Hollow cylinder torsional shear apparatus-based unit test method for gravel pile composite foundation

Abstract

The application discloses a kind of gravel pile composite foundation unit test method based on hollow cylinder torsion shear instrument.The method uses hollow cylinder torsion shear instrument device, the boundary condition of soil between piles of gravel pile composite foundation is simulated by the method that gravel sample is prepared between the inner rubber film and filter layer of the device, hollow cylinder torsion shear test is carried out on this basis, so as to obtain the dynamic characteristics of gravel pile and soil around pile under real earthquake action.The application can well simulate the compaction effect and drainage effect of gravel pile treatment and realize complex dynamic stress condition, more accurately obtain the static and dynamic characteristics and strength index of soil between piles of gravel pile composite foundation.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical laboratory testing methods, and in particular relates to a test method for a composite foundation unit of crushed stone piles based on a hollow cylindrical torsion shear apparatus. Background Technology

[0002] Crushed stone piles are an easy-to-construct, economical, and reliable method for treating soft and liquefiable foundations, and are widely used in civil buildings, airport runways, highway embankments, and reservoir dams.

[0003] Composite foundations made of crushed stone piles, commonly used in engineering projects, exhibit significant differences in scale (coarse and fine), often requiring multi-scale methods for research. The coarse scale represents the entire site, reflecting characteristics such as failure and deformation; the fine scale represents the scale of the crushed stone piles and surrounding soil within a volumetric unit, showcasing the deformation and failure characteristics of the piles or soil. Current research at the fine scale primarily involves indoor unit tests on the crushed stone piles or surrounding soil: dynamic triaxial apparatus can only simulate horizontal shear during earthquakes by applying dynamic deviatoric stress on a 45° inclined plane of the sample; dynamic torsion shear apparatus can only simulate the stress state under pure shear by applying torque.

[0004] The hollow cylinder test apparatus can apply rotational displacement and torque to soil, control the magnitude and direction of the three principal stresses, and achieve various complex stress conditions, including rotation of the principal stress axes. It is currently the most advanced geotechnical testing equipment for studying soil deformation and stability in engineering projects. However, traditional hollow cylinder specimens are tall and thin-walled, resulting in long drainage paths, which do not match the actual drainage paths of the soil between piles in a gravel pile composite foundation. In actual gravel pile composite foundations, there is a compaction effect that increases the density of the soil between piles, and the gravel piles act as good drainage channels, shortening the seepage path and accelerating the dissipation of excess pore pressure. Current technology lacks a unit-based testing method for gravel pile composite foundations based on a hollow cylinder torsion-shear apparatus that can overcome the above problems and comprehensively consider the dynamic characteristics of the soil between piles under complex stress paths under drainage and compaction effects. Summary of the Invention

[0005] To address the problems existing in the background technology, the purpose of this invention is to design a test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus, which can simulate the drainage and compaction effects of crushed stone pile treatment, realize complex dynamic stress paths, and more accurately obtain the static and dynamic characteristics and strength indicators of the soil between the piles in the composite foundation.

[0006] The technical solution of this invention is as follows:

[0007] The method of the present invention uses a hollow cylindrical torsion shear device, which includes a base, a gasket, a retaining ring, a filter layer sealing ring, an inner membrane sealing ring, a lower permeable stone, an annular inner rubber membrane, an annular filter layer, an outer rubber membrane, a three-lobed template, a clamp, a top cover locator, an annular upper permeable stone, a stainless steel top cover, and a cap.

[0008] The base is placed on the bottom surface of the external pressure chamber. A ring-shaped stainless steel top cover is located above the base. The outer circumference of the stainless steel top cover and the outer circumference of the base are connected by a ring-shaped three-lobed template. A cap is placed above the stainless steel top cover. A ring-shaped clamp is fitted on the outer wall of the middle part of the three-lobed template. The outer rubber membrane is fixedly connected to the inner wall of the three-lobed template. A top cover positioner is fixedly connected to the outer wall of the three-lobed template. A groove is provided in the middle of the base. A U-shaped gasket is placed in the groove of the base. The bottom end of the filter layer is set between the side wall of the groove and the gasket, and the outer wall of the filter layer is tightly attached to the groove. The filter layer sealing ring is set between the inner wall of the bottom end of the filter layer and the gasket. The filter layer and the outer rubber membrane are coaxially arranged, and the annular cavity between the filter layer and the outer rubber membrane is used as the device. The soil sample layer is used to place soil samples. The annular lower permeable stone is fixed on the upper surface of the outer periphery of the base and is located at the bottom of the soil sample layer. The retaining ring is placed at the opening of the gasket. The bottom end of the inner rubber membrane is set between the inner side wall of the gasket and the retaining ring, and the outer side wall of the inner rubber membrane is in close contact with the gasket. The inner membrane sealing ring is set between the inner side wall of the bottom end of the inner rubber membrane and the retaining ring. The cavity in the middle of the inner rubber membrane serves as the water storage cavity of the device. The inner rubber membrane and the reverse filter layer are arranged coaxially, and the annular cavity between the inner rubber membrane and the reverse filter layer serves as the gravel drainage layer of the device. The gravel drainage layer is used to place gravel samples. The inner and outer peripheries of the upper permeable stone are fixed on the upper surfaces of the gravel samples and the soil samples, respectively, and the upper surface of the upper permeable stone is in contact with the lower surface of the stainless steel top cover.

