Double-layer anti-filtering pressure limiting well structure and basement temporary passive anti-floating drainage method

Abstract

The invention discloses a double-layer reversed-filtering pressure limiting well structure and a temporary passive anti-floating drainage method for a basement. The double-layer reversed-filtering pressure limiting well structure and the temporary passive anti-floating drainage method are used for short-term anti-floating precipitation before basement roof soil covering is completed. A water collecting pit is arranged below the bottom plate, a double-layer inverted filter layer of 25-40 mm pebbles and 7-15 mm gravel is laid in the pit from bottom to top, and 200 g / m < 2 > geotechnical cloth is arranged between the layers and on the top face; a stainless steel aqueduct with the diameter of 159 mm is embedded in the center of the pit bottom, the top end of the aqueduct is lower than the bottom of the bottom plate by more than or equal to 500 mm, and a passive overflow pressure limiting threshold value And a pressure limiting well chamber communicated with the water guide pipe is arranged above the bottom plate and can be observed by naked eyes and intermittently pumped and drained. And after the bottom plate reaches the strength, the surrounding dewatering wells are closed, underground water passively overflows after being filtered by the double-layer reverse filter only when the water level exceeds a set threshold value, and then is pumped and drained by the small-power movable pump in a short time, so that the conversion from continuous forced drainage to intermittent weak drainage is realized. The system does not need continuous water pumping, disturbance to surrounding underground water level is small, safe overflow can still be achieved during power failure, construction is easy and convenient, manufacturing cost is low, and replicability is high.

Description

Technical Field

[0001] This invention relates to groundwater control technology during the construction of underground buildings. Specifically, it is a system and method for achieving temporary passive anti-buoyancy drainage using a double-layer reverse filter pressure limiting well structure during the "window period" when the basement roof has not yet been covered with soil and the base slab has been completed. Background Technology

[0002] In areas with high groundwater levels, basement construction must go through three stages: excavation of the foundation pit, pouring of the foundation slab, and covering of the top slab with soil. Regulations require that before the top slab is covered with soil, the buoyancy of water on the outside of the basement and under the foundation slab must not exceed the ratio of the structure's self-weight to its anti-buoyancy coefficient; otherwise, continuous dewatering is necessary to ensure safety.

[0003] The current practice involves installing temporary dewatering wells around the foundation pit and using submersible pumps to continuously pump water 24 hours a day, stabilizing the water level at 0.5m to 1.0m below the bottom of the foundation slab. This approach has the following drawbacks: Large pumping volume and high cost – taking a certain project as an example, 27 dewatering wells, each with a depth of 10m. 3 / h, annual pumping capacity approximately 2.36 million m³ 3 The water resource fee alone amounted to 1.53 million yuan; It creates a continuous drop funnel on the surrounding groundwater level, which can easily lead to ground subsidence and uneven deformation of adjacent buildings. The operation and management costs of water pumps, pipelines, and duty stations are high, and there is a risk of instantaneous buoyancy failure once the power is interrupted. The pumping process is "active pumping," which makes it impossible to directly determine the actual water level under the bottom plate, indicating that the monitoring methods are outdated.

[0004] To reduce the frequency of rainfall, some projects embed drainage holes or collection ditches in the foundation slab to directly introduce seepage into the collection pit before pumping it out. However, this approach lacks reverse filtration measures, which can easily lead to the influx and blockage of fine sand. Furthermore, the drainage holes cannot form a controllable "pressure limit" threshold, requiring long-term manual pumping, thus failing to fundamentally solve the aforementioned problems.

