Seepage simulation device and method for preventing and treating seawater invasion through height-increased MICP type seepage interception wall

Abstract

The invention provides a seepage simulation device for preventing and controlling seawater invasion of a lift-height MICP type seepage interception wall and a working method of the seepage simulation device. According to the seepage interception wall forming method, a seepage interception wall forming material can be innovated; the engineering cost is greatly saved, various excavation problems encountered by a traditional seepage interception wall construction technology and disturbance to a coastal aquifer rock-soil body are avoided, the problem of residual salty water pollution caused by traditional seepage interception wall construction is solved, and the height of the seepage interception wall is adjusted according to seawater invasion degree changes caused by future climate and underground water level changes. The device and the method provide reference for construction by using the technology for resisting seawater invasion in a site.

Description

Technical Field

[0001] The invention relates to the technical field of seepage simulation, and in particular to a seepage simulation device for preventing and controlling seawater intrusion with a raised-height MICP type cutoff wall and a working method thereof. Background Art

[0002] Seawater intrusion refers to the excessive exploitation of groundwater in coastal areas, which leads to a drop in groundwater levels. After the balance between seawater and freshwater is destroyed, the salt-fresh water interface moves toward the land. Seawater intrusion can cause groundwater salinization, soil salinization and other consequences, which in turn have an impact on agricultural production, ecological environment, urban water supply, urban buildings, human health and other aspects. In order to resist seawater intrusion, coastal areas generally excavate and build a concrete anti-seepage wall in the downstream coastal area of ​​the aquifer, which is called an underground cut-off wall. The construction of an underground cut-off wall that can completely resist seawater intrusion can only be built through the phreatic aquifer, but this makes it impossible for coastal groundwater to be discharged into the ocean, causing a series of environmental problems. The coastal area has a long coastline and a thick phreatic aquifer. The underground cut-off wall generally needs to be excavated to the aquicluder rock layer, so the amount of concrete consumed is large. This makes the investment cost of this type of project high and the construction period long. Because a foundation pit that meets the height needs to be excavated before the construction of the cut-off wall, the instability of the pit wall during the excavation of the foundation pit is likely to cause the pit wall to collapse, affecting the later concrete pouring.

[0003] Microbial induced carbonate deposition (MICP) is a process in which specific microorganisms (such as urease bacteria) provide the corresponding calcium ions and nutrients required for crystallization to quickly precipitate calcium carbonate crystals with good cementation to fill the pores of rock and soil. This technology has the properties of a natural process, with less disturbance to the treated rock and soil, and has the advantages of low energy consumption and environmental protection. It is widely used to improve foundation soil or other engineering rock and soil. In the process of preventing and controlling seawater intrusion, if MICP technology is used to replace traditional concrete cut-off walls, it will not only greatly reduce the investment in time, money, and energy consumption costs, but also reduce the excavation disturbance of aquifers in coastal areas and reduce pollution to coastal aquifers. Therefore, MICP-type cut-off wall technology for preventing and controlling seawater intrusion has broad prospects.

[0004] Based on similar principles, indoor construction of seepage sand troughs to simulate groundwater flow and solute migration in aquifers is an important means to develop cut-off walls to prevent seawater intrusion. However, there are few studies on MICP-type cut-off walls to prevent seawater intrusion. There is still a lack of experimental devices that can achieve microbial-induced carbonate deposition to form cut-off walls and methods for measuring their seawater intrusion prevention and control effects. This has also become a key bottleneck that hinders the iterative upgrading of MICP-type cut-off walls to prevent seawater intrusion. Therefore, it is necessary to specially design a set of experimental devices and measurement methods that can simulate MICP-type cut-off walls to prevent seawater intrusion to solve the above problems.

[0005] In order to protect underground freshwater resources, coastal areas generally build impermeable cutoff walls that penetrate from the surface to the aquiclude downstream of the aquifer to prevent seawater intrusion. This method of building a cutoff wall can effectively block the intrusion of seawater for a long time, but the existence of the cutoff wall will prevent groundwater from being discharged into the ocean, causing residual salt water retention and enrichment of land-based pollutants, polluting underground freshwater. The original intention of the cutoff wall to protect underground freshwater resources has not been realized.

[0006] The material used for underground water seepage cutoff walls is generally concrete, which is a high-cost material in this construction field, and the construction time is relatively long. At the same time, the construction of underground water seepage cutoff walls requires deeper excavation to reach the target depth. During the excavation process, pit wall collapse is prone to occur, which brings many obstacles to the construction. Summary of the invention

[0007] In order to make up for the shortcomings of the prior art, the present invention provides a seepage simulation device and a working method for preventing seawater intrusion with a raised height MICP type cutoff wall. The present invention innovates an experimental device and method for forming a cutoff wall that is different from the traditional one for resisting seawater intrusion. This method for forming a cutoff wall can innovate the materials for forming the cutoff wall, greatly save engineering costs, avoid various excavation problems encountered in the traditional cutoff wall construction process and disturbance of the rock and soil of the coastal aquifer, remove the residual salt water pollution problems caused by the construction of traditional cutoff walls, and adjust the height of the cutoff wall in response to changes in the degree of seawater intrusion caused by future climate and groundwater level changes. The device and method of the invention provide a reference for the construction of the site using this technology to resist seawater intrusion.