[0009] The method includes the following steps:

[0010] Step 1: Prepare soil samples using a hollow cylindrical torsion shear apparatus.

[0011] Step 2: Prepare a gravel sample between the inner rubber membrane and the filter layer of the device to complete the preparation of the hollow cylindrical sample;

[0012] The hollow cylindrical specimens include soil samples and gravel samples;

[0013] Step 3: Complete the installation of the device;

[0014] Step 4: Place the device in the external pressure chamber, remove the three-lobed template from the device, then close the pressure chamber and saturate the hollow cylindrical sample.

[0015] Step 5: After the hollow cylindrical sample is saturated, the sample is consolidated by applying inner and outer pressures.

[0016] Step 6: Apply dynamic load to the hollow cylindrical specimen using a dynamic hollow cylindrical torsion shear apparatus to simulate and calculate the dynamic characteristics of the specimen under seismic action, thereby realizing the analysis and calculation of the dynamic characteristics of the crushed stone pile and the surrounding soil under real seismic action.

[0017] Step 1 specifically includes:

[0018] Step 11: First, assemble the filter layer sealing ring and gasket. Wet the bottom of the filter layer with water and attach the bottom of the filter layer to the side wall of the base groove. Place the assembled filter layer sealing ring and gasket in the groove of the base so that the filter layer is tightly attached to the side wall of the groove, and at the same time align the holes in the gasket with the holes on the base.

[0019] Step 12: Assemble the inner membrane sealing ring and the retaining ring. Wet the bottom end of the inner rubber membrane with water and attach the bottom end of the inner rubber membrane to the inner side wall of the gasket. Place the assembled inner membrane sealing ring and retaining ring into the opening of the gasket so that the inner rubber membrane is tightly attached to the inner side wall of the gasket. At the same time, align the hole in the retaining ring with the hole on the gasket. Finally, use bolts to fix the base, gasket and retaining ring together.

[0020] Step 13: Place the second compaction shaft in the cavity of the inner rubber membrane, and place the first compaction shaft in the annular cavity between the inner rubber membrane and the filter layer;

[0021] Step 14: Place the permeable stone at the bottom of the filter layer and fix it to the outer periphery of the base, and attach filter paper to the upper surface of the permeable stone.

[0022] Step 15: Install the three-lobed template and clamps:

[0023] An outer rubber membrane is fitted inside the inner wall of the three-lobed template. The height of the outer rubber membrane is greater than the height of the three-lobed template and the outer rubber membrane is tightly attached to the three-lobed template. The part of the upper end of the outer rubber membrane that extends beyond the three-lobed template is turned outward. The part of the lower end of the outer rubber membrane that extends beyond the three-lobed template is turned outward and tightly attached to the three-lobed template. Then, the lower end of the outer rubber membrane and the three-lobed template are fixedly connected by an external outer membrane sealing ring. The three-lobed template with the outer rubber membrane is installed on the base, and a clamp is fitted on the outer wall of the middle part of the three-lobed template.

[0024] Step 16: Place soil into the annular cavity between the outer rubber membrane and the filter layer, and then compact the soil in layers to complete the preparation of the soil sample.

[0025] Step 17: Water is introduced into the soil sample through the water inlet in the base to pre-saturate the soil sample and expel the gas inside the soil sample.

[0026] Step 2 specifically refers to:

[0027] Step 21: Remove the first compacted central shaft from the crushed stone drainage layer of the device;

[0028] Step 22: Fill the crushed stone drainage layer with crushed stone to complete the preparation of the crushed stone sample;

[0029] Step 23: Remove the second compaction shaft b17 from the water storage chamber of the device;

[0030] Step 3 specifically includes:

[0031] Step 31: Fix the bottom of the top cover locator to the base, and then place the permeable stone on top of the sample. The sample mainly consists of soil sample and gravel sample, and the upper surfaces of the soil sample and gravel sample are flush. Place the stainless steel top cover on the permeable stone.

[0032] Step 32: Fill the water storage chamber with de-aired water, then fold the part of the inner rubber membrane that extends beyond the stainless steel top cover outwards and press it tightly against the stainless steel top cover. Then, use a rubber ring to fix the upper end of the inner rubber membrane and the stainless steel top cover together.

