Method for testing bearing capacity of pile foundation under influence of swelling and shrinking deformation of expansive soil

Abstract

The present application belongs to the technical field of swelling soil research, and discloses a kind of swelling soil expansion and contraction deformation on pile foundation bearing performance test method, to solve the problem that existing test device and test method cannot comprehensively and accurately reflect the influence of swelling soil expansion and contraction on pile foundation bearing performance.The present application simulates the environment of the pile foundation to be arranged (i.e., arranging pile foundation in a new site), and simulates the real environment of the pile foundation to be arranged by a test pile in the laboratory, monitors the influence of swelling soil expansion and contraction on the actual bearing performance of the pile foundation in the real environment, thereby establishing scientific and comprehensive test data (i.e., first test data, second test data, third test data and fourth test data), obtaining more accurate and comprehensive data, and comprehensively judging the influence of swelling soil expansion and contraction on the bearing performance of the pile foundation according to the test data.

Description

Technical Field

[0001] This invention belongs to the field of expansive soil research technology, specifically relating to a test method for the effect of expansive soil swelling and shrinkage deformation on the bearing capacity of pile foundations. Background Technology

[0002] Expansive soil is a type of cohesive soil that expands dramatically in volume when soaked in water and shrinks significantly in volume when dehydrated. Due to its high content of clay minerals such as montmorillonite and illite, it is highly hydrophilic. When the natural moisture content is high, the expansion amount and force after soaking are relatively small, while the shrinkage amount and force after dehydration are very large. The larger the natural void ratio, the smaller the expansion amount and force, and the larger the shrinkage amount and force. This type of soil can cause serious damage to buildings. Therefore, there is currently a considerable amount of research on the swelling and shrinkage properties of expansive soil.

[0003] For example, application number 2020111670055 discloses a multifunctional unsaturated expansive soil swelling and shrinkage test device and its test method, including a soil sample chamber unit, an environmental humidity simulation unit, and a data measurement and control unit. This technical document utilizes the wide range of water potential variation of PEG solutions and their ease of adjustment through concentration, supplemented by a soil sample chamber with a semi-permeable membrane as its core and an environmental humidity simulation structure: it can use low-concentration PEG solutions to simulate the ultra-humidity environment of roadbeds or slopes being submerged under heavy rain or storms; it can use medium-concentration PEG solutions to simulate the water absorption or loss environment of roadbeds or slopes under light rain or high-humidity overcast conditions; and it can use high-concentration PEG solutions to simulate the low-humidity, dry environment of roadbeds or slopes losing water under dry conditions such as sunny days. In other words, this invention can conveniently simulate and test the water absorption swelling and water loss shrinkage characteristics of unsaturated expansive soils under different natural environmental humidity levels.

[0004] For example, application number 2016201843790 discloses a test device for measuring the swelling and shrinkage deformation of soil samples during wet-dry cycles. The device includes a test mold; a sample ring placed in the test mold for holding the soil sample to be measured; an upper permeable stone placed above the sample ring for injecting water into the soil sample to conduct a soil swelling test; a lower permeable stone placed below the sample ring for draining excess water injected into the soil sample; a heating component placed around the test mold for heating the swollen soil sample obtained after the swelling test to dry the soil sample and conduct a soil shrinkage test; and a measuring component placed above the upper permeable stone for measuring the soil sample swelling deformation caused by water injection through the upper permeable stone, and / or measuring the soil sample shrinkage deformation caused by heating the swollen soil sample through the heating component. This technical document can perform both swelling and shrinkage deformation tests on soil samples, and can evaluate the water absorption swelling and water loss shrinkage characteristics of soil samples.

[0005] However, the aforementioned technical literature only studies the swelling and shrinkage characteristics of expansive soil itself, and has no significance for the study of the bearing capacity of pile foundations in expansive soil.

[0006] Application No. 201921364772 discloses a multi-directional loading test device for expansive soil pile foundations. This technical document conducts pile foundation tests on expansive soil before and after immersion (i.e., tests are conducted by applying pressure to the pile foundation). However, the pile foundation test data obtained in this way is limited and cannot accurately reflect the influence of the expansion and contraction of expansive soil on the actual bearing capacity of the pile foundation.

