Liquefaction prevention method

Abstract

This invention provides a liquefaction prevention method that enables permanent liquefaction prevention in a short construction period, even in subsoil where existing structures exist. [Solution] A liquefaction prevention method was adopted that includes the following steps. (1) Shaft formation process for forming the shaft T. (2) Installation step of a press-fitting device, which involves installing a drain pipe press-fitting device 40 for press-fitting the drain pipe 10. (3) A drain pipe installation process in which the divided drain pipes 10 are pressed in and joined together in sequence to install the connected drain pipes D on the target ground G. (4) Maintenance pipe connection step, which involves connecting the maintenance pipe 30 to the end of the connected drain pipe D.

Description

Technical Field

[0001] The present invention relates to a construction method for preventing liquefaction of the ground where existing structures are present.

Background Art

[0002] Liquefaction of the ground caused by an earthquake destroys structures on the ground, causing great damage to human lives and property. Conventionally, the inventor has developed a liquefaction prevention method for draining excess interstitial water in the ground that occurs with the liquefaction of the ground assumed during an earthquake. In this method, a nozzle for compressed water that injects compressed water and a nozzle for compressed air that injects compressed air are provided at the lower end of a pile. Compressed water and compressed air are injected from these nozzles and driven into the ground to a predetermined depth to form an excavation hole. By injecting compressed water and compressed air, fine particles in the ground are raised along the pile and discharged to the ground surface. After discharging these fine particles, the injection of compressed air is stopped or the injection pressure is reduced, and the pile is pulled out. At the same time, a backfill material is introduced into the excavation hole during the pulling process to form a foundation column having higher permeability than the surroundings (Patent Document 1).

[0003] However, the above liquefaction prevention method is a method for construction on virgin ground before the construction of a structure. In order to construct on the ground where existing structures are present, the use of life lines such as electricity, gas, and water supply and drainage must be temporarily interrupted, and the structure must be moved to a temporary site. After that, liquefaction countermeasures must be carried out on the virgin ground, and the structure must be moved back to its original position. An empty space equal to or larger than the structure is required in the adjacent area. In addition to the liquefaction prevention construction cost, a large amount of costs such as the cost of securing alternative facilities, the cost of relocating existing structures, and incidental construction costs are required, so it is not realistic in terms of cost-effectiveness.

[0004] On the other hand, a known ground improvement method for horizontal ground involves constructing a shaft surrounded by a wall that prevents the inflow of soil and sand in the soft ground, and using a horizontal drilling device placed in the shaft, inserting short casings, each topped with a soil-removing lid, into the soft ground while rotating them, roughly parallel to the ground surface, and successively adding short casings until the required length is reached. Then, into the added long casings, sequentially insert cylindrical bags filled with quicklime, which are formed by sewing moisture-proof paper together with thread and have both ends closed. Next, leaving the series of cylindrical bags in the long casing in the ground, the casing is rotated and pulled back into the shaft, allowing moisture from the soft ground to gradually permeate into the quicklime in the series of cylindrical bags remaining in the soft ground through the thread holes, thereby causing a saturation reaction (Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2002-322637 [Patent Document 2] Japanese Patent Application Publication No. 59-88525 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, the above-mentioned method of improving horizontal ground may cause a reduction in volume due to the consolidation of soil particles in the soft ground when a drain pipe is driven into the soft ground, potentially inducing settlement of the ground surface or the foundation of structures. In other words, a void is created between the cylindrical bag containing quicklime, which has a smaller diameter compared to the diameter of the casing being pulled back, and the ground, and there is a risk that ground subsidence may occur due to the loosening of the soft ground. Furthermore, when the casing is pushed in, the soil particles of the soft ground in contact with the tip of the casing are subjected to compressive pressure, and as a result of the dissipation of interparticle water and air, soil particles are compressed and solidified on the outer surface of the casing tip, causing solidified soil to adhere to the outer surface of the drain pipe. In addition, due to the frictional resistance caused by the soil pressure between the outer surface of the pushed-in drain pipe and the soft ground, the outer diameter of the casing tip and outer surface expands, and a void is created as described above, which may cause settlement over time.

[0007] Incidentally, in recent years, repeated damage due to re-liquefaction of the ground has become a concern regarding existing structures built on sandy soil with a high liquefaction rate following earthquakes. Specifically, there have been reports of cases where, several years after completing horizontal maintenance work on existing structures that have suffered differential settlement due to liquefaction caused by the initial earthquake, another earthquake causes the ground to liquefy again, resulting in differential settlement of the existing structures. Based on these cases, unless the root cause of the liquefaction in the target ground is eliminated, there is a concern that re-liquefaction damage may occur in the future. However, currently, there is no technology that can prevent reliquefaction in target ground where existing structures exist. Therefore, there is a need for the development of a new technology that is inexpensive, harmless, and can permanently prevent liquefaction with a short construction period.

