CN118704502A - A method for dewatering a deep foundation pit pipe well in a quicksand layer adjacent to an existing building - Google Patents

Abstract

The invention discloses a method for dewatering a deep foundation pit pipe well close to an existing building sand-containing layer, which is characterized by comprising the following steps of; comprises the following steps of; preparing construction, determining a pipe well layout scheme, entering equipment, measuring and paying off, performing foundation pit bracing construction and completing the pipe well construction. The invention effectively ensures the safety of surrounding buildings, ensures that the filter pipe of the pipe well is not blocked in the dewatering process, completes the well sealing of the pressure-bearing and dewatering well once, and prevents the sand flowing phenomenon of the silt stratum.

Description

A method for dewatering a deep foundation pit pipe well in a quicksand layer adjacent to an existing building

Technical Field

A method for dewatering a deep foundation pit pipe well in a quicksand layer adjacent to an existing building

Technical Field

The invention belongs to the technical field of construction, and in particular relates to a pipe well dewatering construction method for a deep foundation pit containing a quicksand layer adjacent to an existing building.

Background Art

With the acceleration of urbanization, urban land is becoming increasingly scarce. Excavating and supporting foundation pits around existing buildings has become the new normal for foundation pit construction. Strata containing quicksand layers are widely distributed, and excavating foundation pits in such strata is very likely to cause construction difficulties such as water and sand gushing in the pit and settlement of existing buildings adjacent to the pit. The existing technology mainly solves the above problems by flexibly selecting precipitation methods and excavation methods. The methods lack universality and ignore the clogging of precipitation wells by sandy soil particles in strata containing quicksand layers.

Summary of the invention

The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a method for effectively ensuring the safety of surrounding buildings, ensuring that the filter pipes of the pipe wells are not blocked during the precipitation process, so that the pressure precipitation wells can be sealed at one time, and preventing the occurrence of sand flow in the silt sand stratum, and preventing the occurrence of sand flow in the silt sand stratum. Deep foundation pit pipe well precipitation construction method containing quicksand layers adjacent to existing buildings.

The purpose of the present invention and the solution of its main technical problems are achieved by adopting the following technical solutions:

The invention discloses a method for dewatering a deep foundation pit pipe well in a quicksand layer adjacent to an existing building, which is characterized by comprising the following steps:

1) Construction preparation;

Combined with the engineering geological conditions, engineering geological survey reports, hydrological and climatic conditions of the project site, and the surrounding buildings and underground cable pipelines of the project to be built, the construction site shall be leveled according to the overall construction arrangement to ensure that the various roads, water and electricity pipelines required for the construction site are ready, and the construction personnel, construction machinery and materials shall be prepared;

2) Determine the layout plan of the tube wells;

Select the dewatering method based on the engineering geological survey report and the excavation depth of the foundation pit;

3) Equipment entering the site;

The construction machinery for the water-blocking curtain includes a three-axis mixer and its supporting equipment, and the closely spaced bored piles that make up the partition wall include a rotary drilling rig and its supporting equipment;

4) Measurement and layout;

According to the provided measurement control points, the well point location is determined by drawing and calculating. A small pit is dug at the well point with a depth of 300mm-500mm. The deviation between the well point location and the designed location should not exceed 50mm.

5) Foundation pit support construction;

The foundation pit retaining construction includes the construction of water-cutting curtain and partition wall. The water-cutting curtain construction uses three-axis mixing piles as the water-cutting curtain, the pile position is marked, the pile driver is in place, the drilling rig is inspected and leveled, the positive cycle drills to the designed depth, the high-pressure grouting pump is turned on, the reverse cycle is lifted while spraying grout to 0.5m above the designed pile surface, the mixing is repeated, the reverse cycle is lifted to the surface of the pile top and then raised to the ground, the pile is completed, the next cycle of cement mixing pile construction, the pile driver is in place, centered, leveled, and corrected for verticality, ensuring that the angle between the machine body and the ground is 90°, and the verticality error of the pile is ensured to be within 1.0%. The required mud is configured according to the design requirements, the mud should be fully stirred to ensure uniformity, and the mud is made The pH, viscosity and sand content parameters meet the design requirements. After the cooling water circulation is normal, start the mixer, loosen the crane wire rope, and let the mixer cut the soil and mix it along the guide frame. The sinking speed is controlled by the motor's current monitoring meter. When encountering a hard soil layer and sinking slowly, flush water to facilitate drilling. After the mixer sinks to the designed depth, turn on the mortar pump, and keep the outlet pressure at 0.4Mpa-0.6MPa to spray cement slurry continuously. The mixer's rotary spraying speed is controlled at 0.4-0.60m/min. When it is lifted to the designed elevation, it is advisable to stop lifting and stirring for a few seconds to ensure that the pile head is evenly compacted. In order to fully mix the cement slurry sprayed into the soil with the soil, repeat the stirring and sinking until the required design depth, then stir and lift, and spray again along the pile body within 1.0m above and below. After the construction is completed;