[0007] For example, application number 202021232050X discloses a static load testing device for pile foundations in expansive soil, including a pressure loading device, a model pile, an expansive soil compaction cylinder, and a drainage valve. The outer surface of the model pile is uniformly covered with a concrete layer, on which multiple strain sensors are fixed at equal intervals from top to bottom. The model pile is vertically inserted into the expansive soil compaction cylinder, with its bottom contacting the pressure sensors inside. The pressure loading end of the pressure loading device contacts the top of the model pile, applying vertical pressure. The drainage valve is located at the bottom of the expansive soil compaction cylinder. The expansive soil compaction cylinder contains gravel, medium sand, and expansive soil samples laid out sequentially from bottom to top, and includes horizontal and vertical sand permeation layers. While this technical document improves the uniformity of water permeation in expansive soil to some extent, the device cannot accurately control the amount of water absorbed, thus failing to comprehensively measure the impact of expansive soil expansion and contraction on the bearing capacity of the pile foundation. Summary of the Invention

[0008] To address the problem that existing testing devices and methods cannot comprehensively and accurately reflect the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations, this invention provides a testing method for the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations. Compared with existing technologies, this method can obtain more accurate and comprehensive data, thereby more realistically reflecting the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations.

[0009] To solve the technical problem, the technical solution adopted by this invention is as follows:

[0010] A test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations, characterized by the following:

[0011] (1) The vertical earth pressure above the expansive soil in the stratum and the data of the pile foundation to be arranged in the stratum are obtained by measurement and calculation. The pile foundation data includes, but is not limited to, pile foundation size, pile foundation material, pile foundation top load, pile foundation vertical ultimate bearing capacity, pile foundation bending strength, etc.

[0012] (2) Based on the geological exploration report, obtain the stratigraphic information of the strata where the pile foundations are to be laid and the water content of each stratum;

[0013] (3) The vertical earth pressure above the expansive soil and the pile foundation data are scaled down proportionally, and test piles are prepared.

[0014] (4) The expansive soil is crushed and naturally dried; the thickness of each stratum in the geological exploration report is reduced according to the reduction factor in step (3), and the material is filled into the test chamber in sequence according to the arrangement order of each stratum below the expansive soil in the stratum information and compacted in layers; then the weight of the expansive soil is weighed and laid on top of the material. When laying the expansive soil in the test chamber, a suction pipe is pre-embedded in the expansive soil, and several water-permeable holes are opened on the circumference of the suction pipe; and several pressure sensors are evenly laid on top of the expansive soil.

[0015] (5) Insert test piles vertically into the test chamber, wherein the lower end of the test pile extends into the lowest layer of the test chamber and is spaced from the bottom of the test chamber.

[0016] (6) After completing step (5), weigh the entire test chamber (including the material filled in the test chamber and the inserted test stakes) and record the initial weight.

[0017] (7) Cover the expansive soil with a pressure plate and set up a pressure mechanism on the pressure plate. The pressure mechanism can apply a constant pressure to the expansive soil. The constant pressure is the pressure value obtained in step (1) after reducing the vertical soil pressure above the expansive soil according to the reduction ratio in step (3).

[0018] (8) According to the set moisture content of the expansive soil, add the weight of water calculated based on the moisture content of the expansive soil in the test chamber in step (4). The added water enters the expansive soil in the test chamber through the suction pipe.

[0019] (9) Conduct corresponding tests on the test piles in the test chamber according to the testing requirements required for the pile foundation to be arranged (such as vertical static load test, high strain test, pull-out test, etc.), and obtain the first test data of the expansive soil at this moisture content based on the data of the monitoring instrument.

[0020] (10) Repeat the test contents of steps (8) and (9) to obtain the second test data of expansive soil at different moisture contents.