[0005] Therefore, there is an urgent need for a temporary anti-buoyancy drainage technology that can be used to close the dewatering wells after the foundation slab construction is completed, requires no power, causes little disturbance to the surrounding water level, and allows for direct monitoring. Summary of the Invention

[0006] This invention aims to provide a double-layer reverse filter pressure-limiting well structure and a corresponding temporary passive anti-buoyancy drainage method that can significantly reduce pumping costs associated with long-term active pumping before the basement roof is covered with soil. The core idea is to construct a coupled drainage system of "deep and shallow double-layer reverse filters + top open pressure-limiting well" under the basement slab, using the natural pressure of groundwater to set the overflow threshold. When the water level rises to approximately 0.5 m below the bottom of the base slab (adjustable according to anti-buoyancy calculations), the groundwater passively overflows into the pressure-limiting well after passing through the double-layer reverse filter, and is then intermittently pumped out by a low-power mobile pump or temporary drainage pipe. This transforms the traditional "continuous strong drainage" into "intermittent weak drainage," achieving a low-energy, low-water-cost, visible, and closable temporary anti-buoyancy measure.

[0007] To achieve the above objectives, the present invention is implemented as follows: A double-layer reverse filter pressure limiting well structure is used for temporary passive anti-buoyancy drainage of basement before the roof slab is covered with soil, including a sump pit with a depth ≥1.5 m and brick formwork / concrete wall. Double-layer reverse filter: 25–40 mm pebbles (lower layer), 7–15 mm crushed stone (upper layer), with 200 g / m² of gravel laid between layers and on the top surface. 2 Geotextiles form a "coarse-to-fine" barrier to prevent fine sand from flowing in and causing blockages; Pressure limiting water pipe: Φ159 mm×4.0 mm stainless steel pipe, with the top end ≥500 mm lower than the bottom of the base plate, forming a passive pressure limiting system that "overflow only occurs when the water level exceeds this threshold"; Perforated cover: ≥20 mm hole diameter, to prevent large debris; Pressure limiting chamber: connected to the top of the water pipe, with visual observation and temporary pumping interface.

[0008] Function: After the foundation slab is completed, the dewatering well is closed, and the groundwater only fluctuates within a local range of 0.5 m, achieving passive anti-buoyancy with low energy consumption and zero water cost.

[0009] Furthermore, the total thickness of the filter layer is limited to 300–500 mm; an additional waterproof membrane is installed between the upper crushed stone layer and the base slab cushion layer. This prevents the concrete slurry from seeping down and contaminating the filter layer, maintaining long-term permeability stability; the thickness range balances water flow capacity and pit space constraints.

[0010] Furthermore, the water inlet section of the guide pipe has Φ8–Φ10 mm perforated holes, spaced 100 mm longitudinally, arranged in four rows circumferentially, and wrapped with 300-mesh stainless steel wire mesh. This ensures even water intake and prevents localized suction from carrying away fine particles; the 300-mesh pore size is slightly smaller than the minimum filter media particle size, forming secondary protection and extending the life of the reverse filter layer.

[0011] Furthermore, the pressure-limiting well chamber uses a Φ600 mm prefabricated PVC / fiberglass inspection well, with an openable well cover at the wellhead and a float valve below. This standardization reduces costs; the float valve automatically closes the pumping interface when the water level in the well falls below the lower limit, preventing the mobile pump from running dry and improving on-site reliability.

[0012] Furthermore, the bottom of the sump pit is 1.9–2.4 m lower than the bottom of the base slab, and a 100 mm thick, 5–10 mm thick leveling gravel layer is laid at the bottom. This 1.9–2.4 m depth provides ≥1 m of depth. 3 Effective volume, thus capable of caching 10 m 3 The typical seepage flow rate is about 2.4 h, allowing time for drainage; the leveling layer ensures the verticality of the water pipe and prevents deviation from causing elevation error at the overflow outlet.

[0013] Meanwhile, this invention also proposes a temporary passive anti-buoyancy drainage method for basements, including... a) A sump and surrounding dewatering wells will be installed simultaneously in front of the foundation slab; b) Lay the filter layer, bury the pipes, and pour the base slab; c) Reduce or stop the dewatering wells after the base slab has reached its strength; d) Passive overflow occurs when the water level rises to the top of the water pipe; e) Monitor the water level in the well chamber and pump water intermittently when it exceeds the warning line; f) The system is sealed once the top slab is covered with soil.

