Active control device for shield construction disturbance in sandy gravel stratum and grouting method

Abstract

The invention discloses a sandy gravel stratum shield construction disturbance active control device and a grouting method, and relates to the technical field of shield construction grouting. Comprising a head sealing pipe, at least one connector pipe, sleeve valve pipes and a bag, every two sleeve valve pipes are fixed in a threaded connection mode, the connector pipes and the head sealing pipe are both in threaded connection with the sleeve valve pipes, the number of the connector pipes is at least one, grouting devices are inserted into the sleeve valve pipes and slide in the sleeve valve pipes, and the two ends of the bag are fixed to the positions of the connector pipes and the head sealing pipe respectively. The bag and the end socket pipe are fixed through a first hose clamp. By means of the grouting device and the using method thereof, the problem of connection of an existing sleeve valve pipe and a bag is solved, stratum disturbance deformation of tunnel shield construction is controlled in a staged and regional mode, the sleeve valve pipe of the existing specification can be directly purchased, a single mold is not needed, application and popularization are easy, connection of the sleeve valve pipe and the bag is reliable, and construction efficiency is improved. The fastening force can be increased, and connection disengagement in the construction process is avoided.

Description

Technical Field

[0001] The invention relates to the technical field of shield construction grouting, in particular to a sand and gravel stratum shield construction disturbance active control device and a grouting method. Background Art

[0002] The shield construction process will inevitably cause disturbance to the surrounding soil, and the loss of soil during the construction process will cause surface settlement and stress changes in the surrounding soil. In order to control surface settlement and soil disturbance, as a post-construction remedial measure, compensatory grouting aims to compensate for the soil loss caused by tunnel excavation, stabilize the tunnel and raise the surface. Water-rich sand and gravel strata are common strata in underground engineering construction. They have the characteristics of loose structure, high permeability and large porosity. They are a typical mechanically unstable stratum. In this stratum, grouting materials are easily diluted when they come into contact with water, and they lose a lot. They are unconstrained and have poor grouting effects. The use of bladders can constrain the scope of grouting. However, the pebble layer is different from soft soil. To achieve the squeezing effect, a larger grouting pressure is generally required. At this time, the connection between the bladder and the grouting device is a key issue; at the same time, the location and time selection of grouting are also difficult to control stratum deformation. At present, it is necessary to develop a device that can be used for fixed-point and multiple grouting, and a construction method for regional grouting in shield construction.

[0003] Regarding the combined use of bladder bags and grouting, (CN105672940A) discloses a grouting water stopper with a membrane bag outside the grouting hole; (CN116220736A) discloses a membrane bag sleeve valve tube secondary grouting device and construction process for dynamic water sealing of karst tunnels; (CN108411920A) discloses a multi-point bladder grouting device and method for controlling soil deformation; (CN113638398A) discloses a bladder grouting soil stress active dynamic control method. The above disclosed technologies involve the principle application of bladder bags and sleeve valve tubes, but do not solve the problem of how to achieve a reliable connection between the bladder bag and the sleeve valve tube, or simply wire binding. However, when the diameter of the bladder bag reaches 250mm and the grouting pressure reaches 2MPa, the reliable connection between the bladder bag and the sleeve valve tube is the core of the problem. The existing sleeve valve tube has a diameter of 48mm. The diameter difference between the bladder bag and the sleeve valve tube is large, so it cannot be tied tightly, causing leakage; the sleeve valve tube is made of PVC material, which has a small bearing capacity and cannot withstand a large binding force; the sleeve valve tube joint is a threaded connection. When the length of the bladder reaches 20m, the joint will fall off when it is lowered vertically.

[0004] Bladder grouting technology can be used to control ground deformation. (CN116641391B) discloses a double-row bladder grouting method for controlling deformation of foundation pits close to tunnels. Multiple rows of bladder bags of different depths are used to grout and control deformation at different excavation stages of the foundation pit, and secondary grouting bags are reserved to achieve multiple grouting. However, the cost of this technical solution is relatively high, and it cannot be implemented in sites with limited space. In addition, no specific connection scheme between the bladder bag and the grouting device is given, only the application scenario of bladder grouting.

