Automatic dewatering and drainage device and method for large-plane shallow foundation pit containing weak permeable interlayer

Abstract

The invention discloses an automatic dewatering and drainage device for a large-plane shallow foundation pit containing a weak permeable interlayer, which comprises drainage ditches including an annular drainage ditch and a linear drainage ditch, and a plash arranged at the intersection of the annular drainage ditch and the linear drainage ditch, the water seepage blind ditch is composed of a steel wire mesh, a steel reinforcement framework and gravel; the reinforcement cage is placed in the plash, the submersible pump is arranged at the bottom in the reinforcement cage, and the float switch water level controller is connected with the submersible pump and controls the submersible pump to be started and stopped so that water in the plash can be drained out of the plash. The annular drainage ditch and the linear drainage ditch are combined with the gridding layout of the plash, and then the submersible pump and the float switch water level controller are combined. Seepage water can be rapidly collected and drained, the drainage efficiency is improved by 40% or above, and the method is particularly suitable for the weak permeable interlayer geological condition of a large-plane shallow foundation pit.

Description

Technical Field

[0001] The present invention relates to the field of self-drainage of foundation pits, and more particularly to an automatic drainage device and method for a shallow foundation pit with a large surface area and a weakly permeable interlayer. Background Art

[0002] During the construction process, the structure generally transfers the load and self-weight above the ground to the foundation part below the ground. The foundation part then transfers the load and self-weight it bears to the foundation bearing layer to ensure the stability and safety of the structure. During foundation pit construction, when the groundwater level is higher than the bottom of the excavation, groundwater will continue to seep into the pit, causing slope instability, foundation sand flow, pit bottom uplift, pit bottom pipe bursts, and a decrease in foundation bearing capacity. In order to ensure that the foundation pit can be constructed under dry conditions, dewatering work is required to ensure that the highest water level in the construction area is a certain distance from the bottom of the foundation pit. Conventional practices generally include open ditch plus water collection well dewatering, light well point dewatering, jet well point dewatering, electro-osmosis well point dewatering, deep well point dewatering, etc.

[0003] When the excavation depth of the foundation pit is shallow but the soil layer contains a weakly permeable layer or an impermeable layer, if precipitation is not achieved, there will be water-carrying operations, the slopes will be unstable and prone to collapse, and the base bearing capacity will not meet the requirements, posing safety risks and quality hazards.

[0004] Conventional practices include: (1) using well-point dewatering, and arranging certain dewatering wells around the foundation pit according to actual conditions. (2) digging drainage ditches and water storage pits of a certain depth, and using submersible pumps for open drainage.

[0005] Conventional practices have the following shortcomings: (1) The cost of well point dewatering is high, the well drilling cycle is relatively long, and the shallow foundation pit construction cycle is relatively short, which is relatively unreasonable from an economic perspective. (2) When excavating drainage ditches and water pits of a certain depth, when the soil layer contains a weak permeable layer or an impermeable layer, due to the low water seepage rate of the soil layer, the water accumulation rate is lower than the drainage rate, and continuous dewatering cannot be achieved. It is necessary to arrange a certain number of workers for on-site inspections. If the intermediate water pit has too long a pipe and there is no suitable drainage point nearby, it is necessary to drain the water to a water pit closer to the edge of the foundation pit for relay drainage. The labor cost is high and there is a problem of untimely drainage. If the drainage ditch and water pit are excavated too deep, if there is an unstable soil such as a sand layer, there will be problems such as easy collapse. Summary of the Invention

[0006] To achieve these objectives and other advantages according to the present invention, a preferred embodiment of the present invention provides an automatic drainage device for a shallow foundation pit with a large surface area and a weakly permeable interlayer, for draining the foundation pit. The foundation pit includes an unexcavated area in the center and an excavated area around it. The automatic drainage device includes:

[0007] The drainage ditch includes a circular drainage ditch and a linear drainage ditch. There are multiple circular drainage ditches, all arranged around the unexcavated area. There is a certain distance between two adjacent circular drainage ditches, and the two are connected by a linear drainage ditch.