[0008] The present invention is realized by the following technical scheme: a seepage simulation device for preventing seawater intrusion with a height-raising MICP type seepage cutoff wall, comprising a seepage trough, wherein two detachable first permeable baffles and a second permeable baffle are installed inside the seepage trough, and the seepage trough is divided into three parts in the horizontal direction, which are a seawater chamber, a reaction chamber, and a fresh water chamber from left to right, a tidal generator is arranged inside the seawater chamber, the bottom of the seawater chamber is connected to a saltwater bucket and is provided with a first peristaltic pump and a first flowmeter, a first overflow pipe is arranged on the upper part of the seawater chamber, and a second flowmeter is arranged on the first overflow pipe, a second peristaltic pump and a third flowmeter are arranged on the bottom of the freshwater chamber, a second overflow pipe is arranged on the upper part of the freshwater chamber, and a fourth flowmeter is arranged on the second overflow pipe, and three rows of pumping well pipes or water injection well pipes are vertically arranged in the reaction chamber near the seawater chamber side; The water injection well pipe is built with a first piston, an air pressure pipe and an injection pipe, the upper and lower parts of the first piston are respectively fixedly installed with the lower part of the first piston and the upper part of the first piston, the lower part of the first piston and the upper part of the first piston are made of a solid stainless steel disc wrapped in a sealing plastic, the outer wall of the first piston is sleeved with a first sealing air cushion, the first sealing air cushion is made of sealing plastic, the upper part of the first sealing air cushion is connected to the upper part of the first piston, and the lower part is connected to the lower part of the first piston, the interior of the first piston is fixedly installed with a first piston middle connector, the first piston middle connector is a hollow stainless steel tube and a 1mm hole is punched every 1cm to form a screen tube, the first piston is built with a first piston inner air pressure pipe and a pressure sensor, the air pressure pipe runs through the first piston, the end position of the water injection pipe can be moved below the first piston, and the left and right parts of the upper surface of the upper part of the first piston are respectively welded with a first left handle and a first right handle; The pumping well pipe is built with a second piston and a water pumping pipe. The lower part of the second piston and the upper part of the second piston are fixedly installed respectively. The lower part of the second piston and the upper part of the second piston are made of a solid stainless steel disc wrapped in a sealed plastic. The outer wall of the second piston is covered with a second sealing air cushion. The upper part of the second sealing air cushion is connected to the upper part of the second piston, and the second lower part is connected to the lower part of the piston. The middle connector of the second piston is fixedly installed inside the second piston. The middle connector of the second piston is a hollow stainless steel tube and a 1mm hole is punched every 1cm to form a screen tube. The second piston is built with a second piston internal air pressure tube. The end position of the water pumping pipe can be moved below the second piston. The left and right parts of the upper surface of the upper part of the second piston are respectively welded with a second left handle and a second right handle; The back of the seepage trough is provided with a sampling and water head measurement system, which includes 40 sampling ports evenly distributed on the back of the seepage trough, and a sampling tube is embedded in the seepage trough at the sampling port, and the other end of the sampling tube is connected to a detachable rubber tube, and the other side of the detachable rubber tube is connected to a pressure measuring tube; A camera is placed directly in front of the seepage trough.

[0009] As a preferred solution, the seepage trough is composed of a hard acrylic plate with a thickness of 10 mm, a length of 220 cm, a width of 60 cm, and a height of 60 cm. The acrylic plate is bonded using glass glue.

[0010] As a preferred solution, the pipe body of the pumping well pipe or the injection well pipe is made of acrylic pipe, with a length of 80 cm and a pipe diameter of 20 mm.

[0011] As a preferred solution, the first sealing air cushion and the second sealing air cushion are made of sealing plastic.

[0012] As a preferred solution, the material of the air pressure pipe is a hard plastic pipe, and the material of the water injection pipe and the water extraction pipe is a transparent plastic pipe.

[0013] As a preferred solution, the sampling tube is an L-shaped hard glass tube with a diameter of 6 mm and a length of 3 cm, and the pressure measuring tube is a hard glass tube with a diameter of 6 mm and different lengths.

