A root-soil composite shear test method and device

Abstract

The present invention discloses a root-soil complex shear test method and device, comprising the following steps: placing a shear box at a selected measurement location, and pressing the shear box downward into the soil; trimming the ground around the shear box to be flush with the shear surface; fixing a clamp to the shear box, and anchoring the shear test device to the ground; applying a thrust to the shear box to make the shear box move a preset distance along a guide rail for the first time, and simultaneously collecting thrust F1 and displacement data signals to obtain a thrust F1-displacement relationship curve; after pulling the shear box back to its original position; moving the shear box along the guide rail for a second time to obtain a thrust F2-displacement relationship curve; obtaining a shear force-displacement relationship curve; and calculating the shear strength of the root-soil complex according to the formula: #imgabs0# The root-soil complex shear test method and device of the present invention obtain accurate root-soil complex shear strength by measuring the thrust when the shear box is moved twice.

Description

Technical Field

[0001] The present invention relates to the technical field of soil mechanics, and more specifically to a root-soil composite shear test method and device thereof. Background Art

[0002] With the deepening of ecological governance, bioengineering measures have received more and more attention. In terms of soil consolidation and slope protection and soil erosion prevention, plant roots play a very important role. They are intricately interspersed in the soil, and through the reinforcement effect of shallow roots and the anchoring effect of deep roots, they play a role in strengthening the soil and improving the soil's resistance to erosion and erosion.

[0003] At present, the evaluation methods and means for the effect of plant roots on soil stability at home and abroad mainly include indoor direct shear test, triaxial compression test, in-situ shear test, etc. Among them, the in-situ shear test is carried out without any disturbance or damage to the roots in the root-soil complex, which can eliminate the influence of sampling or reshaping on the adhesion and friction between the roots and soil, so that its test results are more in line with reality.

[0004] The traditional in-situ shear tester for measuring the shear strength of the root-soil complex has certain limitations during use. The device is large and bulky, and the shear box is mostly controlled by hand-cranked screw feeding, which cannot accurately control the shear speed; data collection is time-consuming and labor-intensive, and it is difficult to synchronously collect thrust and displacement data. Currently, the common shear test device mostly uses upper and lower PVC pipes as shear boxes. Before the experiment, the root-soil complex needs to be planted in two empty PVC pipes and buried vertically in the soil for cultivation. The initial sample preparation of this type of structure is complicated, and the PVC pipe will inevitably restrict the natural growth of the plant root system, which is not suitable for field testing. Summary of the invention

[0005] The object of the present invention is to provide a testing method and a device capable of obtaining accurate shear strength of a root-soil composite.

[0006] According to one aspect of the present invention, a root-soil composite shear test method is provided, comprising the following steps:

[0007] Placing a shear box at a selected measurement location, and pressing the shear box downward into the soil, wherein the bottom surface of the shear box is a shear surface;

[0008] Trimming the ground around the shear box to be flush with the shear surface;

[0009] Putting in a shear test device, fixing a fixture to the shear box, and anchoring the shear test device to the ground;

[0010] Applying thrust to the shear box to make the shear box move a preset distance along the guide rail for the first time, and collecting thrust F1 and displacement data signals at the same time to obtain a thrust F1-displacement relationship curve;

[0011] After the shear box is moved to the preset distance, the shear box is pulled back to its original position;

[0012] Applying thrust to the shear box again, so that the shear box moves the preset distance along the guide rail for the second time, and collecting thrust F2 and displacement data signals at the same time to obtain a thrust F2-displacement relationship curve;

[0013] Obtaining a shear force-displacement relationship curve according to the thrust F1-displacement relationship curve and the thrust F2-displacement relationship curve;

[0014] The shear strength of the root-soil composite is calculated according to formula (1):

[0015]

[0016] Where: E p is the shear strength of the root-soil composite (J / m 2 ), F(x) is the shear force-displacement relationship curve, x is the shear displacement (mm), X P is the displacement corresponding to the peak shear force (mm).