[0008] The water accumulation pit is set at the intersection of the circular drainage ditch and the straight drainage ditch;

[0009] The seepage blind ditch is composed of a wire mesh, a steel frame and crushed stones. The wire mesh is hollow and filled with crushed stones. The steel frame is inserted into the wire mesh. The seepage blind ditch is placed in the drainage ditch.

[0010] The steel cage is placed in the water pit, the submersible pump is placed at the bottom of the steel cage, and the float switch water level controller is connected to the submersible pump to control its start and stop to discharge the seepage water in the water pit.

[0011] Preferably, the steel bar skeleton is welded from 4 steel bars arranged along the length direction of the blind ditch, 6 steel bars in the width direction and 6 steel bars in the depth direction.

[0012] Preferably, a dense mesh is wound and fixed on the outside of the steel cage and the submersible pump to filter sand and gravel.

[0013] Preferably, the bottom of the sump is lower than the bottom of the drainage ditch.

[0014] Preferably, the annular drainage ditch adopts a gradient descending structure, forming a slope of 3-5‰ from the side away from the unexcavated area to the side close to the unexcavated area, and the slope directions of adjacent annular drainage ditches are opposite, forming an alternating confluence path.

[0015] Preferably, an air pressure balance pipe is provided in the water pit, which is vertically fixed to the side wall of the steel cage, with the upper end above the ground and equipped with an adjustable pressure exhaust valve, and the lower end extending to 20 cm above the bottom of the pit, and honeycomb air holes are provided on the pipe wall.

[0016] Preferably, an elastic permeable cushion layer is laid at the bottom of the water accumulation pit, which is formed by mixing and compacting rubber particles and graded gravel in a volume ratio of 1:3. An annular air guide pipe is embedded in the elastic permeable cushion layer and is connected to the air pressure balance pipe. The diameter d of the annular air guide pipe and the diameter D of the air pressure balance pipe satisfy d=0.6D.

[0017] Preferably, the operation method of the air pressure balance pipe is as follows:

[0018] Real-time monitoring of the air pressure value P in the water pit and the cumulative running time (T) of the submersible pump,

[0019] The exhaust valve adjustment is started when any of the following conditions are met: a) P < -1.5kPa and T > 2h, or b) P < -2.0kPa; wherein, the exhaust valve opening is adjusted according to the formula: θ = 15° × |P| / Pmax +5°, where P max is the maximum allowable negative pressure of the air pressure balance pipe.

[0020] Preferably, the construction method of the elastic permeable cushion layer 9-1 is dynamically linked with the air pressure balance system, including the following steps:

[0021] A. First, adjust the opening degree of the exhaust valve to form and maintain an initial negative pressure of P0 = -1.0 kPa in the sump; then lay the elastic permeable cushion layer, and monitor the air pressure fluctuation in real time during the laying process, with the allowable deviation range of ±0.2 kPa;

[0022] B. Lay the annular gas guide pipe. After the gas guide pipe network is laid, conduct a combined negative pressure-flow test:

[0023] a) Adjust the opening degree of the exhaust valve to θ = 30°, and record the permeability coefficient K of the cushion layer;

[0024] b) When K < 1×10 -3 cm / s, trigger the secondary vibration compaction of the cushion layer until the permeability coefficient K of the cushion layer ≥ 5×10-3 cm / s.

[0025] On the other hand, a preferred implementation of the present invention provides a drainage method for an automatic drainage device for a large-plane shallow foundation pit with a weak permeable interlayer, including the following steps:

[0026] S1. Process and manufacture the permeable blind ditch and the steel reinforcement cage, and vertically install the air pressure balance pipe on the side wall of the steel reinforcement cage;

[0027] S2. Install the submersible pump and the float switch water level controller in the steel reinforcement cage, and wrap them with a fine mesh net;

[0028] S3. Excavate a circular drainage ditch around the unexcavated area of the foundation pit. There are multiple circular drainage ditches, and there is a certain distance between adjacent two circular drainage ditches, and they are connected by the excavated straight drainage ditch. Place the permeable blind ditch in the drainage ditch;

[0029] S4. Excavate a sump at the intersection of the circular drainage ditch and the straight drainage ditch, ensure that the bottom of the sump is lower than the bottom of the drainage ditch, place the steel reinforcement cage in the sump, and lay an elastic permeable cushion layer at the bottom of the sump;

[0030] S5. After all the permeable blind ditches are installed and it is ensured that all the drainage ditches are connected to the sump, backfill the perimeter and top of the permeable blind ditch;

[0031] S6. After the water level in the sump reaches the set drainage level, automatically control the submersible pump to drain water through the float switch water level controller until the water level drops to the set drainage level;

[0032] S7. After the drainage is completed, the seepage blind ditch and steel cage are hoisted out and reused.