[0014] A working method of a seepage simulation device for preventing seawater intrusion with a height-raising MICP cutoff wall, specifically comprising the following steps: Step S1: Preparation of experimental materials: The aqueous medium used in the indoor experiment of the reaction chamber is white quartz sand particles. The quartz sand particles are sieved with a set of standard sieves with apertures of 4.75, 2.36, and 1.18. After sieving, they are rinsed with tap water several times and mixed evenly after drying. Deionized water is used as fresh water with a density of 1×103g / L. Seawater is prepared by mixing deionized water with table salt with a concentration of 35g / L and a density of 1025g / L. In order to distinguish between fresh and salt water, food-grade red pigment is used for dyeing. The injected solutions are 1.22×108CFU / mL Bacillus pasteurianus solution and a cementing solution mixed with 1.5 mol / L urea and calcium chloride. Step S2: Sand sample loading: The unsaturated filling mode is adopted, the water level is 10cm below the sand filling position, and the filling height is 50cm; Step S3: Formation of saltwater wedge caused by seawater intrusion: After the sand sample is filled, the first peristaltic pump at the salt water end is turned on to supply salt water to the seawater chamber, and the first overflow pipe of the seawater chamber maintains a seawater head height of 37 cm; the second peristaltic pump at the fresh water end is turned on to supply fresh water to the fresh water chamber, and the second overflow pipe of the fresh water chamber maintains a fresh water head height of 40 cm; the tidal generator is turned on to simulate the formation of tides, and the tidal signal uses a sine wave as the signal value, with a period of 1 s and an amplitude of 2 cm; the test continues until seawater intrusion forms a stable salt water wedge, and the formed salt water wedge is about 70 cm long; Step S4, drilling formation process: According to the state of saltwater wedge formation, the target area of ​​the cut-off wall is determined to be 30cm~50cm away from the first permeable baffle of the seawater chamber, with a height of 15cm; during the drilling process, a 2.2cm diameter spiral drill is used to drill to a depth of 50cm at the bottom of the sand tank to form a well. After the well is completed, an acrylic well pipe is placed in the wellbore; The method optimized by numerical simulation is as follows: in the horizontal direction, the wells are arranged 30 cm away from the first permeable baffle of the seawater chamber, and a row of wells is arranged every 10 cm, and three rows of wells are arranged; in the vertical direction, the wells are arranged at intervals of 10 cm, and a total of 5 rows of wells are arranged; Distribution of pumping and injection wells: The first row is all injection wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom to 20cm near the fresh water chamber to form a screen tube. The piston in the injection well pipe is inserted 5cm from the bottom; a pumping and beating dual-purpose air pump is used to inflate the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston is completely in contact with the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston; The second row is all pumping wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom 50cm away to form a screen tube. The lower part of the piston is inserted into the pumping well pipe 5cm from the bottom. Use a pumping and beating air pump to pump air into the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston; The third column is all water injection wells, and the operation steps are the same as the first column water injection wells; Step S5, MICP type cut-off wall formation process: First grouting: Inject Bacillus pasteurianus solution from the first and third injection wells; inject bacterial solution into the injection well at a rate of 10mL / min. During the injection process, keep the pressure in the lower area of ​​the piston at 0.1~0.15Mpa, and the pressure is monitored by a pressure sensor; the pressure is regulated by connecting a pneumatic tube to a dual-purpose air pump for pumping; the amount of solution injected is equivalent to twice the pore volume of the target area (one third); after 1 / 5 of the solution volume is injected, the pumping well pumps liquid at a rate of 7mL / min. After the bacterial solution injection is completed, the injection well and the pumping well are suspended at the same time; after the grouting is completed, let it stand for 2 hours, and after 2 hours, take samples from 40 sampling ports on the back every 20 minutes to measure the free bacterial biomass in the pore fluid. After the bacterial solution injection was completed and the mixture was left to rest for 22 hours, a cementing solution of 1.5 mol / L urea and calcium chloride was continuously injected into the injection well. The injection speed and amount of the cementing solution were the same as those of the bacterial solution. After 1 / 5 of the cementing solution was injected, the pumping well was pumped at a speed of 7 mL / min. After the injection of the cementing solution was completed, the injection well and the pumping well were suspended at the same time. Second grouting: 22 hours after the grouting is completed, release the pressure on the piston of the water injection well, lift the bottom of the piston 10cm away from the bottom of the water injection well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing isolation of the piston; similarly release the pressure on the piston of the pumping well, lift the bottom of the piston 10cm away from the bottom of the pumping well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing isolation of the piston. The grouting process is the same as the first grouting; The third grouting: The operation of the piston in the pumping well is the same as the operation in the second grouting, and the lower part of the piston is lifted to 15 cm from the bottom of the well pipe; the grouting process is the same as the second grouting process; Step S6: Evaluation of the formation effect of MICP type cut-off wall: 48 hours after the grouting is completed, observe the changes in the shape of the saltwater wedge; during this process, use a high-definition camera 18 to photograph the changes in the shape of the saltwater wedge in the central reaction chamber every 1 minute; take samples from the sampling port on the back every 20 minutes to measure the salt concentration; 70 hours after the grouting is completed, use a spiral drill to drill to a depth of 50 cm at the bottom of the sand tank to form a well on the right side of the cut-off wall area, about 50 cm away from the permeable baffle of the fresh water chamber. After the well is completed, put an acrylic pumping well pipe into the wellbore. The pumping well pipe is set to form a sieve tube with 1 mm holes every 1 cm from the bottom to 40 cm. Use a peristaltic pump to extract the residual salt water at a speed of 20 mL / min until no salt water colored by food dye appears on the right side of the target area, and stop pumping. Observe whether the red salt water on the left will pass through the target area and re-enter the fresh water area on the right, so as to verify the effect of the MICP cut-off wall in preventing seawater intrusion.

[0015] Step S7, MICP type cut-off wall height raising process: The seawater level was raised by 3 cm and the freshwater level was lowered by 4 cm, so that the cut-off wall failed and salt water continued to invade from the top of the target area. At this time, the target area of ​​the cut-off wall became 30 cm to 50 cm from the first permeable baffle (8) of the seawater chamber, with a height of 20 cm. The piston operation in the pumping well was the same as the piston adjustment operation before the second grouting in the above step S5, and the lower part of the piston was raised to 20 cm from the bottom of the well pipe. The grouting process was the same as the second grouting process in the above step S5. The evaluation of the effect of the cut-off wall formation was the same as the above step S6. After the experiment, the supply of seawater and fresh water was stopped, and the salt and fresh water in the seawater chamber and the fresh water chamber were emptied by a water pump. All sampling ports on the back of the seepage trough were opened to discharge the salt and fresh water in the porous medium of the seepage trough. When the surface of the quartz sand in the seepage trough was visibly free of moisture, a CT scanner was used to scan the pores of the porous medium inside the seepage trough, and the porosity of the target area was counted to evaluate the effect of grouting on blocking the pores.

[0016] Due to the adoption of the above technical solution, the present invention has the following beneficial effects compared with the prior art: (1) The device can establish microbial reaction seepage cutoff walls of different sizes and shapes at different locations in response to different degrees of seawater intrusion, lengths and heights of saltwater wedges.

[0017] (2) After the cut-off wall is built, in the face of climate change and groundwater level changes that make the previously built cut-off wall ineffective, the device and method can change the injection position of the microbial liquid into the aquifer by changing the screen position of the pumping well, thereby raising the cut-off wall to resist seawater intrusion.

[0018] (3) Compared with the traditional concrete cut-off wall, the device and method do not require excavation and disturbance of the aquifer medium, and there is no problem of construction difficulty caused by pit wall collapse by drilling a well.

[0019] (4) The device and method use the drilling method to create a well, which has little disturbance to the aquifer medium and less interference to the experiment, thereby improving the accuracy of the experiment.

[0020] (5) The device can simulate the injection of microbial cementing fluid into the coastal aquifer at a laboratory scale, so that it blocks the pores of the aquifer medium and achieves the effect of preventing and controlling seawater intrusion. The injection wells in the device can achieve pressurization and negative pressure, and the pumping wells can achieve directional pumping. The microbial cementing fluid can be directional to deposit calcium carbonate to block the pores in the target area to form a cut-off wall. A new method for forming a cut-off wall to resist seawater intrusion has been innovated. Compared with the traditional method of building a concrete cut-off wall or filling clay, the MICP technology has a lower engineering cost and a shorter construction period to form a cut-off wall to resist seawater intrusion.

