Preparation and removal method of sandy soil sample connected to three-dimensional bending element probe

Abstract

The present invention belongs to the technical field of bending element testing in geotechnical tests, and specifically relates to a method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe. The preparation method includes: evacuating the space between the rubber membrane and the inner wall of the assembled mold tube, and a vertical bending element probe is provided at the bottom of the assembled mold; adding the sand required for the sample into the mold tube in batches, and at the same time adding two pairs of horizontal bending element probes into the mold tube, and compacting the sand layer by layer; adjusting the flatness of the top of the sample, and providing another vertical bending element probe on the top of the sample; saturating the sample to complete the preparation. The removal method includes: removing the confining pressure chamber of the sample after consolidation and bending element testing; collecting and weighing the sand sample; and drying and weighing the collected sand sample. The sand sample connected to the three-dimensional bending element probe prepared by the present invention has high uniformity, which can greatly save test time and improve test efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of bending element testing in geotechnical tests, and in particular relates to a method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe. Background Art

[0002] Among the many mechanical parameters of soil, the small strain shear modulus (G0 or G max ) is an important mechanical index to characterize soil in the small strain range (usually 0.0001%-0.1%), and is often used in soil stability evaluation and numerical simulation. Soil may cause anisotropy during sedimentation or loading, which may lead to the change of soil small strain shear modulus in the horizontal direction (G max,hh ) and vertical direction (G max,vh and G max,hv ) appear different, and accurately describing the anisotropy of the small strain shear modulus of soil is crucial in the analysis of engineering problems mainly based on deformation considerations, such as seismic resistance of underground structures, soil-structure interaction, and deformation prediction of deep foundation pits or tunnel excavations.

[0003] To test the small-strain shear stiffness of sand samples in different directions and thus determine the degree of anisotropy, a triaxial testing system is typically used in conjunction with a three-axis bending element testing system to measure the shear wave velocity in both the horizontal and vertical directions, thereby obtaining the shear modulus in the horizontal and vertical planes. For cohesive soils, the specimen can be prefabricated, with the bending element fixing cap attached to a rubber membrane. After the rubber membrane is applied to the specimen, the horizontal bending element probe is inserted into the specimen and sealed with a rubber ring. However, for sandy soil, it is necessary to prepare samples directly on the base of the triaxial instrument using the currently common pure cylindrical two-petal or three-petal membrane. The rubber membrane cannot be pasted with the bending element fixing cap in advance, and it is difficult to place the bending element probe after the sample preparation is completed; or a hole is dug on the surface of the sample maker, and two pairs of openings are cut in the middle of the rubber membrane to provide space for the bending element fixing cap in advance. Then, liquid rubber is used to fix the bending element sensor on the sample and seal the opening. However, this method cannot achieve vacuum to ensure that the rubber membrane is in close contact with the sample maker, resulting in uneven sample density and prone to necking at the top of the sample, which in turn affects the accuracy and reliability of the test data; and when dismantling samples for ordinary triaxial tests, the samples are generally dismantled directly without performing a final moisture content test, and the test accuracy cannot be verified by combining the data before and after the test. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe, so as to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above object, the present invention provides a method for preparing a sandy soil sample connected to a three-dimensional bending element probe, comprising the following steps:

[0006] Step 1: Assembling the molding die;

[0007] Step 2: Vacuum the space between the rubber film and the inner wall of the mold tube through the vacuum hole to make the rubber film fit tightly against the inner wall of the mold tube;

[0008] Step 3: Add the sand and soil required for the sample into the mold cylinder in batches, compact the sand and soil layer by layer, record the initial mass of the sample, and install the bending element probe at the same time;

[0009] Step 4: Adjust and check the flatness of the sample;

[0010] Step 5: Measure the diameter and height of the sample, calculate the initial volume of the sample, and complete the preparation of the sandy soil sample;

[0011] Step 6: Saturate the sample.