[0005] The existing technical solutions do not provide the connection between the bladder bag and the grouting device and the position and time of grouting during shield construction, so the bladder grouting problem of controlling the disturbance of the sand and gravel stratum by the shield construction cannot be solved. Summary of the invention

[0006] The purpose of the present invention is to provide a device for actively controlling disturbance during shield construction in sandy and gravel strata and a grouting method to solve the problems raised in the above-mentioned background technology.

[0007] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: an active control device for disturbance in shield construction in sand and gravel strata, comprising: a head pipe, a joint pipe, a sleeve valve pipe and a bladder bag; a plurality of sleeve valve pipes are provided, and every two sleeve valve pipes are screwed and fixed, the joint pipe and the head pipe are screwed with the sleeve valve pipe, and at least one joint pipe is provided, a grouting device is inserted into the sleeve valve pipe and slides in the sleeve valve pipe, two ends of the bladder bag are respectively fixed at the joint pipe and the head pipe, the bladder bag is fixed to the head pipe with a throat clamp 1, and the bladder bag is fixed to the joint pipe with a throat clamp 2.

[0008] Furthermore, at least three grooves are provided at the sealing pipe to facilitate positioning of the first throat clamp, and at least three grooves are provided at the joint pipe to facilitate positioning of the second throat clamp.

[0009] Furthermore, one end of the sleeve valve tube is provided with an external thread one, and the other end of the sleeve valve tube is provided with a thickened threaded joint that can be threadedly fixed with the external thread one.

[0010] Furthermore, two ends of the grouting device are respectively screwed with two tightening nuts one and two tightening nuts, wherein a gap is arranged between the tightening nuts one, a rubber plug sleeved on the surface of the grouting device is arranged in the gap, and a rubber head is fixed to the tightening nut two, the rubber head is conical in shape, and the narrower end faces one end of the head pipe, and at least two slurry outlets are opened on the grouting device, and the slurry outlets are located between the rubber head and the rubber plug.

[0011] Furthermore, one end of the sealing pipe connected to the sleeve valve pipe is provided with internal thread 2, which is connected to external thread 1 of the sleeve valve pipe. Both ends of the connecting pipe are respectively provided with external thread 2 and internal thread 1, which is threadedly connected to the thickened part of the threaded joint of the sleeve valve pipe, and the internal thread 1 is threadedly connected to the external thread of the sleeve valve pipe.

[0012] The grouting method of the active control device for disturbance of shield construction in sandy and gravel strata adopts the above-mentioned active control device for disturbance of shield construction in sandy and gravel strata, and comprises the following steps: step one: determine the drilling arrangement and drill holes, and determine the main control area A and the secondary control area B; step two: make the bladder grouting device; step three: lower the bladder grouting device; step four: grouting construction. During the shield construction, synchronous grouting is carried out after the shield tail of the shield machine is separated from the pipe segment. The grouting effect of the water-rich pebble layer is poor, and grouting of the main control area A is carried out at this time.

[0013] Furthermore, in the step 1, determining the drilling arrangement requires delineating the shield construction control sensitive area, setting the soil volume loss rate, performing finite element simulation on the tunnel construction, and determining the depth of the main control area A and the secondary control area B above the tunnel within the tunnel depth range based on the deformation; in the step 1, the bottom elevation of the main control area A should be at least 1.0m below the tunnel bottom, and the top elevation of the main control area A should be at least 1.0m above the tunnel top.

[0014] Furthermore, the drilling depth in step one needs to be determined according to the vertical position of the bag, and the over-drilling depth of the drilling should not be less than the length of the head pipe. From the cross-sectional perspective of the sensitive area of ​​shield construction, the drilling position is located on both sides of the tunnel.

[0015] Furthermore, during the manufacturing process of the bladder grouting device, it is necessary to determine the position of the joint pipe according to the depth of the main control area A and the secondary control area B delineated in step one, one joint pipe is located at the depth intersection of the main control area A and the secondary control area B, and another joint pipe is located at the top elevation of the secondary control area B.

