Method for optimally calculating shear strength of discrete material pile composite foundation

Abstract

The invention relates to the technical field of geotechnical engineering, in particular to a discrete material pile composite foundation shear strength optimization calculation method which comprises the steps that 1, a remolded soil sample preparation device is used for remolding and preparing a matrix sample, a composite soil body is prepared according to different replacement rates, and a back pressure saturation method is adopted for saturating the sample and then conducting consolidation and drainage; 2, carrying out a shear test, and after the shear test is finished, dismounting the sample, weighing the mass and measuring the water content of a matrix; 3, drawing an effective stress path curve, calculating an effective internal friction angle and effective cohesion, and calculating shear strength test indexes of the composite soil body under different replacement rate conditions; and 4, correcting the pile body confining pressure in the composite soil body, inverting the pile-soil interaction coefficient, and calculating the cohesive force and the internal friction angle of the composite soil body based on the same matrix material and the reinforcement filler. The pile-soil interaction is exerted, the action rule of the pile-soil interaction is fully utilized, the calculation result better conforms to the action mechanism of the composite foundation, and the shear strength index value is increased from the engineering perspective.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering technology, specifically to an optimized calculation method for the shear strength of a composite foundation made of granular materials piles. Background Technology

[0002] Composite foundations consist of a matrix and a reinforcement. These two materials interact and jointly bear external loads, making them heterogeneous and anisotropic. Unlike bonded pile composite foundations, granular material pile composite foundations require the confinement of the surrounding soil to form the piles. The matrix provides confining pressure restraint on the reinforcement, while the shear dilatation of the reinforcement disturbs the matrix structure and compacts it. Therefore, the interaction between the granular material piles and the surrounding soil is very significant.

[0003] Patent CN115855686A discloses a method and apparatus for calculating the shear strength of cement-soil mixing pile composite foundations. However, the stiffness parameters assigned to the pile-soil specimen model are difficult to determine. Using Poisson's ratio and elastic modulus calculated through numerical simulation as input parameters for the cement-soil mixing pile composite foundation model carries certain risks. Patent CN1687535A discloses a construction method for pile-soil interactive grout-solidified granular material pile composite foundations. The drilling process allows for only one-time drilling and filling, making it unsuitable for other construction processes such as dynamic compaction replacement. The mechanical performance indicators of the grout-reinforced granular material piles should be calculated according to those of bonded material piles. The cost of grout-reinforced granular material piles after grouting should not be lower than that of cement-soil mixing piles. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide an optimized calculation method for the shear strength index of composite foundations made of granular materials. Starting from the mechanism of lateral interaction between piles and soil, this method explores the confining pressure of the matrix and the compaction effect of the reinforcement, thereby improving the shear strength index of the composite foundation. This method is applicable to the requirements of various granular material pile construction processes. From the perspective of improving the engineering properties of the composite foundation, it can reduce the later-stage project cost. Based on indoor tests, this invention improves the traditional method of calculating the shear strength index using the area replacement rate, proposing a method for calculating the dynamic response shear strength index of piles and soil based on the area replacement rate. For composite foundations with similar or identical matrix material engineering properties, the calculation results are consistent with the shear test results, more accurately reflecting the actual engineering situation.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a method for optimizing the calculation of shear strength of granular material pile composite foundations, comprising the following steps: Step 1: Soil sample preparation. The matrix sample is prepared by remolding in the room using a remolded soil sample preparation device. According to different replacement ratios, hollow tubes of different diameters are used to prepare composite soil. The latex film is evenly placed on the base of the pressure chamber of the triaxial consolidation apparatus for sample loading. The reverse pressure saturation method is used to compress or dissolve the air bubbles remaining inside the composite soil sample in water. After the sample is saturated, the composite soil sample is consolidated and drained. Step 2: Sample shearing. Set the shearing rate and axial displacement reading interval. Conduct the shearing test based on the peak value of the stress gauge reading. After the shearing test, disassemble the pressure chamber, remove the sample, weigh it, and determine the moisture content of the matrix. Step 3: Obtain the effective stress path from the composite soil CU test, plot the effective stress path curve, calculate the effective internal friction angle and effective cohesion through the effective stress path curve, prepare composite soil according to different replacement ratios, repeat the composite soil CU test, and calculate the shear strength test index of the composite soil under different replacement ratios. Step 4: Based on the factors that increase the confining pressure of the pile body after the lateral action of the pile and soil, the confining pressure of the pile body in the composite soil is corrected. The pile-soil interaction coefficient is supplemented according to the replacement rate, the tangent Poisson's ratio of the matrix material, and the stress level. The pile-soil interaction coefficient is inverted, and the relationship between the replacement rate and the pile-soil interaction coefficient is fitted to obtain the optimized internal friction angle and cohesion of the composite soil. The cohesion and internal friction angle of the composite soil based on the same matrix material and reinforcing filler are calculated.

[0006] The above-mentioned method for optimizing the shear strength calculation of granular material pile composite foundations, step 1 includes: Step 1-1: Prepare remolded soil. Use a remolded soil sample preparation device to prepare matrix samples in the room. Take out the consolidated matrix sample from the sample preparation tube. Use the cut upper excess soil to test the moisture content. Weigh the mass of the cut matrix sample including the three-lobed mold and the retaining ring, as well as the mass of the three-lobed mold and the retaining ring. Steps 1-2: Prepare composite soil. According to different replacement rates, use hollow tubes of different diameters. Reserve replacement space in the sample, fill it with bulk material, and then pull out the hollow tube to replace and form composite soil. Weigh the composite soil sample, including a three-lobed mold and a retaining ring. Steps 1-3: Sample loading. Take out the composite soil, remove the three-part mold, evenly place the latex film, and place it on the base of the pressure chamber of the triaxial consolidation apparatus. Apply back pressure from the bottom to remove air bubbles between the composite soil and the latex film. Close the back pressure, use rubber rings to tighten the latex film to the top cover, connect the pipeline, install the outer cover of the pressure chamber of the triaxial consolidation apparatus, inject water from bottom to top to expel the air in the pressure chamber, and seal the pressure chamber. Steps 1-4: Saturate the sample. Use the back pressure saturation method to compress or dissolve the air bubbles remaining inside the composite soil sample in water. Steps 1-5: Specimen consolidation. The composite soil specimens are consolidated and drained using the isotropic consolidation method.

