CN111044388A - Low-stress horizontal direct shear test method - Google Patents
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Abstract
The invention relates to the technical field of testing the shear strength of a soil body by using a direct shear test, in particular to a low-stress level direct shear test testing method. The test contents of the invention mainly comprise no-load test, sample preparation and loading, load selection, method selection, shear test and data processing. The invention provides a method for measuring cohesive force c and internal friction angle of weak soil body with low structural strength, such as loose sand, soft soil and the like
Description
Technical Field
The invention relates to a technology for testing the shear strength of a soil body by using a direct shear test, in particular to a low-stress level direct shear test testing method.
Background
Because the self strength of the weak soil body is lowerThe samples are easily compressed and deformed under the normal load, so that the density difference among the groups of the samples is large, or the samples are easily extruded or sheared to be damaged (such as soft soil), so that the shear strength test result is distorted. The theoretical basis is as follows: according to the Moore coulomb strength theory, when the normal stress is not very large, the shearing strength of the soil unit in a certain plane can be approximated by a linear function
Therefore, for such weak soil, a low stress level direct shear test is preferably used to test the actual strength.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method for measuring cohesive force c and internal friction angle of weak soil bodies with lower structural strength, such as loose sand, soft soil and the like
The test method of (1).
The technical scheme adopted by the invention for solving the technical problems is as follows:
a low stress level direct shear test method mainly comprises the following steps of no-load test, sample preparation and loading, load selection, method selection, shear test and data processing:
(1) and (3) no-load test:
firstly, coating quantitative vaseline between lower shearing boxes, aligning the lower shearing boxes, inserting a fixing pin, placing the shearing boxes without samples on a direct shear apparatus, rotating a hand wheel to enable the shearing boxes to be in close contact with a force measuring ring, enabling a dial indicator to return to zero, then pulling out the fixing pin, starting the apparatus to read, wherein the shearing rate is the same as that of a normal test, measuring and reading the reading of the shearing boxes under the influence of friction through no-load test, and during the test, paying attention to wiping balls below the shearing boxes and coating lubricating oil.
(2) Preparing a sample and loading the sample:
the specimen had a diameter of 61.8mm and a height of 20 mm.
① for soft and weak cohesive soil, cutting an original soil sample from the original soil sample or preparing a disturbed soil sample with given dry density and water content, wherein the number of each group of samples is not less than the number of selected loads, after vaseline with the same amount as that of a no-load test is coated between the shearing boxes, placing the samples and the cutting ring on the well-assembled shearing boxes, and slowly pressing the samples into the boxes with the cutting edge facing upwards;
② for loose sand soil, taking a representative air-dried sample 2mm below the screen, weighing the required air-dried sand amount of each sample according to a given dry density, pouring the prepared sand sample into a well-assembled shear box, compacting the sand sample to a specified dry density, and brushing flat sand surface, wherein the number of each group of samples is not less than the selected pressure number.
Before and after sample loading, permeable stones, hard plastic films or filter papers are added at the upper end and the lower end of the sample according to different sample methods.
(3) Selecting a load:
the vertical pressure can be selected from 1kPa-50kPa according to the principle from small to large, the larger pressure can be applied in stages, the pressure in the range of 1-12.5kPa can be directly applied above the sample by a small weight or a permeable stone, and the pressure in the range of 12.5-50kPa can be applied by a lever by a small weight.
(4) The method comprises the following steps:
according to different practical engineering conditions, a proper test method is preferably selected, wherein the direct shearing test is divided into three types, namely fast shearing, consolidation fast shearing and slow shearing. The rapid shear is suitable for the conditions that the soil body on site is thick, the permeability is small, the construction speed is high, and the soil body is basically not easy to drain and is solidified.
① quick shearing, namely applying shearing stress at a shearing rate of 0.8-1.2mm/min until the sample is damaged, and respectively sticking waterproof films on the upper and lower surfaces of the sample to simulate the condition of no water drainage;
② consolidation fast shearing, wherein the sample is fully consolidated under certain pressure, the upper and lower surfaces of the sample are drained by sticking water permeable filter paper, after the drained water is deformed stably, the drained water is not more than 0.005mm/h, and then shearing stress is applied at a shearing rate of 0.8mm/min until the sample is damaged;
③ slow shearing, namely, the sample is fully drained and consolidated under certain pressure, and then the shearing force is slowly applied at the shearing rate of less than 0.02mm/min, so that the sample has sufficient time to drain water and generate volume deformation in the shearing process.
