A deep foundation pit stability evaluation method based on dynamic load increment displacement response ratio

Abstract

The present application relates to the field of foundation pit construction, specifically to a deep foundation pit stability evaluation method based on dynamic load increment displacement response ratio, which solves the problem of lack of dynamic, quantitative and regional of the existing deep foundation pit stability evaluation method. The specific steps are as follows: according to the geological sampling, the distribution of the foundation pit rock-soil layer is determined, and the excavation working condition and the corresponding excavation depth are divided accordingly; the foundation pit sensor is laid out, and the stress value, strain value and change increment of each working condition monitoring point are calculated through real-time reading; the deep foundation pit excavation dynamic load increment displacement response rate is calculated; the deep foundation pit excavation dynamic load increment displacement response ratio parameter is determined; according to the safety grade of foundation pit engineering, the corresponding critical safety coefficient is determined; based on the calculated early warning threshold, the foundation pit stability judgment criterion is established. The present application realizes the dynamic, quantitative and regional early warning of the deep foundation pit stability state by constructing the quantitative relationship between the dynamic load increment displacement response ratio parameter and the critical safety coefficient K.

Description

Technical Field

[0001] This invention relates to the field of foundation pit construction, specifically to a method for evaluating the stability of deep foundation pits based on the dynamic load displacement response ratio. Background Technology

[0002] With the deepening development of urban underground space, the scale of deep foundation pit projects is increasing daily. The excavation of these pits is essentially a process of unloading the soil and rock mass, which causes stress redistribution and deformation of the retaining structure and surrounding soil, inducing progressive damage and even leading to disasters such as slope instability and support structure failure. Therefore, stability assessment of deep foundation pits can dynamically and quantitatively determine the current degree of damage, allowing for early intervention and prevention of disasters caused by the unloading process of the soil and rock mass.

[0003] Existing methods for evaluating the stability of deep foundation pits, such as the limit equilibrium method or traditional displacement monitoring methods, primarily focus on calculating the overall safety factor of the slope or analyzing the displacement at a single point. These traditional methods struggle to effectively characterize the non-uniform, dynamically evolving instability risks exhibited by different areas of the foundation pit during excavation and unloading due to variations in soil and rock properties and stress states. They also fail to accurately identify and provide early warnings for locally vulnerable areas. Although some studies have introduced damage mechanics into geotechnical engineering and established elastoplastic damage models for rocks, these models primarily focus on material constitutive relationships and are difficult to directly apply to dynamic and regional stability evaluation and early warning in complex construction sites.

[0004] Therefore, there is an urgent need to establish an evaluation model that can dynamically and quantitatively assess the dynamic response and regional damage evolution of soil and rock during the excavation and unloading process, so as to achieve precise control and risk warning of the safety status of deep foundation pit engineering. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for evaluating the stability of deep foundation pits based on the dynamic load displacement response ratio. This method aims to overcome the shortcomings of existing technologies in dynamically and quantitatively evaluating the regional damage evolution and stability status during the excavation of deep foundation pits.

[0006] To achieve the above objectives, the present invention provides a method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio, which specifically includes the following steps:

[0007] (1) Determine the distribution of soil and rock layers in the foundation pit based on geological sampling, and classify the excavation conditions and corresponding excavation depths accordingly;

[0008] (2) Install foundation pit sensors and calculate the stress value of each working condition monitoring point through real-time readings. strain value and its increment and ;

[0009] (3) Calculate the dynamic load displacement response rate of deep foundation pit excavation. ;

[0010] (4) According to Determine the dynamic load displacement response ratio parameter for deep foundation pit excavation ;

[0011] (5) Determine the corresponding critical safety factor based on the safety level of the foundation pit project. ;

[0012] (6) Based on calculate warning threshold Based on this, a criterion for judging the stability of the foundation pit was established.

[0013] This invention constructs a dynamic load displacement response ratio parameter. The quantitative relationship between the critical safety factor K and the regional damage evolution and stability status during the deep foundation pit excavation process is used to dynamically and quantitatively evaluate the regional damage evolution and stability status.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] This invention addresses the shortcomings of traditional stability evaluation in terms of dynamic mechanical characterization by introducing the dynamic load displacement response ratio and its early warning threshold, thereby achieving dynamic and quantitative mechanical characterization of foundation pit stability.