[0007] The above-mentioned method for optimizing the shear strength calculation of granular material pile composite foundations includes steps 1-4: Step a: Apply initial confining pressure and open the drain valve of the pore water pressure gauge to connect it to the atmosphere. Step b: Set up a back pressure loading device at the bottom of the sample, apply back pressure, and wait for water to be continuously discharged from the drain valve of the pore water pressure gauge before closing the drain valve. Step c: After the pore water pressure count value stabilizes, apply the back pressure and confining pressure in stages, keeping the difference between the confining pressure and the back pressure at 5 kPa. Step d: When the ratio of the increment of pore water pressure to the increment of confining pressure ∆u / ∆σ3 > 0.98, the sample is considered saturated; otherwise, the graded loading continues.

[0008] The above-mentioned method for optimizing the shear strength calculation of composite foundation of granular material piles includes steps 1-5 as follows: after applying the confining pressure required for the test, connect the pore water pressure gauge to the pressure chamber. When the reading of the pore water pressure gauge is close to the confining pressure value, open the drain valve to drain the consolidation. When the reading of the pore water pressure gauge dissipates by more than 95%, close the drain valve, and the consolidation is completed.

[0009] In the above-mentioned optimized calculation method for the shear strength of the composite foundation of granular material piles, in step 2, the shear rate is 0.1 mm / min, and the axial displacement is collected once every 0.1 mm. During the shearing process, when the stress gauge reading has no stress peak, the shearing stage is continued until the axial strain reaches 20%; when the stress gauge shows a stress peak, the test is continued until the axial strain reaches 20%. After the shearing is completed, the confining pressure value in the pressure chamber of the triaxial consolidation apparatus is reduced to 0 kPa, and the pressure chamber is disassembled after drainage.

[0010] The above-mentioned method for optimizing the shear strength calculation of granular material pile composite foundations, step 3 includes: Step 3-1: Based on the effective stress path obtained from the composite soil CU test, plot the effective stress path curve with q=(σ1-σ3) / 2 as the ordinate and p=(σ1+σ3) / 2 as the abscissa. The inflection point of the effective stress path is the failure point of the specimen during the shearing process. σ1 represents the axial pressure and σ3 represents the confining pressure. Step 3-2: Calculate the effective internal friction angle and effective cohesion using the effective stress path curve. The formula for the effective internal friction angle is: φ '=arcsin a ,in, a The slope of the fitted line at the failure point on the stress path diagram, and the formula for effective cohesion are: c '= b / cos φ ', b represents the intercept of the fitting line of the failure point on the stress path diagram on the vertical axis; Step 3-3: Prepare composite soil bodies according to different replacement rates, repeat the CU tests of the composite soil bodies, and use the formulas for effective internal friction angle and effective cohesion to calculate the test indexes of the shear strength of the composite soil bodies, namely the effective internal friction angle and effective cohesion, under different replacement rates.

[0011] The above shear strength optimization calculation method for the granular material pile composite foundation, the step 4 includes: Step 4-1: According to the supplemented pile-soil interaction coefficient, obtain the expression of the total axial stress of the composite soil body: , where σ c1 represents the total axial stress of the composite soil body, ξ represents the pile-soil interaction coefficient, m represents the area replacement rate, σ sc represents the test stress of the matrix material, σ g represents the test stress of the reinforcement material; Step 4-2: The matrix material and reinforcement material of the composite foundation are simultaneously damaged under the action of external forces. Apply the Mohr-Coulomb criterion to both materials simultaneously to obtain the strength criterion formula: , where φ represents the internal friction angle; Step 4-3: Substitute the strength criterion formula into the expression of the total axial stress of the composite soil body to obtain , where, c c represents the cohesion of the composite soil body, represents the internal friction angle of the composite soil body, c g represents the cohesion of the reinforcement material, represents the internal friction angle of the reinforcement material, c sc represents the cohesion of the matrix material, represents the internal friction angle of the matrix material; Step 4-4: According to the formula in step 4-3, obtain the calculation index formula for the shear strength of the composite soil body, including: Cohesion formula: , Internal friction angle formula: ; Step 4-5: Invert the pile-soil interaction coefficient through the internal friction angle formula, and it is found by fitting that the pile-soil interaction coefficient decreases in a power function as the replacement rate increases, and the inversion formula is obtained: (0 < k < 1), where, kThis represents the power exponent value of the pile-soil interaction coefficient in the inversion fit; Steps 4-6: Combine the internal friction angle formula and the inversion formula to obtain the optimized formula for calculating the internal friction angle of composite soil: By using the cohesion formula and the internal friction angle formula of composite soil, the cohesion and internal friction angle of composite soil based on the same matrix material and reinforcing filler are calculated respectively.

