A method for calculating shear wave velocity based on matrix suction of unsaturated fine-grained soil

Abstract

The present invention discloses a shear wave velocity calculation method based on the matrix suction of unsaturated fine-grained soil. The method obtains the soil-water characteristic curve of the sample by carrying out a pressure plate instrument test on the unsaturated fine-grained soil sample; then, the test data is fitted based on the VG model to obtain the matrix suction under different saturations; then, the shear wave velocity test of the sample in the continuous dehumidification process under different saturations is carried out based on the bending element method to obtain the evolution law of the shear wave velocity with the saturation; finally, according to the empirical model, a prediction model of the shear wave velocity based on the matrix suction of the unsaturated fine-grained soil is proposed to realize the calculation of the shear wave velocity of the sample in the dehumidification process through the matrix suction. The method avoids the shortcomings of the existing equipment such as complex operation and high cost. The model is applicable to fine-grained soils such as clay and silt, and is helpful to characterize the evolution law of the shear wave velocity of the unsaturated soil with the matrix suction in a wide suction range, which is of great significance for analyzing the change mechanism of the shear wave velocity in the dehumidification process of the unsaturated soil.

Description

Technical Field

[0001] The invention relates to the field of geotechnical test calculation, and in particular to a shear wave velocity prediction calculation method based on unsaturated fine-grained soil matrix suction. Background Art

[0002] Small strain shear stiffness is an important parameter for accurately predicting deformation caused by soil-structure interaction. In arid, semi-arid or tropical areas, the shallow soil layers involved in projects such as highway and railway subgrades, river embankments and shallow foundations are mostly in an unsaturated state. The evaporation and replenishment of water in the soil will cause drastic changes in soil saturation, and the change in saturation will inevitably affect the small strain shear stiffness of the soil. Using shear wave velocity to characterize the small strain shear stiffness of soil is currently a commonly used method in laboratories.

[0003] The main means of testing soil shear wave velocity in the laboratory are resonant columns and bending elements. Resonant column testing of small strain shear stiffness is generally considered to be a more reliable method, but its disadvantage is that the contact problem between the sample and other components of the equipment will affect the determination of the mass moment of inertia of the sample. For soils with large stiffness, the mass moment of inertia of the equipment components must be corrected before each sample loading, and the test operation is cumbersome. Bending element is a piezoelectric ceramic material that can realize the mutual conversion between mechanical energy and electrical energy. Using this property, the bending element can be used as a small sensor to generate and detect shear waves in the soil. It is often implanted in other commonly used equipment (such as triaxial instruments, resonant columns, direct shear instruments, consolidation instruments, centrifuges, etc.) to carry out joint testing, such as the "Unsaturated coarse-grained soil water characteristic curve combined with bending element measurement device" published in Chinese Patent No. CN111811962A.

[0004] Although the use of bending element testing has the advantages of being non-destructive, simple, and can be used in conjunction with other equipment, most of the combined test equipment is complex in structure and expensive, especially for the combined test equipment involving unsaturated soil water retention characteristics and shear wave velocity. The installation and debugging between the clay plate and the bending element is difficult, and there are problems such as the soil properties and suction application range of the equipment. For unsaturated clay or silt, how to predict the shear wave velocity of unsaturated soil at different saturations only through the matrix suction obtained by the pressure plate test and the classical fitting model is still rarely reported, and it is urgent to solve it. Based on the unsaturated soil pressure plate test and the classical soil-water characteristic curve prediction model, combined with the bending element test results of soil shear wave velocity at different saturations, the corresponding relationship between matrix suction and shear wave velocity at different saturations is established, which is crucial to studying the evolution law of unsaturated soil shear wave velocity with water retention characteristics. Summary of the invention

[0005] The purpose of the present invention is to provide a shear wave velocity calculation method based on the matrix suction of unsaturated fine-grained soil. The method can establish the corresponding relationship between the matrix suction and the shear wave velocity during the dehumidification process of unsaturated fine-grained soil, and provide a way to obtain the shear wave velocity of unsaturated soil only through conventional pressure plate tests.