[0014] This method changes the drainage measures from "continuous strong drainage" to "intermittent weak drainage", which can save costs and improve efficiency, and can still safely overflow even in the event of a power outage.

[0015] Furthermore, the warning height is set at 70% of the well chamber depth, the pumping pump is ≤1.1 kW, the pumping time is ≤15 minutes, and the pumping frequency is ≤4 times per day. This minimizes power consumption and time, reducing operating costs and on-site noise, while meeting environmental protection requirements for nighttime construction.

[0016] Furthermore, if the water level rises by less than 100 mm for 48 consecutive hours and there are no signs of the bottom plate rising, the surrounding dewatering wells can be completely shut down. This provides a quantitative indicator for pump shutdown, avoiding safety risks associated with experience-based decisions and facilitating supervision and acceptance.

[0017] Furthermore, pressure-limiting wells are symmetrically arranged at the four corners and the middle of the long side of the basement, with each well controlling an area of ​​≤400 m². 2 This ensures a uniform drop in the groundwater level beneath the foundation slab, preventing sudden changes in bending moment or differential uplift of the foundation slab caused by localized water head differences.

[0018] Finally, an automatic water level recorder is installed above the well chamber, and the data is uploaded to the foundation pit monitoring platform in real time. Alarms are automatically sent when the threshold is exceeded. This enables unattended operation and remote monitoring, reduces the frequency of manual inspections, and improves the level of information management.

[0019] In summary, compared with the prior art, the present invention has the following advantages: (1) Close the dewatering wells in advance: Once the foundation slab is completed, the original "continuous strong drainage" can be changed to "intermittent weak drainage". Taking a certain project as an example, the annual pumping volume is reduced from 2.36 million m³. 3 Dropped to approximately 40,000 m 3 This will save approximately 1.28 million yuan in water costs. (2) Low energy consumption and low water cost: The overflow process requires no power and is only pumped out briefly when the water level in the well chamber exceeds the warning line, reducing energy consumption by more than 90%. (3) Minimal impact on surrounding areas: The groundwater only fluctuates locally within 0.5 m below the bottom slab, without forming a large-area drawdown cone, effectively protecting adjacent buildings and municipal pipelines; (4) Visualized control: The water level in the well chamber can be read by the naked eye, or an automatic water level recorder can be installed to realize remote monitoring and alarm; (5) Simple construction and low cost: It can be formed by using conventional foundation pits, trenches and bottom slabs, without increasing the additional land area. The materials are all common filter materials and pipes on the market. The cost of a single well is less than the monthly operating cost of a dewatering well. (6) High safety redundancy: In the event of a power outage, the system can still automatically drain water through the overflow port to avoid the risk of instantaneous buoyancy failure. Attached Figure Description

[0020] Figure 1 This is a cross-sectional view of the double-layer reverse filter pressure limiting well structure involved in this invention. Detailed Implementation

[0021] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0022] Example 1: I. Project Overview like Figure 1 A project is located in a provincial capital city in Southwest China. The foundation pit covers an area of ​​approximately 14,000 square meters, with two underground levels. The bottom slab elevation is -9.30 m, and the slab thickness is 600 mm. The local stable groundwater level is -2.80 m. Traditional dewatering methods require 32 dewatering wells to be installed and continuously operated until the top slab is covered with soil. The estimated annual pumping volume is 2.8 million cubic meters. 3The total cost of water and daily operating fees is approximately 1.85 million yuan. To save on construction costs and reduce the impact on surrounding buildings, the construction unit adopted the "double-layer reverse filter pressure limiting well structure + temporary passive anti-buoyancy drainage method" of this invention for groundwater control.

