Deep foundation pit construction method based on cooperation of tube well dewatering and graded excavation

Abstract

The invention discloses a deep foundation pit construction method based on cooperation of tube well dewatering and graded excavation, and belongs to the technical field of civil engineering construction. The method comprises the steps that firstly, tube wells are arranged on the periphery of a foundation pit, a water pumping system is started, and the underground water level is lowered to the safe height below the foundation pit bottom elevation; and then based on the stress state of the enclosure structure and the stratum permeability characteristic, the excavation process of the foundation pit is divided into a plurality of graded construction stages, and the water pumping amount and the excavation depth of the tube well are synchronously adjusted in all the stages so as to maintain dynamic balance of water and soil pressure. The rainfall scheme and the excavation rhythm are subjected to feedback regulation and control by monitoring key indexes such as the water level, ground surface settlement and building envelope deformation in real time, and accurate control over the seepage field and the stress field is achieved. According to the method, geological disasters such as water inrush, piping and bottom upheaval of the foundation pit in deep foundation pit construction are effectively prevented, the risk of differential settlement of the surrounding environment is reduced, the stability of a supporting structure and the construction safety are improved, and the method has the advantages of being high in implementation performance and high in engineering application value.

Description

Technical Field

[0001] This invention relates to the field of civil engineering construction technology, and in particular to a deep foundation pit construction method based on the synergy of well dewatering and staged excavation. Background Technology

[0002] Deep foundation pit engineering is a widely adopted foundation construction method in the context of rapid urbanization, commonly used in projects such as rail transit, underground utility tunnels, large underground parking lots, and high-rise building foundations. With the continuous expansion of underground space development, the depth of deep foundation pits is constantly increasing, significantly raising the requirements for stress and deformation control of the retaining structure, and consequently increasing construction safety risks. During the excavation process, the combined effects of external loads, soil properties, and groundwater pressure can easily cause geological disasters such as pit bottom heave, excessive lateral displacement, and surrounding surface subsidence, threatening the safety of surrounding buildings and above-ground and underground pipelines.

[0003] To ensure the stability and construction safety of foundation pits, current projects commonly use diaphragm walls or SMW (subsurface wall) piles as retaining structures, supplemented by internal bracing and reverse construction methods to control deformation. However, when the groundwater level is high, foundation pit excavation can cause soil erosion and pore water pressure redistribution, potentially leading to slope instability, piping, and seepage damage. Therefore, foundation pit dewatering has become a crucial aspect of deep foundation pit construction. Currently, the main dewatering methods include lightweight wellpoint, deep wellpoint, and tubular well dewatering. Among these, tubular well dewatering is more widely used in deep foundation pit projects due to its large drawdown depth, wide impact range, and applicability to medium-to-high permeability strata.

[0004] In existing technologies, dewatering and excavation are often carried out sequentially, lacking coordination. This can easily lead to problems such as dewatering lagging behind excavation, resulting in the risk of sudden water inrush, or excessive dewatering causing additional ground settlement. Furthermore, the dewatering plan is not dynamically adjusted in a timely manner during the excavation phase, making it difficult to balance construction efficiency with environmental control requirements. Especially in situations involving the intermingling of deep aquifers and soft soil layers, traditional construction methods still have shortcomings in terms of surface settlement control, retaining structure stability, and the extent of groundwater disturbance.

[0005] To address the aforementioned issues, there is an urgent need for a deep foundation pit construction method based on the coordination of well dewatering and staged excavation. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of sudden water inrush and uncontrolled deformation caused by the disconnect between dewatering and excavation operations and the imbalance between stress and seepage fields in existing deep foundation pit construction. This invention proposes a dynamic control method for deep foundation pit construction using a wellpoint joint and coordinated approach. By connecting and controlling multiple deep wells in a hydraulically integrated manner, this invention enables the dewatering range to advance synchronously with the excavation sequence, achieving dynamic and stable control of water level drop and soil unloading in a coordinated manner.

