Micro-settlement control construction method for shield tunneling machine to pass through extremely soft soil stratum of river in close range

Abstract

The invention is applicable to the technical field of underground engineering construction, and provides a micro-settlement control construction method for a shield tunneling machine to pass through an extremely soft soil stratum of a river in a close range, which comprises the following steps: S1, preparation before construction: S1-1, geological exploration: adopting various exploration means such as drilling, static sounding and vane shear test, and obtaining a micro-settlement control result; the method comprises the following steps: comprehensively surveying river bottom mud of a crossing section and a lower extremely-soft soil stratum to obtain physical and mechanical parameters of natural water content omega, natural weight gamma, void ratio e, compression modulus Es, cohesive force c and internal friction angle phi of the stratum, and meanwhile, ascertaining hydrogeological conditions of river water level, flow velocity, riverbed deposition thickness and stratum permeability coefficient k. According to the method, the quantitative relation among the tunneling parameters, the grouting parameters and the settlement amount is established, closed-loop control of the construction process can be achieved, the settlement amount can be effectively controlled within a micro-amplitude range, and the safety of a project and the surrounding environment is guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of tunnel engineering technology, and in particular relates to a method for controlling micro-settlement during shield tunneling through extremely soft soil strata in close proximity to rivers. Background Technology

[0002] With the rapid development of urban underground transportation engineering, shield tunneling has been widely used in the construction of underground projects such as subways and tunnels due to its advantages such as minimal impact on the surrounding environment, high construction efficiency, and strong adaptability. However, when shield tunneling needs to pass through extremely soft soil layers of rivers at close range, it faces many technical challenges. Extremely soft soil layers are usually characterized by high natural water content, large porosity, low bearing capacity, high permeability, and high compressibility. During shield tunneling, they are prone to strong disturbance to the soil, leading to damage to the soil structure, changes in effective stress, and consequently, surface and riverbed subsidence. If the subsidence is too large, it will not only affect the safety of surrounding buildings and underground pipelines, but may also cause damage to the riverbed impermeable layer, river water leakage, and even threaten the stability of the river embankment, resulting in serious engineering accidents and economic losses.

[0003] Existing methods for controlling settlement when shield tunnels cross soft soil layers mostly focus on adjusting single parameters or optimizing grouting processes. They lack targeted analysis of the characteristics of extremely soft soil layers and have not established a quantitative relationship between tunneling parameters, grouting parameters, and settlement. This makes it difficult to achieve precise control of micro-settlement. Therefore, it is necessary to provide a new construction method for controlling micro-settlement when shield tunnels cross extremely soft soil layers near rivers to solve the above-mentioned technical problems. Summary of the Invention

[0004] The purpose of this invention is to provide a method that establishes a quantitative relationship between tunneling parameters, grouting parameters, and settlement, thereby enabling closed-loop control of the construction process, effectively controlling settlement within a small range, and ensuring the safety of the project and its surrounding environment. This solves the problems mentioned in the background art.