[0008] This invention was made to solve the above-mentioned problems, and aims to provide a liquefaction prevention method that is inexpensive, harmless, and can permanently prevent liquefaction (re-liquefaction) in a short construction period, even in the subsoil where existing structures exist. [Means for solving the problem]

[0009] To solve the above problems, the present invention provides a liquefaction prevention method (hereinafter sometimes referred to as "this liquefaction prevention method") characterized by including the following steps. (1) A shaft formation step in which a shaft is formed within the area to be liquefaction prevention measures for the target ground or to the side of the said area. (2) A step of installing a drain pipe press-fitting device, in which a drain pipe press-fitting hole is formed in the wall surface of the shaft at a predetermined depth position (if the shaft is located within the target area, it is the wall surface in at least one direction, and if the shaft is located laterally within the target area, it is the wall surface on the target area side; the same applies hereinafter), and a drain pipe press-fitting device is installed for press-fitting the drain pipe. (3) A drain pipe installation process in which the divided drain pipes are pressed into the drain pipe press-in holes of the shaft and joined together sequentially (regardless of the method of connection), thereby being installed to a predetermined position in the horizontal or diagonal direction in the area where liquefaction prevention measures for the target ground are to be implemented. (4) A maintenance pipe connection step, which involves connecting a maintenance pipe, whose opening communicates with the ground surface, to the shaft-side end of the installed drain pipe, for use during maintenance.

[0010] Here, the dimensions and depth of the shaft must be such that a drain pipe press-in device can be installed and workers can perform their duties, and must be appropriately determined according to the ground conditions and the installation method of the drain pipe to be pressed in.

[0011] Furthermore, it is preferable that the drain pipe press-in device includes a rotation mechanism that allows the drain pipe to rotate in at least one direction (to fully rotate in at least one direction, both forward and reverse), and a press-in (pressurizing) mechanism for pressing the drain pipe into the target ground (forcing it in). While there are no restrictions on the installation location of the drain pipe press-in device, installing it on the wall surface opposite the drain pipe press-in hole as a reaction surface allows for easy and accurate penetration of the drain pipe.

[0012] Furthermore, the most suitable ground for applying this liquefaction prevention method is ground containing sandy soil, which is prone to liquefaction. As a characteristic of the sandy soil particles that make up sandy ground, for example, when precast piles are driven, the air and water trapped between the sandy soil particles corresponding to the volume of the precast piles are pushed out, resulting in a consolidation saturation state, making it impossible to drive any more piles.

[0013] As a countermeasure to the above situation, by providing a structure in which the drilling bit, which has drilling wings attached to the tip of the leading edge of the drain pipe to be pressed in (the furthest forward part of the divided drain pipe), is rotatably installed, the drilling bit can be rotated during the drain pipe installation process to agitate the sandy soil that has become consolidated and saturated, and the agitated sandy soil particles can be pressed against the wall of the borehole, thereby making it easier to press in the drain pipe. Furthermore, in the drain pipe installation process described above, if lubricating fluid is injected into the outer surface of the divided drain pipe from the shaft end and the drain pipe is pressed in while rotating it, the press-fitting of the drain pipe into the target area can be easily performed.

[0014] Furthermore, drain pipes are pipes installed to collect and discharge interpore water (groundwater) from the target ground to dissipate the water pressure within the ground. The pipe diameter, length, number, and installation location must be appropriately determined based on various conditions such as ground strength and groundwater level.

[0015] Furthermore, the drain pipe is provided with intake and drainage holes. The number (opening ratio) and dimensions of these holes must be appropriately determined to prevent the accumulation of sediment inside the drain pipe and to ensure that it has the required intake and drainage performance, without causing clogging. For example, it is preferable to provide intake and drainage holes with a diameter of 3 mm to 5 mm at intervals of 5 mm to 15 mm in the circumferential and axial directions. Depending on the soil characteristics of the target ground, it is preferable to decide whether to provide the above-mentioned suction and drainage holes on the entire circumference of the drain pipe or on the lower half of the circumference. Furthermore, the drain pipe may be designed without drainage holes on its upper surface to prevent soil particles from the target ground from entering the pipe.

[0016] In addition, in order to ensure the workability of the drain pipe, the drain pipe preferably has a pipe diameter of 70 mm to 100 mm and a length of 50 cm to 100 cm. This is suitable because it enables downsizing of the drain pipe press-fitting device and allows construction in narrow areas.