[0007] The present invention achieves the above objectives through the following solutions: A deep foundation pit construction method based on the synergy of well dewatering and staged excavation includes: S1: Construct a cofferdam around the foundation pit and set up temporary drainage facilities to form a construction space isolated from external water bodies; S2: Before the excavation of the foundation pit, several deep well points for dewatering are set up on the outer side of the top of the excavation slope and pumping is started to lower the groundwater level to a safe height below the foundation surface of the foundation pit. S3: The foundation pit is excavated in a multi-stage step manner from top to bottom, and a walkway and foundation pit drainage system are set up to achieve unloading and orderly drainage at each stage. S4: Set up a foundation pit monitoring system and dynamically adjust the dewatering and excavation parameters based on the monitoring data during construction to keep the foundation pit under control until the main structure is completed; S5: After the main backfill forms a covering weight, gradually stop the pump and shut down the well point system.

[0008] As a preferred technical solution of the present invention: the cofferdam is an earth-rock cofferdam, and a rock-filled silt-squeezing layer with a thickness of about 1 m and a width of about 16 m is set below the cofferdam axis; the cofferdam is constructed by layering cohesive soil and the boundary line and slope are controlled according to the settlement superelevation.

[0009] As a preferred technical solution of the present invention: the deep well point dewatering wells are arranged 10–30m outside the top of the excavation slope of the foundation pit, with a well spacing of 20–30m, a well depth of 18–22m and the bottom of the well falling into the permeable sand layer; the number of dewatering wells is 24.

[0010] As a preferred technical solution of the present invention: the dewatering well adopts a well pipe with a filter screen, and the space between the well pipe and the well wall is continuously backfilled with sand and gravel filter material with a particle size larger than the filter screen aperture, and the upper part is sealed with clay; before the dewatering well is put into operation, the stable water output and pressure drop parameters are determined by well cleaning / washing and pumping test.

[0011] As a preferred technical solution of the present invention: the dewatering well is equipped with a submersible pump for independent pumping and drainage. The pumped water is treated in a sedimentation tank and then introduced into a ditch or water body outside the cofferdam, and anti-scouring measures are set at the discharge outlet.

[0012] As a preferred technical solution of the present invention: the multi-stage stepped excavation is at least three stages, the slope ratio of each stage is 1:3 to 1:2, the width of the walkway set at each stage is not less than 2 m, and a protective layer is reserved near the foundation surface for cleaning before the main structure construction.

[0013] As a preferred technical solution of the present invention: the foundation pit drainage system includes a water collection ditch arranged around the foundation pit and a water collection well connected thereto. The bottom elevation of the water collection well is at least 1 m lower than the excavation surface. The seepage water in the pit flows into the water collection well and is then discharged from the foundation pit by a water pump.

[0014] As a preferred technical solution of the present invention: the foundation pit monitoring system includes settlement and horizontal displacement monitoring points set on the top of the support, inclination holes set on the outside of the support, and groundwater level observation holes set on the outer edge of the foundation pit; during the excavation period, continuous observation is carried out every 1-3 days, and after stabilization, re-measurement is carried out every 5-7 days, and the observation is intensified in case of rainstorms or abnormalities.

[0015] As a preferred technical solution of the present invention: when the monitoring data is close to the preset warning threshold, the excavation of the lower layer is suspended and coordinated measures are taken, including increasing the pumping volume or adjusting the well point operating parameters, locally reinforcing the slope or supporting it, and construction is resumed after the safety indicators are met by retesting.

[0016] As a preferred technical solution of the present invention: after the well point system is decommissioned, sand and gravel filter material and low-permeability cohesive soil are backfilled in order from bottom to top and compacted in layers to seal the well holes, thereby restoring the continuity of the foundation and avoiding the formation of potential seepage channels.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves synchronized matching between groundwater level decline and soil unloading by coordinating well dewatering and staged excavation of deep foundation pits. This avoids the risks of piping and sudden water inrush caused by delayed dewatering in traditional sequential construction, while also reducing additional settlement caused by excessive dewatering. This coordinated mechanism keeps the water and soil pressure inside the foundation pit in a dynamic equilibrium, significantly improving the safety of foundation pit construction.

[0018] This invention dynamically optimizes the layout of dewatering wells and the control of pumping volume based on the changes in support stress at each excavation stage. This effectively suppresses deformation of the retaining structure, reduces uneven settlement of surrounding roads, buildings, and underground pipelines, and improves environmental friendliness. Compared with traditional fixed dewatering schemes, this invention can more precisely adjust the infiltration field, reduce the range of groundwater disturbance, and achieve foundation stability control.