[0021] Additional aspects and advantages of the present invention will become apparent from the following description or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1 It is a front view structural schematic diagram of the present invention; Figure 2 It is a schematic diagram of the top view of the structure of the present invention; Figure 3 It is a structural schematic diagram of the water injection well pipe; Figure 4 It is a schematic diagram of the structure of the pumping well pipe; Figure 5 Schematic diagram of the sampling and water head measurement system. DETAILED DESCRIPTION

[0023] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0024] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0025] Combine the following Figures 1 to 3The seepage simulation device and working method of the elevated MICP type cut-off wall for preventing and controlling seawater intrusion of the embodiment of the present invention are specifically described. The present invention designs an experimental device and method for simulating the injection of a Bacillus pasteurianus solution and a binder mixed with urea and calcium chloride into a coastal aquifer to generate calcium carbonate precipitation, thereby blocking the pores of the aquifer to achieve the purpose of cutting off the seawater intrusion channel. The pumping and injection wells of the device can be pressurized and negatively pressurized, and the pumping and injection positions are adjustable. In response to different degrees of seawater intrusion, cut-off walls of different shapes and sizes can be formed at directional positions. After the cut-off wall is formed, when a higher cut-off wall needs to be built to deal with future climate change or groundwater level changes that increase the degree of seawater intrusion, the method also has the advantage of raising the cut-off wall. The device can conduct in-depth research on the mechanism and effect of the application of MICP technology in preventing and controlling seawater intrusion. The simulation experiment of the device can provide reference significance for the use of this method in the field to resist seawater intrusion.

[0026] like Figure 1 , Figure 2As shown, the present invention proposes a seepage simulation device for preventing seawater intrusion with a height-raising MICP type seepage cutoff wall, including a seepage trough 1, which is composed of a hard acrylic plate with a thickness of 10 mm, a length of 220 cm, a width of 60 cm, and a height of 60 cm. The acrylic plate is bonded with glass glue and can be removed after the experiment. The seepage trough 1 is equipped with two detachable first permeable baffles 8 and a second permeable baffle 12, which divide the seepage trough 1 into three parts in the horizontal direction from left to right, namely, a seawater chamber 19, a reaction chamber 20, and a freshwater chamber 21. A tidal generator 7 is arranged inside the seawater chamber 19, and the seawater chamber 19 simulates the fixed head boundary of seawater, and the tidal generator simulates the formation of tides. The bottom of the seawater chamber 19 is connected to the saltwater bucket 2 and is provided with a first peristaltic pump 3 and a first flowmeter 4, and seawater is pumped in by the peristaltic pump 3, and the flowmeter 4 monitors the flow of inflowing seawater. A first overflow pipe 6 is provided on the upper part of the seawater chamber 19, and a second flowmeter 5 is provided on the first overflow pipe 6. The overflow pipe 6 controls the seawater level, and the flowmeter 5 monitors the overflow seawater volume. The reaction chamber 20 simulates the coastal phreatic aquifer, and the freshwater chamber 21 simulates the inland boundary of the fixed head. The bottom of the freshwater chamber 21 is connected to the freshwater barrel 14 and is provided with a second peristaltic pump 13 and a third flowmeter 15. Fresh water is pumped in by the peristaltic pump 13, and the pumped water volume is measured by the flowmeter 15. A second overflow pipe 17 is provided on the upper part of the freshwater chamber 21, and a fourth flowmeter 16 is provided on the second overflow pipe 17. The water level is controlled by the overflow pipe 17, and the flowmeter 16 monitors the overflow water volume. Three rows of pumping well pipes 22 or injection well pipes 23 are vertically placed near the side of the seawater chamber 19 in the reaction chamber 20 as a cut-off wall generation system. The cut-off wall formation system is mainly formed by the pumping and injection wells 22. The target area, i.e., the area where the cut-off wall is formed, can be freely adjusted according to actual needs, and the number and position of the placement can be changed according to different design schemes. The reaction chamber 20 simulates the coastal submerged aquifer, and different materials can be filled inside to form a porous medium. The pumping well 22 can be used as an injection well or a pumping well to pump and inject the biological reaction liquid. The sampling port 10 can be sampled at different positions, and the pressure measuring tube 11 can monitor the water pressure at different positions of the aquifer in the reaction chamber; the pipe body of the pumping well pipe 22 or the injection well pipe 23 is made of acrylic pipe, with a length of 80cm and a pipe diameter of 20mm. The pipe body can be selected according to the design parameters. Different diameters and lengths.

[0027] like Figure 3As shown, the water injection well pipe 23 has a first piston 23-1, an air pressure pipe 23-7 and a water injection pipe 23-9 built therein, and the upper and lower parts of the first piston 23-1 are respectively fixedly mounted with a first piston lower part 23-2 and a first piston upper part 23-4, the first piston lower part 23-2 and the first piston upper part 23-4 are made of a sealing plastic-wrapped solid stainless steel disc, and the outer wall of the first piston 23-1 is covered with a first sealing air cushion 23-3, the first sealing air cushion 23-3 is made of sealing plastic, and the upper part of the first sealing air cushion 23-3 is The upper part is connected to the upper part 23-4 of the first piston, and the lower part is connected to the lower part 23-2 of the first piston. The middle connector 23-5 of the first piston is fixedly installed inside the first piston 23-1. The middle connector 23-5 of the first piston is a hollow stainless steel tube and a 1mm hole is punched every 1cm to form a screen tube. The first piston 23-1 is built with a first piston internal air pressure tube 23-6 and a pressure sensor 23-8. The air pressure tube in the piston can be inflated and expanded so that the entire piston fits and closes with the water injection well pipe. It can also be deflated through the air pressure tube so that the piston and the pumping well pipe do not fit and close. The air pressure tube 23-7 runs through the first piston 23-1, and can supply air and deflate to achieve pressurization and pressure relief effects, which is pressurization or decompression for the water injection system. The upper part can be connected to a dual-purpose vacuum pump for pressurization and negative pressure. The end position of the water injection pipe 23-9 can move below the first piston 23-1, and the upper end of the water injection pipe can be connected to a peristaltic pump hose for liquid injection. A first left handle 23-11 and a first right handle 23-10 are welded to the left and right parts of the upper surface of the first piston upper part 23-4, respectively, and the piston can be moved up and down by pulling; like Figure 4As shown, the water pumping well pipe 22 is built with a second piston 22-1 and a water pumping pipe 22-9, and the upper and lower parts of the second piston 22-1 are respectively fixedly installed with a second piston lower part 22-2 and a second piston upper part 22-4, the second piston lower part 22-2 and the second piston upper part 22-4 are made of a sealed plastic-wrapped solid stainless steel disc, and the outer wall of the second piston 22-1 is sleeved with a second sealing air cushion 22-3, the upper part of the second sealing air cushion 22-3 is connected to the second piston upper part 22-4, and the second lower part is connected to the piston lower part 22-2, and the interior of the second piston 22-1 is fixedly installed with a second piston middle connector 22-5, the second piston middle connector 22-5 is a hollow stainless steel tube and a 1mm hole is punched every 1cm to form a screen tube, and the second piston 22-1 is built with a second piston inner air pressure tube 22-6, which can be inflated through the piston inner air pressure tube 6, so that the entire piston is fitted and closed with the water pumping well pipe. It is also possible to deflate the air through the air pressure tube 6 so that the piston and the pumping well pipe are not tightly closed. The end position of the pumping pipe 22-9 can move below the second piston 22-1, and the upper end of the water injection pipe can be connected to a peristaltic pump hose for injection. The left and right parts of the upper surface of the second piston upper part 22-4 are respectively welded with a second left handle 22-11 and a second right handle 22-10, and the piston can be moved up and down by pulling; the first sealing air cushion 23-3 and the second sealing air cushion 22-3 are made of sealing plastic. The material of the air pressure tube 23-7 is a hard plastic tube, and the material of the water injection pipe 23-9 and the pumping pipe 22-9 is a transparent plastic tube.

