Method for obtaining bearing capacity parameters of deep gravel-cobble layer pile foundation through lateral pressure test

Abstract

The invention discloses a method for obtaining pile foundation bearing capacity parameters of a deep gravel-cobble layer through a lateral pressure test, relates to the technical field of pile foundation bearing capacity analysis, is used for determining the pile foundation bearing capacity of the deep gravel-cobble layer, and comprises the following steps: selecting a representative drill hole, selecting a plurality of test depths based on the depth of the deep gravel-cobble layer, and determining the pile foundation bearing capacity of the deep gravel-cobble layer; determining a plurality of measuring point positions representing the drill hole based on the testing depth; lateral pressure measuring point tests are carried out on the multiple measuring point positions, and pressure and volume deformation data are recorded. According to the method, lateral pressure characteristic parameters such as in-situ horizontal pressure, critical plastic pressure and ultimate pressure of the gravel-cobble soil body are obtained, foundation pile bearing capacity design parameters are calculated by referring to corresponding empirical formulas, and pile foundation bearing capacity parameter results are demonstrated through a self-balancing type load test. The difference between the foundation pile bearing capacity design parameters obtained through the lateral pressure test and the self-balancing load test result is small, and the feasibility and accuracy of obtaining the foundation pile bearing capacity design parameters of the deep gravel-cobble layer through the lateral pressure test are proved.

Description

Technical Field

[0001] The present invention relates to the technical field of pile foundation bearing capacity analysis, and particularly to a method for obtaining pile foundation bearing capacity parameters of a deep gravel layer through a pressuremeter test. Background Art

[0002] As a granular structure system, accurately determining the in-situ mechanical properties of a deep sand layer is a difficult problem in geotechnical engineering. With the increasing convenience of transportation development, more and more large bridges have been successfully constructed. To maintain the normal operation and stability of the large bridge, increasing the depth of the bridge pile foundation undoubtedly enables the pile foundation to have sufficient bearing capacity to be transmitted to the large bridge, which also places more stringent requirements on the engineering geological conditions of the foundation rock and soil mass under the bridge. The in-situ mechanical properties of deep rock and soil mass are crucial for maintaining the stability of the upper structure and lower foundation of the bridge.

[0003] The formation of a proposed large bridge is deep gravel filled with silt and fine sand. It is difficult to accurately obtain the physical and mechanical characteristics of the deep gravel layer by using the cone penetration test suitable for testing shallow rock and soil mass. Therefore, it is difficult to obtain the in-situ physical and mechanical strength parameters of this buried stratum for the use of the pile foundation design of the large bridge. At present, the research on the pressuremeter test mainly focuses on the statistics and theoretical analysis of the pressuremeter test results. There is little research on the in-situ physical and mechanical strength properties of such deep and thick gravel soils and their application effects in engineering practice. Summary of the Invention

[0004] In order to solve the technical problem of how to determine the pile foundation bearing capacity of a deep gravel layer, the present invention provides a method for obtaining pile foundation bearing capacity parameters of a deep gravel layer through a pressuremeter test. The following technical solutions are adopted:

[0005] A method for obtaining pile foundation bearing capacity parameters of a deep gravel layer through a pressuremeter test is used to determine the pile foundation bearing capacity of a deep gravel layer. The deep gravel layer refers to a gravel layer with a relatively deep burial depth, a relatively large formation thickness, and all being dense in the cone penetration test. Specifically, it is a gravel layer with a burial depth exceeding 20 meters and a gravel layer thickness greater than 10 meters. The deep gravel layer is the main bearing stratum of the pile. The method includes the following steps:

[0006] Step 1, select a representative borehole, select multiple test depths based on the depth of the deep gravel layer, and determine multiple measuring point positions of the representative borehole based on the test depths respectively;

[0007] Step 2, conduct pressuremeter measuring point tests on multiple measuring point positions respectively, place the pressuremeter probe into the measuring point position for a pressuremeter test, and record the pressure and volume deformation data;

[0008] Step 3, in a rectangular coordinate system, with the volume deformation as the ordinate and the pressure value as the abscissa, draw multiple pressuremeter test result P-V curves of multiple measuring point positions respectively;

[0009] Step 4: Obtain the pressuremeter characteristic parameters of cobble soil at the corresponding measuring point positions through the P-V curves at different measuring point positions respectively. The pressuremeter characteristic parameters include in-situ horizontal pressure, critical pressure and ultimate pressure.

[0010] Step 5: Calculate the pile foundation bearing capacity parameters, including the characteristic value of pile tip bearing capacity and the standard value of ultimate skin friction of the pile side.

[0011] Step 6: Use the self-balanced load test to verify the accuracy of the results of the pile foundation bearing capacity parameters calculated in Step 5.

[0012] By adopting the above technical solutions, the pressuremeter test, as an ideal in-situ deep test method, has its unique advantages in determining the in-situ mechanical properties of rock and soil. Firstly, the pressuremeter tests are respectively carried out at two depth positions of the deep cobble layer, and the variation curve of the lateral pressure of the pressuremeter test with the volume change amount is obtained, and the in-situ horizontal pressure, critical pressure and ultimate pressure and other pressuremeter characteristic parameters of the cobble soil body are obtained. The design parameters of the bearing capacity of the foundation pile are calculated by referring to the corresponding empirical formulas. Finally, the results of the pile foundation bearing capacity parameters are demonstrated by the self-balanced load test.

[0013] The lateral pressure - volume change curve of the pressuremeter test for the cobble layer also has three stages: the initial compaction stage, the quasi-elastic stage and the plastic stage; the bearing capacity strength parameters of the cobble soil have an obvious law of increasing with the increase of depth; the difference between the design parameters of the bearing capacity of the foundation pile obtained by the pressuremeter test and the results of the self-balanced load test is small, which proves the feasibility and accuracy of obtaining the design parameters of the bearing capacity of the foundation pile for the deep cobble layer by the pressuremeter test.

