Method and device for determining collapsibility of large-thickness loess site

Abstract

The invention discloses a method and device for determining collapsibility of a large-thickness loess site, and relates to the technical field of loess detection.The method comprises the steps that collapsibility judgment values of different depths are adopted for conducting collapsibility judgment on soil layer samples of all the depths; taking the soil layer sample with the maximum loess self-weight collapsibility coefficient as a representative sample; determining saturation correction coefficients of the soil layer samples which are judged to be collapsible soil layers by measuring collapsibility coefficients of representative samples under different initial saturations and overburden pressures; preparing a plurality of groups of cutting ring test pieces with different heights by adopting the representative samples, measuring soil sample self-weight collapsibility coefficients of the cutting ring test pieces at different heights, and further determining a size correction coefficient; respectively carrying out a standard collapsibility test and a triaxial compression test on the representative sample to determine a stress correction coefficient; and according to the size correction coefficient, the stress correction coefficient, the collapsible soil layer thickness and the saturation correction coefficient, calculating the self-weight collapsible deformation of the target loess site. According to the invention, the calculation accuracy of the site self-weight collapsibility is improved.

Description

TECHNICAL FIELD

[0001] The application relates to a loess detection technology field, in particular to a method and device for determining the collapsibility of a large-thickness loess site. BACKGROUND

[0002] Loess is a kind of natural soil, accounting for 1 / 10 of the global land area. Special deposition environment and climate conditions have resulted in the characteristics of loess porosity, strong water permeability and metastability. Under the action of water infiltration, the loess structure is easily damaged, causing uneven settlement, collapse, strength degradation and other geotechnical engineering problems. Every year, there are numerous reports of building damage on loess sites due to heavy rain, excessive irrigation or pipeline leakage, resulting in huge economic losses and casualties. Therefore, before engineering construction in loess areas, site collapsibility evaluation must be carried out to identify the thickness of collapsible soil layer and predict the possible site collapsible deformation, so as to provide the necessary basis for the development of subsequent foundation reinforcement, replacement and pre-wetting treatment and other collapsibility elimination schemes.

[0003] Loess site collapsibility evaluation includes site collapsible lower limit depth and self-weight collapsible amount calculation, which is an important basis for determining loess foundation collapsible treatment depth, preliminary treatment scheme and verification. According to the "Building Standard in Collapsible Loess Area" (GB50025-2018), indoor collapsibility test is usually used for calculation, and for major projects, field immersion test is preferably used. Although field immersion test can truly reflect the collapsibility, it is high in cost and requires a lot of manpower and material resources, which makes many engineering projects unable to carry out field detection and can only rely on indoor collapsibility evaluation method.

[0004] With the development of society, the construction of large-thickness collapsible loess sites (loess layer thickness of more than 20m) is increasing. A large number of engineering practices show that the results of indoor test and field test are quite different, the field measured value of site collapsible lower limit depth is 0.11~1.11 times different from the indoor calculated value, and the field measured value of site self-weight collapsible amount is 0.03~3.71 times different from the indoor calculated value.

[0005] The traditional indoor collapsibility evaluation method has obvious differences in stress conditions, immersion conditions and specimen size from the actual field, which makes it difficult to accurately evaluate the site collapsibility and affects the rationality of the selection of subsequent foundation treatment scheme. Therefore, it is urgent to propose a more accurate and reliable collapsibility evaluation method for large-thickness collapsible loess site, to provide more accurate decision basis for engineering design, so as to avoid resource waste or engineering hidden dangers caused by improper treatment. SUMMARY

[0006] The purpose of the present application is to provide a method and device for determining the collapsibility of a large-thickness loess site, which improves the accuracy of site self-weight collapsible amount calculation.

[0007] To achieve the above object, the present application provides the following scheme: in a first aspect, the present application provides a collapsibility determination method for a large-thickness loess site, which comprises: determining geotechnical parameters of soil samples at different depths of a target loess site, the geotechnical parameters including a loess self-weight collapsibility coefficient and an initial saturation degree; the target loess site is a large-thickness loess site; adopting collapsibility discrimination values at different depths, based on the loess self-weight collapsibility coefficients of the soil samples at different depths, judging the collapsibility of the soil samples at different depths, and determining the thickness of the collapsible soil layer of the soil samples determined as collapsible soil layers; selecting the soil sample with the largest loess self-weight collapsibility coefficient among the soil samples determined as collapsible soil layers as a representative sample, and dividing the representative sample into three parts, i.e., a first sub-sample, a second sub-sample and a third sub-sample; changing the initial saturation degree of the first sub-sample, determining the saturation correction coefficient of each soil sample determined as a collapsible soil layer by determining the collapsibility coefficients of the first sub-sample under different initial saturation degrees and overburden pressures; adopting the second sub-sample to prepare multiple groups of ring cutter test pieces with different heights, determining the self-weight collapsibility coefficients of the soil samples of the ring cutter test pieces at different heights by collapsibility test, and determining the size correction coefficient according to the self-weight collapsibility coefficients of the soil samples of the ring cutter test pieces at different heights; respectively carrying out standard collapsibility test and triaxial compression test on the third sub-sample to determine the stress correction coefficient; calculating the self-weight collapsibility deformation amount of the target loess site according to the size correction coefficient, the stress correction coefficient, the thickness of the collapsible soil layer of the soil samples determined as collapsible soil layers and the saturation correction coefficient.