(5) Shear test:
and (3) placing the shearing box which is coated with the lubricant and is filled with the sample on a direct shear apparatus, and adjusting the apparatus to ensure that the shearing box is tightly contacted with a force measuring ring, wherein the maximum measuring range of the force measuring ring is preferably 60-600N.
① quick shear the stopwatch is started and the sample is sheared off within 3-5 minutes at a shear rate of 0.8-1.2 mm/min.
② consolidation fast shearing, namely filling water into a water tank of a shearing box after 5min of consolidation for a saturated sample, covering a wet rag around a pressurizing plate without filling water for an unsaturated sample to prevent water evaporation, considering that the consolidation is stable when the vertical deformation reading of the sample is not more than 0.005mm/h, and recording the reading of a vertical displacement meter, wherein the shearing rate is 0.8-1.2 mm/min;
③ Slow shear-the consolidation step is the same as ②. the shear rate should be less than 0.02mm/min, and the load cell and displacement meter readings are measured at 0.2-0.4mm displacement per shear produced by the sample.
After the shearing was completed, the pressure was removed and the water content in the vicinity of the sheared surface was measured by removing the sample.
(6) Destruction criteria:
① if there is obvious peak value or stable value in the relation curve of shear stress and shear displacement, taking the peak value or stable value as the damage standard;
② if the shear stress increases with the increase of the shear displacement, the maximum shear displacement should be 6mm, and then the curve change condition is observed to determine whether there is a larger turning point, or the stress value when the shear displacement is 4mm is selected to determine the shear strength, the latter is only used when the failure value is difficult to select.
(7) Data processing:
according to data of different load shear tests, reading numbers in no-load are deducted firstly, then shear stress tau and shear displacement delta l are solved respectively, relation curves of tau-delta l and s-delta l are drawn, a shear strength variation curve along with vertical pressure is drawn after shear strength is determined, a proper vertical pressure range is selected according to a Moore coulomb strength theory, and then the shear strength value in the range is fitted according to a least square method, so that cohesive force and an internal friction angle are obtained.
The technical progress achieved by adopting the technical scheme is as follows:
a large number of tests show that when the direct shear test with the stress level of 25kPa-200kPa or 50kPa-400kPa is adopted to test the strength of a soft soil sample according to the requirements of the existing geotechnical test specification on the direct shear test, the sample is easy to extrude or shear damage at the moment of loading, so that the shear strength is difficult to accurately measure. I.e., the stress level of the direct shear test specified by the code exceeds the strength of the weak soil.
In addition, there are various methods for measuring the shear strength of the soil body in the specification, such as an unconfined compressive strength test, a triaxial shear test, a cross plate shear test and the like. The sample of unconfined compression test and triaxial shear test is a cylinder, the height is 80mm, and the self strength of weak soil is lower, so that the regular cylinder sample is difficult to make, and if the loose sand does not have cohesive force, the sample cannot be formed. Then, although the shear strength of the saturated soft cohesive soil without drainage can be measured in the cross plate shear test, the cross plate shear test is inconvenient to use indoors as an in-situ test. Obviously, none of these methods is well suited to indoor testing of shear strength of weak soils. Indeed, there are other test methods for shear strength testing, but overall normalization is not sufficient.
Based on the consideration, the invention takes the Moire coulomb strength theory as the basis, improves the indoor direct shear test method specified by the specification, and provides a low stress level direct shear test method. The low stress level adopted by the method is 1-50kPa, so that the condition that the sample is directly damaged at the moment of loading can be better avoided; the no-load test is adopted while the vaseline is coated between the shearing boxes, so that the influence of the friction effect on the strength test of the weak soil can be effectively reduced; and the force measuring ring with smaller rigidity and higher sensitivity (the maximum measuring range is preferably 60-600N) is replaced, so that the horizontal shear displacement and the shear strength of the test sample are conveniently tested. The most critical point is that: for the relation curve of the shear strength of the soft soil sample obtained by the test along with the change of the vertical stress, the appropriate vertical stress range is flexibly selected according to the Moore coulomb strength theory, namely the requirements of utilization are metLinear function of
The shear strength values within the range can be subjected to linear fitting, so that the cohesive force and the internal friction angle of the weak soil sample are calculated.
According to investigation, many areas in China have soft soil bodies, such as dredger fill sludge in Tianjin areas, deep soft soil in Shanghai areas and the like, the common characteristic of the soft soil bodies is that the strength of the soft soil bodies is low, and the soft soil bodies cannot be easily measured by a conventional test method. The invention aims to effectively test the shear strength of the soft soil body, namely to obtain the cohesive force and the internal friction angle of the soft soil body. The method is simple, flexible, economical and practical, can be realized indoors, can provide convenience for the engineering property research of weak soil in similar areas, and has great value.