[0012] The beneficial effects of the present invention, which is an optimized calculation method for the shear strength of a composite foundation of granular materials piles, are that it leverages the interaction between piles and soil and makes full use of their interaction laws, resulting in calculation results that are more consistent with the action mechanism of the composite foundation. From an engineering perspective, it improves the shear strength index value and reduces the engineering cost. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the overall structure of the reconstituted soil sample preparation device in an embodiment of the present invention; Figure 2 This is a schematic diagram of the upper structure of the automatic loading frame of the reconstituted soil sample preparation device in an embodiment of the present invention; Figure 3 This is a schematic diagram of the lower structure of the automatic loading frame of the reconstituted soil sample preparation device in an embodiment of the present invention; Figure 4 This is a schematic diagram of the sample preparation cylinder structure of the reconstituted soil sample preparation device in an embodiment of the present invention; Figure 5 These are schematic diagrams of hollow tube structures of different diameters in embodiments of the present invention. Figure 6 This is the effective stress path curve of the composite soil sample with a replacement rate of 0% in the embodiments of the present invention; Figure 7 The effective stress path curve of the composite soil sample with a replacement rate of 1.68% in this embodiment of the invention is shown. Figure 8 This is the effective stress path curve of the composite soil sample with a replacement rate of 5.13% in this embodiment of the invention; Figure 9 This is the effective stress path curve of the composite soil sample with a replacement rate of 8.48% in this embodiment of the invention; Figure 10 The effective stress path curve of the composite soil sample with a replacement rate of 15.08% in this embodiment of the invention is shown. Figure 11 The effective stress path curve of the composite soil sample with a replacement rate of 33.90% in this embodiment of the invention is shown. Figure 12 The effective stress path curve of the composite soil sample with a replacement rate of 47.29% in this embodiment of the invention is shown. Figure 13This is a schematic diagram illustrating the response relationship between the pile-soil interaction coefficient and the replacement rate in an embodiment of the present invention. Detailed Implementation

[0014] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described below in conjunction with specific embodiments and accompanying drawings.

[0015] Example 1 A remolded soil sample preparation device for optimizing the calculation method of shear strength of granular material pile composite foundation, such as Figures 1-5 As shown, the system includes an automatic loading frame 100, a sample preparation cylinder 200, a composite soil sample preparation component, and a control panel. The composite soil sample preparation component uses hollow tubes 23 of different diameters. The control panel controls the automatic loading frame and the sample preparation cylinder. The sample preparation cylinder is installed in the middle of the automatic loading frame and is subjected to pressure from both its upper and lower sides. The control panel is equipped with control buttons such as a switch 24, fast rewind 25, stop 26, and fast forward 27. The control panel contains an automatic program that controls the automatic loading frame and the sample preparation cylinder.

[0016] The automatic loading frame includes a crossbeam 1, a sample reaction frame 2, an automatic lifting base 3, an axial pressure rod 4, a pressure sensor 5, a displacement gauge 6, a miniature electronic scale 7, a stepper motor 8, an automatic loading box 9, a lead screw 10, and a fixed connecting rod 11.

[0017] The sample reaction frames are symmetrically arranged on both sides of the crossbeam. The bottom of the sample reaction frames is inserted into the automatic loading box, which contains a stepper motor. A lead screw is installed at the output end of the stepper motor, extending outward from the automatic loading box. The top of the lead screw is connected to an automatic lifting platform. An axial pressure rod is installed below the crossbeam, with its central axis coinciding with that of the lead screw. The axial pressure rod and the lead screw are positioned opposite each other, one above the other. A pressure sensor is installed between the axial pressure rod and the crossbeam. A displacement gauge is mounted on one side of the sample reaction frame using a fixed connecting rod. The displacement gauge is parallel to the axial pressure rod, and its bottom end is lower than the bottom end of the axial pressure rod. The sample preparation tube is placed above the automatic lifting platform, between the axial pressure rod and the automatic lifting platform.

[0018] The sample preparation tube includes a three-valve membrane 12, a stainless steel protective ring 13, a stainless steel protective tube 14, an acrylic water container 15, a valve 16, an upper permeable stone 17, a lower permeable stone 18, a bolt connector 19, a stainless steel top cover 20, a stainless steel base 21, and a sample preparation tube support 22.

[0019] The sample preparation barrel support is installed on a stainless steel base, and the permeable stone is placed on the stainless steel base. A valve is connected to the outside of the base. The three-valve membrane is embedded in the stainless steel base through a stainless steel protective ring. The lower end of the stainless steel protective sleeve is embedded in the top of the three-valve membrane, and the upper end of the stainless steel protective sleeve is connected and fixed to the sample preparation barrel support. The sample preparation barrel support, the stainless steel base, and the stainless steel top cover are connected by bolt connectors. Other connection parts are also connected using bolt and nut structures.

[0020] After the sample is loaded, a permeable stone is placed on top of the stainless steel protective tube, and then a stainless steel top cover is placed on top of it. The sample tube support is fitted inside the plexiglass water-filled tube.

[0021] Example 2 like Figures 6-13 As shown, a method for optimizing the calculation of shear strength of granular material pile composite foundation includes the following steps.

[0022] Step 1: Soil sample preparation. The matrix sample is prepared by remolding in the room using a remolded soil sample preparation device. According to different replacement ratios, hollow tubes of different diameters are used to prepare composite soil. The latex film is evenly placed on the base of the pressure chamber of the triaxial consolidation apparatus for sample loading. The reverse pressure saturation method is used to compress or dissolve the residual air bubbles inside the composite soil sample in water. After the sample is saturated, the composite soil sample is consolidated and drained.

[0023] include: Step 1-1: Prepare remolded soil. Use a remolded soil sample preparation device to prepare matrix samples in the room. Take out the consolidated matrix sample from the sample preparation tube. Test the moisture content of the cut upper excess soil. Weigh the mass of the cut matrix sample including the three-lobed mold and the retaining ring, as well as the mass of the three-lobed mold and the retaining ring.

