A method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas

Abstract

The present invention discloses a method for preventing frost heave, salt expansion, and salt corrosion in shallowly buried drainage pipes in cold and arid regions. The method comprises: determining the freezing depth in the cold and arid region, and thereby determining the depth of a trench excavation in the cold and arid region; excavating the trench, laying a gravel cushion layer at the bottom of the trench, burying a pressure sensor and a first cathode rod at the bottom of the gravel cushion layer, wherein the first cathode rod and the first anode rod are both electrically connected to a first power storage box; lowering the drainage pipe into the trench, filling the trench with fine sand near the drainage pipe and the rest of the trench with the gravel cushion layer; burying a second anode rod and a flow sensor at the top of the gravel cushion layer, wherein the second anode rod and the second cathode rod are both electrically connected to a second power storage box, and controlling the circuit on and off of the second anode rod and the second cathode rod according to the detected seepage flow rate; and installing a power supply device for the power storage box. The present invention also reduces frost heave, salt expansion, and salt corrosion in the drainage pipe, thereby extending the service life of the shallowly buried drainage pipes in cold and arid regions.

Description

Technical Field

[0001] The invention belongs to the technical field of drainage pipe protection, and relates to a method for preventing frost expansion, salt expansion, and salt corrosion of shallowly buried drainage pipes in cold and arid areas. Background Art

[0002] At present, the cold and arid environment has caused serious damage to the drainage pipelines. The drainage pipelines are often damaged by frost heave, and the drainage pipelines in saline areas are also damaged by salt expansion and salt corrosion. In particular, most cold and arid areas have saline areas. The shallow buried drainage pipeline projects in cold and arid areas have a great impact on the construction due to special environmental factors, resulting in huge costs in manpower, machinery, materials, etc.

[0003] During the implementation of the present invention, the inventors discovered the following problems with the prior art. When the upper soil freezes, water at the bottom of the gravel cushion migrates upward. The migrating water flows in the following direction: The soil at the bottom of the gravel cushion migrates a certain distance into the gravel. Then, due to capillary action, the air pressure inside the gravel balances with the surface tension and water pressure. The migrating water cannot migrate vertically upward, but instead migrates to the sides, entering the soil on either side of the gravel. The water then migrates upward and freezes. However, when the upper part freezes, the migration of water from the soil at the bottom of the gravel is accompanied by the formation of a salt crust. Over time, the salt crust at the bottom of the gravel increases, and the resulting salt expansion becomes increasingly severe. Damage to pipelines due to long-term accumulation is only a matter of time. When the frozen area in the upper soil melts, water seeps downward. If the seepage rate is excessive, the seepage water may directly pass through the pipeline, causing salt water to come into contact with the pipeline and cause salt corrosion damage. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a method for preventing frost heave, salt expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas, which reduces the frost heave, salt expansion and salt corrosion of the drainage pipes, improves the service life of shallow buried drainage pipes in cold and arid areas, and solves the frost heave, salt expansion and salt corrosion problems existing in the prior art.

[0005] The technical solution adopted by the present invention is a method for preventing frost heave, salt expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas, comprising the following steps:

[0006] Step 1: Determine the freezing depth in cold and arid areas, and then determine the depth of trench excavation in cold and arid areas;

[0007] Step 2: excavate a pipe trench, lay a gravel cushion layer at the bottom of the trench, and bury a pressure sensor and a first cathode rod at the bottom of the gravel cushion layer. The pressure sensor is used to detect the salt expansion pressure at the bottom of the gravel cushion layer. The first cathode rod and the first anode rod are both electrically connected to the first power storage box. The circuit of the first cathode rod and the first anode rod is controlled to be on and off according to the detected salt expansion pressure. When power is turned on, water flows from the first anode rod to the first cathode rod;

[0008] Step 3: Hang the drain pipe into the trench, fill the area around the drain pipe with a fine sand layer, and fill the rest of the area with a gravel cushion layer;

[0009] Step 4: A second anode rod and a flow sensor are buried on top of the gravel cushion layer. The flow sensor is used to detect the seepage flow rate. The second anode rod and the second cathode rod are electrically connected to the second power storage box. The circuit of the second anode rod and the second cathode rod is controlled to be on and off according to the detected seepage flow rate.