[0028] The back of the seepage trough 1 is provided with a sampling and water head measurement system, which includes 40 sampling ports evenly distributed on the back of the seepage trough 1. The sampling ports are embedded in the seepage trough with a sampling tube 10. The sampling ports are close to the inside of the seepage trough with a gauze adhesion to prevent leakage of the filler. The sampling tube 10 is an L-shaped hard glass tube with a diameter of 6 mm and a length of 3 cm. The other end of the sampling tube 10 is connected to a detachable rubber tube 24, and the other side of the detachable rubber tube 24 is connected to a pressure measuring tube 11. The pressure measuring tube 11 is a hard glass tube with a diameter of 6 mm and different lengths. When sampling, the rubber tube on the side of the pressure measuring tube can be pinched or clamped with a water stop clamp, and the sampling port can be pulled out for sampling.

[0029] A camera 18 is placed directly in front of the seepage trough 1, and the camera position is adjusted to ensure that the seepage trough can be fully photographed.

[0030] Formation principle of biological reaction cut-off wall: According to the formation of saltwater wedge, determine the target area for the cut-off wall. Use the combination of pumping and injection wells to realize directional flow of solution in the target area, and the microbial mixed solution realizes a specific flow field in the aquifer, so that the microbial reaction is carried out in the target area. The calcium carbonate precipitation generated by the microbial reaction blocks the pores in the target area to resist seawater intrusion. When the original cut-off wall cannot resist seawater intrusion under the conditions of climate change or groundwater level change, the piston position of the pumping well can be adjusted to cooperate with the pumping and injection well to continue to raise the cut-off wall to resist seawater intrusion.

[0031] The reaction principle of the microbial mixture is to use the urease synthesized and released by urease bacteria to catalyze the hydrolysis of urea to generate NH4 + and CO3 2- , and then the CO3 generated by hydrolysis 2- and Ca in calcium salts 2+ Combined with the generated CaCO3 precipitation to block the pores of the aquifer, the cut-off wall can prevent seawater intrusion. The chemical reaction chamber is as follows: CO(NH2)2+H2O→H2COOH+2NH3 H2COOH+H2O→NH3+H2CO3 2NH3+2H2O↔2NH4 + +2OH - 2OH - +H2CO3↔CO3 2- +2H2O Ca 2+ +Cell→Cell - Ca 2+ Cell-Ca 2+ +CO3 2- →Cell - CaCO3↓.