[0008] In a second aspect, the present application provides a collapsibility determination device for a large-thickness loess site, which comprises: a geotechnical parameter determination module for determining geotechnical parameters of soil samples at different depths of a target loess site, the geotechnical parameters including a loess self-weight collapsibility coefficient and an initial saturation degree; the target loess site is a large-thickness loess site; a collapsibility discrimination module for adopting collapsibility discrimination values at different depths, based on the loess self-weight collapsibility coefficients of the soil samples at different depths, judging the collapsibility of the soil samples at different depths, and determining the thickness of the collapsible soil layer of the soil samples determined as collapsible soil layers; a representative sample selection module for selecting the soil sample with the largest loess self-weight collapsibility coefficient among the soil samples determined as collapsible soil layers as a representative sample, and dividing the representative sample into three parts, i.e., a first sub-sample, a second sub-sample and a third sub-sample; The saturation correction coefficient determination module is configured to change the initial saturation of the first sub-sample, determine the saturation correction coefficient of each soil layer sample determined as the collapsible soil layer by measuring the collapsibility coefficients of the first sub-sample under different initial saturations and overburden pressures. The size correction coefficient determination module is configured to prepare multiple groups of ring knife test pieces with different heights by using the second sub-sample, determine the self-weight collapsibility coefficients of the soil sample of the ring knife test piece under different heights by means of the collapsibility test, and determine the size correction coefficient according to the self-weight collapsibility coefficients of the soil sample of the ring knife test piece under different heights. The stress correction coefficient determination module is configured to respectively carry out the standard collapsibility test and the triaxial compression test on the third sub-sample, and determine the stress correction coefficient. The target loess site self-weight collapsibility deformation amount calculation module is configured to calculate the target loess site self-weight collapsibility deformation amount according to the size correction coefficient, the stress correction coefficient, the collapsible soil layer thickness of the soil layer sample determined as the collapsible soil layer, and the saturation correction coefficient.

[0009] According to the specific embodiments provided in the present application, the following technical effects are disclosed: The present application provides a large-thickness loess site collapsibility determination method and device, which uses different depth collapsibility discrimination values to determine the collapsibility of each depth soil layer sample, considers the influence of depth on the collapsibility discrimination value, and improves the collapsibility determination accuracy. Meanwhile, the saturation correction coefficient of each soil layer sample determined as the collapsible soil layer is determined by measuring the collapsibility coefficients of the first sub-sample under different initial saturations and overburden pressures; the size correction coefficient is determined according to the self-weight collapsibility coefficients of the soil sample of the ring knife test piece under different heights by means of the collapsibility test; the stress correction coefficient is determined by respectively carrying out the standard collapsibility test and the triaxial compression test on the third sub-sample, so as to consider the stress conditions, immersion conditions and test piece size differences of indoor and outdoor tests, reduce the potential risks caused by the distortion of site evaluation results, improve the accuracy of indoor evaluation results, and improve the accuracy of site self-weight collapsibility calculation, so as to more accurately and reliably determine the site collapsibility lower limit depth and collapsibility deformation, more accurately reflect the actual site collapsibility, and provide accurate and reliable decision basis for subsequent site collapsibility treatment. BRIEF DESCRIPTION OF DRAWINGS

[0010] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0011] Figure 1A flowchart of a method for determining collapsibility of a large-thickness loess site according to an embodiment of the present application is shown.

[0012] Figure 2 A diagram showing the depth distribution of the saturation degree of the ground soil according to an embodiment of the present application is shown.

[0013] Figure 3 A diagram showing the frequency distribution of the saturation degree of the ground soil according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0014] The technical solutions in the embodiments of the present application will be apparently and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0015] The above-mentioned purposes, features and advantages of the present application can be more apparent and easy to understand. The present application will be further described in detail below with reference to the drawings and specific embodiments.

[0016] Through analysis, the differences in the immersion conditions, stress conditions and soil sample sizes between the indoor collapsibility test and the field immersion test are the main reasons for the distortion of the indoor collapsibility evaluation method.

[0017] (1) Difference in immersion conditions. The field immersion test adopts a top-down immersion mode, and the water infiltrates along the vertical joints of the loess in one direction under the action of gravity. Due to the limitation of the water-holding capacity of the soil body, the saturation degree of the ground soil after full immersion is about 80.3%, which is difficult to reach complete saturation. In the indoor collapsibility test, the loess test piece is completely immersed in the water tank, and the water infiltrates from the bottom and the top in two directions to ensure the full saturation of the test piece, and the saturation degree can reach at least 90%. This means that the indoor test overestimates the maximum saturation degree that the soil body can reach in the natural state, which may lead to the deviation of the evaluation result of the site collapsibility from the actual situation.

[0018] (2) Difference in sample size. The loess structure is loose and porous, and has obvious vertical joints, internal structural defects such as pores, cracks and calcareous nuclei. During the collapsibility deformation of the loess ground, these structural defects are smaller than the overall size of the ground and have limited influence on the collapsibility deformation of the ground. In the indoor collapsibility test, the diameter of the test piece is only 61.8mm, and the height is only 20mm, which will undoubtedly magnify the structural defects inside the soil sample, leading to the deviation of the evaluation result of the collapsibility deformation of the soil layer from the actual situation. In addition, the boundary effect of the small-size test piece is significant, and the internal stress distribution of the soil sample is uneven during the test, and the local stress concentration phenomenon is more prominent, which aggravates the collapsibility deformation of the soil sample.

[0019] (3) Stress condition difference analysis. The indoor collapsibility test is usually carried out under the condition of single-axis lateral restraint. During the test, the lateral deformation of the loess is strictly constrained, and the shear slip and relative displacement between soil particles are limited, so that the structural stability and deformation resistance of the loess are effectively enhanced. However, under actual engineering conditions, the deformation of the soil layer is not limited by the lateral deformation, and the collapsibility deformation of the soil is often accompanied by significant lateral and vertical deformation. Therefore, there is a large difference between the indoor collapsibility test results and the in-situ measured results.

[0020] In one exemplary embodiment, the present application provides a method for determining the collapsibility of a large-thickness loess site, as shown in Figure 1 The method for determining the collapsibility of a large-thickness loess site includes steps 101-107.

[0021] Step 101: Determine the geotechnical parameters of the soil samples at different depths of the target loess site, including the loess self-weight collapsibility coefficient and the initial saturation degree; the target loess site is a large-thickness loess site.

[0022] Step 102: Use the collapsibility discrimination values at different depths to determine the collapsibility of the soil samples at each depth based on the loess self-weight collapsibility coefficient of the soil samples at each depth, and determine the thickness of the collapsibility soil layer of the soil samples determined as collapsibility soil layers.

[0023] Step 103: Select the soil sample with the largest loess self-weight collapsibility coefficient among the soil samples determined as collapsibility soil layers as the representative sample, and divide the representative sample into three parts: a first sub-sample, a second sub-sample, and a third sub-sample.

