Underground water control construction method for deep foundation pit in large steel cofferdam of river with deep and thick sand soil layer geology

Abstract

The invention discloses a method for controlling underground water of a deep foundation pit in a large steel cofferdam in a river with a deep and thick sand soil layer geology. Comprising the steps of S1, water pumping testing, S2, site planning, S3, waterproof cloth laying on the outer side of a cofferdam, S4, catchwater construction on the inner side of the cofferdam, S5, tube well dewatering construction, S6, tube well dewatering operation, S7, well point dewatering construction, S8, well point dewatering operation and S9, dismantling and plugging. The surrounding seepage distance is increased outside the cofferdam, the annular intercepting ditch in the cofferdam is used for blocking open water, a circle of dewatering pipe wells on the inner side of the intercepting ditch, two circles of dewatering pipe wells on the slope top of the foundation pit and dewatering well points at the bottom of the foundation pit are combined, the dewatering range is gradually narrowed to control the underground water level, and five-level measures are constructed in total to achieve accurate dewatering. Precipitation blind areas and excessive disturbance are avoided, and the requirement for large-area precipitation under complex working conditions is met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water conservancy engineering, in particular to a groundwater control method for deep foundation pit in large steel cofferdam of deep sand layer geological river. BACKGROUND

[0002] In the construction of water gate engineering, the foundation pit dewatering under the condition of deep sand layer geological river is always the core link related to the safety and progress of the project. As the key control structure of water conservancy hub, the foundation construction of the water gate needs to be carried out on the dry and stable working surface. The deep sand riverbed has strong water permeability and high water enrichment, and the water head pressure caused by the fluctuation of river water level easily leads to the danger of pipe heave and quicksand in the foundation pit, which seriously restricts the construction safety.

[0003] For the complex structure of large steel cofferdam containing deep foundation pit, the traditional dewatering method has the technical problems of insufficient dewatering range and low water level control precision under complex site environment.

[0004] Therefore, how to create a new groundwater control method for deep foundation pit in large steel cofferdam of deep sand layer geological river is one of the current important research and development topics. SUMMARY

[0005] The present application provides a groundwater control method for deep foundation pit in large steel cofferdam of deep sand layer geological river. The first technical problem to be solved is to solve the problems of blind area of dewatering, difficulty in controlling the water level at the bottom of the deep foundation pit and easy over disturbance in the traditional construction method. The dewatering range is gradually reduced by the multi-stage dewatering combination method to meet the dewatering demand in the high permeability environment. The second technical problem to be solved is to realize the accurate positioning of the dewatering range and effectively control the dewatering cost. The third technical problem to be solved is to solve the problem that the traditional dewatering method has single application scene and cannot be applied to complex and large area environment. Thus, the deficiencies of the prior art are overcome and the blank in the technical field is filled.

[0006] To solve the above technical problems, the present application provides a groundwater control method for deep foundation pit in large steel cofferdam of deep sand layer geological river, and the construction steps include: Step S1: pumping test; Step S2: site planning; Planning the position of tube well dewatering and well point dewatering before construction; The position of tube well dewatering includes the cofferdam inner side dewatering well, the embankment side dewatering well and the deep foundation pit slope top dewatering well; The cofferdam inner side dewatering well is arranged on the inner side of the cut-off trench, the embankment side dewatering well is arranged on the bank, and the cofferdam inner side dewatering well and the embankment side dewatering well jointly enclose a closed ring shape; The deep foundation pit slope top dewatering well is arranged at the deep foundation pit slope top position and arranged in an inner and outer two-circle annular shape around the deep foundation pit; Well point dewatering position planning: the well point pipe is located at the deep foundation pit bottom and forms a closed annular shape around the deep foundation pit bottom; Step S3: cofferdam outside waterproof cloth laying; High-strength waterproof cloth is laid outside the cofferdam, the top of the waterproof cloth is tied and fixed with the cofferdam pile top, the bottom extends to the riverbed 5-8m and is pressed by a sandbag, and the waterproof cloth and the sandbag are tied and fixed; Step S4: cofferdam inside water interception ditch construction; The water interception ditch is excavated along the cofferdam inside, the water interception ditch extends to the riverbank at the head and tail, a plurality of water collecting pits are arranged at intervals along the way of the water interception ditch, and a water pump is arranged at the water collecting pit to continuously pump and drain the accumulated water in the pit; Step S5: tube well dewatering construction; The cofferdam inside dewatering well, the embankment side dewatering well and the deep foundation pit slope top dewatering well are constructed according to the planning position in step S2, the construction sequence is from the outer circle to the inner circle, and the observation well is constructed synchronously; Step S6: tube well dewatering operation; After the cofferdam inside dewatering well, the embankment side dewatering well and the deep foundation pit slope top dewatering well are constructed, operation is started, and the operation sequence is the same as the construction sequence, from the outer circle to the inner circle; The cofferdam inside dewatering well and the embankment side dewatering well are operated, the non-deep foundation pit part construction is started after the site is dried, and the deep foundation pit slope top dewatering well is operated, and the deep foundation pit excavation is started according to the water depth in the observation well; Step S7: well point dewatering construction; A closed annular well point pipe is arranged at the deep foundation pit bottom, the well point pipe is 1.5m away from the foundation pit boundary line, the interval is 1m, each well point pipe is connected to the water collecting main pipe through a rubber hose, the water collecting main pipe is sequentially connected to a vacuum pump and a centrifugal pump, and the pump is first discharged to a water tank and then discharged to the cofferdam outside; Step S8: well point dewatering operation; After equipment commissioning and test dewatering, formal continuous dewatering operation is started; Step S9: removal and plugging.