[0006] The present invention provides a shear wave velocity calculation method based on unsaturated fine-grained soil matrix suction, comprising the following steps:

[0007] (1) Sample preparation: The soil-water characteristic curve of the sample is obtained by conducting a pressure plate test on the unsaturated fine-grained soil sample;

[0008] (2) Based on the van Genuchten model (VG model), the soil-water property data of the samples were fitted to obtain the matrix suction of the samples at different saturations;

[0009] (3) Based on the bending element method, the shear wave velocity test of the continuous dehumidification process of the samples at different saturations was carried out to obtain the evolution law of the shear wave velocity with saturation;

[0010] (4) Based on the empirical model of shear wave velocity and effective stress of the Hardin model and the empirical function of porosity, a prediction model for the shear wave velocity of matrix suction of unsaturated fine-grained soils such as clay and silt is proposed, which realizes the calculation of the shear wave velocity of the sample during the dewetting process through matrix suction.

[0011] Furthermore, the specific steps of step (1) are as follows:

[0012] (1.1) Sample preparation, testing the initial moisture content ω of unsaturated fine-grained soil samples 0 , given dry density ρ d ;

[0013] Compact the sample, calculate the mass of the compacted sample soil according to the ring cutter size, and use the static compaction method to prepare the sample;

[0014] (1.2) Place the prepared compacted sample and the clay plate into a vacuum cylinder and evacuate for 8 to 12 hours, then add distilled water to completely immerse the soil sample, and continue evacuating for 18 to 24 hours to saturate the soil sample;

[0015] (1.3) Test on soil-water characteristics of the sample using a pressure plate apparatus. After taking out the saturated sample and weighing its mass, the sample is placed on a saturated clay plate in the pressure plate apparatus. Different suction paths are given to obtain the soil-water characteristic curve of the sample during the dehumidification process.

[0016] When a predetermined matrix suction force is applied, the pressure chamber is sealed, and the mass of the pycnometer connected to the clay plate is weighed to determine whether the sample reaches equilibrium under the suction force at this level;

[0017] After the drainage of each level of the pressure plate instrument is stable and the suction is balanced, open the pressure chamber, take out the sample and weigh its mass, so as to calculate the saturation of the soil under different matrix suctions;

[0018] After weighing, the sample is immediately put back into the pressure chamber, sealed and the next level of suction is applied;

[0019] When the suction force is less than 500 kPa, use a clay board with an air intake value of 500 kPa. When the suction force is greater than 500 kPa, use a clay board with a suction force of 1.5 MPa.

[0020] Furthermore, in step (2), the soil water characteristic data of the sample is fitted based on the VG model. The relationship between matrix suction and saturation in the VG model is Where s is the sample matrix suction, S r is the saturation, α is a parameter related to the air intake value, and m is a fitting parameter. According to the VG model, the matrix suction of the sample at different saturations can be obtained.

[0021] Furthermore, the specific method of step (3) is:

[0022] (3.1) Place the prefabricated parallel samples on a sample plate with a shear wave velocity sensor, and place the sample plate on an electronic balance in a moisturizing cylinder with different relative humidity. Test the shear wave velocity regularly and monitor the change in sample mass to obtain the shear wave velocity of the sample at different saturations.

[0023] (3.2) The upper sample-bearing bending element is set as the receiving end, the lower sample-bearing bending element is set as the excitation end, the excitation frequency is set to 100 Hz, the voltage amplitude is set to 10 Vpp, and the time domain first arrival wave method is used to determine the shear wave propagation time;

[0024] (3.3) Based on the experience of sample thickness and shear wave velocity testing, it is determined that the depth of the bending element sensors at the excitation end and the receiving end to be embedded in the center of the sample is 2 mm. By giving different relative humidity, the sample is dehumidified in a moisturizing container, and the mass data and waveform data are collected at regular intervals to calculate the shear wave velocity of the sample at different saturations;

[0025] (3.4) The signal amplifier is used for filtering. The gain of the amplifier is 20 dB, the high pass is set to 100 kHz, the low pass is 1 MHz, the oscilloscope digital sampling rate is 2.5 GHz, and the signal is stacked 256 times to reduce random noise. The shear wave velocity is equal to the distance L between the sensor vertices. tt The ratio of the propagation time t, that is, the shear wave velocity V S =L tt / t.