[0024] Step 104: Change the initial saturation degree of the first sub-sample, and determine the saturation correction coefficient of each soil sample determined as collapsibility soil layers by measuring the collapsibility coefficient of the first sub-sample under different initial saturation degrees and overburden pressures.

[0025] Step 105: Use the second sub-sample to prepare multiple groups of ring cutter test pieces with different heights, measure the self-weight collapsibility coefficient of the soil samples in the ring cutter test pieces under different heights by collapsibility test, and determine the size correction coefficient according to the self-weight collapsibility coefficient of the soil samples in the ring cutter test pieces under different heights.

[0026] Step 106: Perform standard collapsibility test and triaxial compression test on the third sub-sample respectively, and determine the stress correction coefficient.

[0027] Step 107: Calculate the self-weight collapsibility deformation of the target loess site according to the size correction coefficient, the stress correction coefficient, the thickness of the collapsibility soil layer of the soil samples determined as collapsibility soil layers, and the saturation correction coefficient.

[0028] The loess site targeted by the present application is a large-thickness collapsible loess site. The large-thickness collapsible loess site is a site with a loess layer thickness of more than 20 m.

[0029] The present application discloses a collapsibility evaluation method for a large-thickness collapsible loess site, and relates to the field of loess site investigation and design. The method establishes an indoor determination standard for soil layer collapsibility and proposes a site collapsibility lower limit depth determination method by studying the influence of indoor and outdoor test condition differences on loess collapsibility. Meanwhile, a soil layer collapsibility deformation correction coefficient is proposed, a soil layer collapsibility deformation correction coefficient calibration method is determined, and a site collapsibility deformation calculation method is improved. Finally, a set of loess site collapsibility evaluation method considering indoor and outdoor test condition differences is formed, which effectively improves the accuracy of indoor evaluation results, reduces potential risks caused by site evaluation result distortion, avoids high-cost, long-period and heavy-labor site evaluation tests, and realizes resource conservation and efficiency improvement while ensuring engineering safety.

[0030] The present example is proposed based on the construction of a certain express railway in the Guanzhong region of Shaanxi Province, loess samples along the line are collected, the influence of indoor and outdoor condition differences on loess collapsibility is analyzed, soil layer collapsibility determination standards at different depths and site collapsibility deformation calculation methods are proposed, and site collapsibility evaluation can be more accurately performed, thereby providing a basis for engineering construction in loess regions.

[0031] (1) The soil layer collapsibility determination standards at different depths are proposed. The present example carries out triaxial collapsibility tests to simulate the stress of loess under real conditions, and compares the test results with standard collapsibility test results, thereby studying the influence of stress conditions (triaxial stress state / lateral stress state) and axial stress size (depth) on loess collapsibility, and the results are shown in Table 1.

[0032] Table 1 Comparison of loess collapsibility coefficients under different stress conditions

[0033] Based on the test results, the relationship between the pressure and the collapsibility coefficient ratio is constructed as follows: (1) wherein, δ s C is the loess collapsibility coefficient under triaxial collapsibility test; δ s C is the loess collapsibility coefficient under standard collapsibility test; P P is the axial pressure.

[0034] For a long time, the academic consensus has been that loess is collapsible when the collapsibility coefficient is greater than 0.015. Considering that the stress state of the triaxial collapsibility test is more consistent with the actual soil layer, the triaxial collapsibility test should be used when judging the collapsibility of loess. However, the triaxial collapsibility test has problems such as complicated operation, long cycle and high equipment requirements. In engineering practice, the standard collapsibility test under uniaxial lateral confinement conditions is generally used. In order to facilitate practical application and promotion, the collapsibility judgment index proposed adopts the results of the standard collapsibility test. Therefore, according to formula (1), the collapsibility judgment index under triaxial stress conditions ( δ s The value of '=0.015' is converted to the collapsibility criterion applicable to the standard collapsibility test, as shown in Table 2.

[0035] Table 2 Criteria for determining soil collapsibility at different depths

[0036] (2) The method for calculating site collapse deformation is proposed.

[0037] ① The saturation correction coefficient was proposed: the results of field immersion tests at different sites were statistically analyzed, see... Figure 2 and Figure 3 .

[0038] Under real-world conditions, water infiltrates unidirectionally from top to bottom along the vertical joints of the loess. Due to the soil's water-holding capacity, the saturation of the foundation soil after full immersion is approximately 80.3%, making complete saturation difficult. However, in the indoor collapsibility test, the loess specimens were completely immersed in the water tank, with water infiltrating bidirectionally from the bottom and top, ensuring full saturation of the specimens, reaching at least 90%. Therefore, the effects of soil saturation increment and overburden pressure on the soil collapsibility coefficient were studied, and the results are shown in Table 3.

[0039] Table 3. Loess collapsibility coefficient under different saturation increments and overlying pressures

[0040] A functional model relating saturation increment and collapsibility coefficient was constructed based on the secant modulus method. (2) in, δ s The collapsibility coefficient of loess under the standard collapsibility test; The percentage increase in loess saturation before and after immersion in water is %. P The value is axial pressure, expressed in kPa.

[0041] Depend on Figure 2 and Figure 3It can be seen that the saturation of loess foundation soil after full immersion is about 80.3%, while the saturation of indoor ring shear specimen after full immersion can reach more than 90%. The difference between the two is 9.7%. Substituting this into equation (2), the difference between the loess collapse coefficient under indoor test and actual immersion conditions is Δδ s : (3) The saturation correction coefficient is introduced β sat to reflect the influence of the difference between indoor and outdoor immersion conditions on the collapse coefficient: (4) In the formula: δ s is the loess collapse coefficient under standard test; δ s,80.3% is the loess collapse coefficient under actual immersion conditions (saturation after immersion is 80.3%).

[0042] To weaken the influence of immersion difference on the collapse coefficient, the saturation of indoor ring shear specimen after full immersion is assumed to be 90%. Substituting this into equation (4) gives the loess saturation correction coefficient under different initial saturations. Through regression analysis, there is an approximate relationship between the initial saturation of loess and the saturation correction coefficient: (5) is the initial saturation of loess.

