Method for obtaining water storage coefficient of collapse zone of underground reservoir in coal mine

Abstract

The present invention provides a method for obtaining the water storage coefficient of the collapse zone of the underground water reservoir in a coal mine, comprising: obtaining the maximum surface subsidence value above the goaf area; obtaining the height value of the goaf area; obtaining the first water storage coefficient by the following formula: η1=1-W / H; wherein η1 is the first water storage coefficient, W is the maximum surface subsidence value above the goaf area, and H is the height value of the goaf area. The technical solution of the present invention can effectively solve the problem of inaccurate calculation of the water storage coefficient of the underground water reservoir in the coal mine in the related art.

Description

Technical Field

[0001] The invention relates to the field of construction of underground water reservoirs in coal mines, and in particular to a method for obtaining a water storage coefficient of a collapse zone of an underground water reservoir in a coal mine. Background Art

[0002] After mining, coal mines will form large goafs. The underground water reservoir of a coal mine utilizes the large space area formed by the goaf of the coal mine to form a reservoir. This method can save space on the ground, make full use of the natural evolution of the rock mass in the goaf on mine water, solve the problem of mine water pollution, and realize the recycling of water resources.

[0003] At present, the method for determining the storage capacity of underground water reservoirs in coal mines is often to multiply the water storage volume of the reservoir by the water storage coefficient. The water storage coefficient is affected by many factors such as the fracture block size, accumulation form, and fracture structure of the rock layer in the goaf. The current method for deriving the water storage coefficient is to indirectly estimate it based on the empirical value of the water storage coefficient in the "Regulations on Coal Mine Water Prevention and Control" through influencing factors. Therefore, the water storage coefficient estimated by this method has a large deviation from the actual water storage coefficient, resulting in a large deviation between the calculation of the storage capacity of underground water reservoirs in coal mines and the actual storage capacity. Summary of the invention

[0004] The main purpose of the present invention is to provide a method for obtaining the water storage coefficient of the collapse zone of the coal mine underground water reservoir, so as to solve the problem of inaccurate calculation of the water storage coefficient of the coal mine underground water reservoir in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, there is provided a method for obtaining the water storage coefficient of the collapse zone of an underground water reservoir in a coal mine, comprising: obtaining the maximum subsidence value of the surface above the goaf area; obtaining the height value of the goaf area; obtaining a first water storage coefficient by the following formula: η1=1-W / H; wherein η1 is the first water storage coefficient, W is the maximum subsidence value of the surface above the goaf area, and H is the height value of the goaf area.

[0006] Furthermore, the step of obtaining the height value of the goaf area includes: obtaining the height value of the goaf area through the following formula: H=h×cosα; wherein H is the height value of the goaf area, h is the normal mining thickness of the goaf area, and α is the angle between the normal opening thickness of the goaf area and the vertical direction.

[0007] Furthermore, after the step of obtaining the first water storage coefficient by the following formula: η1=1-W / H, the acquisition method also includes: selecting a section of a tunnel of a predetermined length in the goaf as a test tunnel for conducting a water release experiment, and obtaining the second water storage coefficient η2; taking the minimum value of the first water storage coefficient η1 and the second water storage coefficient η2 as the water storage coefficient of the collapse zone of the coal mine underground reservoir.

[0008] Furthermore, a tunnel of a predetermined length in the goaf is selected as a test tunnel for a water release experiment, and the step of obtaining the second water storage coefficient η2 of the test tunnel through the water release experiment includes: obtaining the water storage capacity of the goaf; obtaining the water storage space of the goaf; obtaining the second water storage coefficient η2 through the following formula: η2=Vwater / V; wherein η2 is the second water storage coefficient, Vwater is the water storage capacity of the goaf, and V is the water storage space of the goaf.

[0009] Furthermore, the step of obtaining the water storage capacity Vwater of the goaf area includes: setting a retaining wall to seal the test tunnel so that the test tunnel forms a closed water storage space; setting a water injection pipe, the first end of which is located outside the test tunnel, and the second end of which is connected to the water storage space, and the water injection pipe is provided with a control valve, a pressure gauge and a flow meter; obtaining the water inflow V1 of the test tunnel and the water injection V2 of the water injection pipe within a predetermined time, the water storage capacity Vwater of the goaf area is equal to the sum of the water inflow V1 of the test tunnel and the water injection V2 of the water injection pipe.

[0010] Furthermore, the step of obtaining the water inflow volume V1 of the test tunnel within a predetermined time includes: digging a ditch outside the test tunnel, and obtaining the water inflow volume V1 of the test tunnel according to the water inflow volume of the ditch.