Detailed Description
Drawings
FIG. 1(a) is a graph of shear stress versus shear displacement for the first embodiment;
FIG. 1(b) is the shear strength vs. vertical pressure relationship of the first embodiment;
FIG. 2(a) is a shear stress versus shear displacement curve for the second embodiment;
FIG. 2(b) is the shear strength versus vertical pressure relationship for the second embodiment;
FIG. 1 is a test result of remolding ultra-soft soil with water content of 63.8% by adopting the invention, and it can be seen from FIG. 1(a) that shear stress and shear displacement are in a weak strain hardening relationship, and the shear failure displacement is about 2.5 mm; as can be seen from FIG. 1(b), when the vertical pressure is too high, the shear strength is rather reduced, so that the appropriate vertical stress range is 2.5 kPa-7.5 kPa, and finally the shear strength parameter, the cohesive force c, is obtainedqAnd internal angle of friction phiqRespectively at 0.95kPa and 2.98 deg.
FIG. 2 is a test result of the present invention for testing the frictional strength of the sand-glass fiber reinforced plastic, in which the shear stress and the shear strain in FIG. 2(a) are in a strong strain hardening relationship, and the shear failure displacement is about 3.0 mm; in fig. 2(b), the intercept of the straight line is almost zero, and the internal friction angle is 21.0 °.
In the first embodiment, a low stress level direct shear test method is adopted to measure the fast shear strength of remolded ultra-soft soil, and the specific process is as follows:
(1) taking a plurality of air-dried soil blocks, testing the air-dried water content of the soil blocks, and then mashing the soil blocks and filling the soil blocks into a plastic barrel; weighing a certain amount of purified water according to the water content of 60%, pouring the purified water into a barrel, and stirring the soil sample at random until the soil sample is uniform. And then measuring the water content of the soil sample again, taking out a part of the soil sample from the barrel, filling the part of the soil sample into a plastic bag, and preparing the soil sample into soil paste with the given water content. In this example, the predetermined water content was 64%, and the measured water content of the paste was 63.8%.
(2) Weighing clean and slightly wet glass sheets and a cutting ring, weighing the quality of the glass sheets and the cutting ring, then uniformly smearing the prepared soil paste into the cutting ring by using a soil adjusting cutter, paying attention to the fact that no gap is reserved and the sample is not excessively extruded, scraping off residual soil after smearing, and weighing the quality of the residual soil to obtain the quality of the sample.
(3) The shear box is firstly wiped clean, a certain amount of vaseline is smeared on the surface of the lower shear box (outside a sample groove), and then the upper shear box and the lower shear box are aligned to insert the fixing bolt. After the balls are cleaned and the lubricating oil is smeared, the assembled shearing box is placed on a direct shear apparatus, a hand wheel is rotated to enable the shearing box to be in close contact with a force measuring ring (the maximum measuring range is 600N), and the reading of a dial indicator is reset to zero. The pin is then removed and the instrument is started to shear the sample between the two shear boxes at a shear rate of 0.8mm/min, while the stopwatch is started to record the dial indicator reading every 15 seconds (i.e. 1 revolution) and the shear is stopped when the shear displacement is greater than 6mm, thereby obtaining the dial indicator reading when the shear boxes are affected by friction.
(4) And (3) assembling a shearing box in the same way as the step (3), putting a hard plastic film at the bottom, carefully removing the sample and a cutting ring from the glass sheet, placing the sample on the shearing box, quickly placing the hard plastic film and the permeable stone on the sample in sequence, and slowly pressing the sample into the shearing box. The apparatus is also adjusted to bring the shear box into intimate contact with the load cell at step (3).
(5) According to the principle of small to large, the vertical pressure is selected from 1-50kPa, and six groups of vertical pressures are selected in the example, namely 2.5kPa, 5kPa, 7.5kPa, 12.5kPa, 25kPa and 50kPa, and then the corresponding loads are selected. If the mass of the permeable stone is not less than 1/10 of the total load, the mass of the permeable stone is also counted. The first three-stage pressure can directly apply load on the sample and the permeable stone, and the second three-stage pressure can be applied by small weights in a grading way through a lever; . Such as 12.5kPa pressure, can be applied by a lever with a small weight of 0.319 kg.
(6) After the sample was loaded, the instrument was started to shear the sample according to step (3) and the dial gauge reading was recorded. The test was stopped when the specimen reached the failure criterion. If the sample is extruded and damaged after being loaded, the damage condition is recorded, and the next stage of pressure is applied instead of another sample.