[0016] This invention can identify and locate local weak areas during the excavation of foundation pits, reveal the differential damage evolution law of soil and rock masses, and achieve a breakthrough from "overall evaluation" to "regional discrimination".

[0017] The data required by this invention only includes stress and displacement information from routine monitoring of foundation pit engineering, requiring no additional equipment, and is easy to promote and apply under complex geological conditions. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the deep foundation pit stability evaluation method based on dynamic load displacement response ratio in an embodiment of the present invention.

[0019] Figure 2 This is a diagram showing the thickness distribution of the soil layer in the foundation pit to be excavated at section 3-3 of Huicheng Road Station in this embodiment of the invention.

[0020] Figure 3 This is a diagram showing the layout of monitoring points in the deep foundation pit according to an embodiment of the present invention;

[0021] Figure 4 This is a diagram showing the layout of horizontal displacement and stress monitoring points in a deep foundation pit according to an embodiment of the present invention.

[0022] Figure 5 This is a schematic diagram of sensor assembly in an embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] This embodiment uses the deep foundation pit project of Huicheng Road Station of Qingdao Metro as the application background. The following will refer to... Figure 1 The flowchart shown is a schematic diagram of the deep foundation pit stability evaluation method based on dynamic load displacement response ratio, which introduces the deep foundation pit stability evaluation method based on dynamic load displacement response ratio.

[0025] A method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio includes the following steps:

[0026] S1: Determination of soil and rock layer parameters for the foundation pit

[0027] In this embodiment, an XY-180 / 200 engineering drilling rig is used for the deep foundation pit to be excavated. Rotary drilling is employed, combined with mud wall support and casing follow-up drilling techniques. The purpose is to delineate the geological structure, soil and rock layer thickness, conduct in-situ borehole sampling and testing, and observe the groundwater level. The borehole diameter is not less than Ф130mm, and the final borehole diameter is Ф91mm (bedrock). The borehole diameter meets the requirements for sampling, testing, and drilling techniques. The drilling depth per pass in the soil layer is 1. 2m, soil core recovery rate not less than 90%; drilling depth per rock stratum run 1 2m. The core recovery rate of completely weathered, strongly weathered and tectonic fractured zones shall not be less than 65%, and the core recovery rate of moderately weathered and slightly weathered strata shall not be less than 80%.

[0028] According to the "Code for Geotechnical Investigation of Urban Rail Transit" (GB50307-2012) and the "Sequence Stratigraphy of Quaternary System in Qingdao Urban Area", the sampling interval is generally 1.5 meters. When the soil layer thickness is greater than 8m, three groups of samples can be taken from the top, middle, and bottom. For soil layers thicker than 0.5m but less than 2m, priority sampling is required. The main sampling locations for underground line exploration points are the soil and rock masses within the tunnel roof, tunnel body, and station pit depth range. Thin-walled samplers are used for pressing cohesive soils, and standard penetration tests (SPTs) are used for sandy soils. Based on borehole sampling analysis, the different soil (rock) layers and their thicknesses in the pit to be excavated are shown in Table 1 and... Figure 2 As shown:

[0029] Table 1 Soil layers and parameters of the foundation pit to be excavated

[0030]

[0031] Note: Based on the geotechnical engineering investigation report of the main structure (YCK28+001.234~YCK28+238.234) of Huicheng Road Station of Phase I Project of Qingdao Metro Line 9, Qingdao Surveying and Mapping Institute, May 2022.

[0032] Based on the completed exploration boreholes and the adoption of the "Qingdao Urban Area Quaternary Sequence Stratigraphy" standard stratigraphic sequence numbering promoted by the Qingdao Municipal Construction Commission, the foundation pit was stratified mainly according to geological age, genesis, and soil and rock types to obtain geometric features and geomorphological information. The stratigraphic structure of this section is complex, with various soil and rock masses distributed from top to bottom, including fill, silty clay, breccia, strongly weathered rhyolite, moderately weathered rhyolite, and slightly weathered rhyolite. The thickness distribution of the soil (rock) layers is as follows: Figure 2 As shown, the entire excavation process is divided into the following sections: Section 1: excavating 1.5m of plain fill; Section 2: excavating 4m of clay; Section 3: excavating 4.3m of angular gravel; Section 4: excavating 3.43m of strongly weathered rock; Section 5: excavating 3m of moderately weathered rock; and Section 6: excavating 3.49m of slightly weathered rock to the bottom of the pit.

