A pore pressure-strength reduction limit analysis method for saturated slope stability evaluation

Abstract

A pore pressure-strength reduction limit analysis method for saturated slope stability evaluation, the calculation method from the rock-soil effective stress principle and the physical and mechanical properties of rock-soil, the relationship between rock-soil solid skeleton deformation, rock-soil deformation and pore water pressure is established, which is specifically manifested as: for the strength reduction method limit analysis of saturated slope, the calculation method of pore water pressure increment of rock-soil is proposed, and the theoretical relationship between pore water pressure and rock-soil shear strength and strength reduction coefficient is established. By means of finite element analysis software, the pore pressure reduction in the process of slope stability calculation is realized, so that the results of rock-soil slope stability analysis are closer to the actual situation. Method: according to the calculation method of pore water pressure increment in the strength reduction method, the cohesion, internal friction angle and pore pressure increment under different reduction coefficients are solved, the pore pressure increment is given as the field load to the calculation unit, and the safety factor of slope stability is obtained.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering technology, and in particular relates to the safety evaluation of fluid-structure interaction limit analysis using the strength reduction method in saturated rock and soil slope engineering. Background Technology

[0002] The stability of a slope is affected not only by the physical and mechanical properties of the soil and rock materials, but also by factors such as the water content, seepage, and groundwater level within the soil and rock. In particular, changes in pore water pressure caused by rainfall and rising groundwater levels introduce uncertainties into the stability analysis of saturated slopes.

[0003] Currently, there are two main methods for calculating the shear strength of soil and rock slopes: the total stress method and the effective stress method. The total stress method does not consider the influence of pore water pressure on the shear strength of the rock mass, leading to discrepancies between the slope stability analysis results and actual conditions. The effective stress method, based on the total stress method, considers the influence of pore water pressure on the shear strength of the soil and rock mass, and can more objectively reflect the stability of soil and rock slopes. In recent years, to better address the stability analysis of fluid-structure interaction slopes or complex slopes, numerical calculation methods have been included in specifications and standards such as the "Technical Specification for Building Slope Engineering" (GB50330). Currently, in fluid-structure interaction slope analysis considering pore water pressure, the initial calculation uses water-soil characteristic curves to change the slope saturation, expressing the influence of pore water pressure on slope stability. However, changes in saturation only macroscopically indicate changes in water content within the slope and cannot directly and accurately identify changes in pore water pressure, thus failing to obtain the effective stress of the soil and rock mass, resulting in deviations between the calculated results and actual conditions.

[0004] The finite element method (FEM) strength reduction method is a relatively mature technique for analyzing the stability of complex rock slopes. It works by altering the strength parameters of the soil and rock mass to bring the anti-sliding force and the sliding force into a balanced or unstable state. However, current FEM methods only reduce the strength parameters of the slope soil and rock mass, such as cohesion and internal friction angle, without considering the influence of pore water pressure. This leads to some discrepancies between the calculated results and actual engineering conditions. Therefore, it is necessary to improve the FEM strength reduction method to adapt to changes in the effective stress of the rock mass caused by variations in pore water pressure, making it suitable for slope stability analysis involving changes in groundwater level. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a pore pressure-strength reduction limit analysis method for evaluating the stability of saturated slopes. This method establishes the relationship between pore water pressure and effective stress, providing an objective approach for evaluating the stability of saturated soil and rock slopes.