[0026] Furthermore, the empirical model of shear wave velocity and effective stress in step (4) is Where A and β are fitting parameters, f(e) is a function of the void ratio, σ' is the effective stress including matrix suction, and 1 kPa is the reference pressure. The purpose is to obtain the required dimension.

[0027] Furthermore, when the unsaturated fine-grained soil sample is silt and clay, the prediction model between the shear wave velocity and matrix suction of silt and clay is: In the formula, is the shear wave velocity at saturation, e 0 is the initial void ratio, s M is matrix suction, P atm is a standard atmospheric pressure, AEV is the intake value, α and β are fitting parameters.

[0028] The method of the present invention obtains matrix suction under different saturations through conventional pressure plate instrument test and VG model, and uses matrix suction to realize a prediction model of shear wave velocity of unsaturated soil in the dehumidification process, thereby avoiding the shortcomings of complex operation and high cost of existing equipment. The model is suitable for fine-grained soils such as clay and silt, and is helpful to characterize the evolution law of shear wave velocity of unsaturated soil with matrix suction in a wide suction range, which is of great significance for analyzing the change mechanism of shear wave velocity in the dehumidification process of unsaturated soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of a method for calculating shear wave velocity based on matrix suction of unsaturated fine-grained soil according to the present invention;

[0030] Figure 2 1 is the clay soil water holding characteristic data and the VG model fitting curve of the embodiment;

[0031] Figure 3 The silt water holding property data and the VG model fitting curve of the embodiment;

[0032] Figure 4 The embodiment is the variation law of the suction and shear wave velocity of clay soil matrix with saturation;

[0033] Figure 5 The embodiment shows the variation of silt matrix suction and shear wave velocity with saturation;

[0034] Figure 6 The embodiment is the measured value and prediction curve of the shear wave velocity of clay soil changing with the matrix suction;

[0035] Figure 7 The present invention shows the measured value and predicted curve of the shear wave velocity of silt in the embodiment changing with the matrix suction. DETAILED DESCRIPTION

[0036] The present invention is further described below with reference to the accompanying drawings and embodiments, but is not intended to limit the present invention:

[0037] Example

[0038] Reference Figure 1 , a shear wave velocity calculation method based on the matrix suction of unsaturated fine-grained soil. Taking pulverulent soil and clay as examples, the calculation method steps are as follows:

[0039] (1) Sample preparation: by conducting pressure plate test on powdery soil and clay soil samples, the soil-water characteristic curve of the samples was obtained;

[0040] (2) Based on the VG model, the soil-water characteristic data of the samples were fitted to obtain the matrix suction of the samples at different saturations;

[0041] (3) Based on the bending element method, the shear wave velocity test of the continuous dehumidification process of the samples at different saturations was carried out to obtain the evolution law of the shear wave velocity with saturation;

[0042] (4) According to the empirical model of shear wave velocity and effective stress of Hardin model and the empirical function of porosity, a prediction model of shear wave velocity based on matrix suction is proposed to realize the calculation of shear wave velocity of the sample in the dehumidification process through matrix suction.

[0043] The specific steps of step (1) are as follows:

[0044] (1.1) Sample preparation, test sample initial moisture content ω 0 , given dry density ρd;

[0045] Compact the sample, calculate the mass of the compacted sample soil according to the ring cutter size, reshape the compacted sample to a height of 20 mm and a diameter of 45 mm, and use the static compaction method to prepare the sample;

[0046] (1.2) The compacted sample and the clay plate are placed in a vacuum cylinder and evacuated for 10 hours. Then, distilled water is added to completely immerse the soil sample. The evacuation is continued for 20 hours until the soil sample reaches saturation.