[0033] Step 33: Fix the cap on the stainless steel top cover, then remove the top cover locator in the device, straighten the part of the outer rubber membrane that extends beyond the three-petal template, and then fix the upper end of the outer rubber membrane and the stainless steel top cover with a rubber ring.

[0034] Step 6 specifically includes:

[0035] Step 61: Apply dynamic load to the specimen continuously using a dynamic hollow cylindrical torsion shear apparatus, and observe the position and morphology of the fracture surface in the specimen in real time during the application of dynamic load. At the same time, use a data acquisition system to monitor the plastic strain, dynamic strength and pore pressure of the specimen in real time. Stop the test when the specimen fails.

[0036] The dynamic loads include a sinusoidal vertical force W and a sinusoidal torque M. T ;

[0037] Step 62: Perform sample unloading operation:

[0038] Open the inlet valve in the pressure chamber and reduce the dynamic load to zero. Then, open the drain valve and air vent valve in the pressure chamber at the same time to drain the pressure chamber. After the water in the pressure chamber is drained, open the pressure chamber, loosen the screws in the device and drain all the water in the water storage chamber. The test is now over.

[0039] By analyzing the dynamic characteristics of the samples obtained during the experiment, the dynamic characteristics of the crushed stone pile and the surrounding soil under real earthquake action can be simulated and calculated.

[0040] In step 13, both the first and second compaction shafts are made of rigid plastic. The first compaction shaft is a hollow cylindrical structure, while the second compaction shaft is a solid cylindrical structure.

[0041] In step 13, the surfaces of the first and second compaction shafts are uniformly coated with talcum powder.

[0042] During soil sample preparation, the hollow cylindrical specimen is divided into two layers: an outer layer of test soil and an inner layer of drainage material. A permeable geotextile separates the soil sample and the drainage material, allowing the inner drainage material to form vertical drainage channels, thus simulating the drainage effect of the gravel pile treatment. A hollow cylindrical torsion shear apparatus is used to apply inner and outer confining pressures to the hollow cylindrical specimen, simulating the compaction effect of the gravel pile treatment on the soil between the piles. The magnitude of the confining pressure is obtained through finite element numerical simulation. A torque is applied using the hollow cylindrical torsion shear apparatus to simulate the horizontal shearing action of an earthquake on the specimen's horizontal plane. Simultaneously, by changing the inner and outer confining pressures, axial force, and torque, various complex dynamic stress conditions such as principal stress axis rotation and seismic loads are achieved, thus simulating the stress path and stress state of the gravel pile composite foundation under dam construction, water storage, and earthquake conditions. This invention can effectively simulate the compaction and drainage effects of gravel pile treatment and achieve complex dynamic stress conditions, more accurately obtaining the static and dynamic characteristics and strength indicators of the soil between the piles in the gravel pile composite foundation.

[0043] The present invention has the following beneficial effects:

[0044] 1. This invention provides a test method for composite foundation units of crushed stone piles based on a hollow cylindrical torsion shear apparatus. It studies the stress path and stress state strength and deformation characteristics of the soil between crushed stone piles under conditions such as dam construction, water impoundment, and earthquakes. Through the indoor dynamic hollow cylindrical torsion shear apparatus test, the dynamic shear stress is directly applied to the horizontal surface of the specimen, rather than the 45° inclined surface of the specimen in traditional dynamic triaxial tests, which better reflects actual engineering conditions.

[0045] 2. This invention provides a test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus. This method simulates the drainage effect of crushed stone pile treatment in engineering practice by filling the inner wall of a traditional hollow cylindrical sample with a layer of permeable material (crushed stone); and simulates the compaction effect of crushed stone pile treatment in engineering practice by applying internal and external pressures to the hollow cylindrical sample. By simulating the drainage and compaction effects, the boundary conditions of the sample are made more consistent with the boundary conditions of the soil between piles in engineering practice, thus more accurately obtaining the static and dynamic characteristics and strength indicators of the soil between piles in the composite foundation, which is helpful for the development of related research on composite foundations.

[0046] 3. This invention provides a test method for a composite foundation unit of crushed stone piles based on a hollow cylinder torsion shear apparatus. This test method uses a conventional dynamic hollow cylinder torsion shear apparatus. Only some tools used for sample preparation and the operation steps in the consolidation process need to be changed. No modification to the hollow cylinder torsion shear apparatus itself is required. It is simple and easy to implement. Attached Figure Description

[0047] Figure 1 This is a diagram of the hollow cylindrical torsion shear device used in this invention;

[0048] Figure 2 To solidify the central axis diagram;

[0049] Figure 3 This is a schematic diagram of the stress at the pile-soil interface;

[0050] Figure 4 This is a schematic diagram of the sample being installed in the pressure chamber.

[0051] Figure 5 This is a schematic diagram of the stress state of the soil.

