CN109946174B - Expansive soil direct shear test device and shear strength measurement method - Google Patents
Expansive soil direct shear test device and shear strength measurement method Download PDFInfo
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Abstract
The device comprises a strain type direct shear apparatus and a loading plate, wherein the loading plate is provided with a limiting structure and a pressure sensor, a large overlying load is applied to the loading plate before a sample is soaked in water, the limiting structure controls the bottom surface of the loading plate to be just contacted with the top surface of a permeable stone on the upper portion of the sample, and the sample is prevented from being acted by the overlying load. The measuring method comprises the following steps: and (3) preparing a sample according to the static pressure of the highway geotechnical test regulation, soaking the sample in constant volume, measuring the vertical expansive force, and carrying out direct shear test by taking the vertical expansive force as an overlying load to obtain the effective cohesive force and the effective internal friction angle of the experimental expansive soil. The test method and the device are simple, convenient to operate, reasonable in structure and economical in manufacturing cost, the microstructure of the sample before shearing under the action of different overlying loads in the direct shear test is consistent, the problem that the shear strength parameter of the expansive soil cannot be accurately obtained for a long time in the field is solved, and accurate design parameters are provided for the structural design of the civil engineering of the expansive soil.
Description
Technical Field
The invention discloses a swelling soil direct shear test device and a shear strength measurement method; belongs to the technical field of geotechnical test measurement.
Background
The expansive soil is spread over more than 40 international countries in six continents around the world, China is one of the countries with the most extensive distribution of the expansive soil, 26 provinces, autonomous regions and direct prefectures exist in the special soil, compared with common clay, the expansive soil is rich in montmorillonite and mixed layer minerals thereof, and when the water content is increased and smaller constraint is received, the expansive soil can generate expansion deformation and rapid strength attenuation; when the wet expansion is restrained to a certain degree, the expansion additional stress which is not negligible is generated on the surrounding soil body and the structure. Therefore, the expansive soil excavation and filling side slopes in the projects of highways, railways, water conservancy, building construction and the like are easy to collapse, the engineering construction and operation safety is seriously threatened, and huge economic losses are caused. Therefore, the reasonability and the accuracy of the design parameters of the expansive soil slope are ensured, and the key for effectively utilizing the expansive soil to fill the embankment and treating the expansive soil cut slope is provided.
Most of the instability of geotechnical engineering is caused by shear failure, and the accurate measurement of the shear strength parameter of the geotechnical body has important significance in geotechnical engineering construction. The direct shear test is most commonly used due to simple operation and wide application range. According to the previous practical engineering, the expansive soil side slope can slide on a gentle slope with the gradient of only 1: 4-1: 5, and the strength index obtained by a direct shear test according to the current specification is not low, and theoretically: the expansive soil slope does not slide. The reason why the above phenomenon occurs is that: the conventional direct shear test method in the existing highway geotechnical test regulation (JTG E40-2007) has essential defects in the measurement process, and cannot truly reflect the shear strength of the expansive soil.
According to a conventional direct shear test method in the existing highway geotechnical test regulation (JTG E40-2007), before a direct shear test, an expansive soil sample needs to be soaked and saturated, in the soaking and saturation process, a cutting ring sample with set initial dry density and water content is placed in a shearing box, permeable stones are arranged above and below the sample, different artificially specified overlying loads (such as 100, 200, 300 and 400kPa overlying loads) are applied to the sample through the action of a loading plate on the upper permeable stones, the sample is soaked to reach a saturation state, and then the direct shear test is carried out; wherein: the radial sizes and areas of the loading plate, the upper permeable stone and the sample are the same. The prior art adopts the loading device, the loading structure and the loading method, and has the following defects: because different overburden loads are applied to the sample before the sample is soaked by the loading plate and the permeable stone which are the same in size and area, the sample is compressed to a certain degree under the action of the overburden loads, and the dry density of the sample is changed relative to the initial dry density set during sample preparation. The expansive soil sample can expand to different degrees under the action of overlying load in the process of water saturation. The most different of expansive and non-expansive soils is that they contain expansive clay mineral particles. The particles expand along with the increase of the water content and compress along with the increase of the overlying load, so that the microstructure of the soil body and the average particle size of the soil body are changed. After the expansive soil absorbs water and expands, clay mineral aggregates become small and tend to disperse, so that the contact between particles in the soil is reduced, the contact between the particles and the clay is increased, and the friction strength between the particles is a main contribution part of the overall friction strength of the expansive soil. Therefore, the expansive deformation may have an influence on the effective stress shear strength index of the expansive soil. The cohesive force and the internal friction angle obtained by fitting by a conventional method are not low, and theoretically, the compacted soil does not have the true cohesive force, so that the actual shear strength index value of the expansive soil is lower than that measured by a conventional test. In a conventional shearing test, the dry density and the microstructure of a saturated expansive soil sample are changed under the action of different overlying loads, the expansive soil sample in different initial states is sheared, so that the intensity envelope shows nonlinearity, and the intercept and the slope obtained by linear fitting are different from the true cohesive force and the internal friction angle.