[0033] S2: Deployment and monitoring of horizontal strain and stress value monitoring system for foundation pit

[0034] The foundation pit for this project is 223m long, 20.1-24m wide, and 18.5-23.0m deep, and will be constructed using the open-cut method. A representative section 3-3 is selected as the research object, and the specific monitoring points are arranged as follows: Figure 3 and Figure 4 As shown.

[0035] Drill holes with a diameter of no less than Ф80mm and a depth of no less than 20m at representative locations such as the external corner of the foundation pit excavation, and clean the hole walls. Use the excavation depth h for each working condition as the preset depth for the monitoring points to obtain 7 monitoring points from top to bottom. The displacement gauge selected is the NDW-100W vibrating wire displacement gauge from NARI Group, and the miniature vibrating wire earth pressure cell is the NZS-15 vibrating wire stress gauge from NARI Group. Before installation, the pressure bladder (sensing surface side) of the earth pressure cell should face upwards. Through the dedicated injection hole, use degassed water (boiled and cooled pure water) and a hand pump to completely expel the air from its cavity until continuous water droplets emerge from the injection hole. Then quickly seal it. After preparation, proceed according to... Figure 5Assemble as shown. Install the vibrating wire displacement gauge and the miniature vibrating wire earth pressure cell side-by-side on the same monitoring section on the sensor side rod at depth point intervals, leaving one monitoring point at each depth. Place the sensor side rod into the hole, apply epoxy resin to the connection between the fixing support and the pre-embedded pipe, and let it cure. Inject cement mortar from the bottom of the hole upwards through the grouting pipe, stopping grouting when grout returns in the vent pipe. After the cement mortar has solidified, fix the sensor to the annular anchor head at the hole opening, connect the cables sequentially, and record the initial readings. After installation, lead the cables out of the hole and install the protective cover.

[0036] When the internal structure of the soil under test undergoes pressure deformation, the pre-installed vibrating wire earth pressure gauge will synchronously transmit the stress change to the vibrating wire through the pressure-bearing membrane, causing the length of the vibrating wire to change. The resulting vibration frequency signal is sensed by the excitation coil and transmitted to the frequency reading instrument through the shielded cable.

[0037] According to the formula Record data using the earth pressure cell reading instrument. The stress value inside the tested rock and soil body .in, The stress value at the monitoring point is given by ; k is the sensitivity coefficient of the vibrating wire earth pressure gauge. F represents the change in the frequency modulus of the vibrating wire earth pressure gauge output. i F0 is the real-time output frequency modulus value of the vibrating wire earth pressure gauge; F0 is the reference value of the output frequency modulus of the vibrating wire earth pressure gauge.

[0038] According to the formula Calculate the stress increment at each monitoring point t under each working condition from the start of damage to the end of excavation. Among these, The stress values ​​at monitoring point t under different working conditions; This represents the stress increment at monitoring point t from the start of damage to the end of excavation under the working condition; The stress value is at monitoring point t under initial operating conditions. The calculation results are shown in Table 2.

[0039] Table 2. Variation of horizontal stress during foundation pit excavation (kN / m) 2 )

[0040]

[0041] According to the formula The data recorded by the earth pressure cell reader is converted into strain values ​​inside the tested soil and rock mass. .in, The stress value at the monitoring point is given by ; k is the sensitivity coefficient of the vibrating wire displacement gauge. F represents the change in the frequency modulus of the vibrating wire displacement gauge. iF0 is the real-time output frequency modulus value of the vibrating wire displacement meter; F0 is the reference value of the output frequency modulus of the vibrating wire displacement meter.

[0042] According to the formula Calculate the strain increment at each monitoring point t under the corresponding stress increment. Among them, The strain values ​​at monitoring point t under different working conditions; The strain increment at monitoring point t under different working conditions under the corresponding stress increment; This represents the initial strain value at monitoring point t under the initial working conditions. The calculation results are shown in Table 3.