Among them; the construction of partition wall is to inject a proper amount of clean water into the aggregate hopper, turn on the mortar pump, clean the residual cement slurry in the pipeline, and remove the soil adhering to the drill bit. The cement content of the mixing pile shall not be less than 15%-20% of the soil weight; the mixing pile adopts P.O. 42.5 ordinary silicate cement, and the water-cement ratio is 1:0.5; the grouting lifting speed is controlled at 0.3-0.5m/min; the verticality deviation of the pile body shall not exceed 1.0%, and the pile position deviation shall not exceed 50mm; the grouting of the pile body is required to be completed in one go and shall not be interrupted. Each pile should be filled with grout once and sprayed and mixed; continuous construction is required, and the piles are overlapped and cross-constructed. The interval between the construction of two adjacent piles shall not exceed 12h; if it exceeds, a pile should be added in the middle of the two piles to remedy the situation; if the grouting is stopped for some reason during the construction process, the mixer should be sunk to 500mm below the grouting stop point, and the grouting and lifting should be resumed after the grouting is restored. The three-spraying and three-mixing construction process should be adopted during mixing;

6) Pipe well construction;

Pipe well construction includes drilling, hole cleaning, backfilling the well bottom sand cushion, declogging the filter pipe section, lowering the well pipe and backfilling the filter material, debugging and test pumping, formal pumping, and well sealing;

A. Drilling: Move the drilling rig to the well site and adjust the drilling rig to ensure that it is balanced and firm, and does not deviate or tilt during the drilling process;

B. Hole cleaning: After the hole is completed, it should be cleaned in time. When cleaning the hole, a soft rubber hose of sufficient length is inserted into the bottom of the well to inject water to clean the hole until clean water flows out. The hole should be cleaned one by one to avoid dead wells;

C. Backfill the sand cushion layer at the bottom of the well; after the hole is cleaned, backfill the sand cushion layer at the bottom of the well. The thickness of the backfill filter material and sand cushion layer should meet the requirements;

D. Anti-clogging of pipe sections: In view of the geological conditions of quicksand layers, the filter pipe section should be treated before the well point pipe is installed, and the outer side of the filter pipe section should be wrapped with a needle-punched non-woven geotextile with two layers of coarse and fine fibers, i.e., geotextile;

E. Lower the well pipe and backfill the filter material; lift the assembled filter pipe by crane to the well hole after the hole is cleaned, and lower the filter material after the well pipe is fixed;

F. Debugging and trial pumping: Before formal pumping, trial pumping must be carried out to check whether the pumping equipment is operating normally. An observation well point is set up in the center of the foundation pit. After debugging and trial pumping are completed, formal pumping will be carried out;

G. Formal pumping: pumping after commissioning and trial pumping are completed;

H. Well sealing; according to the location of the pipe well, well sealing is divided into normal well sealing and bottom sealing of foundation pit pressure dewatering well, the structural bottom plate construction is completed, and the completed part in the foundation pit meets the anti-floating requirements of the main structure; the stable water level in the foundation pit dewatering well is 2m-3m below the foundation pit surface, the bottom of the foundation pit pressure dewatering well is sealed, and the water level of the pressure dewatering well is kept in a static state. The pressure dewatering well is filled with high-quality clay, and the filling height should be 2m-3m higher than the filter tube. After the clay is completely settled and stabilized, the water is pumped out with a water pump, and the pressure water penetration is measured with a water level meter. Clay and cement with a low sand content are fully mixed in a volume ratio of 1:1, and then water is added to mix them into balls, which are filled in the pressure dewatering well. The filling height is about 2m, and the pressure water penetration is detected with a water level meter; a mixture of melon seeds and sand is filled into the pressure dewatering well, and the backfill height of the sand and gravel mixture should be above the foundation pit bottom plate. 2m, fix the grouting pipe to the wellhead with steel bars and start grouting, so that the cement slurry penetrates into the gaps of the well pipe through the gaps of the melon seed slices; after the initial setting time of the cement slurry is reached, cut the well pipe to 0.5m below the bottom plate surface; observe for 2h-4h, if a small amount of water overflows from the well pipe, use quick-drying cement to seal the well pipe; if there is no water overflow from the well pipe and the sealing effect meets the requirements, weld the steel plate to seal the wellhead; roughen the periphery of the concrete bottom plate of the foundation pit reserved for the pressure-bearing dewatering well, and after the garbage is cleaned up, tie the steel bars for secondary concrete pouring.

The three connections and one leveling mentioned in step 1) are water connection, electricity connection, road connection and site leveling.

The precipitation method described in step 2) is open drainage and pipe well precipitation.

The three-spraying and three-mixing described in step 5) is to position the pile driver at the predetermined construction position, then, sink the drill rod to the predetermined depth, after reaching the designed depth, lift the drill rod while stirring, and spray grout at the same time, after lifting, sink to the designed depth again, lift the drill rod while stirring again, and spray grout, after completing the above steps, stop spraying.