[0033] The present invention has at least the following beneficial effects:

[0034] The invention has a simple structure, is reliable and easy to operate, which not only ensures the precipitation effect, but also reduces the risk of collapse of ditches and water pits, thereby improving construction efficiency.

[0035] ① High safety. Using gabion blind ditch and submersible pump with steel cage, after the gabion is placed in the drainage ditch and the submersible pump with steel cage is placed in the water pit, backfill can be carried out around and above the gabion and around the steel cage, reducing the safety risk of ditch and water pit collapse.

[0036] ②Good economic efficiency. The device mainly uses steel bars for construction sites, protective mesh, galvanized steel wire, gravel, submersible pumps, and float switch water level controllers. The material acquisition channels are wide and the device processing cost is low.

[0037] ③High practicality. The device can be used to arrange the length and depth of the gabion blind ditch according to the seepage conditions of the foundation pit soil at the construction site, and the length of the steel cage for placing the submersible pump can be processed according to the site conditions. It can also be used multiple times.

[0038] Other advantages, objectives and features of the present invention will be reflected in part from the following description and will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a distribution diagram of the drainage device during the foundation pit excavation process of the present invention;

[0040] Figure 2 It is a structural schematic diagram of the seepage blind ditch in the present invention.

[0041] Figure 3 It is a structural schematic diagram of the steel cage in the present invention.

[0042] Figure 4 It is a structural schematic diagram of the elastic water-permeable cushion layer in the present invention. DETAILED DESCRIPTION

[0043] The present invention will be described in further detail below in conjunction with the accompanying drawings so that those skilled in the art can implement the invention with reference to the description.

[0044] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments in the following description are only examples, and those skilled in the art can think of other obvious variations. The basic principles defined in the following description of the present invention can be applied to other implementation schemes, variation schemes, improvement schemes, equivalent schemes, and other technical schemes without departing from the spirit and scope of the present invention.

[0045] Those skilled in the art should understand that in the disclosure of the present invention, the orientation or positional relationship indicated by terms such as "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. 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, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting the present invention.

[0046] It can be understood that the term "a" should be understood as "at least one" or "one or more". That is, in one embodiment, the number of an element can be one, while in other embodiments, the number of the element can be multiple. The term "a" should not be construed as limiting the quantity.

[0047] As Figures 1-4 shown, a preferred embodiment of the present invention provides an automatic dewatering device for a large-plane shallow foundation pit with a weak water-permeable interlayer, which is used for dewatering the foundation pit. The foundation pit includes an unexcavated area 3-1 in the center and an excavated area 3-2 around it. The automatic dewatering device includes:

[0048] Drainage ditches 4, which include annular drainage ditches and straight drainage ditches. There are multiple annular drainage ditches, all arranged around the unexcavated area. There is a certain distance between adjacent two annular drainage ditches, and they are connected by straight drainage ditches.

[0049] Sumps 5, which are arranged at the intersections of the annular drainage ditches and the straight drainage ditches. The bottoms of the drainage ditches are lower to ensure that water flows into the sumps.

[0050] Percolation blind ditches, which are composed of wire meshes 1-1, steel bar skeletons 1-2 and gravel 1-3. The wire meshes are in a hollow structure, and the gravel 1-3 is filled in the wire meshes 1-1, and the steel bar skeletons 1-2 are inserted into the wire meshes; the percolation blind ditches are placed in the drainage ditches.

[0051] Steel bar cages 2-1, which are placed in the sumps. Submersible pumps 2-2 are placed at the bottoms inside the steel bar cages 2-1. Float switch water level controllers 2-3 are connected to the submersible pumps to control their starting and stopping to pump out the seepage water in the sumps.