[0014] Optionally, a calibration test is carried out before the pressuremeter measuring point test.

[0015] Optionally, the calibration test includes the following steps:

[0016] Step a: Adjust the pressure difference between the water circuit and the air circuit to make the pressure difference between the air circuit and the water circuit -1 bar.

[0017] Step b: Calibration of the pressuremeter membrane binding force: Place the pressuremeter on the horizontal ground, adjust the pressure difference to -1 bar, set the water circuit switch on the panel to the test on position, all the pressure gauge gears are located on 0 - 25 bar, add water to the instrument and let it stand for 3 min, the water level is at scale 0, and start the calibration; linearly pressurize at intervals of 0.25 bar, complete the counting within 10 s for each pressurization, pressurize once every 1 min, read and record at 30 s and 60 s, terminate the test when the water level drops to scale 650, turn off the air circuit switch, start to return water, when the water level returns to scale 0, turn off the water circuit switch, release the air, and the calibration ends.

[0018] Step c: Horizontally place the dilatometer probe inside the calibration steel pipe and conduct comprehensive calibration of the instrument according to the test method for calibrating the binding force of the dilatometer membrane.

[0019] By adopting the above technical solutions, to ensure the accuracy of the dilatometer test results, a calibration test needs to be carried out before the dilatometer test. The specific process of the calibration test is as follows: (1) Adjust the pressure difference between the water circuit and the gas circuit so that the pressure difference between the gas circuit and the water circuit is -1 bar; (2) Calibrate the binding force of the dilatometer membrane. Place the probe on the horizontal ground, adjust the pressure difference to -1 bar, turn the water circuit switch on the panel to the test position, all the pressure gauge ranges are set at 0 - 25 bar, add water to the instrument and let it stand for 3 minutes. When the water level is at scale 0, start the calibration. Apply pressure linearly at intervals of 0.25 bar. Each time pressure is applied, the counting should be completed within 10 seconds. Apply pressure once every 1 minute and record the readings at 30 seconds and 60 seconds. When the water level drops to around 650 scale, the test can be terminated. Turn off the gas circuit switch and start draining the water. When the water level returns to scale 0, turn off the water circuit switch and release the gas. The calibration is completed. Typical calibration data for the binding force of the dilatometer membrane in this test are shown in Table 1. (3) Comprehensive calibration of the instrument. Horizontally place the dilatometer probe inside the calibration steel pipe and conduct the calibration test according to the test method for calibrating the dilatometer membrane. The difference is that the pressure can be increased to 50 bar or more, and the pressure levels are in the order of 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 bar... to verify whether the instrument can be used normally. Finally, the comprehensive calibration coefficient a of the instrument is obtained as 0.0035 cm 3 / kPa.

[0020] Optionally, the test depth is determined based on the position of the cobble layer. Select two test depths, which are 2 - 3 meters below the top layer of the cobble layer and 2 - 3 meters above the bottom layer of the cobble layer, corresponding to the positions of two measuring points of the borehole.

[0021] By adopting the above technical solutions, selecting the test depths 2 - 3 meters below the top layer of the cobble layer and 2 - 3 meters above the bottom layer can ensure that the obtained test data are more representative. Such a selection can capture the top and bottom characteristics of the cobble layer, thus better understanding the mechanical properties of the entire thick cobble layer.

[0022] Testing at specific depths of the cobble layer can more accurately reflect the bearing capacity and deformation characteristics of the layer at different depths, which is crucial for pile foundation design.

[0023] Selecting two test depths can provide comprehensive mechanical property data of the cobble layer from top to bottom, helping to identify any changes in mechanical properties that may exist within the layer.

[0024] By testing the top and bottom of the cobble layer, the risk of pile foundation design caused by not understanding the mechanical properties of the entire layer thickness can be reduced.

[0025] Optionally, in step 5, the P-V curve is divided into three sections of curves, namely the preliminary stage, the pseudo-elastic stage, and the plastic stage;

[0026] The curve of the pseudo-elastic stage is linearly fitted once to obtain an elastic straight line segment, and the curve of the plastic stage is fitted with a quadratic function to obtain a plastic straight line segment;

[0027] The demarcation point between the preliminary stage curve and the elastic straight line segment is used to make a parallel axis to the vertical coordinate axis and denoted as the P-axis. The straight line segment of the elastic straight line segment is extended to intersect with the vertical coordinate axis, and the corresponding volume value of the intersection point is , from Make a parallel line to the horizontal coordinate axis, and the horizontal coordinate pressure corresponding to the intersection point of the horizontal coordinate parallel line and the P-axis is the in-situ horizontal pressure, denoted as ;

[0028] The demarcation point between the elastic straight line segment and the plastic straight line segment is used to make a parallel axis to the vertical coordinate axis and denoted as the P1-axis. The horizontal coordinate pressure corresponding to the P1-axis is the critical plastic pressure, denoted as ;

[0029] When the plastic straight line segment tends to be parallel to the vertical axis asymptote, the corresponding horizontal coordinate pressure is the ultimate pressure, denoted as .

[0030] By adopting the above technical solution, through the segmentation and fitting of the P-V curve, the pressuremeter characteristic parameters (in-situ horizontal pressure, critical plastic pressure, and ultimate pressure) of the cobble soil can be extracted more accurately. These parameters are crucial for evaluating the bearing capacity of pile foundations. The linear fitting and quadratic function fitting methods provide mathematical rigor, making the parameters extracted from the pressuremeter test data more reliable and scientific. By distinguishing the preliminary stage, the pseudo-elastic stage, and the plastic stage, the mechanical behavior of the cobble soil under different stress states can be understood more clearly, providing deeper insights for pile foundation design. Accurate in-situ horizontal pressure, critical plastic pressure, and ultimate pressure parameters help to calculate the pile tip bearing capacity and the ultimate skin friction of the pile side more accurately, thereby optimizing the pile foundation design and improving the economy and safety of the design.