[0052] In the diagram: 1. Base; 2. Gasket; 3. Snap ring; 4. Reverse filter layer sealing ring; 5. Inner membrane sealing ring; 6. Lower permeable stone; 7. Inner rubber membrane; 8. Reverse filter layer; 9. Outer rubber membrane; 10. Three-lobed template; 11. Clamp; 12. Top cover positioner; 13. Upper permeable stone; 14. Stainless steel top cover; 15. Cap; 16. First compaction shaft; 17. Second compaction shaft. Detailed Implementation

[0053] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0054] The implementation process of the embodiments of the present invention is as follows:

[0055] The method of this invention employs a hollow cylindrical torsion shear apparatus, such as... Figure 1 As shown, the device includes a base 1, a gasket 2, a retaining ring 3, a filter layer sealing ring 4, an inner membrane sealing ring 5, a lower permeable stone 6, an annular inner rubber membrane 7, an annular filter layer 8, an outer rubber membrane 9, a three-lobed template 10, a clamp 11, a top cover positioner 12, an annular upper permeable stone 13, a stainless steel top cover 14, and a cap 15.

[0056] The base 1 is placed on the bottom surface of the external pressure chamber. An annular stainless steel top cover 14 is positioned above the base 1. The outer periphery of the stainless steel top cover 14 and the outer periphery of the base 1 are connected by an annular three-lobed template 10. A cap 15 is placed above the stainless steel top cover 14. An annular clamp 11 is fitted onto the outer wall of the middle section of the three-lobed template 10. An outer rubber membrane 9 is fixedly connected to the inner wall of the three-lobed template 10. A top cover positioner 12 is fixedly connected to the outer wall of the three-lobed template 10. The lower end of 12 is also connected to the outer periphery of the base 1. The top cover locator 12 is used to fix the stainless steel top cover 14 and the base 1. The base 1 has a groove in the middle. The U-shaped gasket 2 is placed in the groove of the base 1. The bottom end of the filter layer 8 is set between the side wall of the groove and the gasket 2, and the outer side wall of the filter layer 8 is in close contact with the groove. The filter layer sealing ring 4 is set between the inner side wall of the bottom end of the filter layer 8 and the gasket 2. Both the gasket 2 and the filter layer sealing ring 4 are used to fix the filter layer 8. The filter layer 8 and the outer rubber The membrane 9 is coaxially arranged, and the annular cavity between the filter layer 8 and the outer rubber membrane 9 serves as the soil sample layer of the device. The soil sample layer is used to place soil samples. The annular permeable stone 6 is fixed to the upper surface of the outer periphery of the base 1, and the permeable stone 6 is located at the bottom of the soil sample layer. The retaining ring 3 is placed at the opening of the gasket 2. The bottom end of the inner rubber membrane 7 is located between the inner side wall of the gasket 2 and the retaining ring 3, and the outer side wall of the inner rubber membrane 7 is in close contact with the gasket 2. The inner membrane sealing ring 5 is located on the inner side wall of the bottom end of the inner rubber membrane 7 and... Between the retaining rings 3, both the retaining rings 3 and the inner membrane sealing ring 5 are used to fix the inner rubber membrane 7. The cavity in the middle of the inner rubber membrane 7 serves as the water storage cavity of the device. The inner rubber membrane 7 and the reverse filter layer 8 are arranged coaxially, and the annular cavity between the inner rubber membrane 7 and the reverse filter layer 8 serves as the crushed stone drainage layer of the device. The crushed stone drainage layer is used to place the crushed stone sample. The inner and outer circumferences of the upper permeable stone 13 are fixed on the upper surface of the crushed stone sample and the soil sample, respectively, and the upper surface of the upper permeable stone 13 is in contact with the lower surface of the stainless steel top cover 14.

[0057] The base 1, the gasket 2, and the retaining ring 3 are all provided with holes for threading bolts.

[0058] The method includes the following steps:

[0059] Step 1: Prepare soil samples using a hollow cylindrical torsion shear apparatus.

[0060] Step 2: Prepare a gravel sample between the inner rubber membrane 7 and the filter layer 8 of the device to complete the preparation of the hollow cylindrical sample. The gravel sample is used to simulate the drainage effect of the gravel pile.

[0061] The hollow cylindrical specimens include soil samples and gravel samples;

[0062] Step 3: Complete the installation of the top of the device;

[0063] Step 4, as follows Figure 4As shown, the device is placed in the external pressure chamber, the three-lobed template 10 in the device is removed, the pressure chamber is then closed, and the hollow cylindrical sample is saturated.

[0064] In step 4, when filling the closed pressure chamber with water during the saturation process, ensure that the valve of the vent pipe at the top of the pressure chamber is open before filling. When the chamber is almost full, reduce the water inlet speed and slowly add water until water flows out slowly and continuously from the vent pipe outside the pressure chamber. Then stop adding water and plug the vent of the pressure chamber.