[0017] Preferably, the speed of the shear box moving along the track for the first time is equal to the speed of the shear box moving along the track for the second time.

[0018] Preferably, the preset distance is 20 cm.

[0019] Preferably, when the shear box is pulled back to its original position, the shear box is separated from the shear surface.

[0020] Preferably, before pressing the shear box downward into the soil, first trim the soil larger than the area of ​​the shear box, and then press the shear box into the soil to be tested by cutting the soil around the shear box while pressing the shear box downward.

[0021] According to another aspect of the present invention, there is provided a root-soil composite shear testing device, comprising:

[0022] A shear box, which is arranged in a rectangular frame shape and is used to be pressed into the soil to be tested to form a shear surface;

[0023] A frame comprising two guide rails arranged in parallel and a bottom plate fixedly connected to the guide rails;

[0024] An electric push rod, disposed on the frame, for driving the shear box to reciprocate along the guide rail;

[0025] A clamp, used for connecting the shear box and the electric push rod;

[0026] A data acquisition system, comprising a pressure sensor for generating a thrust signal of the electric push rod acting on the shear box, a displacement sensor for generating a displacement signal of the shear box, and a data acquisition card;

[0027] The ground anchor rod is used to fix the frame to the ground.

[0028] Preferably, the clamp comprises a front clamp and a rear clamp for clamping the shear box.

[0029] Preferably, the front clamp and the rear clamp are respectively provided with at least two sliding blocks cooperating with the guide rails.

[0030] Preferably, one end of the electric push rod is fixed to the frame, and the other end is fixedly connected to the front clamp via the pressure sensor.

[0031] Preferably, the displacement sensor is a pull-rod type displacement sensor, the pull-rod type displacement sensor is fixed to the guide rail via a bracket, and one end of the pull-rod type displacement sensor is connected to the front clamp.

[0032] The beneficial effects of the present invention are as follows:

[0033] Since the root-soil complex shear test method and device of the present invention measures the thrust when the shear box is moved twice, the friction force of the shear surface can be subtracted from the thrust, thereby obtaining an accurate shear force-displacement curve and further obtaining the precise shear strength of the root-soil complex. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings.

[0035] Figure 1 A schematic structural diagram of the root-soil composite shear testing device of the present invention is shown.

[0036] Figure 2 A schematic structural diagram of a fixture of a root-soil composite shear test device of the present invention is shown.

[0037] Figure 3 The force-displacement relationship curve of the electric push rod of the present invention acting on the shear box is shown.

[0038] Figure 4 The shear force-displacement relationship curve of the present invention is shown. DETAILED DESCRIPTION

[0039] In order to more clearly illustrate the present invention, the present invention is further described below in conjunction with preferred embodiments and accompanying drawings. Similar components in the accompanying drawings are represented by the same reference numerals. It should be understood by those skilled in the art that the content specifically described below is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

[0040] Figure 1 An embodiment of the root-soil composite shear test device of the present invention is shown, and the device includes: a shear box 10, a frame 20, an electric push rod 30, a fixture 40, a data acquisition system 50 and a ground anchor 60. The shear box 10 enters the soil, and its bottom surface forms a shear surface to be tested. The fixture 40 can clamp the shear box 10 and is connected to one end of the electric push rod 30. The other end of the electric push rod 30 is fixed on the frame 20. The electric push rod 30 can apply a force to the shear box 10, thereby driving the shear box 10 to move along the guide rail 21. The data acquisition system 50 includes a pressure sensor 51, a displacement sensor 52 and a data acquisition card. The data acquisition card is not shown in the figure. The pressure sensor 51 generates a thrust signal of the electric push rod 30 acting on the shear box 10, and the displacement sensor 52 generates a displacement signal of the shear box 10. The data acquisition card collects the thrust signal and the displacement signal, and can send the thrust signal and the displacement signal to the computer, thereby generating a thrust-displacement curve.

