Method for constructing water collecting pit in foundation pit of subway entrance in water-rich sand layer

Abstract

The application discloses a method for quickly constructing a water collecting pit of a subway entrance and exit foundation pit in a water-rich sand layer, and comprises the following steps: handing over a subway entrance and exit site, constructing a support pile, a crown beam and a retaining wall; constructing a dewatering well of the subway entrance and exit water collecting pit; excavating foundation pit earthwork according to a design scheme, hanging a steel mesh, and anchoring and spraying concrete to a position 2m above a design foundation pit bottom elevation; excavating the remaining 2m of earthwork, anchoring and spraying concrete, and adding a waterproof agent with a cement mass fraction of 5%; placing a drainage blind pipe in the water collecting pit and draining to the dewatering well in the water collecting pit; excavating an inverted trapezoidal water collecting pit foundation groove at a standard section cushion layer elevation position; constructing a water collecting well structure; blocking the dewatering well in the water collecting pit and backfilling the inverted trapezoidal fat groove around the water collecting well; and the method adopts a construction method of excavating, supporting and draining simultaneously to speed up the construction progress.

Description

Technical Field

[0001] This invention belongs to the field of subway station entrance and exit construction technology, specifically relating to a method for rapidly constructing a water collection pit inside the foundation pit of a subway entrance and exit in a water-rich sandy layer. Background Technology

[0002] The characteristics of subway station entrances and exits under sandy conditions are deep foundation pits and groundwater with high permeability. Since the construction of entrances and exits is in the later stage of the project, they generally face tight acceptance deadlines and the actual situation of owners compressing the construction period.

[0003] Under water-rich sandy conditions, excavation work can only begin when the water level drops to 500mm-1000mm below the bottom slab. The sump pit is located at the lowest point of the foundation pit excavation and is the weakest point in terms of dewatering effect. Poor dewatering and failure to lower the water level require special measures. This is the norm for sump pit dewatering and excavation construction. If not handled properly, it often leads to serious delays in the construction period and safety accidents such as water and sand inrush.

[0004] The construction period for entrances and exits is usually around 75 days. At the same time, dewatering wells need to be installed for periodic dewatering operations, which usually takes about 30 days before the excavation of the foundation pit or sump pit, resulting in a significant waste of construction time.

[0005] For example, Chinese invention patent application number 202110388309.2 discloses a reverse construction method for a water-rich deep foundation pit sump, including the following steps: Construction preparation: Before construction, settlement observation points are installed and experimental data is collected. Based on the experimental data, wells are drilled in advance to dewater the groundwater, ensuring that the groundwater level drops below the foundation level, and dewatering must not stop until the subway station's ancillary structure is capped; Excavation of the foundation soil: Excavation is carried out according to the design requirements, excavating to the foundation pit foundation elevation; The method is characterized by further including the following steps: Construction of the side walls and bottom slabs of the straight and sloping sections: The side walls and bottom slabs of the straight and sloping sections of the foundation pit are constructed according to the design requirements, leaving the soil within the sump area empty, and pre-embedding connecting steel bars in the concrete around the sump area; Reinforcing the sump: After leveling the soil within the sump area, concrete is poured to seal the bottom. The grouting influence radius is used to mark out the locations on the sealing concrete, ensuring the grouting area covers the entire sump and that the grout from adjacent holes overlaps. A drill rod is used to drill through the sealing concrete, reaching below the designed sump base and beyond the designed sidewalls. Grouting pipes are then connected to inject grout into the boreholes, with the drill rod being raised and lowered simultaneously until all grouting holes are filled. Sump construction: The sealing concrete and the soil below it are excavated to the designed sump base elevation. The bottom of the sump is cleaned, shotcrete is applied, and the reinforcing bars for the sidewalls and base slab are tied. The reinforcing bars for the sidewalls are connected to the pre-embedded reinforcing bars. Formwork is erected, and concrete is poured to complete the sump construction.

[0006] The existing technology suffers from the aforementioned problem of slow construction progress. To address this issue, the present invention provides a method for rapidly constructing a sump pit for subway entrances / exits in water-rich sandy layers. Summary of the Invention

[0007] This invention proposes a method for rapidly constructing a water collection pit inside the foundation pit of a subway entrance / exit in a water-rich sandy layer.

