matric suction and corresponding shear strength parameters of each element. Using the average crack length, average aperture, and number of cracks per unit area as statistical criteria, the Monte Carlo method can be used to generate a statistically distributed three-dimensional discrete crack network, which is then embedded as the initial damage model into the slope numerical model. Numerical simulation was performed on the entire process of graded slope excavation to simulate the stress release caused by excavation unloading and the increase of matrix suction caused by water evaporation. The stress intensity factor at the crack tip and the shear stress on the crack surface of each unit were calculated. When both exceeded the fracture toughness of the soil and the shear strength of the crack surface, crack propagation was determined. The maximum length of the extended crack and the crack connectivity rate were tracked and used together as the crack development risk value of the unit.

[0024] After completing the numerical simulation of the entire process of graded slope excavation, the stress state of each element was extracted from the final stress and displacement fields of the numerical model. The overall stability safety factor of the slope and the equivalent plastic strain distribution of each element were calculated using the strength reduction method. The connected regions of elements with equivalent plastic strain greater than 0.02 were divided into shear zones, and the centerline of these shear zones was extracted as the location of the potential sliding surface. The location information of this potential sliding surface will be used to determine the length of micropiles in high-risk areas to ensure that the piles can be inserted below the potential sliding surface.

[0025] Subsequently, a fracture propagation length threshold of 0.5m and a fracture connectivity threshold of 0.3 were set. The fracture development risk value of each element was compared with the threshold one by one: elements with a fracture propagation length value exceeding 0.5m and a fracture connectivity value exceeding 0.3 were designated as high-risk areas, and the remaining elements were designated as low-risk areas.

[0026] During the differentiated support design and construction phase, for high-risk areas, steel pipe grouting micropiles can be used as vertical reinforcements inserted below the potential sliding surface. During micropile construction, holes are drilled on the slope at intervals of 1.5m to 3.0m, and 127mm diameter steel pipes with a wall thickness of 4.5mm are inserted. Grouting holes can be made on the steel pipes, and then pure cement grout with a water-cement ratio of 0.5 is injected to form a grouting body. The pile length is set to exceed the slope height by 1.5m, ensuring that the depth of the pile inserted below the potential sliding surface is not less than 0.4 times the total pile length. The portion of the pile exposed above the slope can be surrounded by U-shaped steel bars with a diameter of 14mm. Subsequently, high-density polyethylene geogrids are laid on the slope as horizontal reinforcement. The tensile strength of the geogrid can be taken as 80kN / m, and the mesh size can be 120mm×120mm. It is laid continuously from top to bottom along the slope, with an overlap width of 300mm between adjacent sheets. U-shaped steel bars are used to hook the transverse ribs of the geogrid to achieve a fixed connection between the pile top and the geogrid. For low-risk areas, full-length bonded anchors can be used as soil nails. The length of the soil nails can be 0.5 times the height of the slope level, with a horizontal and vertical spacing of 1.5m. Cement mortar is injected into the soil nail holes for anchoring. After the soil nailing is completed, a steel mesh with a diameter of 8mm and a mesh size of 250mm×250mm is laid on the slope, and C25 strength grade concrete is sprayed to form a 60mm thick surface layer. Construction proceeds from top to bottom in a graded slope excavation sequence. After each level of slope excavation is completed, micropiles and geogrids are constructed in high-risk areas or soil nails and shotcrete surface layer are constructed in low-risk areas, based on the risk classification results of that level of slope, before proceeding to the next level of excavation.

[0027] The resistance refers to the ultimate bearing capacity of the support structure in the direction of slope sliding. It can be calculated and determined by parameters such as the material strength, cross-sectional dimensions and spacing of the support structure, or it can be measured by the safety factor obtained by limit equilibrium analysis of the slope unit after support.

[0028] This technical solution uses quantifiable indicators such as soil moisture content, liquid and plastic limits, clay content, and average crack length, aperture, and number of cracks to base the prediction of shrinkage crack development risk on measured data, avoiding reliance on experience alone. By combining unsaturated soil mechanical parameters with a three-dimensional discrete crack network for numerical simulation, and using crack propagation length and connectivity as core indicators for risk zoning, the support design for high-risk areas can specifically resist deep sliding caused by crack penetration, while the support for low-risk areas can be simplified accordingly. The connection between micropiles and geogrids allows the vertical reinforcement and horizontal reinforcement to form a synergistic force-bearing whole, jointly constraining deep slope deformation. This differentiated support strategy controls the amount of support work while ensuring the overall stability of the slope.

[0029] In another technical solution, the fracture characteristic parameters characterizing the spatial distribution morphology of fractures include the average fracture length, the average fracture aperture, and the number of fractures per unit area.

[0030] In this technical solution, during field investigation, a representative statistical area can be selected on the slope surface, and the fissure measurement method can be used. Survey lines with a length of 10m can be laid out along the slope's strike and dip directions, and the trace length, opening width, and number of fissures intersecting each survey line are recorded. The arithmetic mean of the trace lengths of all fissures within the survey line area yields the average fissure length; the arithmetic mean of the opening widths of all fissures on the slope surface yields the average fissure aperture; and the number of fissures per unit area is obtained by dividing the total number of fissures counted within the survey line area by the area of ​​the statistical region. These three parameters together constitute the quantitative basis for characterizing the spatial distribution morphology of fissures.

