A direct shear test device and test method with controllable three-dimensional stress boundaries

Abstract

The present invention belongs to the field of geotechnical engineering test technology, and specifically relates to a direct shear test device and test method with controllable three-dimensional stress boundaries. The device of the present invention is composed of a shear box composed of an upper shear box and a lower shear box arranged up and down, an external force loading system, a lateral pressure control system, a fixing device and a water-permeable mechanism. According to the actual stress state of the original body of rock and soil, the present invention adjusts the vertical stress and horizontal bidirectional stress of the sample to enable it to drain and consolidate under the stress boundaries of each direction; it solves the problem that the traditional direct shear test device and method cannot consider the regulation of confining pressure. The sample used in the present invention has a square cross-sectional shape, and the positive stress is kept uniformly distributed during the shear process. By controlling the three-dimensional stress boundary of the sample, the present invention obtains the shear strength parameters of the rock and soil body more realistically, providing a reliable basis for engineering safety and stability analysis.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering tests, and in particular relates to a direct shear test device with controllable three-dimensional stress boundaries and a test method. Background Art

[0002] The direct shear test is simple in structure and easy to operate. It is a common method for determining the shear strength of soil. The principle of the test is Coulomb's law. The internal friction of the soil is proportional to the normal pressure on the shear surface. The soil is prepared into several tests. Under different normal pressures, horizontal shear force is directly applied along the fixed shear surface for shearing. The shear stress when it is sheared is the shear strength. Then, the shear strength index cohesion and friction angle of the soil are determined according to the shear law. Shear strength has important reference value in engineering applications. It can estimate the bearing capacity of the foundation, evaluate the stability of the foundation, calculate the earth pressure of the retaining wall, etc.

[0003] The conventional direct shear test is to place the sample in the fixed upper box and movable lower box of the direct shear apparatus. During the test, vertical pressure is first applied to the soil sample, and then horizontal thrust is applied to the lower box. The displacement between the upper and lower boxes causes the soil sample to be sheared. The test device does not consider the original stress state of the sample and the influence of the confining pressure during the shear test. Therefore, the shear strength parameters obtained from the test are quite different from the actual engineering situation.

[0004] The specimen of the conventional direct shear test is cylindrical, and the shear surface is a circular cross-section. The test process is the process of the upper and lower shear boxes offset from each other. Therefore, with the increase of shear displacement, the upper and lower parts of the cylinder gradually offset, and the effective area of ​​the shear surface decreases. The vertical pressure on it changes from the initial location at the test center to the deviation from the test center. The normal stress of the shear surface changes from the initial uniform distribution to the uneven distribution. During the shear process of the test, the normal stress does not maintain a uniform distribution. Summary of the invention

[0005] The invention provides a direct shear test device and a test method with controllable three-dimensional stress boundaries, aiming to provide a direct shear test device and a test method with simple structure, strong operability and good test effect.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A three-dimensional stress boundary controllable direct shear test device, comprising

[0008] The shear box is square and consists of an upper shear box and a lower shear box arranged up and down, and the middle of the upper shear box and the lower shear box has a hollow square cavity;

[0009] An external force loading system, the external force loading system includes an adjustable horizontal loading mechanism and an adjustable vertical loading mechanism, the horizontal loading mechanism is connected to the outer side wall of the lower shear box, and the vertical loading mechanism is arranged at the upper end opening of the upper shear box;

[0010] A lateral pressure control system, which is connected to the side wall of the shear box and is used to control the pressure of the sample;

[0011] A fixing device, the fixing device is connected to the outer side wall of the upper shear box, and the fixing device is arranged on the opposite side of the horizontal loading mechanism;

[0012] The water-permeable mechanism is arranged in the shear box.

[0013] The upper shear box is a square frame structure without a cover and a bottom; the lower shear box is a box body structure without a cover.

