Method for measuring cohesive force and internal friction angle of soil body under seepage condition by using triaxial apparatus

Abstract

The invention discloses a method for measuring cohesive force and an internal friction angle of a soil body under a seepage condition by utilizing a triaxial apparatus. The method comprises the following steps: S1, adding a back pressure controller on the triaxial apparatus; s2, preparing and mounting a test sample, and mounting the test sample on the triaxial apparatus according to the operation specification of the triaxial apparatus; s3, back pressure saturation is carried out according to the operation standard process of the adopted triaxial apparatus, and drainage consolidation is carried out after the saturation reaches 98% according to the standard requirement; s4, setting back pressure, and simulating equivalent seepage; and S5, shearing and data processing are carried out according to a triaxial apparatus sample specification, and the cohesive force and the internal friction angle under the target hydraulic gradient are solved. The problem that in the prior art, the cohesive force and the internal friction angle of soil cannot be measured under the seepage condition is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of civil engineering, and particularly to a method for measuring the cohesion and internal friction angle of soil under seepage conditions by using a triaxial apparatus. Background Art

[0002] The seepage of water is ubiquitous in projects such as slopes and foundation pits, and the influence of its seepage effect cannot be ignored. In engineering fields such as water conservancy, civil engineering, and mines, the seepage effect will change the mechanical properties of rock and soil masses, causing problems such as foundation settlement, foundation pit collapse, slope instability, and tunnel water inrush. In severe cases, it will not only cause huge economic losses but also result in casualties. In addition, the selection of the mechanical parameters of soil, namely cohesion and internal friction angle, under seepage conditions is crucial for engineering design and numerical simulation analysis. Correctly selecting the cohesion and internal friction angle of soil under seepage conditions can reasonably reduce engineering costs and reduce accidents, which has significant engineering significance.

[0003] Existing research has studied the change of the internal friction angle of sand molds under the action of air flow by using a micro semiconductor stress sensor. The results show that under the action of air flow, the internal friction angle of the sand mold decreases. However, at present, there is little research on the measurement of the internal friction angle and cohesion of soil under the action of water flow. In the geotechnical test code, the main methods for measuring the cohesion and internal friction angle of soil are the direct shear test and the triaxial compression test. However, neither the direct shear test nor the triaxial compression test considers the seepage effect of water and cannot measure the cohesion and internal friction angle of soil under seepage conditions.

[0004] After searching the existing technology, it is found that there is little research on the measurement of the cohesion and internal friction angle of soil under seepage conditions. However, existing research has shown that under the condition of fluid flow, the biting effect between soil particles will weaken, and the fluid plays a certain lubricating role, which will reduce the internal friction angle of the soil. Directly using the parameters obtained by conventional measurement will cause large errors, bringing design risks and cost increases.

[0005] Therefore, in view of the above problems, a method for measuring the cohesion and internal friction angle of soil under seepage conditions by using a triaxial apparatus is provided. Summary of the Invention

[0006] The purpose of the present invention is to overcome the existing defects and provide a method for measuring the cohesion and internal friction angle of soil under seepage conditions by using a triaxial apparatus, so as to solve the problem that the cohesion and internal friction angle of soil cannot be measured under seepage conditions in the existing technology.

[0007] The technical solution for realizing the above purpose is as follows:

[0008] A method for measuring the cohesion and internal friction angle of soil under seepage conditions by using a triaxial apparatus includes:

[0009] Step S1: Add an back pressure controller to the triaxial apparatus.

[0010] Step S2: Prepare and install the specimen. Install the test specimen on the triaxial apparatus according to the operation specifications of the adopted triaxial apparatus.

[0011] Step S3: Conduct back pressure saturation according to the operation specification process of the adopted triaxial apparatus. After the saturation reaches 98% according to the specification requirements, conduct drainage consolidation.

[0012] Step S4: Set the back pressure to simulate equivalent seepage.

[0013] Step S5: Conduct shear and data processing according to the specimen specifications of the triaxial apparatus, and calculate the cohesion and internal friction angle under the modified target hydraulic gradient.