[0031] In another technical solution, based on the aforementioned quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a risk value for crack development, specifically as follows: Based on the natural water content, liquid limit, plastic limit, and clay content of the soil, the matric suction and shear strength parameters of each unit are calculated using unsaturated soil mechanics formulas. The shear strength parameters include effective cohesion c', effective internal friction angle φ', and suction friction angle φ. b ; Using the average crack length, average crack opening, and number of cracks per unit area as statistical basis, a three-dimensional discrete crack network conforming to the statistical distribution is generated by a random simulation method, and the three-dimensional discrete crack network is embedded into the slope numerical model. Numerical simulation was performed on the entire process of graded slope excavation. In each numerical simulation time step simulating excavation unloading and water evaporation, the matrix suction was applied as an external force to the fracture surface, and the shear strength parameter was assigned to the fracture surface contact unit. The stress components around the crack tip in each element are extracted, the equivalent stress intensity factor at the crack tip is calculated, and the normal stress and shear stress on the crack surface are calculated using the contact elements. The calculation process for the normal stress and shear stress of the contact elements on the crack surface involves extracting the stress tensor at the integration point of the contact elements on the crack surface from the stress field of the current numerical simulation time step; projecting the stress tensor onto a local coordinate system defined by the normal and tangential directions of the crack surface to obtain the normal stress σ on the crack surface. n and shear stress τ; Based on the shear strength parameters, the normal stress, and the suction friction angle, the shear strength τ of the crack surface is calculated. f =c'+(σ n +ψ·tanφ b )·tanφ', where σ n Let K be the normal stress on the fracture surface and ψ be the matrix suction; when the equivalent stress intensity factor K at the fracture tip... eq Greater than the soil fracture toughness K IC Furthermore, the shear stress τ on the crack surface reaches the shear strength τ. f When, i.e., τ≥τ f The crack is determined to have expanded in the current time step; the expansion step is advanced along the crack angle direction determined based on the maximum circumferential stress criterion, and the maximum length of the expanded crack is recorded as the crack expansion length value of the element; at the same time, the connection between each crack is tracked, and the ratio of the crack connection area to the total area of ​​the element is taken as the crack connectivity rate value. The fracture propagation length value and the fracture connectivity value are used together as the fracture development risk value.

[0032] In this technical solution, the calculation method for unsaturated soil mechanical parameters involves selecting representative undisturbed soil samples from the site and determining the soil-water characteristic curve in the laboratory using the filter paper method or pressure plate apparatus. Simultaneously, direct shear tests or triaxial tests under different water contents are conducted to calibrate the variation of effective cohesion, effective internal friction angle, and suction friction angle with water content. The generation of the three-dimensional discrete fracture network can be performed using geostatistics software on a standard engineering computer. Using the measured average fracture length, average aperture, and number of fractures per unit area as input statistics, a spatial disk model conforming to the statistical distribution can be generated using the Monte Carlo method. Each fracture surface is discretized into a set of triangular elements, and the fracture network is embedded into the slope finite element mesh through node coordinates and element topology. The contact elements of the fracture surface can be thicknessless joint elements, with their constitutive relation using the Coulomb friction model. The friction coefficient and cohesion are assigned values ​​by the shear strength parameter.

[0033] In another technical solution, the calculation process of the stress intensity factor at the crack tip of each unit during excavation unloading and moisture evaporation is as follows: At each numerical simulation time step of each level of excavation and water evaporation, for each element in the slope numerical model containing a crack tip, based on the stress field and displacement field of the current time step, the stress components around the crack tip are extracted, including the normal stress σ. x σ y σ z and shear stress τ xy τ yz τ zx ; Centered on the crack tip, a closed integral contour is selected. The radius of the integral contour is 0.5 to 2 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. Along the integral contour, the M-integral is calculated using the interaction integration method with preset Type I and Type II fracture auxiliary fields; the M-integral is then separated into Type I stress intensity factors K corresponding to the opening type. I and the Type II stress intensity factor K corresponding to the slip-out type II ; Based on the maximum circumferential stress criterion, K I and K II Synthesized into equivalent stress intensity factor K eq K eq K is determined by the following formula: eq =cos(θ c / 2)[K I ×cos 2 (θ c / 2)-1.5K II ×sin(θ c )], where θ c The crack angle is K. I ×sin(θ c )+K II ×(3cos(θ c )-1)=0; The equivalent stress intensity factor K eq This serves as the stress intensity factor at the crack tip during the numerical simulation time step.

[0034] In this technical solution, the stress intensity factor calculation process utilizes an interaction integration method implemented through a contour integration module developed within the secondary development environment of a general-purpose finite element software. The radius of the integration contour can be taken as 1.0 times the diameter of the smallest circumcircle of the element containing the crack tip, ensuring that the integration path lies entirely within the element and does not cross other crack surfaces. The auxiliary field can be the classical Type I and Type II asymptotic displacement field functions from linear elastic fracture mechanics, using the same material elastic constants as in the parent claim. The stress and displacement fields at each numerical simulation time step are output by the finite element solver and can be used as input data for the contour integration module. When synthesizing the equivalent stress intensity factor, the transcendental equation satisfied by the crack angle is first solved using Newton's iteration method, and then the crack angle is substituted into the synthesis formula.

[0035] In another technical solution, the soil fracture toughness is calculated as follows: during on-site investigation, undisturbed soil samples are taken from the site of the foundation pit and subjected to indoor Brazilian splitting tests to determine the tensile strength σ of the soil. t The measured tensile strength σ t Substituting the pre-defined linear empirical relation K IC =α×σ t The soil fracture toughness K was calculated. IC The coefficient α ranges from 0.30 to 0.40.

[0036] In this technical solution, the Brazilian splitting test, used as an indirect method for estimating soil fracture toughness, can be performed on a universal testing machine. The specimen can be prepared as a disc with a diameter of 50 mm and a thickness of 25 mm, and the loading rate can be controlled at 0.1 mm / min. The peak load at specimen failure is recorded, and the tensile strength is calculated using the Brazilian splitting formula. The coefficient α can be taken as 0.35, and after substituting it into the linear empirical relationship, the estimated value of the soil fracture toughness K_IC is obtained. For the same site, at least five parallel specimens can be prepared for testing, and the average value of the calculated results from each specimen is taken as the final fracture toughness value of the soil.