[0019] This invention incorporates real-time monitoring data into construction decision-making, adjusting construction parameters through feedback from key indicators such as water level, settlement, and foundation pit deformation, forming a closed-loop control system that improves the flexibility and reliability of construction plans. This method boasts advantages such as high construction efficiency, wide applicability, and outstanding engineering economics, and is particularly suitable for deep foundation pit projects under conditions of high groundwater levels, soft soil layers, or complex geological conditions, demonstrating significant potential for widespread application.

[0020] The method of this invention is applicable to the construction of deep foundation pits in subway station foundation pits, deep underground parking lots, large pump gate projects, underground integrated pipe corridors, and high water level areas in soft soil foundations. It can also be extended to reverse construction methods, pile anchor support and steel support systems. It can carry out secondary optimization of design parameters under different strata permeability conditions, and has wide engineering applicability and promotion value. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is an overall flowchart of a deep foundation pit construction method based on the synergy of well dewatering and staged excavation according to the present invention. Figure 2 This is a block diagram of a foundation pit monitoring and control system based on a deep foundation pit construction method that combines well dewatering and graded excavation, according to the present invention. Detailed Implementation

[0023] The following will refer to the appendices in the embodiments of the present invention. Figure 1-2 The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention demonstrates excellent adaptability to deep foundation pit engineering in complex permeable strata. This embodiment uses the construction of a deep foundation pit with well-developed silty soil and sand layers, and a groundwater level approximately 6m above the foundation surface, as an example, the foundation pit of the Yuma Dam main project. Drilling revealed that the strata in this work area mainly consisted of ②-1 silty soil covering ②-3 sandy loam and fine sand layers, exhibiting high permeability. During drilling, the groundwater level remained stable at an elevation of 19.20m, while the designed foundation surface elevation was approximately 13.60~13.90m. After excavation, there was almost no effective cohesive soil impermeable layer. Without dewatering measures, there would inevitably be risks of water inrush, piping, and even pit bottom heave. Therefore, the solution adopted in this invention can maintain a dry construction environment while ensuring safety, achieving high-quality operation.

[0025] Before formal excavation, the construction of a cofferdam around the foundation pit was carried out first. The cofferdam adopted a composite earth-rock structure. A rockfill layer with a thickness of about 1m and a width of about 16m was constructed below the cofferdam axis to solidify the weak foundation and increase its seepage resistance through gravity infiltration. At the same time, the cofferdam filling adopted the method of layered spreading and compaction of clay, with allowance for settlement superelevation, ultimately forming a closed water-blocking protective zone to ensure that the construction area is not affected by external river water. After the cofferdam was completed, drainage facilities were constructed around the construction face to give the working face an independent water safety defense system.

[0026] After the cofferdam is completed, the construction of the foundation pit dewatering system will commence. Twenty-four deep dewatering wells will be installed 10-30m outside the top of the excavated slope, with a well spacing of 20-30m. Each well will be 18-22m deep, penetrating into the sand layer to form an effective pumping channel. The well casing will use sand-free concrete slotted filter pipes to ensure smooth water intake and intercept sand particles. The well walls will be filled with 1-1.5mm gravel filter material with a particle size larger than the filter mesh size. The upper part will be backfilled and sealed with clay to prevent surface water short-circuiting and backflow. Before the dewatering wells are put into operation, well cleaning and pumping tests will be conducted to accurately determine the pumping capacity and stable water output of the well group. The operating conditions will meet the design requirement of a foundation pit inflow of 7078m³ / d. Multiple deep wells will be monitored uniformly through a single control system to monitor pumping volume, drawdown, and hydraulic response between wells, forming a coordinated dewatering mode to avoid localized seepage concentration and uneven settlement caused by single-well pumping. In this invention, the wellpoints form a continuous circular hydraulic curtain in the horizontal direction, and the operating parameters are adjusted by rolling as the excavation range expands, so as to achieve precise water level control that is synchronized with the excavation stages.

[0027] The dewatering system operates on a single-well, single-pump basis, using submersible pumps to draw groundwater back to the surface. After sedimentation in a settling basin, the water is discharged into a river or drainage ditch outside the cofferdam. Erosion control measures are implemented at the discharge outlet to prevent erosion of the surrounding embankment slopes. The dewatering system begins operation before earthwork excavation, gradually lowering the groundwater level to approximately 0.5m below the bottom elevation of the excavation pit, ensuring a dry environment during excavation. The entire dewatering cycle typically lasts about 5 months, until the main backfill provides sufficient weight.

