Unified evaluation method for disturbance degree considering soil stress and structural changes

Abstract

The present invention discloses a unified evaluation method for disturbance degree considering soil stress and structural changes, belonging to the technical field of geotechnical and underground engineering. The method includes calculating the average effective stress of the soil to characterize the soil stress state, as well as the volumetric strain and shear strain to characterize the initial structure of the soil; calculating the structural parameters to characterize the initial structure of the soil; substituting the calculated average effective stress, shear strain, volumetric strain and structural parameters of the soil into the soil disturbance degree evaluation formula to calculate the soil disturbance degree SDD under unloading conditions and loading conditions respectively, so as to evaluate the soil disturbance caused by underground engineering construction. The beneficial effects of the present invention are as follows: comprehensively considering the stress level change and soil structural change caused by underground engineering construction, improving the accuracy of disturbance degree evaluation; being able to accurately predict soil disturbance before construction, which has important guiding significance for actual engineering deformation prediction, reinforcement scheme optimization, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of geotechnical and underground engineering, and particularly relates to a unified evaluation method for disturbance degree considering soil stress and structural changes. Background Art

[0002] Soil disturbance will be caused during underground engineering construction, such as tunnel excavation and foundation pit excavation. After being disturbed, the strength and stiffness characteristics of natural soil will be significantly weakened, and the change of soil structure (including fabric and cementation characteristics) is the main factor leading to this phenomenon. At the same time, construction disturbance will affect the stratum load transfer and deformation, as well as the mechanical response of underground structures near the construction site.

[0003] At present, the evaluation methods for soil disturbance degree include in-situ tests and laboratory tests, mainly through directly measurable soil mechanical parameters, such as cone tip resistance, undrained shear strength, compression index, shear modulus, and structural parameters, etc. However, these mechanical parameters may be affected by additional disturbances during sampling and testing, thus affecting the evaluation accuracy. In addition, the above methods cannot predict the soil disturbance degree before construction, and this prediction is of great significance for evaluating the response of the surrounding stratum and underground structures.

[0004] Therefore, there is an urgent need for a unified evaluation method for disturbance degree considering soil stress and structural changes to solve the above technical problems. Summary of the Invention

[0005] The purpose of the embodiment of the present invention is to provide a unified evaluation method for disturbance degree considering soil stress and structural changes, which is applicable to cohesive soil and non-cohesive soil, and is applicable to unloading (releasing the in-situ soil stress, such as tunnel excavation, foundation pit excavation) and loading (increasing the average effective stress level, such as ground surcharge, grouting with a bladder) conditions; at the same time, it can also be used to predict the soil disturbance caused by construction disturbance, without the need for testing after construction, which has guiding significance for the project, and thus can solve at least one technical problem involved in the background art.

[0006] In order to solve the above technical problems, the present invention is implemented as follows:

[0007] The embodiment of the present invention provides a unified evaluation method for disturbance degree considering soil stress and structural changes, including the following steps:

[0008] Step S1, calculate the average effective stress of the soil to characterize the soil stress state, and the volumetric strain and shear strain to characterize the initial structure of the soil;

[0009] Step S2, calculate the structural parameters to characterize the initial structure of the soil through one-dimensional consolidation tests;

[0010] Step S3: Substitute the calculated average effective stress, shear strain, volumetric strain, and structural parameters of the soil into the soil disturbance degree evaluation formula to calculate the soil disturbance degree SDD under the unloading condition and the loading condition respectively, so as to evaluate the soil disturbance caused by the underground engineering construction, where:

[0011] The soil disturbance degree SDD under the unloading condition is calculated by the following formula:

[0012] ;

[0013] In the formula, represents the average effective stress of the soil after disturbance; represents the average effective stress of the soil before disturbance; represents the structural parameter; represents the generalized shear strain; represents the failure shear strain; represents the volumetric strain; represents the empirical coefficient of the contribution of the volumetric structure to the change of the mechanical properties of the soil;

[0014] The soil disturbance degree SDD under the loading condition is calculated by the following formula:

[0015] .

[0016] Optionally, in step S1, the average effective stress includes the average effective stress of the soil before disturbance and the average effective stress of the soil after disturbance, where:

[0017] The average effective stress is calculated by the following formula:

[0018] ;

[0019] In the formula, , and respectively represent the effective stresses in the three principal stress directions;

[0020] The average effective stress of the soil before disturbance is calculated by the following formula:

[0021] ;

[0022] In the formula, , and respectively represent the effective stresses in the three principal stress directions before disturbance;

[0023] The average effective stress of the soil after disturbance is calculated by the following formula:

[0024] ;

[0025] Wherein, , and respectively represent the effective stresses in the directions of the three principal stresses after disturbance.