[0017] Also, depending on the conditions of the target ground, drain pipes of various materials and forms can be used. In particular, it is preferable for the drain pipe to include a steel pipe provided with water absorption / drainage holes on the circumferential surface and axial direction thereof, and a water permeable sheet adhered to the inner wall of the steel pipe, in order to enhance water collecting performance. In addition, in order to enhance durability, the drain pipe is preferably made of steel, and in order to enhance cost-effectiveness, it is preferably made of synthetic resin.

[0018] Also, the horizontal direction does not refer to a direction without any inclination, and may have a predetermined inclination. Regarding the diagonal direction as well, as long as the effects of the present invention can be achieved, the magnitude of the inclination angle is not a matter of concern.

[0019] The maintenance pipe is a tubular (cylindrical) member in which one end opening is connected to the drain pipe, and the other end opening is exposed directly or indirectly (connected to other members, and a lid or the like may be provided) to the ground surface. As the maintenance pipe, a known sewer pipe can be used, and the mode thereof is not limited, such as a straight pipe, a joint body of a straight pipe and a vent pipe, a flexible body, etc.

[0020] According to this liquefaction prevention construction method, by sequentially joining the drain pipes divided from the shaft, the drain pipes can be laid in the lower ground where existing structures are built, in the current state, simply, with a short construction period and at low cost. Therefore, by burying the drain pipe in the target ground of sandy soil and draining the interstitial water of soil particles having excessive pore water pressure through the drain pipe, the pore water pressure between the soil particles of the sandy soil ground can be dissipated, and liquefaction can be prevented.

[0021] Also, in this liquefaction prevention method, after the drain pipe installation step, a drain pipe internal drain material filling step of filling at least a part of the inside of the joined drain pipe with a drain material for the drain pipe may be performed.

[0022] Furthermore, this liquefaction prevention method may be characterized by including the following steps. (1) A shaft formation step of forming a shaft in the target area for liquefaction prevention measures of the target ground or on the side of the target area. (2) A press-fitting device installation step of forming a drain pipe press-fitting hole on the wall surface at a predetermined depth position of the shaft and installing a drain pipe press-fitting device for press-fitting the drain pipe. (3) A drain pipe installation step of press-fitting and sequentially joining (regardless of the connection method) the divided drain pipes filled with the drain material for the drain pipe from the drain pipe press-fitting hole of the shaft to a predetermined position in the horizontal or diagonal direction in the liquefaction prevention target area. (4) A maintenance pipe connection step of connecting a maintenance pipe for use during maintenance, having an opening communicating with the ground surface, to the end portion on the shaft side of the installed drain pipe.

[0023] Here, the drain material for the drain pipe is a material whose drainage characteristics do not deteriorate during the improvement period, and single substances and composites such as crushed stones, blast furnace slags, and foamed materials with a certain particle size can be used.

[0024] By the way, the target ground often becomes an interbedded ground of fine soil particles such as cohesive soil and sandy silt in addition to sandy soil particles that are the cause of liquefaction. Even in such an interbedded ground, the groundwater level changes up and down due to seasonal variations such as the dry season and the rainy season, and the inside of the drain pipe repeats flooding and drying. In this case, when the inside of the drain pipe is hollow, fine soil particles accumulate during flooding, and the fine soil particles dry and solidify or slime accumulates during the dry season. Along with the above actions, in the worst case, it becomes impossible to remove the solidified soil particles, etc., and the drainage function disappears. Furthermore, soil particles from around the drain pipe can flow into the drain pipe itself, potentially loosening the surrounding ground and causing subsidence of the target ground.

[0025] In this liquefaction prevention method, by filling the drain pipes with drain material, the amount of fine soil particles entering the drain pipes can be reduced, thereby effectively maintaining drainage function and preventing settlement of the target ground.

[0026] Furthermore, this liquefaction prevention method may also be characterized by comprising a drain pipe drain material refilling step, in which, after the maintenance pipe connection step and after a predetermined period of time has elapsed, the drain material for the drain pipe that has been filled is discharged using the maintenance pipe, and the drain pipe is filled (refilled) with the cleaned drain material or new drain material for the drain pipe.

[0027] According to this liquefaction prevention method, by using a maintenance pipe to discharge the drain material that has been filled into the drain pipe, and then filling it with the cleaned drain material or new drain material, it is possible to recover fine soil particles and slime that have accumulated in the drain pipe over time, or to replace the drain material. Therefore, even if the performance of the drain material deteriorates due to aging, etc., it is possible to replace the drain material in a simple and quick manner, so that the drainage function can be effectively maintained over a long period of time, and it is possible to maintain the effect of liquefaction prevention.

[0028] The specified period mentioned above varies depending on the type of ground, but it is usually assumed to be two years or more later, and there may be cases where the drain material refilling process is not required.