[0023] II. Construction Preparation After the foundation pit is excavated to -9.90 m below the bottom of the foundation slab, symmetrical lines are laid out at the four corners and the middle of the long side of the basement to determine eight sump pits. Each pit has a plan dimension of 800 mm × 800 mm and a bottom elevation of -12.30 m (2.40 m lower than the bottom of the foundation slab). This ensures that the controlled area of ​​each pit is ≤400 m². 2 The requirements are as follows: A 120 mm thick brick formwork should be constructed as the pit is excavated to protect the walls and prevent collapse.

[0024] III. Filter Layer Laying First, lay a 100 mm thick layer of leveling crushed stone with a particle size of 5–10 mm at the bottom of the pit and compact it. Then, lay a lower layer of pebbles (25–40 mm, 200 mm thick) and an upper layer of crushed stone (7–15 mm, 200 mm thick) in sequence. A 200 g / m² layer is then laid between the two layers and on top of the upper layer. 2 Geotextile, with a 150 mm overlap, is hot-air welded to form a double-layer filter barrier, preventing fine particles from entering the sump.

[0025] IV. Water pipe installation A Φ159 mm × 4.0 mm stainless steel pipe was selected as the water guide pipe. After sealing the bottom, Φ8 mm quincunx-shaped water inlet holes (100 mm longitudinally, 4 rows circumferentially) were drilled within a range of 0–1.90 m. The hole openings were ground smooth and wrapped with 300-mesh stainless steel wire mesh, and then secured with stainless steel wire. The water guide pipe was placed vertically in the center of the pit, with the top of the pipe at an elevation of -10.40 m (i.e., 0.50 m below the bottom of the base slab). A steel grid frame was used for temporary positioning to ensure that the subsequent concrete pouring would not be misaligned.

[0026] V. Construction of the base slab and well chamber During the construction of the subbase and waterproofing layer, the waterproofing layer extends 300 mm up around the pit opening, overlapping with the geotextile to form a reverse-wrapping joint. The base slab concrete is poured in one go, and after curing for 7 days, a 6 mm thick perforated stainless steel cover plate (with 6 evenly distributed Φ20 mm holes) is installed at the top of the water pipe and spot-welded to the pipe opening. A Φ600 mm high brick well chamber is built around the cover plate, with a Φ50 mm water level observation hole and a Φ75 mm temporary pumping interface reserved in the well wall. The interface is 200 mm above the top of the cover plate to facilitate the insertion of a mobile pump.

[0027] VI. System Operation After the base slab reaches its strength, the dewatering wells are closed one by one, allowing the groundwater level to rise naturally. When the water level rises to -10.40 m from the top of the water pipe, the groundwater overflows into the well chamber after being filtered through a double-layer reverse filter, achieving zero-powered drainage. The water level is recorded daily at 8:00 and 20:00 via observation wells. If the water level rises by less than 100 mm for 48 consecutive hours and the base slab settlement difference is ≤0.5 mm, the remaining dewatering wells are completely closed. If the water level in the well chamber rises to 70% of its depth during this period, a 1.1 kW mobile pump is started for 15 minutes, no more than 4 times a day, to maintain a stable water level.

[0028] VII. Effect Verification Before the roof was covered with soil, the eight pressure-limiting wells operated for a total of 60 days, with a total pumping volume of only 3200 m³. 3 Compared to traditional precipitation methods, it reduced precipitation by approximately 98%; the maximum drop in groundwater level in the surrounding area was 0.6 m, and no ground subsidence alarm was triggered; water and operating costs were saved by approximately NT$1.62 million, achieving the expected goals.

[0029] 8. Capping and Recycling After the top slab is covered with soil and the anti-buoyancy safety factor is ≥1.05, the top of the water pipe is fully welded with steel plate, the well chamber is filled with medium and coarse sand up to the top of the cover plate, and then C15 concrete is poured to be level with the ground. All stainless steel components are recycled and reused, and there are no obstacles left on the site.