[0012] Preferably, the molding mold includes a three-axis pressure chamber base, a rubber membrane and four arc-shaped mold pieces, the four mold pieces constitute a mold cylinder with a circular cross-section, the side of any mold piece is provided with a semi-circular arc-shaped protrusion, the protrusions of two adjacent mold pieces are spliced ​​together to form a horizontal bending element channel with a cylindrical structure, an exhaust hole is opened on one mold piece, the rubber membrane is provided with four ears along the radial direction, the ears are provided corresponding to the horizontal bending element channel, the rubber membrane is provided in the mold cylinder, the top end of the three-axis pressure chamber base is inserted into the bottom end of the mold cylinder, and the three-axis pressure chamber base is sealed with the bottom end of the mold cylinder through the rubber membrane and the O-ring.

[0013] Preferably, in step 1, a negative pressure of 80 kPa is applied through the exhaust hole on the mold connected to a vacuum pump.

[0014] Preferably, in step 3, the mass of the soil samples in each layer is the same, and the height of the drop of the same compacting hammer and the number of hammer blows in each layer are kept the same; a sleeve is placed on the top of the mold cylinder before compacting the last layer of soil sample; after each layer of sample is compacted, the surface of the sand is scraped before proceeding to the next compaction step.

[0015] Preferably, in step 3, the installed bending element probes include a pair of vertical bending element probes and two pairs of horizontal bending element probes.

[0016] Preferably, before adjusting and inspecting the flatness of the top of the sample, a negative pressure of 20 kPa is applied to the sample by a vacuum pump.

[0017] Preferably, in step 6, saturating the sandy soil sample comprises the following steps:

[0018] Step 1: Install the confining pressure chamber and fill the confining pressure chamber with water and exhaust the air;

[0019] Step 2: Adjust the internal pressure of the sample to 0 kPa, and stabilize the external confining pressure of the sample to 20 kPa. Saturate the sample with CO2, water head, and back pressure. When the pore pressure coefficient B value is greater than 0.95, the sample is considered to be fully saturated.

[0020] A method for removing a sandy soil sample connected to a three-dimensional bending element probe, characterized by comprising the following steps:

[0021] Step 1: dismantle the confining pressure chamber of the specimen after the consolidation and bending element tests;

[0022] Step 2: collecting and weighing the sand sample;

[0023] Step 3: Dry the collected sand and soil samples and weigh them.

[0024] Preferably, in step 2, the confining pressure chamber is dismantled, and the water outside the triaxial pressure chamber base and the rubber membrane is wiped dry.

[0025] Preferably, in step 3, the collected samples include sand samples and the residual soil samples on the rubber membrane, the bending element probe, the permeable stone and the filter paper.

[0026] Compared with the prior art, the present invention has the following advantages and technical effects:

[0027] The present invention can ensure that the rubber mold can be tightly attached to the inner wall of the mold tube during the sample preparation process, minimize the necking phenomenon at the top of the sample, and ensure the uniformity of the sample; a horizontal bending element can be installed during the sample preparation process, greatly reducing the difficulty of installing the bending element and saving test time; after the sample preparation is completed, a negative pressure of 20kPa is applied to the sample in advance to ensure that the sample can stand upright and reduce the disturbance of the initial structure of the sample by subsequent operations; after the mold tube is removed, the diameter and height of the sample are measured using a vernier caliper to more accurately obtain the initial porosity of the sample; before the sample is saturated, the internal pressure and confining pressure of the sample are adjusted to reduce the influence of the internal and external pressure difference on the initial structure of the sample; and after the test is completed, the initial porosity of the sample is calculated by back-calculating the final moisture content of the sample, and the initial porosity calculated by two different methods is compared to verify the accuracy and precision of the test. The technical method of the invention can be used to prepare a triaxial sample with high uniformity connected to a three-dimensional bending element probe, which can greatly save test time, improve test efficiency, and ensure test accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive work.

[0029] Figure 1 It is a cross-sectional view of the present invention when using the forming die and sleeve combination to prepare the sample according to the layered compaction method;

[0030] Figure 2 This is a schematic diagram of the structure of the forming die and sleeve combination used in the present invention;

[0031] Among them: 1. Triaxial pressure chamber base; 2. Pressure chamber drain valve; 3. Back pressure valve; 4. Confining pressure valve; 5. O-ring; 6. Permeable stone; 7. Hose clamp; 8. Mould; 9. Rubber membrane; 10. Exhaust hole; 11. Sleeve; 12. Horizontal bending element channel; 13. Vertical bending element probe. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] This embodiment takes a triaxial dry sand sample with a diameter of 50 mm and a height of 100 mm commonly used in a laboratory and connected to a three-dimensional bending element probe as an example to illustrate the technical solution of the present invention.