[0041] The testing device of the present invention provides thrust through the electric push rod 30, avoiding the disadvantages of the traditional hand-cranked screw and improving the stability of the thrust; the pressure sensor 51 and the displacement sensor 52 can simultaneously generate data signals of the thrust and displacement applied to the shear box 10, and the data acquisition system 50 collects the data signals of the thrust and displacement in real time, and can display the change process of the thrust applied to the shear box 10 with the displacement in real time through a computer.

[0042] In this embodiment, the shear box 10 is set to a rectangular frame of 250mm×250mm×150mm, and preferably, it can be made of 304 stainless steel plate with a thickness of 5mm by bending and welding. Using a rectangular shear box to prepare the original soil sample in the field saves the time of early sample preparation. The test device of the present invention can be used to test the root system in the root-soil complex without any disturbance or damage, and can eliminate the influence of sampling or reshaping on the adhesion and friction between the root and soil, so that the test results are more in line with reality.

[0043] like Figure 1 and Figure 2As shown, the clamp 40 includes a front clamp 41 and a rear clamp 42, both of which are semi-rectangular frame-shaped. The front clamp 41 and the rear clamp 42 are fixedly connected by bolts and clamped on the outside of the shear box 10. The front clamp 41 and the rear clamp 42 are made of low-alloy structural steel plates with specifications of 330mm×50mm×10mm, which are bent and welded. A steel plate with specifications of 100mm×50mm×10mm protrudes from the middle part of the front clamp 41 for welding the flange 44.

[0044] The frame 20 includes two parallel guide rails 21 and a bottom plate 22, and the bottom plate 22 is fixed at both ends of the guide rails 21. Preferably, the thrust of the electric push rod 30 is 1600N, the stroke is 0-200mm, and the propulsion speed is adjustable. One end of the electric push rod 30 is hinged to the bottom plate 22, and the other end is connected to the pressure sensor 51, so that it can apply thrust to the shear box 10, thereby driving the shear box 10 to move. A fixing hole is opened on the bottom plate 22, and the ground anchor rod 60 is inserted into the soil through the fixing hole, so as to fix the frame 20 to the ground, ensuring that when the electric push rod 30 pushes the shear box 10 to move, the frame 20 can remain stable and motionless.

[0045] Two sliders 43 are provided on both sides of the front clamp 41 and the rear clamp 42. The two sliders 43 cooperate with the left and right guide rails 21 respectively and move along the guide rails 21, so that the clamp 40 can reciprocate along the guide rails 21 through the sliders 43, and then the shear box 10 can reciprocate along the guide rails 21 through the sliders 43 under the drive of the electric push rod 30.

[0046] Furthermore, the pressure sensor 51 adopts a spoke-type pressure sensor, and the flange 44 is customized according to the size of the spoke-type pressure sensor, with a diameter of 74 mm and a thickness of 10 mm. At a distance of 5.5 mm from the edge thereof, 8 threaded through holes with a diameter of 6 mm are evenly arranged, and the spoke-type pressure sensor is fixed to the flange 44 using an M6 screw. The spoke-type pressure sensor has a measuring range of 0-3000N, is powered by 24V voltage, and outputs a 0-10V voltage signal.

[0047] The displacement sensor 52 is a pull rod type displacement sensor, which has a measuring range of 0-200mm, an accuracy of ±0.1mm, an input voltage of 10V, and an output voltage signal range of 0-10V. The pull rod type displacement sensor is set on the guide rail 21 through a bracket, and the end of the pull rod type displacement sensor is connected to the fixing part 45 on the front clamp 41, so as to generate a displacement signal of the shear box 10.

[0048] Preferably, the data acquisition card uses a USB6005 eight-channel 100K high-speed industrial acquisition card with a voltage signal receiving range of -10V to +10V, and can simultaneously acquire thrust and displacement signals to a laptop computer.