[0007] Further, the step S2 further includes planning a drainage path, specifically: Drainage path planning: the embankment side dewatering well is connected to the river channel through a shallow buried corrugated pipe; The cofferdam inside dewatering well is directly discharged into the river channel in a single-well direct discharge manner across the cofferdam; When there is no construction content outside the deep foundation pit slope top dewatering well, the single-well direct discharge manner is adopted to directly discharge into the river channel across the cofferdam; When there are construction contents outside the deep foundation pit slope top dewatering well, drainage pipes are arranged along the arrangement path of the deep foundation pit slope top dewatering well 3 to access the water collecting tank, and then the water is collected and discharged into the river across the cofferdam; Power supply line planning: reasonably distribute the position of the electric box according to the type of the water pump, avoid the electric circuit passing under the field traffic, and ensure the smooth traffic in the field.

[0008] Further, the step S4 specifically includes the following steps: Step S41: using the excavator to dig a water intercepting trench with a width of 1m and a depth of 0.5m at the designed position, throwing the excavated sediment to the cofferdam side, forming a 1.5m wide slope-shaped protection belt between the water intercepting trench and the cofferdam, and the highest point of the protection belt near the cofferdam side is 30-50cm higher than the bottom of the water intercepting trench, and the height is insufficient to be filled with gravel; Step S42: filling the water retaining platform inside the water intercepting trench, the width of the water retaining platform is not less than 0.8m, and the gravel soil is used for filling; Step S43: laying geotextile in the water intercepting trench, the width of the geotextile is 4m, the part extending to both sides of the water intercepting trench covers the top of the protection belt and the water retaining platform, and 15cm thick gravel is filled and pressed on the geotextile on the bottom of the water intercepting trench, and the gravel naturally adheres to the slope; Step S44: digging 1.5m×1.5m×1.5m water collecting pits along the water intercepting trench, laying geotextile on the side of the water collecting pit, and laying 200mm thick gravel on the geotextile. A steel filter pipe with a diameter of 1-1.5m is inserted into the water collecting pit, the steel filter pipe is wrapped with 80 mesh filter screen, and a water pump is arranged outside the water collecting pit to continuously pump and discharge the accumulated water in the pit; Further, in the step S5, the well spacing of the cofferdam inside dewatering well and the embankment side dewatering well is 10-15m, the well spacing of the deep foundation pit slope top dewatering well is 6-9m, and all the dewatering wells use sandless concrete well pipes, and the specific construction steps are as follows: Step S51: excavate the mud pool according to the site conditions, use the rotary drilling rig to drill the hole, increase the pump capacity, flush the borehole and dilute the mud after the drilling depth reaches the designed depth, and the drilling depth is greater than the designed depth by 0.5-1.0m; Step S52: wrapping the sandless concrete pipe with 80-100 mesh geotextile, injecting clean water to replace the mud before the well pipe is lowered, and pumping out the sediment with a water pump or a sand pipe; Then, the well pipe is slowly lowered, the outside of the well pipe is temporarily fixed in the vertical direction with 2-4 30mm wide bamboo boards, a section of well pipe is connected when the pipe opening and the well opening are different by 200mm, and the well pipe is continuously lowered to 300mm above the existing ground surface; Step S53: after the well pipe is lowered, the filter material is transported to the well opening with a handcart, then filled into the well with a shovel, and the filter material filling amount is recorded, the filter material filling amount is not less than 95% of the calculated amount, the filter material is filled to 1.5m from the existing ground surface, and the well opening position is filled and compacted with cohesive soil; Step S54: After filling the filler, wash the well in time, use intermittent pumping or piston washing method until the water is clear and the sand content is less than 0.01% by weight; Step S55: Observation wells are arranged at intervals of 50-70m, and the observation well is 1 times the drawdown depth away from the well for observing the actual water level.

[0009] Further, the step S6 includes the following specific steps: Step S61: After the well washing is completed, the pumping and drainage equipment is installed uniformly, the suspension support is set up at the well mouth and fixed, and the water pump is lowered to a position 2m away from the bottom of the well by using a fixed length of rope; Step S62: The water level sensor is suspended 1m above the water pump, and the water level sensor parameters are set so that the water pump is automatically turned on when the water level in the well is greater than 3m above the water level sensor position, and the water pump is automatically turned off when the water level sensor is exposed to the water surface; Step S63: The dewatering wells inside the cofferdam and on the embankment side are always operated without interruption during construction, and the dewatering wells on the top of the deep foundation pit are turned off after the construction of the foundation pit is completed, and all dewatering wells are patrolled daily during operation.

[0010] Further, in step S7, the well point pipe is vertically punched according to the marked hole position by using a high-pressure water gun during construction, and the hole depth is 3.5m. A conical punch with a diameter of 100-120mm is arranged at the front end of the high-pressure rubber pipe during punching, and the water pressure is set to 0.4-0.6Mpa; The well point pipe is a water filter pipe with a diameter of 63-90mm and a material of PVC-U, and the length is 3m, and the bottom is sealed. The filter material is medium-coarse sand filled to 1m below the ground, and the upper 1m is sealed with clay; The diameter of the water collection main pipe is 110mm, and the material is PVC; The diameter of the plastic hose is 32mm, and the valve is arranged on the plastic hose. The plastic hose is connected with the water collection main pipe and the well point pipe by socket connection, the interface is sealed by glue adhesive, and is locked by stainless steel clamp; Every 20-30 well point pipes are equipped with a set of vacuum pump and centrifugal pump, the vacuum degree of the vacuum pump is ≥95kPa, the lift of the centrifugal pump is ≥15m, and the vacuum pump and the centrifugal pump are arranged at a distance of ≥5m from the edge of the deep foundation pit.