[0021] (11) Remove the pressure plate in the test chamber and place it above the expansive soil, and separate the test mechanism (such as the pressure equipment or weight used in the vertical static load test) used to test the test pile (such as high strain testing, static load test (vertical static load test, horizontal static load test), pull-out test, etc.) from the test pile. Weigh the entire test chamber, and then alternately spray hot air into the suction pipe and suction the suction pipe. Observe the weight value during the process of spraying hot air and suction, so that the weight value is equal to the initial weight plus the weight of water added according to the moisture content of the expansive soil.

[0022] (12) After completing step (11), repeat the content of step (7).

[0023] (13) Repeat the contents of step (9), and the test performed on the test pile in this step is the same type of test performed on the test pile in step (7) to obtain the third test data of the expansive soil at this moisture content.

[0024] (14) Repeat steps (11) to (13) to obtain the fourth test data of expansive soil at different moisture contents.

[0025] (15) The impact of the swelling and shrinkage of expansive soil on the bearing capacity of pile foundation is comprehensively evaluated based on the first test data, the second test data, the third test data and the fourth test data.

[0026] In some embodiments, the suction pipe is pre-embedded in the expansive soil in a spiral shape within the test chamber, and the suction pipe is covered with filter cloth. By burying the suction pipe in a spiral shape, the uniformity of water distribution in the expansive soil can be improved.

[0027] In some embodiments, two spiral suction pipes are pre-embedded in the expansive soil inside the test chamber, with the two suction pipes spiraling in opposite directions. The uniformity of water distribution in the expansive soil can be further improved by using two suction pipes with opposite spiral directions.

[0028] In some embodiments, in steps (8) to (10), the moisture content of the expansive soil gradually increases according to a set moisture content; in steps (11) to (14), the moisture content of the expansive soil gradually decreases according to a set moisture content.

[0029] In some embodiments, a sealing water-proof plate is provided between the expansive soil in the test chamber and the material below it to prevent water added to the expansive soil from entering the material below.

[0030] In some embodiments, the moisture content of each stratum below the expansive soil in the test chamber is the same as the moisture content of the corresponding stratum obtained from step (2).

[0031] In some embodiments, the stratum information of the stratum where the pile foundation is to be laid in step (2) also includes the density of the expansive soil. When the expansive soil is laid in the test chamber and compacted in layers in step (3), the density of the expansive soil in the test chamber is the same as the density of the expansive soil in the stratum information in step (2).

[0032] In some embodiments, if the density of the expansive soil in the stratum where the pile foundation is to be laid cannot be obtained from the geological exploration report, a core sample of the expansive soil is obtained by core sampling and the expansive soil core sample is measured to obtain the density of the expansive soil.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention provides a method for testing the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations. This method simulates the environment of the proposed pile foundation (i.e., pile foundations placed on a newly constructed site) and simulates the real environment using test piles in an indoor environment. Under these real-world conditions, the method monitors the impact of expansive soil swelling and shrinkage on the actual bearing capacity of the pile foundation, thereby establishing comprehensive and scientific test data (i.e., first test data, second test data, third test data, and fourth test data). The impact of expansive soil swelling and shrinkage on the bearing capacity of the pile foundation is then comprehensively evaluated based on this data. However, existing technologies regarding the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations all rely on controlling the moisture content of the expansive soil to test the bearing capacity of test piles at different moisture contents. However, the applicant's long-term experimental research has revealed that the obtained data does not accurately reflect the impact of expansive soil in the strata on the pile foundation, resulting in a significant discrepancy. The main reason is that many factors influence the bearing capacity of pile foundations. Taking bearing-type pile foundations (most pile foundations are bearing-type) as an example, they are arranged in the strata, and the bearing capacity mainly relies on the bearing layer. However, the strata above the bearing layer (such as expansive soil, clay, etc.) have a certain impact on the bearing capacity of the pile foundation. Therefore, studying the influence of the expansion and contraction of expansive soil on the bearing capacity of pile foundations in isolation is one-sided and cannot play a corrective or guiding role in the design of pile foundations.