Disclosure of Invention
The invention provides the expansive soil direct shear test device and the shear strength measurement method which are simple and reasonable in structure, convenient to operate and capable of strictly controlling the initial dry density state of the sample, and aims to overcome the defects of the existing regulations.
The inventor discovers that: in a conventional direct shear test, the sample may expand or compress to different degrees before shearing after being saturated with water under different overlying loads. For non-expansive soil, the change of expansion and compaction only affects the peak strength of the soil, and has little influence on the limit strength, so the influence of the structural difference of the sample under the action of different overlying loads before shearing on the effective stress shear strength index is generally not considered. However, for expansive soil, the shear strength envelope of expansive soil obtained by the conventional test method has significant nonlinearity (fig. 5), which causes a certain difference between the parameters obtained by linear fitting and the true shear strength index. For example, the cohesion force obtained by fitting in the low stress range (0-50kPa) is, although only a few kPa, as high as 30-40 DEG of internal friction; in a high stress range (more than 50kPa), the cohesive force obtained by fitting is more than 20kPa, and the internal friction angle is about 20 degrees; the cohesive force and the internal friction angle obtained by fitting in the full stress range are not low. Theoretically, the compacted soil has no or extremely small effective cohesive force, the slope slump of the expansive soil has gentle slope property, and the expansive soil still slides when the slope ratio of some slopes is 1:3 or even 1:5, so that the actual shear strength index value of the expansive soil is lower than that measured by a conventional test. The reason for this is that, in the conventional test, after the sample is saturated with water under different confining pressures (overburden loads), the sample expands or compresses to different degrees, and the dry density of the sample before shearing changes significantly. The expansive soil is different from non-expansive soil in the fact that expansive clay mineral particles are contained. The particles expand with the increase of the water content and compress with the increase of the overlying load, so that the microstructure of the soil body and the average particle size of the soil body are changed, as shown in fig. 6. After the expansive soil is expanded or compressed by soaking under the action of different overlying loads, the contact quantity of the particles and the sticky particles in the soil is changed, and the friction strength between the particles is a main contribution part of the overall friction strength of the expansive soil. Therefore, in a conventional direct shear test, the dry density and the microstructure of a saturated expansive soil sample are changed under the action of different overlying loads, the expansive soil samples with different soil body structures are sheared, so that the intensity envelope shows nonlinearity, and the intercept and the slope obtained by linear fitting are different from the true cohesive force and the internal friction angle.
Therefore, the invention designs the loading plate with a limiting structure and a pressure sensor: 1. the test sample is prevented from being compressed due to the action of vertical pressure when an overlying load is applied, and the consistency of the density of the test soil sample is ensured; 2. the vertical expansion force actually measured when the sample expands to a saturated state is used as an overlying load applied to the sample in the shearing process, so that the vertical pressure applied to the sample is constant and no expansion occurs in the shearing process; 3. when the sample absorbs water, the sample is limited in a constant space by applying an external load far greater than the vertical expansive force of expansive soil to the loading plate, the volume expansion and compression cannot occur, and the vertical expansive force is monitored in real time through the pressure sensor. And whether the soil sample completely reaches a saturated state can be judged by monitoring the data change of the pressure sensor.
The invention relates to an expansive soil direct shear test device, which comprises a strain type direct shear instrument and a loading plate, and is characterized in that: the loading plate is provided with a limiting structure, and when the loading plate is installed on a shearing box of the strain type direct shear instrument before a sample is placed in the shearing box of the strain type direct shear instrument for soaking water, the limiting structure controls the bottom surface of the loading plate to be in gapless contact with the sample but not apply load to the sample.
The invention relates to a direct shear test device for expansive soil, wherein a loading plate consists of a lower cylinder and an upper cover, the diameter of the upper cover is larger than that of the lower cylinder, and a positioning boss is formed between a cylinder body of the lower cylinder and the lower bottom surface of the upper cover, so that the cross section of the loading plate is of a T-shaped structure.