[0005] The objective of this invention can be achieved through the following technical solution: the method for controlling micro-settlement during close-range tunneling of shield tunnels through extremely soft soil strata of rivers, provided by this invention, includes the following steps: S1. Pre-construction preparations: S1-1. Geological Exploration: Through drilling, static cone penetration, vane shear test and other exploration methods, a comprehensive exploration of the riverbed sediment and the underlying extremely soft soil strata of the crossing section will be carried out to obtain the physical and mechanical parameters of the strata, such as natural water content ω, natural unit weight γ, void ratio e, compression modulus Es, cohesion c, and internal friction angle φ. At the same time, the hydrogeological conditions of river water level, flow velocity, riverbed sediment thickness and strata permeability coefficient k will be determined. S1-2. Settlement risk prediction: Based on the survey data, a three-dimensional geological model is established, and numerical simulation methods are used to predict the distribution of surface and riverbed settlement during shield tunneling, identify settlement-sensitive areas, and determine the micro-settlement control threshold, which is usually no more than 5mm. S2. Initialization of tunnel boring machine (TBM) parameters: Based on the physical and mechanical parameters of the extremely soft soil strata, the initial TBM parameters are determined using calculation formulas. S2-1. Calculation of Total Shield Thrust: The total shield thrust overcomes the soil frontal resistance, the friction between the shield and the soil, and the cutterhead cutting resistance. The calculation formula is as follows: Where F is the total thrust of the shield (kN) and F1 is the soil resistance at the front (kN). S2-2. Calculation of matching tunneling speed and cutterhead rotation speed: To reduce disturbance to extremely soft soil, it is necessary to control the matching between the cutterhead cutting frequency and the tunneling speed. The matching relationship between the cutterhead rotation speed n (r / min) and the tunneling speed v (mm / min) is as follows: Where f is the cutterhead cutting coefficient, and the value of f in extremely soft soil layers ranges from 0.3 to 0.5. It is adjusted according to the soil moisture content. The higher the moisture content, the smaller the value of f. S3. Dynamically optimize tunneling parameters: S3-1. Real-time data acquisition: During the tunneling process, the shield's built-in sensors collect real-time operating parameters such as total thrust, cutterhead rotation speed, tunneling speed, soil chamber pressure, and excavated soil volume. At the same time, the on-site monitoring equipment collects feedback information on stratum deformation and pore water pressure changes. S3-2. Parameter Adjustment Strategy: When the monitored formation subsidence rate exceeds 0.2 mm / h or the cumulative subsidence reaches 3 mm, the parameters shall be adjusted according to the following principles: (1) If the soil chamber pressure is too low, increase the total thrust of the shield appropriately, with an adjustment range of 5% to 10% of the initial thrust; (2) If the amount of excavated soil is too large, reduce the tunneling speed and increase the cutterhead speed at the same time to ensure The ratio remained stable within the range of 0.3 to 0.5; (3) If signs of soil heave appear, reduce the total thrust and reduce the grouting volume accordingly; S4. Synchronous grouting control: S4-1. Calculation of Synchronous Grouting Volume: The core of synchronous grouting is to promptly fill the annular gap formed after shield tunneling. In extremely soft soil strata, the volume of the annular gap is easily changed due to soil compression. The formula for calculating the grouting volume Q (m³) is as follows: Where K is the grouting coefficient, and in extremely soft soil layers, considering soil compression and grout loss, the value of K ranges from 1.5 to 2.0, d is the outer diameter of the segment (m), and L is the tunneling length (m) corresponding to a single grouting. S4-2. Grouting parameter control: Simultaneous grouting adopts cement-water glass double liquid grout, the grout setting time is controlled at 30~60s, the grouting pressure is controlled at 0.3~0.5MPa, and does not exceed 1.2 times the hydrostatic pressure of the formation, to avoid soil disturbance caused by excessive grouting pressure; S5. Secondary compensation grouting: S5-1. Judgment of grouting timing: When the real-time monitoring shows that the surface or riverbed settlement rate is continuously greater than 0.1 mm / h, or the cumulative settlement reaches 60% of the control threshold, secondary compensation grouting shall be initiated. S5-2. Determination of Secondary Grouting Parameters: Secondary grouting uses ultrafine cement grout. The grouting volume is calculated based on the settlement deficit, using the following formula: Where Q2 is the secondary grouting volume (m³), Δs is the settlement deficit (mm), which is the difference between the control threshold and the current cumulative settlement, A is the grouting influence area (m²), Es is the soil compression modulus (kPa), and e is the soil void ratio. S3. Grouting construction: The secondary grouting holes are set at the grouting holes of the segments. The segmented grouting method is adopted. The grouting pressure is controlled at 0.5~0.8MPa. After the grouting is completed, the grouting holes are sealed and the settlement changes are monitored to ensure that the settlement is stable within the control threshold. S6. Real-time monitoring and closed-loop adjustment: S6-1. Establishment of Monitoring System: Establish a multi-dimensional monitoring system of "surface - riverbed - shield machine body - segment". Surface monitoring and riverbed monitoring points are arranged in a 5m×5m grid, and the density is increased to 3m×3m in key areas. The monitoring indicators include settlement, settlement rate, pore water pressure and segment deformation. The monitoring frequency is once every 30 minutes during tunneling, once every 2 hours after tunneling is completed, and once a day after stabilization. S6-2. Closed-loop adjustment: The monitoring data is transmitted to the construction control center in real time. The settlement change trend is analyzed through the data processing system. If the settlement is close to the control threshold, it is promptly fed back to the tunneling system and grouting system to adjust the tunneling parameters (such as reducing the tunneling speed and increasing the thrust) or grouting parameters (such as increasing the grouting volume and starting secondary grouting in advance) to form a closed-loop control of "monitoring-analysis-adjustment-re-monitoring".