[0035] The present invention provides a test method for the bearing capacity of pile foundations by simulating the expansion and contraction deformation of expansive soil. This method obtains test data that is not only correlated with the expansion and contraction characteristics of expansive soil, but also correlated with the actual environment of the pile foundation in the stratum. This provides data reference for the verification and modification of the pile foundation design, and directly integrates the expansion and contraction performance of expansive soil into the pile foundation design, thereby improving the safety of the pile foundation after use. Detailed Implementation

[0036] The present invention will be further described below with reference to embodiments. These embodiments are merely some, not all, of the embodiments of the present invention. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the protection scope of the present invention.

[0037] The method for testing the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to the present invention includes the following:

[0038] (1) The vertical earth pressure above the expansive soil in the stratum and the data of the pile foundation to be arranged in the stratum are obtained by measurement and calculation. The pile foundation data includes, but is not limited to, pile foundation size, pile foundation material, pile foundation top load, pile foundation vertical ultimate bearing capacity, pile foundation bending strength, pile foundation pull-out capacity, etc. Among them, the pile foundation data can be obtained by querying the relevant documents of the designed pile foundation, so there is no need to design and calculate separately.

[0039] Vertical earth pressure is typically generated by the self-weight of the soil strata and is directed vertically downwards. When the soil strata are very soft and the underground structure is shallowly buried, it can be considered equal to the self-weight of the entire overburden. When the soil strata are relatively stable and the underground structure is deeply buried, only the self-weight of the adjacent overburden plays a role. The former refers to shallow-buried underground structures, while the latter refers to deep-buried underground structures. In engineering design, the latter is often distinguished by the shallow-buried / deep-buried boundary. Therefore, those skilled in the art can easily understand how to obtain vertical earth pressure.

[0040] (2) Based on the geological exploration report, obtain the stratigraphic information of the strata where the pile foundations are to be placed and the water content of each stratum. Since the length of the pile foundations to be placed in the strata is fixed, it is only necessary to obtain the water content of the bearing layer, the thickness and water content of the expansive soil, and the stratigraphic information and water content of each stratum between the bearing layer and the expansive soil. These data can be obtained by consulting the geological exploration report of the site where the pile foundations are to be placed.

[0041] (3) The vertical earth pressure above the expansive soil and the pile foundation data are scaled down proportionally, and test piles are prepared.

[0042] (4) The expansive soil is crushed and naturally dried; the thickness of each stratum in the geological exploration report is reduced according to the reduction factor in step (3), and the material is filled into the test box in sequence according to the arrangement order of each stratum below the expansive soil in the stratum information and compacted in layers; then the weight of the expansive soil is weighed and laid on top of the material. When laying the expansive soil in the test box, a suction pipe is pre-embedded in the expansive soil in a spiral distribution. Several water-permeable holes are opened on the circumference of the suction pipe, and the suction pipe is covered with filter cloth; and several pressure sensors are evenly laid on top of the expansive soil. The stratum information includes at least the type of the stratum (e.g., gravel layer, clay layer, soft soil layer) and the thickness of the stratum; thus, the test pile is used in the sample box to simulate the pile foundation to be arranged. For example, if the soil layers beneath the expansive soil in the strata where the pile foundation is to be installed are, in order, clay, gravel, and the bearing layer, then during the filling of the test chamber, the soil beneath the expansive soil will be filled in the order of clay, gravel, and the bearing layer (e.g., the bearing layer is hard rock). The design specifications have specific requirements for the bearing layer of the pile foundation to ensure that the pile foundation can be inserted into the bearing layer for stabilization; these will not be elaborated upon here. In this invention, to facilitate the insertion of the test pile into the bearing layer within the test chamber, blind holes are provided in the bearing layer corresponding to the location of the test pile.

[0043] The filter cloth effectively blocks the water flow through the permeable holes, resulting in a more uniform spray surface and reducing the impact of the permeable holes on the expansive soil. This prevents the expansive soil from shifting and moving when it passes through the suction pipe. Furthermore, when the expansive soil is sprayed or suctioned through the suction pipe, the filter cloth prevents it from entering the pipe itself. Therefore, the filter cloth covering the suction pipe in this invention has three functions and effects.