[0014] Preferably, in step S1, the triaxial apparatus includes: a confining pressure chamber. A base is fixed at the upper end of the confining pressure chamber. There is a cavity in the middle of the base. An atmospheric pressure port is provided on the upper end surface of the confining pressure chamber. The atmospheric pressure port is connected to the upper end of the base through a pipeline and extends into the cavity. A back pressure port is provided at the upper end of the cavity. The back pressure port is connected to the outside of the confining pressure chamber through a pipeline. Lower drainage ports and pore pressure ports are respectively provided on both sides at the lower end of the cavity. The lower drainage port is connected to the outside of the confining pressure chamber through a pipeline. The pore pressure port is connected to a pore pressure gauge outside the confining pressure chamber through a pipeline.

[0015] Preferably, in step S1, the back pressure port is connected to the outside of the confining pressure chamber and connected to an back pressure controller. On this basis, at the place where the lower drainage port is connected to the outside of the confining pressure chamber through a pipeline, add an back pressure controller and connect it to the lower drainage port through a pipeline for setting the back pressure of the back pressure port and the lower drainage port, achieving the function of simulating upward and downward seepage, and controlling the constancy of the back pressure and the linear change of the pressure.

[0016] Preferably, in step S2, the size of the test specimen is any one of the following three, namely, a diameter of 39.1 mm and a height of 80 mm, a diameter of 61.8 mm and a height of 120 mm, and a diameter of 101 mm and a height of 200 mm.

[0017] Preferably, in step S2, permeable stones and filter papers need to be provided at both the upper and lower parts of the test specimen. The entire specimen side is wrapped with a latex film and installed in the cavity. Among them, the test specimen uses the original soil body or a specimen with little disturbance. The latex film is provided with through holes corresponding to the atmospheric pressure port, the back pressure port, the lower drainage port, and the pore pressure port one by one.

[0018] Preferably, in step S3, after saturation is completed, the drainage valves at the back pressure port and the lower drainage port are opened for drainage consolidation. The back pressure controller pressurizes the cavity through the back pressure port or the lower drainage port to obtain consolidation deformation, and measures the pore pressure and effective stress data through the pore pressure gauge, and then calculates the consolidation coefficient during the consolidation process.

[0019] Preferably, in step S4, the back pressure port and the lower drainage port are set and opened according to requirements. When simulating the downward seepage effect, the back pressure port is set and opened, and the lower drainage port serves as the lower drainage port; when simulating the upward seepage effect, the lower drainage port is set and opened, and the back pressure port serves as the upper drainage port.

[0020] Preferably, in step S4, according to the target hydraulic gradient i to be simulated, the specimen height h, and the shear rate v, the shear process is actually a drained shear, so the shear rate should be 0.003% / min to 0.012% / min. A linear variation back pressure control method is proposed, and its back pressure linear control relationship is obtained in the following specific manner:

[0021] L (t) = h×(1 - v×t);

[0022] In the formula, L (t) is the seepage path length, and t is the cumulative shear time;

[0023] U (t) = γ w ×L (t) ×i;

[0024] In the formula, U (t) is the back pressure value at time t, γ w is the unit weight of water, and i is the target hydraulic gradient, which is a constant after being set.

[0025] Preferably, in step S5, during the shear process, the stress on the soil sample is between the consolidated drained test and the consolidated undrained test. There is a pore pressure change during the shear process, and it is often difficult to draw the strength envelope line for the obtained Mohr stress circle. The values should be taken according to the stress path method, and the cohesion c and the internal friction angle That is:

[0026]

[0027] In the formula, d is the intercept of the average straight line on the vertical axis, and α is the inclination angle of the average straight line.

[0028] Preferably, in step S5, the confining pressure is changed, but the target hydraulic gradient i remains unchanged. Then repeat the above triaxial shear process to obtain at least three groups of cohesion and internal friction angle data, and finally take the average value as the cohesion c and the internal friction angle under the hydraulic gradient i.