[0014] The permeable mechanism includes a drainage hole and a permeable stone; the drainage hole is a blind hole or a through hole, and is arranged on the bottom surface of the lower shear box; the permeable stone includes an upper shear box permeable stone and a lower shear box permeable stone, the upper shear box permeable stone is arranged on the lower surface of the vertical loading mechanism, and the lower shear box permeable stone is arranged between the sample and the drainage hole.

[0015] The external force loading system comprises two sets of horizontal loading mechanisms and one set of vertical loading mechanism; the two sets of horizontal loading mechanisms are respectively connected to two adjacent outer side walls of the lower shear box.

[0016] The horizontal loading mechanisms all include an external force loading indicator, a force adder and a force transmission plate; the external force loading indicator is connected to the force adder, and the force adder is fixed to the outer side wall of the lower shear box through the force transmission plate.

[0017] The vertical loading mechanism comprises an external force loading display, a force booster, a force transmission steel ball and a pressurizing piston; the force transmission steel ball is connected to the pressurizing piston, and the force transmission steel ball is connected to the external force loading display through the force booster.

[0018] There are four sets of lateral pressure control systems; two of them are connected to the adjacent two side surfaces of the upper shear box, and the other two are connected to the adjacent two side surfaces of the lower shear box, and the upper and lower lateral pressure control systems correspond to each other; the lateral pressure control system arranged on the lower shear box is coplanar with the horizontal loading mechanism.

[0019] The lateral pressure control system includes a lateral pressure display, a pressurizer and an air storage chamber; the lateral pressure display and the pressurizer are arranged outside the shear box; the air storage chamber has a rectangular cross-section and is arranged inside the shear box, and the air storage chamber covers the side of the shear box; the lateral pressure display is connected to the pressurizer, and the pressurizer is connected to the air storage chamber in the shear box; a rigid pad is fixedly connected between the air storage chambers in the upper shear box and the lower shear box; a rigid block is fixedly connected at the connection between the adjacent two side walls of the shear box where the air storage chamber is arranged.

[0020] The fixing device is a force-measuring steel ring; the force-measuring steel ring is connected to the upper shear box.

[0021] A direct shear test method with controllable three-dimensional stress boundaries comprises the following steps:

[0022] Step 1: First, place the upper shear box and the lower shear box stably, and then fix the force measuring steel ring to the fixed vertical support;

[0023] Step 2: Place the sample in the shear box and drain and consolidate the sample under the original stress state by adjusting the lateral pressure system and the vertical loading system;

[0024] Step 3: Dynamically control the lateral pressure system on the shear box, apply a set vertical pressure σ to the vertical loading system, and then apply a set shear force τ to the horizontal loading mechanism of the lower shear box (1)x , the sample is sheared at a set shear rate until it breaks;

[0025] Step 4: Repeat steps 1 to 3 to perform 4 sets of shear tests and obtain the vertical pressure σ of the four sets of tests. (1) , σ (2) , σ (3) , σ (4) and four sets of shear forces τ (1)f , τ (2)f , τ (3)f , τ (4)f ;

[0026] Step 5: Based on the vertical pressure values ​​of the four groups of tests obtained in step 4 and the four groups of root shear force values, the relationship between cohesion c and friction angle φ in shear strength is obtained in combination with the Coulomb strength theory;

[0027] Step 6: Determine the soil's ability to resist shear failure using the relationship between cohesion c and friction angle φ obtained in step 5.

[0028] Beneficial effects:

[0029] (1) The present invention adjusts the vertical stress and horizontal bidirectional stress of the sample according to the actual stress state of the original rock and soil body, so that drainage consolidation is carried out under the stress boundaries of each direction, thereby solving the problem that the traditional direct shear test device and method cannot consider the regulation of confining pressure.

[0030] (2) The cross-sectional shape of the specimen used in the present invention is square, so that the normal stress is kept uniformly distributed during the shearing process.

[0031] (3) The present invention controls the three-dimensional stress boundary of the sample, thereby obtaining the shear strength parameters of the rock and soil mass more realistically, providing a reliable basis for engineering safety and stability analysis.