[0044] In some embodiments, two spiral suction pipes are pre-embedded in the expansive soil inside the test chamber, with the two suction pipes spiraling in opposite directions. The uniformity of water distribution in the expansive soil can be further improved by using two suction pipes with opposite spiral directions.

[0045] (5) Insert the test pile vertically into the test chamber, with the lower end of the test pile extending into the lowest layer of the test chamber and having a gap from the inner bottom of the test chamber. Before insertion, pressure sensors are placed at both the bottom and top of the test pile, and monitoring instruments (such as strain gauges, accelerometers, dial gauges, etc.) are placed on the pile body. The monitoring instruments and their placement for the test pile bearing capacity test are existing technologies, understood by those skilled in the art. For example, pile foundation bearing capacity tests mainly include high-strain testing, static load testing (vertical static load test, horizontal static load test), pull-out testing, etc. The sensors required for different tests vary. The "Technical Specification for Testing of Building Foundation Piles" (JGJ 106-2003) specifies requirements for pile foundation testing, and those skilled in the art can conduct corresponding tests according to this specification.

[0046] For example, high-strain testing is a method for determining the vertical compressive bearing capacity and pile integrity of a single pile. During the experiment, a heavy hammer (weighing 1%-1.5% of the estimated ultimate bearing capacity of the single pile) is used to impact the top of the pile, causing sufficient penetration into the prestressed pile. The resulting stress and acceleration response of the pile's mass points are measured, and the vertical bearing capacity and pile integrity are determined through wave theory analysis. Impacting the pile top with a heavy hammer causes sufficient relative displacement between the pile and the soil to fully stimulate the soil resistance around the pile and the end bearing force. Currently, the main high-strain testing methods for prestressed piles include the Case method, the CAPWAPC method, waveform fitting, and wave equation method. Vertical static load testing, on the other hand, is a method of testing pile foundations by slowly increasing the load directly on the pile foundation; it is the most intuitive and accurate method.

[0047] (6) After completing step (5), weigh the entire test chamber (including the material filled in the test chamber and the inserted test stakes) and record the initial weight.

[0048] (7) Cover the expansive soil with a pressure plate and set up a pressure mechanism on the pressure plate. The pressure mechanism can apply a constant pressure to the expansive soil. The constant pressure is the pressure value obtained in step (1) after reducing the vertical soil pressure above the expansive soil according to the reduction ratio in step (3).

[0049] (8) According to the set moisture content of the expansive soil, add the weight of water calculated based on the moisture content of the expansive soil in the test chamber in step (4). The added water enters the expansive soil in the test chamber through a suction pipe. The weight of the expansive soil added according to the weight in the test chamber is used to calculate the weight of water to be added based on the set moisture content, so that the amount of water added can be obtained simply and accurately, which is convenient for precise control of the moisture content of the expansive soil. However, in the existing technology, when realizing the expansion and contraction of expansive soil, only the analysis before and after immersion is performed (for example, the technical documents with application numbers 202021232050X and 201921364772 introduced in the background technology, and these two technical documents cannot obtain accurate data on the amount of water added to the expansive soil according to their structure), and cannot accurately control the influence of expansive soil under different moisture contents.

[0050] (9) Conduct corresponding tests on the test piles in the test chamber according to the testing requirements required for the proposed pile foundation (e.g., vertical static load test, high strain test, pull-out test, etc.), and obtain the first test data of the expansive soil at the moisture content based on the data from the monitoring instrument. When testing the test piles, the applied force is reduced according to the reduction ratio in step (3). For example, if the applied force for the proposed pile foundation in the static load test is N, and it is reduced by 10 times in step (3), then the applied force to the test piles in the test chamber is N / 10.

[0051] (10) Repeat the test contents of steps (8) and (9) to obtain the second test data of expansive soil at different moisture contents.