[0029] The beneficial effects of the present invention are as follows: The method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus provided by the present invention measures the cohesion and internal friction angle of soil under specific hydraulic gradient conditions by simply modifying the triaxial apparatus, which is more in line with engineering practice and better meets the engineering and design requirements. Moreover, it can simulate the working conditions of different seepage directions according to the actual engineering site requirements and set according to the actual situation. The test principle is simple, the operation is convenient, and it is easy to implement. It makes full use of the existing test instrument and equipment resources, with high efficiency and low cost. Brief Description of the Drawings

[0030] Figure 1 is a flow chart of a method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus according to the present invention;

[0031] Figure 2 is a structural diagram of the triaxial apparatus in the present invention;

[0032] Figure 3 is a finite stress path curve graph in the present invention.

[0033] In the figures: 1, confining pressure chamber; 2, base; 3, cavity; 4, atmospheric pressure port; 5, back pressure port; 6, lower drainage port; 7, pore pressure port; 8, pore pressure gauge; 9, latex film. Detailed Embodiment

[0034] Next, the technical solution of the present invention will be clearly and completely described in conjunction with the accompanying drawings. In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0035] Next, the present invention will be further described in conjunction with the accompanying drawings.

[0036] As Figure 1 、 2 shown, a method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus is characterized by including:

[0037] Step S1, adding a back pressure controller to the triaxial apparatus.

[0038] In the embodiment, the triaxial apparatus includes: a confining pressure chamber 1, a base 2 is fixed to the upper end of the confining pressure chamber 1, a cavity 3 is left in the middle of the base 2, an atmospheric pressure port 4 is provided on the upper end surface of the confining pressure chamber 1, the atmospheric pressure port 4 is connected to the upper end of the base 2 through a pipeline and extends into the cavity 3, a back pressure port 5 is arranged at the upper end of the cavity 3, the back pressure port 5 is connected to the outside of the confining pressure chamber 1 through a pipeline, lower drainage ports 6 and pore pressure ports 7 are respectively arranged on both sides at the lower end of the cavity 3, the lower drainage port 6 is connected to the outside of the confining pressure chamber 1 through a pipeline, and the pore pressure port 7 is connected to a pore pressure meter 8 outside the confining pressure chamber 1 through a pipeline.

[0039] In the embodiment, the back pressure port 5 is connected to the outside of the confining pressure chamber 1 through a pipeline and is connected to a back pressure controller. On this basis, at the place where the lower drainage port 6 is connected to the outside of the confining pressure chamber 1 through a pipeline, an additional back pressure controller is added and connected to the lower drainage port 6 through a pipeline, which is used to set the back pressure for the back pressure port 5 and the lower drainage port 6, so as to achieve the function of simulating upward and downward seepage, and control the constancy of the back pressure and the linear change of the pressure.

[0040] Step S2: Prepare the specimen and install it. Install the test specimen on the triaxial apparatus according to the operation specifications of the triaxial apparatus used.

[0041] In the embodiment, the size of the test specimen is any one of the following three, that is, a diameter of 39.1 mm and a height of 80 mm, a diameter of 61.8 mm and a height of 120 mm, and a diameter of 101 mm and a height of 200 mm.

[0042] In the embodiment, permeable stones and filter papers need to be arranged on both the upper and lower parts of the test specimen. The whole specimen side is wrapped with a latex film 9 to prevent the loss of soil particles and affect the test effect, and it is installed in the cavity 3. Among them, the test specimen adopts the original soil body or a specimen with little disturbance degree, and the undisturbed soil taken by a thin-wall soil sampler is the best. The latex film 9 is provided with through holes corresponding to the atmospheric pressure port 4, the back pressure port 5, the lower drainage port 6 and the pore pressure port 7 one by one.

[0043] Step S3: Carry out back pressure saturation according to the operation specification process of the triaxial apparatus used. After the saturation degree reaches 98% according to the specification requirements, carry out drainage consolidation.