[0006] The present invention provides a pore pressure-strength reduction limit analysis method for evaluating the stability of saturated slopes, comprising the following steps: Step 1: Obtain the physical and mechanical parameters of the slope soil and rock mass through indoor and field tests; Step 2: Based on the engineering geological survey and hydrogeological survey, draw the slope profile and the groundwater level of the slope, and calculate the maximum pore water pressure u; Step 3: Calculate the cohesion and internal friction angle of the slope soil and rock mass under different reduction coefficients based on the cohesion and internal friction angle obtained in Step 1. The calculation formula is as follows; c1 = c / F1 (1) φ1=atan(tanφ / F1)(2) In the formula, c and φ are the cohesion and internal friction angle of the soil and rock mass, F1 is the reduction factor, and c1 and φ1 are the cohesion and internal friction angle of the soil and rock mass after strength reduction; Step 4: If the compressibility coefficient of the solid skeleton of the soil and rock mass is not obtained in the test of Step 1, the compressibility coefficient C of the solid skeleton of the soil and rock mass can be calculated by formula (3). s ; C s =3(1-2v) / E s (3) In the formula, v is the Poisson's ratio of the solid skeleton of the rock and soil mass, and E s The elastic modulus of the solid skeleton of the rock mass; Step 5: If the compressibility coefficient of the soil and rock mass is not obtained in the test in Step 1, the compressibility coefficient C of the soil and rock mass can be calculated by formula (4): C = 0.014404 / (1 + 55.8721n) 1.42359 (4) In the formula, n is the porosity of the soil and rock mass, which can be obtained from step one; Step 6: Substitute the solid skeleton compression coefficient and the compression coefficient of the soil and rock mass obtained in Step 1, or the solid skeleton compression coefficient and the compression coefficient of the soil and rock mass calculated in Steps 4 and 5, as well as the reduction factor, maximum pore water pressure, internal friction angle, and other parameters, into formula (5) to calculate the pore water pressure increment: Δu=u(1-C s / C)tanφ(1-1 / F1)(5) In the formula, Δu is the increment of pore water pressure, and C s denoted as the solid skeleton compressibility coefficient of the soil and rock mass, C is the compressibility coefficient of the soil and rock mass, u is the maximum pore water pressure, φ is the internal friction angle of the soil and rock mass, and F1 is the reduction factor. Step 7: Perform strength reduction analysis in finite element software, and assign the pore water pressure increment calculated by formula (5) as the field load to the calculation unit to calculate the stability of the rock mass slope.

[0007] The beneficial effects of this invention are: The present invention provides a pore pressure-strength reduction limit analysis method for evaluating the stability of saturated slopes, which can effectively take into account the effect of pore water pressure in the stability calculation of soil and rock slopes, making the stability analysis results of saturated slopes obtained by the strength reduction method closer to the actual situation. Attached Figure Description

[0008] Figure 1 This is a flowchart of the calculation process of the present invention; Figure 2 This is a diagram showing the location of monitoring points in a specific embodiment of the present invention; Figure 3 This is the s-F1 diagram in a specific embodiment of the present invention. Detailed Implementation

[0009] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0010] A specific embodiment of the pore pressure-strength reduction limit analysis method for evaluating the stability of saturated slopes has the following basic characteristics: the slope is 100m long laterally, 60m high longitudinally, with a slope angle of 38.6°, composed entirely of moderately weathered sandstone, with the groundwater level 5m below the ground surface, an internal friction angle of 22°, a cohesion of 40kPa, and an elastic modulus of 4.09×10⁻⁶. 4 With an MPa, porosity of 14.4%, and Poisson's ratio of 0.22, the stability analysis of this saturated slope was conducted using FLAC3D software. The specific steps are as follows: Step 1: Through indoor and field tests, the internal friction angle of the slope rock mass was obtained to be 22°, the cohesion to be 40 kPa, and the elastic modulus to be 4.09 × 10⁻⁶. 4 MPa, porosity 14.4%, Poisson's ratio 0.22; Step 2: Based on the engineering geological survey and hydrogeological survey, draw the slope profile and the groundwater level of the slope, and calculate the maximum pore water pressure u as 150 kPa; Step 3: Based on the cohesion c and internal friction angle of the slope soil and rock obtained in Step 1, set F1 to a value of 0.5 to 2, and calculate the cohesion c1 and internal friction angle φ1 of the soil and rock under different reduction coefficients according to formula (1) and formula (2), respectively. Step 4: Calculate the compressibility coefficient C of the solid skeleton of the rock and soil using formula (3). s ; Step 5: Calculate the compressibility coefficient C of the soil and rock mass using formula (4); Step 6: Using the solid skeleton compression coefficient of the soil and rock mass, the compression coefficient of the soil and rock mass, the reduction coefficient, the maximum pore water pressure, the internal friction angle and other parameters calculated in Step 4 and Step 5, substitute them into Formula (5) to calculate the pore water pressure increment; Step 7: Use FLAC3D finite element software to build the slope model and set the attachments. Figure 2 At the monitoring point, open the seepage calculation module, assign the pore water pressure increment calculated by formula (5) as the field load to the calculation unit, carry out strength reduction analysis, and calculate the stability of the rock mass slope. In this embodiment, the slope soil layer is composed entirely of moderately weathered sandstone. The longitudinal width of the slope is set to 1m during the calculation process to analyze the two-dimensional slope profile. Based on the actual survey data, the cohesion of the soil and rock mass is 40kPa, the internal friction angle is 22°, the Poisson's ratio is 0.22, and the elastic modulus is 4.09×10⁻⁶. 4 MPa; sandstone porosity 14.4%; groundwater level 15m from bottom of model; FLAC3D software was used for slope strength reduction fluid-structure interaction calculation; 3 monitoring points were set (see attached). Figure 2 The F1 value ranges from 0.5 to 2, and calculations are performed every 0.1. The displacement of the monitoring points obtained from the calculations is used to plot the s-F1 diagram (see Appendix). Figure 3 In this embodiment, when F1 is 1.3, the displacement changes abruptly and increases suddenly, so the safety reduction factor of the slope is 1.3.