[0028] Under stable rainfall conditions, the foundation pit excavation was carried out in three stages from top to bottom. The first stage excavated from the natural ground to the initial design elevation, with a slope ratio of 1:3 to 1:2, forming a walkway more than 2 meters wide. The second stage continued with the same slope parameters until the third stage approached the foundation surface, where unloading occurred layer by layer, simultaneously forming a graded stress-relief structure. After each stage of excavation was completed, the slope was promptly trimmed and shaped, and drainage ditches were installed on the slope surface to allow collected water to flow into collection wells along the ditches, preventing surface water accumulation that could cause softening and slippage. During the excavation process, scouring and deep trenching operations were strictly prohibited to prevent damage to the overall slope stability.

[0029] To further collect seepage water within the foundation pit, collection ditches are constructed around the perimeter, and collection wells are used for low-level collection and drainage. The bottom elevation of the wells is controlled at least 1 meter below the excavation surface, ensuring that any seepage water can be discharged smoothly, forming a complete combined drainage system of "dewatering well - collection ditch - collection well - external drainage". This invention particularly emphasizes the simultaneous existence of dewatering and drainage at two levels, preventing groundwater seepage from forming upward pressure peaks and effectively improving the pit bottom's resistance to sudden inrush.

[0030] To ensure structural safety during the excavation phase, this invention employs a foundation pit monitoring system. This system includes observation points for monitoring the settlement and horizontal displacement of the support top, inclinometer boreholes positioned 1-2m from the outer edge of the support for detecting deep lateral deformation, and groundwater level observation boreholes positioned 2-5m from the outer edge of the support to detect changes in groundwater depth using water level gauges, accurately reflecting the effectiveness of dewatering control. Monitoring is conducted every 1-3 days during excavation, and every 5-7 days after the foundation pit deformation enters a stable phase. In case of abnormal conditions such as heavy rain or increased displacement, the monitoring frequency is immediately increased. The monitoring results form a real-time feedback mechanism, linked to the operation and adjustment of dewatering wells and the excavation rhythm, achieving full-time closed-loop control.

[0031] If monitoring data approaches the preset threshold, such as a sudden increase in support deformation, a delayed drop in water level, or the occurrence of localized sand seepage, excavation of the lower layers should be immediately suspended. Targeted measures should be taken, such as adjusting drainage capacity, adding local support structures, or installing additional water-stopping measures. Construction can only resume after the monitoring data stabilizes, ensuring that each step proceeds within safe control limits. Daily monitoring data is technically analyzed weekly and reported to the supervision unit. The decision-making level reviews the construction status and deformation trends to ensure timely information transmission.

[0032] Once the main structure and its backfill layer provide sufficient weight and cover to effectively contain groundwater, the pumping rate from the dewatering wells is gradually reduced to allow for a safe transition period as groundwater levels rise. During the well shutdown phase, stability is confirmed through borehole-by-bore observation and pressure balance testing, after which all well operations are ceased. To prevent the formation of new seepage channels in the wells, this invention specifies that the well sealing process involves backfilling and compacting in layers of gravel and low-permeability cohesive soil from bottom to top, restoring the integrity of the foundation, achieving the harmless withdrawal of the dewatering system, and finally removing the temporary cofferdam and drainage facilities.

[0033] The results of this invention show that throughout the entire deep foundation pit construction cycle, the working area maintained a dry and safe operating environment. No piping, sand inrush, or heave occurred at the bottom of the pit. The deformation of the support structure remained within permissible standards, dewatering operations were stable, and all monitoring indicators were within the pre-controlled range. Compared to traditional strategies that rely solely on dewatering wells or passive support, this invention, through a full-chain active control system of "dewatering—excavation—drainage—monitoring—feedback," significantly reduces construction risks, shortens the amount of dewatering and construction period, reduces energy costs, and improves the overall structural construction quality and efficiency. It has significant engineering application value and promotional significance.