[0011] Further, the step S8 includes the following specific construction steps: Step S81: Equipment commissioning; Start the vacuum pump, and the system vacuum degree should reach 70-80kPa within 30min, and the pressure drop should be ≤5kPa within 30min; Step S82: Trial dewatering; The trial dewatering time is 24 hours, and the vacuum degree is maintained at ≥60kPa during the period, and the formal dewatering is carried out after the water is clear. Step S83: uninterrupted formal precipitation.

[0012] Further, the specific construction method of step S9 is: After the well point dewatering operation is completed, the water collection main, vacuum pump and centrifugal pump are removed, and the well point pipe 4 is removed or directly poured into the main structure cushion layer; After the pipe well dewatering operation is completed, graded sand is poured into the dewatering well, and cement, sand and gravel are mixed in a ratio of 2:2:1 at the well mouth to form dry material; Then all the water in the well is pumped out, the dry material is poured into the well after the water pump is quickly removed, and the dry material is poured into the well while tamping to half the well depth, then stop pouring dry material; A small amount of water is added to the dry material to form wet material, which is poured into the well to a distance of 1.5m from the ground, and C20 plain concrete is poured on the top of the wet material to the ground.

[0013] After such a design, the present application has at least the following advantages.

[0014] 1. The present application uses a steel cofferdam to increase the seepage distance and a ring-shaped multi-stage continuous dewatering method to gradually create a construction surface for dry trench operation, avoiding the problem of pipe gushing sand in deep sand riverbed conditions.

[0015] 2. Traditional small steel cofferdams use concrete bottom sealing or self-locking to close to form a complete water-blocking operation surface, but for large steel cofferdams, due to their large area, the concrete bottom sealing construction volume is large, the self-locking bottom leakage points are many, the initial drainage difficulty is high, frequent leakage needs to be repaired in the later period, and the maintenance cost is high. Both traditional construction methods are difficult to form a complete water-blocking operation surface; the present construction method uses geotextile to increase the seepage distance and gravel blind ditch to cut off the leakage, which not only creates effective conditions for the initial dewatering of the cofferdam, but also effectively reduces the influence of cofferdam leakage on foundation pit dewatering.

[0016] 3. The traditional single-row well point dewatering construction method cannot meet the dewatering requirements in high-permeability coefficient environments such as sand-rich strata. The present application uses a combination of multi-layer dewatering wells outside the foundation pit and well point dewatering inside the foundation pit to meet the dewatering requirements in high-permeability environments. At the same time, by gradually reducing the dewatering range from the outside to the inside, precise dewatering is achieved, avoiding the occurrence of dewatering blind areas, and effectively controlling the dewatering cost.

[0017] 4. The application scenario of the present application varies greatly, making it difficult to form a stable seepage method, and it is difficult to manually adjust the opening and closing of each dewatering well, which greatly affects the site construction. Therefore, an automatic dewatering switch is used in combination with the water pump to control the water pump to be automatically started when the water level in the well reaches the preset value, which can effectively reduce the amount of manual work, and at the same time avoid the situation that individual well sites are damaged due to insufficient seepage.

[0018] 5、The application is suitable for the scene of cross construction between the precipitation part and the construction part in a large space, emphasizes the flexible and changeable well site arrangement mode, completes the drainage path, road pipe position and power line planning in advance, optimizes the well site in time according to the site construction progress during the construction process, adjusts the use of the well site in time according to the position of the construction operation surface, realizes the precipitation effect while reducing the precipitation cost under the condition of not affecting the construction operation. BRIEF DESCRIPTION OF DRAWINGS

[0019] The above is only a summary of the technical scheme of the application, in order to enable the technical means of the application to be more clearly understood, the application is further described in detail below in combination with the drawings and specific embodiments.

[0020] Figure 1 It is a construction process schematic diagram of the application.

[0021] Figure 2 It is a plane arrangement schematic diagram of the application.

[0022] Figure 3 It is Figure 2 A-A cross-sectional schematic diagram in the application.

[0023] Figure 4 It is Figure 3 A local enlarged schematic diagram at B in the application.

[0024] Marked for explanation: 1, cofferdam inside dewatering well; 2, embankment side dewatering well; 3, deep foundation pit top dewatering well; 4, well point pipe; 5, cofferdam; 6, water intercepting ditch; 7, water collecting pit; 8, protection belt; 9, water retaining platform; 10, gravel. DETAILED DESCRIPTION

[0025] The application is suitable for the river channel with rich sand stratum with a water depth of 3-5m, carries out dewatering operation under the working condition of deep foundation pit in a large area steel cofferdam, the dewatering area is planar and the dewatering area is >20000m 2 , the foundation pit depth is 3-6m, and the cumulative precipitation water head difference reaches 7-10m. Under such complex working conditions, the traditional single dewatering mode is difficult to meet the dewatering demand, therefore, the application provides a deep thick sand river large steel cofferdam deep foundation pit dewatering construction method, and the specific construction steps are as follows.

[0026] Step S1: pumping test.