[0029] Furthermore, if this liquefaction prevention method includes a shaft drain material filling step, which involves filling the shaft with shaft drain material, after or simultaneously with the maintenance pipe connection step, it becomes possible to easily and effectively fill the shaft with drain material and connect the shaft to the drain pipe in the target ground. [Effects of the Invention]

[0030] According to the present invention, it is possible to provide a liquefaction prevention method that is inexpensive, harmless, and can permanently prevent liquefaction (re-liquefaction) in a short construction period, even in the subsoil where existing structures exist. [Brief explanation of the drawing]

[0031] [Figure 1] This is a schematic side cross-sectional view illustrating the liquefaction prevention method of the present invention, where (a) shows the shaft formation process and (b) shows the press-in device installation process. [Figure 2] This is a side view showing a drain pipe press-fitting device. [Figure 3] (a) and (b) are schematic side cross-sectional views illustrating the drain pipe installation process. [Figure 4] This is a side view showing a portion of the divided drain pipe at the tip cut out. [Figure 5] This is a cutaway side view of a drain pipe, showing the process of injecting lubricating water during the drain pipe installation process. [Figure 6] This is a perspective view showing the drain pipe and drain material unit. [Figure 7] (a) and (b) are schematic side cross-sectional views illustrating the drain material filling process inside the drain pipe. [Figure 8] (a) is a schematic side cross-sectional view showing the maintenance pipe connection process, and (b) is a schematic side cross-sectional view showing the shaft drain material filling process. [Figure 9] (a) and (b) are schematic side cross-sectional views illustrating the drain material refilling process inside the drain pipe. [Figure 10]This is a schematic side cross-sectional view showing a target ground to which the liquefaction prevention method of the present invention has been applied. [Modes for carrying out the invention]

[0032] The following describes in detail an example of an embodiment of this liquefaction prevention method, with reference to the drawings. In the explanation based on the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted. Furthermore, in this embodiment, for the sake of explanation, we will use the simplest case as an example, where a single connected drain pipe is installed horizontally (in the following description, unless otherwise specified, the divided drain pipe will simply be referred to as "drain pipe 10," and the completed drain pipe formed by connecting the drain pipes 10 will be referred to as "connected drain pipe D").

[0033] [This liquefaction prevention method] This liquefaction prevention method targets sandy soil, which is prone to liquefaction, and assumes that the area below where an existing structure S (house) is constructed is designated as the target area for liquefaction prevention (hereinafter referred to as "target area G").

[0034] This liquefaction prevention method comprises (1) a shaft formation process, (2) a press-in device installation process, (3) a drain pipe installation process, (4) a drain material filling process inside the drain pipe, (5) a maintenance pipe connection process, (6) a drain material filling process inside the shaft, and (7) a drain material refilling process inside the drain pipe.

[0035] (1) Shaft formation process This process involves forming a shaft T to the side of the target area G for liquefaction prevention in the target ground (Figure 1(a)). In this process, a shaft T of predetermined dimensions and depth is excavated using an excavation device (not shown), and the surrounding wall is protected by installing steel sheet piles 5 (or earth retaining steel plates) on the surrounding wall, which serve as earth retaining walls and reaction plates for the drain pipe press-in device 40. Furthermore, the dimensions of the shaft T must be such that the drain pipe press-in device 40 can be installed and workers can work on it. Generally, it is preferable for the shaft to be 120 cm or larger.

[0036] (2) Press-fitting device installation process This process involves forming a drain pipe insertion hole T1 at the depth position of the connecting drain pipe D to be installed in the target area G on the wall surface of the shaft T, leveling the bottom of the shaft T, and installing a drain pipe insertion device 40 for pressing in the drain pipe 10 (Figure 1(b)).

[0037] The drain pipe press-fitting device 40 includes a drain pipe press-fitting mechanism, the main component of which is a hydraulic jack 41 for press-fitting the drain pipe 10, and a drain pipe rotation mechanism, the main component of which is a hydraulic motor 45 for rotating the drain pipe 10. The drain pipe press-fitting mechanism and the drain pipe rotation mechanism are mounted on a frame 47. The height of the frame 47 can be adjusted using a level adjustment jack 48 (Figure 2). Since known technologies can be used for the hydraulic jack 41 and hydraulic motor 45, a detailed description will be omitted.

[0038] The hydraulic jack 41 is equipped with a piston rod 42 having a rotation prevention plate 43 at its tip, and the piston rod 42 can move back and forth horizontally by hydraulic pressure. The hydraulic jack 41 is installed so that its base end is in contact with the steel sheet pile 5 which is the reaction plate (reaction surface) on the wall surface facing the drain pipe press hole T1 of the shaft T, and so that the piston rod 42 is oriented in a way that allows it to be inserted into the drain pipe press hole T1 and is aligned with the height of the drain pipe press hole T1.