[0031] Through the mathematical fitting method, the subjective error in the artificial interpretation of the curve can be reduced, and the objectivity and repeatability of the test results can be improved.

[0032] The determination of the critical plastic pressure and the ultimate pressure helps to predict the failure behavior of the soil under load, providing a basis for avoiding engineering accidents.

[0033] Optionally, the method for determining the ultimate pressure is:

[0034] Use the plastic straight line segment to extrapolate the curve to the maximum volume increment value , , is the initial volume of the middle cavity of the dilatometer, is the difference between the cavity volume and the initial volume, and the corresponding pressure is taken as the ultimate pressure .

[0035] By adopting the above technical solution, by extrapolating the plastic straight line segment to the maximum volume increment value, the ultimate pressure of the soil can be estimated more accurately, which is a key parameter for evaluating the ultimate bearing capacity of pile foundations. The determination of the ultimate pressure helps to predict that the soil will fail when the pressure is exceeded, which is crucial for ensuring the safety of engineering structures. By accurately knowing the ultimate pressure of the soil, exceeding this pressure can be avoided in the design stage, thereby improving the safety factor of pile foundation design.

[0036] Optionally, in step 5, the method for calculating the pile foundation bearing capacity parameters is as follows:

[0037] The calculation formula for the characteristic value of pile tip bearing capacity is:

[0038] ;

[0039] where is the characteristic value of pile tip bearing capacity, is the critical pressure, is the in-situ horizontal pressure;

[0040] The calculation formula for the standard value of ultimate skin friction of the pile side is:

[0041] ;

[0042] where is the standard value of ultimate skin friction of the pile side, is the ultimate pressure.

[0043] Optionally, in step 6, the self-balanced static load test is to embed a load cell in the pile body and use the self-weight of the pile body, the skin friction of the pile side and the end resistance of the pile to provide reaction forces for the test.

[0044] By adopting the above technical solution, the self-balanced load test is used for verification. The self-balanced static load test is a test method in which a load cell is embedded in the pile body and the self-weight of the pile body, the skin friction of the pile side and the end resistance of the pile are used to provide reaction forces for each other. This test method can monitor the change values of the resistance on the side of the pile foundation and the change values of the end resistance of the pile in real time and truly, and has obvious advantages in verifying the mechanical properties of deep rock and soil masses.

[0045] In summary, the present invention includes at least one of the following beneficial technical effects:

[0046] The present invention can provide a method for obtaining the bearing capacity parameters of pile foundations in deep gravelly soil layers through pressuremeter tests. Firstly, pressuremeter tests are respectively carried out at two depth positions in the deep gravelly soil layer to obtain the curve of the lateral pressure varying with the volume change in the pressuremeter test, and the in-situ horizontal pressure, critical pressure, ultimate pressure and other pressuremeter characteristic parameters of the gravelly soil are obtained. The design parameters of the pile foundation bearing capacity are calculated by referring to the corresponding empirical formulas. Finally, the results of the pile foundation bearing capacity parameters are verified through self-balanced load tests.

[0047] The curve of the lateral pressure - volume change in the pressuremeter test of the gravelly soil layer also has three stages: the initial compaction stage, the pseudo-elastic stage, and the plastic stage; the bearing capacity strength parameters of the gravelly soil have an obvious law of increasing with the increase in depth; the difference between the design parameters of the pile foundation bearing capacity obtained from the pressuremeter test and the results of the self-balanced load test is small, which proves the feasibility and accuracy of obtaining the design parameters of the pile foundation bearing capacity in the deep gravelly soil layer through the pressuremeter test. Description of the Drawings

[0048] Figure 1 is a schematic flow chart of the method for obtaining the bearing capacity parameters of pile foundations in deep gravelly soil layers through the pressuremeter test of the present invention;

[0049] Figure 2 is a schematic diagram of the P-V curve at a depth of 26 m in a specific embodiment of the present invention;

[0050] Figure 3 is a schematic diagram of the P-V curve at a depth of 32 m in a specific embodiment of the present invention;

[0051] Figure 4 is a schematic diagram of the segmented fitting result of the P-V curve at a depth of 26 m in a specific embodiment of the present invention;

[0052] Figure 5 is a schematic diagram of the segmented fitting result of the P-V curve at a depth of 32 m in a specific embodiment of the present invention;

[0053] Figure 6 is a schematic diagram of the obtained result of the pressuremeter characteristic parameters at a depth of 26 m in a specific embodiment of the present invention;

[0054] Figure 7 is a schematic diagram of the obtained result of the pressuremeter characteristic parameters at a depth of 32 m in a specific embodiment of the present invention;

[0055] Figure 8 is a schematic diagram of the fitting result of the pressuremeter characteristic parameters in a specific embodiment of the present invention;

[0056] Figure 9 is a schematic diagram of the curve of the pile settlement - incremental load change in a specific embodiment of the present invention;

[0057] Figure 10It is a schematic diagram of the axial force distribution of the 8-4 pile in a specific embodiment of the present invention;

[0058] Figure 11 It is a schematic diagram of the side friction-displacement curve of the 8-4 pile in a specific embodiment of the present invention. Specific embodiments

[0059] The present invention will be further described in detail below with reference to the accompanying drawings.

[0060] An embodiment of the present invention discloses a method for obtaining pile foundation bearing capacity parameters in a deep gravel layer by a pressuremeter test.