[0065] In step 4, the sample is saturated using the back-pressure saturation method. To avoid unstable pressure application that could cause the back pressure to exceed the confining pressure and lead to sample rupture, a certain confining pressure, typically 20 kPa, is applied beforehand. Simultaneously, the inner confining pressure, outer confining pressure, and back pressure are increased until the back pressure reaches the preset back pressure threshold pr and the inner and outer confining pressures reach pr + 20 kPa. The current pressure is maintained constant for 24 hours to saturate the sample. The sample is considered saturated when the value B = Δu / Δσ, where Δu is the pore pressure increment caused by the confining pressure increment and Δσ is the confining pressure increment > 0.95.

[0066] Step 5: After the hollow cylindrical sample is saturated, the sample is consolidated by applying inner and outer pressures to simulate the compaction effect of crushed stone piles.

[0067] In step 5, when consolidating the sample by applying inner and outer confining pressures, the back pressure is kept constant. The inner and outer confining pressures are gradually increased from pr + 20 kPa to the preset inner confining pressure threshold pi and outer confining pressure threshold po, ​​respectively. The axial force is gradually increased from zero until the stress on the sample is in the in-situ stress state. The back pressure, inner and outer confining pressures, and axial force are kept constant, and the sample is consolidated while recording the water displacement ΔV during the consolidation process. Consolidation is complete when the excess pore water pressure dissipates to 0 and the volumetric strain rate of the sample does not exceed 0.003% / h. In step 5, inner and outer confining pressures are applied to the hollow cylindrical sample to simulate the compaction effect of a crushed stone pile, and the consolidation time is approximately 12 hours.

[0068] Step 6: Apply dynamic load to the hollow cylindrical specimen using a dynamic hollow cylindrical torsion shear apparatus to simulate and calculate the dynamic characteristics of the specimen under seismic action, thereby realizing the analysis and calculation of the dynamic characteristics of the crushed stone pile and the surrounding soil under real seismic action.

[0069] Step 1 is as follows:

[0070] Step 11: First, assemble the filter layer sealing ring 4 and the gasket 2. Wet the bottom end of the filter layer 8 with water and attach the bottom end of the filter layer 8 to the side wall of the groove in the base 1. Place the assembled filter layer sealing ring 4 and gasket 2 in the groove of the base 1 so that the filter layer 8 is tightly attached to the side wall of the groove, and at the same time align the holes in the gasket 2 with the holes on the base 1. Both the filter layer sealing ring 4 and the gasket 2 are used to fix the filter layer 8.

[0071] Step 12: Assemble the inner membrane sealing ring 5 and the retaining ring 3. Wet the bottom end of the inner rubber membrane 7 with water and attach the bottom end of the inner rubber membrane 7 to the inner side wall of the gasket 2. Place the assembled inner membrane sealing ring 5 and retaining ring 3 into the opening of the gasket 2 so that the inner rubber membrane 7 is tightly attached to the inner side wall of the gasket 2. At the same time, align the hole in the retaining ring 3 with the hole on the gasket 2. Finally, use bolts to fix the base 1, gasket 2 and retaining ring 3 together.

[0072] Both the retaining ring 3 and the inner membrane sealing ring 5 are used to fix the inner rubber membrane 7;

[0073] Step 13: Place the second compaction shaft 17 in the cavity of the inner rubber membrane 7 (i.e., the water storage cavity), and place the first compaction shaft 16 in the annular cavity between the inner rubber membrane 7 and the reverse filter layer 8 (i.e., the gravel drainage layer).

[0074] Step 14: Place the permeable stone 6 on the bottom of the filter layer 8 and fix it to the outer periphery of the base 1 to prevent the permeable stone 6 from squeezing the filter layer 8. Attach filter paper to the upper surface of the permeable stone 6. The filter paper is used to isolate the permeable stone 6 from the soil sample.

[0075] Step 15: Install the three-lobed template 10 and the clamp 11:

[0076] An outer rubber membrane 9 is fitted inside the inner wall of the three-lobed template 10. The height of the outer rubber membrane 9 is greater than the height of the three-lobed template 10 and the outer rubber membrane 9 is tightly attached to the three-lobed template 10. The part of the upper end of the outer rubber membrane 9 that extends beyond the three-lobed template 10 is turned outward. The part of the lower end of the outer rubber membrane 9 that extends beyond the three-lobed template 10 is turned outward and tightly attached to the three-lobed template 10. Then, the lower end of the outer rubber membrane 9 and the three-lobed template 10 are fixedly connected by an external outer membrane sealing ring. The three-lobed template 10 with the outer rubber membrane 9 is installed on the base 1, and a clamp 11 is fitted on the outer wall of the middle part of the three-lobed template 10.

[0077] The three-lobed template 10 is a ring-shaped thin metal plate;

[0078] Step 16: Place soil into the annular cavity (i.e., soil sample layer) between the outer rubber membrane 9 and the filter layer 8, and then compact the soil in layers. After each layer of soil is compacted, roughen the surface to complete the preparation of the soil sample and ensure the integrity of the hollow cylindrical sample structure.