[0032] A working method of a seepage simulation device for preventing seawater intrusion with a height-raising MICP cutoff wall, characterized in that it specifically comprises the following steps: Step S1: Preparation of experimental materials: The aqueous medium used in the experiment in reaction chamber 20 was white quartz sand particles. The quartz sand particles were sieved using a set of standard sieves with apertures of 4.75, 2.36, and 1.18, rinsed several times with tap water after sieving, and mixed evenly after drying. Deionized water was used as fresh water with a density of 1×103 g / L. Seawater was prepared by mixing deionized water with table salt with a concentration of 35 g / L and a density of 1025 g / L. In order to distinguish between fresh and salt water, food-grade red pigment was used for dyeing. The injected solutions were 1.22×10 8CFU / mL Bacillus pasteurianus solution and a cementing fluid mixed with 1.5 mol / L urea and calcium chloride; Step S2: Sand sample loading: The unsaturated filling mode is adopted, the water level is 10cm below the sand filling position, and the filling height is 50cm; Step S3: Formation of saltwater wedge caused by seawater intrusion: After the sand sample is filled, the first peristaltic pump 3 at the salt water end is turned on to supply salt water to the seawater chamber 19, and the first overflow pipe 6 of the seawater chamber maintains the seawater head height of 37 cm; the second peristaltic pump 13 at the fresh water end is turned on to supply fresh water to the fresh water chamber 21, and the second overflow pipe 17 of the fresh water chamber maintains the fresh water head height of 40 cm; the tidal generator 7 is turned on to simulate the formation of tides, and the tidal signal uses a sine wave as the signal value, with a period of 1 s and an amplitude of 2 cm; the test is continuously run until the seawater intrusion forms a stable salt water wedge, and the formed salt water wedge is about 70 cm long; Step S4, drilling formation process: According to the state of saltwater wedge formation, the target area of ​​the cut-off wall is determined to be 830cm~50cm away from the first permeable baffle of the seawater chamber, with a height of 15cm; during the drilling process, a 2.2cm diameter spiral drill is used to drill to a depth of 50cm at the bottom of the sand tank to form a well. During the drilling process, care should be taken to prevent the quartz sand well wall from collapsing. After the well is completed, an acrylic well pipe is placed in the wellbore; The method optimized by numerical simulation is as follows: in the horizontal direction, the wells are arranged 830 cm away from the first permeable baffle of the seawater chamber, and a row of wells is arranged every 10 cm, and three rows of wells are arranged; in the vertical direction, the wells are arranged at intervals of 10 cm in the width direction of the device, and a total of 5 rows of wells are arranged; Distribution of pumping and injection wells: The first row is all injection wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom to 20cm near the fresh water chamber 21 to form a screen tube. The piston in the injection well pipe 23 is inserted 5cm from the bottom. Use a pumping and beating dual-purpose air pump to pump air into the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston is completely attached to the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston; The second row is all pumping wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom 50cm away to form a screen tube. The lower part of the piston is inserted into the pumping well pipe 5cm from the bottom. Use a pumping and beating air pump to pump air into the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston; The third column is all water injection wells, and the operation steps are the same as the first column water injection wells; Step S5, MICP type cut-off wall formation process: First grouting: inject Bacillus pasteurianus solution from the first and third injection wells; inject bacterial solution into the injection well at a rate of 10mL / min, and keep the pressure in the lower area of ​​the piston at 0.1~0.15Mpa during the injection process. The pressure is monitored by pressure sensor 23-8; the pressure is regulated by air pressure tube 23-7 connected to a dual-purpose air pump for pumping; the amount of solution injected is equivalent to 2 times the pore volume of the target area (one third); after 1 / 5 of the solution volume is injected, the pumping well pumps liquid at a rate of 7mL / min. After the bacterial solution injection is completed, the injection well and the pumping well are suspended at the same time; after the grouting is completed, let it stand for 2h, and after 2h, take samples from 40 sampling ports on the back every 20min to measure the free bacterial biomass in the pore fluid. After the bacterial solution injection was completed and the mixture was left to rest for 22 hours, a cementing solution of 1.5 mol / L urea and calcium chloride was continuously injected into the injection well. The injection speed and amount of the cementing solution were the same as those of the bacterial solution. After 1 / 5 of the cementing solution was injected, the pumping well was pumped at a speed of 7 mL / min. After the injection of the cementing solution was completed, the injection well and the pumping well were suspended at the same time. Second grouting: 22 hours after the grouting is completed, release the pressure on the piston of the water injection well, lift the bottom of the piston 10cm away from the bottom of the water injection well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing isolation of the piston; similarly release the pressure on the piston of the pumping well, lift the bottom of the piston 10cm away from the bottom of the pumping well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing isolation of the piston. The grouting process is the same as the first grouting; The third grouting: The operation of the piston in the pumping well is the same as the operation in the second grouting, and the lower part of the piston is lifted to 15 cm from the bottom of the well pipe; the grouting process is the same as the second grouting process; Step S6: Evaluation of the formation effect of MICP type cut-off wall: 48 hours after the grouting is completed, observe the changes in the shape of the saltwater wedge; during this process, use a high-definition camera 18 to photograph the changes in the shape of the saltwater wedge in the central reaction chamber every 1 minute; take samples from the sampling port on the back every 20 minutes to measure the salt concentration; 70 hours after the grouting is completed, use a spiral drill to drill to a depth of 50 cm at the bottom of the sand tank to form a well on the right side of the cut-off wall area, about 50 cm away from the permeable baffle of the fresh water chamber. After the well is completed, put an acrylic pumping well pipe into the wellbore. The pumping well pipe is set to form a sieve tube with 1 mm holes every 1 cm from the bottom to 40 cm. Use a peristaltic pump to extract the residual salt water at a speed of 20 mL / min until no salt water colored by food dye appears on the right side of the target area, and stop pumping. Observe whether the red salt water on the left will pass through the target area and re-enter the fresh water area on the right, so as to verify the effect of the MICP cut-off wall in preventing seawater intrusion.

[0033] Step S7, MICP type cut-off wall height raising process: The seawater level was raised by 3 cm and the freshwater level was lowered by 4 cm, so that the cut-off wall failed and salt water continued to invade from the top of the target area. At this time, the target area of ​​the cut-off wall became 30 cm to 50 cm from the first permeable baffle (8) of the seawater chamber, with a height of 20 cm. The piston operation in the pumping well was the same as the piston adjustment operation before the second grouting in the above step S5, and the lower part of the piston was raised to 20 cm from the bottom of the well pipe. The grouting process was the same as the second grouting process in the above step S5. The evaluation of the effect of the cut-off wall formation was the same as the above step S6. After the experiment, the supply of seawater and fresh water was stopped, and the salt and fresh water in the seawater chamber and the fresh water chamber were emptied by a water pump. All sampling ports on the back of the seepage trough were opened to discharge the salt and fresh water in the porous medium of the seepage trough. When the surface of the quartz sand in the seepage trough was visibly free of moisture, a CT scanner was used to scan the pores of the porous medium inside the seepage trough, and the porosity of the target area was counted to evaluate the effect of grouting on blocking the pores.

