CN101092818A - Method for lowering water by combining phreatic water in footing groove with artesian aquifer - well stratifications - Google Patents

Abstract

The invention relates to a method for combined dewatering of foundation pit diving and a well layered dewatering of a confined aquifer in the technical field of underground engineering construction. It includes the following steps: the dewatering well adopts a well structure; the controllable water stop valve is closed when the water is drained; it is determined by the three-dimensional groundwater seepage analysis finite element, and the determined water pumping volume and the influence radius are determined; when the excavation goes deep into the aquifer , extracting water in the confined aquifer to reduce the water pressure; according to the plane size of the foundation pit, excavation depth, buried depth of the underground wall, and aquifer structure conditions; the finite element calculation based on the three-dimensional groundwater seepage and ground subsidence is applied. Methods To determine the excavation depth, pumping volume, diameter and quantity of wells, influence radius of hydraulic gradient and surrounding land subsidence when the confined water is lowered. Compared with the existing method, the present invention can reduce the number of wells by 1/3-1/2, save 30-50% of the dewatering construction cost, and is suitable for foundation pits containing submerged layers-water-resisting layers-confined aquifers and need to be lowered. Precipitation engineering of pressurized water head.

Description

The Method of Combined Dewatering of Foundation Pit Diving and Confined Aquifer One Well Separately

technical field

The invention relates to a construction method in the technical field of underground engineering construction, in particular to a method for joint dewatering of foundation pit diving and confined aquifer-well stratification.

Background technique

Underground construction in soft soil foundation with high groundwater level, such as foundation pit excavation, the biggest problem in construction is that the existence of groundwater will cause water gushing at the bottom of the foundation pit. Therefore, using well points or large-diameter tube wells to artificially lower the groundwater level is a common and effective method in excavation construction of foundation pits. The method of foundation pit excavation generally adopts dry excavation, that is, the water in the soil body that needs to be excavated is drained first (keep the groundwater level at 1m below the excavation surface), and then the method of excavating the soil. In addition, when the excavation depth of the foundation pit is relatively large, the water-resisting layer between the submerged layer and the confined aquifer becomes very thin after excavation, and the self-weight of the residual water-resisting layer at the base cannot balance the pressure of the confined water, which occurs Quicksand or piping damage. At this time, the method of reducing the water head of the confined aquifer should be adopted to reduce the risk of the foundation pit. For the dewatering of foundation pits that require simultaneous pumping of phreatic water and confined water, there are two dewatering methods to lower the groundwater level: the first method is the double-well dewatering method; the second method is the one-well dewatering method. The so-called double-well dewatering method is to set a group of wells to the submerged layer (or impermeable water layer) to drain the shallow water. This well is called a drain well; A group of large-diameter tube wells are set up to reduce the groundwater pressure head in the confined aquifer, and the wells are called depressurization wells. The so-called one-well dewatering method is to directly set up a group of large-caliber tube well groups to simultaneously drain shallow water and reduce the groundwater pressure head of the confined aquifer. However, when the foundation pit maintenance water-stop structure cannot pass through all the confined aquifers (that is, the confined aquifer is connected inside and outside the foundation pit), the precipitation in the foundation pit will cause the drop in the groundwater head of the surrounding confined aquifers to be transmitted to the larger far away. Due to consolidation, the surrounding foundations will consolidate and settle; the longer the precipitation time, the greater the settlement, which will cause environmental problems. Relatively speaking, the first method above takes a shorter time to extract the confined aquifer, and has less impact on the environment than the second method; however, the construction cost is doubled due to the addition of a group of wells, which is not very economical. A good construction technology should not only minimize the impact on the environment, but also meet the requirements of economy.

After searching the existing technical literature, it is found that the application number is 200610086139.8, and the name of the patent application is "the method of combined vacuum dewatering with layered and divided electroosmosis for the treatment of large area soft ground". The vacuum precipitation method is applied to the reinforcement treatment of large-area soft ground. In this method, the electroosmotic well point tubes are arranged in the deep layer for electroosmotic dewatering, the vacuum well point tubes are arranged for vacuum dewatering in the shallow layer, the deep layer electroosmotic well point tubes are arranged at intervals from the shallow layer vacuum well point tubes, and the dewatering construction is carried out in two steps: the first pass Use deep electroosmotic well point tubes for dewatering, and then use shallow vacuum well point tubes for dewatering in the electroosmotic dewatering construction area. This dewatering method has a large number of wells, a small treatment depth, and high engineering cost. It is only limited to the dewatering reinforcement treatment of soft soil foundations, and is not suitable for foundation pit dewatering.