[0079] The soil sample is a standard hollow cylindrical soil sample with an inner diameter, outer diameter, and height of 60 mm, 100 mm, and 200 mm, respectively. Before loading the soil sample into the soil layer, determine the number of compaction layers and the mass of soil sample to be added to each layer.

[0080] Step 17: Water is introduced into the soil sample through the water inlet in the base 1 to pre-saturate the soil sample and expel the gas inside the soil sample.

[0081] like Figure 2 As shown, the first compaction shaft 16 and the second compaction shaft 17 in step 13 are both made of rigid plastic. The first compaction shaft 16 is a hollow cylindrical structure with an outer diameter of 60 mm, an inner diameter of 40 mm, and a wall thickness of 10 mm. The second compaction shaft 17 is a solid cylindrical structure with a diameter of 40 mm.

[0082] In step 13, the surfaces of the first compaction shaft 16 and the second compaction shaft 17 are evenly coated with talcum powder to facilitate the removal operation in subsequent steps.

[0083] Step 2 is as follows:

[0084] Step 21: Remove the first compaction shaft 16 from the crushed stone drainage layer of the device, while the second compaction shaft 17 remains in the water storage chamber;

[0085] Step 22: Fill the crushed stone drainage layer with crushed stone to complete the preparation of the crushed stone sample. The second compaction shaft 17 remains in the water storage cavity, supporting the inner wall of the hollow cylindrical sample during this process.

[0086] In step 22, the gravel used should be highly rounded to avoid puncturing the rubber membrane during the experiment.

[0087] Step 23: Remove the second compaction shaft b17 from the water storage chamber of the device;

[0088] Step 31: Fix the bottom of the top cover locator 12 to the base 1, and then place the upper permeable stone 13 on the top of the sample. The sample mainly consists of soil sample and gravel sample, and the upper surfaces of the soil sample and gravel sample are flush. Place the stainless steel top cover 14 on the upper permeable stone 13 and connect the stainless steel top cover 14 to the top of the top cover locator 12.

[0089] Step 32: Fill the water storage chamber with de-aired water, then fold the part of the inner rubber membrane 7 that extends beyond the stainless steel top cover 14 outward and press it tightly against the stainless steel top cover 14, and then use a rubber ring to fix the upper end of the inner rubber membrane 7 and the edge of the stainless steel top cover 14 together.

[0090] Step 33: Use bolts to fix the cap 15 onto the stainless steel top cover 14, then remove the top cover locator 12 from the device, straighten the part of the upper end of the outer rubber membrane 9 that extends beyond the three-petal template 10, that is, fold the part of the upper end of the outer rubber membrane 9 that extends beyond the three-petal template 10 inward, and then fix the upper end of the outer rubber membrane 9 and the stainless steel top cover 14 with a rubber ring.

[0091] When sealing the outer rubber membrane 9 in step 33, the sealing position should be higher than the height of the interface between the upper permeable stone 13 and the stainless steel top cover 14.

[0092] Step 6 specifically involves:

[0093] Step 61: Apply dynamic load to the specimen continuously using a dynamic hollow cylindrical torsion shear apparatus, and observe the position and morphology of the fracture surface in the specimen in real time during the application of dynamic load. At the same time, use a data acquisition system to monitor the dynamic characteristics of the specimen, such as plastic strain, dynamic strength and pore pressure, in real time to study the development trend of plastic strain, dynamic strength and pore pressure with the number of vibrations. Stop the test when the specimen is completely destroyed.

[0094] Dynamic loads include sinusoidal vertical force W and sinusoidal torque M. T That is, the combination of axial force and torsional shear cyclic variable load, sinusoidal vertical force W and sinusoidal torque M. T The size relationship must satisfy σ calculated according to the following formula. z -σ θ / 2 and τ θz Equal, such as Figure 5 As shown:

[0095]

[0096]

[0097]

[0098] Where, σ θ For the inner pressure p i and the surrounding pressure p o The resulting circumferential stress; σ z The vertical force W and the inner and inner confining pressures p i and the surrounding pressure p o The axial stress generated by the combination; τ θz For torque M T The resulting shear stress, where Ro is the outer diameter of the soil sample and Ri is the inner diameter of the soil sample;

[0099] The amplitude and frequency of the dynamic load are determined by the actual magnitude of the earthquake in the project.

[0100] Step 62: Perform sample unloading operation.

[0101] Slightly open the inlet valve in the pressure chamber and reduce the dynamic load to zero at a certain speed. Then, open the drain valve and vent valve in the pressure chamber at the same time to drain the pressure chamber. To speed up the drainage, apply a positive pressure of about 300 kPa at the vent end. After the water in the pressure chamber is drained, open the pressure chamber, loosen the screws in the device and drain all the water in the water storage chamber. The test is now over.