[0011] Furthermore, the retaining wall includes a first cast wall, a second cast wall and a sandbag wall. The second cast wall is located on a side of the first cast wall away from the water storage space and is spaced apart from the first cast wall. The sandbag wall is located on a side of the second cast wall away from the water storage space and is closely attached to the second cast wall.

[0012] Furthermore, the cross section of the sandbag wall is a trapezoidal structure.

[0013] Furthermore, the cross-sectional slope of the trapezoidal structure is between 2 and 3.

[0014] Furthermore, the average density of the sand in the sandbag wall is 1.8 g / cm 3 Up to 2.5g / cm 3 between.

[0015] By applying the technical solution of the present invention, since the sum of the water storage coefficient η and the sedimentation coefficient q is equal to 1, the first water storage coefficient η1 of the collapse zone of the underground reservoir of the coal mine can be calculated by calculating the sedimentation coefficient q. The sedimentation coefficient q is equal to the ratio of the maximum surface subsidence value W above the goaf area to the height value H of the goaf area. Therefore, the first water storage coefficient η1 of the collapse zone of the underground reservoir of the coal mine can be calculated by calculating the sedimentation coefficient q, and the first water storage coefficient η1 is used as the water storage coefficient of the collapse zone of the underground reservoir of the coal mine. The above-mentioned method for obtaining the water storage coefficient is obtained by using the parameters of the coal mine itself, and has higher accuracy than the empirical value described in the background technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0017] Figure 1 A flow chart showing an embodiment of a method for obtaining a water storage coefficient of a collapse zone of an underground water reservoir in a coal mine according to the present invention;

[0018] Figure 2 A schematic diagram of the structure of the collapse zone test tunnel is shown.

[0019] The above drawings include the following reference numerals:

[0020] 10. Test tunnel; 20. Retaining wall; 21. First pouring wall; 22. Second pouring wall; 23. Sandbag wall; 30. Water injection pipe. DETAILED DESCRIPTION

[0021] 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, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. 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.

[0022] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0023] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0024] After long-term research, the inventors found that after the underground coal seam is mined, the surrounding rock strata will lose support and gradually move, bend and break into the goaf area. As the coal mining face continues to advance, this process gradually expands outward and upward (roof) to the ground until it reaches the ground, causing the surface to sink. The degree of surface subsidence can be measured by the subsidence coefficient, which refers to the ratio of the maximum surface subsidence value W to the vertical projection length of the normal mining thickness H of the coal seam when fully mined, also known as the sinking activation coefficient. Its size is mainly related to geological mining factors such as the properties of the overlying rock strata, the stratigraphic structure, the coal mining method, the roof management method and the mining nature. For mines where the surface subsidence time of goaf is more than 5 years, the surface subsidence is only within 2% of the total surface subsidence. Therefore, the difference between the total surface subsidence and the overall space of coal seam mining thickness is approximately the water storage space, that is, the sum of the water storage coefficient η and the sedimentation coefficient q is equal to 1. The water storage coefficient η of the collapse zone of the coal mine underground reservoir can be calculated by calculating the sedimentation coefficient q.

[0025] Based on the above principle, Figure 1 As shown, the method for obtaining the water storage coefficient of the collapse zone of the coal mine underground reservoir of the present application includes:

[0026] Step S10: obtaining the maximum ground subsidence value above the goaf area;

[0027] Step S20: obtaining the height value of the goaf area;

[0028] Step S30: Obtain the first water storage coefficient by the following formula: η1=1-W / H; wherein η1 is the first water storage coefficient, W is the maximum subsidence value of the surface above the goaf area, and H is the height value of the goaf area.

[0029] By applying the technical solution of this embodiment, since the sum of the water storage coefficient η and the sedimentation coefficient q is equal to 1, the first water storage coefficient η1 of the collapse zone of the underground reservoir of the coal mine can be calculated by calculating the sedimentation coefficient q. The sedimentation coefficient q is equal to the ratio of the maximum surface subsidence value W above the goaf area to the height value H of the goaf area. Therefore, the first water storage coefficient η1 of the collapse zone of the underground reservoir of the coal mine can be calculated by calculating the sedimentation coefficient q, and the first water storage coefficient η1 is used as the water storage coefficient of the collapse zone of the underground reservoir of the coal mine. The above method for obtaining the water storage coefficient is obtained by using the parameters of the coal mine itself, and has higher accuracy than the empirical value described in the background technology.