[0032] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0034] Figure 1 It is a schematic elevation view of the present invention;

[0035] Figure 2 is a cross-sectional schematic diagram of the upper shear box of the present invention;

[0036] Figure 3 is a cross-sectional schematic diagram of the lower shear box of the present invention;

[0037] Figure 4 It is a schematic diagram of the shear strength relationship of the present invention.

[0038] In the figure: 1-shear box; 2-horizontal loading mechanism; 3-vertical loading mechanism; 4-sample; 5-upper shear box; 6-lower shear box; 7-drainage hole; 8-upper shear box permeable stone; 9-lower shear box permeable stone; 10-external force loading indicator; 11-force booster; 12-force transmission plate; 13-pressurizing piston; 14-lateral pressure indicator; 15-pressurizer; 16-air storage chamber; 17-force measuring steel ring; 18-fixed vertical support; 19-rigid pad; 20-rigid block; 21-force transmission steel ball. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] Embodiment 1:

[0041] according to Figure 1-3 A three-dimensional stress boundary controllable direct shear test device is shown, comprising

[0042] The shear box 1 is square and is composed of an upper shear box 5 and a lower shear box 6 arranged up and down, and the middle of the upper shear box 5 and the lower shear box 6 has a hollow square cavity;

[0043] The external force loading system includes an adjustable horizontal loading mechanism 2 and an adjustable vertical loading mechanism 3. The horizontal loading mechanism 2 is connected to the outer side wall of the lower shear box 6, and the vertical loading mechanism 3 is arranged at the upper end opening of the upper shear box 5;

[0044] A lateral pressure control system, which is connected to the side wall of the shear box 1 and is used to control the pressure of the sample 4;

[0045] A fixing device, the fixing device is connected to the outer side wall of the upper shear box 5, and the fixing device is arranged on the opposite side of the horizontal loading mechanism 2;

[0046] The water-permeable mechanism is arranged in the shear box 1 .

[0047] In actual use, the upper shear box 5 and the lower shear box 6 are first placed stably, and then the fixing device is fixed to the fixed vertical support 18; then the sample 4 is placed in the upper shear box 5 and the lower shear box 6, and the sample 4 is first drained and consolidated under the original stress state by adjusting the lateral pressure system and the vertical loading mechanism 3; then the lateral pressure system on the shear box is dynamically adjusted, and the set vertical pressure σ is applied to the vertical loading mechanism 3 (1) , and then apply the set shear force τ to the loading mechanism 2 through the level of the lower shear box 6 (1)x , make sample 4 shear at the set shear rate until it breaks; repeat 4 groups of shear tests to obtain the vertical pressure σ of the four groups of tests (1) , σ (2) , σ (3) , σ (4) and four sets of shear forces τ (1)f , τ (2)f , τ (3)f , τ (4)f; Then, based on the vertical pressure and four groups of shear force values ​​obtained from the four groups of tests, combined with the Coulomb strength theory and the existing technical methods, the relationship between cohesion c and friction angle φ in shear strength is obtained; through the obtained relationship between cohesion c and friction angle φ, the resistance of the soil to shear failure is determined, providing a reliable basis for subsequent engineering safety and stability analysis.

[0048] The present invention regulates the vertical stress and horizontal bidirectional stress of the specimen to enable drainage and consolidation under the stress boundaries of each direction; it solves the problem that the traditional direct shear test device and method cannot consider the regulation of confining pressure. In addition, the shear box 1 adopts a square technical solution, which is convenient for controlling the confining pressure of the present invention; the cross-sectional shape of the specimen 4 placed in the hollow square cavity is square, and during the shear process, it is ensured that the normal stress is uniformly distributed. This device controls the three-dimensional stress boundary of the specimen, and it obtains the shear strength parameters of the rock and soil mass more realistically, providing a reliable basis for engineering safety and stability analysis.

[0049] Embodiment 2:

[0050] according to Figure 1 and Figure 2 The three-dimensional stress boundary controllable direct shear test device shown is different from the first embodiment in that: the upper shear box 5 is a square frame structure without a cover and a bottom; the lower shear box 6 is a box structure without a cover.