[0043] Table 3. Horizontal strain variation values ​​during foundation pit excavation (×10) -3 )

[0044]

[0045] S3: Dynamic Load Displacement Response Rate During Deep Foundation Pit Excavation The determination

[0046] The stress-strain curves and failure characteristics of soil and rock mass elements show that, as a nonlinear system, the material can measure its strain change before complete instability and failure. With stress change proportionality coefficient As a quantitative characterization of its stability and tendency to instability, this proportionality coefficient is defined. The dynamic load displacement response rate is expressed mathematically as follows:

[0047] ,

[0048] Let the variation of the generalized load on the foundation pit be... The change in response under load is R is the dynamic displacement response rate of the system under load. This can be expressed quantitatively as:

[0049] ,

[0050] Based on the relationship between slope load and response, the process of the foundation pit under generalized load can be divided into two cases: initial elastic loading deformation and plastic loading deformation, which are denoted as follows: The dynamic load displacement response rate of each monitoring point under various working conditions in the initial stage, The dynamic load displacement response rate of the monitoring points at the end of excavation under each working condition.

[0051] Record the horizontal stress change during excavation in working condition 1 as the initial stress increment, and the corresponding horizontal strain change as the initial strain increment.

[0052] According to the formula Calculate the initial dynamic load displacement response rate .in, The initial dynamic load displacement response rate at monitoring point t; The stress increment at monitoring point t at the initial moment; This represents the increase in material strain caused by the stress increment at the initial monitoring point t. The calculation results are shown below:

[0053] , ,

[0054] , ,

[0055] , ,

[0056] .

[0057] According to the formula: Calculate the mean initial dynamic load displacement response rate for each excavation depth. Among them, Z represents the initial dynamic load displacement response rate at monitoring point t; t n' represents the depth of measuring point t; n' represents the number of monitoring points within the cumulative excavation depth H of the foundation pit. n represents the total number of rock strata. The calculation results are shown below:

[0058] , , ,

[0059] , , .

[0060] According to the formula Calculate the dynamic load displacement response rate of monitoring points after excavation for each working condition. Among them, This represents the strain increment at each monitoring point t under the corresponding stress increment. This represents the stress increment at each monitoring point t during the period from the start of damage to the end of excavation. The calculation results are shown in Table 4.

[0061] Table 4 Dynamic load displacement response rate at monitoring points under various operating conditions (×10) -3 )

[0062]

[0063] According to the formula: Calculate the overall dynamic load response rate after each excavation condition is completed. Among them, Z represents the dynamic load displacement response rate of the monitoring points at the end of excavation under each working condition; t n' is the depth of measuring point t; n' is the cumulative excavation depth of the foundation pit. The number of monitoring points within the area. The calculation results are shown below:

[0064] , , ,

[0065] , , .

[0066] S4: Determination of the dynamic loading displacement response ratio parameter for deep foundation pit excavation

[0067] The increment of horizontal stress generated during excavation and unloading is selected as the dynamic loading parameter of the deep foundation pit evaluation system, and the increment of horizontal displacement is selected as the dynamic response parameter of the deep foundation pit evaluation system. The dynamic loading displacement response rate within any deformation stage of the material is then used. Response rate of dynamic loading displacement during the initial deformation stage The ratio is defined as the dynamic load increase response ratio, as shown in the following formula:

[0068] = / ,

[0069] According to the formula = / The dynamic load displacement response ratio parameters of each monitoring point under various working conditions were calculated, and the results are shown in Table 5.

[0070] Table 5. Dynamic load displacement response ratio parameter values ​​at monitoring points under various working conditions during foundation pit excavation.

[0071]

[0072] According to the formula The overall dynamic load displacement response ratio parameter was calculated under different excavation conditions. The calculation results are shown below:

[0073] , , , , , .

[0074] S5: Determination of Stability Criteria and Early Warning Threshold for Foundation Pit Excavation

[0075] Using the horizontal stress of the surrounding soil and rock during the excavation of the foundation pit as the load increase, and the horizontal displacement of the surrounding soil and rock caused by the stress change during the excavation as the response, we can obtain:

[0076] ,

[0077] Define the dynamic load response ratio reciprocal Then we have:

[0078] ,

[0079] The deep foundation pit excavation process is actually a damage development process of the soil and rock mass. Therefore, a damage variable D is introduced to represent the degree of damage, and the elastic modulus E of the soil and rock mass unloading due to damage is considered. i The effects are as follows:

[0080] E i =(1-D)E0,

[0081] The relationship between elastic modulus damage and the reciprocal of the dynamic loading response ratio are used. By combining the equations, we can obtain:

[0082] ,

[0083] Based on the relationship between damage mechanics and the probability of foundation pit failure, a limit safety factor for foundation pits is proposed. For the maximum damage variable and the maximum allowable damage variable The ratio of, among which The maximum value is 1, that is:

[0084] ,

[0085] Further, its response ratio with dynamic load increase was obtained. The relationship is:

[0086] ,

[0087] When the maximum allowable damage variable When it reaches its maximum, It also corresponds to its maximum value, therefore... As a criterion for determining the stability of a slope:

[0088] ,

[0089] according to Calculate the early warning threshold for the dynamic load displacement response ratio of this foundation pit project. .