Compared with the prior art, the present invention has obvious beneficial effects. It can be seen from the above technical scheme that the pipe well dewatering is to remove the moisture in the soil with a water pump, reduce or eliminate the dynamic water pressure in the soil, and improve the stability of the foundation pit. In addition, due to the downward seepage, under the action of dynamic water pressure, the pipe well dewatering can make the soil layer at the bottom of the pit tend to be dense. The use of pipe well dewatering to lower the groundwater level allows construction personnel to carry out earthwork excavation and foundation construction in a waterless and dry environment, which can not only avoid water gushing, sand gushing, mud bubbling, and slurry overflow, but also effectively prevent the occurrence of sand flow in silt sand formations. When constructing water-cutting curtains such as high-pressure jet grouting piles and mixing piles, due to the large permeability coefficient of the layer containing quicksand, the dynamic water pressure of the groundwater in the layer is large, and the water flow speed is fast, resulting in the cement slurry being dispersed by groundwater before it hardens and forms strength, and the groundwater connection inside and outside the pit cannot be completely cut off, causing the settlement of the existing buildings adjacent to it. By setting up partition walls (closely spaced bored piles) between the existing buildings and the foundation pit support structure, and double-liquid grouting (Portland cement, water glass) between the partition walls and the foundation pit retaining structure, effective protection can be formed between the building and the foundation pit, isolating the groundwater connection inside and outside the pit, effectively reducing the impact of foundation pit precipitation on the existing buildings adjacent to it, and preventing building settlement. Wrapping the filter pipe section of the pipe well with two layers of needle-punched nonwoven geotextiles (geotextiles) of different coarse and fine dimensions can effectively prevent the filter pipe section from being blocked by silt sand. The outer 8D needle-punched nonwoven geotextile provides the first layer of barrier for silt sand, and the inner 6D needle-punched nonwoven geotextile provides the second layer of barrier for silt sand. D (denier) is a unit of density used to measure fiber fineness, referring to the mass of 9000m long fiber at the standard moisture regain. The 8D needle-punched nonwoven geotextile is made of 8D fiber. The mass of 9000m long 8D fiber at the standard regain is 8g. The particle size of silt particles varies. The larger particles are blocked by the outer 8D needle-punched nonwoven geotextile. After the particles with moderate particle size pass through the outer geotextile, they are blocked by the inner 6D needle-punched nonwoven geotextile. The particles with smaller particle size enter the well pipe through two layers of nonwoven geotextile without causing clogging. The permeability coefficient of the layer containing quicksand is large, which causes the groundwater to be replenished faster in the well pipe of the pressure dewatering well. After the well pumping stops, the water level in the well rises rapidly. The cement mortar at the wellhead dissolves in water and cannot harden, which seriously slows down the construction progress and reduces the construction quality. This method uses clay mixed with cement to make balls to fill the pressure dewatering well, which can prevent the groundwater in the well pipe from rising and create favorable conditions for the smooth sealing of the well. In conclusion, the present invention effectively ensures the safety of surrounding buildings and ensures that the filter pipes of the pipe wells are not blocked during the precipitation process, and can seal the pressure precipitation wells in one go to prevent the occurrence of sand flow in silt sand formations.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a process flow chart of the present invention;

Fig. 2 is a schematic diagram of the principle of water reduction in a tube well of the present invention;

FIG3 is a schematic diagram a of the anti-clogging method of the present invention;

FIG4 is a schematic diagram b of the anti-clogging method of the present invention;

FIG5 is a schematic diagram of reinforcement of the present invention;

Fig. 6 is a cross-sectional view of a tube well of the present invention;

FIG. 7 is a schematic diagram of sealing a pressure dewatering well according to the present invention.

DETAILED DESCRIPTION

The specific implementation methods, structures, features and functions of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

Example 1

A method for dewatering a deep foundation pit pipe well in a quicksand layer adjacent to an existing building, characterized by comprising the following steps:

1) Construction preparation

Combined with the engineering geological conditions, engineering geological survey reports, hydrological and climatic conditions of the project site, and surrounding buildings and underground cables and pipelines of the project to be built, the construction site is "three connections and one leveling" according to the overall construction arrangement, and the construction team is invited to bid to ensure that the various roads, water and electricity pipelines required for the construction site precipitation construction are available. After the construction team is determined, a construction plan is prepared in combination with the opinions of the construction team, and the construction team is required to prepare construction personnel, construction machinery and tools, construction materials, etc. in accordance with the construction plan after approval. Before construction, relevant technical personnel should prepare safety technical disclosures, give written disclosures to construction workers, and sign and keep a record.