[0030] IX. Comparison of Economic Benefits Traditional rainwater well solutions (not using this invention) Rainfall cycle: 365 days (from excavation to backfilling completion) Number of rainwater wells: 32 Single well flow rate: 10 m³ 3 / h Annual pumping volume = 32 × 10 × 24 × 365 = 2.803 million m³ 3 Water resource fee: 0.65 yuan / m³ 3 → Total water bill = 280.3 × 0.65 ≈ 1,822,000 yuan Water pump shift: 32 units × 365 days × 1 shift / day × 150 yuan / shift → Daily shift fee = 1.752 million yuan The total cost of the traditional solution is approximately 182.2 + 175.2 = 357.4 million yuan. This invention presents a dual-layer reverse filter pressure-limiting well scheme. Rainfall cycle: 60 days (only from structural completion to soil covering stage) Number of pressure-limiting wells: 8 (replacing 32 dewatering wells) Passive overflow pumping rate: 53 m³ (measured average) 3 / d·seat, 60d total displacement = 8 × 53 × 60 ≈ 0.32 million m 3 Water resource fee: 0.65 yuan / m³ 3 → Water fee = 0.32 × 0.65 ≈ 0.21 million yuan Intermittent ventilation shift: 8 seats × 60 days × 1 shift / day × 150 yuan / shift → Daily shift fee = 72,000 yuan Well construction cost (materials + labor): 8 × 0.45 million yuan = 3.6 million yuan The total cost of this invention is approximately 0.21 + 7.2 + 3.6 = 10.97 million yuan. Saving effect Direct cost savings: 357.4 - 10.97 ≈ 346.4 million yuan Savings percentage: 346.4 / 357.4 ≈ 97% Savings per cubic meter of water: Traditional price: 1.27 yuan / m 3 → This invention costs 0.034 yuan / m 3 It decreased by 97%, in line with the reduction in daily shift fees.

[0031] Hidden benefits The groundwater level in the surrounding area dropped from 4.5 m to 0.6 m, avoiding a ground subsidence. The compensation cost was approximately 800,000 yuan (based on experience).

[0032] The dewatering wells were shut down 305 days in advance, releasing power, pipelines, and personnel on duty, with indirect costs amounting to approximately 250,000 yuan.

[0033] in conclusion Within the 14,000㎡ foundation pit of this project, the use of this invention resulted in a one-time direct operating cost saving of 3.46 million yuan, and with the added implicit benefits, the total savings amounted to ≥ 4.5 million yuan, with an investment payback period of < 0.5 months, demonstrating significant economic benefits.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A double-layer reverse filter pressure-limiting well structure for temporary passive anti-buoyancy drainage of basements before the roof slab is covered with soil, characterized in that: include: The sump pit is located below the basement floor slab and has a depth of ≥1.5 m. The pit walls are formed by brick formwork or concrete lining. Double-layer filter layers are laid sequentially from the bottom of the pit upwards: —Lower filter layer: pebbles, 150–250 mm thick, 25–40 mm in diameter. —Upper filter layer: 150–250 mm thick, 7–15 mm particle size crushed stone, A layer of 200 g / m² is laid between the two layers and on the top surface of the upper layer. 2 Geotextiles are used to prevent fine particles from seeping into the sump. The pressure-limiting water pipe is vertically embedded in the center of the pit bottom. It is a Φ159 mm × 4.0 mm stainless steel pipe. Its inlet section has a plum blossom-shaped water inlet hole and is wrapped with a nylon filter screen. The top of the water pipe is ≥500 mm lower than the bottom surface of the basement floor slab to form a passive overflow pressure limit threshold. A perforated cover plate is fixed to the top of the water pipe, with an opening diameter ≥20 mm, allowing groundwater overflow while preventing debris from entering; The pressure-limiting well chamber is located above the bottom plate and connected to the top of the water pipe. The side wall of the well chamber is reserved with water level observation holes and temporary pumping interfaces for intermittent pumping of water collected in the pit.