[0061] Refer to Figures 1-11 , Embodiment 1, a method for obtaining pile foundation bearing capacity parameters in a deep gravel layer by a pressuremeter test, used to determine the pile foundation bearing capacity of a deep gravel layer. The method includes the following steps:

[0062] Step 1, select a representative borehole, select multiple test depths based on the depth of the deep gravel layer, and determine multiple measuring point positions of the representative borehole based on the test depths;

[0063] Step 2, conduct pressuremeter test points tests on multiple measuring point positions respectively. Place the pressuremeter probe into the measuring point position for a pressuremeter test, and record the pressure and volume deformation data;

[0064] Step 3, in a rectangular coordinate system, with the volume deformation as the ordinate and the pressure value as the abscissa, draw multiple P-V curves of the pressuremeter test results at multiple measuring point positions respectively;

[0065] Step 4, obtain the pressuremeter characteristic parameters of the gravel soil at the corresponding measuring point positions through the P-V curves at different measuring point positions respectively. The pressuremeter characteristic parameters include in-situ horizontal pressure, critical pressure, and ultimate pressure;

[0066] Step 5, calculate the pile foundation bearing capacity parameters, which include the characteristic value of the pile tip bearing capacity and the standard value of the ultimate side friction of the pile;

[0067] Step 6, use a self-balanced load test to verify the accuracy of the pile foundation bearing capacity parameter results calculated in Step 5.

[0068] As an ideal in-situ test method for deep layers, the pressuremeter test has its unique advantages in determining the in-situ mechanical properties of rock and soil. First, pressuremeter tests were conducted on two depth positions of the deep gravel layer respectively, and the variation curves of the lateral pressure of the pressuremeter test with the volume change were obtained, and the in-situ horizontal pressure, critical pressure, and ultimate pressure and other pressuremeter characteristic parameters of the gravel soil were obtained. The design parameters of the pile foundation bearing capacity were calculated by referring to the corresponding empirical formulas, and finally the pile foundation bearing capacity parameter results were demonstrated by a self-balanced load test.

[0069] The lateral pressure - volume change curve of the pressuremeter test in the cobble - gravel layer also has three stages: the initial compaction stage, the pseudo - elastic stage, and the plastic stage; the bearing capacity strength parameters of cobble - gravel soil have an obvious law of increasing with the increase in depth; the design parameters of the bearing capacity of the foundation pile obtained from the pressuremeter test have little difference from the results of the self - balanced load test, which proves the feasibility and accuracy of obtaining the design parameters of the bearing capacity of the foundation pile in the deep cobble - gravel layer by the pressuremeter test.

[0070] Example 2: A calibration test is carried out before the pressuremeter test at the measuring point.

[0071] Example 3: The calibration test includes the following steps:

[0072] Step a: Adjust the pressure difference between the water circuit and the air circuit so that the pressure difference between the air circuit and the water circuit is - 1 bar.

[0073] Step b: Calibration of the binding force of the pressuremeter membrane: Place the pressuremeter on the horizontal ground, adjust the pressure difference to - 1 bar, set the water - circuit switch on the panel to the test position, all the pressure - gauge gears are set at 0 - 25 bar, add water to the instrument and let it stand for 3 min. When the water level is at scale 0, start the calibration; linearly pressurize at intervals of 0.25 bar. Each time after pressurization, complete the counting within 10 s. Pressurize once every 1 min and record the readings at 30 s and 60 s. Terminate the test when the water level drops to the 650 scale. Turn off the air - circuit switch and start the water return. When the water level returns to the 0 scale, turn off the water - circuit switch and release the air. The calibration is completed.

[0074] Step c: Horizontally place the pressuremeter probe in the calibration steel pipe and conduct the comprehensive calibration of the instrument according to the test method of the calibration of the binding force of the pressuremeter membrane.

[0075] To ensure the accuracy of the pressuremeter test results, a calibration test needs to be carried out before the pressuremeter test. The specific process of the calibration test is as follows: (1) Adjust the pressure difference between the water circuit and the air circuit so that the pressure difference between the air circuit and the water circuit is -1 bar; (2) Calibration of the pressuremeter cell binding force. Place the probe on the horizontal ground, adjust the pressure difference to -1 bar, turn the water circuit switch on the panel to the test position, all the pressure gauge ranges are set at 0 - 25 bar, add water to the instrument and let it stand for 3 min, the water level is at scale 0, and start the calibration. The pressure is linearly increased at intervals of 0.25 bar. Each time the pressure is increased, the counting should be completed within 10 s, and the pressure is increased every 1 min. Readings are recorded at 30 s and 60 s. When the water level drops to about 650 scale, the test can be terminated. Turn off the air circuit switch and start the water return. When the water level returns to scale 0, turn off the water circuit switch and release the air, and the calibration ends. The typical calibration data of the pressuremeter cell binding force in this test is shown in Table 1. (3) Comprehensive calibration of the instrument. Place the pressuremeter probe horizontally in the calibration steel pipe and conduct the calibration test according to the test method of the pressuremeter cell calibration. The difference is that the pressure can be increased to 50 bar or more, and the pressure levels are in the order of 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 bar...... to verify whether the instrument can be used normally. Finally, the comprehensive calibration coefficient a of the instrument is obtained as 0.0035 cm 3 / kPa.

[0076] In Example 4, the test depth is determined based on the position of the cobble layer. Two test depths are selected, which are 2 - 3 m below the top layer of the cobble layer and 2 - 3 m above the bottom layer of the cobble layer, corresponding to the positions of two measuring points of the representative borehole.