[0037] In another technical solution, the calculation process for the fracture propagation length value and the fracture connectivity value is as follows: At each time step of the numerical simulation simulating the excavation unloading and moisture evaporation process, the equivalent stress intensity factor K at the crack tip calculated at the current time step is used. eq With soil fracture toughness K IC Compare the shear stress τ on the crack surface with the shear strength τ. f Compare; When K eq >K IC And τ≥τ f When the crack propagates at the current time step, the crack angle θ is determined based on the maximum circumferential stress criterion.c Determine the expansion direction, and advance the crack tip forward along this direction by an expansion step Δa. The expansion step Δa is 0.1 to 0.3 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. When dividing the slope elements, the characteristic length of the element is 0.06m to 0.2m. Update the fracture geometry information by discretizing each fracture surface in the three-dimensional discrete fracture network into a set of fracture polygons, extracting the skeleton lines of each fracture polygon as its geometric segments, and recording the total length of the expanded fracture as the fracture expansion length value L of the unit. i ; Tracing the interconnections between each fracture, and detecting the shortest distance between any two fracture polygons in the updated fracture network, if this distance is less than the critical distance d... c The critical distance d c If the value is between 0.01 and 0.05 times the average length of the crack, it is determined that a through connection has occurred at this location, and the two cracks are merged into a single crack polygon. Calculate the ratio of the fracture connectivity area to the total area of ​​the element, and then calculate the fracture connectivity value C of the element using the following formula. i =A crack,i / A i ;where A crack,i Let A be the union area of ​​the areas of all crack polygons within the i-th cell on the projection plane. i The total area of ​​the i-th unit; The crack propagation length value L i and the fracture connectivity value C i Together, they serve as the risk value for the development of the aforementioned cracks.

[0038] In this technical solution, the calculation process for the crack propagation length and connectivity rate is as follows: in each numerical simulation time step, the propagation step size Δa can be taken as 0.2 times the diameter of the smallest circumscribed circle of the element containing the crack tip. Updating the crack geometry can be achieved by modifying the triangular element mesh of the crack surface, adding new nodes and elements along the crack angle direction at the original crack tip. When the crack surface is discretized into polygons, the boundaries of all triangular elements constituting each crack surface can be projected onto the slope plane to extract the outer polygons. Skeleton line extraction can use a median transformation algorithm to extract the centerline as a geometric line segment from the crack polygons; its total length is the crack propagation length. Detection of the connection between multiple crack polygons can be achieved by calculating the nearest point pair distance between any two polygon boundaries using a geometric calculation library; the critical distance dc is taken as 0.02 times the average crack length. The merging operation of two crack polygons is implemented using Boolean union operations. The ratio of the union area of ​​all crack polygons within a cell to the total area of ​​the cell is the crack connectivity rate.

[0039] In another technical solution, based on the crack development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk and low-risk areas, specifically: Set the crack propagation length threshold L th and the gap connectivity threshold C th The crack propagation length L of the i-th element is... i With L th Compare the values ​​and assign the fracture connectivity value C of the unit to the comparison. i With C th Compare; When L i >L th And C i >C th At that time, the unit was designated as a high-risk area; When L i ≤L th Or C i ≤C th At that time, the unit was designated as a low-risk area; The L th The value range is from 0.3m to 1.0m, and the value of C is... th The value range is from 0.2 to 0.5.

[0040] In this technical solution, regarding the setting of the grading threshold and the process of region division, the crack propagation length threshold L... th The threshold value can be set to 0.5m, and the fracture connectivity threshold C can be set to 0.5m. th It can be taken as 0.3. After calculating the crack propagation length and connectivity of all elements, the L value of each element can be... i and C i Store them in list form. When comparing them one by one, the L of the i-th unit is... i Compared to 0.5m, C i Compared to 0.3, when a unit has a crack propagation length of 0.62m and a connectivity rate of 0.38, it meets the double exceedance condition and is designated as a high-risk area; when a unit has a crack propagation length of 0.45m or a connectivity rate of 0.25, at least one of them does not reach the threshold, and the unit is designated as a low-risk area. The classification results can be marked with different colors on the surface mesh map of the slope numerical model.

[0041] In another technical solution, a first support structure combining deep reinforcement and slope protection is used for the high-risk area, specifically: The vertical reinforcement is a micropile, which is a steel pipe grouting micropile. The diameter of the steel pipe ranges from 100mm to 200mm, and the water-cement ratio of the grout ranges from 0.45 to 0.55. The micropile is arranged at a spacing of 1.5m to 3.0m along the slope direction. The pile length is set to exceed the slope height by 1.0m to 2.0m. The depth of the pile body inserted below the potential sliding surface is not less than 0.4 times the total length of the pile body. The horizontal reinforcement is a high-density polyethylene geogrid with a tensile strength of not less than 80kN / m and a mesh size range of 100mm×100mm to 150mm×150mm. It is laid along the entire length of the slope, and the overlap width between two adjacent geogrids is not less than 300mm. The top of the micropile is fixed to the geogrid by a connector, which is a U-shaped steel bar with a diameter of 12mm to 16mm. The U-shaped steel bar surrounds the top of the micropile and hooks the transverse ribs of the geogrid.

[0042] In this technical solution, regarding the specific construction of the first support structure, the diameter of the steel pipe for the grouting micropiles can be 150mm, with a wall thickness of 5mm. Grout outlet holes of 10mm diameter are staggered every 500mm along the length of the steel pipe. The grouting material can be pure cement grout with a water-cement ratio of 0.5, and the grouting pressure can be controlled between 0.5MPa and 1.0MPa. The spacing of the micropiles along the slope direction can be 2.0m, and the pile length exceeds the slope height by 1.5m. The depth of the pile insertion below the potential sliding surface can be 0.5 times the total pile length. The tensile strength of the high-density polyethylene geogrid can be 100kN / m, the mesh size is 120mm×120mm, and it is laid continuously along the slope from top to bottom, with an overlap width of 350mm between adjacent sections. The diameter of the U-shaped steel bar can be 14mm, and the bending radius matches the outer diameter of the micropile steel pipe. The two limbs of the U-shaped steel bar pass through the sides of the micropile and hook onto the transverse ribs of the geogrid.