[0044] In the embodiment, after saturation is completed, open the drainage valves at the back pressure port 5 and the lower drainage port 6 to carry out drainage consolidation. The back pressure controller pressurizes the cavity 3 through the back pressure port 5 or the lower drainage port 6 to obtain consolidation deformation, and measures the pore pressure and effective stress data through the pore pressure meter 8, and then calculates the consolidation coefficient during the consolidation process.

[0045] Step S4: Set the back pressure to simulate equivalent seepage.

[0046] In the embodiment, back pressure is set to ensure a constant hydraulic gradient during the shearing process. The back pressure port 5 and the lower drainage port 6 are set and opened according to requirements. When simulating downward seepage, the back pressure port 5 is set and opened, and the lower drainage port 6 serves as the lower drainage port; when simulating upward seepage, the lower drainage port 6 is set and opened, and the back pressure port 5 serves as the upper drainage port.

[0047] In the embodiment, according to the target hydraulic gradient i, specimen height h, and shearing rate v to be simulated, the shearing process is actually a drained shearing process. Therefore, the shearing rate should be 0.003% / min to 0.012% / min. A linearly varying back pressure control method is proposed, and its specific back pressure linear control relationship is obtained as follows:

[0048] L (t) = h×(1 - v×t);

[0049] In the formula, L (t) is the seepage path length, and t is the cumulative shearing time;

[0050] U (t) = γ w ×L (t) ×i;

[0051] In the formula, U (t) is the back pressure value at time t, γ w is the unit weight of water, and i is the target hydraulic gradient, which is a constant after being set. When the target hydraulic gradient is equal to 1 and the shearing rate is 0.005% / min, the back pressure control equations for the three specimens are shown in Table 1 below.

[0052]

[0053]

[0054] Table 1

[0055] In the embodiment, after setting the back pressure, the back pressure switch and the shearing switch should be opened synchronously to ensure that the hydraulic gradient of the soil sample remains constant during the shearing process.

[0056] Step S5: Perform shearing and data processing according to the triaxial apparatus specimen specifications, and obtain the cohesion and internal friction angle under the target hydraulic gradient.

[0057] In the embodiment, during the shearing process, the stress on the soil sample is between the consolidated drained test and the consolidated undrained test. There is a pore pressure change during the shearing process, and it is often difficult to draw the strength envelope line for the obtained Mohr stress circle. The values should be taken according to the stress path method. As Figure 3 shown, the cohesion c and the internal friction angle are calculated as follows:

[0058]

[0059] In the formula, d is the intercept of the average straight line on the vertical axis, and α is the inclination angle of the average straight line.

[0060] In the embodiment, the confining pressure is changed, but the target hydraulic gradient i remains unchanged. Then, the above triaxial shear process is repeated to obtain at least three groups of cohesion and internal friction angle data. Finally, the average value is taken as the cohesion c and the internal friction angle under the condition of the hydraulic gradient i.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus, characterized in that, Including: Step S1: Add an additional back pressure controller to the triaxial apparatus. Step S2: Prepare and install the specimen. Install the test specimen on the triaxial apparatus in accordance with the operating specifications of the triaxial apparatus used. Step S3: Conduct back pressure saturation in accordance with the operating specification process of the triaxial apparatus used. After the saturation degree reaches 98% as required by the specification, conduct drainage consolidation. Step S4: Set the back pressure to simulate equivalent seepage. Step S5: Conduct shearing and data processing in accordance with the specimen specifications of the triaxial apparatus, and determine the cohesion and internal friction angle under the modified target hydraulic gradient.

2. The method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 1, wherein In the said step S1, the triaxial apparatus includes: a confining pressure chamber (1), a base (2) is fixed at the upper end of the confining pressure chamber (1), a cavity (3) is left in the middle of the base (2), an atmospheric pressure port (4) is provided on the upper end face of the confining pressure chamber (1), the atmospheric pressure port (4) is connected to the upper end of the base (2) through a pipeline and extends into the cavity (3), a back pressure port (5) is provided at the upper end of the cavity (3), the back pressure port (5) is connected to the outside of the confining pressure chamber (1) through a pipeline, lower drainage ports (6) and pore pressure ports (7) are respectively provided on both sides at the lower end of the cavity (3), the lower drainage port (6) is connected to the outside of the confining pressure chamber (1) through a pipeline, and the pore pressure port (7) is connected to a pore pressure gauge (8) outside the confining pressure chamber (1).