[0016] Furthermore, the lowering of the bladder grouting device in step three includes the following steps: P: using a car crane to turn over and erect the bladder grouting device; P: moving the bladder grouting device to the drilling position and lowering it, and after the lowering is completed, the gaps in the hole are backfilled with fine sand; P: after the bladder bag is lowered, it is stabilized for at least three days, during which time the gaps in the hole are backfilled with fine sand, and the gaps are filled until they can no longer be filled.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The bladder grouting device and method for controlling the disturbance of sand and gravel strata during shield construction solves the problem of the connection between the existing sleeve valve tube and the bladder bag through the grouting device and the use method thereof, and realizes the stage-by-stage and regional control of the disturbance deformation of the stratum during tunnel shield construction. The technology of the present invention can directly purchase sleeve valve tubes of existing specifications without the need for a separate mold, and is easy to promote and apply. The sleeve valve tube and the bladder bag are reliably connected, and the tightening force can be increased to avoid connection separation during construction. The stage-by-stage and regional control of the disturbance deformation of the stratum can be realized, and complex control can be achieved with a simple process. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the bladder grouting device of the present invention; Figure 2 It is a schematic diagram of the disassembled structure of the bladder grouting device of the present invention; Figure 3 It is a schematic diagram of grouting in shield construction of the present invention.

[0019] In the figure: 1. bladder bag; 2. sleeve valve tube; 201. external thread one; 202. thickened threaded joint; 3. joint tube; 301. external thread two; 302. internal thread one; 4. end tube; 401. internal thread two; 5. throat clamp one; 6. throat clamp two; 7. grouting device; 701. slurry outlet; 702. tightening nut one; 703. rubber plug; 704. tightening nut two; 705. rubber end tube; 8. stratified sedimentation instrument. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] like Figure 1 and Figure 2 As shown, the present invention provides a technical solution: an active control device for disturbance in shield construction in sand and gravel strata, comprising a head pipe 4, a joint pipe 3, a sleeve valve pipe 2 and a bag 1.

[0022] The sleeve valve tube 2 is provided with several sections, and every two sleeve valve tubes 2 are screwed and fixed, the joint tube 3 and the head tube 4 are screwed with the sleeve valve tube 2, and the joint tube 3 is provided with at least one, the grouting device 7 is inserted into the sleeve valve tube 2 and slides in the sleeve valve tube 2, the two ends of the bag 1 are respectively fixed at the positions of the joint tube 3 and the head tube 4, and are fixed to the head tube 4 with a throat clamp 5, and are fixed to the joint tube 3 with a throat clamp 6, wherein the sleeve valve tube 2 is a grouting equipment, which consists of a tube body and a one-way sleeve valve to realize a one-way grouting function. In this embodiment, the sleeve valve in the sleeve valve tube 2 is arranged near the external thread 201. Since the sleeve valve is in the tube body, it is a prior art and is not shown in the figure.

[0023] In the above embodiment, the grouting device 7 can be moved to each sleeve valve pipe 2 and perform the grouting action.

[0024] like Figure 2As shown, in order to ensure the smooth implementation of the above embodiment, it is necessary to understand that at least three grooves are provided on the outer surface of the sealing tube 4 to facilitate the positioning of the throat clamp 5 and to facilitate the throat clamp 5 to bind and fix the bag 1. At least three grooves are also provided on the joint pipe 3 to facilitate the positioning of the throat clamp 6 and to facilitate the throat clamp 26 to bind and fix the bag 1. That is to say, the two ends of the bag 1 are fixed to the sealing tube 4 and the joint pipe 3 respectively by the throat clamp 1 5 and the throat clamp 2 6. In this case, the bag 1 is in a basically sealed state.

[0025] like Figure 2 As shown, in order to ensure the smooth implementation of the above embodiment, it should be understood that one end of the sleeve valve tube 2 is provided with an external thread 201, and the other end is provided with a thickened threaded joint 202 which can be screwed and fixed with the external thread 201, that is, every two sleeve valve tubes 2 can be screwed and fixed.