[0010] Step 5: Install the power supply devices of the first and second power storage boxes.

[0011] Furthermore, the freezing depth in the cold and arid areas is the depth H from the ground surface to 0°C underground; the depth of the trench excavated in the cold and arid areas is H+2R+2h, where R is the radius of the transmission pipeline and h is the thickness of the laid gravel cushion layer.

[0012] Furthermore, the thickness h of the gravel cushion layer is determined according to the capillary principle, h=h1+h2+h3, where h1 is the capillary water wetted area, i.e., the first area; h2 is the capillary water continuous area, i.e., the second area; and h3 is the residual water area in the pores of the gravel cushion layer, i.e., the third area. The equation for the equilibrium state between the second area and the third boundary area is: U a -U w =2T a / r, where U a is the pore pressure, U w is the water pressure, T a is the surface tension of the concave liquid surface, and r is the radius of curvature of the concave liquid surface.

[0013] Furthermore, in step 2, the pressure sensor is arranged at the junction of the bottom of the gravel cushion layer and the soil layer, the first cathode rod is 8 cm-15 cm away from the bottom of the gravel cushion layer; and the first anode rod is arranged outside the gravel cushion layer.

[0014] Furthermore, in step three, the particle size of the fine sand ranges from 0.125 mm to 0.5 mm, and the particle size of the gravel ranges from 5 mm to 25 mm.

[0015] Furthermore, in the step 3, a fine sand layer is filled within a distance L1 around the drain pipe, and the value of L1 is 20 cm to 30 cm greater than the outer diameter of the drain pipe.

[0016] Furthermore, in step 4, the flow sensor is 15 cm to 25 cm away from the top of the gravel cushion layer, and the second anode rod is arranged between the flow sensor and the top of the gravel cushion layer.

[0017] Furthermore, the power supply devices of the first power storage box and the second power storage box are solar charging panels or wind generators.

[0018] Furthermore, protection devices are provided outside the first power storage box and the second power storage box, and inert gas is filled between the first power storage box and the second power storage box and the corresponding protection devices respectively.

[0019] Furthermore, the thickness of the gravel cushion layer ranges from 30 cm to 40 cm, and the first anode rod and the second cathode rod are arranged at a position 40 cm to 50 cm away from the outermost side of the gravel cushion layer.

[0020] The beneficial effects of the present invention are:

[0021] The present invention uses capillary action and electrophoresis to fill the drain pipe with gravel and sand around it, and topsoil on top of the gravel. When external water seeps into the filled gravel area, the capillary action prevents the water from flowing into the gravel area, thereby reducing frost heave, salt expansion, and salt corrosion caused by external water inflow, thereby protecting the drain pipe. Water cannot flow into the interior and will not freeze. The frost heave pressure on the drain pipe caused by the surrounding frost heave is reduced by the gravel. When water evaporates and condenses in the drain pipe, the gravel cushion layer at the bottom can drain the condensed water, thereby avoiding salt corrosion and salt expansion damage caused by water and steel. This extends the service life of shallow-buried drain pipes in cold and arid regions and improves the economic benefits of the project. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 any creative work.

[0023] Figure 1 It is a structural diagram of an embodiment of the present invention.

[0024] Figure 2 Schematic diagram of the water-solid-air force structure in an embodiment of the present invention.

[0025] Figure 3 This is a control flow diagram of an embodiment of the present invention.

[0026] Figure 4 is a flow chart of an embodiment of the present invention.

[0027] In the figure, 1. wind turbine, 2. solar charging panel, 3. flow sensor, 4. second anode rod, 5. first cathode rod, 6. first anode rod, 7. drain pipe, 8. gravel cushion, 9. second cathode rod, 10-1. first power storage box, 10-2. second power storage box, 11. inert gas, 12. protective equipment, 13. power supply line, 14. pressure sensor, 15. fine sand layer. DETAILED DESCRIPTION

[0028] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described 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 making creative efforts are within the scope of protection of the present invention.