[0006] As a further aspect of the present invention, F1 is obtained by the following formula: in, The diameter of the tunnel boring machine (m) is the shield diameter. The ultimate bearing capacity of the soil on the front side (kPa) in extremely soft soil strata You can calculate it using the following formula: in, The depth of the center of the tunnel section (m).

[0007] As a further aspect of the present invention, F2 is the frictional force (kN) between the shield and the soil, and the calculation formula is as follows: in, The shield shell length (m) is the length of the tunnel boring machine. The frictional stress (kPa) between the shield and the soil. =0.5c.

[0008] As a further aspect of the present invention, F3 is the cutting resistance of the cutter head (kN), and the calculation formula is as follows: in, The cutterhead torque is (kN·m). In extremely soft soil strata, M can be calculated as follows: Estimate.

[0009] As a further aspect of the present invention, the formula for calculating the formation hydrostatic pressure is as follows: in, ρ is the specific weight of water, taken as 10 kN / m³, and h is the burial depth of the grouting point (m).

[0010] As a further aspect of the present invention, the criteria for determining the grouting timing are as follows: (1) Settlement rate index: When the settlement rate of the surface or riverbed is continuously greater than 0.1 mm / h in real time, and this trend is maintained stably for 2 monitoring cycles (calculated based on monitoring once every 30 minutes, i.e., for more than 1 hour), after excluding external interference factors such as monitoring equipment error and sudden changes in river water load, it is determined that secondary grouting needs to be started. This threshold is determined based on the test data of the compression rate of extremely soft soil in the later stage. When the rate exceeds 0.1 mm / h, if it is not compensated in time, the subsequent cumulative settlement is likely to exceed the control threshold. (2) Cumulative settlement index: When the cumulative settlement of a certain monitoring point reaches 60% of the preset micro-settlement control threshold, regardless of the settlement rate, a secondary grouting plan must be initiated. For sensitive areas such as river embankments and underground pipelines, this ratio is strictly adjusted to 50% to ensure sufficient settlement compensation space is reserved. The judgment process requires comprehensive analysis of data from multiple monitoring points. Grouting can only be formally implemented when more than three adjacent monitoring points in the same area simultaneously meet any of the above conditions, in order to avoid misjudgment caused by abnormal data from a single point.

[0011] As a further aspect of the present invention, the formula for calculating the grouting-affected area (m²) is as follows: Where D is the outer diameter of the segment, and 0.5m is the effective diffusion radius of the grouting, which is determined based on grouting tests on extremely soft soil.

[0012] As a further aspect of the present invention, the segmented grouting method is as follows: (1) The length of each grouting section is controlled between 0.5 and 1.0 m, and the grouting is carried out section by section from the bottom of the hole upwards; (2) After each grouting is completed, let it stand for 5 to 10 minutes until the grout has initially solidified before proceeding to the next section. This is to prevent the grout from flowing back along the hole wall. The grouting sequence follows the principle of "downstream first, then upstream; outside first, then inside; symmetrical and synchronous". The four grouting holes of each ring segment are started simultaneously. The difference in grouting volume is controlled within 10% to prevent the segments from deforming due to uneven stress.

[0013] As a further embodiment of the present invention, the surface monitoring extends 30m on each side of the shield tunnel axis and is evenly distributed in a 5m×5m grid. For key sensitive areas such as riverbanks, areas with dense underground pipelines, and areas around the foundations of existing buildings, the monitoring points are densified to 3m×3m. The monitoring points are reinforced concrete stakes embedded 0.8~1.2m below the ground surface, with stainless steel probes installed on the top to avoid river erosion and human disturbance. The riverbed monitoring uses dedicated underwater monitoring stakes, with the bottom of the stake embedded 1.5m below the riverbed and the top flush with the riverbed surface. A protective sleeve is installed on the outside to prevent water flow impact. The layout of the monitoring points corresponds one-to-one with the surface monitoring points to ensure the longitudinal comparability of settlement data.