Contents of the invention

Aiming at the deficiencies and defects of the prior art, the present invention proposes a method for joint dewatering of foundation pit diving and confined aquifer-well layering, so that it can be aimed at foundation pits composed of submerged layers-water-resisting layers-confined aquifers The dewatering project realizes the use of one well to solve the problem of dehydration of diving and depressurization of confined water. And can try to avoid the impact of precipitation on the surrounding environment.

The present invention is achieved through the following technical solutions, and the present invention comprises the following steps:

(1) One well is used for the dewatering well, and the depth reaches the confined aquifer. The filter pipe of the dewatering well is designed in two sections, the upper and lower sections. water pressure; the water-resisting layer uses well pipes to connect the upper and lower water filter pipes.

A controllable water stop valve is installed in the aquifer inside the well pipe to isolate submerged and confined water in a well.

As for the controllable water stop valve, the controllable water stop valve is closed to maintain the pressure in the pressurized water during dry diving; when the pressurized water needs to be lowered, the controllable water stop valve is opened.

(2) Close the controllable water stop valve when draining and diving to maintain the water pressure in the confined water layer. In this way, the water level in the foundation pit and its surrounding confined water layer is kept, no unstable seepage occurs in the confined water layer, and the ground outside the foundation pit will not sink when it is drained and dived.

(3) Drill several complete wells that pass through the submerged layer in the proposed site, and conduct field pumping tests to determine the pumping volume and influence range when the submerged wells are drained. Pumping volume and radius of influence.

(4) When the excavation goes deep into the aquifer and the thickness of the aquifer from the excavation surface to the confined water roof is not enough to resist the water head of the confined aquifer and there is a risk of inrush, open the controllable water stop valve and turn the vacuum pump It is lowered into the lower section of the dewatering well, and the water in the confined aquifer is pumped to reduce the water pressure.

(5) According to the plane size of the foundation pit, the excavation depth, the buried depth of the underground wall, and the structural conditions of the aquifer; the finite element calculation method based on the three-dimensional groundwater seepage and land subsidence is used to determine the excavation depth when the confined water is reduced , water pumping rate, well diameter and quantity, influence radius of hydraulic slope and surrounding land subsidence.

The water-blocking effect of retaining structures such as underground diaphragm walls should be considered in the calculation, and the above-mentioned relevant parameters determined should be used as the basis for determining the construction plan for confined water precipitation.

The finite element calculation for the determination of the three-dimensional groundwater seepage and land subsidence is as follows:

1) The modeling range of the finite element: the length in the horizontal direction is greater than (L ₁ +2R) meters, the width is greater than (L _w +2R) meters, and the vertical direction should be greater than (H _cp +H _cd1 +H _cf +H _cd2 ) meters, where _L1 is the length of the foundation pit, _Lw is the width of the foundation pit, R is the influence radius, _Hcp is the thickness of the unconfined aquifer, _Hcd1 is the thickness of the first aquifer, _Hcf is the thickness of the confined aquifer, H _cd2 is the thickness of the second water barrier layer.

2) Initial condition setting: the groundwater table of the phreatic aquifer is set as the initial water head, and the pressure head of each confined aquifer is the initial water head of each confined aquifer; the initial water head in the aquifer and the Confined aquifers have the same hydraulic head.

3) Boundary condition setting in the calculation: constant water head is adopted for the periphery and bottom of the calculation domain.

4) Mesh division: The grid division on the plane should be within 20 meters around the foundation pit with a relatively fine grid and gradually enlarged outward.

The vertical division of the grid should be uniform.

5) Water intake points for calculation: the amount of groundwater taken in the calculation is allocated according to the grid unit nodes;

According to the allocation of grid unit nodes, when the water is drained, there is no water volume distribution on the nodes of the confined water layer; when the pressure of the confined water is reduced, the water volume nodes in the water layer are closed.