[0029] Example 1,

[0030] A method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas, such as Figure 1 、 3 , 4, including the following steps:

[0031] Step 1: Determine the freezing depth in cold and arid regions, i.e., the depth H from the ground surface to 0°C underground. The location of 0°C underground is determined by burying temperature sensors at different depths. After determining the freezing depth, determine the depth of the trench for excavation in the cold and arid regions (i.e., H + 2R + 2h), where H is the freezing depth, R is the radius of the pipeline, and h is the thickness of the laid gravel cushion layer 8.

[0032] The thickness h is determined according to the capillary principle, specifically h=h1+h2+h3, where h1 is the capillary water wetted area, that is, when external water flows into the first area contacting the gravel, this part is the saturated area; h2 is the capillary water continuous area, that is, when external water contacts the first area of ​​the gravel and then enters the second area of ​​the gravel, and when the external water reaches the boundary between the second area and the third area, there is a state of equilibrium; h3 is the third area, which is the residual water area in the pores of the gravel itself, and its own pore pressure U a Balanced with the upper part. The formula for the equilibrium state of the second area and the third boundary area is: U a -U w =2T a / r, where U a is the pore pressure, U w is the water pressure, T a is the surface tension of the concave liquid surface, r is the radius of curvature of the concave liquid surface, see Figure 2 .

[0033] In step two, a pressure sensor 14 and a first cathode rod 5 are buried at the bottom of the gravel cushion layer 8, respectively. The pressure sensor 14 is connected to the first power storage box 10-1. When the upper part freezes, moisture migrates upward, salt precipitates at the bottom of the gravel cushion layer 8, and salt expansion occurs. The pressure sensor 14 is subjected to pressure, reaches a predetermined value, and is transmitted to the first power storage box 10-1. The switch controller inside the first power storage box 10-1 controls the first power storage box 10-1 to immediately turn on the power supply, and the first anode rod 6 is electrically connected to the first cathode rod 5. Water flows from the first anode rod 6 to the first cathode rod 5 to reduce salt precipitation. When the pressure on the pressure sensor 14 is reduced to a predetermined value, the switch controller inside the first power storage box 10-1 controls the first power storage box 10-1 to immediately disconnect the power supply.

[0034] In this embodiment of the present invention, pressure sensor 14 is model M5156-000002. When the pressure generated by salt expansion causes the pressure detected by pressure sensor 14 to reach a predetermined value, the pressure is transmitted to the first power storage box 10-1 on the right side. The signal output by pressure sensor 14 is a current. The signal output by pressure sensor 14 is transmitted to the first power storage box 10-1 via wires. The first power storage box 10-1 has a switch controller inside. When the current reaches the predetermined value, the switch controller controls the first power storage box 10-1 to open and turn on the power supply. Moisture migrates from the first anode rod 6 on the right side of the gravel area to the area near the first cathode rod 5 at the bottom of the gravel cushion 8. The migrated water dissolves the salt crust, reducing salt expansion. When the pressure decreases, causing the current signal output by pressure sensor 14 to reach or below the predetermined value, the switch controller controls the first power storage box 10-1 to open and disconnect the power supply.

[0035] Because salt precipitation mainly occurs in the soil at the bottom of the gravel cushion layer 8, the pressure sensor 14 is arranged at the junction of the gravel and the soil layer at the bottom of the gravel cushion layer 8. The first cathode rod 5 does not contact the bottom of the gravel cushion layer 8 and should be 8 cm-15 cm away from the bottom of the gravel cushion layer. If the distance is less than 8 cm, on the one hand, the water migrating from the anode to the cathode will first dissolve the salt crust generated near the pressure sensor 14, and the generated pressure value will quickly drop to a predetermined value, and then the power will be cut off immediately, and the generated salt crust will not be dissolved well, and the effect of reducing salt expansion will be poor. On the other hand, if the distance is less than 8 cm, too much water will migrate into the cathode, which will affect the balance between water vapor and matrix suction, and the water will rise to contact the pipeline, corroding the pipeline.