[0014] As a further aspect of the present invention, the monitoring of the shield machine body utilizes the shield machine's built-in attitude monitoring system and the added strain sensors to collect parameters such as shield axis deviation, cutterhead attitude, and shield shell stress in real time. The sensor spacing is 2m, and the measurement accuracy is controlled within ±0.1mm. The monitoring of the tunnel segments is based on the deployment of 4 settlement monitoring points (1 each on the top, bottom, left, and right) and 4 strain monitoring points on each ring of tunnel segments. The monitoring points use adhesive sensors, which are fixed to the inside of the tunnel segments to avoid interference with grouting holes and bolts.

[0015] Compared with related technologies, the micro-settlement control construction method for shield tunneling through extremely soft soil strata near rivers provided by this invention has the following beneficial effects: 1. This invention addresses the characteristics of extremely soft soil strata in rivers by establishing a quantitative relationship between shield tunneling parameters, grouting parameters, and settlement. A series of calculation formulas enable the determination of construction parameters, thereby avoiding the problem of unstable settlement control caused by empirical operation.

[0016] 2. This invention adopts a collaborative control strategy of "dynamic tunneling parameter optimization + synchronous grouting + secondary compensation grouting", combined with multi-dimensional real-time monitoring and closed-loop adjustment, which can respond to dynamic changes in the strata in a timely manner, effectively control the surface and riverbed subsidence within the corresponding range, and ensure the safety of the surrounding environment and river embankment of the crossing section. 3. The construction process of this invention is clear and highly operable, and it can be applied to river crossing projects in extremely soft soil strata under different diameter shield tunnels and different burial depths, thus having wide applicability and promotional value. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process of the present invention; Figure 2 This is a schematic diagram illustrating the construction steps of the present invention. Figure 3 This is a schematic diagram of the shield machine of the present invention crossing a river and extremely soft soil; Figure 4 This is a schematic diagram of the measured stratigraphic and hydrogeological data in Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of the linkage data between tunneling parameters and settlement monitoring in Embodiment 1 of the present invention; Figure 6 This is a schematic diagram of the measured data of grouting effect in Embodiment 1 of the present invention; Figure 7 This is a schematic diagram of the segment deformation monitoring data in Embodiment 1 of the present invention; Figure 8 This is a schematic diagram of the special monitoring data for sensitive areas in Embodiment 2 of the present invention; Figure 9 This is a schematic diagram of the measured data of the grouting system in Embodiment 2 of the present invention; Figure 10 This is a schematic diagram of the linkage data between soil excavation volume and parameter adjustment in Embodiment 3 of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0020] Example 1 A subway shield tunnel project in a certain city needs to cross a river at close range. The crossing section is 85m long, the shield diameter D=6.2m, the segment outer diameter d=6.0m, the shield shell length L=8.5m, the riverbed depth is 5m, the riverbed silt thickness is 1.8m, and the underlying extremely soft soil layer is mainly silty clay. Exploration parameters obtained are: natural water content ω=45%, natural unit weight γ=17kN / m³. 3 The formation has a void ratio e = 1.25, a compression modulus Es = 3.5 MPa, a cohesion c = 8 kPa, an internal friction angle φ = 6°, and a formation permeability coefficient k = 1.2 × 10⁻⁶. -6 cm / s, the center burial depth of the tunnel section is h=12m, the micro-settlement control threshold is set at 5mm, there are no special sensitive structures in the crossing area, which is a conventional crossing condition.