[0039] Furthermore, a hydraulic motor 45 is provided on the front surface of the anti-rotation plate 43. The hydraulic motor 45 is capable of holding the drain pipe 10 at its front surface and rotating the drain pipe 10 in both forward and reverse directions. With this structure, the drain pipe press-fitting device 40 can move the drain pipe 10 horizontally using a hydraulic jack 41 while rotating the drain pipe 10 in both forward and reverse directions using the hydraulic motor 45, and press-fit it into the target ground G.

[0040] (3) Drain pipe installation process In this process, the drain pipes 10 are sequentially connected from the drain pipe insertion hole T1 in the shaft T and pressed into place until they reach a predetermined horizontal position in the target area G (until they reach the planned length) (Figures 3(a), (b)).

[0041] The drain pipe 10 comprises a steel pipe 11 having rectangular-shaped intake and drainage holes 11a on its circumferential surface and in the axial direction, and a permeable sheet 12 attached to the inner wall of the steel pipe 11 (Figure 6). Furthermore, a drilling bit 15 is provided at the tip of the most advanced drain pipe 10 (hereinafter, the most advanced drain pipe 10 will be referred to as "the most advanced drain pipe 10A").

[0042] The drilling bit 15 has a conical tip 15a, a cylindrical portion 15b with drilling blades 16 attached to its circumferential wall, and a frustoconical base portion 15c, the base portion 15c being connected to the leading edge drain pipe 10A. Each drain pipe 10 (including the leading edge drain pipe 10A) is rotatable together with the drilling bit 15 by the drain pipe press-fitting device 40.

[0043] Furthermore, the rotation radius of the drilling blade 16 is the same as the radius of the drain pipe 10. Furthermore, for ease of installation, the length of the drain pipe 10 is determined to match the dimensions of the shaft and the maximum effective length of the press-fitting stroke of the hydraulic jack 41 of the drain pipe press-fitting device 40.

[0044] Furthermore, a fixed pulley 13 is attached to the inner tip of the most advanced drain pipe 10A for use when filling and discharging the drain material unit 20, and for guiding the lubricating water injection device 50 as needed. This fixed pulley 13 is made of stainless steel and has an internal wire 14 wound around it that is long enough to reach at least the shaft T when the connected drain pipe D is installed (Figure 4).

[0045] In this process, when the drain pipe 10 is pressed into the target ground G, the hydraulic motor 45 rotates the drain pipe 10 in forward and reverse directions, stirring the soil particles with the drilling bit 15 (performing friction cutting), and the rear end of the drain pipe 10 is pressurized by the hydraulic jack 41. At this time, the stirred and loosened soil particles are compacted along the drilling bit 15 and into the circumferential wall of the borehole due to the rotational force and pressing force.

[0046] At this time, depending on the condition of the target ground G, when the drain pipe 10 is pressed in, a suitable amount of lubricating water (lubricating liquid) can be injected into the outer surface of the drain pipe 10 from the end on the shaft T side using the lubricating water injection device 50 (Figure 5).

[0047] The lubricating water injection device 50 is connected to a lubricating water supply pump (not shown) via a hose 52, and is capable of discharging water from a discharge section 51 near its tip. When using the lubricating water injection device 50, one end of the internal wire 14 wound around the fixed pulley 13 of the leading edge drain pipe 10A is connected to the tip of the lubricating water injection device 50, and the other end of the internal wire 14 is pulled towards the ground surface by human power or a pulling device (not shown), thereby moving the lubricating water injection device 50 to a predetermined position inside the drain pipe 10 and discharging water from the discharge section 51 (the discharged lubricating water is injected into the outer side surface of the drain pipe 10 through the suction and drainage holes 11a of the drain pipe 10).

[0048] Each drain pipe 10 is joined by connecting the end of the previously pressed-in drain pipe 10 with the tip of the drain pipe 10 to be pressed in using a connecting fitting (not shown). The same procedure is then repeated until the planned length is reached.

[0049] Furthermore, in order to connect the maintenance pipe 30, the end portion 10a of the drain pipe 10 closest to the shaft T (Figure 8(a), (b)) is left slightly protruding from the drain pipe press-fit hole T1. In addition, to prevent the connecting drain pipe D from coming out together when the drain material unit 20 is withdrawn during the shaft drain material filling process and the drain material refilling process in the drain pipe described below, the drain pipe press-fit hole T1 and the vicinity of the end portion 10a of the drain pipe 10 closest to the shaft T are joined by welding or the like.

[0050] (4) Drain material filling process inside the drain pipe This process involves filling the inside of the connected drain pipe D with a drain material unit for drain pipes (hereinafter referred to as "drain material unit 20") (Figure 6), which contains crushed stone 23 (drain material for drain pipes) inside (Figures 7(a), (b)).