[0026] like Figure 2 As shown, what needs to be understood about this solution is that two ends of the grouting device 7 are respectively screwed with two tightening nuts 1 702 and tightening nuts 2 704, wherein a gap is provided between the two tightening nuts 1 702, and a rubber plug 703 sleeved on the surface of the grouting device 7 is provided in the gap, and a rubber seal 705 is fixed to the tightening nut 2 704, and the rubber seal 705 is conical in shape, and the narrower end faces one end of the seal pipe 4, and sealing can be achieved between the rubber plug 703 and the rubber seal 705, and at least two slurry outlets 701 are provided on the grouting device 7, and the outlet The slurry outlet 701 is located between the rubber seal 705 and the rubber plug 703, wherein the grouting device 7 is sealed at one end facing the seal pipe 4, and is connected to the grouting equipment at the end away from the seal pipe 4. In this case, when the grouting device 7 performs grouting, the slurry only spreads between the rubber seal 705 and the rubber plug 703, and the grouting device 7 is located in the sleeve valve tube 2. That is to say, when the slurry overflows from the grouting device 7, the slurry overflows in the sleeve valve tube 2, and the area in the sleeve valve tube 2 between the rubber seal 705 and the rubber plug 703 is sealed.

[0027] like Figure 2 As shown, in order to ensure the smooth implementation of this embodiment, it is also necessary to know that one end of the sealing tube 4 connected to the sleeve valve tube 2 is provided with an internal thread 2 401, and the internal thread 2 401 is connected to the external thread 1 201 of the sleeve valve tube 2. The two ends of the connecting tube 3 are respectively provided with an external thread 2 301 and an internal thread 1 302. The external thread 2 301 is threadedly connected to the thickened thread joint 202 of the sleeve valve tube 2, and the internal thread 1 302 is threadedly connected to the external thread 1 201 of the sleeve valve tube 2. Through this structure, the connection between the sleeve valve tube 2, the connecting tube 3 and the sealing tube 4 can be realized.

[0028] In summary, the bladder grouting device can realize segmented grouting and adjustment of the grouting position. After the sleeve valve tube 2, the joint tube 3 and the head tube 4 are connected and fixed, the grouting device 7 can slide in the inner wall of the entire sleeve valve tube 2 and perform grouting at any time. After the grouting device 7 discharges the slurry, the slurry overflows from the grouting device 7 and spreads to the bladder bag 1 through the one-way overflow effect of the sleeve valve tube 2, thereby filling the bladder bag 1.

[0029] by Figure 3 As shown, based on the above-mentioned capsule grouting device, we propose a new grouting method for active control of disturbance of shield construction in sandy and gravel strata, which includes the following steps: Step 1: Determine the drilling layout and drill the holes.

[0030] The shield construction control sensitive area is delineated, and then the soil volume loss rate is set. The finite element simulation of the tunnel construction is carried out, and the depth of the main control area A within the tunnel depth range and the secondary control area B above the tunnel are determined according to the deformation.

[0031] Regarding step one, in the gravel stratum, due to its strong permeability, the change of groundwater level will affect the stability of the stratum. Therefore, combined with hydrogeological data, with the tunnel as the center, a sensitive area is delineated along a certain range in the longitudinal and lateral directions of the tunnel. In the longitudinal direction, it starts from 3-5 meters in front of the tunnel face and extends backward to 6-10 meters behind the shield tail. In the lateral direction, it extends 1-3 meters on both sides from the outer contour of the tunnel as the sensitive area range.

[0032] Drilling uses numerical simulation software (such as FLAC-3D or MIDAS-GTS, etc.) to establish a shield tunnel construction model, taking into account various parameters of the gravel stratum, including particle size distribution, porosity, internal friction angle, cohesion, etc., and simulating the stratum deformation and structural stress conditions under different soil volume loss rates. The soil volume loss rate is set to different values ​​such as 0.5%, 1%, 1.5%, and 2%, and the corresponding surface settlement values, tunnel convergence values, and the response of surrounding buildings and pipelines are observed. The depth of the main control area A within the tunnel depth range and the secondary control area B above the tunnel are planned and drilled accordingly.

[0033] Among them, the bottom elevation of the main control area A should be at least 1.0m below the bottom of the tunnel, and the top elevation of the main control area A should be at least 1.0m above the top of the tunnel.