2. The dual-layer reverse filter pressure-limiting well structure according to claim 1, characterized in that: The total thickness of the double-layer filter layer is 300–500 mm, and an additional waterproof membrane is installed between the upper layer of crushed stone and the bottom slab to prevent concrete slurry from seeping into the filter layer.

3. The dual-layer reverse filter pressure-limiting well structure according to claim 1 or 2, characterized in that: The length of the water inlet section of the water pipe is equal to the depth of the water collection pit. The diameter of the water inlet hole is Φ8–Φ10 mm, the longitudinal spacing is 100 mm, and there are 4 rows evenly distributed in the circumference. The nylon filter screen is a 300-mesh stainless steel wire mesh.

4. The dual-layer reverse filter pressure-limiting well structure according to claim 1, characterized in that: The pressure-limiting well chamber uses a Φ600 mm prefabricated PVC or fiberglass inspection well. The wellhead is equipped with an openable well cover, and a detachable float valve is installed below the well cover. When the water level in the well is lower than the set lower limit, the pumping interface will be automatically closed to prevent dry pumping.

5. The dual-layer reverse filter pressure-limiting well structure according to claim 1, characterized in that: The bottom elevation of the sump pit is 1.9–2.4 m lower than the bottom elevation of the basement floor slab. A 100 mm thick layer of leveling gravel with a particle size of 5–10 mm is laid at the bottom of the pit to ensure the verticality of the water pipe and the uniformity of water inflow.

6. A temporary passive anti-buoyancy drainage method for basements, characterized in that: The dual-layer reverse filter pressure-limiting well structure as described in any one of claims 1–5 is adopted and implemented according to the following steps: a) After the foundation pit is excavated to the design elevation, sump pits are arranged at intervals of 10–25 m within the basement floor area, and temporary dewatering wells are constructed around the perimeter at the same time; b) Lay a double-layer filter layer and geotextile according to the design requirements, pre-embed pressure limiting water pipes, pour the bottom slab concrete, and expose the top of the water pipes and cover them for protection. c) After the foundation slab concrete reaches the design strength, close or reduce the number of surrounding dewatering wells in operation to allow the groundwater level to rise naturally. d) When the groundwater level rises to the top elevation of the water pipe, the groundwater enters the sump after being filtered by the double-layer filter layer and then passively overflows into the pressure limiting chamber through the water pipe. e) The water level is monitored in real time through the water level observation hole of the pressure limiting well chamber. When the water level in the well chamber reaches the warning height, a mobile low-power water pump is used to intermittently pump the water to the site drainage network. f) After the top slab is covered with soil and meets the anti-buoyancy design requirements, the top of the water pipe is sealed, the pressure limiting chamber is backfilled, and the permanent dewatering system is put into operation.

7. The temporary passive anti-buoyancy drainage method for basements according to claim 6, characterized in that: In step e), the warning height is 70% of the well depth, the pump power is ≤1.1 kW, the single well pumping time is ≤15 min, and the number of pumping times per day is ≤4.

8. The temporary passive anti-buoyancy drainage method for basements according to claim 6 or 7, characterized in that: After step c), the groundwater level rise rate is recorded through the water level observation well. When the water level rise is less than 100 mm for 48 consecutive hours and there is no sign of the bottom structure floating, the surrounding dewatering wells are completely shut off.

9. The temporary passive anti-buoyancy drainage method for basements according to claim 6, characterized in that: The double-layer reverse filter pressure limiting well structure is symmetrically arranged at the four corners and the middle of the long side of the basement, with a single well controlling an area of ​​≤400 m². 2 This is to ensure that the groundwater level under the foundation is uniformly controlled.

10. The temporary passive anti-buoyancy drainage method for basements according to claim 6, characterized in that: An automatic water level recorder is installed above the pressure-limiting well chamber. The data is uploaded to the foundation pit monitoring system in real time. When the water level exceeds the set threshold, an alarm message is automatically pushed to the mobile terminal of the management personnel.

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