[0034] In the description of the present invention, the term "plurality" refers to two or more than two. Unless otherwise clearly defined, the orientation or positional relationship indicated by the terms "upper" and "lower" is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation of the present invention; the terms "connection", "installation", "fixation", etc. should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0035] In the description of this specification, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0036] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A seepage simulation device for preventing seawater intrusion with a height-raising MICP cut-off wall, comprising a seepage trough (1), characterized in that The seepage trough (1) is provided with two detachable first permeable baffles (8) and a second permeable baffle (12), which divide the seepage trough (1) into three parts in the horizontal direction, which are a seawater chamber (19), a reaction chamber (20), and a fresh water chamber (21) from left to right. A tidal generator (7) is provided inside the seawater chamber (19). The bottom of the seawater chamber (19) is connected to the salt water bucket (2) and is provided with a first peristaltic pump (3) and a first flow meter (4). The upper part of the seawater chamber (19) is provided with a first overflow pipe (6), a second flow meter (5) being arranged on the first overflow pipe (6), a bottom of the fresh water chamber (21) being connected to the fresh water bucket (14) and being provided with a second peristaltic pump (13) and a third flow meter (15), a second overflow pipe (17) being arranged on the top of the fresh water chamber (21), a fourth flow meter (16) being arranged on the second overflow pipe (17), and three rows of pumping well pipes (22) or water injection well pipes (23) being vertically arranged in the reaction chamber (20) near the sea water chamber (19); The water injection well pipe (23) is built with a first piston (23-1), an air pressure pipe (23-7) and a water injection pipe (23-9); the first piston lower part (23-2) and the first piston upper part (23-4) are respectively fixedly mounted on the upper and lower parts of the first piston (23-1); the first piston lower part (23-2) and the first piston upper part (23-4) are made of a sealing plastic-coated solid stainless steel disc; the first piston (23-1) is sheathed with a first sealing air cushion (23-3); the first sealing air cushion (23-3) is made of sealing plastic; the upper part of the first sealing air cushion (23-3) is connected to the upper part of the first piston (23-4), and the lower part is connected to the lower part of the first piston (23-2). ), a first piston middle connector (23-5) is fixedly installed inside the first piston (23-1), the first piston middle connector (23-5) is a hollow stainless steel tube and has 1 mm holes punched at intervals of 1 cm to form a screen tube, the first piston (23-1) is built with a first piston internal air pressure tube (23-6) and a pressure sensor (23-8), the air pressure tube (23-7) runs through the first piston (23-1), the end position of the water injection tube (23-9) can move below the first piston (23-1), and the left and right parts of the upper surface of the first piston upper part (23-4) are respectively welded with a first left handle (23-11) and a first right handle (23-10); The pumping well pipe (22) is provided with a second piston (22-1) and a pumping pipe (22-9). The second piston (22-1) is respectively fixedly provided with a second piston lower part (22-2) and a second piston upper part (22-4). The second piston lower part (22-2) and the second piston upper part (22-4) are made of a solid stainless steel disc wrapped in a sealing plastic. The outer wall of the second piston (22-1) is provided with a second sealing air cushion (22-3). The upper part of the second sealing air cushion (22-3) is connected to the second piston upper part (22-4). The lower part of the second piston (22-1) is connected to the lower part of the second piston (22-4). 2-2), a second piston middle connector (22-5) is fixedly installed inside the second piston (22-1), the second piston middle connector (22-5) is a hollow stainless steel tube and a 1 mm hole is punched every 1 cm to form a screen tube, a second piston internal air pressure tube (22-6) is built into the second piston (22-1), the end position of the water pumping pipe (22-9) can be moved below the second piston (22-1), and a second left handle (22-11) and a second right handle (22-10) are welded to the left and right parts of the upper surface of the second piston upper part (22-4), respectively; The back of the seepage trough (1) is provided with a sampling and water head measurement system, the sampling and water head measurement system comprising 40 sampling ports evenly distributed on the back of the seepage trough (1), a sampling tube (10) embedded in the seepage trough at the sampling port, the other end of the sampling tube (10) being connected to a detachable rubber tube (24), and the other side of the detachable rubber tube (24) being connected to a pressure measuring tube (11); A camera (18) is placed directly in front of the seepage trough (1).

2. A seepage simulation device for preventing seawater intrusion by using a MICP type cut-off wall with a raised height according to claim 1, characterized in that The seepage trough (1) is composed of a hard acrylic plate with a thickness of 10 mm, a length of 220 cm, a width of 60 cm, and a height of 60 cm. The acrylic plate is bonded using glass glue.

3. A seepage simulation device for preventing seawater intrusion by using a height-raising MICP cut-off wall according to claim 1, characterized in that The body of the water pumping well pipe (22) or the water injection well pipe (23) is made of acrylic tube, with a length of 80 cm and a tube diameter of 20 mm.

4. A seepage simulation device for preventing seawater intrusion by using a MICP type cut-off wall with a raised height according to claim 1, characterized in that The first sealing air cushion (23-3) and the second sealing air cushion (22-3) are made of sealing plastic.

5. The seepage simulation device for preventing seawater intrusion by using a MICP cut-off wall with a raised height according to claim 1 is characterized in that The material of the air pressure pipe (23-7) is a hard plastic pipe, and the material of the water injection pipe (23-9) and the water extraction pipe (22-9) is a transparent plastic pipe.

6. The seepage simulation device for preventing seawater intrusion by using a MICP cut-off wall with a raised height according to claim 1 is characterized in that The sampling tube (10) is an L-shaped hard glass tube with a diameter of 6 mm and a length of 3 cm, and the pressure measuring tube (11) is a hard glass tube with a diameter of 6 mm and different lengths.

7. The working method of the seepage simulation device for preventing seawater intrusion by using a height-raising MICP cut-off wall as described in claims 1 to 6 is characterized in that , specifically including the following steps: Step S1: Preparation of experimental materials: The aqueous medium used in the indoor experiment of the reaction chamber (20) was white quartz sand particles. The quartz sand particles were sieved using a set of standard sieves with apertures of 4.75, 2.36, and 1.