[0051] The drain material unit 20 comprises a perforated underground drain pipe 21 (underground buried permeable pipe) made of synthetic resin with a predetermined length and a flat inner surface, and crushed stone materials 23 that constitute the drain material for the drain pipe, which are filled into a cylindrical metal mesh body 22 (Figure 7). The crushed stone materials 23 are made up of materials that have a high permeability coefficient and do not deteriorate over time, and include at least one of the following: crushed stone, recycled crushed stone, blast furnace slag, and permeable foamed material, all of a predetermined particle size.

[0052] Multiple rectangular holes 21a are formed on the wall surface of the perforated drainage pipe 21. Therefore, the interpore water from the target ground G that flows in from the suction and drainage holes 11a of the drain pipe 10 can be guided to the mesh structure 22 through the rectangular holes 21a. Furthermore, a conical wire attachment section 21b is provided at the tip of the perforated drainage pipe 21.

[0053] In this process, first, the drain pipe press-in device 40 is removed from the shaft T. Next, one end of the internal wire 14, which is wound around the fixed pulley 13 of the leading drain pipe 10A, is connected to the wire attachment portion 21b at the tip of the perforated pipe 21 in the drain material unit 20. Then, the other end of the internal wire 14 is pulled towards the ground surface by hand or by a pulling device (not shown), thereby inserting the drain material unit 20 into the connecting drain pipe D and filling the inside of the connecting drain pipe D with the drain material unit 20. The internal wire 14 is left in place as it will be used when refilling the drain material unit 20 during the drain material refilling process inside the drain pipe.

[0054] (5) Maintenance pipe connection process This process involves connecting a maintenance pipe 30, whose opening 30a communicates with the ground surface, to the end of the connected drain pipe D on the shaft T side (Figure 8(a)).

[0055] In this process, a maintenance pipe 30 is connected to the end portion 10a of the drain pipe T protruding into the shaft T, and the pipe is positioned so that the opening 30a at the end reaches the ground surface.

[0056] The maintenance pipe 30 is used in the drain material refilling process described below, by passing through the pipe to discharge and refill the drain material unit 20. Therefore, it is necessary for the pipe to have a larger diameter than the perforated drain pipe 21 of the drain material unit 20 (in this embodiment, a drain pipe made of synthetic resin is used). In addition, to facilitate the discharge and refilling of the drain material unit 20, the maintenance pipe 30 has a curved section at the connection point between the shaft T and the connecting drain pipe D, and is positioned to have a gentle slope. After the work is completed, a cover 31 is placed over the opening 30a.

[0057] (6) Drain material filling process inside the shaft This process involves filling the shaft T with shaft drain material 25, and backfilling the shaft T to the ground surface while compacting the shaft drain material 25 (Figure 8(b)). The shaft drain material 25 is composed of crushed stone, recycled crushed stone, blast furnace slag, and permeable foam material, all of a predetermined particle size, which have a high permeability coefficient and do not deteriorate over time. The shaft drain material 25 may be the same type of crushed stone as the crushed stone 23 of the drain material unit 20, or it may be a different type of crushed stone.

[0058] Furthermore, if any problems occur with the press-fitted connecting drain pipe D, it is possible to address the issue by redrilling the shaft T, which is filled with the shaft drain material 25.

[0059] (7) Drain material refilling process inside the drain pipe This process involves, after a predetermined period of time has elapsed, discharging the filled drain material unit 20 to the ground surface using the shaft T, and then refilling it with the cleaned drain material unit 20'.

[0060] In this process, first, the opening 30a of the maintenance pipe 30 exposed on the ground surface is opened, and a sample of the crushed stone 23 from the drain material unit 20 is taken to check for the presence or absence of fine soil particles and slime. If fine soil particles or the like are attached to the crushed stone 23, the drainage performance of the drain material unit 20 is reduced, and the drain material unit 20 is removed using the following method.

[0061] First, to ensure smooth operation, a work wire 61 is connected to the rear end of the drain material unit 20 (reference numeral 21c indicates the attachment point), and an extension wire 62 is connected to the end of the internal wire 14. Next, the work wire 61 is pulled out towards the ground by hand or by a pulling device, and the drain material unit 20 is removed (discharged) to the ground (Figure 9(a)).

[0062] Next, if the removed drain material unit 20 is undamaged, the crushed stone 23 is washed with water. If the drain material unit 20 is damaged, new crushed stone is placed in the mesh and a drain material for the drain pipe (not shown) is prepared.

[0063] Subsequently, by pulling the extension wire 62 towards the ground surface, the drain material unit 20', which has been washed and cleaned, is pushed through the maintenance pipe 30 towards the drain pipe D, thereby refilling the inside of the connecting drain pipe D (Figure 9(b)). Then, the extension wire 62 is removed from the insertion wire 14, and the opening 30a of the maintenance pipe 30 is closed.