[0052] In the specific implementation process, the set moisture content can be set manually, for example, by measuring the data of test piles of expansive soil at moisture contents of 5%, 10%, 20%, 30%, and 50%. Then, in the first test, the moisture content of the expansive soil is 5%; in the second test, the moisture content of the expansive soil is 10%; in the third test, the moisture content of the expansive soil is 20%; in the fourth test, the moisture content of the expansive soil is 30%; and in the fifth test, the moisture content of the expansive soil is 50%. Among them, the set moisture content should at least include data consistent with the moisture content of the expansive soil in the stratum where the pile foundation is to be placed. For example, if the moisture content of the expansive soil where the pile foundation is to be placed is 17%, then the moisture content set in step (8) must include the value of 17% moisture content.

[0053] (11) Remove the pressure plate from the test chamber above the expansive soil, and separate the test mechanism (such as the pressure equipment or weight used in the vertical static load test) used for testing the test pile (e.g., high strain testing, static load test (vertical static load test, horizontal static load test), pull-out test, etc.) from the test pile. Weigh the entire test chamber, and then alternately spray hot air into the suction pipe and suction. Observe the weight value during the process of spraying hot air and suction, so that the weight value is equal to the initial weight plus the weight of water added at the corresponding moisture content of the expansive soil. For example, if the initial weight of the entire test chamber is G0, and the weight of water added when the moisture content of the expansive soil is 50% is G1, then the weight value G = G0 + G1; if the weight of water added when the moisture content of the expansive soil is 30% is G2, then the weight value G = G0 + G2.

[0054] The specific structure of the test mechanism for testing the bearing capacity of pile foundations (test piles) is existing technology, which can be understood by those skilled in the art, and will not be described in detail here.

[0055] (12) After completing step (11), repeat the contents of step (7). Since a constant pressure value has been obtained in step (7), apply pressure to the pressure plate according to the constant pressure value in step (7).

[0056] (13) Repeat the contents of step (9), and the test performed on the test pile in this step is the same type of test performed on the test pile in step (7). For example, if the test pile is subjected to a vertical static load test in step (7), then the test pile is subjected to a vertical static load test in this step (13) in the same way, so as to obtain the third test data of the expansive soil at this moisture content.

[0057] (14) Repeat steps (11) to (13) to obtain the fourth test data of expansive soil at different moisture contents.

[0058] (15) The impact of the swelling and shrinkage of expansive soil on the bearing capacity of pile foundation is comprehensively evaluated based on the first test data, the second test data, the third test data and the fourth test data.

[0059] In the experiment, the present invention first gradually increases the moisture content of the expansive soil to obtain the effect of different moisture contents on the bearing capacity of the pile foundation (second test data); then gradually decreases the moisture content of the expansive soil to obtain the effect of different moisture contents on the bearing capacity of the pile foundation again (fourth test data). This allows for comparison of the two sets of data, which can reflect the effects of the expansive soil on the bearing capacity of the pile foundation during the expansion and contraction processes.

[0060] Current technologies rely on testing expansive soil before and after immersion in water. However, this approach only reflects the impact of soil expansion on pile foundation performance. Furthermore, data solely on soil expansion or contraction cannot accurately reflect the influence of expansive soil within the strata on the pile foundation.

[0061] In some embodiments, in steps (8) to (10), the moisture content of the expansive soil gradually increases according to a set moisture content, that is, water is gradually added in steps (8) to (10), thereby causing the moisture content of the expansive soil to gradually increase. In steps (11) to (14), the moisture content of the expansive soil gradually decreases according to a set moisture content, that is, water is gradually discharged from the test chamber through a suction pipe in steps (11) to (14), thereby causing the moisture content of the expansive soil to gradually decrease.

[0062] In some embodiments, a sealing water-proof plate is provided between the expansive soil in the test chamber and the material below it to prevent water added to the expansive soil from entering the material below.

[0063] In some embodiments, the moisture content of each stratum (i.e., material) below the expansive soil in the test chamber is the same as the moisture content of the corresponding stratum obtained from step (2). In specific implementation, if the moisture content of each stratum (material) below the expansive soil in the test chamber differs greatly, a sealing water-proof plate is also arranged.

[0064] The sealing waterproof plate has a light-transmitting opening in the middle to allow the test pile to pass through.

