A triaxial apparatus for testing volume change of unsaturated soil specimens and its testing method

Abstract

The present invention discloses a triaxial apparatus for testing the volume change of unsaturated soil specimens and a testing method thereof. The apparatus includes a gas storage vessel and a triaxial apparatus. The first interface at the top of the gas storage vessel is connected to the specimen exhaust hole of the triaxial apparatus through a pipeline, and the second interface at the bottom of the gas storage vessel is connected to the back pressure controller of the triaxial apparatus. During the test, the pressure of the back pressure controller is set slightly greater than the water head pressure in the gas storage vessel. During the test process, the temperature is kept constant and the pressure of the back pressure controller is kept constant. At this time, the volume of the inlet and outlet gas in the specimen pores is approximately equal to the change in the water volume in the gas storage vessel. By detecting the change in the water volume in the gas storage vessel, the change in the void gas volume of the unsaturated soil specimen can be obtained. The present invention overcomes the defect that the conventional triaxial apparatus cannot directly measure the change in the gas volume in the unsaturated soil specimen, realizes high-precision measurement of the volume change of the unsaturated soil specimen, and has sufficient precision.

Description

Technical Field

[0001] The present invention belongs to the technical field of static and dynamic triaxial test in geotechnical tests, and particularly relates to a triaxial apparatus for testing the volume change of unsaturated soil specimens and a testing method thereof. Background Art

[0002] A triaxial apparatus is a scientific research instrument often used in geotechnical tests. By monitoring the volume change, confining pressure, stress, etc. of soil specimens with specific dimensions, the mechanical characteristics of this type of soil can be obtained. In the saturated soil compression / shear test using a conventional triaxial apparatus, the soil specimen is first completely saturated, and the pores in the specimen are filled with water and do not contain gas. Since the compressibility of soil particles is very small, the volume change of the specimen is considered to be the change of the moisture inside the specimen, and it can be known by measuring the amount of water discharged during the test.

[0003] However, if the specimen uses unsaturated soil, the pores in the specimen contain not only moisture but also gas. At this time, if the volume change of the specimen is to be obtained, it is necessary to simultaneously monitor the volume of gas discharged from the specimen and the volume of water discharged. In the unsaturated state, it is very difficult for moisture to be discharged, and in most cases, the volume change of the specimen is reflected as the change of pore gas. The previous method was to use a communicating vessel vertical tube and visually read the water level change to record the volume change of the discharged gas. The accuracy was low and the operation was cumbersome. It was not suitable for electronic measurement and was only applicable to the case where the specimen volume was compressed. If the specimen expands, water will enter the specimen, resulting in the failure of the test conditions. Generally, for unsaturated soil specimens, due to the sensitivity of gas to temperature, pressure, etc., the volume change of gas should be measured by precise instruments, such as using a specially designed triaxial test apparatus for unsaturated soil, but this involves a large amount of equipment costs. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defect that the conventional triaxial and dynamic triaxial test instruments cannot directly measure the volume of unsaturated soil specimens, and to provide a triaxial apparatus for testing the volume change of unsaturated soil specimens and a testing method thereof. A gas storage dish is added to the original triaxial apparatus measurement system, thus forming a new measurement system, and realizing high-precision measurement of the volume change of unsaturated soil specimens.

[0005] The present invention is realized by the following technical solutions:

[0006] A triaxial apparatus for testing the volume change of unsaturated soil specimens includes a gas storage dish and a triaxial apparatus. The gas storage dish is cylindrical, with a first interface provided at the top and a second interface provided at the bottom. The first interface at the top of the gas storage dish is connected to the specimen exhaust hole of the triaxial apparatus through a pipeline, and the second interface at the bottom of the gas storage dish is connected to the back pressure controller of the triaxial apparatus.

[0007] The testing method of this apparatus is as follows:

[0008] Step 1: Install the unsaturated soil specimen on the triaxial apparatus, add a certain volume of degassed water into the gas storage vessel, and keep the water level below the first interface to prevent water from flowing back into the specimen.

[0009] Step 2: Connect the first interface of the gas storage vessel to the lower exhaust hole of the triaxial apparatus through a pipeline, and connect the second interface of the gas storage vessel to the back pressure controller of the triaxial apparatus.

[0010] Step 3: Set the pressure of the back pressure controller slightly greater than the water head pressure in the gas storage vessel. At this time, the pressure of the gas in the specimen is the difference between the pressure of the back pressure controller and the water head pressure in the gas storage vessel.

[0011] Step 4: Start the triaxial apparatus and conduct a compression test on the unsaturated soil specimen to discharge the gas in the pores of the unsaturated soil specimen. During the test, keep the temperature constant and keep the pressure of the back pressure controller unchanged. At this time, the volume of the inlet and outlet gas in the pores of the specimen is approximately equal to the change in the water volume in the gas storage vessel. By detecting the change in the water volume in the gas storage vessel, the change in the void gas volume of the unsaturated soil specimen can be obtained.