[0090] S6: Differential Stability Evaluation and Early Warning of Foundation Pit Excavation

[0091] The excavation level of the foundation pit is Class II. By consulting the "Technical Specification for Foundation Pit Support" (JGJ 120-2012), the safety factor K for the excavated foundation pit slope is determined. cr =1.30, according to the formula Calculate the early warning threshold for the dynamic load displacement response ratio of the foundation pit project. ;

[0092] Based on the dynamic load displacement response ratio parameter values ​​at different depths under different excavation conditions in Table 5, local excavation stability evaluation and early warning are carried out.

[0093] Local differential damage analysis: Ratio of local dynamic load response at different depths during foundation pit excavation. Although the pressure was between 1.000 and 3.677, it did not reach the point of failure. However, the local dynamic load response ratio increased rapidly when the excavation depth was between 16.23m and 19.72m, indicating that this area was a weak link in the stability of the foundation pit. This area can be closely monitored and its support strengthened during subsequent construction, thus achieving early warning of potential risk areas.

[0094] The overall excavation stability is evaluated and early warning is given based on the overall dynamic load displacement response ratio parameter value under different excavation conditions.

[0095] , , , , , .

[0096] Overall stability damage analysis: Overall dynamic load response ratio during foundation pit excavation The value remained between 1.0 and 1.36 throughout the excavation process, indicating that the foundation pit was in a generally stable state throughout the construction period.

[0097] In summary, the method of this invention successfully achieves dynamic and quantitative evaluation and early warning of the overall stability and local differential damage of deep foundation pit engineering.

[0098] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio, characterized in that, Includes the following steps: S1: Determine the distribution of soil and rock layers in the foundation pit based on geological sampling, and classify the excavation conditions and corresponding excavation depths accordingly; S2: Install sensors in the foundation pit and calculate the stress value at each monitoring point based on real-time readings. strain value and its increment and ; S3: Calculate the dynamic load displacement response rate during deep foundation pit excavation. ; S4: According to Determine the dynamic load displacement response ratio parameter for deep foundation pit excavation ; S5: Determine the corresponding critical safety factor based on the safety level of the foundation pit project. ; S6: Based on calculate warning threshold Based on this, a criterion for judging the stability of the foundation pit was established.

2. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio as described in claim 1, characterized in that, The foundation pit sensor in S2 includes a miniature vibrating wire earth pressure cell and a vibrating wire displacement gauge installed on the same monitoring section.

3. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, The dynamic load displacement response rate in S3 The calculation formula is defined as strain increment. With stress increment The ratio of .

4. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, The dynamic load displacement response rate in S3 Specifically, this includes: initial dynamic load displacement response rate Mean initial dynamic load displacement response rate at each excavation depth Dynamic load displacement response rate at monitoring points after excavation under various working conditions is completed. Overall dynamic load response rate after completion of each excavation condition .

5. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, In S4, the dynamic load displacement response ratio parameter Including dynamic load displacement response ratio parameters at various working condition monitoring points Overall dynamic load displacement response ratio parameters under different excavation conditions .

6. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, The safety factor in S5 The safety level of the foundation pit is determined according to the "Technical Specification for Foundation Pit Support" JGJ 120-2012.

7. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, The warning threshold in S6 Based on the critical safety factor Determined according to the following formula: 。 8. The method for evaluating the stability of deep foundation pits based on the dynamic loading displacement response ratio according to claim 1, characterized in that, The stability criteria for the foundation pit in S6 are as follows: (1) If Less than If the monitoring points are locally stable during the foundation pit excavation process, then it is determined that the monitoring points are stable. Greater than or equal to This indicates that the monitoring point was locally unstable during the foundation pit excavation process; (2) If Less than If the excavation process of the foundation pit is deemed to be generally stable, then the excavation process of the foundation pit is determined to be Greater than or equal to Therefore, it is determined that there is a risk of overall instability during the excavation process of the foundation pit.

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