[0008] The present invention adopts the following technical solution:

[0009] A method for rapid construction of a sump pit within the foundation pit of a subway entrance / exit in a water-rich sandy layer includes:

[0010] Step 1: Handover of the subway entrance / exit site, construction of retaining piles, capping beams, and retaining walls;

[0011] Step 2: Construction of dewatering wells to lower groundwater levels at the sump.

[0012] Step 3: Excavate the foundation pit according to the design plan, hang steel mesh anchors and spray concrete to 2m above the designed bottom elevation of the foundation pit;

[0013] Step 4: Excavate the remaining 2m of soil, anchor and spray concrete, and add 5% waterproofing agent by weight of cement.

[0014] Step 5: Place a drainage blind pipe in the foundation pit to divert water to the dewatering well in the foundation pit;

[0015] Step 6: Excavate an inverted trapezoidal water collection pit trench downwards at the standard section subbase elevation;

[0016] Step 7: Construct the water collection pit structure;

[0017] Step 8: Seal the dewatering wells in the foundation pit and backfill the inverted trapezoidal trench around the water collection pit structure.

[0018] Furthermore, step 2 includes the construction of surface dewatering wells. Two dewatering wells are set on each side of the foundation pit, for a total of four wells, with a spacing of 15m-20m. The dewatering wells are made of sand-free concrete pipes with a diameter of 500mm, and the depth of the dewatering wells extends at least 10 meters beyond the bottom of the sump pit.

[0019] Furthermore, step 2 includes the construction of a dewatering well within the foundation pit. A dewatering well is installed 2m outside the sump. The dewatering well is made of sand-free concrete pipe with a diameter of 500mm, and the depth of the dewatering well extends at least 10 meters beyond the bottom of the sump.

[0020] Furthermore, in step 4, the anchor-sprayed concrete is applied using a wet spraying method. Pre-mixed concrete is used, and 5% of the cement mass of waterproofing agent is added to the original mix proportion. The waterproofing agent is added after the cement and is forcibly stirred evenly.

[0021] Furthermore, in step 4, the remaining 2m is excavated in layers, with each layer having a depth of no more than 1m and a length of no more than 10m. After the excavation is completed, anchor-sprayed concrete with added waterproofing agent is sprayed. After one layer is completed, the next layer is excavated.

[0022] Furthermore, step 4 includes the treatment of the dewatering wells in the foundation pit: as the excavation process progresses, the sand-free concrete pipes are gradually exposed on the excavation surface, and the sand-free concrete pipes are removed section by section in 0.5m lengths to reduce the top height of the dewatering wells in the foundation pit until the remaining 2m of excavation is completed.

[0023] Furthermore, step 5 includes:

[0024] Step 5.1: When the earthwork is excavated to the bottom of the standard section cushion layer, a square trench of 250*250mm is excavated along the root of the retaining piles, and two flexible permeable blind pipes with a diameter of 100mm are placed side by side in the trench.

[0025] Step 5.2: Pour a 100mm cushion layer to cover two flexible permeable blind pipes. After the cushion layer is completed, spray waterproof anchored concrete on the cushion layer and install additional drainage blind pipes to complete the bottom waterproof sealing of the standard section of the foundation pit.

[0026] Furthermore, in step 5.1, the horizontal position of the water collection pit is located at the middle of the total length of the flexible permeable blind pipe, and the flexible permeable blind pipe extends at least 2m beyond the longitudinal edge of the water collection pit on each side.

[0027] Furthermore, in step 5.1, the flexible permeable blind pipe slopes at 5% from one end toward the dewatering well in the foundation pit, and a T-joint is used at the dewatering well in the foundation pit to add two horizontal drainage pipes to connect the flexible permeable blind pipe to the dewatering well.

[0028] Compared with the prior art, the superior effects of the present invention are as follows:

[0029] 1. The method for rapid construction of a water sump pit in the foundation pit of a subway entrance in a water-rich sandy layer, as described in this invention, adopts a construction method of excavation, support and drainage at the same time. The rapid construction shortens the excavation and exposure time of the water sump pit and completes all construction procedures under the condition of dewatering required for an entrance water sump pit.