[0043] According to equation (5), the soil layer saturation correction coefficient under different saturation intervals is determined, as shown in Table 4.

[0044] Table 4 Value table of soil layer saturation correction coefficient

[0045] ② Introduction of size correction coefficient.

[0046] Small size samples (φ61.8mm×h20mm) can amplify the internal structural defects of the soil sample and have a certain influence on the test results. Therefore, the influence of specimen size on the loess collapse coefficient is studied. The specimen diameter φ is uniform at 61.8mm, and the height h is 20mm, 40mm, 60mm and 80mm. The results are shown in Table 5.

[0047] Table 5 Loess self-weight collapse coefficient under different specimen heights

[0048] The collapse coefficient measured by the standard indoor collapse test specimen (h=20mm) is taken as the normalized reference value δ s2 , and the normalized collapse coefficient δsh / δ s2 The relationship between the test piece height h and the relationship: (6) In the formula: δ s2 is the standard collapsibility coefficient of loess under the standard collapsibility test; δ sh is the collapsibility coefficient of loess when the test piece height h is h; h is the test piece height.

[0049] As can be seen from formula (6), as the test piece height increases, the normalized self-weight collapsibility coefficient gradually converges to 0.20. When the test piece height tends to infinity, the influence of internal structural defects of the soil sample on the collapse is weakened, the stress distribution during the collapse of the soil sample gradually approaches the actual engineering condition, and the test result tends to be more true value.

[0050] Therefore, it is recommended to introduce a size effect coefficient β size = 0.20 when calculating the collapsibility deformation of the soil layer to eliminate the interference of the test boundary condition and the internal structural defects of the test piece on the evaluation result of the collapsibility deformation.

[0051] ③ The stress correction coefficient is proposed.

[0052] Different depth soil samples were collected, and the difference between the standard collapsibility test and the triaxial collapsibility test under self-weight stress condition was compared, as shown in Table 6.

[0053] Table 6 Loess self-weight collapsibility coefficient under different stress conditions

[0054] As can be seen from Table 5, there is a relatively obvious linear relationship between the test results of the standard collapsibility test and the triaxial collapsibility test: (7) It can be seen that the intercept of the regression equation is small, and the self-weight collapsibility coefficient measured under triaxial stress condition is approximately 1.81 times that under uniaxial lateral restraint condition, which is basically consistent with the existing research results. According to formula (7), it is recommended to introduce a stress condition correction coefficient β str = δ zs ’ / δ zs ≈1.81.

[0055] In an exemplary embodiment, step 101 specifically comprises: drilling and sampling per meter to obtain soil samples at different depths in the target loess site.

[0056] According to the "Standard for Soil Test Methods" (GBT 50123-2019), undisturbed loess is collected, and the soil parameters of the soil samples at different depths are determined.

[0057] In an exemplary embodiment, step 102 specifically includes: determining the collapsibility of the soil layer according to the self-weight collapsibility coefficient of loess at different depths, and determining the thickness of the collapsible soil layer (i.e., the lower limit of collapsibility). More specifically, due to the difference in soil distribution, collapsible soil layers and non-collapsible soil layers may be distributed in a staggered manner. The collapsibility of the soil layer is determined according to the dominant type of soil layer in the staggered range. The 2-3m (non-) collapsible soil layer is ignored.

[0058] When the depth range is [0, 5), the collapsibility discrimination value is 0.015; when the depth range is [5, 10), the collapsibility discrimination value is 0.016; when the depth range is [10, 15), the collapsibility discrimination value is 0.017; when the depth range is [15, 20), the collapsibility discrimination value is 0.022; when the depth range is [20, 25), the collapsibility discrimination value is 0.032; when the depth range is [25, 30), the collapsibility discrimination value is 0.047; when the depth range is [30, 35), the collapsibility discrimination value is 0.058; when the depth range is [35, 40), the collapsibility discrimination value is 0.061; and when the depth range is [40, +∞), the collapsibility discrimination value is 0.062. In each depth range, if the self-weight collapsibility coefficient of loess is greater than the collapsibility discrimination value, the corresponding soil sample is determined to be a collapsible soil layer. The range of each depth range described above is meters.

[0059] In an exemplary embodiment, step 103, the loess at the depth of the maximum self-weight collapsibility coefficient within the lower limit of collapsibility is selected as the representative sample for the correction coefficient calibration. The cutting ring sample is prepared, and the maximum allowable difference of the soil sample density is required to be ±0.03g / cm 3 If the requirement is not met, the sample is re-prepared, or the surface pits and grooves of the test piece are filled according to the specification.

[0060] The existing indoor site collapsibility evaluation method does not consider the stress condition difference between indoor and outdoor tests when determining the lower limit of site collapsibility, resulting in a large error between the lower limit of site collapsibility and the actual measurement result. Therefore, considering the influence of the depth and stress condition difference of the soil sample on the collapsibility coefficient of the soil sample, a collapsibility determination standard for soil layers at different depths is proposed, and the collapsibility discrimination values at different depths are shown in Table 1, which provides a theoretical basis for determining the lower limit of site collapsibility.

[0061] In an exemplary embodiment, step 104 specifically includes: changing the initial saturation of the first sub-sample, and constructing the relationship between the saturation increment, the overburden stress, and the collapsibility coefficient by determining the collapsibility coefficient of the first sub-sample under different initial saturations and overburden pressures.

[0062] Based on the relationship between saturation increment, overlying stress, and collapsibility coefficient, a relationship between the saturation correction coefficient and the initial saturation is constructed. Based on this relationship, the saturation correction coefficient for each soil sample identified as collapsible is determined.

[0063] Specifically, changing the initial saturation of the first sub-sample includes: preparing the first sub-sample into a standard ring cutter specimen, and using a spray humidification method to perform 3-5 stages of humidification on the standard ring cutter specimen (each stage increasing the saturation by 3-5%), thereby changing the initial saturation of the ring cutter specimen. S r Water required for each stage △m Calculate according to formula (8): (8) In the formula: △ m The amount of water added is in grams; S r,step For humidification intervals, % δ w The density of water, in g / cm³ 3 ; V The volume of a standard ring cutter is in cm. 3 ; e The void ratio of the soil sample.