According to the expansive soil direct shear test device, the diameter of the lower cylinder of the loading plate is the same as the inner diameter of the shear box of the strain type direct shear apparatus, and the diameter of the upper cover of the loading plate is larger than the inner diameter of the shear box of the strain type direct shear apparatus.
The invention relates to a direct shear test device for expansive soil.
The invention relates to a expansive soil direct shear test device.A groove extending to the bottom surface is formed on a cylinder at the lower part of a loading plate, a pressure sensor is arranged in the groove, and the pressure sensor is electrically connected with a data acquisition device; the input end of the data acquisition device is respectively connected with the pressure sensor and the vertical displacement meter, and the output end of the data acquisition device is electrically connected with an external computer.
The invention relates to an expansive soil direct shear test device, wherein a sample is arranged in a shearing box of a strain type direct shear apparatus, 1 permeable stone is respectively arranged at the upper part and the lower part, a loading plate is clamped on the shearing box, and a pressure sensor arranged in a cylindrical groove at the lower part of the loading plate is in gapless contact with a gasket arranged on the upper surface of the upper permeable stone.
The invention relates to a swelling soil direct shear test device, wherein the diameter of the upper top surface of a loading plate is at least 5mm larger than the inner diameter of a shear box of a strain type direct shear apparatus.
The invention relates to a direct shear test device for expansive soil.
The invention relates to a direct shear test device for expansive soil.
According to the expansive soil direct shear test device, the loading plate is clamped on the shear box and is in contact with the upper end face of the shear box through the limiting snap ring, so that the axial positioning of the loading plate is realized.
According to the soil body vertical expansion force measuring device, the depth of the groove is matched with the thickness of the pressure sensor.
The invention relates to a method for measuring the shear strength of expansive soil, which comprises the following steps:
the first step is as follows: sample preparation
Preparing not less than four compaction samples with the same dry density but different saturation degrees by static pressure according to the direct shear test regulation in the highway geotechnical test regulation JTG E40-2007;
the second step is that: overburden load measurement
After sample preparation is finished, placing a soil sample into a shearing box, respectively placing 1 porous stone on the upper part and the lower part of the sample, clamping a loading plate on the shearing box by using a limiting clamping ring, contacting a pressure sensor arranged in a cylindrical groove on the lower part of the loading plate with a gasket arranged on the upper surface of the upper porous stone, and applying a load larger than the maximum vertical expansion force of the sample to the loading plate; then, injecting water into the shearing box until the reading of the pressure sensor is stable (the change value of the reading of the pressure sensor within 2 hours is less than or equal to 1 kPa), and reading the reading of the pressure sensor;
the third step: slow shear
Taking the reading of the pressure sensor obtained in the second step as an overlying load, applying the overlying load on a loading plate, and removing the limiting snap ring; performing a direct shear test according to the specification of a highway geotechnical test regulation JTG E40-2007, obtaining the ultimate shear strength of the sample after the shearing is finished, and simultaneously measuring the moisture content of the sheared sample; performing data processing according to the regulation of JTG E40-2007 to obtain the effective cohesive force and the effective internal friction angle of the expansive soil;
during slow shearing, the fixed pin is pulled out, and the shearing rate of the strain type direct shear apparatus is set to be 0.02 mm/min. Starting the direct shear apparatus, recording the shear deformation and the reading of the dynamometer dial gauge. Obtaining a change curve of the shear stress under the action of the overlying load along with the shear displacement, and taking the shear stress when the shear displacement reaches 7mm as the ultimate strength of the sample under the action of the overlying load; after shearing, sucking accumulated water in the box, removing shearing force and vertical pressure, moving the pressure frame, taking out the sample, and measuring the water content. And the saturation is calculated. The shear stress is calculated as follows: τ ═ CR
In the formula: τ -shear stress (kPa), calculated to 0.1;
c-dynamometer force measuring ring coefficient (kPa/0.01mm)
R-dynamometer dial indicator reading (mm)
The test was carried out according to the second and third steps for another sample in the saturation state prepared in the first step. Thereby obtaining the ultimate shear strength of the sample under the action of different vertical loads (expansion forces).
And performing linear fitting on the overburden load (expansion force) and the corresponding limit shear strength to obtain the effective cohesive force and the effective internal friction angle in the dry density state.
According to the method for measuring the shear strength of the expansive soil, when a sample is prepared by static pressure, the dry density of the sample is selected within the range of the maximum dry density and the expansion saturation dry density of the test expansive soil, and the water content of the sample is determined according to the saturation of the test expansive soil, namely 50% -95%.