[0077] Selecting the test depths 2 - 3 m below the top layer of the cobble layer and 2 - 3 m above the bottom layer can ensure that the obtained test data is more representative. Such a selection can capture the top and bottom characteristics of the cobble layer, thus better understanding the mechanical properties of the entire thick cobble layer.

[0078] Testing at specific depths of the cobble layer can more accurately reflect the bearing capacity and deformation characteristics of the layer at different depths, which is crucial for pile foundation design.

[0079] Selecting two test depths can provide comprehensive mechanical property data of the cobble layer from top to bottom, helping to identify any possible changes in mechanical properties within the layer.

[0080] By testing the top and bottom of the cobble layer, the risk of pile foundation design caused by not understanding the mechanical properties of the entire layer thickness can be reduced.

[0081] In Example 5, in step 5, the P - V curve is divided into three segments, namely the initial stage, the pseudo - elastic stage, and the plastic stage;

[0082] The curve in the pseudo-elastic stage is linearly fitted once to obtain the elastic straight-line segment, and the curve in the plastic stage is fitted with a quadratic function to obtain the plastic straight-line segment;

[0083] The demarcation point between the initial-stage curve and the elastic straight-line segment is used to draw a parallel axis to the vertical coordinate axis, denoted as the P-axis. The straight-line segment of the elastic straight-line segment is extended to intersect with the vertical coordinate axis, and the volume value corresponding to the intersection point is , from draw a parallel line to the abscissa. The horizontal pressure corresponding to the intersection point of the horizontal abscissa line and the P-axis is the in-situ horizontal pressure, denoted as ;

[0084] The demarcation point between the elastic straight-line segment and the plastic straight-line segment is used to draw a parallel axis to the vertical coordinate axis, denoted as the P1-axis. The horizontal pressure corresponding to the P1-axis is the critical plastic pressure, denoted as ;

[0085] When the plastic straight-line segment approaches the asymptote parallel to the vertical axis, the horizontal pressure corresponding to it is the ultimate pressure, denoted as .

[0086] By segmenting and fitting the P-V curve, the pressuremeter characteristic parameters (in-situ horizontal pressure, critical plastic pressure, and ultimate pressure) of cobble and gravel soil can be extracted more accurately. These parameters are crucial for evaluating the bearing capacity of pile foundations. The linear fitting and quadratic function fitting methods provide mathematical rigor, making the parameters extracted from pressuremeter test data more reliable and scientific. By distinguishing the initial stage, pseudo-elastic stage, and plastic stage, the mechanical behavior of cobble and gravel soil under different stress states can be understood more clearly, providing deeper insights for pile foundation design. Accurate in-situ horizontal pressure, critical plastic pressure, and ultimate pressure parameters help to calculate the pile tip bearing capacity and the ultimate side friction resistance of the pile more accurately, thereby optimizing the pile foundation design and improving the economy and safety of the design.

[0087] Through the mathematical fitting method, the subjective error in manually interpreting the curve can be reduced, and the objectivity and repeatability of the test results can be improved.

[0088] The determination of the critical plastic pressure and the ultimate pressure helps to predict the failure behavior of the soil under load, providing a basis for avoiding engineering accidents.

[0089] Example 6. The method for determining the ultimate pressure is as follows:

[0090] Use the plastic straight-line segment and extrapolate the curve to the maximum volume increment value , , is the initial volume of the middle chamber of the pressuremeter, is the difference between the cavity volume and the initial volume. Take the pressure corresponding to as the ultimate pressure .

[0091] By extrapolating the plastic straight line segment to the maximum volume increment value, the ultimate pressure of the soil can be estimated more accurately, which is a key parameter for evaluating the ultimate bearing capacity of pile foundations. The determination of the ultimate pressure helps to predict the failure of the soil when the pressure is exceeded, which is crucial for ensuring the safety of engineering structures. By accurately knowing the ultimate pressure of the soil, exceeding this pressure can be avoided in the design stage, thereby increasing the safety factor of pile foundation design.

[0092] In Example 7, in step 5, the method for calculating the pile foundation bearing capacity parameters is as follows:

[0093] The calculation formula for the characteristic value of pile tip bearing capacity is:

[0094] ;

[0095] where is the characteristic value of pile tip bearing capacity, is the critical pressure, is the in-situ horizontal pressure;

[0096] The calculation formula for the standard value of the ultimate skin friction of the pile is:

[0097] ;

[0098] where is the standard value of the ultimate skin friction of the pile, is the ultimate pressure.

[0099] In Example 8, in step 6, the self-balanced static load test is to embed a load cell in the pile body and use the self-weight of the pile body, the skin friction of the pile and the tip resistance of the pile to provide reaction forces for the test.

[0100] The self-balanced load test is used for verification. The self-balanced static load test is a test method in which a load cell is embedded in the pile body and the self-weight of the pile body, the skin friction of the pile and the tip resistance of the pile are used to provide reaction forces for each other. This test method can monitor the change values of the resistance on the side of the pile foundation and the change values of the tip resistance of the pile in real time and truly, and has obvious advantages in verifying the mechanical properties of deep geotechnical bodies.