2) Determine the layout plan of the pipe well

Select the dewatering method according to the engineering geological survey report and the excavation depth of the foundation pit. Pipe well dewatering is suitable for the following situations: For soil layers with a permeability coefficient greater than 10-4cm/s and abundant groundwater, such as sandy soil, sandy silt, silt and gravel soil, open drainage is prone to cause a large amount of soil particles to be lost, and it is difficult to meet the dewatering requirements with light well points. Pipe well dewatering is adopted. Pipe wells are generally arranged in a ring around the foundation pit 0.5m-1.5m away from the upper edge of the slope; when the foundation pit is narrow, it is arranged in a straight line on one side; when it is an independent foundation pit with a small area, a point arrangement is adopted. The distance from the center of the well to the edge of the foundation pit depends on the drilling method of the drilling rig used; the maximum burial depth of the pipe well is 10m, the spacing is 10m-50m, and the dewatering depth is 5m.

3) Equipment arrival

The construction machinery for water-blocking curtains includes three-axis mixers and supporting equipment. The densely-packed bored piles that make up the partition wall use rotary drilling rigs and supporting equipment. The pipe well dewatering equipment mainly includes well pipes and pumping equipment. The pumping equipment is a vacuum pump. The vacuum pump is equipped with a distribution box cable and a spare diesel generator. The well-forming equipment includes a rotary drilling rig and supporting equipment, and auxiliary equipment includes a truck crane, a transport vehicle, a total station, and a theodolite.

4) Measurement and layout

According to the provided measurement control points, the well point location is determined by drawing and calculating. A small pit is dug at the well point location with a depth of 300mm-500mm. The deviation between the well point location and the designed location shall not exceed 50mm.

) Water cut-off curtain construction

Compared with double-axis mixing piles, three-axis mixing piles have fast construction speed, uniform mixing, no mud, and good finished product quality. Therefore, three-axis mixing piles are used as water-cutting curtains. The construction process of water-cutting curtains is as follows: pile position layout, pile driver positioning, inspection, leveling of drilling rigs, positive cycle drilling to the designed depth, turning on the high-pressure grouting pump, reverse circulation while spraying and lifting to 0.5m above the designed pile surface, repeated mixing and drilling, reverse circulation lifting the drill to the surface of the pile top and then lifting it out of the ground, the pile is completed, and the next cycle of cement mixing piles is constructed.

A. Pile driver in place: After the pile driver is in place, check the drill rod length and drill bit diameter, then center, level and correct the verticality to ensure that the angle between the machine body and the ground is 90° and that the pile verticality error is within 1.0%.

Preparation of mud: The required mud is prepared according to the design requirements, and the mud should be fully stirred to ensure uniformity. The pH, viscosity, sand content and other parameters of the prepared mud meet the design requirements.

Stirring and sinking: After the cooling water circulation is normal, start the mixer, loosen the crane wire rope, and let the mixer cut the soil and sink along the guide frame. The sinking speed is controlled by the motor's current monitoring meter. It is not advisable to flush water during pre-mixing; when encountering a hard soil layer and the sinking is slow, flush water to facilitate drilling.

Mixing and lifting: After the mixer sinks to the designed depth, start the mortar pump and keep the outlet pressure at 0.4Mpa-0.6MPa to spray cement slurry continuously. The mixer's rotary spraying speed is controlled at about 0.5m/min. When it is lifted to the designed elevation, it is advisable to stop lifting and stir for a few seconds to ensure that the pile head is evenly compacted.

Repeated mixing, sinking and lifting: In order to fully mix the cement slurry sprayed into the soil with the soil, repeatedly stir and sink until the required depth. Then stir and lift, and spray again within 1.0m above and below the pile body. After the construction is completed, inject an appropriate amount of clean water into the aggregate hopper, turn on the mortar pump, clean the residual cement slurry in the pipeline, and remove the soil adhering to the drill bit.

The cement content of the mixing pile shall not be less than 12% of the soil weight (mass ratio). According to experience, it is recommended to adjust the cement dosage to 15%-20%. The mixing pile adopts P.O. 42.5 ordinary silicate cement with a water-cement ratio of 1:0.5. The grouting and lifting speed is controlled within 0.5m/min. The verticality deviation of the pile body shall not exceed 1.0%, and the pile position deviation shall not be greater than 50mm. The grouting of the pile body is required to be completed in one go without interruption. Each pile should be filled with grout once and sprayed. Continuous construction is required, and the piles are overlapped and cross-constructed. The construction interval between two adjacent piles shall not exceed 12h. If exceeded, a pile should be added in the middle of the two piles to remedy the situation. If the grouting is stopped for some reason during the construction process, the mixer should be sunk to 500mm below the grouting stop point, and the grouting and lifting should be resumed after the grouting is restored. The "three spraying and three mixing" construction process should be adopted during mixing. The number and width of the blades of the mixing head should match the vertical angle of the mixing shaft, the number of rotations of the mixing head, and the lifting speed. Ensure that the mixing times of the soil within the reinforcement depth range meet the design requirements and the support structure is constructed; bored cast-in-place piles have strong applicability, small vibration and low noise, so bored cast-in-place piles are used as the support structure.