[0012] In the above technical solution, in Step 1, the volume of degassed water is preferably 1 / 2 of the total capacity of the gas storage vessel.

[0013] In the above technical solution, the pressure of the back pressure controller is 1 kPa - 5 kPa greater than the water head pressure in the gas storage vessel, aiming to make the gas bear a pressure not greater than 5 kPa. The water head pressure in the gas storage vessel can be calculated according to the water column height. Generally, it is considered that the pressure of a 10 cm water column is 1 kPa.

[0014] In the above technical solution, the change in the water volume in the gas storage vessel is measured and automatically recorded by a high-precision back pressure controller.

[0015] The advantages and beneficial effects of the present invention are as follows:

[0016] The present invention overcomes the disadvantage that the conventional triaxial apparatus cannot directly measure the change in the gas volume of the unsaturated soil specimen. The device adds a gas storage vessel on the basis of the original triaxial apparatus measurement system, thus forming a new measurement system, realizing high-precision measurement of the volume change of the unsaturated soil specimen, having sufficient precision, and also being applicable to measuring the volume change of the unsaturated soil in the dynamic triaxial test. Description of the Drawings

[0017] Figure 1 It is a schematic structural diagram of the triaxial apparatus device for testing the volume change of the unsaturated soil specimen of the present invention.

[0018] For those of ordinary skill in the art, without creative efforts, other relevant drawings can be obtained according to the above drawings. Detailed Embodiments

[0019] To enable those skilled in the art to better understand the solution of the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0020] Referring to the attached drawings, a triaxial apparatus for testing the volume change of unsaturated soil specimens includes a gas storage vessel 1 and a triaxial apparatus 2. The gas storage vessel 1 is cylindrical, with a first interface 1.1 provided at its top and a second interface 1.2 provided at its bottom. The first interface at the top of the gas storage vessel is connected to the specimen exhaust hole of the triaxial apparatus through a pipeline 3, and the second interface at the bottom of the gas storage vessel is connected to the back pressure controller of the triaxial apparatus.

[0021] The following specifically introduces the testing method of this device:

[0022] Step 1: Install an unsaturated soil specimen on the triaxial apparatus, add a certain volume of degassed water into the gas storage vessel, and keep the water level lower than the first interface to prevent water from flowing back into the specimen; the volume of degassed water is preferably 1 / 2 of the total volume of the gas storage vessel. For example, the height of the degassed water in the gas storage vessel is 10 cm, and the head pressure is about 10 kPa.

[0023] Step 2: Connect the first interface of the gas storage vessel to the lower exhaust hole of the triaxial apparatus through a pipeline, and connect the second interface of the gas storage vessel to the back pressure controller of the triaxial apparatus.

[0024] Step 3: Set the pressure of the back pressure controller slightly greater than the head pressure in the gas storage vessel (the pressure of the back pressure controller is 1 kPa - 5 kPa greater than the head pressure in the gas storage vessel). At this time, the pressure of the gas in the specimen is the difference between the pressure of the back pressure controller and the head pressure in the gas storage vessel. For example, if the pressure of the back pressure controller is 15 kPa, then the gas pressure in the specimen is 15 - 10 = 5 kPa.

[0025] Step 4: Start the triaxial apparatus and conduct a compression test on the unsaturated soil specimen to expel the gas in the pores of the unsaturated soil specimen. During the test, keep the temperature constant and keep the pressure of the back pressure controller constant. At this time, the volume of the inlet and outlet gas in the pores of the specimen can be approximately equal to the change in the water volume in the gas storage vessel, and the change in water volume is measured and automatically recorded by a high-precision back pressure controller.

[0026] Under the condition that the total head pressure (15 kPa, that is, the pressure of the back pressure controller) remains constant, during the test, the water level in the gas storage vessel changes, so the gravitational head will change, and the gas pressure in the gas storage vessel will also change. Therefore, there is a certain error in approximately using the change in water volume to represent the volume change of the specimen as described above. The error analysis is as follows:

[0027] I. Test conditions

[0028] 1) The specimen has a diameter of 3.91 cm, a height of 8 cm, a volume of 96 mL, and a porosity of 0.5 (relatively large). Therefore, the gas volume in the specimen is 48 mL.

[0029] 2) The inner diameter of the gas storage vessel is 8 cm, the height is 20 cm, the water storage is 10 cm, and the gas storage volume is 3.14 * 4 2 * 10 = 502.4 mL. The gas in the pipeline (i.e., the pipeline between the first interface of the gas storage vessel and the sample exhaust hole of the triaxial apparatus) is about 20 mL. The total gas volume = 48 + 502.4 + 20 = 570.4 mL.

[0030] 3) The laboratory temperature is 20 °C, and the atmospheric pressure is 1 standard atmospheric pressure (101.33 kPa).