[0047] (1.3) Test of soil-water characteristics of the sample in the pressure plate apparatus. After taking out the saturated sample and weighing its mass, place it on the saturated clay plate in the pressure plate apparatus. In order to ensure the continuity of the dehumidification process of the sample in the pressure plate apparatus, it is necessary to ensure that the ring knife sample is in good contact with the clay plate. Different suction paths are given to obtain the soil-water characteristic curve of the sample dehumidification process.

[0048] The pressure plate instrument adjusts the matrix suction by controlling the air pressure. When a predetermined matrix suction is applied, the pressure chamber is sealed, and the mass of the pycnometer connected to the clay plate is weighed to determine whether the soil sample reaches equilibrium under this level of suction.

[0049] After the drainage of each level of the pressure plate instrument is stable and the suction is balanced, open the pressure chamber, take out the sample and weigh its mass, so as to calculate the saturation of the soil under different matrix suctions;

[0050] After weighing, the sample is immediately put back into the pressure chamber, sealed and the next level of suction is applied;

[0051] When the suction force is less than 500 kPa, a clay plate with an air intake value of 500 kPa is used, and when the suction force is greater than 500 kPa, a clay plate with a suction force of 1.5 MPa is used; the suction path is shown in Table 1;

[0052]

[0053] Step (2) is to fit the soil water characteristic data of the sample based on the VG model. The relationship between matrix suction and saturation in the VG model is Where s is the sample matrix suction, S r is the saturation, α is a parameter related to the air intake value, and m is a fitting parameter. According to the VG model, the matrix suction of the sample at different saturations can be obtained.

[0054] The specific method of step (3) is:

[0055] (3.1) Place the prefabricated parallel samples on a sample plate with a shear wave velocity sensor, and place the sample plate on an electronic balance in a moisturizing cylinder with different relative humidity. The electronic balance is peeled in advance to obtain the actual mass of the sample; the shear wave velocity is tested regularly and the change in the sample mass is monitored to obtain the shear wave velocity of the sample at different saturations;

[0056] (3.2) The upper sample-bearing bending element is set as the receiving end, and the lower sample-bearing bending element is set as the excitation end. Using a square wave with a wider frequency range as the excitation end input waveform will increase the amplitude of the received signal, making it easier to interpret and process the signal. The excitation frequency is set to 100 Hz, the voltage amplitude is 10 Vpp, and the time domain first arrival wave method can effectively determine the shear wave propagation time.

[0057] (3.3) Based on the test experience related to sample thickness and shear wave velocity, it is determined that the depth of the bending element sensors at the excitation end and the receiving end to be embedded in the center of the sample is 2 mm. By giving different relative humidity, the sample is dehumidified in a moisturizing container, and the mass data and waveform data are collected at regular intervals, and then the shear wave velocity of the sample at different saturations is calculated;

[0058] (3.4) In order to obtain a higher quality received signal, a signal amplifier is used for filtering. The gain of the amplifier is 20 dB, the high pass is set to 100 kHz, the low pass is 1 MHz, the oscilloscope digital sampling rate is 2.5 GHz, and the signal is stacked 256 times to reduce random noise. The wave speed is equal to the distance L between the sensor vertices. tt The ratio of the propagation time t, that is: V S =L tt / t.

[0059] Empirical model of shear wave velocity and effective stress in step (4) Where A and β are fitting parameters, f(e) is a function of the void ratio, and σ' is the effective stress including matrix suction.

[0060] Table 2 gives the commonly used forms of the void ratio function f(e) proposed by some scholars, but they need to be modified in specific applications to be suitable for different soil properties;

[0061]

[0062] In this embodiment, based on the experimental calculation results of shear wave velocity, the prediction model between shear wave velocity and matrix suction applicable to silt and clay is: In the formula, is the shear wave velocity at saturation, e 0 is the initial void ratio, s M is matrix suction, P atm is a standard atmospheric pressure, AEV is the intake value, α and β are fitting parameters.