[0024] Steps 1-2: Prepare composite soil. Use hollow tubes of different diameters according to different replacement rates. Reserve replacement space in the sample, fill it with bulk material, and then pull out the hollow tube to replace and form a composite soil. Weigh the composite soil sample, which includes a three-lobed mold and a retaining ring.

[0025] Steps 1-3: Sample loading. Remove the composite soil, remove the three-part mold, evenly place the latex film, and put it on the base of the pressure chamber of the triaxial consolidation apparatus. Apply back pressure from the bottom to remove air bubbles between the composite soil and the latex film. Close the back pressure, use rubber rings to tighten the latex film to the top cover, connect the pipeline, install the outer cover of the pressure chamber of the triaxial consolidation apparatus, inject water from bottom to top to expel the air in the pressure chamber, and seal the pressure chamber.

[0026] Steps 1-4: Saturating the sample. Using the reverse pressure saturation method, residual air bubbles inside the composite soil sample are compressed or dissolved in water, including: Step a: Apply initial confining pressure and open the drain valve of the pore water pressure gauge to connect it to the atmosphere.

[0027] Step b: Set up a back pressure loading device at the bottom of the sample, apply back pressure, and wait for water to be continuously discharged from the drain valve of the pore water pressure gauge before closing the drain valve.

[0028] Step c: After the pore water pressure count value stabilizes, apply the back pressure and confining pressure in stages, keeping the difference between the confining pressure and the back pressure at 5 kPa.

[0029] Step d: When the ratio of the increment of pore water pressure to the increment of confining pressure ∆u / ∆σ3 > 0.98, the sample is considered saturated; otherwise, the graded loading continues.

[0030] Steps 1-5: Sample consolidation. The isotropic consolidation method is used to consolidate and drain the composite soil sample. After applying the required confining pressure, the pore water pressure gauge is connected to the pressure chamber. When the pore water pressure gauge reading is close to the confining pressure value, the drain valve is opened to drain the sample during consolidation. When the pore water pressure gauge reading has dissipated by more than 95%, the drain valve is closed, and consolidation is complete.

[0031] Step 2: Sample shearing. Set the shear rate and axial displacement reading interval. Conduct the shear test based on the peak value of the stress gauge reading. After the shear test, disassemble the pressure chamber, remove the sample, weigh it, and determine the moisture content of the matrix.

[0032] The shear rate was 0.1 mm / min, and the axial displacement was measured every 0.1 mm. During the shearing process, when the stress gauge reading had no stress peak, the shearing stage continued until the axial strain reached 20%. When the stress gauge showed a stress peak, the test continued until the axial strain reached 20%. After the shearing was completed, the confining pressure in the pressure chamber of the triaxial consolidation apparatus was reduced to 0 kPa, and the pressure chamber was disassembled after drainage.

[0033] Step 3: Obtain the effective stress path from the composite soil CU test, plot the effective stress path curve, calculate the effective internal friction angle and effective cohesion through the effective stress path curve, prepare composite soil according to different replacement ratios, repeat the composite soil CU test, and calculate the shear strength test index of the composite soil under different replacement ratios.

[0034] include: Step 3-1: Based on the effective stress path obtained from the composite soil CU test, plot the effective stress path curve with q=(σ1-σ3) / 2 as the ordinate and p=(σ1+σ3) / 2 as the abscissa. The inflection point of the effective stress path is the failure point of the specimen during the shearing process. σ1 represents the axial pressure and σ3 represents the confining pressure.

[0035] Step 3-2: Calculate the effective internal friction angle and effective cohesion using the effective stress path curve. The formula for the effective internal friction angle is: φ '=arcsin a,in, a The slope of the fitted line at the failure point on the stress path diagram, and the formula for effective cohesion are: c '= b / cos φ ', b This represents the intercept of the fitted line at the failure point on the stress path diagram on the vertical axis.

[0036] Step 3-3: Prepare composite soils according to different replacement ratios and repeat the CU test of the composite soils. Calculate the effective internal friction angle and effective cohesion of the composite soils under different replacement ratios using the formulas for the effective internal friction angle and effective cohesion.

[0037] Step 4: Based on the factors that increase the confining pressure of the pile body after the lateral action of the pile and soil, the confining pressure of the pile body in the composite soil is corrected. The pile-soil interaction coefficient is supplemented according to the replacement rate, the tangent Poisson's ratio of the matrix material, and the stress level. The pile-soil interaction coefficient is inverted, and the relationship between the replacement rate and the pile-soil interaction coefficient is fitted to obtain the optimized internal friction angle and cohesion of the composite soil. The cohesion and internal friction angle of the composite soil based on the same matrix material and reinforcing filler are calculated.

[0038] include: Step 4-1: Based on the supplemented pile-soil interaction coefficient, obtain the expression for the total axial stress of the composite soil: , where σ c1 ξ represents the total axial stress in the composite soil, and ξ represents the pile-soil interaction coefficient. m σ represents the area replacement ratio. sc σ represents the test stress of the matrix material. g This indicates the test stress of the reinforcing material.

[0039] Step 4-2: The matrix material and reinforcement material of the composite foundation fail simultaneously under external force. Applying the Mohr-Coulomb criterion to both materials, the strength criterion formula is obtained: ,in φ This represents the internal friction angle.

[0040] Step 4-3: Substitute the strength criterion formula into the expression for the total axial stress of the composite soil to obtain... ,in, c c Indicates the cohesion of composite soil. Indicates the internal friction angle of the composite soil. c g Indicates the cohesive strength of the reinforcing material. Indicates the internal friction angle of the reinforcing material. c sc Indicates the cohesive force of the matrix material. Denotes the internal friction angle of the matrix material.