[0030] 2. The method for rapid construction of a water collection pit in the foundation pit of a subway entrance in a water-rich sandy layer, as described in this invention, uses shotcrete anchoring and waterproofing agent to seal the pore water around the bottom of the foundation pit. Water seeping between piles is sealed by the shotcrete anchoring and the foundation concrete with added waterproofing agent. The water flows to the double-row drainage blind pipe under the foundation and is then diverted to the dewatering well in the pit through the double-row drainage blind pipe, resulting in good drainage effect. Detailed Implementation

[0031] To better understand the above-mentioned objectives, features and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0032] Example

[0033] The method for rapid construction of a sump pit for subway entrance / exit foundation pits in water-rich sandy layers includes:

[0034] Step 1: Handover of the subway entrance / exit site, construction of retaining piles, capping beams, and retaining walls;

[0035] Step 2: Construction of dewatering wells to lower groundwater levels at the sump.

[0036] Step 3: Excavate the foundation pit according to the design plan, hang steel mesh anchors and spray concrete to 2m above the designed bottom elevation of the foundation pit;

[0037] Step 4: Excavate the remaining 2m of soil, anchor and spray concrete, and add 5% waterproofing agent by weight of cement.

[0038] Step 5: Place a drainage blind pipe in the foundation pit to divert water to the dewatering well in the foundation pit;

[0039] Step 6: Excavate an inverted trapezoidal water collection pit trench downwards at the standard section subbase elevation;

[0040] Step 7: Construct the water collection pit structure;

[0041] Step 8: Seal the dewatering wells in the foundation pit and backfill the inverted trapezoidal trench around the water collection pit structure;

[0042] Step 9: Comparison of construction period and advantages / disadvantages;

[0043] In steps 1 to 2 of the above embodiment, due to the limitations of the entrance and exit site, the capping beam and retaining wall are constructed first, followed by the construction of the dewatering well.

[0044] In step 2 of the above embodiment, the construction includes the installation of surface dewatering wells and dewatering wells inside the foundation pit. When installing surface dewatering wells, two dewatering wells are set on each of the left and right sides of the foundation pit, for a total of four wells. The spacing between them should be 15m, and the longest spacing should not exceed 20m. The dewatering wells use sand-free concrete pipes with a diameter of 500mm. The depth of the dewatering wells should extend at least 10 meters beyond the bottom of the sump. A water pump is installed at the bottom of the dewatering wells. The water pump specifications are 50 cubic meters / h. The length of the sand-free concrete pipe is the standard length of 1.5m. When installing dewatering wells inside the foundation pit, one well is set at a position 2m outside the sump. The construction method is the same as that for surface dewatering wells. The length of the sand-free concrete pipe is the standard length, and the length of the sand-free concrete pipe is a special length of 0.5m. The wells are installed before the foundation pit is excavated. A water pump with a capacity of 50 cubic meters / h is used.

[0045] In step 4 of the above embodiment:

[0046] The anchor shotcrete is applied using a wet spraying method and is pre-mixed by a commercial concrete plant. 5% waterproofing agent by weight of cement is added to the original mix proportion, and the addition order is after the cement. The mixture is then forcibly stirred until uniform.

[0047] The remaining 2m will be excavated in two stages, with each layer no more than 1m deep and no more than 10m long. After the excavation is completed, sprayed concrete with waterproofing agent will be sprayed immediately. After one stage is completed, the next stage or the next section will be excavated.

[0048] As the dewatering wells are constructed, the sand-free concrete pipes are gradually exposed on the excavation surface during the excavation process. The sand-free concrete pipes are removed section by section at a length of 0.5m to reduce the top height of the dewatering wells in the pit.

[0049] Until the earthwork at the standard section subgrade of the entrance and exit is completed;

[0050] In step 5 of the above embodiment:

[0051] When the earthwork is excavated to the bottom of the standard section cushion layer, a square trench of 250*250mm is manually excavated along the base of the retaining piles, and two flexible permeable blind pipes with a diameter of 100mm are placed side by side in the trench.