(7) And (3) arranging all the data into excel, calculating to obtain the shear stress and the shear displacement under the action of different vertical pressures, drawing a relation curve of the shear stress and the shear displacement, selecting a corresponding shear strength value according to the destruction standard, and drawing a shear strength and vertical pressure curve. The water content of the remolded ultra-soft soil is 63.8%, and the relation curve of the shear strength and the vertical pressure is shown in figure 1. It can be seen that the shear strength of the test piece decreases when the vertical stress is greater than 7.5 kPa. According to the Moore coulomb strength theory, the vertical pressure range is selected to be 2.5-7.5 kPa, and then a linear function is utilized
Fitting the obtained product to obtain the fast shearing and adhesive force c of the remolded ultra-soft soilqAnd angle of internal friction
Respectively at 0.95kPa and 2.98 deg.
In the second embodiment, a low stress level direct shear test method is used for measuring the friction strength of the loose sand-glass fiber reinforced plastic interface, and the specific process is as follows:
(1) taking a plurality of dried sand samples, preparing the dried sand samples into sand samples with the water content of 11%, filling the sand samples into plastic bags, putting the plastic bags into a moisture retention device, standing for 24 hours, and simultaneously keeping the area of the sample to be 30cm2Four pieces of round glass fiber reinforced plastics with the same size are cut. (the glass fiber reinforced plastic is a glass fiber reinforced plastic plate, the color is green, the thickness is 5-6 mm, the surface is rough and uneven, and the texture is regular)
(2) No-load test procedure was the same as in the first (3) step of the example
(3) According to the density of the sample of 2.02g/cm3Weighing a sand sample with a half mass of the sample, pouring the sand sample into a clean shearing box in which permeable stones and filter paper are placed, brushing the flat surface, placing a hard wood plate, beating the sand sample by hands until the surface of the sand sample is flush with the shearing surface of the shearing box, taking down the hard wood plate, placing a round glass fiber reinforced plastic, smearing a fixed amount of vaseline outside a sample groove of the shearing box, aligning the sand sample to the upper shearing box and the lower shearing box, inserting a fixing bolt, weighing the other half of the sand sample, pouring the sand sample into the shearing box, beating the surface of the sand sample by the same method to enable the sample to reach a specified dry density, placing the filter paper and the permeable stones, but not placing an upper pressure.
(4) Firstly, cleaning the balls on the direct shear apparatus, smearing a certain amount of lubricating oil, then placing the assembled shear box on the shear apparatus, rotating the hand wheel to make the shear box closely contact with the force measuring ring, and zeroing the dial indicator.
(5) According to the principle of increasing from small to large, the vertical pressure is selected from 1-50kPa, three groups of vertical pressures are selected, namely 5.1kPa, 9.5kPa and 13.6kPa respectively, the mass of the permeable stone is not counted, and the three levels of pressure are all that the load is directly added on the sample and the permeable stone.
(6) After the sample was loaded, the deadbolt was quickly pulled out, the instrument was started to shear at a shear rate of 0.8mm/min, while the stopwatch was started, the dial indicator reading was recorded every 15 seconds (1 revolution), and the test was stopped when the sample reached the failure criterion.
(7) And (3) sorting all the data into excel, firstly deducting the reading measured in no-load, then calculating to obtain the shear stress and the shear displacement under the action of different vertical pressures, drawing a relation curve between the shear stress and the shear displacement, selecting a corresponding shear strength value according to a destruction standard, and drawing a shear strength and vertical pressure curve. The relationship curve between the shear strength and the vertical pressure of the sand loosening pipe-soil interface of the embodiment is shown in figure 2, and then a linear function is utilized
Fitting the obtained product to obtain the internal friction angle of the strength parameter of the loose sand-glass fiber reinforced plastic interface
At 21.0 deg., the cohesion is negligible.