[0034] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A deep foundation pit construction method based on pipe well dewatering and staged excavation cooperation, characterized in that, The method comprises the following steps: S1: building a cofferdam around the foundation pit and setting temporary drainage facilities to form a construction space isolated from the external water body; S2: before the foundation pit is excavated, a plurality of deep well point dewatering wells are arranged outside the excavation slope top and are started to be pumped to lower the underground water level to a safe height below the foundation level of the foundation pit; S3: the foundation pit is excavated in a multi-stage stepped manner from top to bottom, and a horse path and a foundation pit drainage system are arranged to realize step-by-step unloading and orderly drainage; S4: a foundation pit monitoring system is arranged, and the dewatering and excavation parameters are dynamically adjusted based on the monitoring data during the construction process to keep the foundation pit in a controlled state until the main structure is completed; S5: after the main backfilling forms a cover pressure, the pump is gradually stopped and the well point system is closed.

2. The deep foundation pit construction method based on the cooperation of tube well dewatering and staged excavation according to claim 1, characterized in that: The cofferdam is a soil and rock cofferdam, and a riprap silt-extruding layer is arranged below the axis of the cofferdam, with a thickness of about 1 m and a width of about 16 m; the cofferdam is filled with layered cohesive soil and the edge line and slope are controlled according to the settlement super-high.

3. The deep foundation pit construction method based on the cooperation of tube well dewatering and staged excavation according to claim 1, characterized in that: The deep well point dewatering wells are arranged at a position 10-30 m outside the excavation slope top of the foundation pit, with a well spacing of 20-30 m, a well depth of 18-22 m, and a well bottom falling into a permeable sand layer; the number of dewatering wells is 24.

4. The deep foundation pit construction method based on pipe well dewatering and staged excavation cooperation according to claim 3, characterized in that: The dewatering wells are provided with filter screen well pipes, the well pipe and the hole wall are once continuously backfilled with sand and gravel filter material with a particle size greater than the filter screen aperture, and the upper part is protected with clay; before the dewatering wells are put into operation, the hole is cleaned / washed and water pumping test is conducted to determine the stable water yield and pressure drop parameters.

5. The deep foundation pit construction method based on pipe well dewatering and staged excavation cooperation according to claim 3, characterized in that: The dewatering wells are independently pumped by a matching submersible pump, the pumped water is treated in a sand settling tank and then introduced into a ditch or water body outside the cofferdam, and anti-scouring measures are arranged at the discharge port.

6. The deep foundation pit construction method based on the cooperation of tube well dewatering and staged excavation according to claim 1, wherein, The multi-stage stepped excavation is at least three stages, the slope ratio of each stage is 1:3-1:2, the width of the horse path arranged at each stage is not less than 2 m, and a protection layer is reserved near the foundation level and cleaned before the main structure construction.

7. The deep foundation pit construction method based on pipe well dewatering and staged excavation cooperation according to claim 1, characterized in that: Among them, The foundation pit drainage system comprises a water collecting ditch arranged along the periphery of the foundation pit and a water collecting well connected thereto, the bottom elevation of the water collecting well is at least 1 m lower than the excavation surface, and the seepage water in the pit is discharged out of the foundation pit by a water pump after flowing into the water collecting well.

8. The deep foundation pit construction method based on pipe well dewatering and staged excavation cooperation according to claim 1, characterized in that: Among them, The foundation pit monitoring system comprises a settlement and horizontal displacement monitoring point arranged at the top of the support, an inclinometer hole arranged outside the support, and an underground water level observation hole arranged at the outer edge of the foundation pit; during excavation, continuous observation is carried out every 1-3 days, re-measurement is carried out every 5-7 days after stabilization, and observation is intensified when it rains or is abnormal.

9. The deep foundation pit construction method based on the cooperation of tube well dewatering and staged excavation according to claim 8, characterized in that: When the monitoring data approaches the preset warning threshold, the lower excavation is suspended and coordinated treatment is performed, including increasing the pumping amount or adjusting the well point operation parameters, locally reinforcing the slope or support, and continuing the construction after the re-measurement meets the safety index.

10. The deep foundation pit construction method based on the cooperation of tube well dewatering and staged excavation according to claim 1, characterized in that: After the well point system is disabled, the sand and gravel filter material and the low-permeability cohesive soil are backfilled in a layer-by-layer manner from bottom to top to close the well hole, so as to restore the continuity of the foundation and avoid forming a potential seepage channel.

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