[0026] Optionally, in step S1, the volumetric strain is calculated by the following formula:

[0027] ;

[0028] Wherein, represents the unloading and reloading Poisson's ratio; ， and represent the stress increments in the directions of the three principal stresses after disturbance; , and represent the moduli in the directions of the three principal stresses; represents the maximum shear strain; represents the dilation angle.

[0029] Optionally, in step S1, the shear strain is characterized by the normalized shear strain obtained from the generalized shear strain and the failure shear strain, where:

[0030] Generalized shear strain is calculated by the following formula:

[0031] ;

[0032] Wherein, , , represent the strain increments in the directions of the three principal stresses after disturbance;

[0033] The failure shear strain is obtained through a triaxial consolidated undrained compression test.

[0034] Optionally, step S2 specifically includes:

[0035] Step S21, conduct a one-dimensional consolidation test on the soil before disturbance to obtain the e - p curve and e -log p curve of the soil before disturbance. Calculate the structural index from the e - p curve, and calculate the inherent structural index from the e -log p curve, wherein:

[0036] Structural index It is calculated by the following formula:

[0037] ;

[0038] In the formula, represents the vertical effective stress; represents the additional void ratio, that is, a function of the vertical effective stress ; represents the soil yield stress;

[0039] Intrinsic structural index It is calculated by the following formula:

[0040] ;

[0041] In the formula, represents the void ratio of the corresponding ideal remolded soil;

[0042] Step S22, through the intrinsic structural index Normalized structural index to obtain the structural parameter , which is represented by the following formula:

[0043] .

[0044] Optionally, in step S3, for cohesionless soil = 34, for cohesive soil = 31.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] (1) The method provided by the present invention comprehensively considers the stress level change and soil structure change caused by underground engineering construction, and improves the accuracy of disturbance degree evaluation;

[0047] (2) The method provided by the present invention introduces a structural parameter to characterize the initial strength of the soil structure, and uses volumetric strain (including dilatancy) and shear strain to reflect the structural change of the soil after disturbance; the empirical parameters related to the soil type are obtained through back-analysis of cohesionless soil and cohesive soil underground engineering cases , and the applicability of this method is verified by applying this method to actual cases;

[0048] (3) The method provided by the present invention is applicable to cohesionless soils and cohesive soils with different structural states. At the same time, by considering the change of stress level, this method is applicable to the evaluation of disturbance degree under unloading (releasing in-situ soil stress, such as tunnel excavation, foundation pit excavation) and loading (increasing the average effective stress level, such as ground surcharge, grouting in bags) conditions. The evaluation results under different conditions are in good agreement with the measured results based on in-situ tests or experiments.

[0049] (4) The parameters in the method provided by the present invention can be obtained through theoretical methods and numerical simulations for the evaluation of disturbance degree of actual underground engineering. Therefore, this method can accurately predict soil disturbance before construction, which has important guiding significance for the prediction of actual engineering deformation and the optimization of reinforcement schemes. Description of the Drawings

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts, where:

[0051] Figure 1 Schematic diagram of the key parameters for evaluating soil disturbance degree provided by the present invention;

[0052] Figure 2 Schematic diagram of the calculation of structural parameters provided by the present invention;

[0053] Figure 3 Schematic diagram of the calculation of the disturbance degree at the bottom of the foundation pit during excavation in sandy soil stratum provided by the present invention;

[0054] Figure 4 Schematic diagram of the calculation of the disturbance degree of the underlying stratum during tunnel excavation in soft clay stratum provided by the present invention;

[0055] Figure 5 Schematic diagram of the calculation of the disturbance degree of the side soil during grouting in bags in the silt-sand stratum provided by the present invention. Detailed Embodiments

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0057] The terms "first", "second", etc. in the description and claims of the present invention are used to distinguish similar objects, rather than to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present invention can be implemented in an order other than those illustrated or described herein, and the objects distinguished by "first", "second", etc. are generally of the same type, and the number of objects is not limited. For example, the first object can be one or more. In addition, "and / or" in the description and claims means at least one of the connected objects, and the character " / ", generally represents an "or" relationship between the associated objects before and after.