[0021] Construction process and parameter calculation: 1. Preparations before construction: By employing a combination of drilling, static cone penetration, and vane shear test methods, the geological and hydrogeological survey of the crossing section was completed. Based on the survey data, a three-dimensional geological model was established, and the maximum surface subsidence was predicted to be approximately 4.8 mm through numerical simulation. The central area of ​​the riverbed was identified as a subsidence-sensitive area. 2. Initialize tunneling parameters; 3. Synchronous grouting; 4. Dynamic Adjustment and Secondary Grouting: At 30m, the riverbed settlement rate was monitored at 0.18mm / h, with a cumulative settlement of 2.8mm (56% of the control threshold), not meeting the conditions for initiating secondary grouting. After continuing excavation for another 5m, the settlement rate increased to 0.22mm / h, and the initiation parameters were adjusted: the soil chamber pressure was too low, so the total thrust was increased by 8%, adjusted to 15650×1.08≈16902kN; after adjustment, the settlement rate decreased to 0.15mm / h. At 60m, the cumulative settlement at a certain monitoring point reached 3.0mm (60% of the control threshold), and secondary grouting was initiated. The settlement deficit Δs = 5 - 3.0 = 2 mm, the grouting influence area A = π × (D / 2 + 0.5)² = π × (3.0 + 0.5)² ≈ 38.5 m², the secondary grouting volume Es is in kPa, Δs is in m, Q2 = 2 × 10⁻³ m × 38.5 m² × 3500 kPa / (1 + 1.25) = 2 × 10⁻³ × 38.5 × 3500 / 2.25 ≈ 119 × 10⁻³ m³ = 0.119 m³, segmented grouting is adopted, each segment is 0.8 m long, and the grouting pressure is 0.6 MPa.

[0022] 5. Monitoring results: During the entire crossing process, the maximum cumulative settlement of the surface and riverbed was 3.2 mm, which was controlled within the 5 mm micro-settlement threshold. The maximum deformation of the tunnel segments was 0.8 mm, and there were no leaks or abnormalities.

[0023] Example 2 A shield tunnel for an intercity railway crosses a navigable river. The crossing section is 120m long, the shield diameter is D=10.0m, the segment outer diameter is d=9.8m, the shield shell length is L=11.0m, the riverbed depth is 8m, the riverbed silt thickness is 2.5m, and the lower part is fluid plastic extremely soft soil. Exploration parameters: ω=52%, γ=16kN / m³, e=1.42, Es=2.8MPa, c=6kPa, φ=4°, k=8×10⁻⁶. -7 cm / s, the center burial depth of the tunnel section is h=18m, the crossing area is close to the river embankment and is a sensitive area, the micro settlement control threshold is 5mm, and the cumulative settlement ratio after the second grouting is started is strictly taken as 50%.

[0024] Construction process and key controls 1. Initial total thrust calculation: σ=5c+γh=5×6+16×18=318kPa; F1=π×10² / 4×318≈24923kN; τ=0.5×6=3kPa, F2=π×10×11×3≈1036.7kN; M=0.15×F×10 (estimated F=35000kN)=52500kN·m, F3=π×10 / 2×52500≈82467kN; total thrust F=24923+1036.7+82467≈108427kN.

[0025] 2. Synchronous grouting: K=2.0, L=1.0m, Q=2.0×π×(10²−9.8²)×1.0 / 4≈6.2m³; grout setting time 35s, grouting pressure 0.18MPa (static water pressure 180kPa, 180×1.2=216kPa=0.216MPa); 3. Secondary grouting initiation: When the excavation reaches 45m, the cumulative settlement of three adjacent monitoring points near the embankment reaches 2.5mm (50% of the control threshold), and secondary grouting is initiated: Δs=5-2.5=2.5mm, A=π×(9.8 / 2+0.5)²=π×(4.9+0.5)²≈73.9m², Q2=2.5×10⁻³×73.9×2800 / (1+1.42)≈0.21m³; segmented grouting is adopted, with each segment being 0.6m in length, and left to stand for 8 minutes. The grouting sequence follows the principle of "outer side first, then inner side, symmetrical and synchronous". 4. Monitoring results: After the crossing was completed, the maximum cumulative settlement near the embankment was 3.8 mm and the maximum settlement at the riverbed was 4.2 mm, both of which met the requirements for micro-settlement control. There were no cracks or displacements in the embankment.