[0043] In another technical solution, a second support structure mainly consisting of shallow reinforcement is used for the low-risk area, specifically: The second support structure includes soil nails and a steel mesh shotcrete surface layer, wherein the soil nails are full-length bonded anchors, the length of the soil nails is 0.4 to 0.7 times the height of the slope level, and the spacing between the soil nails in the horizontal and vertical directions is 1.2m to 2.0m. The thickness of the steel mesh shotcrete surface layer ranges from 50mm to 80mm, the concrete strength grade is not less than C20, the diameter of the steel bars in the steel mesh ranges from 6mm to 10mm, and the mesh size ranges from 200mm×200mm to 300mm×300mm. After the soil nailing is completed, the steel mesh is laid on the slope and shotcrete is sprayed to form the steel mesh shotcrete surface layer.

[0044] In this technical solution, regarding the specific construction of the second support structure, the soil nail drilling diameter can be 100mm, the soil nail reinforcement uses 25mm diameter threaded steel bars, the grouting material is cement mortar with a water-cement ratio of 0.45, and the fineness modulus of the sand can be 2.5. The length of the soil nail is 0.5 times the height of the slope level, and the horizontal and vertical spacing is 1.5m. The reinforcing mesh can use 8mm diameter HPB300 plain round steel bars, with a mesh size of 250mm×250mm, and the intersections of the reinforcing bars are tied and fixed with wire. The shotcrete can be of C25 strength grade, with a maximum aggregate particle size not exceeding 15mm. During the shotcrete operation, the nozzle is kept 0.8m to 1.0m away from the sprayed surface, and the shotcrete is applied in layers to a design thickness of 60mm. The ends of the soil nails are equipped with hooks to overlap with the reinforcing mesh, so that the soil nails and the shotcrete surface layer form an integral load-bearing structure.

[0045] In the crack propagation determination of this invention, an equivalent stress intensity factor criterion (K) based on fracture mechanics is also introduced. eq >K IC ) and the shear strength criterion based on unsaturated soil mechanics (τ≥τ) f Both criteria must be met simultaneously for crack propagation to be considered. This dual criterion design is not a simple superposition, but is based on a deep understanding of the mechanism of soil shrinkage crack propagation in arid regions.

[0046] (1) Analysis of the limitations of a single criterion If only the stress intensity factor criterion K is used eq >K IC If the stress concentration at the crack tip exceeds the material's resistance to fracture, the crack is considered to have propagated. This criterion stems from linear elastic fracture mechanics, which assumes the crack surface is an ideally smooth surface with no shear resistance. However, in arid regions, soil shrinkage cracks often exhibit irregular serrated surfaces. Interlocking, friction, and localized bonding exist between the soil layers on either side of the crack. Even if the crack tip meets the fracture conditions, the crack surface can still transmit some shear force through these mechanisms, preventing shear slippage. Therefore, a single K... eq In this case, the criterion can lead to over-prediction of the propagation behavior, misclassifying some cracks that have not actually propagated as propagation.

[0047] If only the shear strength criterion τ≥τ is used fThis criterion focuses solely on whether the shear stress on the fracture surface exceeds the material's shear capacity. This criterion originates from the Mohr-Coulomb failure criterion in geotechnical engineering and is applicable to describing shear failure across a structural surface. However, it neglects the fundamental driving force of fracture propagation: stress concentration at the fracture tip. When there is high stress concentration at the fracture tip, but insufficient shear stress on the fracture surface to drive slip, a single shear strength criterion will miss fractures that are about to propagate but have not yet initiated slip, which is dangerous in engineering.

[0048] (2) The synergistic and complementary mechanism of dual criteria This invention connects two criteria via an AND statement, forming a collaborative decision mechanism of "crack tip drive (whether it can expand) + sliding surface constraint (whether sliding is allowed)", the physical meaning of which is as follows: K eq >K IC This answers the question of whether the fracture tip has a sufficient energy release rate to propel the fracture forward. This is a necessary condition for fracture propagation and characterizes the sufficiency of the driving force.

[0049] τ≥τ f This answers the question of whether the interlocking friction between the soil on both sides of the fracture surface can be overcome, thus allowing relative sliding of the fracture surface. This is a sufficient condition for fracture propagation, indicating that the resistance on the sliding surface has been overcome.

[0050] This synergistic mechanism can accurately capture the unique "cracked but not broken, slippery but not penetrated" state of soil fissures in arid regions—that is, although the fissures have opened, the equivalent normal stress (σ) on the fissure surface is maintained due to the contribution of matrix suction (ψ) under unsaturated conditions. n +ψ·tanφ b The higher shear strength τ results in a higher shear strength. f As the surface of the fracture increases, it becomes difficult for the fracture surface to truly slip. This invention addresses this by applying the matrix suction force ψ to the suction friction angle φ. b Introducing the shear strength formula τ f =c'+(σ n +ψ·tanφ b )·tanφ', such that τ f It is highly sensitive to changes in moisture content, thus accurately reflecting the key phenomenon that as soil loses water and matrix suction increases during drought evaporation, the shear strength of the fracture surface is enhanced.

[0051] (3) Example of the determination process for typical units Taking a high-risk unit in this embodiment as an example, the working process of the dual criteria is explained. After simulating the second stage of excavation and experiencing 30 days of evaporation, the equivalent stress intensity factor K is calculated at the tip of a crack within the unit. eq =15.2 kPa·m 1 / 2Greater than the soil fracture toughness K IC = 11.375 kPa·m 1 / 2 The first criterion is met. If only this criterion is used, the crack will be judged as an extension.