[0064] [Effects and benefits of this liquefaction prevention method in target area G] In the target area G to which this liquefaction prevention method is applied, there is a connected drain pipe D filled with a drain material unit 20 containing crushed stone 23 (drain material for drain pipes) that has water permeability, and a shaft T filled with shaft drain material 25 that is connected to the connected drain pipe D (Figure 10). Therefore, when an earthquake occurs in the target area G, pore water between soil particles flows into the drain pipe 10 through the intake and drainage holes 11a of the connected drain pipe D, passes through the rectangular holes 21a of the perforated underground pipe 21, passes between the crushed stone 23, and is discharged into the shaft T installed at the end of the connected drain pipe D. In this way, the rise in excess pore water pressure, which is the cause of liquefaction in the target ground G, can be quickly dissipated.

[0065] Furthermore, since the shaft T is filled with a shaft drain material 25 containing crushed stone 23, it is easier to excavate compared to the target ground G. Therefore, if any problem occurs with the press-fitted drain pipe 10, it is possible to address the issue by re-excavating the shaft T.

[0066] [Effects and Effects of This Liquefaction Prevention Method] According to this liquefaction prevention method, drain pipes 10 are sequentially connected from a shaft T and pressed into the target area G, and drain material units 20 are filled inside the drain pipes 10 to prevent liquefaction. Therefore, even in the subsoil where existing structures S exist, liquefaction prevention measures can be implemented without changing the current conditions.

[0067] Furthermore, the drilling bit 15, which has drilling blades 16 attached to the tip of the leading-edge drain pipe 10A to be pressed in, is rotatably mounted. During the drain pipe installation process, friction can be reduced by rotating the drilling bit 15, allowing the compacted and saturated sandy soil to be agitated. This makes it possible to efficiently insert the drain pipe 10, and by pressing the agitated ground against the wall of the borehole, it is possible to prevent voids from forming in the target ground G.

[0068] Furthermore, since the drain pipe 10 includes a steel pipe 11 with numerous intake and drainage holes 11a on its circumferential surface and in the axial direction, and a permeable sheet 12 attached to the inner wall of the steel pipe 11, it can be made into a structure that provides water permeability simply and effectively.

[0069] Furthermore, by filling the inside of the drain pipe 10 with the drain material unit 20, the amount of fine soil particles entering the connected drain pipe D can be reduced, thereby effectively maintaining the drainage function and preventing the settlement of the target ground G. Furthermore, the crushed stone 23 that constitutes the drain material unit 20 is filled into the perforated underground pipe 21 and the mesh body 22. Therefore, the filling of crushed stone 23 in the drain material filling process for drain pipes, and the discharge and refilling of crushed stone 23 in the drain material refilling process inside drain pipes can be carried out efficiently.

[0070] Furthermore, by using the shaft T to discharge the filled drain material unit 20 to the outside and filling it with a cleaned drain material unit 20' or a new drain material unit, it is possible to recover slime that has accumulated over time in the drain pipe 10 or replace the drain material unit 20. Therefore, even if the performance of the crushed stone 23 deteriorates due to aging or other reasons, the drain material unit 20 can be replaced easily and in a short amount of time, thus maintaining the liquefaction prevention effect for a long period of time.

[0071] Furthermore, since the system includes a shaft drain material filling process in which shaft drain material 25 is filled into the shaft T after the maintenance pipe connection process, it is possible to easily and effectively fill the shaft T with shaft drain material 25 in the target ground G and connect the shaft T to the drain pipe 10.

[0072] Although an example of a preferred embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and the design of each element can be modified as appropriate without departing from the spirit of the present invention.

[0073] The liquefaction prevention method described in this embodiment is merely an example, and can be implemented in an appropriate manner depending on the construction site. Therefore, the structure, material, shape, dimensions, etc., of each component in the above description can be appropriately determined according to the construction site. Furthermore, this liquefaction prevention method only illustrates the minimum necessary components, and other necessary components may be added as long as they do not hinder the effects of the present invention.

[0074] Furthermore, while this liquefaction method is particularly useful in confined spaces and areas approximately 3 meters below the surface where liquefaction is said to frequently occur, there are no restrictions on the construction area of ​​the target ground or the size of the drain pipe, so it can be applied to large-scale target ground and at great depths. Furthermore, this liquefaction method can be implemented in the same way as the existing structures, regardless of the type of structure. This includes preventing surface liquefaction in existing structures that do not have fixed piles, as well as in existing structures that do have solid piles, and preventing steps caused by liquefaction directly beneath paved surfaces such as parking lots and roads, and in bridge abutment access roads.