[0030] In order to further improve the accuracy of the first water storage coefficient η1, in this embodiment, step S10 includes: obtaining the height value of the goaf area by the following formula: H=h×cosα; wherein H is the height value of the goaf area, h is the normal mining thickness of the goaf area, and α is the angle between the normal opening thickness of the goaf area and the vertical direction. In the above structure, the height value H of the goaf area is the projection value of the normal mining thickness of the goaf area in the vertical direction. When the tunnel does not extend in the horizontal direction, the height value H of the goaf area needs to be corrected to obtain a more accurate first water storage coefficient η1.

[0031] Although the method of measuring the sedimentation coefficient to convert the water storage coefficient utilizes the actual parameters of the coal mine, the above calculation method still has errors. In order to further obtain a more accurate water storage coefficient, the actual goaf area can be used to conduct a waterproof test to measure the water storage coefficient. However, the actual goaf area is too large, and the cost of conducting a water release experiment on the entire goaf area is too high. Therefore, a part of the goaf area tunnel can be selected for a water release experiment, and the parameters obtained from the experiment can be used to convert the water storage coefficient of the entire goaf area. The water release test is to strengthen and seal the relatively independent goaf area closed wall that has been formed, and to store water in the goaf area through water gushing in the goaf area and water injection from the external water supply pipeline, and to monitor the internal water level changes through an external pressure gauge.

[0032] Specifically, after step S30, the method for obtaining the water storage coefficient further includes:

[0033] Step S40: selecting a tunnel of a predetermined length in the goaf as a test tunnel 10 to perform a water discharge experiment, and obtaining a second water storage coefficient η2;

[0034] Step S50: taking the minimum value of the first water storage coefficient η1 and the second water storage coefficient η2 as the water storage coefficient of the collapse zone of the coal mine underground reservoir.

[0035] In the above structure, the water storage coefficient of the test tunnel 10 is measured by the water discharge experiment, and the water storage coefficient of the test tunnel 10 is used as the water storage coefficient of the goaf area, so that the water storage coefficient obtained is closer to the actual one. And it can effectively save the testing cost of the water storage coefficient of the goaf area. In addition, the selection of the water storage coefficient can be based on the principle of "low rather than high", that is, a lower value is used as the water storage coefficient of the final coal mine underground reservoir collapse zone.

[0036] Further, step S40 includes: step S41: obtaining the water storage capacity of the goaf area; step S42: obtaining the water storage space of the goaf area; step S43: obtaining the second water storage coefficient η2 by the following formula: η2=Vwater / V; wherein η2 is the second water storage coefficient, Vwater is the water storage capacity of the goaf area, and V is the water storage space of the goaf area. In the above structure, the water release test is to reinforce and seal the relatively independent goaf closed wall that has been formed, and to realize water storage in the goaf through water gushing in the goaf and water injection through external water supply pipelines, and to monitor the internal water level changes through an external pressure gauge. When the water level in the goaf rises to a certain height, the water storage capacity Vwater in the goaf is counted by releasing water, and the water storage space V in the goaf is calculated. The ratio between the two is the water storage coefficient of the goaf.

[0037] Further, step S41 includes: step S411: setting a water retaining wall 20 to seal the test tunnel 10, so that the test tunnel 10 forms a closed water storage space; step S412: setting a water injection pipe 30, the first end of the water injection pipe 30 is located outside the test tunnel, the second end of the water injection pipe is connected to the water storage space, and the water injection pipe 30 is provided with a control valve, a pressure gauge and a flow meter; step S413: obtaining the water inflow volume V1 of the test tunnel and the water injection volume V2 of the water injection pipe within a predetermined time, and the water storage volume Vwater of the goaf area is equal to the sum of the water inflow volume V1 of the test tunnel and the water injection volume V2 of the water injection pipe. Among them, step S413 includes: digging a ditch outside the test tunnel, and obtaining the water inflow volume V1 of the test tunnel according to the water inflow volume of the ditch.

[0038] Specifically, in step S412, the water injection pipe is a DN100mm seamless steel pipe with a length of 6000mm; the pressure gauge has a range of 0-0.2MPa (with flange); the flow meter is an electromagnetic flow meter that can display instantaneous flow and total flow and has a zeroing function (with flange); the control valve is a gate valve (with flange). In addition, a water thermometer and a stopwatch are required.