[0031] According to the maximum error analysis, when the sample volume changes by 50%, that is, when the exhausted sample gas reaches 48 mL, it is used as the measurement error of the system.

[0032] II. Analysis process

[0033] 1) Calculation basis and gas constant. According to pV = nRT (Clapeyron equation), under standard conditions (0 °C, 101.33 kPa), the volume occupied by 1 mole of any ideal gas is about 22.4 L, and the molar volume of the gas is 22.4 L / mol. The constant R = 101.33 × 10 3 Pa × 22.4 L / (1.0 mol × 273 k) = 8314.256 (Pa·L) / (mol·k)

[0034] 2) The amount of gas in the gas storage vessel, sample, and pipeline. At a general laboratory temperature (20 °C, the standard temperature is 273 + 20 = 293 k) and 1 standard atmospheric pressure, the volume of 1.0 mol of gas = 1 × 8314.256 × 293 / (101.33 × 10 3 ) = 24.041 L = 24041 mL. Then the number of moles of the above gas n = 570.4 / 24041 = 0.02373 mol.

[0035] 3) Gas pressure change. When the inner diameter (radius) of the gas storage vessel is 4 cm and the sample volume changes by 40 mL, if the gas pressure remains unchanged, the water level drops / rises by 48 / (3.14 × 4 2 ) = 0.955 cm, that is, the water head changes by 1 cm, corresponding to a change in the gravity water head pressure of ρgh = 100 Pa. Since the total water head remains unchanged (the water pressure of the back pressure controller is 15 kPa), the actual gas pressure change is 100 Pa. At this time, the measured water volume of the back pressure controller is caused by the rise and fall of the water level and is 48 mL.

[0036] 4) Gas volume change and error caused by pressure change. At 20 °C (standard temperature 273 + 20 = 293 k), the volume change of the gas caused by the pressure change is

[0037] dv = -dp / p2 ×nRT = -100 / (1.05×10 5 ) 2 ×n×R×(273 + 20) = 5.243*10 -4 L = -0.524 mL (The negative sign indicates that the volume decreases due to the increase in pressure)

[0038] That is, the volume change is 0.524 mL. Therefore, the maximum measurement error caused is 0.524 / 48 = 1.09%.

[0039] III. Analysis Conclusion

[0040] Considering the maximum volume change of a conventional triaxial specimen is 50%, the specimen volume changes by 48 mL, and the measurement error of the gas volume change in the above system is 1.09%. It can be approximately considered that the change in water volume is the same as the change in specimen volume.

[0041] Note: Generally, the size of a triaxial specimen is a diameter of 3.91 cm and the height is 2 - 2.5 times the diameter, such as 8.0 cm. If a large specimen is used, the size of the gas storage dish should be appropriately increased.

[0042] The above is an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification, or equivalent substitution that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A testing method for a triaxial apparatus for testing volume change of unsaturated soil specimens, characterized in that: The triaxial apparatus includes an air storage vessel and a triaxial apparatus. The air storage vessel is cylindrical, with a first interface provided at the top and a second interface provided at the bottom. The first interface at the top of the air storage vessel is connected to the specimen exhaust hole of the triaxial apparatus through a pipeline, and the second interface at the bottom of the air storage vessel is connected to the back pressure controller of the triaxial apparatus; The testing method includes the following steps: Step 1: Install an unsaturated soil specimen on the triaxial apparatus, add a certain volume of degassed water into the air storage vessel, and keep the water level lower than the first interface to prevent water from flowing back into the specimen; Step 2: Connect the first interface of the air storage vessel to the lower exhaust hole of the triaxial apparatus through a pipeline, and connect the second interface of the air storage vessel to the back pressure controller of the triaxial apparatus; Step 3: Set the pressure of the back pressure controller slightly greater than the water head pressure in the air storage vessel. At this time, the pressure of the gas in the specimen is the difference between the pressure of the back pressure controller and the water head pressure in the air storage vessel; Step 4: Start the triaxial apparatus and conduct a compression test on the unsaturated soil specimen to discharge the gas in the pores of the unsaturated soil specimen. During the test, keep the temperature constant and keep the pressure of the back pressure controller constant. At this time, the volume of the in-and-out gas in the pores of the specimen is approximately equal to the change in the water volume in the air storage vessel. By detecting the change in the water volume in the air storage vessel, the change in the volume of the void gas in the unsaturated soil specimen can be obtained; In Step 1, the volume of degassed water is preferably 1 / 2 of the total capacity of the air storage vessel; The pressure of the back pressure controller is 1 kPa to 5 kPa greater than the water head pressure in the air storage vessel.

2. The testing method for a triaxial apparatus for testing volume change of unsaturated soil specimens according to claim 1, characterized in that: The change in the water volume in the air storage vessel is measured and automatically recorded by a high-precision back pressure controller.

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