[0064] In an exemplary embodiment, the relationship between the saturation correction coefficient and the initial saturation is constructed based on the relationship between the saturation increment, overburden stress, and collapsibility coefficient. Specifically, this includes determining different initial saturations through collapsibility tests. S r and overlying stress P Collapse coefficient of soil sample δ s The recommended pressure level is 3-5, and the increase in soil saturation Δ before and after the indoor test should be recorded. S r Establish saturation increment Δ S r Overburden stress P With the collapsibility coefficient δ s The relationship between them.

[0065] The relationship between saturation increment, overlying stress, and collapsibility coefficient is expressed as: (9) in, This is the collapsibility coefficient; The percentage increase in soil saturation before and after the indoor collapsibility test is %. The overburden stress is expressed in kPa. , and are regression coefficients, which are calibrated by experiments if engineering conditions permit; or recommended constant values are used if engineering conditions are limited.

[0066] then different initial saturations S r The saturation correction coefficient of lower loess β sat (10) In the formula, δ zs,field is the self-weight collapsibility coefficient of soil under field water immersion conditions; S r,field is the maximum saturation degree that the soil can reach under field water immersion conditions, and is taken as 80% when there is a lack of measured data; S r is the initial saturation degree of the soil sample; P z is the overburden stress of the soil sample, kPa; δ zs is the self-weight collapsibility coefficient of soil under standard collapse test.

[0067] Thus, the relationship between the saturation correction coefficient and the initial saturation degree is established as: (11) Considering that the regression equation has a negative value, the saturation correction coefficient is taken as: (12) wherein, is the saturation correction coefficient, is the initial saturation degree, , and are regression coefficients, which are calibrated by experiments if engineering conditions permit; or recommended constant values are used if engineering conditions are limited.

[0068] In an exemplary embodiment, step 105 specifically comprises: using the first sub-sample to prepare 4 groups of cutting ring test pieces with different heights, and the heights of the 4 groups of cutting ring test pieces are 20 mm, 40 mm, 60 mm and 80 mm respectively. The self-weight collapsibility coefficient of the soil sample of the cutting ring test piece with a height of 20 mm is taken as a normalized reference value. Based on the normalized reference value, a relationship between the normalized self-weight collapsibility coefficient and the height of the cutting ring test piece is constructed. According to the relationship between the normalized self-weight collapsibility coefficient and the height of the cutting ring test piece, a size correction coefficient is determined.

[0069] ​In one exemplary embodiment, the relationship between the normalized self-weight collapse coefficient and the height of the cutting ring specimen is expressed as: (13) wherein, is the normalized self-weight collapse coefficient, is the self-weight collapse coefficient of the soil sample of the cutting ring specimen, is the normalized reference value, is the size correction coefficient, is the height of the cutting ring specimen, is the regression coefficient.

[0070] In one exemplary embodiment, step 106 specifically comprises: according to the principle of the "double-line method" collapse test, respectively carrying out the standard collapse test and the triaxial compression test on the third sub-sample to obtain the self-weight collapse coefficient of the soil body under the standard collapse test and the self-weight collapse coefficient of the soil body under the triaxial compression test.

[0071] The triaxial compression test suggests using the consolidation undrained method (CU), and the principal stress ratio K 0 of the test can be determined by the static lateral pressure coefficient test or the Jaky formula: (14) wherein, is the internal friction angle of the soil sample.

[0072] The stress correction coefficient is calculated according to formula (15).

[0073] (15) wherein, is the stress correction coefficient, is the self-weight collapse coefficient of the soil body under the triaxial compression test, is the self-weight collapse coefficient of the soil body under the standard collapse test.

[0074] The existing indoor site collapsibility evaluation method does not consider the stress conditions, soaking conditions and specimen size differences between indoor and outdoor tests when determining the site collapsibility deformation, resulting in a large error between the site collapsibility deformation and the measured results. Therefore, considering the influence of the above differences on the collapsibility of the soil layer, the corresponding soil layer collapsibility deformation correction coefficient is proposed, and the site collapsibility deformation calculation method is improved.

[0075] In one exemplary embodiment, the formula for calculating the self-weight collapsibility deformation amount of the target loess site in step 107 is: (16) wherein, is the self-weight collapsibility deformation amount of the target loess site, The non-collapsible soil layer is not accumulated to the collapsible lower limit depth, and the unit is m. n is the number of soil samples determined as collapsible soil layers; is the thickness of the i-th layer of collapsible soil, and the unit is m.

[0076] is the soil correction coefficient number, which is taken from Table 6 according to the “Code for Building in Collapsible Loess Regions” (GB 50025-2018). is the size correction coefficient, and the recommended value 0.20 is taken when the test conditions are lacking. is the stress correction coefficient, and the recommended value 1.81 is taken when the test conditions are lacking. is the saturation correction coefficient of the i-th layer of collapsible soil, and the recommended value is taken when the test conditions are lacking, as shown in Table 7. is the soil self-weight collapsibility coefficient of the i-th layer of collapsible soil.

[0077] Table 7 Soil Correction Coefficient

[0078] Example 1 The DK125+000~135+000 section of the West Korea Intercity Railway control project carried out collapsibility evaluation of large thickness collapsible loess site, and selected DK130 (Yuexing Village, Pucheng, Shaanxi) as a representative site for indoor and outdoor site collapsibility evaluation to verify the reliability of the method.

[0079] Step 1: Drill every meter in the loess site, collect undisturbed loess according to “Standard for Soil Test Methods” (GB / T 50123-2019), and measure soil parameters such as loess self-weight collapsibility coefficient, saturation, and density at different depths. The results are shown in Table 8.

[0080] Table 8 Site Loess Soil Parameters

[0081] Step 2: Determine the collapsibility of the soil layer according to the loess self-weight collapsibility coefficient at different depths, and determine the thickness of the collapsible soil layer (i.e. the collapsible lower limit depth). The results are shown in Table 9.