[0039] In the above structure, the water entering the test tunnel during the water release experiment includes two parts. One part is the water inflow of the test tunnel, which mainly records the water storage time and calculates the water storage volume in the goaf according to the original measured ditch flow rate. The other part is the water injection from the water injection pipe. By installing a gate valve and an electromagnetic flowmeter, the water injection time is controlled, and the total amount of water injected is recorded, and the water pressure value during the period is also recorded. The water injection time is recorded, and the water injection is stopped when the final water pressure reaches 0.051MPa and the water storage height is 3.2m. After the water injection is completed, the control valve is opened to release the water. During the water release period, the installed electromagnetic flowmeter is used to record the water release volume per hour, and the water pressure value is also recorded to facilitate the subsequent application of the above parameters.

[0040] Furthermore, if Figure 2 As shown, the retaining wall 20 includes a first pouring wall 21, a second pouring wall 22 and a sandbag wall 23. The second pouring wall 22 is located on a side of the first pouring wall 21 away from the water storage space and is spaced apart from the first pouring wall 21. The sandbag wall 23 is located on a side of the second pouring wall 22 away from the water storage space and is closely attached to the second pouring wall. The above structure can improve the strength of the retaining wall, thereby ensuring the airtightness of the test tunnel.

[0041] Specifically, the cross section of the sandbag wall 23 is a trapezoidal structure. This arrangement can improve the strength of the sandbag wall 23, thereby reinforcing the second pouring wall 22 and improving the ability of the entire retaining wall 20 to bear water pressure.

[0042] It should also be noted that the cross-sectional slope of the trapezoidal structure is between 2 and 3. Preferably, the cross-sectional slope of the trapezoidal structure is 2.5. The average density of the sand in the sandbag wall is 1.8 g / cm 3 Up to 2.5g / cm 3 between.

[0043] The following takes the coal mine goaf area of ​​a coal mining area in Ningxia as an example to explain the method of obtaining the water storage coefficient of the underground reservoir collapse zone. Specifically:

[0044] 1. Calculate the first water storage coefficient η1 using the surface settlement method

[0045] This mining area has been mined out for more than ten years, and the surface subsidence has basically stabilized. Therefore, the difference between the total surface subsidence and the overall space of coal seam mining thickness is approximately the water storage space. The water storage coefficient η of the collapse zone of the coal mine underground reservoir can be calculated by calculating the settlement coefficient q. The calculation method of the surface settlement coefficient q is shown in the following table:

[0046]

[0047] According to the surface subsidence monitoring data of the first mining area of ​​Lingxin Coal Mine, the subsidence coefficient in the mining area is about 0.76, that is, the first water storage coefficient η1 is about 0.24.

[0048] 2. Using the water release test method to obtain the second water storage coefficient η2

[0049] The goaf area has a strike length of 893m and a face length of 185m. The mining was completed in 2003-2004. A water release experiment was carried out at the place where the roof rock stratum in the goaf area had settled and stabilized.

[0050] The test tunnel floor elevation is +1101.31m. The test tunnel section is a semicircular arch section with a net width of 2800mm and a net height of 2500mm. The support method is anchor mesh spraying support. The ditch is located at the lower left corner of the tunnel section with a size of 400×400mm. The water level in the ditch is about 200mm. At the same time, the seal at the intersection with the main inclined shaft is intact. The tunnel is a semicircular arch section with a net width of 5200mm and a net height of 4000mm.

[0051] In step S411, the first casting wall and the second casting wall are cast with hollow bricks and concrete, and the lateral compressive strength is relatively small. In order to ensure the safety of underground workers, the second casting wall needs to be reinforced by stacking sandbag walls before the test.

[0052] Calculation of the dimensions of the sandbag wall 23:

[0053] According to the Mohr-Coulomb criterion:

[0054]

[0055]

[0056] αPA1=F

[0057] Among them, A1 is the area of ​​the enclosed wall, which is 6.16m2, and A is the bottom area of ​​the sandbag wall.

[0058] For safety reasons, the safety factor α was set to 1.5 when designing the size of the sandbag wall. At the same time, the slope of the trapezoidal section was preliminarily determined to be 2.5:1.

[0059] but

[0060] again b=L-0.44,A=Ld

[0061] So W = (L-0.22)hγd

[0062] so

[0063] After finishing, we can get:

[0064]

[0065] The sandbag is filled with coarse sand, the average density of the coarse sand is 2.0g / cm3, the cohesion c is 0.05MPa, and the internal friction angle Take 30°. The tunnel section consists of a rectangular section and a semicircular section. The radius of the semicircular section is 1.4m, the maximum height is 2.5m, the maximum width is 2.8m, and the expected maximum head height is 5m, so h is taken as 2.5m, d is taken as 2.8m, and P is taken as 0.05MPa.

[0066] After calculation, we can get: L = 2.68m. In fact, the cross section of the sandbag wall is designed as a trapezoid, and the L value should be greater than 1m.