The invention relates to a method for measuring the shear strength of expansive soil.
According to the method for measuring the shear strength of the expansive soil, in a direct shear test, the samples with different initial saturation degrees are applied with the overlying load with the same size as the vertical expansive force of the samples, the samples cannot expand in the saturation process, and the dry density of each sample and the soil body structure are kept unchanged before direct shear.
According to the method for measuring the shear strength of the expansive soil, an overlying load equal to the vertical expansive force of the sample is applied; the sample does not expand in the saturation process; the microstructure remained consistent with the dry density of each sample prior to shearing.
In the test process, taking compacted expansive soil according to engineering requirements to prepare a sample with the soil sample thickness being the same as that of a special cutting ring, wherein the sample thickness is 20mm, the sampling number of the sample is 4-6 parts, preparing the sample with specific water content and dry density by adopting static pressure sample preparation, and the sample preparation process does not allow water to seep out or the mass loss of the soil sample to exceed 0.1 g;
after the sample preparation is completed, permeable stone filter paper is placed in the shearing box, the upper portion of the shearing box is covered with the filter paper and the permeable stone, the loading plate is placed down, the pressure sensor in the groove of the loading plate is just in contact with the permeable stone on the upper layer, appropriate load is applied, it is guaranteed that the sample cannot expand under the action of the load on the upper layer, and the sample cannot be compressed due to the vertical pressure effect because the loading plate is clamped on the inner diameter of the shearing box.
And opening the data acquisition instrument to set the sampling frequency to be 1Hz and the sampling time to be 2d, and starting to monitor the vertical expansion force in real time. And the water surface is always kept to exceed the upper surface of the sample by 5-8mm, so that the sample is soaked from bottom to top;
checking the instantaneous value of the expansion force every 2h, if the reading difference value of the instantaneous values of the expansion force of two adjacent times is less than or equal to 1kPa, the expansion is considered to be stable, and the vertical expansion force instantaneous value P recorded at the last timetI.e. instantaneous value PtIs the vertical expansion force which is saturated and stable in water absorption.
Replacement of overburden load by sensor reading P at expansion stabilitytAnd pulling out the fixed bolt of the shearing box, and setting the shearing rate to be 0.02 mm/min. Direct shear was started and the peak intensity was recorded as well as the ultimate intensity of the shear displacement of 7 mm.
The device and the method are adopted to measure the shear strength of the expansive soil under the condition of constant initial dry density, and provide important parameters for the analysis and design of the stability of the expansive soil slope. The special loading plate with the limiting structure and the groove are adopted, so that on one hand, the microstructure of the grain diameter of the sample can be kept unchanged before and after the application of the overlying load which is equal to the vertical expansive force of the sample, and on the other hand, the vertical expansive force of the expansive soil can be measured under the constant volume condition. According to the test method, whether the test sample is saturated or not in the test process is monitored in real time by adopting the button strain type pressure sensor, so that the vertical expansion force, the saturation and the shear strength under the condition of initial dry density are accurately measured. The invention is mainly characterized in that different initial saturation (water content) states of the sample are controlled.
TABLE 1 showsThe difference between the test method and the conventional direct shear test method in the highway soil engineering test regulation (JTG E40-2007) is that the test device is adopted to ensure that the dry density of the expansive soil before shearing is consistent under different stress states, and the microstructure of the expansive soil is not changed, so that the dry density is consistent according to the coulomb strength formula
Effective internal friction angle obtained by the test
The effective cohesion force c' is a strength parameter for a specific dry density in the true sense.
TABLE 1 comparison of the test method with the conventional method
Compared with the existing direct shear test device, the direct shear test device has the advantages that:
1. the loading plate with the limiting snap ring structure is adopted to apply load equal to the vertical expansion force of the sample to the sample, so that the sample is prevented from expanding and compressing in the saturation process, the consistency of the dry density and the microstructure of each sample before direct shearing is ensured, and each sample is in the same state. The shear strength characteristic of the expansive soil in a specific dry density state can be accurately reflected by using the test result of the expansive soil sample in the same state, the nonlinearity of a shear strength envelope (a curve that the ultimate shear strength changes along with an overlying load) obtained by a conventional test is avoided, the obtained effective cohesive force and the obtained effective internal friction angle are consistent with the theory and the engineering practice, and the method can be more used for accurately evaluating the stability of the expansive soil slope.
2. The sample keeps invariable volume at saturation in-process, through the button strain gauge pressure sensor who settles in load plate concave groove can be effective, convenient direct measurement sample vertical expansive force under the constant volume condition. And whether the sample is saturated or not can be accurately judged by monitoring the reading change of the sensor.