[0058] Please refer to Figure 1 As shown, an embodiment of the present invention provides a unified evaluation method for disturbance degree considering soil stress and structural changes, including the following steps:

[0059] Step S1, calculate the average effective stress of the soil mass used to characterize the soil stress state, and the volumetric strain and shear strain used to characterize the initial structure of the soil mass;

[0060] Step S2, calculate the structural parameters used to characterize the initial structure of the soil mass through one-dimensional consolidation tests;

[0061] Step S3, substitute the calculated average effective stress, shear strain, volumetric strain and structural parameters of the soil mass into the soil disturbance degree evaluation formula, and calculate the soil disturbance degree SDD under the unloading condition and the loading condition respectively, so as to evaluate the soil disturbance caused by underground engineering construction, where:

[0062] The soil disturbance degree SDD under the unloading condition is calculated by the following formula:

[0063] ;

[0064] In the formula, represents the average effective stress of the soil mass after disturbance; represents the average effective stress of the soil mass before disturbance; represents the structural parameter; represents the generalized shear strain; represents the failure shear strain; represents the volumetric strain; represents the empirical coefficient of the contribution of the volumetric structure to the change of the mechanical properties of the soil mass;

[0065] The soil disturbance degree SDD under the loading condition is calculated by the following formula:

[0066] .

[0067] In step S1, the average effective stress including the average effective stress of the soil mass before disturbance and the average effective stress of the soil mass after disturbance , where:

[0068] The average effective stress is calculated by the following formula:

[0069] ;

[0070] In the formula, , and respectively represent the effective stresses in the directions of the three principal stresses, which can be obtained from the measured values of the earth pressure sensors and pore water pressure sensors before and after the in-situ tests or the relevant soil arch theory;

[0071] The average effective stress of the soil mass before disturbance is calculated by the following formula:

[0072] ;

[0073] In the formula, , and respectively represent the effective stresses in the directions of the three principal stresses before disturbance;

[0074] The average effective stress of the soil mass after disturbance is calculated by the following formula:

[0075] ;

[0076] In the formula, , and respectively represent the effective stresses in the directions of the three principal stresses after disturbance.

[0077] The volumetric strain is calculated by the following formula:

[0078] ;

[0079] In the formula, represents the unloading and reloading Poisson's ratio; ， and represent the stress increments in the directions of the three principal stresses after disturbance; , and represent the moduli in the directions of the three principal stresses (unloading and reloading modulus or compression modulus, depending on the unloading or loading conditions); represents the maximum shear strain, ; Denoted as the dilatancy angle, it can be obtained through a triaxial consolidated drained compression test.

[0080] The shear strain is characterized by the normalized shear strain obtained from the generalized shear strain and the failure shear strain, where:

[0081] Generalized shear strain Is calculated by the following formula:

[0082] ;

[0083] In the formula, , , Denote the strain increments in the directions of the three principal stresses after disturbance;

[0084] The failure shear strain is used to evaluate the shear strain state. When the shear strain reaches the failure shear strain, it indicates that the cementation between soil particles has been completely destroyed. The failure shear strain can be obtained through a triaxial consolidated undrained compression test.

[0085] Step S2 specifically includes:

[0086] Step S21, conduct a one-dimensional consolidation test on the soil before disturbance to obtain the e - p Curve and e -log p Curve, specifically as shown in Figure 2 Shown. The structural index is calculated from the e - p Curve, and the inherent structural index is calculated from the e -log p Curve, where:

[0087] Structural index , that is, Figure 2 The area of CDE in, is calculated by the following formula:

[0088] .

[0089] In the formula, Denotes the vertical effective stress; Denotes the additional void ratio, that is, a function of the vertical effective stress ; Denotes the yield stress of the soil;

[0090] It can be seen from the above formula that the structural index Is the integral value of the additional void ratio In the interval [0, .

[0091] Inherent structural index , that is, Figure 2The area of ABCD is calculated by the following formula:

[0092] ;

[0093] In the formula, represents the void ratio of the corresponding ideal remolded soil;

[0094] It can be seen from the above formula that the inherent structural index is e - p the area enclosed by the ideal remolded soil and the horizontal axis on the curve of the

[0095] Step S22, obtaining the structural parameter by normalizing the structural index with the inherent structural index , which is expressed by the following formula:

[0096] .

[0097] In step S3, the cohesionless soil = 34 is obtained by back-analysis of the vertical unloading test of Fujian standard sand, and the cohesive soil = 31 is obtained by back-analysis of the shield tunneling of Shanghai soft clay.

[0098] The following takes a specific example to verify the feasibility of the unified evaluation method of disturbance degree considering soil stress and structural changes provided by the present invention.