[0075] Furthermore, the above embodiment described an example where a single connecting drain pipe is installed horizontally. However, various arrangements of connecting drain pipes are possible, such as installing multiple connecting drain pipes side by side at the same depth, laying multiple connecting drain pipes in different depth directions, or using both in combination.

[0076] Furthermore, in the above embodiment, the case in which a shaft is formed laterally in the area to be treated for liquefaction prevention measures was described as an example. However, it is also possible to form a shaft within the area to be treated, form multiple drain pipe insertion holes in the wall surface of the shaft, and insert drain pipes through these holes in multiple directions within the area to be treated.

[0077] Furthermore, in the above embodiment, after the drain pipe installation process, the drain material is filled into the drain pipe during the drain material filling process. However, the process of filling drain material inside the drain pipes may be omitted (the drain pipe installation process and the drain material filling process inside the drain pipes may be performed simultaneously) by pressing the divided drain pipes, which are pre-filled with drain material, into the target ground. In this case, the drain material for the drain pipes that is filled in each divided drain pipe can be connected to adjacent drain pipes, making it easier to discharge and refill the drain material for the drain pipes during the drain material filling process inside the drain pipes.

[0078] Furthermore, in the above embodiment, the shaft drain material filling process is performed after the maintenance pipe connection process, but the shaft drain material filling process may be performed simultaneously with the maintenance pipe connection process. Furthermore, there are no restrictions on the method of filling the drain material for drain pipes, the method of refilling the drain material for drain pipes, or the form of the drain material for drain pipes and the drain material unit for drain pipes.

[0079] G Target area S Existing structure T shaft T1 Drain pipe press-fit hole D Connecting drain pipe 10. Drain pipe (divided drain pipe) 10A State-of-the-art drain pipe 11a Suction and drainage hole 13 Fixed pulley 14. Insertion wire 15 drilling bits 16 drilling blades 20,20' Drain material unit (Drain material unit for drain pipes) 21 Underdrain perforated pipe 22 Reticular 23. Crushed stone (drain material for drain pipes) 25 Drain material for vertical shafts 30 Maintenance pipes 40. Drain pipe press-fitting device 41 Hydraulic jack 42 Piston Rod 45 Hydraulic motor 50 Lubricating water injection device 61 Work Wire 62 Extension wires

Claims

1. A liquefaction prevention method characterized by including the following steps. (1) A shaft formation step in which a shaft is formed within or to the side of the area to be treated for liquefaction prevention measures. (2) A step of installing a drain pipe press-in device, in which a drain pipe press-in hole is formed in the wall surface at a predetermined depth position of the shaft and a drain pipe press-in device is installed for press-in the drain pipe. (3) A drain pipe installation step, in which the divided drain pipes are pressed into the drain pipe press-fit hole of the shaft and joined together in sequence, thereby being installed to a predetermined position in the horizontal or diagonal direction in the target area. (4) A maintenance pipe connection step, in which a maintenance pipe having an opening that communicates with the ground surface is connected to the shaft-side end of the installed drain pipe.

2. The liquefaction prevention method according to claim 1, characterized in that, after the drain pipe installation step, a drain pipe internal drain material filling step is performed, in which drain material for drain pipes is filled inside the joined drain pipe.

3. A liquefaction prevention method characterized by including the following steps. (1) A shaft formation step in which a shaft is formed within or to the side of the area to be treated for liquefaction prevention measures. (2) A step of installing a drain pipe press-in device, in which a drain pipe press-in hole is formed in the wall surface at a predetermined depth position of the shaft and a drain pipe press-in device is installed for press-in the drain pipe. (3) A drain pipe installation step, in which the divided drain pipes, which are filled with drain material for drain pipes, are pressed into the drain pipe press-fit hole of the shaft and joined together in sequence, thereby installing them to a predetermined position in the horizontal or diagonal direction in the target area. (4) A maintenance pipe connection step, in which a maintenance pipe having an opening that communicates with the ground surface is connected to the shaft-side end of the installed drain pipe.

4. After the maintenance pipe connection process, and after a predetermined period of time has elapsed, The liquefaction prevention method according to claim 2 or 3, further comprising a drain pipe drain material refilling step, in which the drain material filled in the drain pipe is discharged using the maintenance pipe, and the drain material is refilled with the cleaned drain material or new drain material.

5. The liquefaction prevention method according to claim 2 or 3, further comprising a shaft drain material filling step, which involves filling the shaft with shaft drain material, after or simultaneously with the maintenance pipe connection step.

6. The liquefaction prevention method according to any one of claims 1 to 3, characterized in that the tip of the divided drain pipe at the far end opposite to the shaft, used in the drain pipe installation process, is provided with a conical bit with spiral wings.

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