3. The test method is simple, convenient to implement, reasonable in structure, low in cost and practical. The test sample size of the invention is consistent with the conventional test sample size, and the expansive soil sample can be saturated by water absorption in a short time, so that the vertical expansive force is stable. The change of the expansion force is monitored automatically, and the whole process of the tester is not needed.
In conclusion, the test method and the device are simple, convenient and reasonable, have low manufacturing cost, overcome the great disturbance caused by loading to the internal structure of the sample in the conventional direct shearing method and the device, ensure the similarity of the microstructure of the sample before shearing under the action of different overlying loads, obtain the parameter capable of truly representing the shear strength performance of the expansive soil in a specific dry density state, solve the problem that the stability of the expansive soil side slope after rainfall humidification expansion is expected to be reasonably evaluated by accurately obtaining the shear strength parameter of the expansive soil for a long time in the field, provide accurate design parameters for the civil engineering structure design of the expansive soil area, and have simple and practical devices, thereby being suitable for popularization and application in engineering.
Drawings
FIG. 1a is a schematic structural diagram of a direct shear test apparatus and a loading plate thereof according to the present invention.
FIG. 1b is a schematic structural view of a limit snap ring matched with the loading plate.
FIG. 2 is a schematic view of a conventional direct shear test apparatus and a loading plate thereof.
FIG. 3 shows example 2 and comparative example 1 with an initial dry density of 1.40g/cm3Modified and conventional shear strength measurements under conditions.
FIG. 4 shows example 3 and comparative example 2 with an initial dry density of 1.60g/cm3Modified and conventional shear strength measurements under conditions.
FIG. 5 is a strength envelope of Guangxi Bai color compacted expansive soil.
FIG. 6 is an electron micrograph of the microstructure characteristics of the expansive soil after the expansive soil expands to different degrees.
In the drawings:
in the attached figure 1a, 1-shear box, 2-permeable stone, 3-sample, 4-loading plate, 5-pressure sensor, 6-gasket, 7-limiting snap ring, 8-data acquisition device and 9-vertical displacement meter.
In the attached figure 1a, the outer diameter of the upper cover of the loading plate is 66.8mm, the outer diameter of the lower cylinder is 61.8mm, the height is 25mm, a groove with the height of 9mm and the radius of 30mm is arranged in the loading plate, and the cross section of the loading plate is of a T-shaped structure. The inner diameter of the limiting snap ring shown in the attached drawing 1b is 61.mm, the outer diameter is 67.8mm, the height is 5mm, the limiting snap ring can be clamped on a cylinder at the lower part of the loading plate, so that an overlying load is borne by the shearing box, the dry density of a sample is not changed in the saturation process, and the limiting snap ring is provided with an opening-closing seam to facilitate disassembly.
FIG. 2 is a schematic diagram of a conventional direct shear test apparatus and its load plate. The radial sizes and areas of the loading plate, the upper permeable stone and the sample are the same.
The following drawbacks exist: because different overlying loads are applied to the sample by the loading plate and the permeable stone with the same size and area before the sample is soaked, at the moment, the dry density of the sample is changed relative to the initial dry density set during sample preparation under the action of the overlying loads, and the expansive soil sample can expand or compress to different degrees under the action of the overlying loads in the water saturation process.
As can be seen from FIG. 3, the initial dry density is 1.40g/cm3The expansive soil sample obtained by the test method provided by the invention has good linear correlation of the measurement result. The internal friction angle is 13.5 degrees, the cohesive force is 0.8kPa, R20.9989. The measurement results obtained by the conventional test method have poor linear correlation. The internal friction angle is 14.7 degrees, the cohesive force is 10.1kPa, R2＝0.9591
As can be seen from FIG. 4, the initial dry density is 1.60g/cm3The expansive soil sample obtained by the test method provided by the invention has good linear correlation of the measurement result. The internal friction angle is 18.9 degrees, and the cohesive force is 4.0Pa, R20.9912. The measurement result obtained by the conventional test method has poor linear correlation, the internal friction angle is 22.8 degrees, the cohesive force is 10.7kPa, and R is20.9894. Comparing fig. 3, fig. 4 shows that the internal friction angle and the cohesion are larger in the conventional test. The reason is that the samples are compressed and deformed to different degrees when the overlying load is applied, and the saturated process is expanded to different degreesAnd (4) swelling. The microstructure of each soil sample particle size changes, which results in distorted test data.