[0036] If the distance is greater than 15 cm, the water that migrates in due to electrophoresis is limited and cannot dissolve the salt crust formed between the pressure sensor 14 and the first cathode rod 5 . This results in poor dissolution of the salt crust and poor salt expansion reduction effect.

[0037] The thickness of the crushed stone cushion layer 8 is in the range of 30cm-40cm. The radial distance along the drainage pipe 7 is constructed according to the circumscribed rectangle for the convenience of construction. Figure 1The first anode rod 6 is arranged on the right side of the gravel area and 40cm-50cm away from the outermost side of the gravel cushion layer 8. The amount of water migration under electrophoresis is affected by the distance. If the distance is too far, the migration of water will be greatly hindered.

[0038] Considering evaporation, when the evaporated water encounters the pipeline and condenses, it can be discharged through the gravel cushion layer 8, so that salt corrosion and salt swelling will not occur at the bottom of the pipeline.

[0039] Step three, after determining the depth to be excavated, excavate the trench, specifically by large-scale machines. No slope excavation is performed here, as slope excavation in cold regions is extremely expensive. To save costs, straight trench excavation is adopted, and layered excavation is performed until the excavation reaches the designed elevation. The drain pipe 7 is lowered into the trench, and gravel and fine sand are filled around the drain pipe 7. A fine sand layer 15 is filled at a distance of L1 around the drain pipe 7, and a gravel cushion layer 8 is filled at other distances. The role of the gravel is as follows: first, when the upper part is frozen, moisture migrates upward, and the gravel cushion layer has large pores. Due to capillary action, moisture migrates to a certain distance and turns to the sides, changing its path to migrate upward, thereby protecting the drain pipe 7 from salt corrosion, and the salt swelling generated at the bottom can be reduced by the gravel; second, when the upper frozen area melts, it cannot seep into the drain pipe 7 due to capillary action.

[0040] The particle size of fine sand ranges from 0.125mm to 0.5mm, and the particle size of gravel ranges from 5mm to 25mm. The fine sand filled here is mainly used to protect the pipe from being scratched or punctured by gravel. The distance L1 is set to be 20cm to 25cm greater than the outer diameter of the drainage pipe 7 (the distance along the radial direction of the drainage pipe 7). If the distance is too small, the pipe may be damaged during the crushed stone filling and rolling. If the distance is too large, the matrix suction is related to the pore size of the gravel area. If the pore size is too small, the matrix suction is greater, the capillary water pressure head is higher, and the excavation and gravel filling area will be expanded, increasing the cost. It is also very likely that the pipe will come into contact with capillary water.

[0041] Step four, bury the second anode rod 4 on the top of the gravel, the second anode rod 4 is connected to the second power storage box 10-2, the second cathode rod 9 is connected to the second power storage box 10-2, the flow sensor 3 is connected to the switch of the second power storage box 10-2 through the PLC single chip or circuit design, and the electrical switch function of the second power storage box 10-2 is controlled. When the flow sensor 3 (melting stage) reaches the predetermined value, the second power storage box 10-2 is controlled to open the switch through the PLC single chip or circuit design, so that the second anode rod 4 is electrically connected to the second cathode rod 9, and water flows from the second anode rod 4 to the second cathode rod 9. The drain pipe 7 is protected by the dual action of the capillary of the gravel and the current drainage, and then the layered filling and covering compaction are controlled.