[0027] It should be noted that the present application is applicable to the dewatering construction scene with large site area, particularly complex situation, long construction period and multiple site conversions during the construction period. The conventional dewatering method is mainly applicable to the scene with relatively small area, without site conversion, stable water level and seepage. Therefore, the pumping test in the present application is different from the conventional dewatering test, and is mainly used for determining the dewatering basic parameters and the required water pump model of the pipe well dewatering. Since the applicable working condition of the present application is the dewatering with large area, the river bank dewatering and the dewatering of the cofferdam, etc. multiple dewatering wells with different depths, and the dewatering scene will change several times during the construction process with the adjustment of the construction site, the single-hole dewatering test has large deviation from the actual situation, and the dewatering parameters obtained by the single-hole test cannot be applied to the actual dewatering construction. The in-situ multi-hole pumping test should be carried out, and the well depth, well spacing, dewatering pump model and other parameters are determined based on the test.

[0028] The test pipe well is constructed according to the same process as the formal pipe well (detailed below), the spacing of the test pipe well is not greater than the actual dewatering depth (5-12m), the number of test holes is not less than 5, and a separate observation well is arranged at a distance of 1 times the dewatering depth in the middle position for observing the water level.

[0029] The pumping test should be carried out for 3 times of drawdown, each time of drawdown is 1 / 3 of the maximum drawdown depth, and the stable duration time is 8h-24h.

[0030] The dynamic water level and water yield are observed at 5min, 10min, 15min, 20min, 25min, 30min after the start of pumping, and then every 30min or 60min. The water temperature and air temperature are measured synchronously every 2h-4h. After the pumping stops, the recovery water level observation should be carried out, and the observation time interval is the same as that of the dynamic water level observation.

[0031] According to the water level in the observation well, whether to adjust the well depth and well spacing is determined. According to the pumping amount, the size of the water pump is determined, and the pumping capacity of the water pump is greater than the average maximum seepage capacity of the dewatering well, that is, the water pump has the ability to control the water level in the well to be above 2m above the well bottom when it works continuously.

[0032] Step S2: site planning.

[0033] The position of the pipe well dewatering, the position of the well point dewatering, the drainage path and the power supply line are planned before the construction.

[0034] The position of the pipe well dewatering includes the cofferdam inner side dewatering well 1, the embankment side dewatering well 2 and the deep foundation pit slope top dewatering well 3.

[0035] The cofferdam inner side dewatering well 1 is arranged on the inner side of the water intercepting ditch 6, the embankment side dewatering well 2 is arranged on the bank, and the cofferdam inner side dewatering well 1 and the embankment side dewatering well 2 jointly enclose a closed ring shape.

[0036] The inner side dewatering well 1 and the embankment side dewatering well 2, together with the cofferdam and other water interception facilities and interception ditches, form an outer ring water interception system to block water leakage from the cofferdam and seepage in all directions around the cofferdam, and control the groundwater depth to 1-2m below the non-deep foundation pit site, providing dry operation conditions for the non-deep foundation pit area inside the cofferdam.

[0037] The deep foundation pit slope top dewatering well 3 is located at the top of the deep foundation pit slope and is arranged in two concentric rings around the deep foundation pit. The main dewatering range is a local area of ​​the deep foundation pit, which controls the groundwater at the bottom of the deep foundation pit and provides dry working conditions for subsequent deep foundation pit excavation and other construction.

[0038] The locations of the dewatering well 1 on the inner side of the cofferdam, the dewatering well 2 on the embankment side, and the dewatering well 3 on the top of the deep foundation pit slope should avoid the main traffic roads within the site to prevent repeated adjustments to the location of the dewatering wells due to traffic disruption.

[0039] Wellpoint dewatering location planning: Wellpoint dewatering pipe 4 is located at the bottom of the foundation pit, forming a closed ring around the bottom of the deep foundation pit.

[0040] Drainage path planning: Dewatering well 2 on the embankment side discharges into the river channel via shallow-buried corrugated pipes; dewatering well 1 inside the cofferdam discharges into the river channel by crossing cofferdam 5 using a single-well direct discharge method; dewatering well 3 on the top of the deep foundation pit slope discharges according to the following path: When there is no construction work outside the dewatering well 3 at the top of the deep foundation pit, the single well is used to directly discharge water into the river across the cofferdam. When there is construction work outside the dewatering well 3 at the top of the deep foundation pit, a drainage pipe is installed along the layout path of the dewatering well 3 at the top of the deep foundation pit to connect to the collection tank, and then the water is discharged into the river channel by crossing the cofferdam.

[0041] Power supply line planning: The location of electrical boxes should be reasonably allocated according to the water pump model to avoid the circuit passing under the traffic within the site and to ensure smooth traffic flow within the site.

[0042] Step S3: Lay waterproof fabric on the outside of the cofferdam.

[0043] A high-strength waterproof fabric is laid on the outside of cofferdam 5. The top of the waterproof fabric is tied to the top of the cofferdam 5 piles and the bottom extends to the riverbed 5-8m and is weighed down with sandbags. The waterproof fabric and sandbags are tied together.

[0044] Step S4: Construction of the intercepting ditch inside the cofferdam.

[0045] A water interception ditch 6 is excavated along the inner side of the cofferdam 5, with both ends of the water interception ditch 6 extending to the riverbank. Figure 2 (Dotted line on the left side of the middle section) Multiple water collection pits 7 are arranged at intervals along the intercepting ditch 6. Water pumps are installed at the water collection pits 7 to continuously pump out the accumulated water. The specific steps are as follows.