[0052] However, further calculation of the stress state on the crack surface yields the normal stress σ. n =8.5 kPa, shear stress τ = 12.3 kPa. At this time, the matrix suction ψ = 85.7 kPa. Substituting into the shear strength formula: τ f =c'+(σ n +ψ·tanφ b )·tanφ' = 15 + (8.5 + 85.7 × tan15°) × tan24° = 15 + (8.5 + 85.7 × 0.268) × 0.445 = 15 + (8.5 + 22.97) × 0.445 = 15 + 31.47 × 0.445 = 15 + 14.0 = 29.0 kPa Calculation results show that τ = 12.3 kPa < τ f =29.0 kPa, the second criterion was not met. Therefore, although the stress concentration at the crack tip was sufficient, the crack surface was "locked" in place by the contribution of matrix suction to the shear strength of the crack surface, and the crack did not propagate at this time step.

[0053] However, as evaporation continues, assuming that the soil moisture content decreases further by day 60, two key changes occur: ① the soil becomes brittle, K... IC Decreased to 10.1 kPa·m 1 / 2 ② Stress redistribution at the crack tip, K eq Rising to 17.8 kPa·m 1 / 2 Meanwhile, due to the continuous deformation of the slope, the shear stress τ at the crack surface rises to 31.5 kPa. τ is then recalculated. f =28.8 kPa. The result satisfies: K eq (17.8) > K IC (10.1) and τ (31.5) ≥ τ f (28.8) If both criteria are met simultaneously, the crack is accurately determined to have expanded at that time step.

[0054] This example clearly illustrates that the dual criterion avoids problems caused by a single K criterion in the early stages of evaporation. eqThe "false alarm" caused by the criterion accurately captured the real expansion moment when the slope deformation intensified in the later stage of evaporation, demonstrating the technical advantage of timely early warning.

[0055] Example 1 This example uses a large loess foundation pit project in an arid region of Northwest my country as a case study. The foundation pit was designed to be 12m deep and excavated using a three-stage slope, with each stage having a height of 4m and a slope ratio of 1:0.75. The site soil was Q3 Malan loess, characterized by collapsibility and high evaporation sensitivity, with a visible network of drying shrinkage fissures on the surface.

[0056] Step 1: On-site investigation and acquisition of quantitative indicators Engineering geological investigation was conducted on the foundation pit site. Uncirculated soil samples were taken by drilling, and laboratory geotechnical tests were conducted according to the "Standard for Geotechnical Testing Methods" (GB / T 50123). The quantitative indicators of the soil at this site were as follows: natural water content w=8.5%, liquid limit wL=26.2%, plastic limit wP=16.8%, and clay content (particle size <0.005mm) of 22%. Based on particle size and mineral composition analysis, the soil is classified as sandy silty clay with a high montmorillonite content, which is the material basis for drying shrinkage cracking.

[0057] Meanwhile, in typical areas of the site, a combination of linear surveying and high-precision UAV photogrammetry was used to obtain characteristic parameters of the fracture spatial distribution. Statistical results show that the average fracture length is 0.52m, the average aperture is 3.2mm, and the number of fractures per unit area is 15 / m. 2 These three measured parameters will serve as the statistical basis for the subsequent generation of the three-dimensional discrete fracture network.

[0058] In addition, undisturbed soil samples were taken for Brazilian splitting tests to measure the tensile strength σ of the soil. t = 32.5 kPa. Based on the empirical formula K... IC =α×σ t Taking a coefficient α = 0.35, the fracture toughness K of the soil at this site was calculated. IC =11.375 kPa·m 1 / 2 This value will be used as one of the criteria for determining whether the crack will propagate.

[0059] Step 2: Slope unit division and crack development risk value assignment The entire numerical simulation and analysis process was implemented on the general-purpose finite element software ABAQUS platform, combined with Python secondary development.

[0060] (1) Establishing the numerical model and unsaturated mechanical parameter field of the slope: Based on the design dimensions for graded slope protection of the foundation pit, a three-dimensional finite element model of the slope was established. The model was meshed, and the element type was C3D8P (three-dimensional eight-node pore pressure element), with an element feature length of 0.1m.

[0061] The soil-water characteristic curve (SWCC) measured in the laboratory was fitted using the Van Genuchten model in unsaturated soil mechanics. Combined with Fredlund's dual-stress variable strength theory, initial matric suction field and shear strength parameters varying with water content were calculated and assigned to each element in the model. For the soil in this embodiment, the initial matric suction zone is 85.7 kPa, with corresponding parameters: effective cohesion c' = 15 kPa, effective internal friction angle φ' = 24°, and suction friction angle φ... b =15°.

[0062] (2) Generate and embed a three-dimensional discrete fracture network: Based on the aforementioned measured average crack length (0.52m), average aperture (3.2mm), and number of cracks per unit area (15 cracks / m), 2 Assuming that the fracture length follows an exponential distribution and the dip and dip angle follow a uniform distribution, a three-dimensional discrete fracture network represented by a spatial disk model is generated using the Monte Carlo stochastic simulation method.

[0063] The generated fracture network geometry is converted into a finite element mesh by writing a program. Specifically, each fracture surface is discretized into a set of contact elements without thickness and embedded into the slope finite element mesh established in step (1). These contact elements will be used for subsequent calculation of the normal stress σ on the fracture surface. n and shear stress τ.

[0064] (3) Numerical simulation and risk value calculation of the entire process of excavation unloading and moisture evaporation: In the simulation, three stages of excavation and unloading were simulated stepwise, with a moisture evaporation step added after each stage of excavation. The growth of matrix suction was simulated by modifying the pore pressure boundary conditions of the surface layer. The following calculations were performed in each numerical simulation time step: a. Calculate the equivalent stress intensity factor K at the crack tip. eq : Extract the stress component (σ) around each element containing the crack tip. x , σ y , σ z , τ xy , τ yz , τ zx A closed integral contour is selected centered at the crack tip, with a radius equal to 1.0 times the characteristic length of the element. Along this contour, the M-integral is calculated using the interaction integral method, and the Type I stress intensity factor K is obtained.I and Type II stress intensity factor K II Based on the maximum circumferential stress criterion, from formula K... eq =cos(θ c / 2)[K I ×cos 2 (θ c / 2)-1.5K II ×sin(θ c )] (where θ c Satisfy K I ×sin(θ c )+K II ×(3cos(θ c The equivalent stress intensity factor K used in this embodiment is calculated as follows: (-1)=0) eq .