[0052] A 100mm thick cushion layer was poured to cover two flexible permeable blind pipes. After the cushion layer was completed, waterproof anchored shotcrete was sprayed on the cushion layer and additional drainage blind pipes were installed to complete the bottom waterproof sealing of the standard section of the foundation pit.

[0053] The horizontal position of the sump is located in the middle of the total length of the two flexible permeable blind pipes, and the two flexible permeable blind pipes extend at least 2m beyond the longitudinal edge of the sump on each side.

[0054] The flexible permeable blind pipe slopes at 5% from one end toward the dewatering well in the pit, and a T-joint is used at the dewatering well in the pit to add two horizontal drainage pipes to connect the longitudinal flexible permeable blind pipe to the dewatering well.

[0055] In particular, at the bottom of the foundation pit near the sump pit, due to the short dewatering cycle, there will be continuous seepage of residual pore water between the sand layers between the retaining piles. The water seeping between the piles is sealed by the anchor sprayed concrete and the cushion concrete with added waterproofing agent, flows to the double-row drainage blind pipe under the cushion, and is diverted to the dewatering well in the pit by the double-row drainage blind pipe and discharged by the water pump.

[0056] In step 6 of the above embodiment:

[0057] Check the dynamic water level of the four wells around the foundation pit and the one well inside the foundation pit. The check cycle is three consecutive days, twice a day, with an interval of about 12 hours. If the dynamic water level in the well is 6 meters lower than the bottom elevation of the dewatering well, then the conditions for excavating the sump pit are met.

[0058] Excavation is carried out in layers, with each layer not exceeding 0.5m in depth. After each layer is excavated, wet spraying is immediately used to apply premixed shotcrete mixed with waterproofing agent.

[0059] The sump pit is excavated in an inverted trapezoidal shape, with a longitudinal slope of 1:1.5. The slope surface is sealed with 100*100mm steel mesh and premixed concrete mixed with waterproofing agent.

[0060] As the earthwork is excavated in increments of 0.5m, the dewatering wells inside the pit are lowered section by section, with each lowering depth not exceeding one section, or 0.5m.

[0061] As the earthwork elevation and the dewatering well sections in the pit continue to decrease, the longitudinal and transverse drainage blind pipes also decrease layer by layer with the excavation face, continuously diverting water into the dewatering wells in the pit.

[0062] Repeat the above steps multiple times until the excavation reaches the bottom elevation of the sump pit;

[0063] The water collection well is excavated in layers using a method of opening a trench in the middle and leaving slope protection soil on both sides, with the slope protection soil being excavated alternately. That is, first, the middle third of the transverse earthwork is excavated; then, the middle third of the slope protection soil on one side is excavated, and waterproof shotcrete is applied; then, the remaining middle third of the earthwork on the other side is excavated, and waterproof shotcrete is applied; then, the next layer is excavated, and when excavating the earthwork on both sides of the next layer, the excavation sequence is the reverse of the previous layer.

[0064] After excavating to the bottom of the sump, a 200mm thick concrete cushion layer with waterproofing agent is poured, and two flexible permeable blind pipes are covered. After the cushion layer is completed, waterproof anchor shotcrete is sprayed on the cushion layer and additional drainage blind pipes are installed to complete the bottom waterproof sealing of the standard section of the foundation pit.

[0065] The method of excavating at a height of 0.5m per layer and excavating the middle soil first and then the slope protection soil shortens the exposure time of the foundation pit. This allows for waterproof shotcrete sealing within 2 hours after the retaining piles are exposed, achieving a rapid water-stopping effect. The purpose of alternating excavation on the left and right sides is to minimize the exposure time of the foundation pit on one side of the retaining piles and reduce the deep water volume between the piles to a minimum.

[0066] In step 7 of the above embodiment: after the waterproof cushion layer is completed, the bottom slab of the water collection well is constructed first, and then the side walls and top slab are constructed.

[0067] In step 8 of the above embodiment:

[0068] After the water sump structure was completed, the water pump was removed and the sump was backfilled and sealed with C30 expansion and impermeable concrete.