[0041] Step 4-4: Obtain the calculation index formula for the shear strength of the composite soil mass according to the formula in Step 4-3, including: Cohesion formula: , where c c Denotes the cohesion of the composite soil mass, Denotes the internal friction angle of the composite soil mass, c sc Denotes the cohesion of the matrix material, Denotes the internal friction angle of the matrix material.

[0042] Internal friction angle formula: , where Denotes the internal friction angle of the composite soil mass, Denotes the internal friction angle of the reinforcement material, Denotes the internal friction angle of the matrix material, ξ denotes the pile-soil interaction coefficient, and m denotes the area replacement ratio.

[0043] Step 4-5: Invert the pile-soil interaction coefficient through the internal friction angle formula. It is found by fitting that the pile-soil interaction coefficient decreases in a power function with the increase of the replacement ratio, and the inversion formula is obtained: (0 < k < 1), where k Denotes the power exponent value of the pile-soil interaction coefficient obtained by inversion fitting.

[0044] Step 4-6: Combine the internal friction angle formula and the inversion formula to obtain the optimized calculation formula for the internal friction angle of the composite soil mass: , and calculate the cohesion and internal friction angle of the composite soil mass based on the same matrix material and reinforcement filler through the cohesion formula and the internal friction angle formula of the composite soil mass.

[0045] Example 3 In this example, according to the operation steps of the technical solution, the specific implementation manner is described in detail.

[0046] Test materials: The test site is a foundation treatment project on the north bank of Jiaozhou Bay. The matrix soil sample of the composite foundation is silty clay with silt. The soil sampling depth is 2.0 - 3.0 m, gray to grayish black, in a fluid plastic state, with a shiny cut surface, uniform particles, and locally containing shell debris. The main physical property indexes are shown in Table 1. The reinforcement is fine gravel with a diameter of 2 - 5 mm, good gradation, cohesion c g = 0, and the internal friction angle = 38°.

[0047] Table 1: Basic physical and mechanical indexes of silty clay with silt .

[0048] Sample preparation: The matrix sample was prepared by remolding silty clay in the room using a remolded soil sample preparation device. The sample size was: diameter × height = 61.8 mm × 125 mm.

[0049] Install the sample preparation bucket bracket 22 on the stainless steel base 21, place the permeable stone 18 on the stainless steel base 21, and connect and embed the three-lobed membrane 12 into the stainless steel base 21 through the stainless steel protective ring 13. The lower end of the stainless steel protective sleeve 14 is embedded into the top of the three-lobed membrane 12, and the upper end of the stainless steel protective sleeve 14 is connected and fixed to the sample preparation bucket bracket 22. The sample preparation bucket is now installed.

[0050] To ensure the silty clay is easily mixed evenly, the moisture content is adjusted to 1.5 times the liquid limit. After thorough mixing, the mixture is filled into a sample preparation tube, with each filling being 920g. After the sample is filled, a permeable stone 17 is placed on top, followed by a stainless steel top cover 20. Airless water is injected through the top of the acrylic water-holding tube 15 into the gap between the acrylic water-holding tube 15 and the three-lobed mold 12, up to above the junction of the top of the three-lobed mold and the protective tube, to prevent rapid evaporation of moisture from the sample during the consolidation process.

[0051] The automatic loading frame consists of an automatic loading box 9 connected to a sample reaction frame 2 and a crossbeam 1, forming a fixed frame for the force-bearing system. When using the slow consolidation method for graded loading, the switch 24 is first turned on, and the stepper motor 8 drives the lead screw 10 to lift the automatic lifting platform 3, which in turn lifts the sample preparation container via the stainless steel base 21 placed on top. When the stainless steel top cover 20 of the sample preparation container contacts the axial pressure rod 4, the pressure sensor 5 fixed to the crossbeam 1 displays the applied load data. Simultaneously, the displacement gauge 6, fixed to the sample reaction frame 2 via the fixed connecting rod 11, is adjusted to contact the top surface of the stainless steel top cover 20.

[0052] The graded loading amount of the slow consolidation method is displayed by pressure sensor 5. The graded loading amounts designed for the experiment are: 1 kPa, 6 kPa, 12.5 kPa, 25 kPa, and 50 kPa. During loading, the fast forward key 27 can be pressed, and the stop key 26 is pressed when the graded loading amount displayed by pressure sensor 5 is reached. During loading, water discharged from the soil during the consolidation process flows into the miniature electronic scale 7 on the automatic loading box 9 through the permeable stone 18 and valve 16, and the mass of the discharged water is weighed. The displacement value of displacement gauge 6 is used to determine whether the consolidation of the silty clay under each load level has stabilized; the criterion is a displacement rate < 0.01 mm / h. Similarly, during unloading, the fast rewind key 25 can be pressed, and the stop key 26 is pressed when the graded unloading amount displayed by pressure sensor 5 is reached. The moisture content of the remolded silty clay sample is maintained at 35-37%, and the density is maintained at 1.95 g / cm³. 3 about.

[0053] The reshaped silty clay sample (including the three-lobed mold 12 and the stainless steel retaining ring 13) is removed from the sample preparation tube. The excess soil at the top is cut off and its moisture content is measured. The mass of the cut silty clay sample (including the three-lobed mold 12 and the stainless steel retaining ring 13) is also measured. The mass of the three-lobed mold 12 and the stainless steel retaining ring 13 is also measured. To ensure that the silty clay sample is not damaged during the replacement to form a composite soil body, hollow tubes 23 with outer diameters of 8mm, 14mm, 18mm, 24mm, 36mm, and 42.5mm are designed before replacement. Replacement space is reserved in the silty clay sample. Well-graded fine gravel is loaded into the hollow tube 23. The hollow tube 23 is then pulled out to fill the replacement space with fine gravel, completing the sample preparation of the composite soil body. The mass of the composite soil body (including the three-lobed mold 12 and the stainless steel retaining ring 13) is measured.