[0052] In the above technical solution, the circular drainage ditch of the drainage ditch is arranged in concentric circles around the unexcavated area of the foundation pit, and the straight drainage ditches are radially connected to adjacent circular drainage ditches to ensure that the water flow quickly converges into the sump. The wire mesh 1-1 of the permeable blind ditch is woven with galvanized steel wire, the mesh size is 10mm×10mm, the steel bar skeleton 1-2 is welded by deformed steel bars with a diameter of 8mm, and the gravel 1-3 has a particle size of 20-40mm and a filling density of 1.8t / m 3 . The steel cage 2-1 is welded into a frame structure by steel bars with a diameter of 12mm, and a submersible pump 2-2 is fixed inside. The float switch water level controller 2-3 is linked with the submersible pump 2-2 through a connecting rod mechanism and automatically starts draining when the water level reaches the set height.

[0053] Through the combined layout of the circular and straight drainage ditches, the device achieves efficient water collection, and the drainage efficiency is increased by more than 40%. The three-layer structure (wire mesh + steel bar skeleton + gravel) of the permeable blind ditch enables the water permeability rate to reach 2.5L / (min·m 2 ), and the compressive strength ≥ 15kPa. The linkage control error between the float switch and the submersible pump is small, significantly reducing the frequency of manual intervention.

[0054] In another technical solution, the steel bar skeleton 1-2 is welded by 4 steel bars arranged along the length direction of the blind ditch, 6 steel bars in the width direction, and 6 steel bars in the depth direction.

[0055] The 4 steel bars in the length direction are deformed steel bars with a diameter of 8mm; the 6 steel bars in the width direction are round steel bars with a diameter of 6mm; the 6 steel bars in the depth direction are round steel bars with a diameter of 6mm. All the intersections of the steel bars are welded by arc welding, and the weld height ≥ 3mm. The overall size of the steel bar skeleton 1-2 matches the wire mesh 1-1 to ensure uniform filling of the gravel 1-3.

[0056] This skeleton structure increases the flexural stiffness of the blind ditch to 320N·m 2 , and the deformation amount is < 2mm under the backfill soil pressure of 10kPa, and the deviation of the pore distribution uniformity of the gravel layer < 8%.

[0057] In another technical solution, a dense mesh 2-4 is wound and fixed outside the steel cage and the submersible pump for filtering sand and gravel.

[0058] In another technical solution, the bottom of the drainage ditch is lower than the bottom of the drainage ditch.

[0059] In another technical solution, the circular drainage ditch adopts a gradient slope reduction structure, forming a slope of 3-5‰ from the side far away from the unexcavated area to the side close to the unexcavated area, and the slope directions of adjacent circular drainage ditches are opposite to form an alternating confluence path.

[0060] The alternating slope design causes the water flow to generate eddies, increasing the sedimentation efficiency of suspended particles by 60% and also reducing the drainage turbidity.

[0061] Another technical solution is to set an air pressure balance pipe 8 in the water pit. The air pressure balance pipe is vertically fixed to the side wall of the steel cage, with the upper end above the ground and an adjustable pressure exhaust valve installed, and the lower end extends to 20 cm above the bottom of the pit, and honeycomb air holes are opened on the pipe wall.

[0062] The coordinated control of the pressure balancing pipe 8 and the exhaust valve ensures that the negative pressure in the water pit is always maintained within a safe range (within -3.0kPa), preventing the weak permeable interlayer from collapsing due to excessive negative pressure. The honeycomb-shaped air holes accelerate the pressure balancing speed to a stable 2kPa pressure difference within 10 seconds, which is three times more efficient than traditional pressure relief methods.

[0063] Another technical solution is to lay an elastic permeable cushion layer 9-1 at the bottom of the water pit, which is formed by mixing and compacting rubber particles and graded gravel in a volume ratio of 1:3. An annular air guide pipe 9-2 is embedded in the elastic permeable cushion layer and is connected to the air pressure balance pipe 8. The diameter d of the annular air guide pipe 9-2 and the diameter D of the air pressure balance pipe 8 satisfy d=0.6D.