[0046] Step S41: Use an excavator to dig a drainage ditch 6 with a width of 1m and a depth of 0.5m at the designed location. Throw the excavated mud and sand to the side of the cofferdam 5 to form a 1.5m wide sloping protective belt 8 between the drainage ditch 6 and the cofferdam 5. The highest point of the protective belt 8 near the cofferdam 5 is 30-50cm higher than the bottom of the drainage ditch 6. Fill in any gaps in the height with gravel.

[0047] Step S42: Construct a water-retaining platform 9 inside the intercepting ditch 6. The width of the water-retaining platform 9 shall not be less than 0.8m, and it shall be constructed using crushed stone and soil.

[0048] Step S43: Lay geotextile 11 in the intercepting ditch 6. The geotextile 11 is 4m wide and extends to both sides of the intercepting ditch 6 to cover the top of the protective belt 8 and the water retaining platform 9. Fill the bottom of the intercepting ditch 6 with 15cm thick gravel 10 on top of the geotextile 11. The gravel 10 naturally adheres to the slope.

[0049] Step S44: Along the intercepting ditch 6, dig 1.5m×1.5m×1.5m water collection pits at intervals. Lay geotextile around the water collection pits, and then lay a 200mm thick layer of gravel on top of the geotextile. Insert steel filter pipes with a diameter of 1-1.5m into the water collection pits, and wrap the steel filter pipes with an 80-mesh filter screen. Install water pumps outside the water collection pits 7 to continuously pump out the water accumulated in the pits.

[0050] It should be noted that the function of the waterproof cloth outside the cofferdam 5 in step S3 is to increase the seepage distance, and the function of the intercepting ditch 6 in step S4 is to cut off the open water. The two work together to effectively block the leakage of open water and ensure that there is no open water in the working environment of the central area.

[0051] Step S6: Well dewatering construction.

[0052] According to the planned locations in step S2, construct the inner side dewatering well 1, the embankment side dewatering well 2, and the deep foundation pit slope top dewatering well 3. The construction sequence is from the outer circle to the inner circle, and the observation wells are constructed simultaneously.

[0053] The spacing between the dewatering wells 1 on the inner side of the cofferdam and 2 on the embankment side is 10-15m, and the spacing between the dewatering wells 3 on the top of the deep foundation pit slope is 6-9m. All dewatering wells use sand-free concrete well pipes.

[0054] It should be noted that if the deep foundation pit adopts the zonal excavation construction, the dewatering well 3 at the top of the deep foundation pit slope needs to be adjusted according to the excavation range to ensure that it surrounds the outer perimeter of the single area to be excavated.

[0055] The specific construction steps for well dewatering are as follows.

[0056] Step S51: Excavate a mud pit 3m away from the drilling point according to the site conditions, and use a rotary drilling rig for wet drilling. After the drilling depth reaches the designed hole depth, increase the pump flow, flush the borehole, and dilute the mud. The hole depth should ensure that the well depth after flushing is not less than the designed well depth, but it should not be too deep. In this embodiment, the drilling depth is 0.5 to 1.0m greater than the designed depth.

[0057] Step S52: Wrap the sand-free concrete pipe with 80-100 mesh geotextile. Before lowering the well pipe, inject clean water to replace the mud and use a water pump or sand removal pipe to extract the sediment.

[0058] Then, the well pipe is slowly lowered. When it is lowered, 2 to 4 bamboo boards 30mm wide are used to temporarily fix the well pipe vertically. When the pipe opening is 200mm away from the well opening, the next section of well pipe is connected. The well pipe is continuously lowered until it is 300mm above the current ground level.

[0059] Step S53: After the well casing is lowered, use a wheelbarrow to transport the filter material to the wellhead, then use a shovel to fill it into the well and record the amount of filler. The amount of filter material filled should not be less than 95% of the calculated amount. Fill the filter material to a height of 1.5m from the existing ground level, and fill the wellhead area with cohesive soil to make it compact.

[0060] Step S54: After filling is completed, flush the well in time by intermittent pumping or piston flushing until the water is clear and the sand content by weight is less than one ten-thousandth.

[0061] Step S55: Arrange observation wells at intervals of 50-70m, with the distance between the observation wells being one time the precipitation depth, to observe the actual water level.

[0062] Step S6: Well dewatering operation.

[0063] After the construction of the inner side dewatering well 1, the embankment side dewatering well 2, and the deep foundation pit slope top dewatering well 3 is completed, they will be put into operation. The operation sequence is the same as the construction sequence, that is, they will be operated one by one from the outer circle to the inner circle. After the inner side dewatering well 1 and the embankment side dewatering well 2 are in operation, construction of non-deep foundation pit parts will begin after the site dries. After the deep foundation pit slope top dewatering well 3 is in operation, the foundation pit will be excavated according to the water depth in the observation well. The specific operation steps are as follows.

[0064] Step S61: After the well is cleaned, the pumping and drainage equipment will be installed. A suspension support will be erected and fixed at the wellhead. The water pump will be lowered to a position 2m from the bottom of the well using a rope of fixed length.

[0065] Step S62: Suspend the water level sensor 1m above the water pump, set the water level sensor parameters so that the water pump automatically starts when the water level in the well is 3m above the water level sensor position, and automatically shuts off when the water level sensor is exposed above the water surface.