Among them; the hole diameter and verticality of the bored pile construction process must meet the requirements; when lowering the steel cage, care must be taken not to let the steel cage collide with the hole wall, and the steel cage must be firmly fixed in place; the thickness of the sediment after cleaning the hole must meet the requirements, and underwater concrete pouring must be carried out in time after cleaning the hole; if for some reason underwater concrete cannot be poured for more than 4 hours, the hole should be cleaned again; when pouring underwater concrete, technical indicators such as the quality of concrete, the particle size of coarse aggregate, and the slump of concrete should meet the requirements; during the concrete pouring process, the speed of pulling out the catheter must be strictly controlled, and it is strictly forbidden to pull the catheter out of the mud surface; the catheter is buried 0.5m-1m deep during initial pouring, and 2m-6m deep during normal pouring; the height after concrete pouring must exceed the design elevation by 0.5m-1m; special attention must be paid to the impact of the quicksand layer on the quality of the pile.

6) Partition wall construction

In order to prevent the settlement of the existing buildings adjacent to the foundation pit within the influence range of precipitation, a partition wall is set between the foundation pit support structure and the adjacent existing building. The depth of the partition wall is more than 2m deeper than the depth of the foundation pit retaining structure, and the partition wall adopts densely arranged bored piles. Double liquid grouting construction: after the construction of the partition wall is completed, double liquid grouting is performed between the partition wall and the foundation pit support structure. Double liquid grout is made of cement slurry and water glass. Cement slurry is composed of silicate cement and water. The initial setting time of the mixture composed of cement slurry and water glass is 0.5min-1.5min. The ratio of silicate cement, water and water glass in the mixture is 1:0.5:0.05. The prepared double liquid slurry is used for grouting reinforcement between the partition wall and the foundation pit support structure. The grouting is determined to be one row or more rows according to the distance to the adjacent existing building.

Among them; During the double-liquid grouting construction process, a geological drill is used to drill vertically from the ground. The hole-making speed is fast, the reliability is high, and the verticality is easy to control. It can be used to find out the leakage and the stratum in the area to be reinforced. It can be known whether the borehole is connected to the leakage point to be treated, and whether there is hollowing phenomenon during the drilling process; different grouting parameters are adopted for different strata to improve the uniformity of grouting reinforcement and ensure the grouting effect;

Adopt appropriate slurry ratio, control grouting pressure index, ensure grouting effect, and control surface uplift. After grouting is completed, as the foundation pit dewatering and excavation proceed, conduct settlement observations on adjacent existing buildings. If the settlement change exceeds the expected range, increase the double-liquid grouting volume between the partition wall and the foundation pit support structure.

7) Drilling

Move the drilling rig to the well point, adjust the drilling rig, ensure that the drilling rig is balanced and firm, and there is no deviation or tilt during the drilling process. The tube well hole is drilled with mud wall protection, and a casing is set at the hole mouth to prevent the hole mouth from collapsing. A mud drainage ditch and a mud pool are set on one side. After drilling, ensure that the distance between the well point pipe and the pit wall is not less than 1.0m-1.5m (if the distance is too small, it is easy to leak), and the spacing between the well points after drilling is 0.8m-1.6m. The depth of the well point pipe buried in the ground should be determined according to the precipitation depth and the location of the aquifer, but the filter pipe must be buried in the aquifer and 0.9m-1.2m deeper than the bottom of the foundation pit (ditch, trench). The burial depth of the well point pipe should be calculated and determined. The spacing between the precipitation well points is about 10m-15m. After the tube well is drilled, the hole diameter and the verticality of the tube well should be checked to ensure that the well point pipe can be smoothly inserted to the bottom of the hole after drilling. There should be a gap between the well point pipe and the hole wall for backfilling filter material. After drilling, check the mud properties in the hole to ensure that there is no hole collapse or diameter shrinkage.

8) Cleaning holes

After the hole is completed, it should be cleaned in time. When cleaning the hole, a soft rubber hose of sufficient length is inserted into the bottom of the well to inject water to clean the hole until clear water flows out. Cleaning should be carried out hole by hole to avoid dead wells.

9) Backfill the sand cushion at the bottom of the well

After the hole is cleaned, the sand cushion layer at the bottom of the well should be backfilled. The thickness of the backfill filter material and sand cushion layer should meet the requirements.

10) Anti-clogging of filter tube section

In view of the geological conditions of the quicksand layer, the filter pipe section should be treated before the well point pipe is installed, and the outer side of the filter pipe section should be wrapped with two layers of coarse and fine fiber needle-punched nonwoven geotextiles (geotextiles). Among them, the outer coarse fiber needle-punched nonwoven geotextile is made of 8D fiber, and the inner fine fiber needle-punched nonwoven geotextile is made of 6D fiber. The two layers of needle-punched nonwoven geotextiles of different coarse and fine dimensions should be tied separately. After the inner layer of 6D dimensional needle-punched nonwoven geotextile is tied firmly, the outer layer of 8D dimensional needle-punched nonwoven geotextile should be tied. The seams between the two layers of needle-punched nonwoven geotextiles of different coarse and fine dimensions cannot overlap, and the seams should be staggered by more than 500mm.