It can be seen from the Guangxi Bai color compacted expansive soil strength envelope of FIG. 5: the shear strength envelope of expansive soils has significant nonlinearity.
FIG. 6a shows the microstructure of the soil body before swelling and the average particle size (average particle size 3.1 μm) of the soil body; FIG. 6b shows the microstructure of the soil body and the average particle size (average particle size 1.3 μm) of the soil body after 50% swelling of the swelling soil; FIG. 6c shows the microstructure of the soil mass and the average particle size of the soil mass (average particle size 0.7 μm) after swelling of the swelling soil by 75%; as can be seen from FIG. 6a \ b \ c: after the expansive soil absorbs water and expands, clay mineral aggregates become small and tend to disperse, so that the contact between particles in the soil is reduced, the contact between the particles and the clay is increased, and the friction strength between the particles is a main contribution part of the overall friction strength of the expansive soil. Therefore, the expansive deformation can affect the effective stress shear strength index of the expansive soil.
Detailed Description
In the embodiment of the invention, the initial dry density of the sample is 1.40g/cm by the test device3，1.60g/cm3The shear strength parameter of the remolded soil sample is measured.
Example 1:
referring to the attached drawing 1a, the expansive soil direct shear test device comprises a strain type direct shear apparatus and a loading plate 4, wherein the loading plate 4 is provided with a limiting structure, and when a sample is placed in a shearing box 1 of the strain type direct shear apparatus before water is soaked, and the loading plate 4 is installed on the shearing box 1 of the strain type direct shear apparatus, the limiting structure controls the bottom surface of the loading plate 4 to realize gapless contact with the sample 3 but not apply load to the sample;
the loading plate 4 consists of a lower cylinder and an upper cover, the diameter of the upper cover is larger than that of the lower cylinder, and a positioning boss is formed between the column body of the lower cylinder and the lower bottom surface of the upper cover, so that the cross section of the loading plate is of a T-shaped structure; the diameter of the lower cylinder of the loading plate is the same as the inner diameter of the shearing box 1 of the strain type direct shear apparatus, and the diameter of the upper cover of the loading plate 4 is 5mm larger than the inner diameter of the shearing box 1 of the strain type direct shear apparatus; a limiting snap ring 7 is sleeved on the lower cylinder, and the loading plate 4 is clamped on the shearing box 1 and is in contact with the upper end face of the shearing box 1 through the limiting snap ring 7, so that the axial positioning of the loading plate is realized; a groove extending to the bottom surface is formed in the cylinder at the lower part of the loading plate 4, a pressure sensor 5 is arranged in the groove, the depth of the groove is matched with the thickness of the pressure sensor 5, and the pressure sensor 5 is electrically connected with a data acquisition device 8; the input end of the data acquisition device 8 is respectively connected with the pressure sensor and the vertical displacement meter, and the output end is electrically connected with an external computer; a sample 3 is arranged in a shearing box 1 of a strain type direct shear apparatus, 1 permeable stone 2 is respectively arranged at the upper part and the lower part, a loading plate 4 is clamped on the shearing box 1, and a pressure sensor 5 arranged in a cylindrical groove at the lower part of the loading plate is in gapless contact with a gasket 6 arranged on the upper surface of the upper permeable stone 2; the pressure sensor 5 in the groove measures the vertical expansion force formed when the sample swells when immersed in water.
The outer diameter of the upper top surface of the loading plate is 66.8mm, the outer diameter of the lower bottom surface of the loading plate is 61.8mm, the height of the loading plate is 25mm, a groove with the height of 9mm and the radius of 30mm is arranged in the loading plate, the limiting structure is formed by a convex edge formed by the fact that the diameter of the upper top surface of the loading plate is larger than that of the lower bottom surface of the loading plate, and the cross section of the loading plate is in a T-shaped structure.
Example 2:
and (3) acquiring original soil samples and disturbed soil samples of expansive soil on the scene of the Guangxi Baise expressway, and performing basic physical property indexes and engineering property tests. The indoor test results show that: the plasticity index of the expansive soil sample of the multicolored highway is 33.5 percent, and the free expansion rate is 63 percent. According to the current expansive soil distinguishing and classifying standard of the highway, the soil sample is medium expansive soil. By adopting the expansive soil direct shear test device and the shear strength test method, the sample preparation and the direct shear test are carried out on the expansive soil in Guangxi province, and the specific implementation steps are as follows:
according to an initial dry density of 1.40g/cm3Five soil samples with different water contents are prepared according to the saturation degrees of 85%, 75%, 60%, 55% and 50%. A compaction test piece is prepared by adopting a static pressure sample preparation method, and the top surface of the test piece is smooth and neat and the height of the test piece is 20mm after static pressure.