[0066] Step S63: The inner dewatering well 1 and the embankment dewatering well 2 are operated continuously during the construction period. The deep foundation pit slope top dewatering well 3 is shut down after the foundation pit construction is completed. All dewatering wells are inspected daily during operation.

[0067] It should be noted that if the deep foundation pit is constructed using a zoned excavation method, the dewatering well 3 at the top of the deep foundation pit slope will be shut down as the zone construction is completed.

[0068] There are other construction works outside the dewatering well 3 on the top of some deep foundation pit slopes. Before operation, a drainage pipe needs to be laid to collect the water from multiple wells and connect them to the collection tank. Then, the water is discharged to the cofferdam 5 on the side with the flow through secondary pumping.

[0069] When the drainage pipe route intersects with the site access road, a 6m long, 0.6m wide and 0.5m deep crossing trench is excavated. After the drainage pipe is laid in the trench, loose plain soil is backfilled to the level of the access road, and a steel plate is laid to cover it to protect the drainage pipe below from being crushed.

[0070] Each water collection tank has a volume of not less than 25 cubic meters and is placed near the cofferdam on the downstream side and at a distance of more than 2 meters from the intercepting ditch.

[0071] An overflow hole with a diameter greater than 100mm is reserved at the top of the water collection tank. The overflow hole is discharged into the intercepting ditch 6 on the side of the water flow using a rigid pipe. A steel plate is laid at the discharge point to protect the intercepting ditch 6 and prevent the water flow from directly scouring the ditch.

[0072] Each water collection tank collects water from no fewer than 6 wells. The water collection tank is equipped with a large pump (50 cubic meters) and a small pump (10 cubic meters). The water level in the water tank is adjusted by switching the small pump (10 cubic meters).

[0073] Step S7: Wellpoint dewatering construction.

[0074] A closed loop of wellpoint pipes 4 is installed at the bottom of the deep foundation pit. The wellpoint pipes 4 are 1.5m away from the edge of the foundation pit and spaced 1m apart.

[0075] Each wellpoint pipe is connected to the main water collection pipe via a rubber hose. The main water collection pipe is then connected to a vacuum pump and a centrifugal pump in sequence. After pumping, the water is first discharged into a water tank and then discharged outside the cofferdam.

[0076] It should be noted that if the deep foundation pit adopts the sectional excavation construction, the well point pipe 4 is always placed at the bottom of the excavated area. As each new block is excavated, the space enclosed by the well point pipe 4 also expands.

[0077] During construction, the well point pipe 4 is vertically punched with a high-pressure water gun according to the marked hole position, with a hole depth of 3.5m. When punching, the front end of the high-pressure hose is equipped with a conical punch with a diameter of 100-120mm, and the water pressure is set to 0.4-0.6Mpa.

[0078] Wellpoint pipe 4 is a PVC-U filter pipe with a diameter of 63-90mm and a length of 3m. It is wrapped with two layers of 60-mesh nylon mesh and sealed at the bottom.

[0079] The filter media consists of medium-coarse sand, filled to 1m below ground level, and covered with clay to 1m above.

[0080] The main water collection pipe has a diameter of 110mm and is made of PVC.

[0081] The plastic hose has a diameter of 32mm and is equipped with a valve. The plastic hose is connected to the main water collection pipe and well point pipe via a socket joint. The joint is sealed with adhesive and locked with stainless steel clamps.

[0082] Wellpoint dewatering has a short service life. To avoid insufficient vacuum, two wellpoint pipes can be configured simultaneously during actual construction, one for use and one for backup. If the airtightness of one wellpoint pipe decreases and cannot meet the dewatering requirements, it can be quickly switched to the other wellpoint pipe to maintain the dewatering capacity.

[0083] One set of vacuum pump and centrifugal pump is provided for every 20 to 30 well points. The vacuum pump has a vacuum degree of ≥95kPa and the centrifugal pump has a head of ≥15m. The vacuum pump and centrifugal pump are located ≥5m from the edge of the deep foundation pit.

[0084] Step S8: Wellpoint dewatering operation.

[0085] Step S81: Equipment trial operation.

[0086] Start the vacuum pump. The system vacuum should reach 70-80 kPa within 30 minutes, and the pressure drop should be ≤5 kPa within 30 minutes.

[0087] Step S82: Trial precipitation.

[0088] The trial precipitation lasted for 24 hours, during which a vacuum of ≥60 kPa was maintained. Formal precipitation occurred after the water clarified.

[0089] Step S83: Continuous formal precipitation.

[0090] Step S9: Removal and sealing.

[0091] After the wellpoint dewatering operation is completed, the main water collection pipe, vacuum pump and centrifugal pump are removed, and the wellpoint pipe 4 is removed or directly poured into the main structure cushion layer.

[0092] After the well dewatering operation is completed, graded sand and gravel are first poured into the dewatering well, and cement, sand and gravel are mixed evenly at the wellhead in a ratio of 2:2:1 to make dry material.

[0093] Then, pump all the water out of the well, quickly pull out the water pump, and put the dry material into the well. While putting the material in, compact it until it reaches half the depth of the well, then stop putting in the dry material.

[0094] Add a small amount of water to the dry material to make a wet material, and put it into the well to a height of 1.5m from the ground. Pour C20 plain concrete on top of the wet material to the ground.