[0082] Table 9 Site Collapsible Soil Layer Thickness Determination

[0083] Step 3: The loess at a depth of 12m is the most collapsible, and is selected as a representative soil sample for correction coefficient calibration. Prepare a cutting tool sample, and the maximum allowable difference in soil sample density should be ±0.03g / cm 3 .

[0084] Step 4: The standard cutter samples (φ61.8mm x h20mm) were prepared by 4-stage humidification using spray humidification method, and the soil samples with 53.1%, 58.1% and 63.1% saturation were prepared. The water required for each stage is △ m : (17) Step 5: The saturation increment △ S r and overburden stress P The collapsibility of the soil sample under different saturation increments and overburden stress δ s , as shown in Table 10.

[0085] Table 10 The collapsibility of the soil sample under different saturation increments and overburden stress

[0086] Saturation increment △ S r , overburden stress P and collapsibility δ s , and the relationship is: (18) The maximum saturation of the soil under the field water immersion condition is 80%, and the saturation correction coefficient of the loess under different initial saturation S r is calculated according to formula (18) and (19). β sat The results are shown in Table 11.

[0087] (19) In the formula: δ zs,field —The self-weight collapsibility of the soil under the field water immersion condition; S r,field —The maximum saturation of the soil under the field water immersion condition, %; S r —The initial saturation of the soil sample, %; P z —The overburden stress of the soil sample, kPa; δ zs—— The self-weight collapsibility of the soil under the standard collapsibility test; Table 11 The saturation correction coefficient of the soil sample under different initial saturation

[0088] The relationship between the saturation correction coefficient and the initial saturation is established as: (20) The value mode of the saturation correction coefficient for calculating the site collapsible deformation is: (21) Step 6: Self-weight collapsible coefficient of soil samples under different specimen heights δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ z The results are shown in Table 12.

[0089] Table 12 Self-weight collapsible coefficient of soil samples under different specimen heights

[0090] The normalized self-weight collapsible coefficient δ zsh / δ zs2 The relationship between and the specimen height h is: (22) Therefore, the size correction coefficient β size is 0.21.

[0091] Step 7: The internal friction angle of the site loess is about 31.1°, and the principal stress ratio of the triaxial compression test The self-weight collapsible coefficient of the soil measured by the triaxial compression test is 0.070.

[0092] Therefore, the stress correction coefficient .

[0093] Step 8: Calculate the site self-weight collapsible deformation amount according to formula (16), and the results are shown in Table 13.

[0094] Table 13 Calculation results of site self-weight collapsible deformation amount (method of the present application)

[0095] Comparison of reliability of evaluation results: According to the results of the site test pit immersion test, the measured site collapsible lower limit depth is 15 m, the predicted depth by the method of the present application is 13 m; the measured site self-weight collapsible amount is 0.0460 m, and the predicted result by the method of the present application is 0.0495 m. It can be seen that the site collapsible lower limit depth and the site self-weight collapsible amount determined by the method of the present application are consistent with the site measured results.

[0096] The site collapsible lower limit depth determined by the calculation method of the “Code for Building in Collapsible Loess Area” (GB 50025-2018) is 23 m, the site self-weight collapsible amount is 0.2592 m, and the site collapsible evaluation result has a large error, and the calculation process is shown in Table 14.

[0097] Table 14 Calculation results of site self-weight collapsible deformation amount (standard method)

[0098] Example 2 The self-weight collapsible deformation amount of the DK246+500 site of Zhengzhou-Xi'an Railway was determined, and the results are shown in Table 15.

[0099] Table 15: Calculation results of self-weight collapsible deformation amount of site (method of the present application)

[0100] Comparison of reliability of evaluation results: According to the results of the immersion test of the test pit on site, the measured collapsible lower limit depth of the site is 17 m, the predicted depth by the method of the present application is 21 m, the measured self-weight collapsible amount of the site is 0.1720 m, and the predicted result by the method of the present application is 0.1327 m. It can be seen that the collapsible lower limit depth and the self-weight collapsible amount of the site determined by the method of the present application are consistent with the measured results on site. The collapsible lower limit depth of the site determined by the calculation method of the “Code for Building in Collapsible Loess Area” (GB50025-2018) is 26 m, the self-weight collapsible amount of the site is 0.5868 m, and the evaluation result of the collapsibility of the site has a large error, and the calculation process is shown in Table 16.

[0101] Table 16: Calculation results of self-weight collapsible deformation amount of site (standard method)

[0102] Example 3 The self-weight collapsible deformation amount of the site of a project in Weiyu Township, Huayin, Shaanxi was determined, and the results are shown in Table 17.

[0103] Table 17: Calculation results of self-weight collapsible deformation amount of site (method of the present application)

[0104] Comparison of reliability of evaluation results: According to the results of the immersion test of the test pit on site, the measured collapsible lower limit depth of the site is 22 m, the predicted depth by the method of the present application is 21 m, the measured self-weight collapsible amount of the site is 0.1763 m, and the predicted result by the method of the present application is 0.1631 m. It can be seen that the self-weight collapsible amount of the site determined by the method of the present application is consistent with the measured results on site.

[0105] The collapsible lower limit depth of the site determined by the calculation method of the “Code for Building in Collapsible Loess Area” (GB50025-2018) is 21 m, the self-weight collapsible amount of the site is 0.5391 m, and the evaluation result of the collapsibility of the site has a large error, and the calculation process is shown in Table 18.

[0106] Table 18: Calculation results of self-weight collapsible deformation amount of site (standard method)

[0107] Example 4 The self-weight collapsible deformation amount of the site of a project in Shenheya, Xi'an, Shaanxi was determined, and the results are shown in Table 19.

[0108] Table 19 Calculation results of self-weight collapsible deformation of site (method of the application)

[0109] Reliability comparison of evaluation results: According to the results of the site immersion test, the measured site collapsible lower limit depth is 18 m, the predicted depth by the method of the application is 18 m; the measured site self-weight collapsible amount is 0.0409 m, and the predicted result by the method of the application is 0.0486 m. It can be seen that the site self-weight collapsible amount determined by the method of the application is consistent with the site measured results.