[0026] Example 3 A municipal drainage shield tunnel crosses a small river. The crossing section is 60m long, the shield diameter D=4.5m, the segment outer diameter d=4.3m, and the shield shell length L=6.8m. The riverbed depth is 3m, the riverbed silt thickness is 1.2m, and the lower part is silty clay. Exploration parameters: ω=40%, γ=17.5kN / m³, e=1.18, Es=4.2MPa, c=10kPa, φ=8°, k=2×10⁻⁶. -6 cm / s. The burial depth at the center of the tunnel section is h=8m, and the micro-settlement control threshold is 5mm.

[0027] Construction process and characteristics 1. Initial parameters: σ = 5 × 10 + 17.5 × 8 = 190 kPa; F1 = π × 4.5² / 4 × 190 ≈ 3044 kN; τ = 0.5 × 10 = 5 kPa, F2 = π × 4.5 × 6.8 × 5 ≈ 480 kN; M = 0.15 × F × 4.5 (estimated F = 5000 kN) = 3375 kN·m, F3 = π × 4.5 / 2 × 3375 ≈ 23864 kN. Correction: The estimated F was too small, so we re-estimated F = F1 + F2 + F3 ≈ 3044 + 480 + F3, F3 =π×4.5 / 2×0.15F×4.5=π×4.5×0.15F×4.5 / 2≈4.77F, therefore F=3044+480+4.77F→3.77F=-3524, which is incorrect. Correct the cutter head torque estimation: M=0.15×D×F1 (related to frontal resistance), M=0.15×4.5×3044≈2050kN·m, F3=π×4.5 / 2×2050≈14580kN, total thrust F=3044+480+14580≈18104kN; 2. Key Controls: Due to the shallow burial depth and significant impact from river water load, monitoring frequency was increased to once every 20 minutes; synchronous grouting K=1.6, grouting pressure 0.1MPa (static water pressure 80kPa, 0.1MPa≤80×1.2=96kPa); excessive soil excavation occurred during tunneling, so the tunneling speed was reduced from 15mm / min to 12mm / min, and the cutterhead speed was increased from 2.2r / min to 2.8r / min to ensure that v / πDn=12000 / (π×4500×2.8)≈0.303, and remained stable within the range of 0.3~0.5. 3. Monitoring results: The maximum cumulative settlement was 2.9 mm, the maximum settlement rate was 0.16 mm / h, there was no soil heave or leakage, and the construction quality was stable.

[0028] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling micro-settlement during shield tunneling through extremely soft soil strata near a river, characterized in that, Includes the following steps: S1. Pre-construction preparation; S2. Initialization of shield tunneling parameters, determining initial tunneling parameters through calculation formulas based on the physical and mechanical parameters of extremely soft soil strata; S3. Dynamic optimization of tunneling parameters; S4. Synchronous grouting control; S5. Secondary compensation grouting; and S6. Real-time monitoring and closed-loop adjustment. Among them, S1-1. Geological exploration: By using drilling, static cone penetration, vane shear test and other exploration methods, a comprehensive exploration of the riverbed sediment and the underlying extremely soft soil strata of the crossing section will be carried out to obtain the physical and mechanical parameters of the strata, such as natural water content ω, natural unit weight γ, void ratio e, compression modulus Es, cohesion c, and internal friction angle φ. At the same time, the hydrogeological conditions of river water level, flow velocity, riverbed sediment thickness and strata permeability coefficient k will be explored. Among them, S1-2. Settlement risk prediction: Based on the survey data, a three-dimensional geological model is established, and numerical simulation method is used to predict the settlement distribution of the surface and riverbed during the shield tunneling process, identify settlement sensitive areas, and determine the micro-settlement control threshold, which is usually no more than 5mm. Among them, S2-1. Calculation of total shield thrust, the total shield thrust overcomes the frontal resistance of the soil, the friction between the shield shell and the soil, and the cutting resistance of the cutterhead. The calculation formula is as follows: Where F is the total thrust of the shield (kN) and F1 is the soil resistance at the front (kN). S2-2. Calculation of matching tunneling speed and cutterhead rotation speed: To reduce disturbance to extremely soft soil, it is necessary to control the matching between the cutterhead cutting frequency and the tunneling speed. The matching relationship between the cutterhead rotation speed n (r / min) and the tunneling speed v (mm / min) is as follows: Where f is the cutterhead cutting coefficient. In extremely soft soil layers, the value of f ranges from 0.3 to 0.5, and is adjusted according to the soil moisture content. The higher the moisture content, the smaller the value of f.

2. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: in, S3-1. Real-time data acquisition: During the tunneling process, the shield's built-in sensors collect real-time operating parameters such as total thrust, cutterhead rotation speed, tunneling speed, soil chamber pressure, and excavated soil volume. At the same time, the on-site monitoring equipment collects feedback information on stratum deformation and pore water pressure changes. Specifically, S3-2. Parameter adjustment strategy: When the monitored formation subsidence rate exceeds 0.2 mm / h or the cumulative subsidence reaches 3 mm, the parameters shall be adjusted according to the following principles: (1) If the soil chamber pressure is too low, increase the total thrust of the shield appropriately, with an adjustment range of 5% to 10% of the initial thrust; (2) If the amount of excavated soil is too large, reduce the tunneling speed and increase the cutterhead speed at the same time to ensure The ratio remained stable within the range of 0.3 to 0.5; (3) If signs of soil heave appear, reduce the total thrust and reduce the grouting volume accordingly; Among them, S4-1. Calculation of synchronous grouting volume: The core of synchronous grouting is to fill the annular gap formed after shield tunneling in a timely manner. In extremely soft soil strata, the volume of the annular gap is easily changed due to soil compression. The formula for calculating the grouting volume Q (m³) is as follows: Where K is the grouting coefficient, and in extremely soft soil layers, considering soil compression and grout loss, the value of K ranges from 1.5 to 2.0, d is the outer diameter of the segment (m), and L is the tunneling length (m) corresponding to a single grouting. Among them, S4-2. Grouting parameter control: synchronous grouting adopts cement-water glass double liquid grout, the grout setting time is controlled at 30~60s, the grouting pressure is controlled at 0.3~0.5MPa, and does not exceed 1.2 times the hydrostatic pressure of the formation, so as to avoid soil disturbance caused by excessive grouting pressure.

3. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: in, S5-1. Judgment of grouting timing: When the real-time monitoring shows that the surface or riverbed settlement rate is continuously greater than 0.1 mm / h, or the cumulative settlement reaches 60% of the control threshold, secondary compensation grouting shall be initiated. Among them, S5-2. Determination of secondary grouting parameters: Ultrafine cement grout is used for secondary grouting, and the grouting volume is calculated based on the settlement deficit. The calculation formula is as follows: Where Q2 is the secondary grouting volume (m³), Δs is the settlement deficit (mm), which is the difference between the control threshold and the current cumulative settlement, A is the grouting influence area (m²), Es is the soil compression modulus (kPa), and e is the soil void ratio. Among them, S3. Grouting construction: The secondary grouting holes are set at the grouting holes of the segments. The segmented grouting method is adopted, and the grouting pressure is controlled at 0.5~0.8MPa. After the grouting is completed, the grouting holes are sealed, and the settlement changes are monitored to ensure that the settlement is stable within the control threshold. Among them, S6-1. Establishment of monitoring system: Establish a multi-dimensional monitoring system of "surface - riverbed - shield machine body - segment". The surface monitoring and riverbed monitoring points are arranged in a 5m×5m grid, and the density is increased to 3m×3m in key areas. The monitoring indicators include settlement, settlement rate, pore water pressure and segment deformation. The monitoring frequency is once every 30 minutes during tunneling, once every 2 hours after tunneling is completed, and once a day after stabilization. Among them, S6-2. Closed-loop adjustment: The monitoring data is transmitted to the construction control center in real time. The settlement change trend is analyzed through the data processing system. If the settlement is close to the control threshold, it is fed back to the tunneling system and grouting system in a timely manner to adjust the tunneling parameters (such as reducing the tunneling speed and increasing the thrust) or grouting parameters (such as increasing the grouting volume and starting the secondary grouting in advance) to form a closed-loop control of "monitoring-analysis-adjustment-re-monitoring".

4. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: F1 is obtained through the following formula: in, The diameter of the tunnel boring machine (m) is the shield diameter. The ultimate bearing capacity of the soil on the front side (kPa) in extremely soft soil strata You can calculate it using the following formula: in, The burial depth at the center of the tunnel section (m); F2 is the frictional force (kN) between the shield and the soil, calculated using the following formula: in, The shield shell length (m) is the length of the tunnel boring machine. The frictional stress (kPa) between the shield and the soil. =0.5c; F3 represents the cutting resistance of the cutter head (kN), calculated using the following formula: in, The cutterhead torque is (kN·m). In extremely soft soil strata, M can be calculated as follows: Estimate.

5. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The formula for calculating the formation hydrostatic pressure is as follows: in, ρ is the specific weight of water, taken as 10 kN / m³, and h is the burial depth of the grouting point (m).

6. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The criteria for determining the timing of grouting are as follows: (1) Settlement rate index: When the settlement rate of the surface or riverbed is continuously greater than 0.1 mm / h in real time, and this trend is maintained stably for 2 monitoring cycles (calculated based on monitoring once every 30 minutes, i.e., for more than 1 hour), after excluding external interference factors such as monitoring equipment error and sudden changes in river water load, it is determined that secondary grouting needs to be started. This threshold is determined based on the test data of the compression rate of extremely soft soil in the later stage. When the rate exceeds 0.1 mm / h, if it is not compensated in time, the subsequent cumulative settlement is likely to exceed the control threshold. (2) Cumulative settlement index: When the cumulative settlement of a certain monitoring point reaches 60% of the preset micro-settlement control threshold, regardless of the settlement rate, a secondary grouting plan must be initiated. For sensitive areas such as river embankments and underground pipelines, this ratio is strictly adjusted to 50% to ensure sufficient settlement compensation space is reserved. The judgment process requires comprehensive analysis of data from multiple monitoring points. Grouting can only be formally implemented when more than three adjacent monitoring points in the same area simultaneously meet any of the above conditions, in order to avoid misjudgment caused by abnormal data from a single point.

7. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The formula for calculating the grouting-affected area (m²) is as follows: Where D is the outer diameter of the segment, and 0.5m is the effective diffusion radius of the grouting, which is determined based on grouting tests on extremely soft soil.

8. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The segmented grouting method is as follows: (1) The length of each grouting section is controlled between 0.5 and 1.0 m, and the grouting is carried out section by section from the bottom of the hole upwards; (2) After each grouting is completed, let it stand for 5 to 10 minutes until the grout has initially solidified before proceeding to the next section. This is to prevent the grout from flowing back along the hole wall. The grouting sequence follows the principle of "downstream first, then upstream; outer side first, then inner side; symmetrical and synchronous". The four grouting holes of each ring segment are started simultaneously. The difference in grouting volume is controlled within 10% to prevent the segments from deforming due to uneven stress.

9. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The surface monitoring extends 30m on each side of the shield tunnel axis, and is evenly distributed in a 5m×5m grid. For key sensitive areas such as riverbanks, areas with dense underground pipelines, and areas around the foundations of existing buildings, the monitoring points are densified to 3m×3m. The monitoring points are reinforced concrete stakes, embedded 0.8~1.2m below the ground surface, with stainless steel probes installed on the top to avoid river erosion and human disturbance. The riverbed monitoring uses dedicated underwater monitoring stakes, with the bottom of the stake embedded 1.5m below the riverbed and the top flush with the riverbed surface. External protective sleeves are installed to prevent water flow impact. The layout of the monitoring points corresponds one-to-one with the surface monitoring points to ensure the longitudinal comparability of settlement data.

10. The method for controlling micro-settlement during close-range tunneling of a shield tunnel through extremely soft soil strata of a river, as described in claim 1, is characterized in that: The monitoring of the tunnel boring machine body utilizes the machine's built-in attitude monitoring system and added strain sensors to collect parameters such as shield axis deviation, cutterhead attitude, and shield shell stress in real time. The sensors are spaced 2m apart, and the measurement accuracy is controlled within ±0.1mm. The monitoring of the tunnel segments is based on the deployment of 4 settlement monitoring points (1 each on the top, bottom, left, and right) and 4 strain monitoring points on each ring of tunnel segments. The monitoring points use adhesive sensors, which are fixed to the inside of the tunnel segments to avoid interference with grouting holes and bolts.

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