11) Lower the well pipe and backfill the filter material

Lift the assembled filter pipe by crane into the wellbore after the hole is cleaned. Prevent the filter pipe from colliding with the well wall during lifting. The verticality of the well point pipe should meet the requirements, ensure that the center of the well point pipe coincides with the center of the wellhead, and the spacing between the well point pipe and the well wall is uniform, meeting the requirements for lowering the filter material. When the well point pipe is lowered in sections, there should be fixing measures between each section (3-4 bamboo strips or wooden squares with a length of 100mm are used between the sections and fixed by steel wire binding) to prevent the upper and lower sections from being misaligned. Well point pipes are mostly metal pipes, sandless concrete pipes, and PVC pipes. There is a filter pipe section with an opening at the bottom of the well point pipe, and the length of the filter pipe section is generally 1m. The filter pipe section and the well point pipe and the well point pipes are connected by hoses. The connection must be firm and tested to ensure that there is no air leakage. The main water collection pipe is generally a PVC pipe, and the elevation should be as close to the groundwater level as possible. During installation, the upward slope should be maintained at 0.25%-0.5% along the direction of the pumping water flow. After the well pipe is assembled in place, the well pipe should be more than 500mm above the ground and covered to prevent rainwater or muddy water from entering the well pipe. After the well pipe is fixed, the filter material is placed down. The selected filter material should have good water seepage and permeability. The specific requirements are as follows:

A. The sand and gravel must be coarse sand with a diameter of 2mm-4mm;

B. The filter tube should be placed in the middle of the wellbore, and the thickness of the sand and gravel filter layer should reach the original ground to improve water permeability and prevent soil particles from penetrating into the filter tube and blocking the filter mesh; the sand filling thickness should be uniform and the speed should be fast to squeeze the mud out of the wellbore; the filler should be placed with a shovel to prevent uneven placement or impact on the wellbore wall; the sand filling process should not be interrupted to prevent the hole wall from collapsing;

C. The filling height of the sand and gravel filter layer must be at least 1000mm-1800mm above the top of the filter pipe, and should generally be filled above the original groundwater level to ensure smooth flow of water in the soil layer.

12) Debugging and trial pumping

Check whether the joints of the hose connecting the water collecting main pipe and the well point pipe are leaking during the test pumping. If there is a leak, reconnect or plug it with putty, and re-tighten the flange bolts and hose lead wire until there is no leak. Before formal pumping, a test pumping must be carried out to check whether the pumping equipment is operating normally and whether there is any leakage in the pipeline. Install a vacuum gauge on the water pump inlet pipe and a pressure gauge on the water pump outlet pipe. In order to observe whether the precipitation depth reaches the required precipitation depth, an observation well point is set at the center of the foundation pit so that the precipitation curve can be drawn by observing the water level at the well point. During the test pumping, the vacuum degree of the entire pipe network should be checked. The vacuum degree should reach 550mmHg (73.33kPa) before pumping. Observe the amount of water discharged and the quality of the water discharged. If the water quality is continuously turbid, the cause should be investigated. After the cause is found out and handled, the test pumping can be continued. Formal pumping can be carried out only after the test pumping is qualified.

13) Formal pumping

During the formal pumping process, the pumping speed, water output and water quality should be recorded continuously. If any problems are found, they should be dealt with promptly. Pumping should be continued to ensure that the groundwater level drops to 0.5m below the bottom of the foundation pit. During the formal pumping process, attention should be paid to the following:

A. Ensure power outage during pumping;

B. Check regularly, the water discharge pattern should be "large first then small, turbid first then clear";

C. Check and adjust the outlet valve of the centrifugal pump frequently to control the water output. When the groundwater level drops to the required level, reduce the water output of the outlet valve to keep the suction and drainage as even as possible;

D. During winter construction, the main water collection pipe should be well insulated to prevent freezing;

E. A drainage ditch should be dug around the upper part of the foundation pit to prevent rainwater from flowing into the foundation pit.