[0034] In step 1, the drilling depth needs to be determined according to the vertical position of the bag 1, and the over-drilling depth of the drilling should not be less than the length of the head pipe 4. The plane position of the drilling hole is set on both sides of the tunnel, and the drilling diameter is 150mm. That is to say, from the cross-sectional perspective of the sensitive area of ​​shield construction, the drilling position is located on both sides of the tunnel.

[0035] Step 2: Preparation of bladder grouting device.

[0036] F1: Determine the position of the joint pipe 3 according to the depth of the main control area A and the secondary control area B delineated in step 1. One joint pipe 3 is located at the depth boundary of the main control area A and the secondary control area B, and another joint pipe 3 is located at the top elevation of the secondary control area B. That is, the main control area A and the secondary control area B are separated into two independent filling areas A and B by the bladder grouting device, which control the deformation of the main control area A and the secondary control area B respectively.

[0037] F2: Connect the sleeve valve tube 2 and the joint tube 3, insert the bottom end of the connected sleeve valve tube 2 into the sealing tube 4, set three throat clamps 5 in the groove position of the sealing tube 4, tighten them, and set three throat clamps 2 6 in the groove of the joint tube 3, tighten them.

[0038] Step 3: Lower the bladder grouting device.

[0039] P1: Use a truck crane to turn over and erect the bladder grouting device.

[0040] P2: Move the bladder grouting device to the drilling position and lower it. After lowering, fill the gaps in the hole with fine sand.

[0041] P3: After the bag 1 is lowered, it is recommended to stabilize it for at least three days. During this period, fill the gaps in the hole with fine sand several times until the gaps can no longer be filled, ensuring that the bag 1 is in close contact with the surrounding soil without affecting the control effect.

[0042] Step 4: Grouting construction. During shield construction, synchronous grouting will be carried out after the tail of the shield machine is separated from the segment. The grouting effect of the water-rich pebble layer is poor, so grouting in the main control area A is carried out at this time.

[0043] G1: Insert the grouting device 7 into the sleeve valve tube 2 to perform grouting in the bladder bag 1 at the depth of the main control area A.

[0044] G2: After the grouting is completed, observe the monitoring data of the stratified settlement instrument 8. When the soil settlement above the tunnel occurs, grouting is carried out in the secondary control area B.

[0045] After each grouting is completed, the sleeve valve pipe 2 is cleaned to facilitate subsequent grouting.

[0046] During the start-up of bladder grouting and within two days after the completion of grouting, the monitoring frequency should be maintained at once every two hours.

[0047] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is limited by the attached embodiments and their equivalents.

Claims

1. An active control device for disturbance during shield construction in sandy and gravel strata, characterized in that: include: A sealing pipe (4), a joint pipe (3), a sleeve valve pipe (2) and a bag (1); a plurality of sleeve valve pipes (2) are provided, and every two sleeve valve pipes (2) are screwed and fixed to each other; the joint pipe (3) and the sealing pipe (4) are both screwed to the sleeve valve pipe (2), and the joint pipe (3) is provided with at least one; a grouting device (7) is inserted into the sleeve valve pipe (2) and slides in the sleeve valve pipe (2); two ends of the bag (1) are respectively fixed to the joint pipe (3) and the sealing pipe (4); the bag (1) and the sealing pipe (4) are fixed by a first throat clamp (5), and the bag (1) and the joint pipe (3) are fixed by a second throat clamp (6); The two ends of the grouting device (7) are respectively provided with a rubber plug (703) and a rubber sealing head (705); the grouting device (7) is sealed at one end facing the sealing head pipe (4), and the end away from the sealing head pipe (4) is connected to the grouting equipment; After the sleeve valve tube (2), the joint tube (3) and the sealing tube (4) are connected and fixed, the grouting device (7) slides in the inner wall of the entire sleeve valve tube (2) and performs segmented grouting at any time. When the slurry overflows from the grouting device (7), the area between the rubber sealing head (705) and the rubber plug (703) in the sleeve valve tube (2) is sealed, and the slurry overflows from the grouting device (7) and flows through the sleeve valve tube (2) into the bag (1).

2. The active control device for disturbance during shield construction in sandy and gravel strata according to claim 1 is characterized by: The sealing pipe (4) is provided with at least three grooves for positioning the first throat clamp (5), and the joint pipe (3) is also provided with at least three grooves for positioning the second throat clamp (6).