[0065] b. Calculate the stress state and shear strength at the crack surface: Simultaneously, the stress tensor at the integration point of the contact element on the fracture surface is extracted and projected onto a local coordinate system defined by the normal and tangential directions of the fracture surface to obtain the normal stress σ. n And shear stress τ. According to the formula τ f =c'+(σ n +ψ·tanφ b The shear strength τ of the fracture surface at the current moment is calculated by tanφ'. f .

[0066] c. Perform crack propagation and penetration determination: Calculate K eq With soil fracture toughness K IC (11.375 kPa·m 1 / 2 Compare and pair τ with τ f Comparison. When K is satisfied. eq >K IC And τ≥τ f When the crack tip begins to propagate, it is determined that propagation has occurred. The propagation direction is determined by the crack initiation angle θ, which is based on the maximum circumferential stress criterion. c The expansion step size Δa is taken as 0.2 times the feature length of the unit.

[0067] d. Update the fracture network and calculate the element risk value: After crack propagation, the geometric information of the crack surface is updated to track the connection between cracks. After completing all simulation steps, the final result for each element is output. Taking a representative element as an example, its maximum crack propagation length L... i Reaching 0.61m, the fracture connectivity C i It reached 0.42.

[0068] Step 3: Set grading thresholds and divide risk areas Set the crack propagation length threshold L th =0.5m, fracture connectivity threshold C th =0.3. The program will automatically determine whether L is satisfied. i >0.5m and C i Elements with a fracture density >0.3 are classified as high-risk areas. In these areas, fractures have shown significant expansion and interconnection, potentially forming slip channels. Elements not meeting these two conditions are designated as low-risk areas.

[0069] Simultaneously, a strength reduction analysis was performed on the final model to calculate the overall stability safety factor of the slope. The equivalent plastic strain penetration zone was extracted to obtain the potential sliding surface, which mainly passes through the delineated high-risk area, verifying the rationality of the risk delineation.

[0070] Step 4: Carry out differentiated support construction from top to bottom. Following the designed graded slope excavation sequence, earthwork was excavated from top to bottom for the first-level slope (0-4m below the ground surface). After excavation, based on the risk classification results of the units on the slope surface, precise layout was carried out, and construction was carried out separately for high-risk and low-risk areas.

[0071] First-level slope construction High-risk areas: Deep reinforcement: Steel pipe grouting micropiles are used as vertical reinforcement. Steel pipes with an outer diameter of 140mm and a wall thickness of 4.5mm are used, with a pile length of 6.0m (exceeding the first-level slope height by 2.0m, ensuring the pile body inserts approximately 3.0m below the potential sliding surface, which is 0.5 times the total pile length). They are arranged at 2.0m intervals along the slope direction. After drilling, the steel pipes are inserted, and pure cement grout with a water-cement ratio of 0.5 is injected, with the grouting pressure controlled at 0.8~1.0MPa.

[0072] Slope protection: After pile construction, the slope is trimmed, and a continuous high-density polyethylene (HDPE) unidirectional geogrid with a tensile strength of 100 kN / m and a mesh size of 120mm×120mm is laid. The overlap width between adjacent geogrids is 350mm.

[0073] Pile top connection: A U-shaped connector made of HPB300 steel bars with a diameter of 14mm is used to surround the top of the micropile and hook its two limbs to the transverse ribs of the geogrid to achieve a reliable connection between the pile and the geogrid and form a cooperative stress-bearing structure.

[0074] Low-risk areas: Shallow reinforcement: Full-length bonded soil nails are used. The soil nails are made of 25mm diameter HRB400 threaded steel bars, with a 100mm diameter borehole. The soil nail length is 2.4m (0.6 times the slope height), and they are arranged at horizontal and vertical spacings of 1.5m x 1.5m. M30 cement mortar with a water-cement ratio of 0.45 is injected into the borehole.

[0075] Slope protection: After all soil nails are installed and tensioned, a steel mesh made of 8mm diameter HPB300 steel bars is hung on the slope, with a mesh size of 250mm×250mm. Then, C25 grade fine aggregate concrete is sprayed to a thickness of 60mm to form a complete shallow reinforcement surface layer.

[0076] Only after the differential support of the first-level slope is completed can the excavation of the second-level slope (4-8m below the ground surface) be carried out, and the support construction of this level be completed in the same manner. This process is continued step by step until the support of the third-level slope is completed and the foundation pit is excavated to the bottom.

[0077] Security Comparison Using the three-dimensional finite element strength reduction method, the overall stability safety factor (FoS) of the slope in Example 1 was calculated under three different schemes: Comparative Example 1 (Traditional Unified Conventional Support): The entire section adopts soil nailing + shotcrete surface support, with parameters the same as the low-risk area support of this invention.

[0078] Comparative Example 2 (Traditional Unified Reinforced Support): The entire section adopts micropiles + geogrid support, with parameters the same as those for the high-risk area support of this invention.

[0079] Example 1 (Differentiated support): Corresponding support methods for Comparative Example 1 and Comparative Example 2 were adopted according to risk zones.

[0080] The calculation results are shown in Table 1.