[0069] After the sump dewatering well is sealed, the trapezoidal trench around the sump should be backfilled immediately until the subgrade elevation is reached.

[0070] The applicant needs to explain that the conventional dewatering method involves excavating to the subgrade elevation and then dewatering for about 15 days; the dewatering plan needs to be designed and determined, and 8-10 dewatering wells are required.

[0071] In the scheme described in this embodiment: after excavation to the subgrade elevation, a sump pit is immediately excavated; and only 5 dewatering wells are required, which improves construction efficiency and saves construction costs.

[0072] This invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims.

Claims

1. A method for rapidly constructing a sump pit within the foundation pit of a subway entrance / exit in a water-rich sandy layer, characterized in that, include: Step 1: Handover of the subway entrance / exit site, construction of retaining piles, capping beams, and retaining walls; Step 2, Construction of dewatering wells to lower groundwater levels at the sump: Construct surface dewatering wells, with two dewatering wells on each side of the foundation pit, for a total of four wells, spaced 15m-20m apart. The dewatering wells shall be constructed using sand-free concrete pipes with a diameter of 500mm, and the depth of the dewatering wells shall extend at least 10 meters beyond the bottom of the sump pit. Construct a dewatering well inside the foundation pit. Set up one dewatering well 2m outside the sump pit. The dewatering well is made of sand-free concrete pipe with a diameter of 500mm. The depth of the dewatering well is not less than 10 meters beyond the bottom of the sump pit. Step 3: Excavate the foundation pit according to the design plan, hang steel mesh anchors and spray concrete to 2m above the designed bottom elevation of the foundation pit; Step 4: Excavate the remaining 2m of soil, apply shotcrete, and add 5% (by weight of cement) of waterproofing agent. The anchor shotcrete adopts the wet spraying method, using pre-mixed concrete, and adding 5% waterproofing agent by weight of cement to the original mix proportion. The waterproofing agent is added after the cement and is forcibly mixed evenly. The remaining 2m will be excavated in layers, with each layer no more than 1m deep and no more than 10m long. After excavation, anchor-sprayed concrete with waterproofing agent will be sprayed. The next layer will be excavated after the first layer is completed. As the excavation process progresses, the sand-free concrete pipe is gradually exposed on the excavation surface. The sand-free concrete pipe is removed section by section in 0.5m lengths to reduce the top height of the dewatering well in the foundation pit until the remaining 2m of excavation is completed. Step 5: Place a blind drainage pipe inside the foundation pit to divert water to the dewatering well inside the pit. Step 6: Excavate an inverted trapezoidal water collection pit trench downwards at the standard section subbase elevation; Step 7: Construct the water collection pit structure; Step 8: Seal the dewatering wells in the foundation pit and backfill the inverted trapezoidal trench around the water collection pit structure.

2. The method for rapid construction of a water collection pit in the foundation pit of a subway entrance / exit in a water-rich sandy layer according to claim 1, characterized in that, Step 5.1: When the earthwork is excavated to the bottom of the standard section cushion layer, a square trench of 250*250mm is excavated along the root of the retaining piles, and two flexible permeable blind pipes with a diameter of 100mm are placed side by side in the trench. Step 5.2: Pour a 100mm cushion layer to cover two flexible permeable blind pipes. After the cushion layer is completed, spray waterproof anchored concrete on the cushion layer and install additional drainage blind pipes to complete the bottom waterproof sealing of the standard section of the foundation pit.

3. The method for rapid construction of a water sump pit within the foundation pit of a subway entrance / exit in a water-rich sandy layer, as described in claim 2, is characterized in that... In step 5.1, the horizontal position of the water collection pit is at the middle of the total length of the flexible permeable blind pipe, and the flexible permeable blind pipe extends at least 2m beyond the longitudinal edge of the water collection pit on each side.

4. The method for rapidly constructing a sump pit within the foundation pit of a subway entrance / exit in a water-rich sandy layer, as described in claim 2, is characterized in that... In step 5.1, the flexible permeable blind pipe slopes at 5% from one end toward the dewatering well in the pit, and a T-joint is used at the dewatering well in the pit to add two horizontal drainage pipes to connect the flexible permeable blind pipe to the dewatering well.