After sample preparation is finished, permeable stones and filter paper are sequentially placed in the shearing box, then the soil sample is pushed into the shearing box, and the filter paper and the permeable stones are placed on the upper portion of the soil sample. The pressure sensor is placed in a concave groove of the loading plate, the button strain type pressure sensor wire section is connected with a computer through a data acquisition instrument, and then the loading plate is placed on the shearing box, so that the sample and the loading plate are on the same axis.
And (3) opening data acquisition software to set the sampling frequency to be 1Hz and the sampling time to be 2d, starting to monitor the vertical expansion force in real time, and realizing automatic acquisition through computer acquisition software. And applying a proper load, injecting water into the consolidation apparatus, and keeping the water level to exceed the upper surface of the sample by 5-8mm all the time so that the sample is soaked from bottom to top.
And when the reading of the pressure sensor is stable, the soil sample is completely saturated. And replacing the load applied on the loading plate with the reading size of the sensor, disassembling the limiting snap ring, and performing direct shear test. The shear rate of the strain type direct shear apparatus was set to 0.02 mm/min. Starting the direct shear apparatus, and recording the shear deformation and the reading of the dynamometer. After the test is finished, data are stored and exported, the sample is taken down, weighed and dried, and the field is cleaned. The final test results are shown in fig. 2. The peak values of the vertical expansion force of the cutting ring soil sample with the saturation of 85.7, 74.9, 60.1, 55.2 and 50.3 percent are respectively 49.8kPa, 68.1kPa, 94.7kPa, 121.1kPa and 141.9kPa, the internal friction angle is 13.5 degrees, the cohesive force is 0.8kPa, and the correlation coefficient R is2＝0.9989。
Example 3
And (3) acquiring original soil samples and disturbed soil samples of expansive soil on the scene of the Guangxi Baise expressway, and performing basic physical property indexes and engineering property tests. The indoor test results show that: the plasticity index of the expansive soil sample of the multicolored highway is 33.5 percent, and the free expansion rate is 63 percent. According to the current expansive soil distinguishing and classifying standard of the highway, the soil sample is medium expansive soil. By adopting the expansive soil direct shear test device and the shear strength test method, the sample preparation and the direct shear test are carried out on the expansive soil in Guangxi province, and the specific implementation steps are as follows:
according to an initial dry density of 1.60g/cm3Preparing cutting ring soil samples with the saturation degrees of 95%, 90%, 85%, 80% and 75%. The disturbed soil test piece is prepared by adopting a static pressure sample preparation method, so that the surface of the test piece is ensured to be smooth and regularThe initial height was 20 mm.
After sample preparation is finished, permeable stones and filter paper are sequentially placed in the shearing box, then the soil sample is pushed into the shearing box, and the filter paper and the permeable stones are placed on the upper portion of the soil sample. The pressure sensor is placed in a concave groove of the loading plate, the button strain type pressure sensor wire section is connected with a computer through a data acquisition instrument, and then the loading plate is placed on the shearing box, so that the sample and the loading plate are on the same axis.
And (3) opening data acquisition software to set the sampling frequency to be 1Hz and the sampling time to be 2d, starting to monitor the vertical expansion force in real time, and realizing automatic acquisition through computer acquisition software. And applying a proper load, injecting water into the consolidation apparatus, and keeping the water level to exceed the upper surface of the sample by 5-8mm all the time so that the sample is soaked from bottom to top.