[0051] In actual use, the upper shear box 5 and the lower shear box 6 adopt the technical solution, which can conveniently apply force and pressure to the sample 4, ensuring that accurate data is obtained in the direct shear test.

[0052] Embodiment three:

[0053] according to Figure 1 The three-dimensional stress boundary controllable direct shear test device shown in the figure is different from the embodiment 1 in that: the permeable mechanism includes a drainage hole 7 and a permeable stone; the drainage hole 7 is a blind hole or a through hole, and is arranged on the bottom surface of the lower shear box 6; the permeable stone includes an upper shear box permeable stone 8 and a lower shear box permeable stone 9, the upper shear box permeable stone 8 is arranged on the lower surface of the vertical loading mechanism 3, and the lower shear box permeable stone 9 is arranged between the specimen 4 and the drainage hole 7.

[0054] In actual use, the upper shear box permeable stone 8 is arranged between the sample 4 and the vertical loading mechanism 3. The permeable mechanism adopts the technical solution, which can fully remove the moisture in the sample and ensure the reliability of the obtained sample direct shear test data.

[0055] Whether the drainage hole 7 is a blind hole or a through hole can be selected according to the actual situation of the sample. When the water content of the sample 4 is relatively high, a through hole can be selected; when the water content of the sample 4 is relatively low, a blind hole can be selected, or even no drainage hole 7 can be set.

[0056] In this embodiment, the side wall and the bottom plate of the shear box are both made of rigid materials. The use of rigid materials can ensure the deformation of the shear box during the test, making the test data more accurate.

[0057] Embodiment 4:

[0058] according to Figure 1 A three-dimensional stress boundary controllable direct shear test device as shown in 3 is different from Example 1 in that: the external force loading system includes two sets of horizontal loading mechanisms 2 and one set of vertical loading mechanisms 3; the two sets of horizontal loading mechanisms 2 are respectively connected to the two adjacent outer walls of the lower shear box 6.

[0059] In actual use, the use of this technical solution to set up the external force loading system can ensure that the specimen can be loaded with forces in all directions, making the obtained test data more reliable and trustworthy.

[0060] Embodiment five:

[0061] according to Figure 1 and Figure 3 The three-way stress boundary controllable direct shear test device shown is different from the first embodiment in that: the horizontal loading mechanism 2 includes an external force loading display 10, a force adder 11 and a force transmission plate 12; the external force loading display 10 is connected to the force adder 11, and the force adder 11 is fixed to the outer wall of the lower shear box 6 through the force transmission plate 12.

[0062] In actual use, the force applied by the force adder 11 is transmitted to the sample 4 through the force transmission plate 12, and the external force loading display instrument 10 displays the magnitude of the force applied by the force adder 11, so that the applied force value can be obtained conveniently.

[0063] Embodiment six:

[0064] according to Figure 1 The three-way stress boundary controllable direct shear test device shown is different from the embodiment 1 in that: the vertical loading mechanism 3 includes an external force loading display 10, a force adder 11, a force transmission steel ball 21 and a pressurizing piston 13; the force transmission steel ball 21 is connected to the pressurizing piston 13, and the force transmission steel ball 21 is connected to the external force loading display 10 through the force adder 11.

[0065] In actual use, the force applied by the force adder 11 is transmitted to the sample 4 through the force transmission steel ball 21, and the external force loading display 10 displays the magnitude of the force applied by the force adder 11, so that the force value can be obtained conveniently. The force added by the force adder 11 through the force transmission steel ball 21 ensures that the force applied to the sample 4 is always kept in the normal direction.

[0066] Embodiment seven:

[0067] according to Figure 1 , Figure 2 and Figure 3 The three-dimensional stress boundary controllable direct shear test device shown is different from the embodiment 1 in that: there are four sets of lateral pressure control systems; two sets of lateral pressure control systems are connected to the adjacent two side surfaces of the upper shear box 5, and the other two sets of lateral pressure control systems are connected to the adjacent two side surfaces of the lower shear box 6, and the upper and lower lateral pressure control systems correspond respectively; the lateral pressure control system arranged on the lower shear box 6 is coplanar with the horizontal loading mechanism 2.