[0065] In some embodiments, the stratum information of the stratum where the pile foundation is to be laid in step (2) also includes the density of the expansive soil. When the expansive soil is laid in the test chamber and compacted in layers in step (3), the density of the expansive soil in the test chamber is the same as the density of the expansive soil in the stratum information in step (2), which provides a theoretical basis for the compaction of the expansive soil in the test chamber. At the same time, it makes the environment of the test pile in the test chamber closer to the real environment, further improving the authenticity of the test data.

[0066] In some embodiments, if the density of the expansive soil in the stratum where the pile foundation is to be installed cannot be obtained from the geological exploration report, core sampling is used to obtain expansive soil core samples, and the density of the expansive soil is measured from the expansive soil core samples. Furthermore, by measuring the density of the expansive soil through core sampling, the density of each stratum below the expansive soil can also be measured during the core sampling process, thereby further improving the accuracy of the data.

[0067] The present invention provides a method for testing the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations. This method simulates the environment of the proposed pile foundation (i.e., pile foundations placed on a newly constructed site) and simulates the real environment using test piles in an indoor environment. Under these real-world conditions, the method monitors the impact of expansive soil swelling and shrinkage on the actual bearing capacity of the pile foundation, thereby establishing comprehensive and scientific test data (i.e., first test data, second test data, third test data, and fourth test data). The impact of expansive soil swelling and shrinkage on the bearing capacity of the pile foundation is then comprehensively evaluated based on this data. However, existing technologies regarding the impact of expansive soil swelling and shrinkage on the bearing capacity of pile foundations all rely on controlling the moisture content of the expansive soil to test the bearing capacity of test piles at different moisture contents. However, the applicant's long-term experimental research has revealed that the obtained data does not accurately reflect the impact of expansive soil in the strata on the pile foundation, resulting in a significant discrepancy. The main reason is that many factors influence the bearing capacity of pile foundations. Taking bearing-type pile foundations (most pile foundations are bearing-type) as an example, they are arranged in the strata, and the bearing capacity mainly relies on the bearing layer. However, the strata above the bearing layer (such as expansive soil, clay, etc.) have a certain impact on the bearing capacity of the pile foundation. Therefore, studying the influence of the expansion and contraction of expansive soil on the bearing capacity of pile foundations in isolation is one-sided and cannot play a corrective or guiding role in the design of pile foundations.

[0068] The present invention provides a test method for the bearing capacity of pile foundations by simulating the expansion and contraction deformation of expansive soil. This method obtains test data that is not only correlated with the expansion and contraction characteristics of expansive soil, but also correlated with the actual environment of the pile foundation in the stratum. This provides data reference for the verification and modification of the pile foundation design, and directly integrates the expansion and contraction performance of expansive soil into the pile foundation design, thereby improving the safety of the pile foundation after use.

[0069] In a preferred embodiment of the present invention, multiple identical test piles are arranged in the test chamber to facilitate different tests on the pile foundation using the testing mechanism (e.g., test pile number 1 undergoes a vertical static load test, test pile number 2 undergoes a horizontal static load test, test pile number 3 undergoes a high-strain test, etc.). This allows for the acquisition of test data for different test types (e.g., vertical static load test, high-strain test, horizontal static load test, etc.) of the test piles under the same conditions. This improves testing efficiency and reduces the influence of external factors (e.g., even when filling with expansive soil, the uniformity of two fillings cannot be guaranteed, and there may be some deviation in the thickness of the material filling the test chamber) on the test data, thus improving the accuracy and comprehensiveness of the test data.