[0101] The following uses specific examples to illustrate the implementation principle of the method for obtaining pile foundation bearing capacity parameters by the pressuremeter test of the present invention in a deep cobble layer:

[0102] The test site of a certain bridge is located in an alluvial plain of a river, and the ground elevation is mainly between 31.30 - 33.50 m. The overlying strata in this area are mainly fill layers with a relatively thin thickness, ranging from about 0 - 2 m; the middle strata are mainly silty clay and silty sand layers with an average thickness of about 10 m; the underlying strata are mainly gravel layers, and the main components of the bedrock are sandstone, granite and siliceous rock, filled with silt and fine sand. The general particle size of the drilling samples is 1 - 5 cm, and the visible maximum particle size can reach 15 cm. The rounding is good and the sorting is average, with a thickness greater than 30 meters. The main faults near the field area are the concealed Jiazhuyuan Fault, the Gong'an - Jianli Fault Zone and relatively far - away faults. The seismic fortification intensity of this area is degree 6, the thickness of the soil layer coverage is greater than 150 m, and no structural anomalies are found within 200 m below the ground surface. The groundwater mainly includes pore phreatic water and pore confined water, with weak water permeability, mainly receiving atmospheric precipitation recharge and mainly discharging by evaporation. In order to determine the pile foundation bearing capacity of the deep gravel layer, representative boreholes are selected for pressuremeter tests, and a total of 2 measuring points in the gravel layer are completed, located at the depths of 26 m and 32 m respectively.

[0103] Pressuremeter test plan and process:

[0104] To ensure the accuracy of the pressuremeter test results, a calibration test needs to be carried out before the pressuremeter test. The specific process of the calibration test is as follows: (1) Adjust the pressure difference between the water circuit and the air circuit so that the pressure difference between the air circuit and the water circuit is - 1 bar; (2) Calibration of the pressuremeter membrane restraint force. Place the probe on the horizontal ground, adjust the pressure difference to - 1 bar, set the water circuit switch on the panel to the test position, all the pressure gauge gears are located on 0 - 25 bar, add water to the instrument and let it stand for 3 min, the water level is at the 0 scale, and start the calibration. The pressure is linearly pressurized at intervals of 0.25 bar. Each time the pressure is increased, the counting should be completed within 10 s, and the pressure is increased every 1 min, and the readings are recorded at 30 s and 60 s. When the water level drops to about 650 scale, the test can be terminated. Turn off the air circuit switch and start to drain the water. When the water level returns to the 0 scale, turn off the water circuit switch and release the air, and the calibration ends. The typical calibration data of the pressuremeter membrane restraint force for this test are shown in Table 1. (3) Comprehensive calibration of the instrument. Place the pressuremeter probe horizontally in the calibration steel pipe and carry out the calibration test according to the test method of the pressuremeter membrane calibration. The difference is that the pressure can be increased to 50 bar or more, and the pressure levels are carried out in the order of 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 bar...... to verify whether the instrument can be used normally. Finally, the comprehensive calibration coefficient a of the instrument is obtained as 0.0035 cm 3 / kPa.

[0105] Table 1 Data table for calibration of pressuremeter membrane restraint force

[0106]

[0107] After the above instrument inspection and calibration are completed, the measuring point test can be carried out. This time, the cobble layer is mainly selected as the test object, and the test depths are 26m and 32m respectively. First, place the probe at the measuring point position, pay attention to protecting the pipeline and the probe, turn on the water and gas circuit switches, the water level should remain basically unchanged, and then pressurize every 1 minute in sequence according to 0, 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0 bar... For each pressurization, read and record the readings at 15s, 30s, and 60s. When the water level drops to around the 600 - 650 scale, return water and gas to make it rise steadily. When the gas circuit pressure drops below 5 bar, turn off the gas circuit switch and return water. When the water level reaches the 0 scale, turn off the water circuit switch. Turn on the gas circuit switch, exhaust until completely exhausted, let it stand for 3 minutes, gently lift the probe out of the borehole, and the measurement is completed.

[0108] Results of the pressuremeter test: After data calibration, the results of the pressuremeter test at the measuring points with buried depths of 26m and 32m in the cobble layer are shown in Figure 2 、 Figure 3 . In the rectangular coordinate system, with the volume deformation V (cm 3 ) as the ordinate and the pressure P (kPa) as the abscissa, the pressuremeter test results P (kPa)-V (cm 3 ) curves at the positions of 26m and 32m are respectively plotted.

[0109] In the figure, the P (kPa)-V (cm 3 ) curve can be divided into three segments: (1) The initial curve segment is the preliminary stage; (2) The quasi-elastic stage, where the pressure and the volume change are approximately in a linear relationship; (3) The tail curve segment is in the plastic stage, and with the increase of pressure, the volume change increases rapidly. It can be seen from Figure 2 、 Figure 3 that these three stages of the curve are relatively obvious.

[0110] For Figure 2 、 Figure 3 , polynomial piecewise fitting is carried out for the quasi-elastic process and the plastic yield process. For the quasi-elastic process of the test, first-order linear fitting is adopted, and for the plastic yield process of the test, quadratic function fitting is adopted. The collective results are shown in Figure 4 、 Figure 5 . It can be known from Figure 4 and Figure 5 that the correlation coefficients R2 of the fitting results of the approximate elastic process are 0.985 and 0.963 respectively, and the correlation coefficients R2 of the fitting results of the plastic yield process are 0.999 and 0.998 respectively, indicating that the test data effects at these two measuring points are ideal.

[0111] Analysis of test results shows that the in-situ horizontal pressure, critical pressure, and ultimate pressure. In fact, with the increase in depth, the mechanical properties of cobble soil should gradually become stronger. As shown in Table 2, there is also a depth effect on the in-situ mechanical properties of cobble soil, that is, the mechanical strength parameters of cobble soil at deeper positions are higher.

[0112] According to the pressuremeter test results of P(kPa)-V(cm 3 ) curve, three important mechanical parameters of cobble soil can be obtained, namely the in-situ horizontal pressure, critical pressure, and ultimate pressure of the soil mass. The in-situ horizontal stress of cobble soil is the stress value in the horizontal direction when the soil mass is in stable equilibrium at a certain depth. The results are shown in Table 2; extend the straight line segment of the pressuremeter curve to intersect with the vertical axis, and the intersection point is , from draw a parallel line to the P-axis to intersect with a point on the curve, and the corresponding pressure is value, as shown specifically in Figure 6 , Figure 7 . The critical pressure reflects the stress boundary value at which the soil mass changes from the elastic deformation state to the plastic yield state. The results are shown in Table 2.