18. After sieving, they were rinsed several times with tap water, dried, and mixed evenly. The fresh water used was deionized water with a density of 1×10 3 g / L; seawater is prepared by mixing deionized water and salt with a concentration of 35g / L and a density of 1025g / L. In order to distinguish between salty and fresh water, food-grade red pigment is used for dyeing. The injection solutions are 1.22×10 8 CFU / mL Bacillus pasteurianus solution and a cementing fluid mixed with 1.5 mol / L urea and calcium chloride; Step S2: Sand sample loading: The unsaturated filling mode is adopted, the water level is 10cm below the sand filling position, and the filling height is 50cm; Step S3: Formation of saltwater wedge caused by seawater intrusion: After the sand sample is filled, the first peristaltic pump (3) at the salt water end is turned on to supply salt water to the sea water chamber (19), and the first overflow pipe (6) of the sea water chamber maintains a sea water head height of 37 cm; the second peristaltic pump (13) at the fresh water end is turned on to supply fresh water to the fresh water chamber (21), and the second overflow pipe (17) of the fresh water chamber maintains a fresh water head height of 40 cm; the tidal generator (7) is turned on to simulate the formation of tides, and the tidal signal uses a sine wave as a signal value, with a period of 1 s and an amplitude of 2 cm; the test is continuously run until the sea water invades to form a stable salt water wedge, and the formed salt water wedge is about 70 cm long; Step S4, drilling formation process: According to the state of the saltwater wedge formation, the target area of ​​the cut-off wall is determined to be 30 cm to 50 cm away from the first permeable baffle (8) of the seawater chamber, with a height of 15 cm; during the drilling process, a 2.2 cm diameter spiral drill is used to drill to a depth of 50 cm at the bottom of the sand tank to form a well. After the well is completed, an acrylic well pipe is placed in the wellbore; The method optimized by numerical simulation is as follows: in the horizontal direction, the wells are arranged starting from 30 cm away from the first permeable baffle (8) of the seawater chamber, and a row of wells is arranged every 10 cm, and three rows of wells are arranged; in the vertical direction (the width direction of the device), the wells are arranged at intervals of 10 cm in sequence, and a total of 5 rows of wells are arranged; Distribution of pumping and injection wells: The first row is all injection wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom to 20cm near the fresh water chamber (21) to form a screen pipe. The piston in the injection well pipe (23) is inserted 5cm from the bottom. A pump is used to pump air into the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston is completely attached to the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston. The second row is all pumping wells. Each acrylic well pipe is punched with 1mm holes every 1cm from the bottom 50cm away to form a screen tube. The lower part of the piston is inserted into the pumping well pipe 5cm from the bottom. Use a pumping and beating dual-purpose air pump to pump air into the air pressure pipe in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the wall of the injection well pipe to achieve the upper and lower sealing isolation of the piston; The third column is all water injection wells, and the operation steps are the same as the first column water injection wells; Step S5, MICP type cut-off wall formation process: First grouting: inject Bacillus pasteurianus solution from the first and third injection wells; inject bacterial solution into the injection well at a rate of 10 mL / min, and keep the pressure in the lower area of ​​the piston at 0.1~0.15 MPa during the injection process. The pressure is monitored by a pressure sensor (23-8); the pressure is adjusted using an air pressure tube (23-7) connected to a dual-purpose air pump for pumping; the amount of solution injected is equivalent to twice the pore volume of the target area (one third); after 1 / 5 of the solution volume is injected, the pumping well is pumped at a rate of 7 mL / min; after the bacterial solution injection is completed, the injection well and the pumping well are suspended at the same time; after the grouting is completed, let it stand for 2 hours, and after 2 hours, take samples from 40 sampling ports on the back every 20 minutes to measure the free bacterial biomass in the pore fluid; after the bacterial solution injection is completed and let stand for 22 hours, continue to inject 1.5 The cementing liquid is a mixture of 1.5 mol / L urea and calcium chloride. The injection speed and amount of the cementing liquid are the same as those of the bacterial liquid. After 1 / 5 of the amount of the cementing liquid is injected, the pumping well is pumped at a speed of 7 mL / min. After the injection of the cementing liquid is completed, the injection well and the pumping well are suspended at the same time. Second grouting: 22 hours after the grouting is completed, release the pressure on the piston of the water injection well, lift the bottom of the piston 10cm away from the bottom of the water injection well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing and isolation effect of the piston; similarly release the pressure on the piston of the pumping well, lift the bottom of the piston 10cm away from the bottom of the pumping well, and use a dual-purpose air pump to pump air into the air pressure tube in the piston until the air pressure reaches 0.5Mpa, so that the sealing air cushion around the piston completely fits the pipe wall of the water injection well to achieve the upper and lower sealing and isolation effect of the piston; the grouting process is the same as the first grouting; The third grouting: The operation of the piston in the pumping well is the same as the operation in the second grouting, and the lower part of the piston is lifted to 15 cm from the bottom of the well pipe; the grouting process is the same as the second grouting process; Step S6: Evaluation of the formation effect of MICP type cut-off wall: 48 hours after the completion of grouting, observe the changes in the shape of the salt water wedge; during this process, use a high-definition camera 18 to photograph the changes in the shape of the salt water wedge in the central reaction chamber every 1 minute; take samples from the sampling port on the back every 20 minutes to measure the salt concentration; 70 hours after the completion of grouting, use a spiral drill to drill to a depth of 50 cm at the bottom of the sand tank to form a well on the right side of the cut-off wall area, about 50 cm away from the permeable baffle of the fresh water chamber. After the well is completed, put an acrylic pumping well pipe into the wellbore; the pumping well pipe is set to form a sieve tube with 1 mm holes every 1 cm from the bottom to 40 cm; use a peristaltic pump to extract the residual salt water at a speed of 20 mL / min until no salt water colored by food dye appears on the right side of the target area, and stop pumping; observe whether the red salt water on the left will pass through the target area and re-enter the fresh water area on the right, thereby verifying the effect of the MICP cut-off wall in preventing seawater intrusion; Step S7, MICP type cut-off wall height raising process: The seawater level was raised by 3 cm and the freshwater level was lowered by 4 cm, so that the cut-off wall failed and salt water continued to invade from the top of the target area. At this time, the target area of ​​the cut-off wall became 30 cm to 50 cm from the first permeable baffle (8) of the seawater chamber, with a height of 20 cm. The piston operation in the pumping well was the same as the piston adjustment operation before the second grouting in step S5, and the lower part of the piston was raised to 20 cm from the bottom of the well pipe. The grouting process was the same as the second grouting process in step S5. The evaluation of the effect of the cut-off wall was the same as step S6. After the experiment, the supply of seawater and fresh water was stopped, and the salt and fresh water in the seawater chamber and the fresh water chamber were emptied by a water pump. All sampling ports on the back of the seepage trough were opened to discharge the salt and fresh water in the porous medium of the seepage trough. When the surface of the quartz sand in the seepage trough was free of moisture, a CT scanner was used to scan the pores of the porous medium inside the seepage trough, and the porosity of the target area was counted to evaluate the effect of grouting on blocking the pores.

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