[0042] The second cathode rod 9 is set on the left side of the gravel area and is 40cm-50cm away from the outermost side of the gravel cushion layer 8. The flow sensor 3 model is YF-2102-A. The setting position of the flow sensor 3 should be 15cm-25cm away from the top of the gravel cushion layer 8, and the second anode rod 4 should be set in the middle position between the flow sensor 3 and the top of the gravel cushion layer 8. When the seepage rate reaches a predetermined value, the flow sensor 3 transmits a pulse signal to the PLC microcontroller or circuit design. The PLC microcontroller or circuit design controls the power supply of the second power storage box 10-2 according to the transmitted pulse signal. Water migrates from the anode rod at the top of the gravel area to the second cathode rod 9 area on the left side of the gravel area, reducing the possibility of water directly passing through the pipeline due to repeated freezing and thawing. Before the water seeps into the gravel area, part of the water is migrated away by electrophoresis. When the generated seepage value decreases so that the seepage detected by the flow sensor 3 reaches or is lower than the predetermined value, the second power storage box 10-2 is controlled to disconnect the switch and cut off the power supply. External water cannot flow in and will be balanced in the third and second junction areas, and then dispersed and flow out to both sides. If water cannot flow into the drain pipe 7, no frost heave damage will occur. Even if ice pressure is generated by external freezing, it will be reduced by gravel with large pores. When external water cannot flow into the area near the drain pipe 7, salt expansion and salt corrosion will not occur. The above considerations are about seepage.

[0043] Under the action of capillary and electrophoretic forces, when the frozen area above the soil melts, the water at the top of the gravel area will seep downward. The direction of the seepage water is that the soil layer at the top of the gravel area seeps to a certain distance inside the gravel. Then, due to the capillary action, the air pressure inside the gravel is balanced with the surface tension and water pressure of the liquid surface. The seepage water cannot seep vertically downward but seeps to both sides, then enters the soil on both sides of the gravel, and then seeps downward. However, with repeated freeze-thaw effects over time and when the seepage rate reaches a relatively large value, it is possible that water will directly pass through the pipeline, causing salt corrosion damage to the pipeline.

[0044] In this embodiment, the control of the two sets of electrode rods does not interfere with each other. If only the upper set of electrode rods is installed, it can only address the situation where the frozen area above the soil is thawing. When the upper soil layer freezes, moisture migrates upward, and a salt crust forms at the bottom of the gravel cushion along the longitudinal direction of the pipeline, which in turn causes increasing salt expansion. Over time, damage to the pipeline is only a matter of time. If only the lower set of electrode rods is installed, it can only address the situation where the upper soil is frozen. When the frozen area above the soil layer melts, moisture seeps downward. If the seepage rate is too large, the seepage water may directly pass through the pipeline, allowing the salt water to come into contact with the pipeline and cause salt corrosion damage to the pipeline.

[0045] Step 5: Solar charging panels 2 and wind turbine 1 are installed on top, charging first and second energy storage boxes 10-1, 10-2 via power lines 13. Protective devices 12 are installed on the exterior of first and second energy storage boxes 10-1, 10-2, and an inert gas 11 (such as carbon dioxide) is filled between them to protect the boxes. This fully utilizes wind and solar energy for electricity storage, responding to the national call for the utilization of new energy.

[0046] This embodiment of the present invention considers the effects of water, heat, steam, salt, and force on shallow-buried drain pipes 7. Firstly, the hydrothermal effects prevent water from flowing into the drain pipes 7, potentially corroding them. Secondly, steam condenses, preventing water from accumulating at the bottom of the pipes 7 and instead draining through the gravel cushion. Thirdly, the larger-pore gravel mitigates the pressure from salt expansion and frost heave caused by salt precipitation in cold and arid regions (mostly saline soils). Through capillary action and electrophoresis, the pipes encased in gravel are protected from salt and frost heave damage. This approach addresses the issues of frost heave, salt expansion, and salt corrosion in shallow-buried drain pipes 7 in cold and arid regions, extending the pipeline's useful life in these regions and improving the economic benefits of the pipeline project.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas, characterized in that: The following steps are involved: Step 1: Determine the freezing depth in cold and arid areas, and then determine the depth of trench excavation in cold and arid areas; Step 2: excavating a pipe trench, laying a gravel cushion layer (8) at the bottom of the pipe trench, burying a pressure sensor (14) and a first cathode rod (5) at the bottom of the gravel cushion layer (8), respectively, the pressure sensor (14) is used to detect the salt expansion pressure at the bottom of the gravel cushion layer (8), the first cathode rod (5) and the first anode rod (6) are both electrically connected to the first power storage box (10-1), and the circuit of the first cathode rod (5) and the first anode rod (6) is controlled to be on and off according to the detected salt expansion pressure, and after power is turned on, water flows from the first anode rod (6) to the first cathode rod (5); Step 3: Hang the drain pipe (7) into the trench, fill the area around the drain pipe (7) with a fine sand layer (15), and fill the rest of the area with a gravel cushion layer (8); Step 4: A second anode rod (4) and a flow sensor (3) are buried on the top of the gravel cushion layer (8). The flow sensor (3) is used to detect the seepage flow rate. The second anode rod (4) and the second cathode rod (9) are electrically connected to the second power storage box (10-2). The circuit of the second anode rod (4) and the second cathode rod (9) is controlled to be on or off according to the detected seepage flow rate. Step 5: Install the power supply devices of the first power storage box (10-1) and the second power storage box (10-2).