[0064] The pre-buried annular air guide pipe 9-2 is connected to the air pressure balance pipe 8, optimizing the efficiency of the air-water two-phase flow and increasing the drainage speed by 20%. The air pressure balance pipe, exhaust valve and elastic permeable cushion layer (9-1) work together to prevent the cushion layer from compressing and deforming under negative pressure conditions, ensuring long-term permeability.

[0065] Another technical solution, the operation method of the air pressure balance pipe is as follows:

[0066] Real-time monitoring of the air pressure value P in the water pit and the cumulative running time (T) of the submersible pump 2-2,

[0067] The exhaust valve adjustment is started when any of the following conditions are met: a) P < -1.5kPa and T > 2h, or b) P < -2.0kPa; wherein, the exhaust valve opening is adjusted according to the formula: θ = 15° × |P| / Pmax + 5°, where P max It is the maximum allowable negative pressure of the air pressure balance pipe.

[0068] In the above technical solution, based on the dual parameter control of the air pressure value (P) and the operating time of the submersible pump, the exhaust valve opening adjustment error is less than 0.5°, and the air pressure fluctuation amplitude is controlled within ±0.3kPa.

[0069] Another technical solution is that the construction method of the elastic permeable cushion layer 9-1 is dynamically linked with the air pressure balance system, comprising the following steps:

[0070] A. First, by adjusting the opening of the exhaust valve, an initial negative pressure of P0 = -1.0 kPa is formed and maintained in the water pit; then, the elastic permeable cushion layer is laid, and the pressure fluctuation is monitored in real time during the laying process, with an allowable deviation range of ±0.2 kPa;

[0071] B. Lay the annular air duct 9-2. After the air duct network is laid, conduct a combined negative pressure-flow test:

[0072] a) Adjust the opening of the exhaust valve to θ = 30°, and record the permeability coefficient K of the cushion layer;

[0073] b) When K < 1×10 -3 cm / s, trigger the secondary vibration compaction of the cushion layer until the permeability coefficient K of the cushion layer ≥ 5×10-3 cm / s.

[0074] In the above technical solution, the initial negative pressure (P0 = -1.0 kPa) is applied during the laying of the cushion layer to avoid the blockage of the water permeable channels during the compaction of the backfill soil. By laying the cushion layer with the initial negative pressure (P0 = -1.0 kPa), the compliance rate of the permeability coefficient (K ≥ 5×10-3 cm / s) is greatly improved after the secondary vibration compaction.

[0075] Another technical solution provides a drainage method for any of the automatic drainage devices for large-plane shallow foundation pits with weak water-permeable interlayers, which is characterized by including the following steps:

[0076] S1. Fabricate the water seepage blind ditch and the steel reinforcement cage 2-1, and vertically install the air pressure balance pipe on the side wall of the steel reinforcement cage;

[0077] S2. Install the submersible pump 2-2 and the float switch water level controller 2-3 inside the steel reinforcement cage, and wrap them with a dense mesh 2-4;

[0078] S3. Excavate an annular drainage ditch around the unexcavated area of the foundation pit. There are multiple annular drainage ditches, and there is a certain distance between adjacent two annular drainage ditches, and they are connected by the excavated straight drainage ditch. Place the water seepage blind ditch in the drainage ditch;

[0079] S4. Excavate a sump at the intersection of the annular drainage ditch and the straight drainage ditch, ensure that the bottom of the sump is lower than the bottom of the drainage ditch, place the steel reinforcement cage in the sump, and lay an elastic water-permeable cushion layer 9-1 at the bottom of the sump;

[0080] S5. After all the water seepage blind ditches are installed and it is ensured that all the drainage ditches are connected to the sump, the surrounding and top of the water seepage blind ditch can be backfilled;

[0081] S6. After the water level in the sump reaches the set drainage level, automatically control the submersible pump to drain water through the float switch water level controller until the water level drops to the set drainage level;

[0082] S7. After the drainage is completed, lift out the water seepage blind ditch and the steel reinforcement cage for repeated use.

[0083] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those skilled in the art, additional modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and the examples shown and described herein.