[0063] Figure 2 The measured data and fitting curve of the water holding characteristics of the clay soil sample in the embodiment are shown in the figure. The test data of the soil water characteristics of the sample are fitted according to the VG model to obtain the correlation coefficient R 2 is 0.922, α is 0.05, and m is 0.3. The matrix suction gradually increases with the decrease of saturation. The initial stage is saturation S r ≥97%, the matrix suction increases sharply with the decrease of saturation. When 20%≤S r When S is less than 97%, it is the transition stage. The matrix suction gradually increases with the decrease of saturation. r When the saturation is ≤20%, it is the residual stage. According to the VG model fitting results, the matrix suction will continue to increase with the decrease of saturation.

[0064] Figure 3 The measured data and fitting curve of the water holding characteristics of the silt sample in the example are shown in Figure 2. The test data of the soil water characteristics of the sample were fitted according to the VG model to obtain the correlation coefficient R 2is 0.982, α is 0.06, and m is 0.35. The matrix suction gradually increases with the decrease of saturation. The initial stage is saturation S r ≥95%, the matrix suction increases sharply with the decrease of saturation. When 20%≤S r ≤95% is the transition stage, and the matrix suction gradually increases with the decrease of saturation. r When the saturation is ≤20%, it is the residual stage. According to the VG model fitting results, the matrix suction will continue to increase with the decrease of saturation.

[0065] Figure 4 The following is the variation law of shear wave velocity and matrix suction of clay soil sample with saturation. Both shear wave velocity and matrix suction increase continuously with the decrease of saturation, and can be divided into initial stage, transition stage and residual stage. r ≥97% is the initial stage, and the two increase sharply with the decrease of saturation. r ≤97% is the transition stage, and the two gradually increase with the decrease of saturation. r When ≤30%, it is the residual stage, and both continue to increase as the saturation decreases.

[0066] Figure 5 The following is the variation law of shear wave velocity and matrix suction of silt sample with saturation. Both shear wave velocity and matrix suction increase continuously with the decrease of saturation, and can be divided into initial stage, transition stage and residual stage. r ≥95% is the initial stage, and the two increase sharply with the decrease of saturation. r When the saturation decreases, it is the transition stage. r When ≤20%, it is the residual stage, and both continue to increase as the saturation decreases.

[0067] Figure 6 The matrix suction prediction formula of the shear wave velocity of the clay sample in the embodiment is obtained, and the correlation coefficient R 2 is 0.989, α is 0.12, and β is 0.34. The shear wave velocity increases with the increase of matrix suction. r When the shear wave velocity is less than 97%, it increases rapidly with the decrease of matrix suction. r When the shear wave velocity is >97%, it increases gradually with the decrease of matrix suction.

[0068] Figure 7 The matrix suction prediction formula of the shear wave velocity of the silt sample in the embodiment is obtained by 2 is 0.944, α is 0.52, and β is 0.33. The shear wave velocity increases with the increase of matrix suction. rWhen the shear wave velocity is less than 95%, it increases rapidly with the decrease of matrix suction. r When the shear wave velocity is >95%, it increases gradually with the decrease of matrix suction.

[0069] The embodiments of the present invention are described in words above in conjunction with the accompanying drawings, but the present invention is not limited to the embodiments, and various changes can be made according to the purpose of the present invention. Any changes, modifications, combinations, substitutions, and simplifications made according to the spirit and principle of the technical solution of the present invention should be considered as equivalent replacement methods, and as long as they meet the purpose of the present invention and do not deviate from the purpose of the present invention, they belong to the protection scope of the present invention.