3. A method for determining the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 2, characterized in that, In the said step S1, the back pressure port (5) is connected to the outside of the confining pressure chamber (1) through a pipeline and is connected to an additional back pressure controller. On this basis, an additional back pressure controller is added at the position where the lower drainage port (6) is connected to the outside of the confining pressure chamber (1) through a pipeline, and is connected to the lower drainage port (6) through a pipeline, for setting the back pressure of the back pressure port (5) and the lower drainage port (6), achieving the function of simulating upward and downward seepage, and controlling the constancy of the back pressure and the linear change of the pressure.

4. A method for determining the cohesive force and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 1, characterized in that, In the said step S2, the size of the test specimen is any one of the following three, namely, a diameter of 39.1 mm and a height of 80 mm, a diameter of 61.8 mm and a height of 120 mm, and a diameter of 101 mm and a height of 200 mm.

5. A method for measuring the cohesive force and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 2, characterized in that, In the said step S2, permeable stones and filter papers need to be provided at both the upper and lower parts of the test specimen. The entire specimen side is wrapped with a latex film (9) and installed in the cavity (3). Among them, the test specimen uses the original soil body or a specimen with little disturbance degree. The latex film (9) is provided with through holes corresponding one by one to the atmospheric pressure port (4), the back pressure port (5), the lower drainage port (6), and the pore pressure port (7).

6. A method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 2, characterized in that, In the said step S3, after saturation is completed, open the drainage valves at the back pressure port (5) and the lower drainage port (6) for drainage consolidation. The back pressure controller pressurizes the cavity (3) through the back pressure port (5) or the lower drainage port (6) to obtain consolidation deformation, and measures the pore pressure and effective stress data through the pore pressure gauge (8), and then calculates the consolidation coefficient during the consolidation process.

7. A method for measuring the cohesive force and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 2, characterized in that In the step S4, set and open the back pressure port (5) and the lower drain port (6) according to requirements. When simulating the downward seepage effect, set and open the back pressure port (5), and the lower drain port (6) serves as the lower drain port. When simulating the upward seepage effect, set and open the lower drain port (6), and the back pressure port (5) serves as the upper drain port.

8. A method for determining the cohesive force and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 7, characterized in that, In the step S4, according to the target hydraulic gradient i, specimen height h, and shear rate v to be simulated, the shear process is actually drained shear. Therefore, the shear rate should be 0.003% / min to 0.012% / min. A linear variation back pressure control method is proposed, and its back pressure linear control relationship is obtained in the following specific way: L (t) = h × (1 - v × t); where L (t) is the seepage path length, and t is the cumulative shear time; U (t) = γ w × L (t) × i; Where U (t) is the back pressure value at time t, γ w is the unit weight of water, and i is the target hydraulic gradient, which is a constant after being set.

9. A method for measuring the cohesion and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 1, characterized in that In the step S5, during the shearing process, the stress on the soil sample is between the consolidated drained test and the consolidated undrained test. There is a pore pressure change during the shearing process, and it is often difficult to draw the strength envelope for the obtained Mohr stress circle. The values should be taken according to the stress path method, and the cohesion c and the internal friction angle are calculated That is: In the formula, d is the intercept of the average straight line on the vertical axis, and α is the inclination angle of the average straight line.

10. A method for determining the cohesive force and internal friction angle of soil under seepage conditions using a triaxial apparatus according to claim 9, characterized in that, In the step S5, the confining pressure is changed, but the target hydraulic gradient i remains unchanged. Then, the above triaxial shear process is repeated to obtain at least three groups of cohesion and internal friction angle data. Finally, the average values are taken as the cohesion c and internal friction angle under the hydraulic gradient i.

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