Claims (1)
1. A low stress level direct shear test method mainly comprises no-load test, sample preparation and loading, load selection, method selection, shear test and data processing, and specifically comprises the following steps:
(1) and (3) no-load test:
firstly, coating a quantitative lubricant between lower shearing boxes, aligning the lower shearing boxes with the shearing boxes, inserting a fixing pin, placing the shearing boxes without samples on a direct shear apparatus, rotating a hand wheel to enable the shearing boxes to be in close contact with a force measuring ring, enabling a dial indicator to return to zero, then pulling out the fixing pin, starting the apparatus to read, wherein the shearing rate is the same as that of a normal test, measuring and reading the reading of the shearing boxes under the influence of friction through no-load test, and wiping balls under the shearing boxes and coating lubricating oil during the test;
(2) preparing a sample and loading the sample:
the diameter of the sample is 61.8mm, and the height is 20 mm;
① for soft cohesive soil, cutting an original soil sample from the original soil sample or preparing a disturbed soil sample with given dry density and water content, wherein the number of each group of samples is not less than the number of selected loads, after a lubricant with the same amount as that of a no-load test is smeared between the shearing boxes, placing the samples and the cutting ring on the well-assembled shearing boxes, the cutting edge of the cutting ring faces upwards, and slowly pressing the samples into the boxes;
② for loose sandy soil, taking a representative air-dried sample screened by 2mm, weighing the air-dried sand amount required by each sample according to a given dry density, pouring the prepared sand sample into a well-assembled shear box, compacting the sand sample to a specified dry density, and brushing a flat sand surface, wherein the number of each group of samples is not less than the selected load number;
before and after sample loading, adding permeable stones, hard plastic films or filter paper on the upper end and the lower end of the sample according to different test methods;
(3) selecting a load:
the vertical pressure can be selected from 1kPa-50kPa according to the principle from small to large, the larger pressure can be applied in stages, the pressure within the range of 1-12.5kPa can be directly applied above the sample by a small weight or a permeable stone, and the like, and the pressure within the range of 12.5-50kPa can be applied by a lever by a small weight;
(4) the method comprises the following steps:
according to different practical engineering conditions, a proper test prescription is preferably selected, wherein the direct shear test is divided into three types, namely fast shear, consolidation fast shear and slow shear, and the fast shear is preferably selected for the conditions that the soil body on site is thick, the permeability is small, the construction speed is high, and the consolidation is not performed in time of water drainage basically;
① quick shearing, namely applying shearing stress at a shearing rate of 0.8-1.2mm/min until the sample is damaged, and respectively sticking hard plastic films on the upper and lower surfaces of the sample to simulate the condition of no water drainage;
② consolidation fast shearing, wherein the sample is fully consolidated under certain pressure, the upper and lower surfaces of the sample are drained by sticking water permeable filter paper, after the drained water is deformed stably, the drained water is not more than 0.005mm/h, and then shearing stress is applied at a shearing rate of 0.8-1.2mm/min until the sample is damaged;
③ slow shearing, namely, fully draining water and solidifying the sample under a certain pressure, and slowly applying shearing force at a shearing rate of less than 0.02mm/min to ensure that the sample has sufficient time to drain water and generate volume deformation in the shearing process;
(5) shear test:
placing the shearing box which is coated with the lubricant and is filled with the sample on a direct shear apparatus, and adjusting the apparatus to ensure that the shearing box is tightly contacted with a force measuring ring, wherein the maximum measuring range of the force measuring ring is preferably 60-600N;
① quick shearing, starting a stopwatch, and shearing the sample within 3-5 minutes at a shearing rate of 0.8-1.2 mm/min;
② consolidation fast shearing, namely filling water into a water tank of a shearing box after 5min of consolidation for a saturated sample, covering a wet rag around a pressurizing plate without filling water for an unsaturated sample to prevent water evaporation, considering that the consolidation is stable when the vertical deformation reading of the sample is not more than 0.005mm/h, and recording the reading of a vertical displacement meter, wherein the shearing rate is 0.8-1.2 mm/min;
③ slow shearing, namely, the consolidation step is ②, the shearing rate is less than 0.02mm/min, and when the shearing displacement of the sample is 0.2-0.4mm, the reading of the dynamometer and the displacement meter is measured and recorded;
after the shearing was completed, the pressure was released and the sample was taken out to measure the water content in the vicinity of the sheared surface.
(6) Destruction criteria:
① if there is obvious peak value or stable value in the relation curve of shear stress and shear displacement, taking the peak value or stable value as the damage standard;
② if the shear stress increases with the increase of the shear displacement, the maximum shear displacement should reach 6mm, then according to the curve change condition to observe whether there is a larger turning point to determine, or choose the stress value when the shear displacement is 4mm to determine the shear strength, the latter only when the damage value is difficult to choose to adopt;
(7) data processing:
according to data of different load shear tests, reading numbers in no-load are deducted firstly, then shear stress tau and shear displacement delta l are solved respectively, relation curves of tau-delta l and s-delta l are drawn, a shear strength variation curve along with vertical pressure is drawn after shear strength is determined, a proper vertical pressure range is selected according to a Moore coulomb strength theory, and then the shear strength value in the range is fitted according to a least square method, so that cohesive force and an internal friction angle are obtained.
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