[0049] The test device of the present invention is small in size and light in weight (25KG), and is easy to disassemble and carry; a data acquisition card is used to synchronously collect voltage signals of the pressure sensor 51 and the displacement sensor 52, and the thrust and displacement of the shear box 10 are displayed in real time on the computer screen; the test is carried out without any disturbance or damage to the root system in the root-soil complex, so that the test results are more practical and accurate.

[0050] The present invention also discloses a root-soil composite shear test method, comprising the following steps:

[0051] S1. Determine the specific measurement location in the selected experimental plot, press the shear box 10 downward into the soil, and the bottom surface of the shear box 10 is the shear surface.

[0052] In order to reduce the disturbance that may be caused to the soil during the installation of the shear box 10, before pressing the shear box 10 downward into the soil, first trim the soil slightly larger than the area of ​​the shear box 10, and then press the shear box 10 into the soil to be tested by cutting the soil around the shear box 10 while pressing the shear box 10 downward, so that the soil surface is flush with the 10 cm scale line in the shear box 10. At this time, the soil on the bottom of the shear box 10 is the shear surface of the shear test. It can be understood that the height of the shear box 10 of this embodiment is 15 cm. In other embodiments, the soil surface can be flush with the top surface of the shear box 10, that is, the soil height in the shear box 10 is 15 cm. Those skilled in the art can select the soil height in the shear box 10 within the range of 0-15 cm according to the actual depth required for testing.

[0053] In order to ensure the accuracy of the test results, three measurement sites were selected for each experimental plot, that is, three tests were conducted on each experimental plot, and the depth of the test soil could be 0-10 cm, 10-20 cm, 20-30 cm, 30-40 cm or 40-50 cm respectively.

[0054] S2. After the installation of the shear box 10 is completed, the site is cleaned so that the soil around the shear box 10 is flush with the bottom surface of the shear box 10, that is, the ground around the shear box 10 is flush with the shear surface, leaving space for placing the test device.

[0055] S3. Place the test device in the field, fix the fixture 40 to the shear box 10, and anchor the test device to the bottom surface.

[0056] Specifically, the front clamp 41 and the rear clamp 42 are connected by bolts so that the front clamp 41 and the rear clamp 42 clamp the shear box 10, and then the ground anchor 60 is inserted into the soil to fix the test device to the ground.

[0057] S4, applying thrust to the shear box 10, so that the shear box 10 moves a preset distance along the guide rail for the first time, and at the same time collecting thrust F1 and displacement data signals to obtain a thrust F1-displacement relationship curve.

[0058] In this embodiment, a thrust is applied to the shear box 10 by an electric push rod 30 fixed on the base plate 22. Since the slider 43 on the clamp 40 is slidably connected to the guide rail 21, the clamp 40 can slide along the guide rail 21, thereby ensuring that the shear box 10 slides a distance of 20 cm along the guide rail 21. The moving speed of the shear box 10 is approximately 0.7 mm / s.

[0059] Since the electric push rod 30 is connected to the fixture 40 through the pressure sensor 51, and the fixture 40 is also connected to the displacement sensor 52, the pressure sensor 51 generates a data signal of the thrust F1, and the displacement sensor 52 generates a data signal of the displacement distance. The data acquisition card collects the data signals and saves them to the laptop computer with an acquisition frequency of 100 Hz, and then uses the mapping software to generate the thrust F1-displacement relationship curve.

[0060] S5. After the shear box 10 moves a preset distance, the shear box 10 is pulled back to its original position.

[0061] In the implementation manner, after the electric push rod 30 pushes the shear box 10 to move a distance of 20 cm, the electric push rod 30 pulls the shear box 10 to the initial position. In order to prevent the shear surface from being damaged when the shear box 10 is pulled back, the bottom plate 22 is slightly raised when the shear box 10 is pulled back so that the bottom surface of the shear box 10 is separated from the shear surface to avoid contact between the shear box 10 and the shear surface. After the shear box 10 is completely pulled back, the bottom plate 22 is adjusted back to its original position so that the bottom surface of the shear box 10 is fitted with the shear surface.