14) Sealing the well

According to the location of the pipe well, well sealing is divided into normal well sealing and bottom sealing of foundation pit pressure dewatering well. The well sealing operation can be carried out after the normal wellhead sealing meets the following requirements:

A. Construction of the structural base plate is completed;

B. The completed part of the foundation pit meets the anti-floating requirements of the main structure;

C. The stable water level in the foundation pit dewatering well is 2m-3m below the foundation pit surface. The steps for sealing the bottom of the foundation pit pressure dewatering well are as follows:

D. Keep the water level of the pressure well static, fill it with high-quality clay, and the filling height should be 2m-3m higher than the filter tube; after the clay is completely settled and stabilized, use a water pump to pump out the water, and measure the pressure water penetration with a water level meter;

E. Mix clay and cement with a low sand content in a volume ratio of 1:1, add water and mix to form a ball, fill it into the pressure dewatering well, the filling height is about 2m, and use a water level meter to detect the penetration of pressure water; fill the pressure dewatering well with a mixture of melon seed slices and sand, and the backfill height of the sand and gravel mixture should be 2m above the bottom plate of the foundation pit; fix the grouting pipe to the wellhead with steel bars and start grouting, so that the cement slurry penetrates into the gap of the well pipe through the gap of the melon seed slices; after the initial setting time of the cement slurry is reached, cut the well pipe to 0.5m below the bottom plate surface; observe for 2h-4h, if a small amount of water overflows from the well pipe, use quick-drying cement to seal the well pipe; if there is no water overflowing from the well pipe and the sealing effect meets the requirements, weld the steel plate to seal the wellhead; roughen the periphery of the concrete bottom plate of the foundation pit reserved for the pressure dewatering well, tie the steel bars after the garbage is cleaned up, and then pour the secondary concrete, and the construction is completed.

The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims (4)

1. A method for dewatering construction of a deep foundation pit pipe well close to an existing building sand-containing layer is characterized by comprising the following steps of; comprises the following steps of;

1) Preparing construction;

Combining engineering geological conditions, engineering geological survey reports, hydrologic climate conditions of the engineering location and conditions of surrounding buildings and underground cable pipelines of the engineering to be built, carrying out three-way leveling on a construction site according to the overall arrangement of the construction, and ensuring that various roads and water and electricity pipelines required by precipitation construction on the construction site have conditions, and preparing work of constructors, construction machines and construction materials;

2) Determining a tubing well layout scheme;

Selecting a precipitation mode according to the engineering geological investigation report and the excavation depth of the foundation pit;

3) Entering the equipment;

The water curtain construction machine is provided with a triaxial stirrer and matched equipment, and a rotary drilling rig and matched equipment for forming the close-packed bored pile of the partition wall are arranged;

4) Measuring and paying off;

Determining the position of a well point through drawing calculation according to the provided measurement control points, digging a small soil pit at the well point, wherein the depth is 300-500 mm, and the deviation between the well point position and the design point position is not more than 50mm;

5) Performing foundation pit enclosure construction;

The foundation pit enclosure construction comprises water interception curtain construction and partition wall construction, wherein; the construction of the water interception curtain adopts a triaxial mixing pile as the water interception curtain, the pile position is lofted, a pile machine is in place, a testing and leveling drilling machine is used for forward circulation drilling to the designed depth, a high-pressure grouting pump is started, reverse circulation is performed while grouting is performed, lifting is performed to 0.5m above the designed pile surface, repeated stirring is performed, the reverse circulation is performed, the ground is lifted after the drilling is performed to the surface of the pile top, the pile forming is finished, the next circulation cement mixing pile is constructed, the pile machine is in place for centering, leveling and correcting verticality, the included angle between the machine body and the ground is ensured to be 90 degrees, the pile verticality error is ensured to be within 1.0 percent, the required slurry is configured according to the design requirement, the slurry is sufficiently stirred and ensured to be uniform, the parameter indexes of pH, viscosity and sand content of the prepared slurry are in accordance with the design requirement, the stirring machine is started after the cooling water circulation is normal, loosening a steel wire rope of a crane to enable a stirrer to cut soil along a guide frame, stir and sink, wherein the sinking speed is controlled by a current monitoring meter of a motor, when a hard soil layer is sunk slowly, flushing can be performed to facilitate drilling, a mortar pump is started after the stirrer is sunk to a designed depth, the outlet pressure is kept at 0.4Mpa-0.6Mpa, cement paste is continuously sprayed, the rotary spraying speed of the stirrer is controlled to be 0.4-0.60m/min, when the stirring is lifted to a designed elevation, lifting is stopped and stirring is performed for a plurality of seconds to ensure that a pile head is uniformly compact, and in order to enable cement paste sprayed into the soil and the soil body to be fully stirred, stirring and sinking are repeated until the designed required depth, stirring and lifting are performed again within a range of 1.0m along the lower part of the pile body, and after construction is finished;

Wherein; the partition wall construction is to inject a proper amount of clear water into the collection hopper, start the mortar pump, clean the residual cement slurry in the pipeline, and simultaneously clean the drill bit adhesion soil, wherein the cement content mass ratio of the stirring pile is not less than 15% -20% of the soil; the stirring pile adopts P.O. 42.5 ordinary Portland cement, and the water-cement ratio is 1:0.5; the grouting lifting speed is controlled to be 0.3-0.5m/min; the deviation of the perpendicularity of the pile body is not more than 1.0%, and the deviation of the pile position is not more than 50mm; the grouting requirement of the pile body is finished at one time without interruption, and each pile is preferably filled with slurry once and sprayed and stirred; continuous construction is required, pile lap joint is inserted and crossed, and the construction interval between two adjacent piles is not more than 12 hours; if the pile number exceeds the preset value, pile adding and remedying are adopted in the middle of the two piles; stopping slurry according to reasons in the construction process, sinking the stirrer to 500mm below the slurry stopping point, and stirring and lifting after slurry supply is recovered, wherein a three-spraying and three-stirring construction process is adopted during stirring;