And when the reading of the pressure sensor is stable, the soil sample is completely saturated. And replacing the load applied on the loading plate with the reading size of the sensor, disassembling the limiting snap ring, and performing direct shear test. The shear rate of the strain type direct shear apparatus was set to 0.02 mm/min. Starting the direct shear apparatus, and recording the shear deformation and the reading of the dynamometer. After the test is finished, data are stored and exported, the sample is taken down, weighed and dried, and the field is cleaned. The final test results are shown in fig. 2. The peak values of the vertical expansion force of the cutting ring soil sample with the saturation degrees of 94.1%, 91.4%, 84.2%, 80.2% and 76.5% are respectively 132.3kPa, 153.6kPa, 178.9kPa, 215.5kPa and 242.1kPa, the internal friction angle is 18.9 degrees, the cohesive force is 4kPa, and the correlation coefficient R is2＝0.9912。
Comparative example 1:
taking a soil sample in the same state as in example 2, preparing 5 cutting ring test pieces by a static pressure sample preparation method, wherein the initial dry density is controlled to be 1.40g/cm3And the water content is 30 percent. According to the direct shear test method in the current highway soil engineering test regulation (JTG E40-2007), overlaying loads of 12.5kPa, 25kPa,50kPa, 100kPa and 200kPa are applied to the test sample, and the internal friction angle is 14.7 degrees, the cohesive force is 10.1kPa, and R is obtained by regression fitting2＝0.9591。
Comparative example 2:
taking a soil sample in the same state as in example 2, preparing 5 cutting ring test pieces by a static pressure sample preparation method, and initially preparingThe dry density was controlled to 1.60g/cm3And the water content is 26%. According to the direct shear test method in the current highway soil engineering test regulation (JTG E40-2007), overlaying loads of 12.5kPa, 25kPa,50kPa, 100kPa and 200kPa are applied to the test sample, and the internal friction angle is 22.8 degrees, the cohesive force is 10.7kPa, R is obtained by regression fitting2＝0.9894。
Claims (5)
1. The expansive soil shear strength measurement method is characterized in that a expansive soil direct shear test device is adopted for measurement, and the expansive soil direct shear test device comprises a strain type direct shear instrument and a loading plate, and is characterized in that: the loading plate consists of a lower cylinder and an upper cover, the diameter of the upper cover is larger than that of the lower cylinder, and a positioning boss is formed between the column body of the lower cylinder and the lower bottom surface of the upper cover, so that the cross section of the loading plate is of a T-shaped structure; the diameter of the lower cylinder of the loading plate is the same as the inner diameter of a shearing box of the strain type direct shear apparatus, and the diameter of the upper cover of the loading plate is larger than the inner diameter of the shearing box of the strain type direct shear apparatus; a limit snap ring is sleeved on the lower cylinder;
the measuring method comprises the following steps:
the first step is as follows: sample preparation
Preparing at least four compacted samples with the same dry density but different saturation degrees by static pressure according to the direct shear test regulation in the highway geotechnical test regulation JTG E40-2007;
the second step is that: overburden load measurement
After sample preparation is finished, putting a compacted sample into a shearing box, respectively placing 1 porous stone on the upper part and the lower part of the compacted sample, clamping a loading plate on the shearing box by using a limiting clamping ring, and controlling the bottom surface of the loading plate to be in gapless contact with the porous stones on the top of the compacted sample without applying load on the compacted sample by using the limiting clamping ring; the pressure sensor arranged in the groove of the lower cylinder of the loading plate is contacted with the gasket arranged on the upper surface of the upper permeable stone, and a load larger than the maximum vertical expansion force of the compacted sample is applied to the loading plate; then, injecting water into the shearing box until the reading of the pressure sensor is stable, and reading the reading of the pressure sensor;
the third step: slow shear
Taking the reading of the pressure sensor obtained in the second step as an overlying load, applying the overlying load on a loading plate, and removing the limiting snap ring; performing a direct shear test according to the specification of a highway soil engineering test regulation JTG E40-2007, obtaining the ultimate shear strength of the compacted sample after shearing is finished, and simultaneously measuring the water content of the compacted sample after shearing; and (3) carrying out data processing according to the specification of JTG E40-2007 to obtain the effective cohesive force and the effective internal friction angle of the expansive soil.
2. The expansive soil shear strength measuring method according to claim 1, wherein: a groove extending to the bottom surface is formed in the cylinder at the lower part of the loading plate, a pressure sensor is arranged in the groove, and the pressure sensor is electrically connected with the data acquisition device; the input end of the data acquisition device is respectively connected with the pressure sensor and the vertical displacement meter, and the output end of the data acquisition device is electrically connected with an external computer.
3. The expansive soil shear strength measuring method according to claim 1, wherein: when the sample is prepared by static pressure, the dry density of the compacted sample is selected within the range of the maximum dry density and the expansion saturation dry density of the test expansive soil, and the water content of the compacted sample is determined according to the saturation of the test expansive soil, namely 50-95%.
4. The expansive soil shear strength measuring method according to claim 1, wherein: and in the process of soaking and saturating the compacted sample, measuring the vertical expansion force by adopting a constant volume method.
5. The expansive soil shear strength measurement method according to any one of claims 3 to 4, wherein: when a load is applied, the compaction sample cannot be compressed, in the direct shear test, the compaction sample with different initial saturation degrees is applied with an overlying load equal to the vertical expansion force of the compaction sample, the compaction sample does not expand in the saturation process, and the dry density and the soil body structure of each compaction sample before direct shear are kept unchanged.
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