[0099] (1) Centrifugal model test for foundation pit excavation in dry sand stratum

[0100] This test was carried out on a ZJU-400 geotechnical centrifuge in a certain university. The effective rotation radius of the centrifuge is 4.5 m, and the centrifugal acceleration is maintained at 60g during the test. The internal size of the rigid model box used in the test is 1000 mm × 400 mm × 1000 mm (length × width × height). The prototype sizes of the foundation pit depth and width are 24 m and 15 m respectively. The soil used in the test is Fujian standard sand with a relative density of 75%. Miniature earth pressure sensors are used to measure the earth pressure in the test, and the measurement accuracy is 1 kPa. Multiple pairs of bending elements are installed to measure the shear wave velocity, and the corresponding shear modulus can be obtained from the measured shear wave velocity.

[0101] Vertical and horizontal earth pressure sensors are buried at 1.2 m, 6 m, and 11 m below the foundation pit bottom to monitor the earth pressure changes during the test; one-dimensional consolidation tests are carried out on the soil before the test to obtain its e - p curve and e -log pFor the curve, its structural parameter can be obtained as 0.067; the reference unloading and reloading modulus of Fujian standard sand is measured as 103 MPa, from which the unloading and reloading moduli at each stress level can be obtained; the volumetric strain and shear strain can be calculated through the calculation formula; according to the back analysis results, for the empirical coefficient of Fujian standard sand take 34, substitute the required parameters into the soil disturbance degree calculation formula proposed by this method, and the corresponding disturbance degree can be obtained. Compare the results obtained by this method with the measured disturbance degree based on the shear modulus and the disturbance degree obtained only considering the change of stress level, as Figure 3 shown, it can be seen that the method provided by the present invention has a good fitting effect with the measured disturbance degree and overcomes the limitation of underestimating soil disturbance only considering the stress level.

[0102] (2) On-site test case of shield tunneling in soft clay stratum

[0103] Combined with Figure 4 as shown, the shield tunnel of a certain subway adopts the staggered joint assembly of precast reinforced concrete segments. The outer diameter of the tunnel lining is 6.2 m and the thickness is 0.35 m. At the monitoring section, the buried depth at the top of the tunnel is 5.5 m. The construction site stratum is mainly divided into four soil layers. The shallow layer is the fill soil with a thickness of 1.9 m, the silt clay layer is from 1.9 m to 10 m, the silty clay layer is from 10 m to 18.5 m, and the silty clay layer is below. The tunnel is mainly located in the silt clay layer and the silty clay layer. Static cone penetration tests (CPT) were carried out on the soil body along the tunnel center line before and after tunneling. According to the cone tip resistance results, the measured strength attenuation rates of the soil body at depths of 13 m and 15.1 m from the ground surface can be obtained. The soil body here is the silty clay layer.

[0104] The average effective stress of the soil body after tunnel excavation is obtained through the numerical simulation of the shield tunnel excavation process. One-dimensional consolidation tests were carried out on the silty clay before the test to obtain its e - p curve and e - log p curve, and its structural parameter can be obtained as 0.304; the reference unloading and reloading modulus of silty clay is measured as 21.1 MPa, from which the unloading and reloading moduli at each stress level can be obtained; the volumetric strain and shear strain can be calculated through the calculation formula; according to the back analysis results, for the empirical coefficient of silty clay take 31, substitute the required parameters into the soil disturbance degree calculation formula, and the disturbance degrees at depths of 13 m and 15.1 m from the ground surface can be obtained. Compare the results obtained by this method with the measured disturbance degree calculated based on the undrained shear strength and the disturbance degree obtained only considering the change of stress level. Specifically, as Figure 4 shown, it can be seen that the method provided by the present invention has a good fitting effect with the measured disturbance degree, and the maximum relative error is 3.5%.

[0105] (3) Case of bag grouting in silt-sand stratum

[0106] The soil used in the test was a remolded silt-sand sample, which was obtained by constant-temperature drying, crushing of soil blocks, grinding of blocks, vacuum pumping and stirring, and consolidation of the soil samples taken from the site. The void ratio e 0 = 0.8, and the natural water content w = 28.7%. The built indoor grouting experimental system can restore the in-situ stratum stress, simulate the actual boundary conditions, and monitor in real time the physical and mechanical properties of the soil such as stress, deformation, and stiffness. The model test profile is as Figure 5 shown (the figure shows the right half of the model test). The model barrel is 600 mm high and 600 mm in inner diameter (radius is 300 mm). The front end of the grouting pipe is evenly provided with grouting holes, and a rubber bag is sleeved outside the pipe. The bag is fixed to the groove on the outer wall by iron wire, and the upper and lower regions can be expanded separately in time segments. The lengths of the upper and lower grouting regions are both 100 mm. The buried position of the grouting pipe is as Figure 5 shown (the top of the grouting region is 100 mm from the pressure plate, and the bottom of the grouting region is 300 mm from the bottom of the model barrel). A servo loading device applies pressure to the pressure plate at the top of the model barrel P v . Before and after the bag grouting, a piezocone penetration test (CPTU) was carried out to obtain the cone tip resistance and excess pore water pressure of the soil, so as to determine the undrained shear strength of the soil.