[0102] By analyzing and calculating the dynamic characteristics of the samples obtained during the test, the dynamic characteristics of the crushed stone pile and the surrounding soil under real earthquake action can be realized.

[0103] In step 61, when the sample is on the verge of failure, the pore pressure in the sample will rise sharply, the axial strain will increase dramatically, and the axial force of the sample will decrease rapidly. At this point, after loading the sample for one or two more cycles, the vibration can be stopped, and the test ends. During the sample removal process in step 62, care should be taken to protect the rubber diaphragm to prevent it from leaking. The removed samples can be recycled, and if the particle size distribution of the samples is similar, they can be reused.

[0104] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A test method for a composite foundation unit of crushed stone piles based on a hollow cylindrical torsion shear apparatus, characterized in that: The method uses a hollow cylindrical torsion shear device, which includes a base (1), a gasket (2), a retaining ring (3), a filter layer sealing ring (4), an inner membrane sealing ring (5), a lower permeable stone (6), an annular inner rubber membrane (7), an annular filter layer (8), an outer rubber membrane (9), a three-lobed template (10), a clamp (11), a top cover locator (12), an annular upper permeable stone (13), a stainless steel top cover (14), and a cap (15). The base (1) is placed on the bottom surface of the external pressure chamber. The annular stainless steel top cover (14) is located above the base (1). The outer periphery of the stainless steel top cover (14) and the outer periphery of the base (1) are connected by an annular three-lobed template (10). The cap (15) is placed above the stainless steel top cover (14). An annular clamp (11) is fitted on the outer wall of the middle part of the three-lobed template (10). The outer rubber membrane (9) is fixedly connected to the inner wall of the three-lobed template (10). A top cover locator (12) is fixedly connected to the outer wall of the base (1). A groove is provided in the middle of the base (1). A U-shaped gasket (2) is placed in the groove of the base (1). The bottom end of the filter layer (8) is set between the side wall of the groove and the gasket (2), and the outer wall of the filter layer (8) is close to the groove. The filter layer sealing ring (4) is set between the inner side wall of the bottom end of the filter layer (8) and the gasket (2). The filter layer (8) and the outer rubber membrane (9) are coaxially arranged and the filter layer (8) and the outer rubber membrane (9) are connected together. The annular cavity between 9) serves as the soil sample layer of the device, which is used to place soil samples. The annular permeable stone (6) is fixed on the upper surface of the outer periphery of the base (1) and the permeable stone (6) is located at the bottom of the soil sample layer. The retaining ring (3) is placed at the opening of the gasket (2). The bottom end of the inner rubber membrane (7) is set between the inner side wall of the gasket (2) and the retaining ring (3), and the outer side wall of the inner rubber membrane (7) is in close contact with the gasket (2). The inner membrane sealing ring (5) is set at the bottom end of the inner rubber membrane (7). Between the inner sidewall and the retaining ring (3), the cavity in the middle of the inner rubber membrane (7) serves as the water storage cavity of the device. The inner rubber membrane (7) and the reverse filter layer (8) are arranged coaxially, and the annular cavity between the inner rubber membrane (7) and the reverse filter layer (8) serves as the crushed stone drainage layer of the device. The crushed stone drainage layer is used to place crushed stone samples. The inner and outer circumferences of the upper permeable stone (13) are fixed on the upper surfaces of the crushed stone sample and the soil sample, respectively, and the upper surface of the upper permeable stone (13) is in contact with the lower surface of the stainless steel top cover (14). The method includes the following steps: Step 1: Prepare soil samples using a hollow cylindrical torsion shear apparatus. Step 2: Prepare a gravel sample between the inner rubber membrane (7) and the filter layer (8) of the device to complete the preparation of the hollow cylindrical sample; The hollow cylindrical specimens include soil samples and gravel samples; Step 3: Complete the installation of the device; Step 4: Place the device in the external pressure chamber, remove the three-lobed template (10) from the device, then close the pressure chamber and saturate the hollow cylindrical sample. Step 5: After the hollow cylindrical sample is saturated, the sample is consolidated by applying inner and outer pressures. Step 6: Apply dynamic load to the hollow cylindrical specimen using a dynamic hollow cylindrical torsion shear apparatus to simulate and calculate the dynamic characteristics of the specimen under seismic action, thereby realizing the analysis and calculation of the dynamic characteristics of the crushed stone pile and the surrounding soil under real seismic action.

2. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 1, characterized in that: Step 1 specifically includes: Step 11: First, assemble the filter layer sealing ring (4) and the gasket (2). Wet the bottom of the filter layer (8) with water and attach the bottom of the filter layer (8) to the side wall of the groove of the base (1). Place the assembled filter layer sealing ring (4) and gasket (2) in the groove of the base (1) so that the filter layer (8) is tightly attached to the side wall of the groove, and at the same time align the hole in the gasket (2) with the hole on the base (1). Step 12: Assemble the inner membrane sealing ring (5) and the retaining ring (3). Wet the bottom end of the inner rubber membrane (7) with water and attach the bottom end of the inner rubber membrane (7) to the inner side wall of the gasket (2). Place the assembled inner membrane sealing ring (5) and retaining ring (3) in the opening of the gasket (2) so that the inner rubber membrane (7) is tightly attached to the inner side wall of the gasket (2). At the same time, align the hole in the retaining ring (3) with the hole on the gasket (2). Finally, use bolts to fix the base (1), gasket (2) and retaining ring (3) together. Step 13: Place the second compaction shaft (17) in the cavity of the inner rubber membrane (7), and place the first compaction shaft (16) in the annular cavity between the inner rubber membrane (7) and the filter layer (8); Step 14: Place the permeable stone (6) on the bottom of the filter layer (8) and fix it to the outer periphery of the base (1), and attach filter paper to the upper surface of the permeable stone (6). Step 15: Install the three-lobed template (10) and clamp (11): An outer rubber membrane (9) is fitted inside the inner wall of the three-lobed template (10). The height of the outer rubber membrane (9) is greater than the height of the three-lobed template (10), and the outer rubber membrane (9) is tightly attached to the three-lobed template (10). The part of the upper end of the outer rubber membrane (9) that extends beyond the three-lobed template (10) is turned outward. The part of the lower end of the outer rubber membrane (9) that extends beyond the three-lobed template (10) is turned outward and tightly attached to the three-lobed template (10). Then, the lower end of the outer rubber membrane (9) and the three-lobed template (10) are fixedly connected by an outer membrane sealing ring. The three-lobed template (10) fitted with the outer rubber membrane (9) is installed on the base (1), and a clamp (11) is fitted on the outer wall of the middle part of the three-lobed template (10). Step 16: Place soil in the annular cavity between the outer rubber membrane (9) and the filter layer (8), and then compact the soil in layers to complete the preparation of the soil sample; Step 17: Water is introduced into the soil sample through the water inlet in the base (1) to pre-saturate the soil sample and expel the gas inside the soil sample.

3. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 1, characterized in that: Step 2 specifically refers to: Step 21: Remove the first compaction shaft (16) from the crushed stone drainage layer of the device; Step 22: Fill the crushed stone drainage layer with crushed stone to complete the preparation of the crushed stone sample; Step 23: Remove the second compaction shaft (17) from the water storage chamber of the device.

4. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 1, characterized in that: Step 3 specifically includes: Step 31: Fix the bottom of the top cover locator (12) to the base (1), and then place the permeable stone (13) on the top of the sample. The sample is mainly composed of soil sample and gravel sample, and the upper surfaces of the soil sample and gravel sample are flush. Place the stainless steel top cover (14) on the permeable stone (13). Step 32: Fill the water storage chamber with de-aired water, then fold the part of the inner rubber membrane (7) that extends beyond the stainless steel top cover (14) outward and stick it tightly to the stainless steel top cover (14), and then use a rubber ring to fix the upper end of the inner rubber membrane (7) and the stainless steel top cover (14) together. Step 33: Fix the cap (15) on the stainless steel top cover (14), then remove the top cover locator (12) in the device, straighten the part of the upper end of the outer rubber membrane (9) that extends beyond the three-petal template (10), and then fix the upper end of the outer rubber membrane (9) and the stainless steel top cover (14) with a rubber ring.

5. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 1, characterized in that: Step 6 specifically includes: Step 61: Apply dynamic load to the specimen continuously using a dynamic hollow cylindrical torsion shear apparatus, and observe the position and morphology of the fracture surface in the specimen in real time during the application of dynamic load. At the same time, use a data acquisition system to monitor the plastic strain, dynamic strength and pore pressure of the specimen in real time. Stop the test when the specimen fails. The dynamic loads include a sinusoidal vertical force W and a sinusoidal torque M. T ; Step 62: Perform sample unloading operation: Open the inlet valve in the pressure chamber and reduce the dynamic load to zero. Then, open the drain valve and air vent valve in the pressure chamber at the same time to drain the pressure chamber. After the water in the pressure chamber is drained, open the pressure chamber, loosen the screws in the device and drain all the water in the water storage chamber. The test is now over. By obtaining the dynamic characteristics of the specimens during the test, the dynamic characteristics of the crushed stone pile and the surrounding soil under real earthquake action can be analyzed and calculated.

6. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 2, characterized in that: The first compaction shaft (16) and the second compaction shaft (17) in step 13 are both made of hard plastic. The first compaction shaft (16) is a hollow cylindrical structure, and the second compaction shaft (17) is a solid cylindrical structure.

7. The test method for a composite foundation unit based on a hollow cylindrical torsion shear apparatus according to claim 2, characterized in that: In step 13, the surfaces of the first compaction shaft (16) and the second compaction shaft (17) are uniformly coated with talcum powder.

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