[0081] Table 1 Support scheme Overall stability safety factor FoS evaluate Comparative Example 1 1.18 The safety requirements for a Class I foundation pit are not met (FoS≥1.35). Comparative Example 2 1.56 Meeting the requirements, but having too much surplus, is uneconomical. Example 1 1.42 Meets requirements, achieving a balance between safety and economy. While Comparative Example 1 is simple to construct and low in cost, conventional soil nailing cannot effectively prevent deep sliding caused by fracture penetration in high-risk areas with dense fracture development, resulting in a safety factor lower than the standard requirements. Comparative Example 2, through full-section reinforced support, achieved a safety factor as high as 1.56, meeting the safety requirements, but resulted in severe over-support in low-risk areas. Example 1, by precisely applying reinforced support in high-risk areas, suppressed the main instability factor of fracture propagation and penetration, significantly increasing the safety factor from 1.18 to 1.42, meeting the standard requirements and avoiding the safety hazards of Comparative Example 1. Furthermore, compared to Comparative Example 2, although the safety factor decreased from 1.56 to 1.42, it remained within the safe range, demonstrating the feasibility of reducing support in low-risk areas.

[0082] Validation of the accuracy of crack propagation prediction To verify the accuracy of this method in predicting crack propagation, during construction, 10 pre-determined "high-risk" (predicted propagation) and 10 pre-determined "low-risk" (predicted no propagation) crack observation points were selected on the slope surface for 60 days of on-site monitoring. The actual crack propagation was recorded using crack gauges and digital image correlation (DIC) technology. The results are shown in Table 2.

[0083] Table 2 Prediction Category Number of observation points Actual expansion Actually not expanded Prediction accuracy High risk (forecast extension) 10 9 1 90% Low risk (forecast not extended) 10 0 10 100% overall 20 - - 95% The overall prediction accuracy reached 95%, indicating that the method of this invention can accurately predict the actual expansion behavior of soil shrinkage cracks under excavation unloading and water evaporation. The only "false positive" point (predicted expansion but no actual expansion) occurred near the drainage ditch at the top of the slope. The reason for this was that the local shading effect of the drainage ditch slowed down the evaporation rate. This factor was not carefully considered in the model and can be optimized in subsequent studies.

[0084] The above comparative analysis proves that the method of the present invention achieves the following technical effects through the strategy of "numerical simulation to predict risks → differentiated matching of support strength": compared with traditional conventional support, the overall stability safety factor is increased from 1.18 to 1.42, an increase of about 20.3%; the prediction accuracy rate verified by on-site monitoring reaches 95%, ensuring the pertinence and effectiveness of support design.

[0085] Although 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 and examples shown and described herein.

Claims

1. A method for graded slope excavation of deep foundation pits in arid regions, characterized in that... Includes the following steps: The soil at the site of the foundation pit is investigated to obtain quantitative indicators that can predict the risk of soil shrinkage crack development in arid areas. The quantitative indicators include the natural water content, liquid limit, plastic limit, clay content, and crack characteristic parameters that characterize the spatial distribution of cracks. Based on the quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a crack development risk value. Based on the crack development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk areas and low-risk areas. For the high-risk areas, a first support structure combining deep reinforcement and slope protection is adopted. The first support structure includes a vertical reinforcement inserted below the potential sliding surface and a horizontal reinforcement covering the slope. For the low-risk areas, a second support structure mainly based on shallow reinforcement is adopted. The resistance of the second support structure is lower than that of the first support structure. Following the graded slope sequence, earthwork excavation is carried out step by step from top to bottom. After each level of excavation is completed, the construction of the first and second support structures is completed according to the division of high-risk and low-risk areas corresponding to that level of slope, and then the next level of excavation is carried out until the bottom of the foundation pit is reached.

2. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 1, characterized in that, The fracture characteristic parameters that characterize the spatial distribution of fractures include the average fracture length, the average fracture aperture, and the number of fractures per unit area.

3. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 2, characterized in that, Based on the aforementioned quantitative indicators, the slope after graded excavation is divided into units, and each unit is assigned a risk value for crack development, specifically: Based on the natural water content, liquid limit, plastic limit, and clay content of the soil, the matric suction and shear strength parameters of each unit are calculated using unsaturated soil mechanics formulas. The shear strength parameters include effective cohesion c', effective internal friction angle φ', and suction friction angle φ. b ; Using the average crack length, average crack opening, and number of cracks per unit area as statistical basis, a three-dimensional discrete crack network conforming to the statistical distribution is generated by a random simulation method, and the three-dimensional discrete crack network is embedded into the slope numerical model. Numerical simulation was performed on the entire process of graded slope excavation. In each numerical simulation time step simulating excavation unloading and water evaporation, the matrix suction was applied as an external force to the fracture surface, and the shear strength parameter was assigned to the fracture surface contact unit. The stress components around the crack tip in each element are extracted, the equivalent stress intensity factor at the crack tip is calculated, and the normal stress and shear stress on the crack surface are calculated using the contact elements. The calculation process for the normal stress and shear stress of the contact elements on the crack surface involves extracting the stress tensor at the integration point of the contact elements on the crack surface from the stress field of the current numerical simulation time step; projecting the stress tensor onto a local coordinate system defined by the normal and tangential directions of the crack surface to obtain the normal stress σ on the crack surface. n and shear stress τ; Based on the shear strength parameters, the normal stress, and the suction friction angle, the shear strength τ of the crack surface is calculated. f =c'+(σ n +ψ·tanφ b )·tanφ', where σ n Let K be the normal stress on the fracture surface and ψ be the matrix suction; when the equivalent stress intensity factor K at the fracture tip... eq Greater than the soil fracture toughness K IC Furthermore, the shear stress τ on the crack surface reaches the shear strength τ. f When, i.e., τ≥τ f The crack is determined to have expanded in the current time step; the expansion step is advanced along the crack angle direction determined based on the maximum circumferential stress criterion, and the maximum length of the expanded crack is recorded as the crack expansion length value of the element; at the same time, the interconnection between each crack is tracked, and the ratio of the crack interconnection area to the total area of ​​the element is taken as the crack connectivity rate value. The fracture propagation length value and the fracture connectivity value are used together as the fracture development risk value.

4. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 3, characterized in that, The calculation process for the stress intensity factor at the crack tip of each unit during excavation unloading and moisture evaporation is as follows: At each numerical simulation time step of each level of excavation and water evaporation, for each element in the slope numerical model containing a crack tip, based on the stress field and displacement field of the current time step, the stress components around the crack tip are extracted, including the normal stress σ. x σ y σ z and shear stress τ xy τ yz τ zx ; Centered on the crack tip, a closed integral contour is selected. The radius of the integral contour is 0.5 to 2 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. Along the integral contour, the M-integral is calculated using the interaction integration method with preset Type I and Type II fracture auxiliary fields; the M-integral is then separated into Type I stress intensity factors K corresponding to the opening type. I and the Type II stress intensity factor K corresponding to the slip-out type II ; Based on the maximum circumferential stress criterion, K I and K II Synthesized into equivalent stress intensity factor K eq K eq K is determined by the following formula: eq =cos(θ c / 2)[K I ×cos 2 (θ c / 2)-1.5K II ×sin(θ c )], where θ c The crack angle is K. I ×sin(θ c )+K II ×(3cos(θ c )-1)=0; The equivalent stress intensity factor K eq This serves as the stress intensity factor at the crack tip during the numerical simulation time step.

5. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 3, characterized in that, The method for calculating the fracture toughness of the soil is as follows: During the on-site investigation, samples of the undisturbed soil at the site of the foundation pit are taken and subjected to an indoor Brazilian splitting test to determine the tensile strength σ of the soil. t The measured tensile strength σ t Substituting the pre-defined linear empirical relation K IC =α×σ t The soil fracture toughness K was calculated. IC The coefficient α ranges from 0.30 to 0.

40.

6. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 3, characterized in that, The calculation process for the fracture propagation length and fracture connectivity is as follows: At each numerical simulation time step simulating the excavation unloading and moisture evaporation process, the equivalent stress intensity factor K at the crack tip calculated at the current time step is used. eq With soil fracture toughness K IC Compare the shear stress τ on the crack surface with the shear strength τ. f Compare; When K eq >K IC And τ≥τ f When the crack propagates at the current time step, the crack angle θ is determined based on the maximum circumferential stress criterion. c Determine the expansion direction, and advance the crack tip forward along this direction by an expansion step Δa. The expansion step Δa is 0.1 to 0.3 times the characteristic length of the element where the crack tip is located. The characteristic length of the element is the diameter of the smallest circumcircle of the element. When dividing the slope elements, the characteristic length of the element is 0.06m to 0.2m. Update the fracture geometry information by discretizing each fracture surface in the three-dimensional discrete fracture network into a set of fracture polygons, extracting the skeleton lines of each fracture polygon as its geometric segments, and recording the total length of the expanded fracture as the fracture expansion length value L of the unit. i ; Tracing the interconnections between each fracture, and detecting the shortest distance between any two fracture polygons in the updated fracture network, if this distance is less than the critical distance d... c The critical distance d c If the value is between 0.01 and 0.05 times the average length of the crack, it is determined that a through connection has occurred at this location, and the two cracks are merged into a single crack polygon. Calculate the ratio of the fracture connectivity area to the total area of ​​the element, and then calculate the fracture connectivity value C of the element using the following formula. i =A crack,i / A i ;where A crack,i Let A be the union area of ​​the areas of all crack polygons within the i-th cell on the projection plane. i The total area of ​​the i-th unit; The crack propagation length value L i and the fracture connectivity value C i Together, they serve as the risk value for the development of the crack.

7. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 6, characterized in that, Based on the fracture development risk value of each unit, a grading threshold is set to divide each unit of the slope into high-risk and low-risk areas, specifically as follows: Set the crack propagation length threshold L th and the gap connectivity threshold C th The crack propagation length L of the i-th element is... i With L th Compare the values ​​and assign the fracture connectivity value C of the unit to the comparison. i With C th Compare; When L i >L th And C i >C th At that time, the unit was designated as a high-risk area; When L i ≤L th Or C i ≤C th At that time, the unit was designated as a low-risk area; The L th The value range is from 0.3m to 1.0m, and the value of C is... th The value range is from 0.2 to 0.

5.

8. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 1, characterized in that, The high-risk area employs a first support structure combining deep reinforcement and slope protection, specifically: The vertical reinforcement is a micropile, which is a steel pipe grouting micropile. The diameter of the steel pipe ranges from 100mm to 200mm, and the water-cement ratio of the grout ranges from 0.45 to 0.

55. The micropile is arranged at a spacing of 1.5m to 3.0m along the slope direction. The pile length is set to exceed the slope height by 1.0m to 2.0m. The depth of the pile body inserted below the potential sliding surface is not less than 0.4 times the total length of the pile body. The horizontal reinforcement is a high-density polyethylene geogrid with a tensile strength of not less than 80kN / m and a mesh size range of 100mm×100mm to 150mm×150mm. It is laid along the entire length of the slope, and the overlap width between two adjacent geogrids is not less than 300mm. The top of the micropile is fixed to the geogrid by a connector, which is a U-shaped steel bar with a diameter of 12mm to 16mm. The U-shaped steel bar surrounds the top of the micropile and hooks the transverse ribs of the geogrid.

9. The method for graded slope excavation of deep foundation pits in arid regions as described in claim 1, characterized in that, For the low-risk area, a second support structure mainly consisting of shallow reinforcement is adopted, specifically: The second support structure includes soil nails and a steel mesh shotcrete surface layer, wherein the soil nails are full-length bonded anchors, the length of the soil nails is 0.4 to 0.7 times the height of the slope level, and the spacing between the soil nails in the horizontal and vertical directions is 1.2m to 2.0m. The thickness of the steel mesh shotcrete surface layer ranges from 50mm to 80mm, the concrete strength grade is not less than C20, the diameter of the steel bars in the steel mesh ranges from 6mm to 10mm, and the mesh size ranges from 200mm×200mm to 300mm×300mm. After the soil nailing is completed, the steel mesh is laid on the slope and shotcrete is sprayed to form the steel mesh shotcrete surface layer.

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