3. The active control device for disturbance during shield construction in sandy and gravel strata according to claim 1 is characterized by: One end of the sleeve valve tube (2) is provided with an external thread one (201), and the other end of the sleeve valve tube (2) is provided with a thickened threaded joint (202) that can be threadedly fixed to the external thread one (201).

4. The active control device for disturbance during shield construction in sandy and gravel strata according to claim 1 is characterized by: Two tightening nuts (702) and two tightening nuts (704) are respectively screwed at both ends of the grouting device (7), wherein a gap is provided between the tightening nuts (702), a rubber plug (703) is provided at the gap and sleeved on the surface of the grouting device (7), a rubber seal (705) is fixed at the tightening nut (704), the rubber seal (705) is in a cone shape, and the narrower end faces one end of the seal pipe (4), and at least two slurry outlets (701) are provided on the grouting device (7), and the slurry outlets (701) are located between the rubber seal (705) and the rubber plug (703).

5. The active control device for disturbance during shield construction in sandy and gravel strata according to claim 3 is characterized by: One end of the sealing tube (4) connected to the sleeve valve tube (2) is provided with an internal thread No. 2 (401), and the internal thread No. 2 (401) is connected to an external thread No. 1 (201) of the sleeve valve tube (2). Both ends of the joint tube (3) are respectively provided with an external thread No. 2 (301) and an internal thread No. 1 (302), and the external thread No. 2 (301) is threadedly connected to the thickened thread joint (202) of the sleeve valve tube (2), and the internal thread No. 1 (302) is threadedly connected to the external thread No. 1 (201) of the sleeve valve tube (2).

6. A grouting method for a sand and gravel stratum shield construction disturbance active control device, using the sand and gravel stratum shield construction disturbance active control device according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: Step 1: determining the arrangement of drilling holes and drilling holes, determining the main control area A and the secondary control area B; Step 2: manufacturing the bladder grouting device; Step 3: lowering the bladder grouting device; Step 4: grouting construction. During the shield construction, synchronous grouting is carried out after the shield tail of the shield machine is separated from the segment. The grouting effect of the water-rich pebble layer is poor. At this time, grouting is carried out in the main control area A.

7. The grouting method of the active control device for disturbance of shield construction in sandy and gravel strata according to claim 6 is characterized by: In the step 1, the drilling arrangement needs to be determined by delineating the shield construction control sensitive area, setting the soil volume loss rate, performing finite element simulation on the tunnel construction, and determining the depth of the main control area A and the secondary control area B above the tunnel within the tunnel depth range based on the deformation; in the step 1, the bottom elevation of the main control area A should be at least 1.0m below the tunnel bottom, and the top elevation of the main control area A should be at least 1.0m above the tunnel top.

8. The grouting method of the active control device for disturbance of shield construction in sandy and gravel strata according to claim 6 is characterized by: The drilling depth in step 1 needs to be determined according to the vertical position of the bag (1), and the over-drilling depth of the drilling should not be less than the length of the head pipe (4). From the perspective of the cross-section of the sensitive area of ​​shield construction, the drilling position is located on both sides of the tunnel.

9. The grouting method of the active control device for disturbance of shield construction in sandy and gravel strata according to claim 6 is characterized by: During the manufacturing process of the bladder grouting device, the position of the joint pipe (3) needs to be determined according to the depths of the main control area A and the secondary control area B delineated in step 1. One joint pipe (3) is located at the depth boundary of the main control area A and the secondary control area B, and another joint pipe (3) is located at the top elevation of the secondary control area B.

10. The grouting method of the active control device for disturbance of shield construction in sandy and gravel strata according to claim 6, characterized in that: The lowering of the bladder grouting device in step three comprises the following steps: P1: using a car crane to turn over and erect the bladder grouting device; P2: moving the bladder grouting device to the position of the drill hole (12) and lowering it, and after the lowering is completed, backfilling the gaps in the hole with fine sand; P3: after the bladder bag (1) is lowered, it is stabilized for at least three days, during which time the gaps in the hole are backfilled with fine sand, filling the gaps until the gaps cannot be filled further.

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