2. The method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas according to claim 1, characterized in that: The freezing depth in the cold and arid regions is the depth H from the ground surface to 0°C underground; the depth of the trench excavated in the cold and arid regions is H+2R+2h, where R is the radius of the transmission pipeline and h is the thickness of the gravel cushion layer (8) to be laid.

3. The method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas according to claim 2, characterized in that: The thickness h of the crushed stone cushion layer (8) is determined according to the capillary principle, h=h1+h2+h3, wherein h1 is the capillary water wetted area, i.e., the first area; h2 is the capillary water continuous area, i.e., the second area; h3 is the residual water area in the pores of the crushed stone cushion layer (8), i.e., the third area; the equation for the equilibrium state between the second area and the third boundary area is: U a -U w =2T a / r, where U a is the pore pressure, U w is the water pressure, T a is the surface tension of the concave liquid surface, and r is the radius of curvature of the concave liquid surface.

4. The method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas according to claim 1, characterized in that: In step 2, the pressure sensor (14) is arranged at the junction of the bottom of the gravel cushion layer (8) and the soil layer, the first cathode rod (5) is 8 cm to 15 cm away from the bottom of the gravel cushion layer (8), and the first anode rod (6) is arranged outside the gravel cushion layer (8).

5. The method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid areas according to claim 1, characterized in that: In the step 3, the particle size of the fine sand ranges from 0.125 mm to 0.5 mm, and the particle size of the gravel ranges from 5 mm to 25 mm.

6. The method for preventing frost heave and salt corrosion of shallow buried drainage pipes in cold and arid regions according to claim 1, characterized in that: In the step 3, a fine sand layer (15) is filled within a distance L1 around the drain pipe (7), and the value of L1 is 20 cm to 30 cm greater than the outer diameter of the drain pipe (7).

7. The method for preventing frost heave and salt corrosion of shallow buried drainage pipes in cold and arid regions according to claim 1, characterized in that: In step 4, the flow sensor (3) is 15 cm to 25 cm away from the top of the gravel cushion layer (8), and the second anode rod (4) is arranged between the flow sensor (3) and the top of the gravel cushion layer (8).

8. The method for preventing frost expansion and salt corrosion of shallow buried drainage pipes in cold and arid regions according to claim 1, characterized in that: The power supply devices of the first power storage box (10-1) and the second power storage box (10-2) are solar charging panels (2) or wind generators (1).

9. The method for preventing frost heave and salt corrosion of shallow buried drainage pipes in cold and arid regions according to claim 1, characterized in that: Protection equipment (12) is provided outside the first electricity storage box (10-1) and the second electricity storage box (10-2), and inert gas (11) is filled between the first electricity storage box (10-1) and the second electricity storage box (10-2) and the corresponding protection equipment (12).

10. A method for preventing frost heave and salt corrosion of shallow buried drainage pipes in cold and arid regions according to claim 1 or 3, characterized in that: The thickness of the crushed stone cushion layer (8) ranges from 30 cm to 40 cm, and the first anode rod (6) and the second cathode rod (9) are arranged at a position 40 cm to 50 cm away from the outermost side of the crushed stone cushion layer (8).

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