[0054] The following test procedures for sample loading, sample saturation, sample consolidation, and sample shearing shall be performed in accordance with the consolidation test procedure.

[0055] Sample loading: Remove the composite soil sample, remove the three-part mold 12, evenly place the latex film, and put it on the base of the pressure chamber of the triaxial consolidation apparatus. Apply a 5 kPa back pressure from below to remove air bubbles between the composite soil and the latex film; close the back pressure, use rubber rings to tighten the latex film to the top cover of the triaxial consolidation apparatus, connect the pipelines, install the outer cover of the pressure chamber of the triaxial consolidation apparatus, and then inject water from bottom to top to expel the air in the pressure chamber and seal the pressure chamber.

[0056] Sample saturation: Because the composite soil sample contains fine gravel reinforcement, if air saturation is used, some water will be lost from the gravel during the sample loading process. Therefore, counter-pressure saturation is used. To prevent the sample height from changing during saturation, the triaxial consolidation apparatus lifting platform is finely adjusted so that the top cover of the sample contacts the axial compression rod, applying a certain axial pressure to the sample.

[0057] To compress or dissolve the residual air bubbles inside the composite soil sample in water, the reverse pressure saturation method is used.

[0058] The operation steps of the reverse pressure saturation method are as follows: First, apply the initial confining pressure σ. 30 =20kPa, σ 30To introduce the initial confining pressure, open the drain valve of the pore water pressure gauge to connect it to the atmosphere, then apply a back pressure of 15 kPa. Once water continuously drains from the drain valve, close it. The back pressure loading device is located at the bottom of the sample, allowing for maximum expulsion of air bubbles between gravel particles from bottom to top. After the pore water pressure gauge value stabilizes, apply both back pressure and confining pressure in stages to minimize disturbance to the sample. The increments of both confining pressure and back pressure are 30 kPa, with the difference between the two pressures maintained at 5 kPa. This step aims to compress or dissolve any remaining air bubbles inside the sample. When the ratio of the pore water pressure increment to the confining pressure increment, ∆u / ∆σ3, is greater than 0.98, the sample is considered saturated; otherwise, the staged loading continues.

[0059] Specimen Consolidation: The isotropic consolidation method is used, where σ1 = σ3, σ1 represents the axial pressure, and σ3 represents the confining pressure. Consolidation drainage is performed on the composite soil specimen. After applying the required confining pressure, the pore water pressure gauge is connected to the pressure chamber of the triaxial consolidation apparatus. When the pore water pressure gauge reading approaches the confining pressure value, the drain valve is opened to drain the consolidation pressure. When more than 95% of the pore water pressure gauge reading has dissipated, consolidation is complete, and the drain valve is closed. The triaxial consolidation apparatus platform is adjusted so that the axial pressure rod just contacts the specimen cap at the top of the specimen.

[0060] Specimen shearing: The shearing rate was set at 0.1 mm / min, and the axial displacement was automatically recorded every 0.1 mm. During the shearing process, when the stress gauge reading showed no stress peak, the shearing phase continued until the axial strain reached 20%; when the stress gauge showed a stress peak, the test continued until the axial strain reached 20%, facilitating comparative analysis. After shearing, the confining pressure in the triaxial consolidation apparatus was reduced to 0 kPa, the pressure chamber was drained, the pressure chamber was disassembled, the specimen was removed, and its mass and moisture content were determined.

[0061] Shear strength test index: The CU test can measure pore water pressure during shearing to obtain the effective stress path.

[0062] Based on the effective stress path obtained from the composite soil CU test, an effective stress path curve was plotted with q = (σ1 - σ3) / 2 as the ordinate and p = (σ1 + σ3) / 2 as the abscissa, as shown below. Figure 6As shown. The effective stress path of the composite foundation is relatively complex. When the replacement rate is low, the effective stress path under different confining pressures shows an inverted "S-shaped" development trend. As the replacement rate increases, the effective stress path transforms into an "S-shaped" path. When the replacement rate increases to 15.08%, the effective stress path rises approximately linearly, but still retains an inflection point at the end. The inflection point of the effective stress path is the failure point of the sample during shearing. As the replacement rate increases, the axial strain corresponding to the appearance of the inflection point is larger, indicating that the composite soil has a greater ability to resist failure, and its shear strength increases with the increase of the replacement rate. The effective internal friction angle and effective cohesion are calculated through the effective stress path curve, as shown in formulas (1) and (2).

[0063] φ '=arcsin a (1), c '= b / cos φ '(2),where: a This represents the slope of the fitted line at the failure point on the stress path diagram. b This represents the intercept (kPa) of the fitted line at the failure point on the stress path diagram on the vertical axis. φ � represents the effective internal friction angle (°) of the composite soil. c ' represents the effective cohesion of the composite soil (kPa).

[0064] Composite soils were prepared according to different replacement ratios, and the above CU test was repeated. The shear strength test index of the composite soils under different replacement ratios was calculated using formulas (1) and (2). φ ' 、c ').

[0065] According to formulas (1) and (2), combined with Figure 13 Effective stress path curve, calculation of the effective internal friction angle of composite soil shear strength index φ The effective cohesion c' is calculated, and the results are shown in Table 2. A schematic diagram is shown below. Figures 6-12 As shown.

[0066] Table 2: Test parameters for shear strength of composite soil .