6) Constructing a pipe well;

the construction of the pipe well comprises drilling, hole cleaning, backfilling of a sand cushion layer at the bottom of the well, dredging and plugging of a filter pipe section, well casing and backfilling of filter materials, debugging and trial pumping, formal pumping and Feng Jing;

A. drilling holes; the drilling machine moves to the well point position, the drilling machine is adjusted, the drilling machine is balanced and firm, and no deviation and inclination occur in the drilling process;

B. clearing holes; after the hole forming is finished, cleaning holes in time, and inserting a soft rubber pipe with enough length into the bottom of the well to fill water to clean the holes until clean water flows out, wherein holes are cleaned one by one to avoid dead well;

C. Backfilling a sand cushion layer at the bottom of the well; backfilling a sand cushion layer at the bottom of the well after the hole cleaning is completed, the thickness of the backfill filter material and the sand cushion layer should meet the requirement;

D. The pipe section is prevented from being blocked; aiming at the geological condition of the layer containing the flowing sand, the filter pipe section is treated before the well point pipe is installed, and the outer side of the filter pipe section is wrapped with a thick layer and a thin layer of fiber to form a needled non-woven geotextile, namely geotextile;

E. Setting the well pipe and backfilling filter materials; hoisting the assembled filter tube into the tube well hole after hole cleaning by a crane, and lowering filter materials after the well tube is fixed;

F. Debugging and water pumping trial; the test pumping is needed before the formal pumping to check whether the pumping equipment operates normally, an observation well point is arranged in the center of the foundation pit, and the positive pumping is performed after the debugging and test pumping are completed;

G. formally pumping water; pumping water after debugging and water trial pumping are completed;

H, sealing the well; according to the position of the pipe well, the well sealing is divided into normal well sealing and bottom sealing of the foundation pit pressure-bearing dewatering well, the construction of the structural bottom plate is completed, and the completed part in the foundation pit meets the anti-floating requirement of the main structure; the stable water level in the pit dewatering well is 2m-3m below the pit surface, the bottom of the pit pressure-bearing dewatering well is sealed, the water level of the pressure-bearing dewatering well is kept in a static state, high-quality clay is adopted to fill the pressure-bearing dewatering well, the filling height is 2m-3m higher than that of a filter tube, when the clay is completely settled and stable, water is pumped out by a water pump, the pressure-bearing water permeation condition is measured by a water level meter, clay and cement with small sand content are fully mixed according to the volume ratio of 1:1, then water is added and mixed into a sphere shape, the pressure-bearing dewatering well is filled, the filling height is about 2m, and the pressure-bearing water permeation condition is detected by the water level meter; filling the pressure-bearing dewatering well with a mixture of melon seed pieces and sand, wherein the backfilling height of the sand-stone mixture is 2m above the foundation pit bottom plate, fixing the grouting pipe with a wellhead by using steel bars, and grouting to enable cement slurry to permeate into a well pipe gap through gaps of the melon seed pieces; after the initial setting time of the cement paste is reached, the well pipe is cut off to be 0.5m below the bottom plate surface; observing for 2-4 hours, and sealing the well pipe by using quick-setting cement if a small amount of water overflows in the well pipe; if no water overflows in the well pipe and the sealing effect meets the requirement, welding a steel plate to seal the wellhead; and roughening the periphery of a foundation pit concrete bottom plate reserved by the pressure-bearing dewatering well, binding reinforcing steel bars after garbage cleaning is completed, and performing secondary concrete pouring.

2. The method for dewatering a deep foundation pit pipe well immediately adjacent to an existing building sand-containing layer according to claim 1, wherein the three-way level in step 1) is water-through, electric-through, road-through and field-leveling.

3. The method for dewatering a deep foundation pit pipe well immediately adjacent to an existing building sand-containing layer according to claim 1, wherein the dewatering mode in the step 2) is open drainage and pipe well dewatering.

4. The method of dewatering construction of a deep foundation pit pipe well immediately adjacent to a sand-flowing layer of an existing building according to claim 1, wherein in the step 5), three-spraying three stirring is ‌ to position a pile driver at a predetermined construction position, ‌ then ‌ is sunk to a predetermined depth through a drill pipe, after ‌ reaches a design depth, ‌ is lifted while stirring the drill pipe, ‌ is sprayed with slurry at the same time, ‌ is lifted, ‌ is sunk to the design depth again, ‌ is lifted while stirring the drill pipe again, ‌ is sprayed with slurry, and ‌ is completed, and then ‌ is stopped.