[0035] The method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe according to the present invention requires the following auxiliary devices in addition to conventional test auxiliary parts such as a permeable stone 6 with a circular hole in the middle, filter paper, a compacting hammer, a water-absorbing ball, silicone ester, a vernier caliper, a vertical bending element probe, and a horizontal bending element probe:

[0036] 1) If Figures 1 to 2As shown, the molding mold includes a triaxial pressure chamber base 1, a rubber membrane 9 and four arc-shaped mold pieces 8. The four mold pieces 8 constitute a mold cylinder with a circular cross-section. The side of any mold piece 8 is provided with a semicircular arc-shaped protrusion. The protrusions of two adjacent mold pieces are spliced ​​together to form a horizontal bending element channel 12 with a cylindrical structure. An exhaust hole 10 is opened on one mold piece. The rubber membrane 9 is provided with four ears along the radial direction. The ears are provided corresponding to the horizontal bending element channel 12. The rubber membrane 9 is provided in the mold cylinder, and the top end of the triaxial pressure chamber base 1 is inserted into the bottom end of the mold cylinder, and the triaxial pressure chamber base 1 is sealed with the bottom end of the mold cylinder through the rubber membrane 9 and the O-ring 5; the height of the rubber membrane 9 is slightly larger than the height of the mold piece 8, and the diameter of the ear is equal to the inner diameter of the horizontal bending element channel 12. The rubber membrane 9 is integrally formed, which can ensure the sealing of the sample and improve the efficiency of sample preparation.

[0037] 2) The confining pressure chamber and the vacuum pump with adjustable pressure are both commercially available, and the operating principle is based on existing technology.

[0038] Reference Figures 1 to 2 As shown, the method for preparing a sandy soil sample using the above-mentioned forming mold and auxiliary parts includes the following steps:

[0039] Step 1. Clean the base 1 of the triaxial pressure chamber, evenly apply an appropriate amount of silicone ester on the side of the base 1 of the triaxial pressure chamber, then place the permeable stone 6 with a round hole in the middle and the filter paper in sequence, use a vernier caliper to measure the thickness of the special rubber membrane 9 with four ears, measure once at each end, then put the rubber membrane 9 on the base 1 of the triaxial pressure chamber, and tighten it with the O-ring 5; put the four ears of the rubber membrane 9 on the four horizontal curved channels 12 on the mold cylinder respectively, and fix them with small O-rings.

[0040] Step 2: Connect a vacuum pump through the exhaust hole 10 to apply a negative pressure of 80 kPa to the space between the mold tube and the rubber film 9, so that the rubber film 9 is tightly attached to the inner wall of the mold tube.

[0041] Step 3: Prepare the sample using a layered compaction method. A certain mass of sand and soil is added to the mold cylinder in five batches and evenly tapped with a compaction tool. To ensure uniformity, the mass and height of each layer are maintained as much as possible, while the height and number of hammer strikes are kept consistent. After each layer is compacted, the surface of the sand and soil is scraped before proceeding to the next compaction step to ensure sample continuity and prevent stratification. When the sample is installed close to the horizontal bending element channel 12 of the mold, place the bending element probe on the four ears of the rubber membrane 9, and put the four ears on the rubber membrane 9 on the bending element probe respectively, and fasten it with a small O-ring, then pour a certain mass of sample, and compact the next layer of sample. After the last layer of sample is compacted, place a permeable stone on the top of the sample, a top cap with a bending element, turn up the rubber membrane, and fix it on the top cap of the sample with a small O-ring, and record the initial mass m1 of the sample, adjust the vacuum pump pressure to 20kPa, connect the vacuum pump to the back pressure channel of the base 1 of the triaxial pressure chamber, apply a negative pressure of 20kPa to the sample, keep the sample upright, and reduce the disturbance of the subsequent operations to the sample.