[0067] As shown in Table 2, as the replacement rate increases from 0 to 47.29%, the effective internal friction angle of the composite soil increases from 24.017° to 35.451°, and the effective cohesion decreases from 10.376 kPa to 0 kPa.

[0068] Shear strength calculation index: Due to the dilatation of granular materials, the radial stress on the pile-soil contact surface is higher than the confining pressure. Therefore, the stress of the composite soil is not a simple combination of the stress of each material under the same test conditions according to the area replacement ratio. Factors such as the increase of the pile confining pressure after the pile-soil lateral action should be considered to correct the pile confining pressure in the composite soil and supplement the pile-soil interaction coefficient (ξ). The expression of the total axial stress of the composite soil is shown in formula (3).

[0069] (3), where: σ c1 ξ represents the total axial stress of the composite soil (kPa), and ξ represents the pile-soil interaction coefficient. m σ represents the area replacement ratio. sc σ represents the test stress (kPa) of the matrix material. g This represents the test stress (kPa) of the reinforcing material.

[0070] Assuming that the matrix material and reinforcement material of the composite foundation fail simultaneously under external force, i.e., the Mohr-Coulomb criterion is applied to both materials simultaneously. Applying formula (4) to the Mohr-Coulomb strength criterion, we obtain: (4) Substitute into formula (3) to obtain formula (5). (5), where: c c This represents the cohesion of the composite soil (kPa). Indicates the internal friction angle (°) of the composite soil. c g This indicates the cohesive strength (kPa) of the reinforcing material. Indicates the internal friction angle (°) of the reinforcing material. c sc This represents the cohesive strength of the matrix material (kPa). Indicates the internal friction angle (°) of the matrix material.

[0071] Formula (6) and formula (7) are obtained from formula (5) to calculate the shear strength of composite soil.

[0072] (6), (7).

[0073] The value of the pile-soil interaction coefficient is related to factors such as the replacement rate, the tangent Poisson's ratio of the matrix material, and the stress level. The replacement rate is an adjustable macroscopic factor with the greatest impact. Other factors are properties of the soil and rock mass, and their effects vary depending on the replacement rate. Therefore, the pile-soil interaction coefficient with the same matrix material exhibits a strong response relationship with the replacement rate. Based on the aforementioned CU test, this invention obtains the shear strength test parameters of composite soil under different replacement rates. c＇、 ＇ ), the pile-soil interaction coefficient is inverted through formula (7). It is found by fitting that as the replacement ratio increases, the pile-soil interaction coefficient decreases in a power function, as shown in formula (8). This phenomenon conforms to the actual working situation of granular material piles.

[0074] (0 < k < 1) (8), where: k represents the power exponent value of the pile-soil interaction coefficient obtained by inverse fitting. The power exponent values of composite soils with the same matrix material and reinforcing filler are the same.

[0075] By combining formula (7) and formula (8), the optimized calculation formula (9) for the internal friction angle of the composite soil can be obtained.

[0076] (9).

[0077] In actual engineering, formula (6) and formula (9) can be used to calculate the cohesion and internal friction angle of composite soils based on the same matrix material and reinforcing filler, respectively.

[0078] Specifically, using the test data of the shear strength of the composite soil in Table 2, the pile-soil interaction coefficient is inverted through formula (7). The degree of pile-soil interaction conforms to the power function law as Figure 4 shown. The pile-soil interaction coefficient of this project ξ is shown in formula (10), and the power exponent value k = 0.41.

[0079] (10).

[0080] Therefore, the shear strength indexes ( c c 、 ) of the composite foundation based on the same matrix material and reinforcing filler in the test are optimized and calculated by formula (11) and formula (12).

[0081] (11), (12).

[0082] The above embodiments are only for explaining the structural concept and characteristics of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It is not intended to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for optimizing the calculation of shear strength of granular material pile composite foundations, characterized in that, Includes the following steps: Step 1: Soil sample preparation. The matrix sample is prepared by remolding in the room using a remolded soil sample preparation device. According to different replacement ratios, hollow tubes of different diameters are used to prepare composite soil. The latex film is evenly placed on the base of the pressure chamber of the triaxial consolidation apparatus for sample loading. The reverse pressure saturation method is used to compress or dissolve the air bubbles remaining inside the composite soil sample in water. After the sample is saturated, the composite soil sample is consolidated and drained. Step 2: Sample shearing. Set the shearing rate and axial displacement reading interval. Conduct the shearing test based on the peak value of the stress gauge reading. After the shearing test, disassemble the pressure chamber, remove the sample, weigh it, and determine the moisture content of the matrix. Step 3: Obtain the effective stress path from the composite soil CU test, plot the effective stress path curve, calculate the effective internal friction angle and effective cohesion through the effective stress path curve, prepare composite soil according to different replacement ratios, repeat the composite soil CU test, and calculate the shear strength test index of the composite soil under different replacement ratios. Step 4: Based on the factors that increase the confining pressure of the pile body after the lateral action of the pile and soil, the confining pressure of the pile body in the composite soil is corrected. The pile-soil interaction coefficient is supplemented according to the replacement rate, the tangent Poisson's ratio of the matrix material, and the stress level. The pile-soil interaction coefficient is inverted, and the relationship between the replacement rate and the pile-soil interaction coefficient is fitted to obtain the optimized internal friction angle and cohesion of the composite soil. The cohesion and internal friction angle of the composite soil based on the same matrix material and reinforcing filler are calculated.

2. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 1, characterized in that, Step 1 includes: Step 1-1: Prepare remolded soil. Use a remolded soil sample preparation device to prepare matrix samples in the room. Take out the consolidated matrix sample from the sample preparation tube. Use the cut upper excess soil to test the moisture content. Weigh the mass of the cut matrix sample including the three-lobed mold and the retaining ring, as well as the mass of the three-lobed mold and the retaining ring. Steps 1-2: Prepare composite soil. According to different replacement rates, use hollow tubes of different diameters. Reserve replacement space in the sample, fill it with bulk material, and then pull out the hollow tube to replace and form composite soil. Weigh the composite soil sample, including a three-lobed mold and a retaining ring. Steps 1-3: Sample loading. Take out the composite soil, remove the three-part mold, evenly place the latex film, and place it on the base of the pressure chamber of the triaxial consolidation apparatus. Apply back pressure from the bottom to remove air bubbles between the composite soil and the latex film. Close the back pressure, use rubber rings to tighten the latex film to the top cover, connect the pipeline, install the outer cover of the pressure chamber of the triaxial consolidation apparatus, inject water from bottom to top to expel the air in the pressure chamber, and seal the pressure chamber. Steps 1-4: Saturate the sample. Use the back pressure saturation method to compress or dissolve the air bubbles remaining inside the composite soil sample in water. Steps 1-5: Specimen consolidation. The composite soil specimens are consolidated and drained using the isotropic consolidation method.

3. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 2, characterized in that, Steps 1-4 include: Step a: Apply initial confining pressure and open the drain valve of the pore water pressure gauge to connect it to the atmosphere. Step b: Set up a back pressure loading device at the bottom of the sample, apply back pressure, and wait for water to be continuously discharged from the drain valve of the pore water pressure gauge before closing the drain valve. Step c: After the pore water pressure count value stabilizes, apply the back pressure and confining pressure in stages, keeping the difference between the confining pressure and the back pressure at 5 kPa. Step d: When the ratio of the increment of pore water pressure to the increment of confining pressure ∆u / ∆σ3 > 0.98, the sample is considered saturated; otherwise, the graded loading continues.

4. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 2, characterized in that, Steps 1-5 include: after applying the confining pressure required for the test, connecting the pore water pressure gauge to the pressure chamber; when the reading of the pore water pressure gauge is close to the confining pressure value, opening the drain valve to drain the solidification; when the reading of the pore water pressure gauge dissipates by more than 95%, closing the drain valve, and the solidification is completed.

5. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 1, characterized in that, In step 2, the shear rate is 0.1 mm / min, and the axial displacement is collected every 0.1 mm. During the shearing process, when the stress gauge reading has no stress peak, the shearing stage ends when the axial strain reaches 20%. When the stress gauge shows a stress peak, the test continues until the axial strain reaches 20%. After shearing, the confining pressure in the pressure chamber of the triaxial consolidation apparatus is reduced to 0 kPa, and the pressure chamber is disassembled after drainage.

6. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 1, characterized in that, Step 3 includes: Step 3-1: Based on the effective stress path obtained from the composite soil CU test, plot the effective stress path curve with q=(σ1-σ3) / 2 as the ordinate and p=(σ1+σ3) / 2 as the abscissa. The inflection point of the effective stress path is the failure point of the specimen during the shearing process. σ1 represents the axial pressure and σ3 represents the confining pressure. Step 3-2: Calculate the effective internal friction angle and effective cohesion using the effective stress path curve. The formula for the effective internal friction angle is: φ '=arcsin a ,in, a The slope of the fitted line at the failure point on the stress path diagram, and the formula for effective cohesion are: c '= b / cos φ ', b This represents the intercept of the fitted line at the failure point on the stress path diagram on the vertical axis. Step 3-3: Prepare composite soils according to different replacement ratios and repeat the CU test of the composite soils. Calculate the effective internal friction angle and effective cohesion of the composite soils under different replacement ratios using the formulas for the effective internal friction angle and effective cohesion.

7. The method for optimizing the calculation of shear strength of granular material pile composite foundation according to claim 1, characterized in that, Step 4 includes: Step 4-1: Based on the supplemented pile-soil interaction coefficient, obtain the expression for the total axial stress of the composite soil: , where σ c1 ξ represents the total axial stress in the composite soil, and ξ represents the pile-soil interaction coefficient. m σ represents the area replacement ratio. sc σ represents the test stress of the matrix material. g Indicates the test stress of the reinforcing material; Step 4-2: The matrix material and reinforcement material of the composite foundation fail simultaneously under external force. Applying the Mohr-Coulomb criterion to both materials, the strength criterion formula is obtained: ,in φ Indicates the angle of internal friction; Step 4-3: Substitute the strength criterion formula into the expression for the total axial stress of the composite soil to obtain... ,in, c c Indicates the cohesion of composite soil. Indicates the internal friction angle of the composite soil. c g Indicates the cohesive strength of the reinforcing material. Indicates the internal friction angle of the reinforcing material. c sc Indicates the cohesive force of the matrix material. Indicates the internal friction angle of the matrix material; Step 4-4: Obtain the formula for calculating the shear strength index of composite soil based on the formula in Step 4-3, including: Cohesion formula: , Formula for internal friction angle: ; Step 4-5: Invert the pile-soil interaction coefficient through the internal friction angle formula. It is found by fitting that the pile-soil interaction coefficient decreases in a power function as the replacement ratio increases, and the inversion formula is obtained: (0 < k < 1), where k represents the power exponent value of the pile-soil interaction coefficient obtained by inversion fitting; Steps 4-6: Combine the internal friction angle formula and the inversion formula to obtain the optimized formula for calculating the internal friction angle of composite soil: By using the cohesion formula and the internal friction angle formula of composite soil, the cohesion and internal friction angle of composite soil based on the same matrix material and reinforcing filler are calculated respectively.

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