[0062] S6. Apply thrust to the shear box 10 again, so that the shear box 10 moves the preset distance along the guide rail 21 for the second time, and at the same time collect thrust F2 and displacement data signals to obtain a thrust F2-displacement relationship curve.

[0063] The electric push rod 30 applies thrust to the shear box 10 for the second time, so that the shear box 10 slides along the guide rail for a distance of 20 cm, and the moving speed is also maintained at 0.7 mm / s. The data acquisition card collects the thrust F2 and displacement data signals generated by the pressure sensor 51 and the displacement sensor, and saves the data signals to the laptop computer, and then uses the mapping software to generate the thrust F2-displacement relationship curve. The shear box 10 moves the same distance and speed twice before and after, so that the thrust displacement relationship curves before and after correspond to each other.

[0064] S7, such as Figure 3 As shown, the shear force-displacement relationship curve is obtained according to the thrust F1-displacement relationship curve and the thrust F2-displacement relationship curve.

[0065] During the shear failure of the root-soil complex, the thrust measured by the pressure sensor 51 includes shear force and friction force, wherein the shear force mainly includes the tensile force when the root system breaks, the friction force between the root system and the lower soil when the root system is pulled out, and the cohesion between soil particles, while the friction force mainly comes from the friction between the upper and lower shear surfaces after the soil structure is destroyed, and between the guide rail 21 and the slider 43. The friction force between the upper and lower shear surfaces has nothing to do with the shear strength of the root-soil complex, so it needs to be subtracted from the total thrust-displacement curve to obtain the actual shear force-displacement curve of the root-soil complex.

[0066] The thrust F1 in the thrust F1-displacement relationship curve generated when the electric push rod 30 pushes the shear box 10 to move for the first time includes shear force and friction force, among which the friction force between the guide rail 21 and the slider 43 can be reduced as much as possible when debugging the device, so that it can be ignored, that is, the thrust F1 includes the pulling force when the root system is broken, the friction between the root system and the lower soil when the root system is pulled out and the cohesion between soil particles, and the friction between the upper and lower shear surfaces after the soil structure is destroyed.

[0067] Since the soil has been sheared and damaged when the electric push rod 30 pushes the shear box 10 to move for the second time, the thrust F2 in the thrust F2-displacement relationship curve generated at this time only includes the friction between the upper and lower shear surfaces, so the shear force-displacement relationship curve is obtained by subtracting the thrust F2-displacement relationship curve from the thrust F1-displacement relationship curve.

[0068] Theoretically, the thrust F2-displacement relationship curve should be a horizontal straight line, but in reality it is not. Instead, it shows a slowly rising trend. The fundamental reason for this result is that, although the upper and lower soil bodies and the main root system have been cut off after the first shearing, the soil shear surface is not very flat. In addition, due to the influence of the root system, loose soil particles will accumulate unidirectionally during the movement of the shear box 10, thereby slightly increasing the shear force. This influence may be related to the soil texture, root characteristics and soil moisture content, and needs to be gradually corrected later. However, overall, the increase in friction is small and will not affect the results of this test.

[0069] S8. Calculate the shear strength of the root-soil composite according to formula (1):

[0070]

[0071] Where: E p is the shear strength of the root-soil composite (J / m 2 ), F(x) is the shear force-displacement relationship curve, x is the shear displacement (mm), X P is the displacement corresponding to the peak shear force (mm).