Claims

1. A shear wave velocity calculation method based on matrix suction of unsaturated fine-grained soil, characterized in that: The steps include: (1) Sample preparation: The soil-water characteristic curve of the sample is obtained by conducting a pressure plate test on the unsaturated fine-grained soil sample; (2) Based on the VG model, the soil-water characteristic data of the samples were fitted to obtain the matrix suction of the samples at different saturations; (3) Based on the bending element method, the shear wave velocity test of the continuous dehumidification process of the samples at different saturations was carried out to obtain the evolution law of the shear wave velocity with saturation; (4) Based on the empirical model of shear wave velocity and effective stress of the Hardin model and the empirical function of porosity, a prediction model of shear wave velocity based on matrix suction of unsaturated fine-grained soil is proposed to calculate the shear wave velocity of the sample in the dehumidification process through matrix suction; The empirical model of shear wave velocity and effective stress In the formula, A and β are fitting parameters, f(e) is a function of the porosity ratio, σ' is the effective stress including matrix suction, and 1 kPa is the reference pressure, the purpose is to obtain the required dimension; When the unsaturated fine-grained soil samples are silt and clay, the prediction model between the shear wave velocity and matrix suction of silt and clay is: In the formula, is the shear wave velocity at saturation, e0 is the initial porosity, s M is matrix suction, P atm is a standard atmospheric pressure, AEV is the intake value, α and β are fitting parameters.

2. The shear wave velocity calculation method according to claim 1, characterized in that: The specific steps of step (1) are as follows: (1.1) Sample preparation, test the initial moisture content of unsaturated fine-grained soil sample ω0, given dry density ρ d ; Compact the sample, calculate the mass of the compacted sample soil according to the ring cutter size, and use the static compaction method to prepare the sample; (1.2) Place the prepared compacted sample and the clay plate into a vacuum cylinder and evacuate for 8 to 12 hours, then add distilled water to completely immerse the soil sample, and continue evacuating for 18 to 24 hours to saturate the soil sample; (1.3) Test on soil-water characteristics of the sample using a pressure plate apparatus. After taking out the saturated sample and weighing its mass, the sample is placed on a saturated clay plate in the pressure plate apparatus. Different suction paths are given to obtain the soil-water characteristic curve of the sample during the dehumidification process. When a predetermined matrix suction force is applied, the pressure chamber is sealed, and the mass of the pycnometer connected to the clay plate is weighed to determine whether the sample reaches equilibrium under the suction force at this level; After the drainage of each level of the pressure plate instrument is stable and the suction is balanced, open the pressure chamber, take out the sample and weigh its mass, so as to calculate the saturation of the soil under different matrix suctions; After weighing, the sample is immediately put back into the pressure chamber, sealed and the next level of suction is applied; When the suction force is less than 500 kPa, use a clay board with an air intake value of 500 kPa. When the suction force is greater than 500 kPa, use a clay board with a suction force of 1.5 MPa.

3. The shear wave velocity calculation method according to claim 1, characterized in that: The specific method of step (2) is: Using the relationship between matrix suction and saturation in the VG model Where s is the sample matrix suction, S r is the saturation, α is a parameter related to the air intake value, and m is a fitting parameter. According to the VG model, the matrix suction of the sample at different saturations can be obtained.

4. The shear wave velocity calculation method according to claim 1, characterized in that: The specific method of step (3) is: (3.1) Place the prefabricated parallel samples on a sample plate with a shear wave velocity sensor, and place the sample plate on an electronic balance in a moisturizing cylinder with different relative humidity. Test the shear wave velocity regularly and monitor the change in sample mass to obtain the shear wave velocity of the sample at different saturations. (3.2) The upper sample-bearing bending element is set as the receiving end, the lower sample-bearing bending element is set as the excitation end, the excitation frequency is set to 100 Hz, the voltage amplitude is set to 10 Vpp, and the time domain first arrival wave method is used to determine the shear wave propagation time; (3.3) Based on the experience of sample thickness and shear wave velocity testing, it is determined that the depth of the bending element sensors at the excitation end and the receiving end to be embedded in the center of the sample is 2 mm. By giving different relative humidity, the sample is dehumidified in a moisturizing container, and the mass data and waveform data are collected at regular intervals to calculate the shear wave velocity of the sample at different saturations; (3.4) The signal amplifier is used for filtering. The gain of the amplifier is 20 dB, the high pass is set to 100 kHz, the low pass is 1 MHz, the oscilloscope digital sampling rate is 2.5 GHz, and the signal is stacked 256 times to reduce random noise. The shear wave velocity is equal to the distance L between the sensor vertices. tt The ratio of the propagation time t, that is, the shear wave velocity V S =L tt / t.

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