[0011] The solutions described in the embodiments are not intended to limit the scope of patent protection of this invention. All equivalent implementations or modifications that do not depart from the scope of this invention are included in the patent scope of this case.

Claims

1. A pore pressure-strength reduction limit analysis method for evaluating the stability of saturated slopes, characterized in that: The relationship between pore water pressure, strength reduction factor, and compressibility factor was established. The calculation of pore pressure increment includes the compressibility factor of the solid skeleton of the soil and rock mass, the compressibility factor of the soil and rock mass, the strength reduction factor, the internal friction angle, and the maximum pore pressure. The pore pressure increment is the change of pore pressure of the soil and rock mass with the strength reduction factor during the finite element strength reduction fluid-structure interaction limit analysis. Includes the following steps: Step 1: Obtain the physical and mechanical parameters of the slope soil and rock mass through indoor and field tests; Step 2: Based on the engineering geological survey and hydrogeological survey, draw the slope profile and the groundwater level of the slope, and calculate the maximum pore water pressure u; Step 3: Based on the cohesion and internal friction angle of the slope soil and rock mass obtained in Step 1, calculate the cohesion and internal friction angle of the soil and rock mass under different reduction coefficients. The calculation formula is as follows; c1 = c / F1 (1) φ1=atan(tanφ / F1)(2) In the formula, c and φ are the cohesion and internal friction angle of the soil and rock mass, F1 is the reduction factor, and c1 and φ1 are the cohesion and internal friction angle of the soil and rock mass after strength reduction; Step 4: If the compressibility coefficient of the solid skeleton of the rock and soil was not obtained in the test in Step 1, the compressibility coefficient C of the solid skeleton of the rock and soil can be calculated by formula (3). s : W s =3(1-2v) / E s (3) In the formula, v is the Poisson's ratio of the solid skeleton of the rock and soil mass, and E s The elastic modulus of the solid skeleton of the rock mass; Step 5: If the compressibility coefficient of the soil and rock mass is not obtained in the test in Step 1, the compressibility coefficient C of the soil and rock mass can be calculated by formula (4): C=0.014404 / (1+55.8721n) 1.42359 (4) In the formula, n is the porosity of the soil and rock mass, which can be obtained from step one; Step 6: Substitute the solid skeleton compression coefficient and the compression coefficient of the soil and rock mass obtained in Step 1, or the solid skeleton compression coefficient and the compression coefficient of the soil and rock mass calculated in Steps 4 and 5, as well as the reduction factor, maximum pore water pressure, and internal friction angle parameter, into formula (5) to calculate the pore water pressure increment: Δu=u(1-C s / C)tanφ(1-1 / F1)(5) In the formula, Δu is the increment of pore water pressure, and C s denoted as the solid skeleton compressibility coefficient of the soil and rock mass, C is the compressibility coefficient of the soil and rock mass, u is the maximum pore water pressure, φ is the internal friction angle of the soil and rock mass, and F1 is the reduction factor. Step 7: Perform strength reduction analysis in finite element software, and assign the pore water pressure increment calculated by formula (5) as the field load to the calculation unit to calculate the stability of the rock mass slope.

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