[0042] Step 4: Place the micro level on top of the sample and check and fine-tune the flatness of the sample by observing the vial.

[0043] Step 5. After the sample is leveled, loosen the upper and lower throat clamps of the mold barrel and remove the four mold pieces 8 around the sample; use a vernier caliper to measure the diameters of the sample at the top, middle, and bottom positions respectively, take the average value and subtract twice the thickness of the rubber film 9, which is the true diameter (d) of the sample; and take the average value (h) of the height on both sides of the sample, which is the true height of the sample, and calculate the initial volume of the sample, where the true diameter of the sample is the average value of the diameter minus twice the thickness of the rubber film.

[0044] Step 6: Install the confining pressure chamber. After the confining pressure chamber is filled with water and the bubbles are discharged, close the exhaust valve at the top of the confining pressure chamber.

[0045] The present invention describes a method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe. In order to reduce the impact of the pressure unloading process on the initial structure of the sample, the following gradual pressure unloading method is adopted: first, close the back-pressure valve 3 and the confining pressure valve 4, adjust the vacuum pump pressure to 10kPa, connect the confining pressure valve through the outlet pipe of the three-axis confining pressure controller, and adjust the confining pressure to 10kPa. After the confining pressure stabilizes, open the back-pressure valve 3 and the confining pressure valve 4 at the same time. At this time, the negative pressure inside the sample is reduced to 10kPa, the confining pressure outside the sample is 10kPa, and the overall pressure of the sample is still 20kPa; after the internal and external pressures of the sample stabilize, close the back-pressure valve 3 and the confining pressure valve 4 again, turn off the vacuum pump, and adjust the confining pressure to 20kPa. After the confining pressure stabilizes, open the back-pressure valve and the hole pressure valve connected to the inside of the sample. Finally, the internal pressure of the sample is 0, and the external confining pressure of the sample is 20kPa.

[0046] Then, according to the conventional triaxial operation, the sample is saturated with CO2, water head and back pressure until the pore pressure coefficient B value is greater than 0.95. The sample is considered to be fully saturated, that is, the preparation of the sandy soil sample connected to the three-dimensional bending element probe is completed, and then consolidation and bending element tests can be carried out.

[0047] The present invention provides a method for removing a sandy soil sample connected to a three-dimensional bending element probe, comprising the following steps:

[0048] Step 1: After the consolidation and bending element tests are completed, close the backpressure valve and unload the confining pressure and backpressure in sequence. Drain the confining pressure chamber through pressure chamber drain valve 2, dismantle the confining pressure chamber, and wipe dry the specimen base and rubber membrane.

[0049] Step 2: Prepare a soil sample bowl and weigh the bowl (m2). Remove the sample, along with the permeable stone and rubber membrane, from the base, minimizing the amount of sample falling from the rubber mold. Place the sample into the bowl, remove the permeable stone and filter paper, and weigh the wet mass of the sample (m3).

[0050] Step 3: Place the soil sample in the bowl into the oven for drying and record the mass of the soil sample after drying (m4). f =(m3-m4) / (m4-m2).

[0051] Evaluation of the test accuracy of a triaxial specimen connected to a three-axis bending probe. The specific method is as follows:

[0052] The present invention adopts two methods to verify the accuracy of the test, taking the dry sand sample as an example. First, the initial specific volume v of the sample is calculated using formulas (1) and (2) respectively. i The accuracy of the test is verified by comparing the difference between the two and the average value. Generally, when the difference is within ±0.02, it means that the test accuracy is high and the test data is true and reliable.

[0053]

[0054] Among them, v i is the initial specific volume of the sample; G s is the particle density of the sand sample, which is generally 2.65 for quartz sand; γ w is the bulk density of water, 9.8kN / m 3 ; γ di is the initial dry bulk density of the sample; w f is the final moisture content of the sample; ε vol It is the volume change of the sample during the consolidation process and can be obtained by the back pressure volume controller of the triaxial apparatus.

[0055] In the description of the present invention, it should be understood that the terms "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention, 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 cannot be understood as a limitation on the present invention.