[0067] Therefore, L can be taken as 3m for the design of the sandbag wall. At this time, b is 2m, the height is 2.5m, and the width is 2.8m. By reverse calculation, when the water pressure that the sandbag wall can withstand is 0.05MPa, the safety factor is 1.68.

[0068] The sandbags are flood-control expansion sandbags, and are available in two specifications: 400×400mm and 400×200mm. The sandbags are filled with coarse sand and sealed tightly.

[0069] According to the method in step S41, the water storage capacity of the goaf is measured to be 3976.8m 3 , water injection volume is 1860m 3 , the total water volume is 5836.8m 3 The total water storage space is 25777m 3 After deducting the bypass water volume and space, it can be inferred that the water storage coefficient of the collapse zone of the coal mine underground reservoir is 0.173.

[0070] In the description of the present invention, it is necessary to understand that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, perpendicular, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the devices or elements referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention. The directional words "inside and outside" refer to the inside and outside relative to the contours of each component itself.

[0071] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0072] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

Claims

1. A method for obtaining the water storage coefficient of a collapse zone of an underground reservoir in a coal mine, characterized in that: include: Get the maximum ground subsidence value above the mined-out area; Get the height value of the goaf area; The first water storage coefficient is obtained by the following formula: η1=1-W / H; Where η1 is the first water storage coefficient, W is the maximum ground subsidence above the goaf area, and H is the height of the goaf area; After the step of obtaining the first water storage coefficient by the following formula: η1=1-W / H, the obtaining method further includes: A tunnel of a predetermined length in the goaf is selected as a test tunnel (10) to perform a water discharge experiment, and a second water storage coefficient η2 is obtained; Taking the minimum value of the first water storage coefficient η1 and the second water storage coefficient η2 as the water storage coefficient of the collapse zone of the coal mine underground reservoir; The steps of selecting a tunnel of a predetermined length in a goaf area as a test tunnel (10) for a water discharge experiment, and obtaining a second water storage coefficient η2 of the test tunnel (10) through the water discharge experiment include: Obtaining the water storage capacity of the goaf area; Acquire the water storage space of the goaf area; The second water storage coefficient η2 is obtained by the following formula: η2=Vwater / V; Among them, η2 is the second water storage coefficient, Vwater is the water storage capacity of the goaf area, and V is the water storage space of the goaf area.

2. The acquisition method according to claim 1, characterized in that: The steps for obtaining the height value of the goaf area include: The height value of the goaf area is obtained by the following formula: H = h × cos α; Among them, H is the height of the goaf area, h is the normal mining thickness of the goaf area, and α is the angle between the normal opening thickness of the goaf area and the vertical direction.

3. The acquisition method according to claim 1, characterized in that: The step of obtaining the water storage volume Vwater of the goaf area comprises: A water retaining wall (20) is provided to seal the test tunnel (10), so that the test tunnel (10) forms a closed water storage space; A water injection pipe (30) is provided, wherein a first end of the water injection pipe (30) is located outside the test tunnel (10), a second end of the water injection pipe (30) is communicated with the water storage space, and a control valve, a pressure gauge and a flow meter are provided on the water injection pipe (30); The water inflow volume V1 of the test tunnel (10) and the water injection volume V2 of the water injection pipe (30) within a predetermined time are obtained, and the water storage volume Vwater of the goaf area is equal to the sum of the water inflow volume V1 of the test tunnel (10) and the water injection volume V2 of the water injection pipe (30).

4. The acquisition method according to claim 3, characterized in that: The step of obtaining the water inflow volume V1 of the test tunnel (10) within a predetermined time comprises: A ditch is dug outside the test tunnel (10), and the water inflow volume V1 of the test tunnel (10) is obtained based on the water inflow volume of the ditch.

5. The acquisition method according to claim 3, characterized in that: The water retaining wall (20) comprises a first cast wall (21), a second cast wall (22) and a sandbag wall (23); the second cast wall (22) is located on a side of the first cast wall (21) away from the water storage space and is spaced apart from the first cast wall (21); the sandbag wall (23) is located on a side of the second cast wall (22) away from the water storage space and is closely attached to the second cast wall (22).

6. The acquisition method according to claim 5, characterized in that: The cross section of the sandbag wall (23) is a trapezoidal structure.

7. The acquisition method according to claim 6, characterized in that: The cross-sectional slope of the trapezoidal structure is between 2 and 3.

8. The acquisition method according to claim 5, characterized in that: The average density of the gravel in the sandbag wall (23) is 1.8 g / cm 3 Up to 2.5g / cm 3 between.

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