[0113] Table 2 Summary of strength parameters of pressuremeter test results

[0114]

[0115] Generally speaking, the end point of the straight line segment of the pressuremeter test P(kPa)-V(cm 3 ) curve, that is, the pressure corresponding to the tangent point of the curve and the straight line segment is Pf, as shown specifically in Figure 6 , Figure 7 . After the curve passes the critical pressure and tends to be parallel to the vertical axis asymptote, the corresponding pressure is the ultimate pressure Pl value. The quadratic function fitting curve extrapolation method can be used to the maximum volume increment value , where , is the initial volume of the middle chamber of the pressuremeter, is the difference between the cavity volume and the initial volume, and the pressure corresponding to the obtained is taken as the ultimate pressure .

[0116] Furthermore, a simple linear fitting analysis is carried out on the in-situ horizontal pressure, critical pressure, and ultimate pressure obtained from the tests at the 26m and 32m positions. The specific results are shown in Figure 8 . From Figure 8 , the in-situ pressuremeter stress values of the cobble layer at different depths can be preliminarily estimated, and then the pile foundation bearing capacity strength parameters at different depth positions can be calculated using empirical formulas.

[0117] It should be noted that the test is a pre-bored pressuremeter test. Considering that the test soil is gravelly soil, during the process of drilling, due to the characteristics of its granular structure, the gravel is extremely prone to borehole collapse, which will cause differences between the measured horizontal stress value and the in-situ stress value at the test point. Therefore, special attention should be paid at the beginning of the test to minimize the disturbance to the structure and stress state of the soil around the borehole, ensure that the measured horizontal stress value in the test is close to the true stress value, and at the same time ensure the accuracy of the obtained critical plastic pressure and ultimate pressure values.

[0118] The design parameters of pile foundation bearing capacity mainly include the characteristic value of pile tip bearing capacity and the standard value of ultimate skin friction of pile side . The standard value of foundation bearing capacity and the standard value of ultimate skin friction of pile side are calculated according to the following formulas respectively.

[0119] ;

[0120] ;

[0121] It can be seen from Table 3 that the bearing capacity strength of the gravel layer increases with the increase of depth, that is, the bearing capacity strength shows a depth effect.

[0122] Table 3 Results of pressuremeter test strength parameters

[0123]

[0124] In order to verify the accuracy of the obtained pile foundation bearing capacity parameter values from the in-situ pressuremeter test, a self-balanced load test is adopted in this paper for verification. The self-balanced static load test is a test method in which a load cell is embedded in the pile body and the self-weight of the pile body, the skin friction of the pile side and the end resistance of the pile provide reaction forces for each other. This test method can monitor the change values of the side resistance and the end resistance of the pile foundation at any time and truly, and has obvious advantages in verifying the mechanical properties of deep rock and soil masses.

[0125] Test scheme and process: The test pile in this test is numbered as test pile 8-4, the pile type is cast-in-place pile, the concrete grade used is C35, the pile diameter is 2200mm, the pile top elevation is about 31.387m, and the pile bottom elevation is -3.513m.

[0126] The specific test process is as follows: The load cell of test pile 8-4 was loaded. When the load reached 15 levels of 2×14400 kN, the total downward displacement was greater than 40 mm, so the loading was terminated. Then, the upper load cell of test pile 8-4 was loaded. When the load reached 12 levels of 2×8000 kN, the downward displacement increased steeply. The displacement of this load level was more than 5 times that of the previous level, and the total displacement was greater than 40 mm. The bearing capacity of the middle section of the pile reached the limit. At this time, the lower load cell was closed, and the middle and lower sections of the pile jointly provided the reaction force to continue loading and test the bearing capacity of the upper section of the pile. When the upper load cell continued to be loaded to 16 levels of 2×10677 kN, the upward displacement of the upper load cell increased suddenly. The total displacement was greater than 40 mm, and the displacement of this load level was more than 5 times that of the previous level. The bearing capacity of the upper section of the pile reached the limit and the load could not be stabilized, so the loading was terminated. The specific load level-displacement change process is shown in Figure 9 。

[0127] Figure 10 The curve showing the variation of the axial force at each part of the pile body is presented. Analyzing from the perspective of the load application level, it can be seen from the figure that as the load level increases, the axial force at each part of the pile body also shows a gradually increasing trend. This is because as the load increases, the strength of the pile itself to resist the load also comes into play. Analyzing from the perspective of the pile body elevation, from the pile top to the pile bottom, the axial force of the pile also shows a gradually increasing trend, which also indicates that the pile has the function of transmitting the load deep into the ground.

[0128] Distribution results of the lateral friction resistance of the pile foundation: Figure 11 The distribution change results of the lateral friction resistance of the pile foundation with the settlement displacement of the pile are presented. It can be seen from Figure 11 that as the load level at the pile top increases, the settlement displacement of the pile also increases, and the lateral friction resistance at each part of the pile side shows an increasing trend. This is because as the load increases, the resistance of the soil layer around the pile to the sinking of the pile body also comes into full play.

[0129] In addition, in order to quantify the relationship between the lateral friction resistance of the pile and the pile body displacement, a power function model was used to perform nonlinear fitting on the friction resistance change curve. The results show that the fitting degree is basically stable above 0.9, and the fitting effect is good.