[0107] The average effective stress of the soil after tunnel excavation was obtained through the buried earth pressure sensors. One-dimensional consolidation tests were carried out on the soil before the test to obtain its e - p curve and e -log p curve, and its structural parameter was found to be 0.213; the compression modulus of the remolded silt-sand soil was measured to be 4.75 MPa; the volumetric strain and shear strain can be calculated through the calculation formula; according to the back analysis results, for the silt-sand empirical coefficient take 31, substitute the required parameters into the soil disturbance degree calculation formula, and the disturbance degree at point A (as Figure 5 shown, the radial distance between point A and the expansion point is 240 mm) was found to be -60.6%. The measured disturbance degree based on the undrained shear strength calculation was -53.0%. It can be seen that the method provided by the present invention has a good fitting effect with the measured disturbance degree, and the relative error is 14.3%.

[0108] It should be noted that in this article, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method, article or device. Without further limitations, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, method, article or device including that element.

[0109] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative rather than restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the purpose of the present invention and the scope protected by the claims, and all of them fall within the protection scope of the present invention.

Claims

1. A unified evaluation method for disturbance degree considering soil stress and structural changes, characterized in that: The steps include: Step S1, calculating the average effective stress of the soil used to characterize the stress state of the soil, and the volume strain and shear strain used to characterize the initial structural properties of the soil; Step S2, calculating structural parameters characterizing the initial structural properties of the soil through a one-dimensional consolidation test; Step S3, substituting the calculated soil average effective stress, shear strain, volume strain and structural parameters into the soil disturbance degree evaluation formula, respectively calculating the soil disturbance degree SDD under unloading conditions and loading conditions, to evaluate the soil disturbance caused by underground engineering construction, wherein: The soil disturbance degree SDD under unloading conditions is calculated by the following formula: ； In the formula, represents the average effective stress of soil after disturbance; represents the average effective stress of soil before disturbance; represents structural parameters; represents the generalized shear strain; represents the failure shear strain; represents body strain; An empirical coefficient that represents the contribution of body structure to the change of soil mechanical properties; The soil disturbance degree SDD under loading conditions is calculated by the following formula: 。 2. The method according to claim 1, characterized in that In step S1, the average effective stress Including the average effective stress of the soil before disturbance and the average effective stress of the soil after disturbance ,in: Mean effective stress Calculated by the following formula: ； In the formula, , and Represent the effective stresses in the three principal stress directions respectively; Average effective stress of soil before disturbance Calculated by the following formula: ； In the formula, , and They represent the effective stresses in the three principal stress directions before the disturbance; Average effective stress of soil after disturbance Calculated by the following formula: ； In the formula, , and They represent the effective stresses in the three principal stress directions after disturbance.

3. The method according to claim 2, characterized in that In step S1, the volume strain Calculated by the following formula: ； In the formula, represents the loading and unloading Poisson’s ratio; ， and represents the stress increments in the three principal stress directions after the disturbance; , and represents the modulus in the three principal stress directions; represents the maximum shear strain; represents the dilatancy angle.

4. The method according to claim 3, characterized in that: In step S1, the shear strain is characterized by the normalized shear strain obtained by the generalized shear strain and the failure shear strain, where: Generalized shear strain Calculated by the following formula: ； In the formula, , , represents the strain increments in the three principal stress directions after disturbance; The failure shear strain is obtained through triaxial consolidated undrained compression tests.

5. The method according to claim 4, characterized in that Step S2 specifically includes: Step S21, a one-dimensional consolidation test is performed on the soil before disturbance to obtain the soil before disturbance. e - p Curve and e -log p The curve, e - p The structural index is calculated by e -log p The inherent structural index is calculated from the curve, where: Structural Index Calculated by the following formula: ； In the formula, represents the vertical effective stress; represents the additional void ratio, i.e., vertical effective stress Function of represents the yield stress of soil; Intrinsic structural index Calculated by the following formula: ； In the formula, represents the void ratio of the corresponding ideal remolded soil; Step S22, using the intrinsic structural index Normalized Structural Index Get structural parameters , expressed by the following formula: 。 6. The method according to claim 5, characterized in that In step S3, there is no clay soil =34, clay soil =31.

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