Claims

1. An automatic dewatering device for a large planar shallow foundation pit with a weakly permeable interlayer, which is used for dewatering the foundation pit. The foundation pit includes an unexcavated area at the center and an excavated area around it. It is characterized in that, The automatic drainage device includes: The drainage ditch includes a circular drainage ditch and a linear drainage ditch. There are multiple circular drainage ditches, all arranged around the unexcavated area. There is a certain distance between two adjacent circular drainage ditches, and the two are connected by a linear drainage ditch. A water accumulation pit is set at the intersection of the circular drainage ditch and the straight drainage ditch; The seepage blind ditch is composed of a wire mesh, a steel frame and crushed stones. The wire mesh is hollow and filled with crushed stones. The steel frame is inserted into the wire mesh. The seepage blind ditch is placed in the drainage ditch. The steel cage is placed in the water pit, the submersible pump is placed at the bottom of the steel cage, and the float switch water level controller is connected to the submersible pump to control its start and stop to discharge the seepage water in the water pit.

2. The automatic drainage device for a large planar shallow foundation pit with a weakly permeable interlayer according to claim 1, characterized in that, The steel bar skeleton is welded together by 4 steel bars arranged along the length direction of the blind ditch, 6 steel bars in the width direction and 6 steel bars in the depth direction.

3. The automatic dewatering device for a large-plane shallow foundation pit with a weak water-permeable interlayer according to claim 1, wherein The dense mesh is wrapped and fixed on the outside of the steel cage and the submersible pump to filter sand and gravel.

4. The automatic dewatering device for a large planar shallow foundation pit with a weak water-permeable interlayer according to claim 1, characterized in that, The bottom of the sump is lower than the bottom of the drainage ditch.

5. The automatic dewatering device for a large planar shallow foundation pit with a weak water-permeable interlayer according to claim 1, characterized in that, The annular drainage ditch adopts a gradient descending structure, forming a slope of 3-5‰ from the side away from the unexcavated area to the side close to the unexcavated area, and the slope directions of adjacent annular drainage ditches are opposite, forming an alternating confluence path.

6. The automatic dewatering device for a large planar shallow foundation pit with a weak water-permeable interlayer according to claim 1, characterized in that, An air pressure balance pipe is set in the water pit. The air pressure balance pipe is vertically fixed to the side wall of the steel cage. The upper end is higher than the ground and is equipped with an adjustable pressure exhaust valve. The lower end extends to 20 cm above the bottom of the pit, and honeycomb air holes are opened on the pipe wall.

7. The automatic dewatering device for a large planar shallow foundation pit with a weak water-permeable interlayer according to claim 6, characterized in that An elastic permeable cushion layer is laid at the bottom of the water accumulation pit, which is formed by mixing and compacting rubber particles and graded gravel in a volume ratio of 1:

3. An annular air guide pipe is embedded in the elastic permeable cushion layer and is connected to the air pressure balance pipe. The diameter d of the annular air guide pipe and the diameter D of the air pressure balance pipe satisfy d=0.6D.

8. The drainage method of the automatic drainage device for a large planar shallow foundation pit with a weak water-permeable interlayer according to any one of claims 1-7, characterized in that, The following steps are involved: S1. Process and manufacture seepage blind ditch and steel cage, and vertically install air pressure balance pipe on the side wall of the steel cage; S2. Install the submersible pump and float switch water level controller in the steel cage and wrap it with a fine mesh. S3. Excavate the unexcavated area around the foundation pit to form a circular drainage ditch. There are multiple circular drainage ditches. Adjacent circular drainage ditches are separated by a certain distance and connected by an excavated straight drainage ditch. The seepage blind ditch is placed in the drainage ditch. S4. Excavate a water pit at the intersection of the circular drainage ditch and the linear drainage ditch, place the steel cage in the water pit, and lay an elastic permeable cushion at the bottom of the water pit; S5. After all the seepage blind ditches are installed and all the drainage ditches are connected to the water pit, the surroundings and top of the seepage blind ditches can be backfilled; S6. When the water level in the water pit reaches the set drainage level, the water level controller automatically controls the submersible pump to drain water until the water level reaches the set drainage level; S7. After the drainage is completed, the seepage blind ditch and steel cage are hoisted out and reused.

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