[0110] The site collapsible lower limit depth determined by the calculation method of the “Code for Building in Collapsible Loess Area” (GB50025-2018) is 18 m, the site self-weight collapsible amount is 0.1755 m, and the site collapsibility evaluation result has a large error, and the calculation process is shown in Table 20.

[0111] Table 20 Calculation results of self-weight collapsible deformation of site (standard method)

[0112] Example 5 A project in He Ping Town, Yuzhong, Lanzhou, the site self-weight collapsible deformation results are shown in Table 21.

[0113] Table 21 Calculation results of self-weight collapsible deformation of site (method of the application)

[0114] Reliability comparison of evaluation results: According to the results of the site immersion test, the measured site collapsible lower limit depth is 24 m, the predicted depth by the method of the application is 24 m; the measured site self-weight collapsible amount is 0.6560 m, and the predicted result by the method of the application is 0.4894 m. It can be seen that the site self-weight collapsible amount determined by the method of the application is consistent with the site measured results.

[0115] The site collapsible lower limit depth determined by the calculation method of the “Code for Building in Collapsible Loess Area” (GB50025-2018) is 29 m, the site self-weight collapsible amount is 1.6470 m, and the site collapsibility evaluation result has a large error, and the calculation process is shown in Table 22.

[0116] Table 22 Calculation results of self-weight collapsible deformation of site (standard method)

[0117] The engineering example shows that: the maximum error of the site collapsible lower limit depth obtained by the method is 4m, and the maximum error of the existing indoor site collapsibility evaluation method is 9m. The error of the site collapsible deformation obtained by the method is between-25% and 19%, and the error of the existing site collapsibility evaluation result is-7% to 463% times of the site measured value, which has obvious error.

[0118] Based on the same inventive concept, the embodiment of the present application also provides a large-thickness loess site collapsibility determination device for implementing the large-thickness loess site collapsibility determination method.

[0119] The present application provides a large-thickness loess site collapsibility determination device, which comprises a geotechnical parameter determination module for determining the geotechnical parameters of soil samples at different depths of a target loess site, wherein the geotechnical parameters include loess self-weight collapsibility coefficient and initial saturation.

[0120] The collapsibility discrimination module is used to determine the collapsibility of soil samples at different depths based on the loess self-weight collapsibility coefficient of each depth soil sample, and determine the collapsible soil layer thickness of the soil sample determined as collapsible soil layer.

[0121] The representative sample selection module is used to select the soil sample with the largest loess self-weight collapsibility coefficient in the soil sample determined as collapsible soil layer as the representative sample, and divide the representative sample into three parts: first sub-sample, second sub-sample and third sub-sample.

[0122] The saturation correction coefficient determination module is used to change the initial saturation of the first sub-sample, and determine the saturation correction coefficient of each soil sample determined as collapsible soil layer by determining the collapsibility coefficient of the first sub-sample under different initial saturations and overburden pressure.

[0123] The size correction coefficient determination module is used to prepare multiple groups of ring knife test pieces with different heights using the second sub-sample, determine the soil sample self-weight collapsibility coefficient of the ring knife test piece under different heights by collapsibility test, and determine the size correction coefficient according to the soil sample self-weight collapsibility coefficient of the ring knife test piece under different heights.

[0124] The stress correction coefficient determination module is used to carry out standard collapsibility test and triaxial compression test on the third sub-sample respectively, and determine the stress correction coefficient.

[0125] The target loess site self-weight collapsible deformation calculation module is used to calculate the target loess site self-weight collapsible deformation according to the size correction coefficient, the stress correction coefficient, the collapsible soil layer thickness of the soil sample determined as collapsible soil layer and the saturation correction coefficient.

[0126] Any combination of the technical features in the above embodiments can be made. For the sake of brevity, the foregoing description has not described all possible combinations of the technical features in the above embodiments, however, it is understood that any combination of the technical features is within the scope of the present disclosure as long as there is no contradiction.

[0127] The principles and implementation manners of the present application are described herein by using specific examples, and the above embodiments are only used to help understand the method of the present application and its core idea; meanwhile, according to the idea of the present application, the specific implementation manners and application ranges will be changed by those skilled in the art. In conclusion, the content of the present description should not be understood as a limitation of the present application.

Claims

1. A method for determining the collapsibility of a thick loess site, characterized in that, The method for determining the collapsibility of thick loess sites includes: Geotechnical parameters of soil samples at different depths in the target loess site were determined. The geotechnical parameters included the loess self-weight collapsibility coefficient and initial saturation. The target loess site was a thick loess site. Using collapsibility discrimination values ​​at different depths, based on the loess self-weight collapsibility coefficient of soil samples at each depth, the collapsibility of soil samples at each depth is judged, and the overall thickness of soil samples judged as collapsible soil layers is determined as the thickness of collapsible soil layers. The soil sample with the largest self-weight collapsibility coefficient among the soil samples identified as collapsible soil layers was selected as the representative sample, and the representative sample was divided into three parts: the first sub-sample, the second sub-sample, and the third sub-sample. By changing the initial saturation of the first sub-sample, and by measuring the collapsibility coefficient of the first sub-sample under different initial saturation and overburden pressure, the saturation correction coefficient of each soil layer sample identified as collapsible soil layer is determined. Multiple sets of ring cutter specimens at different heights were prepared using the second sub-sample. The soil self-weight collapse coefficient of the ring cutter specimens at different heights was determined by the collapse test. The size correction coefficient was determined based on the soil self-weight collapse coefficient of the ring cutter specimens at different heights. Standard wet collapse tests and triaxial compression tests were conducted on the third sub-sample to determine the stress correction factor; The self-weight collapsing deformation of the target loess site is calculated based on the size correction factor, stress correction factor, thickness of collapsible soil sample determined to be collapsible soil layer, and saturation correction factor.

2. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, Geotechnical parameters of soil samples at different depths in the target loess site were determined, specifically including: Drilling was conducted at the target loess site to collect soil samples at each meter depth, obtaining soil samples at different depths. Geotechnical parameters were determined for soil samples at different depths.

3. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, By changing the initial saturation of the first sub-sample and measuring the collapsibility coefficient of the first sub-sample under different initial saturation and overlying pressure, the saturation correction coefficient of each soil layer sample identified as collapsible is determined, specifically including: By changing the initial saturation of the first sub-sample, and measuring the collapsibility coefficient of the first sub-sample under different initial saturation and overburden pressure, the relationship between saturation increment, overburden stress and collapsibility coefficient is constructed. Based on the relationship between saturation increment, overlying stress and collapsibility coefficient, the relationship between saturation correction coefficient and initial saturation is constructed. Based on the relationship between the saturation correction factor and the initial saturation, the saturation correction factor for each soil sample identified as collapsible soil layer is determined.

4. The method for determining the collapsibility of thick loess sites according to claim 3, characterized in that, The relationship between saturation increment, overlying stress, and collapsibility coefficient is expressed as: ； in, This is the collapsibility coefficient. For saturation increment, For overlying stress, , and All are regression coefficients; The formula for determining the saturation correction factor for each soil sample classified as collapsible is as follows: ； in, This is the saturation correction factor. The initial saturation, , and All are regression coefficients.

5. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, Multiple sets of ring cutter specimens at different heights were prepared using the second sub-sample. The soil self-weight collapsibility coefficient of the ring cutter specimens at different heights was determined through collapsibility tests. Based on the soil self-weight collapsibility coefficient of the ring cutter specimens at different heights, a size correction factor was determined, specifically including: Four sets of ring cutter specimens with different heights were prepared using the first sub-sample. The heights of the four sets of ring cutter specimens were 20 mm, 40 mm, 60 mm and 80 mm, respectively. The self-weight collapsibility coefficient of the soil sample from the ring cutter specimen with a height of 20 mm was used as the normalized benchmark value. Based on the normalized baseline value, a relationship between the normalized self-weight collapse coefficient and the height of the ring cutter specimen is constructed. The dimensional correction factor is determined based on the relationship between the normalized self-weight sinking coefficient and the height of the ring cutter specimen.

6. The method for determining the collapsibility of thick loess sites according to claim 5, characterized in that, The relationship between the normalized self-weight collapse coefficient and the height of the ring cutter specimen is expressed as: ； in, This is the normalized self-weight collapse coefficient. The collapsibility coefficient of the soil sample by its self-weight is given by the ring cutter specimen. As the baseline value for normalization, This is a size correction factor. The height of the ring cutter specimen. It is a constant.

7. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, Standard wet collapse tests and triaxial compression tests were conducted on the third sub-sample to determine the stress correction factor, specifically including: Standard collapsibility test and triaxial compression test were carried out on the third subsample respectively to obtain the soil self-weight collapsibility coefficient under standard collapsibility test and the soil self-weight collapsibility coefficient under triaxial compression test; According to the formula Calculate the stress correction factor; in, This is the stress correction factor. This represents the soil's self-weight collapsibility coefficient under triaxial compression testing. This is the soil self-weight collapse coefficient under the standard collapse test.

8. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, The formula for calculating the self-weight collapsibility deformation of the target loess site is: ； in, Let n be the self-weight collapsibility deformation of the target loess site, and n be the number of soil samples identified as collapsible soil layers. Soil quality correction factor. This is a size correction factor. This is the stress correction factor. This is the saturation correction factor for the i-th collapsible soil layer. Let be the soil collapsibility coefficient based on its self-weight in the i-th collapsible soil layer. denoted as the thickness of the i-th collapsible soil layer.

9. The method for determining the collapsibility of thick loess sites according to claim 1, characterized in that, Using collapsibility discrimination values ​​at different depths, and based on the loess self-weight collapsibility coefficient of soil samples at each depth, the collapsibility of soil samples at each depth is judged, specifically including: When the depth range is [0,5), the collapsibility discrimination value is 0.015; When the depth range is [5, 10), the collapsibility discrimination value is 0.016; When the depth range is [10, 15), the collapsibility discrimination value is 0.017; When the depth range is [15, 20), the collapsibility discrimination value is 0.022; When the depth range is [20, 25), the collapsibility discrimination value is 0.032; When the depth range is [25, 30), the collapsibility discrimination value is 0.047; When the depth range is [30, 35), the collapsibility discrimination value is 0.058; When the depth range is [35, 40), the collapsibility discrimination value is 0.061; When the depth range is [40, +∞), the collapsibility discrimination value is 0.

062.

10. A device for determining the collapsibility of a thick loess field, characterized in that, The device for determining the collapsibility of thick loess sites includes: The geotechnical parameter measurement module is used to measure the geotechnical parameters of soil samples at different depths in the target loess site. The geotechnical parameters include the loess self-weight collapsibility coefficient and the initial saturation. The target loess site is a thick loess site. The collapsibility discrimination module is used to use collapsibility discrimination values ​​at different depths, based on the loess self-weight collapsibility coefficient of soil samples at each depth, to judge the collapsibility of soil samples at each depth, and to determine the thickness of collapsible soil layers in soil samples that are judged to be collapsible soil layers. The representative sample selection module is used to select the soil sample with the largest self-weight collapsibility coefficient of loess among the soil samples that are determined to be collapsible soil layers as the representative sample, and divide the representative sample into three parts: the first sub-sample, the second sub-sample, and the third sub-sample. The saturation correction coefficient determination module is used to change the initial saturation of the first sub-sample and determine the saturation correction coefficient of each soil layer sample that is determined to be collapsible by measuring the collapsibility coefficient of the first sub-sample under different initial saturation and overburden pressure. The size correction coefficient determination module is used to prepare multiple sets of ring cutter specimens of different heights using the second sub-sample, determine the soil sample self-weight collapse coefficient of the ring cutter specimens at different heights through collapse test, and determine the size correction coefficient based on the soil sample self-weight collapse coefficient of the ring cutter specimens at different heights. The stress correction factor determination module is used to conduct standard wet collapse tests and triaxial compression tests on the third sub-sample to determine the stress correction factor. The self-weight collapsibility deformation calculation module for the target loess site is used to calculate the self-weight collapsibility deformation of the target loess site based on the size correction factor, stress correction factor, thickness of collapsible soil layer and saturation correction factor of soil layer sample determined to be collapsible soil layer.

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