[0068] In actual use, the lateral pressure control system adopts this technical solution, so that the shear test of sample 4 is more perfect and the obtained test value is more scientific.

[0069] Embodiment eight:

[0070] according to Figure 1 The three-dimensional stress boundary controllable direct shear test device shown is different from the embodiment 1 in that: the lateral pressure control system includes a lateral pressure display 14, a pressurizing device 15 and an air storage chamber 16; the lateral pressure display 14 and the pressurizing device 15 are arranged outside the shear box 1; the cross-section of the air storage chamber 16 is rectangular and is arranged in the shear box 1, and the air storage chamber 16 covers the side of the shear box 1; the lateral pressure display 14 is connected to the pressurizing device 15, and the pressurizing device 15 is connected to the air storage chamber 16 in the shear box 1; a rigid pad 19 is fixedly connected between the air storage chambers 16 in the upper shear box 5 and the lower shear box 6; a rigid block 20 is fixedly connected at the connection between the adjacent two side walls of the shear box where the air storage chamber 16 is arranged.

[0071] In actual use, the pressurizer 15 charges the air storage chamber 16, thereby pressurizing the sample 4, and the pressure value is displayed by the pressure side pressure display instrument 14 to ensure that the intensity of the pressure is mastered. The rigid pad 19 and the rigid block 20 are set to eliminate the influence of the air storage chamber 16 in the upper and lower shear boxes and between adjacent air storage chambers 16, so as to ensure the authenticity and validity of the test data.

[0072] The selection of the rigid pad 19 and the rigid block 20 only needs to ensure that the rigid pad 19 and the rigid block 20 do not deform during the test.

[0073] The gas storage chamber 16 is an expandable and contractible structure for storing and releasing gas. The gas storage chamber 16 has a rectangular cross section and is arranged in the shear box 1. The gas storage chamber 16 covers the side of the shear box 1, ensuring that the test can evenly simulate the surrounding rock pressure.

[0074] Embodiment nine:

[0075] according to Figure 1 and Figure 2 The three-dimensional stress boundary controllable direct shear test device shown is different from the first embodiment in that the fixing device is a force measuring steel ring 17; the force measuring steel ring 17 is connected to the upper shear box 5.

[0076] In actual use, the force measuring steel ring 17 adopts the existing technology, and its function is to measure the shear force, and needs to be fixed with the fixed vertical support 18 to ensure that the present invention is carried out in a stable condition and the shear box is evenly stressed. The fixed vertical support 18 can be a wall or a fixed object.

[0077] Embodiment ten:

[0078] A direct shear test method with controllable three-dimensional stress boundaries comprises the following steps:

[0079] Step 1: First, place the upper shear box 5 and the lower shear box 6 stably, and then fix the force measuring steel ring 17 to the fixed vertical support 18;

[0080] Step 2: Place the sample 4 in the shear box 1, and first drain and consolidate the sample 4 under the original stress state by adjusting the lateral pressure system and the vertical loading mechanism 3;

[0081] Step 3: Dynamically control the lateral pressure system 3 on the shear box, apply a set vertical pressure σ1 vertically to the loading mechanism 3, and then apply a set shear force τ horizontally to the loading mechanism 2 through the lower shear box 6 1x , causing the sample 4 to be sheared at a set shear rate until it is sheared;

[0082] Step 4: Repeat steps 1 to 3 to perform 4 sets of shear tests and obtain the vertical pressure σ of the four sets of tests. (1) , σ (2) , σ (3) , σ (4) and four sets of shear forces τ (1)f , τ (2)f , τ (3)f , τ (4)f ;

[0083] Step 5: Based on the vertical pressure values ​​of the four groups of tests obtained in step 4 and the four groups of root shear force values, the relationship between cohesion c and friction angle φ in shear strength is obtained in combination with the Coulomb strength theory;

[0084] Step 6: Determine the soil's ability to resist shear failure through the relationship between cohesion c and friction angle φ obtained in step 5, providing a basis for engineering safety and stability analysis.