Claims

1. A test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations, characterized in that, Including the following: (1) The vertical earth pressure above the expansive soil in the stratum and the data of the pile foundation to be arranged in the stratum are obtained by measurement and calculation. The pile foundation data includes the pile foundation size, pile foundation material, pile foundation top load, pile foundation vertical ultimate bearing capacity and pile foundation bending strength. (2) Based on the geological exploration report, obtain the stratigraphic information of the strata where the pile foundations are to be laid and the water content of each stratum; (3) The vertical earth pressure above the expansive soil and the pile foundation data are scaled down proportionally, and test piles are prepared; (4) The expansive soil is crushed and dried naturally; the thickness of each stratum in the geological exploration report is reduced according to the reduction factor in step (3), and the material is filled into the test box and compacted in layers according to the arrangement order of each stratum below the expansive soil in the geological information; then the weight of the expansive soil is weighed and laid on top of the material; when laying the expansive soil in the test box, a suction pipe is pre-embedded in the expansive soil, and several water-permeable holes are opened on the circumference of the suction pipe; and several pressure sensors are evenly laid on top of the expansive soil. (5) Insert test piles vertically into the test chamber, wherein the lower end of the test pile extends into the lowest layer of the test chamber and is spaced from the bottom of the test chamber; (6) After completing step (5), weigh the entire test chamber and record the initial weight; (7) Cover the expansive soil with a pressure plate and set up a pressure mechanism on the pressure plate. The pressure mechanism can apply a constant pressure to the expansive soil. The constant pressure is the pressure value obtained in step (1) after reducing the vertical soil pressure above the expansive soil according to the reduction ratio in step (3). (8) According to the set moisture content of the expansive soil, add the weight of water calculated according to the moisture content based on the weight of the expansive soil in the test chamber in step (4). The added water enters the expansive soil in the test chamber through the suction pipe. (9) Conduct corresponding tests on the test piles in the test chamber according to the testing requirements required for the pile foundation to be arranged, and obtain the first test data of the expansive soil at this moisture content based on the data of the monitoring instrument; (10) Repeat the test contents of steps (8) and (9) to obtain the second test data of expansive soil at different moisture contents; (11) Remove the pressure plate in the test chamber and place it above the expansive soil, and separate the test mechanism used to test the test pile from the test pile. Weigh the entire test chamber, and then alternately spray hot air into the suction pipe and suction the suction pipe. Observe the weight value during the process of spraying hot air and suction, so that the weight value is equal to the initial weight plus the weight of water added according to the moisture content of the expansive soil. (12) After completing step (11), repeat the content of step (7); (13) Repeat the contents of step (9), and the test performed on the test pile in this step is the same type of test performed on the test pile in step (7) to obtain the third test data of the expansive soil at this moisture content; (14) Repeat steps (11) to (13) to obtain the fourth test data of expansive soil at different moisture contents; (15) The impact of swelling and shrinkage of expansive soil on the bearing capacity of pile foundation is comprehensively evaluated based on the first test data, the second test data, the third test data and the fourth test data.

2. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 1, characterized in that, The suction pipe is pre-embedded in the expansive soil in the test chamber in a spiral shape, and the outside of the suction pipe is covered with filter cloth.

3. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 2, characterized in that, Two spiral suction pipes are pre-embedded in the expansive soil inside the test chamber, with the spiral directions of the two suction pipes being opposite.

4. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 1, characterized in that, In steps (8) to (10), the moisture content of the expansive soil gradually increases according to the set moisture content; in steps (11) to (14), the moisture content of the expansive soil gradually decreases according to the set moisture content.

5. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 1, characterized in that, A sealing water-proof plate is installed between the expansive soil in the test chamber and the material below it to prevent water added to the expansive soil from entering the material below.

6. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 1, characterized in that, The moisture content of each stratum below the expansive soil in the test chamber is the same as the moisture content of the corresponding stratum obtained from step (2).

7. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 1, characterized in that, The stratum information of the stratum where the pile foundation is to be laid in step (2) also includes the density of the expansive soil. When the expansive soil is laid in the test box and compacted in layers in step (3), the density of the expansive soil in the test box is the same as the density of the expansive soil in the stratum information in step (2).

8. The test method for the effect of swelling and shrinkage deformation of expansive soil on the bearing capacity of pile foundations according to claim 7, characterized in that, If the density of the expansive soil in the stratum where the pile foundation is to be laid cannot be obtained from the geological exploration report, core sampling is used to obtain expansive soil core samples, and the density of the expansive soil is obtained by measuring the expansive soil core samples.