[0095] This invention employs a five-level system: increasing the seepage distance outside the cofferdam and blocking open water with a ring-shaped intercepting ditch inside the cofferdam. It combines a ring of dewatering wells inside the intercepting ditch, two rings of dewatering wells at the top of the foundation pit slope, and dewatering well points at the bottom of the foundation pit to gradually reduce the dewatering range and control the groundwater level. This system achieves precise dewatering, avoids dewatering blind spots and excessive disturbance, and meets the large-area dewatering needs of complex working conditions.

[0096] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, or alterations made by those skilled in the art using the disclosed technical content shall fall within the protection scope of the present invention.

Claims

1. A method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers with thick sandy soil layers, characterized by: The construction steps include: Step S1: Pumping test; Step S2: Site planning; Plan the locations of well dewatering and wellpoint dewatering before construction; Planning for the location of well dewatering: including dewatering wells inside the cofferdam, dewatering wells on the embankment side, and dewatering wells at the top of the deep foundation pit slope; The inner dewatering well of the cofferdam is located inside the intercepting ditch, and the dewatering well on the bank side is located on the bank. The inner dewatering well of the cofferdam and the dewatering well on the bank side together form a closed ring. The dewatering wells at the top of the deep foundation pit are located at the top of the deep foundation pit slope and are arranged in two concentric rings around the deep foundation pit. Wellpoint dewatering location planning: The wellpoint pipes are located at the bottom of the deep foundation pit, forming a closed loop around the bottom of the deep foundation pit; Step S3: Laying waterproof fabric on the outside of the cofferdam; A high-strength waterproof fabric is laid on the outside of the cofferdam. The top of the waterproof fabric is tied to the top of the cofferdam piles and the bottom extends 5-8m to the riverbed and is weighed down with sandbags. The waterproof fabric and the sandbags are tied together. Step S4: Construction of the intercepting ditch inside the cofferdam; A water interception ditch is excavated along the inner side of the cofferdam. The water interception ditch extends to the riverbank at both ends. Multiple water collection pits are arranged at intervals along the water interception ditch. Water pumps are installed at the water collection pits to continuously pump out the water accumulated in the pits. Step S5: Well dewatering construction; According to the planned locations in step S2, construct dewatering wells on the inner side of the cofferdam, dewatering wells on the embankment side, and dewatering wells on the top of the deep foundation pit slope. The construction sequence is from the outer circle to the inner circle, and observation wells are constructed simultaneously. Step S6: Well dewatering operation; The dewatering wells on the inner side of the cofferdam, the dewatering wells on the embankment side, and the dewatering wells on the top of the deep foundation pit slope are put into operation after construction is completed. The operation sequence is the same as the construction sequence, and they are operated one by one from the outer circle to the inner circle. The dewatering wells on the inner side of the cofferdam and the dewatering wells on the embankment side are in operation. Construction of non-deep foundation pit parts will begin after the site dries. After the dewatering wells on the top of the deep foundation pit are in operation, the deep foundation pit excavation will begin according to the water depth in the observation wells. Step S7: Wellpoint dewatering construction; A closed loop of wellpoint pipes is installed at the bottom of the deep foundation pit. The wellpoint pipes are 1.5m away from the edge of the foundation pit and spaced 1m apart. Each wellpoint pipe is connected to the main water collection pipe through a rubber hose. After the main water collection pipe, a vacuum pump and a centrifugal pump are connected in sequence. After the pumps, the water is first discharged to a water tank and then discharged to the outside of the cofferdam. Step S8: Wellpoint dewatering operation; After the equipment trial run and trial precipitation, continuous formal precipitation operation began. Step S9: Removal and sealing.

2. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers in deep sandy soil layers, as described in claim 1, is characterized in that... Step S2 also includes planning the drainage path, specifically: Drainage path planning: The dewatering wells on the embankment side will be drained into the river channel through shallow-buried corrugated pipes; The dewatering wells inside the cofferdam use a single-well direct discharge method to cross the cofferdam and discharge into the river channel; When there is no construction work outside the dewatering well at the top of the deep foundation pit, the single well is used to directly discharge water into the river across the cofferdam. When there is construction work outside the dewatering well at the top of the deep foundation pit, a drainage pipe is installed along the layout path of the dewatering well 3 at the top of the deep foundation pit to connect to the collection tank, and then the water is discharged into the river channel by crossing the cofferdam. Power supply line planning: The location of electrical boxes should be reasonably allocated according to the water pump model to avoid the circuit passing under the traffic within the site and to ensure smooth traffic flow within the site.

3. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers with thick sandy soil layers, as described in claim 1, is characterized in that... The specific construction steps in step S4 include: Step S41: Use an excavator to dig a 1m wide and 0.5m deep intercepting ditch at the designed location. Throw the excavated mud and sand toward the cofferdam side to form a 1.5m wide sloping protective belt between the intercepting ditch and the cofferdam. The highest point of the protective belt near the cofferdam side is 30-50cm higher than the bottom of the intercepting ditch. Fill in any gaps in height with gravel. Step S42: Construct a water-retaining platform inside the intercepting ditch. The width of the water-retaining platform shall not be less than 0.8m, and it shall be constructed using crushed stone and soil. Step S43: Lay geotextile in the intercepting ditch. The geotextile is 4m wide and extends to both sides of the intercepting ditch to cover the protective belt and the top of the water retaining platform. Fill the geotextile at the bottom of the intercepting ditch with 15cm thick gravel, and let the gravel naturally adhere to the slope. Step S44: Excavate 1.5m×1.5m×1.5m water collection pits at intervals along the intercepting ditch, lay geotextile around the water collection pits, and lay 200mm thick gravel on top of the geotextile. A steel filter pipe with a diameter of 1-1.5m is inserted into the water collection pit. The steel filter pipe is wrapped with an 80-mesh filter screen. A water pump is installed outside the water collection pit to continuously pump out the water accumulated in the pit.

4. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers with thick sandy soil layers, as described in claim 1, is characterized in that... In step S5, the spacing between the dewatering wells on the inner side of the cofferdam and the dewatering wells on the embankment side is 10-15m, and the spacing between the dewatering wells at the top of the deep foundation pit slope is 6-9m. All dewatering wells use sand-free concrete well pipes. The specific construction steps are as follows: Step S51: Excavate a mud pit according to site conditions, use a rotary drilling rig for wet drilling, and after the drilling depth reaches the designed hole depth, increase the pump flow, flush the borehole, and dilute the mud. The drilling depth is 0.5 to 1.0 m greater than the designed depth. Step S52: Wrap the sand-free concrete pipe with 80-100 mesh geotextile. Before lowering the well pipe, inject clean water to replace the mud and use a water pump or sand-removing pipe to extract the sediment. Then, the well pipe is slowly lowered. When it is lowered, 2 to 4 bamboo boards 30mm wide are used to temporarily fix the well pipe vertically. When the pipe opening is 200mm away from the well opening, the next section of the well pipe is connected. The well pipe is continuously lowered until it is 300mm above the current ground level. Step S53: After the well casing is lowered, use a wheelbarrow to transport the filter material to the wellhead, then use a shovel to fill it into the well and record the amount of filler. The amount of filter material filled should not be less than 95% of the calculated amount. Fill the filter material to a height of 1.5m from the existing ground level, and fill the wellhead area with cohesive soil to make it compact. Step S54: After filling is completed, flush the well in time by intermittent pumping or piston flushing until the water is clear and the sand content by weight is less than one ten-thousandth. Step S55: Arrange observation wells at intervals of 50-70m, with the distance between the observation wells being one time the precipitation depth, to observe the actual water level.

5. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers with thick sandy soil layers according to claim 1, characterized in that, The specific steps of step S6 include: Step S61: After the well is cleaned, the pumping and drainage equipment will be installed. A suspension support will be erected and fixed at the wellhead. The water pump will be lowered to a position 2m from the bottom of the well using a rope of fixed length. Step S62: Suspend the water level sensor 1m above the water pump, set the water level sensor parameters so that the water pump automatically turns on when the water level in the well is 3m above the water level sensor position, and automatically turns off when the water level sensor is exposed above the water surface. Step S63: The dewatering wells on the inner side of the cofferdam and the dewatering wells on the embankment side shall be operated continuously during the construction period. The dewatering wells on the top of the deep foundation pit shall be shut down after the foundation pit construction is completed. All dewatering wells shall be inspected daily during the operation period.

6. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers with thick sandy soil layers according to claim 1, characterized in that, In step S7, when constructing the well point pipe, a high-pressure water gun is used to vertically punch holes according to the marked hole positions, with a hole depth of 3.5m. When punching, a conical punch with a diameter of 100-120mm is attached to the front end of the high-pressure hose, and the water pressure is set to 0.4-0.6Mpa. The well point pipe is a PVC-U filter pipe with a diameter of 63-90mm and a length of 3m. It is wrapped with two layers of 60-mesh nylon mesh and sealed at the bottom. The filter media consists of medium-coarse sand filled to 1m below ground level, with the top 1m sealed with clay; The main water collection pipe has a diameter of 110mm and is made of PVC. The plastic hose has a diameter of 32mm and is equipped with a valve. The plastic hose is connected to the main water collection pipe and well point pipe via a socket connection. The joint is sealed with adhesive and locked with stainless steel clamps. One set of vacuum pump and centrifugal pump is provided for every 20 to 30 well points. The vacuum pump has a vacuum degree of ≥95kPa and the centrifugal pump has a head of ≥15m. The vacuum pump and centrifugal pump are located ≥5m from the edge of the deep foundation pit.

7. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers in deep sandy soil layers according to claim 1, characterized in that, The specific construction steps in step S8 include: Step S81: Equipment trial operation; Start the vacuum pump. The system vacuum should reach 70-80 kPa within 30 minutes, and the pressure drop should be ≤5 kPa within 30 minutes. Step S82: Trial precipitation; The trial precipitation lasted for 24 hours, during which a vacuum of ≥60 kPa was maintained. Formal precipitation occurred after the water clarified. Step S83: Continuous formal precipitation.

8. The method for controlling groundwater in deep foundation pits within large steel cofferdams for rivers in deep sandy soil layers according to claim 1, characterized in that, The specific construction method for step S9 is as follows: After the wellpoint dewatering operation is completed, the main water collection pipe, vacuum pump and centrifugal pump are removed, and the wellpoint pipe is removed or directly poured into the main structure cushion layer; After the well dewatering operation is completed, graded sand and gravel are first poured into the dewatering well, and cement, sand and crushed stone are mixed evenly at the wellhead in a ratio of 2:2:1 to make dry material; Then, pump all the water out of the well, quickly pull out the water pump, and put the dry material into the well. While putting the material in, compact it until it reaches half the depth of the well, then stop putting in the dry material. Add a small amount of water to the dry material to make a wet material, and put it into the well to a height of 1.5m from the ground. Pour C20 plain concrete on top of the wet material to the ground.

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