[0130] The measured value of the friction resistance of the cobble layer was calculated to be 134 kPa - 166 kPa, as shown in Table 4:

[0131] Table 4 Results Table of Ultimate Lateral Friction Resistance of Self-Balanced Static Load Test

[0132]

[0133] The table shows that as the depth of the cobble layer increases, the lateral friction resistance also increases, which also indirectly verifies the depth effect revealed by the pressuremeter test results. According to the pressuremeter test, the lateral friction resistance value at a depth of 26 m is 134.5 kPa, and the lateral friction resistance value at an elevation of 32 m is 154.15 kPa. By comparison, it can be seen that the measured values of the lateral friction resistance have little difference. Therefore, the pressuremeter test is completely suitable for determining the bearing capacity parameters of the cobble layer pile foundation.

[0134] The above are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, all equivalent changes made according to the structure, shape, and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. Method for obtaining pile foundation bearing capacity parameters of deep gravel layer by pressuremeter test, characterized in that, The method for determining the bearing capacity of pile foundations in deep gravel layers comprises the following steps: Step 1: Select a representative borehole, select multiple test depths based on the depth of the thick gravel layer, and determine the positions of multiple measuring points of the representative borehole based on the test depths; Step 2: Perform lateral pressure test at multiple measuring points respectively, place the lateral pressure probe at the measuring point to perform the lateral pressure test, and record the pressure and volume deformation data; Step 3: In a rectangular coordinate system, with the volume deformation as the ordinate and the pressure value as the abscissa, draw multiple PV curves of the lateral pressure test results at multiple measuring points. Step 4, obtaining the lateral pressure characteristic parameters of the gravel soil at the corresponding measuring point through the PV curves at different measuring points, the lateral pressure characteristic parameters including in-situ horizontal pressure, plastic pressure and ultimate pressure; Step 5: Calculate the pile foundation bearing capacity parameters, which include the pile end bearing capacity characteristic value and the pile side ultimate friction resistance standard value; Step 6: Use a self-balancing load test to verify the accuracy of the pile foundation bearing capacity parameter results calculated in step 5.

2. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer by a lateral pressure test according to claim 1, characterized in that A calibration test is performed before the lateral pressure measuring point test.

3. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer through a pressuremeter test according to claim 2, characterized in that, The calibration test consists of the following steps: Step a, adjusting the pressure difference between the water channel and the gas channel so that the pressure difference between the gas channel and the water channel is -1 bar; Step b, calibrating the restraining force of the lateral pressure membrane: Place the lateral pressure gauge on a horizontal surface, adjust the pressure difference to -1 bar, set the water circuit switch on the panel to the test state, and set all the pressure gauges to 0-25 bar. Fill the instrument with water and let it stand for 3 minutes. When the water level is at the 0 mark, start calibration. Increase the pressure linearly in intervals of 0.25 bar, counting the pressure within 10 seconds for each increase. Increase the pressure once every 1 minute, and record the readings at 30 seconds and 60 seconds. The test is terminated when the water level drops to 650. Set the air circuit switch to off, start water return, and when the water level returns to the 0 mark, close the water circuit switch, bleed the air, and the calibration is complete. Step c: Place the lateral pressure gauge probe horizontally in the calibration steel pipe and perform comprehensive calibration of the instrument according to the test method for lateral pressure membrane restraint force calibration.

4. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer through a lateral pressure test according to claim 1, characterized in that The test depth is determined based on the position of the gravel layer. Two test depths are selected: 2-3 meters below the top layer of the gravel layer and 2-3 meters above the bottom layer of the gravel layer, corresponding to the two measuring point positions representing the borehole.

5. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer through a lateral pressure test according to claim 1, characterized in that, In step 5, the PV curve is divided into three segments, namely the initial stage, the quasi-elastic stage and the plastic stage; The curve of the quasi-elastic stage is fitted with a linear function to obtain the elastic straight line segment, and the curve of the plastic stage is fitted with a quadratic function to obtain the plastic straight line segment; The demarcation point between the initial stage curve and the elastic straight line segment is used to draw a parallel axis to the vertical coordinate axis, denoted as the P-axis. The straight line segment of the elastic straight line segment is extended to intersect with the vertical coordinate axis, and the corresponding volume value at the intersection point is , and from draw a parallel line to the abscissa. The horizontal pressure corresponding to the intersection point of the horizontal abscissa line and the P-axis is the in-situ horizontal pressure, denoted as ; The parallel axis to the vertical coordinate axis passing through the demarcation point between the elastic straight line segment and the plastic straight line segment is denoted as the P1 axis, and the horizontal coordinate pressure corresponding to the P1 axis is the critical plastic pressure, denoted as ; When the plastic straight-line segment approaches an asymptote parallel to the vertical axis, the corresponding horizontal-axis pressure is the ultimate pressure, denoted as .

6. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer through a lateral pressure test according to claim 5, characterized in that The ultimate pressure is determined by: Use the plastic straight-line segment and extrapolate the curve to the maximum volume increment value , , is the initial volume of the middle cavity of the dilatometer, is the difference between the cavity volume and the initial volume, and the corresponding pressure is taken as the limit pressure .

7. The method for obtaining pile foundation bearing capacity parameters of a deep cobble layer by a lateral pressure test according to claim 6, characterized in that In step 5, the method for calculating the pile foundation bearing capacity parameters is: The calculation formula for the characteristic value of pile tip bearing capacity is: ； wherein is the characteristic value of pile tip bearing capacity, is the critical edge pressure, is the in-situ horizontal pressure; The calculation formula for the standard value of the ultimate friction resistance of the pile side is: ； Among them is the standard value of the ultimate skin friction of the pile is the ultimate pressure 8. The method for obtaining pile foundation bearing capacity parameters of deep cobble-gravel layer by pressuremeter test according to claim 7, characterized in that, In step 6, the self-balancing static load test is performed by embedding a load box in the pile body and utilizing the pile body's deadweight, pile side resistance, and pile end resistance to provide reaction forces to each other.

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