[0085] In actual use, firstly, the upper shear box 5 and the lower shear box 6 are stably placed, and then the fixing device is fixed to the fixed vertical support 18; then the sample 4 is placed in the upper shear box 5 and the lower shear box 6, and when the lateral pressure system and the vertical loading mechanism 3 are started, the sample 4 is acted on with the cooperation of the fixing device, i.e., the force-measuring steel ring 17, so that the sample 4 is first drained and consolidated under the original stress state; then the lateral pressure system on the shear box is dynamically regulated, and the set vertical pressure σ is applied through the vertical loading mechanism 3 (1) , and then apply the set shear force τ to the loading mechanism 2 through the level of the lower shear box 6 (1)x , make sample 4 shear at the set shear rate until it breaks; repeat 4 groups of shear tests to obtain the vertical pressure σ of the four groups of tests (1) , σ (2) , σ (3) , σ (4) and four sets of shear forces τ (1)f , τ (2)f , τ (3)f , τ (4)f ; Then, according to the vertical pressure and four sets of shear force values ​​obtained from the four groups of tests, the relationship between cohesion c and friction angle φ in shear strength is obtained by combining Coulomb strength theory and using the existing technology method; By obtaining the relationship between cohesion c and friction angle φ (such as Figure 4 As shown in the figure, the soil's resistance to shear failure is determined, providing a reliable basis for subsequent engineering safety and stability analysis.

[0086] In summary, the present invention adjusts the vertical stress and horizontal bidirectional stress of the sample according to the actual stress state of the original rock and soil body, so that drainage and consolidation are carried out under the stress boundaries of each direction; thus solving the problem that the traditional direct shear test device and method cannot consider the regulation of confining pressure. The sample used in the present invention has a square cross-sectional shape, and the positive stress is kept uniformly distributed during the shear process. By controlling the three-dimensional stress boundary of the sample, the present invention can more realistically obtain the shear strength parameters of the rock and soil body, providing a reliable basis for engineering safety and stability analysis.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

[0088] In the absence of conflicts, technicians in this field can combine the relevant technical features in the above examples according to actual conditions to achieve corresponding technical effects. The specific combinations are not described here one by one.

[0089] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0090] The above are only preferred embodiments of the present invention. The present invention will not be limited to the embodiments shown herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein. Any simple modification, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A three-dimensional stress boundary controllable direct shear test device, characterized in that: include A shear box (1), the shear box (1) is square in shape and is composed of an upper shear box (5) and a lower shear box (6) arranged one above the other, wherein the middle parts of the upper shear box (5) and the lower shear box (6) have a hollow square cavity; An external force loading system, the external force loading system comprising an adjustable horizontal loading mechanism (2) and an adjustable vertical loading mechanism (3), the horizontal loading mechanism (2) being connected to the outer side wall of the lower shear box (6), and the vertical loading mechanism (3) being arranged at the upper end opening of the upper shear box (5); A lateral pressure control system, the lateral pressure control system being connected to the side wall of the shear box (1) and being used to control the pressure of the sample (4); A fixing device, the fixing device being connected to the outer side wall of the upper shear box (5), and the fixing device being arranged on the opposite side of the horizontal loading mechanism (2); A water permeable mechanism, the water permeable mechanism being arranged in the shear box (1); The external force loading system comprises two sets of horizontal loading mechanisms (2) and one set of vertical loading mechanisms (3); the two sets of horizontal loading mechanisms (2) are respectively connected to two adjacent outer side walls of the lower shear box (6); The lateral pressure control system is provided in four sets; two sets of the lateral pressure control system are connected to the adjacent two side surfaces of the upper shear box (5), and the other two sets of the lateral pressure control system are connected to the adjacent two side surfaces of the lower shear box (6), and the upper and lower lateral pressure control systems correspond to each other; the lateral pressure control system provided on the lower shear box (6) is coplanar with the horizontal loading mechanism (2); The lateral pressure control system comprises a lateral pressure display (14), a pressurizing device (15) and an air storage chamber (16); the lateral pressure display (14) and the pressurizing device (15) are arranged outside the shear box (1); the air storage chamber (16) has a rectangular cross section and is arranged inside the shear box (1), and the air storage chamber (16) covers the side surface of the shear box (1); the lateral pressure display (14) is connected to the pressurizing device (15), and the pressurizing device (15) is connected to the air storage chamber (16) inside the shear box (1); a rigid cushion block (19) is fixedly connected between the air storage chambers (16) inside the upper shear box (5) and the lower shear box (6); a rigid block (20) is fixedly connected to the connection between the adjacent two side walls of the shear box where the air storage chamber (16) is arranged; The cross-section shape of the specimen used is square, so that the normal stress remains evenly distributed during the shearing process.