[0072] For the index of the shear strength of the root-soil complex, the maximum shear force per unit area commonly used in soil mechanics and material mechanics is not used. This is mainly because the shear failure process of the root-soil complex is an energy-consuming process, and the changes in shear force and displacement during the shear failure process of the root-soil complex need to be considered at the same time. Therefore, by integrating the shear force-displacement relationship curve of the root-soil complex, the consumed energy (shear work per unit area) is obtained as an index for evaluating the shear strength of the root-soil complex. Specifically, the turning point (X) on the shear force-displacement curve where the shear force increases rapidly and decreases significantly is found. P , T P ), and the shear strength of the root-soil composite is calculated according to formula (1).

[0073] Since the root-soil composite shear test method of the present invention measures the thrust when the shear box 10 is moved twice, the thrust can be subtracted from the friction of the shear surface, thereby obtaining an accurate shear force-displacement curve and further obtaining the precise shear strength of the root-soil composite.

[0074] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A root-soil composite shear test method, characterized in that: The steps include: Placing a shear box at a selected measurement location, and pressing the shear box downward into the soil, wherein the bottom surface of the shear box is a shear surface; Trimming the ground around the shear box to be flush with the shear surface; Putting in a shear test device, fixing a fixture to the shear box, and anchoring the shear test device to the ground; Apply thrust to the shear box, so that the shear box moves a preset distance along the guide rail for the first time, and at the same time collects thrust F 1 and displacement data signal to obtain thrust F 1-displacement relationship curve; After the shear box is moved to the preset distance, the shear box is pulled back to its original position; Apply thrust to the shear box again, so that the shear box moves the preset distance along the guide rail for the second time, and at the same time collects thrust F 2 and displacement data signal to obtain thrust F 2-displacement relationship curve; According to the thrust F 1- Displacement relationship curve and thrust F 2-displacement relationship curve to obtain the shear force-displacement relationship curve; The shear strength of the root-soil composite is calculated according to formula (1): (1) Where: is the shear strength of the root-soil composite (J / m 2 ), is the shear force-displacement relationship curve, is the shear displacement (mm), is the displacement corresponding to the peak shear force (mm).

2. The root-soil composite shear test method according to claim 1, characterized in that: The speed at which the shear box moves along the track for the first time is equal to the speed at which the shear box moves along the track for the second time.

3. The root-soil composite shear test method according to claim 1, characterized in that: The preset distance is 20 cm.

4. The root-soil composite shear test method according to claim 1, characterized in that: When the shear box is pulled back to its original position, the shear box is separated from the shear surface.

5. The root-soil composite shear test method according to claim 1, characterized in that: Before pressing the shear box downward into the soil, first trim the soil larger than the area of ​​the shear box, and then press the shear box into the soil to be tested by cutting the soil around the shear box while pressing the shear box downward.

6. A root-soil composite shear test device, applied to the test method according to claim 1, characterized in that: include: A shear box, which is set in a rectangular frame shape and is used to be pressed into the soil to be tested to form a shear surface; A frame comprising two guide rails arranged in parallel and a bottom plate fixedly connected to the guide rails; An electric push rod, disposed on the frame, for driving the shear box to reciprocate along the guide rail; A clamp, used for connecting the shear box and the electric push rod; A data acquisition system, comprising a pressure sensor for generating a thrust signal of the electric push rod acting on the shear box, a displacement sensor for generating a displacement signal of the shear box, and a data acquisition card; and The ground anchor rod is used to fix the frame to the ground.

7. The root-soil composite shear test device according to claim 6, characterized in that: The clamp includes a front clamp and a rear clamp for clamping the shear box.

8. The root-soil composite shear test device according to claim 7, characterized in that: The front clamp and the rear clamp are respectively provided with at least two sliding blocks matched with the guide rails.

9. The root-soil composite shear test device according to claim 7, characterized in that: One end of the electric push rod is fixed on the frame, and the other end is fixedly connected to the front clamp through the pressure sensor.

10. The root-soil composite shear test device according to claim 7, characterized in that: The displacement sensor is a pull-rod type displacement sensor, which is fixed on the guide rail, and one end of the pull-rod type displacement sensor is connected to the front clamp.

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