[0056] The above are only preferred specific implementation methods of the present application, but the scope of protection of the present application is not limited thereto. Any changes or replacements that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed in this application should be covered by the scope of protection of the present application.

Claims

1. A method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe, characterized in that: The following steps are involved: Step 1: Assembling the molding die; In step 1, the forming mold includes a triaxial pressure chamber base, a rubber membrane, and four arc-shaped mold pieces. The four mold pieces constitute a mold cylinder with a circular cross-section. The side of any mold piece is provided with a semicircular arc-shaped protrusion. The protrusions of two adjacent mold pieces are spliced ​​together to form a horizontal bending element channel with a cylindrical structure. An exhaust hole is opened on one mold piece. The rubber membrane is provided with four ears along the radial direction. The ears are provided corresponding to the horizontal bending element channels. The rubber membrane is provided in the mold cylinder. The top end of the triaxial pressure chamber base is inserted into the bottom end of the mold cylinder. The triaxial pressure chamber base is sealed with the bottom end of the mold cylinder through the rubber membrane and an O-ring. Step 2: Vacuum the space between the rubber film and the inner wall of the mold cylinder through the vacuum hole; Step 3: Add the soil samples required for the sample into the mold cylinder in batches, complete the compaction of the soil samples layer by layer and record the initial mass of the samples, and install the bending element probe at the same time; In step 3, the installed bending element probes include a pair of vertical bending element probes and two pairs of horizontal bending element probes; Step 4: Adjust and check the flatness of the top of the sample; Step 5: Measure the diameter d and height h of the sample and calculate the initial volume v1 of the sample; Step 6: Saturate the sample, complete the preparation of the sandy soil sample and perform consolidation and bending element tests; In step 6, saturating the sandy soil sample comprises the following steps: Step 1) Install the confining pressure chamber and fill the confining pressure chamber with water and exhaust the air; Step 2) Adjust the internal pressure of the sample to 0 kPa, stabilize the external confining pressure of the sample to 20 kPa, and perform CO2 saturation, water head saturation, and back pressure saturation on the sample. When the pore pressure coefficient B value is greater than 0.95, the sample is saturated; The soil sample removal method comprises the following steps: Step a: After the consolidation and bending element tests, the confining pressure chamber is removed from the specimen, and the water outside the triaxial pressure chamber base and the rubber membrane is wiped dry; Step b: Prepare a soil sample bowl and weigh the bowl m2; remove the sample from the base together with the bottom permeable stone and rubber membrane, and try to minimize the scattering of the sample inside the rubber mold during this process, and put them into the bowl together, remove the permeable stone and filter paper, and weigh the wet weight of the sample m3; Step c, drying the sample collected in step b and weighing it, and recording the mass m4 of the dried soil sample; The experimental accuracy evaluation method is as follows: Where m1 is the initial mass of the sample; v i is the initial specific volume of the sample; G s is the particle density of the sand sample, which is generally 2.65 for quartz sand; γ w is the bulk density of water, 9.8kN / m 3 ; γ di is the initial dry bulk density of the sample; w f is the final moisture content of the sample; ε vol is the volume change of the sample during the consolidation process, which is obtained by the back pressure volume controller of the triaxial apparatus; the initial specific volume v of the sample is calculated using formulas (1) and (2) respectively i , and the accuracy of the test is verified by comparing the difference between the two and the average value.

2. The method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe according to claim 1 is characterized in that: In step 2, a negative pressure of 80 kPa is applied by connecting the exhaust hole on the mold to a vacuum pump.

3. The method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe according to claim 1 is characterized in that: In step 3, the mass of the soil samples of each layer is the same, and the height of the compacting hammer and the number of hammer blows are kept the same for each layer. Before the last layer of soil sample is compacted, a sleeve is placed on the top of the mold tube. After each layer of sample is compacted, the surface of the soil sample is scraped before the next step of compaction.

4. The method for preparing and removing a sandy soil sample connected to a three-dimensional bending element probe according to claim 1 is characterized in that: Before adjusting and inspecting the flatness of the top of the sample, a negative pressure of 20 kPa is applied to the sample.

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