2. A three-dimensional stress boundary controllable direct shear test device as claimed in claim 1, characterized in that: The upper shear box (5) is a square frame structure without a cover and a bottom; the lower shear box (6) is a box body structure without a cover.

3. A three-dimensional stress boundary controllable direct shear test device as claimed in claim 1, characterized in that: The water-permeable mechanism comprises a drainage hole (7) and a permeable stone; the drainage hole (7) is a blind hole or a through hole, and is arranged on the bottom surface of the lower shear box (6); the permeable stone comprises an upper shear box permeable stone (8) and a lower shear box permeable stone (9); the upper shear box permeable stone (8) is arranged on the lower surface of the vertical loading mechanism (3), and the lower shear box permeable stone (9) is arranged between the sample (4) and the drainage hole (7).

4. A three-dimensional stress boundary controllable direct shear test device as claimed in claim 1, characterized in that: The horizontal loading mechanism (2) comprises an external force loading display (10), a force adder (11) and a force transmission plate (12); the external force loading display (10) is connected to the force adder (11), and the force adder (11) is fixed to the outer side wall of the lower shear box (6) via the force transmission plate (12).

5. A three-dimensional stress boundary controllable direct shear test device as claimed in claim 1, characterized in that: The vertical loading mechanism (3) comprises an external force loading display (10), a force adding device (11), a force transmission steel ball (21) and a pressurizing piston (13); the force transmission steel ball (21) is connected to the pressurizing piston (13), and the force transmission steel ball (21) is connected to the external force loading display (10) via the force adding device (11).

6. A three-dimensional stress boundary controllable direct shear test device as claimed in claim 1, characterized in that: The fixing device is a force-measuring steel ring (17); the force-measuring steel ring (17) is connected to the upper shear box (5).

7. The direct shear test method of the direct shear test device with controllable three-dimensional stress boundaries as claimed in claim 6, characterized in that: The steps include: Step 1: First, place the upper shear box (5) and the lower shear box (6) stably, and then fix the force measuring steel ring (17) connected to the side wall of the upper shear box 5 to the fixed vertical support member (18); Step 2: placing the sample (4) in the shear box (1), and adjusting the lateral pressure system and the vertical loading mechanism (3) to first drain and consolidate the sample (4) under the original stress state; Step 3: Dynamically control the lateral pressure system (3) on the shear box by applying a set vertical pressure σ to the vertical loading mechanism (3) （1） , and then the set shear force τ is applied to the loading mechanism (2) through the lower shear box (6) （1）x , causing the sample (4) to be sheared at a set shear rate until it is sheared; Step 4: Repeat steps 1 to 3 to perform 4 sets of shear tests and obtain the vertical pressure σ of the four sets of tests. （1） , σ （2） , σ （3） , σ （4） and four sets of shear forces τ （1）f , τ （2）f , τ （3）f , τ （4）f ; Step 5: Based on the four groups of vertical pressure and four groups of shear force values ​​obtained in step 4, the relationship between cohesion c and friction angle φ in shear strength is obtained in combination with Coulomb strength theory; Step 6: Determine the soil's ability to resist shear failure using the relationship between cohesion c and friction angle φ obtained in step 5.

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