6) Use the above model to calculate the range of water level drop and the range of settlement deformation in the surrounding strata during the groundwater pumping process, and determine the dewatering construction plan such as excavation depth, water pumping volume, well diameter and number of tube wells when the confined water is reduced.

The present invention adopts a one-well layered joint dewatering scheme for the dewatering problem in the foundation pit excavation with a confined aquifer underneath. The surrounding environmental factors are considered during the pumping process, which can minimize the impact on the surrounding environment during the precipitation process. At the same time, the foundation deformation analysis finite element model based on the three-dimensional groundwater seepage theory is used to determine the excavation depth, water pumping volume, well diameter and number and other dewatering construction schemes when reducing the confined water. Compared with the existing construction methods, this method can reduce the number of wells by 1/3 to 1/2, save 30 to 50% of the dewatering construction cost, and the excavation construction efficiency is also improved to varying degrees, and the economic benefits are very high. significantly. The invention is applicable to the dewatering engineering of the foundation pit containing a submerged layer-a water-resisting layer-a pressurized aquifer and needing to depressurize the water head.

Description of drawings

Figure 1 Plane view of the modeling range of the foundation pit

Figure 2 Sectional view of the modeling depth of the foundation pit

Fig.3 3D finite element mesh of foundation pit

Detailed ways

The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings: this embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided, but the protection scope of the present invention is not limited to the following the described embodiment.

Embodiment Take the foundation pit dewatering project of a subway station in Shanghai as an example.

A subway station in Shanghai has two floors underground, and the main body is a three-story two-span (partial three-story three-span, second-story two-span) cast-in-place reinforced concrete structure. The main structure of the station is 313.5 meters long and 22.8 meters wide. The excavation of the foundation pit of the station is divided into two areas. The excavation depth is 22.39 meters in the south end of the well, 23.01 meters in the north end of the well, and 21.06 to 22.26 meters in the standard section. The underground diaphragm wall is used for enclosure, and the depth of the underground diaphragm wall is 36.50 meters. The project site is distributed with diving and confined water. The phreatic aquifer is mainly composed of silty soil, the water level is buried at a depth of 1.2 meters; the top surface of the confined aquifer is buried at a depth of 19.59 to 23.03 meters, and its thickness is 10.5 to 41.80 meters. The soil quality is mainly gray sandy silt, and the confined water head is 4.5 About meters; between the unconfined aquifer and the confined aquifer, there is a cohesive soil aquifer with a thickness of 9.0-12.0 meters. During the construction of this project, it is necessary to drain the groundwater in the phreatic aquifer, and lower the water level of the confined aquifer to 0.5-1.5 meters below the excavation bottom of the foundation pit. Since the foundation pit project of the station is located in the downtown area of Shanghai, the surrounding environment is complex, and there are many underground pipelines and ground buildings, so the impact of precipitation on the surrounding environment should be minimized as much as possible. The foundation pit dewatering adopts a combined dewatering scheme of one well layered phreatic dewatering and confined water depressurization. The specific implementation scheme is as follows.

As shown in Figure 1, one well is used as the dewatering well, and the diameter of the well pipe is determined to be 250m, the wall thickness is 3mm, and the length is 9.0 to 12.0 meters. The diameter of the water filter pipe is the same as that of the well pipe; the upper part of the water filter pipe is placed in the submerged layer, and its length is equivalent to the thickness of the submerged layer, which is 8 to 11 meters; the lower part of the water filter pipe is placed in the confined aquifer, and the length is 8 meters. The depth of the tube well is 28.3-29.3 meters. Install a plate-type controllable water-stop valve at a depth of about 19.0 meters in the well pipe (water-resisting layer).

Three-dimensional finite element modeling: In the design stage of the dewatering scheme, the 6eoGlow3D three-dimensional porous media seepage model was used to simulate and analyze the dewatering process during the excavation of the foundation pit. According to the field pumping test, the influence radius of the submerged water is about 60 meters, and the influence range of the confined water is 300 meters; in order to eliminate the boundary influence, the distance of 400 meters from the entire foundation pit and the outer boundary of the foundation pit is taken as the finite element modeling range . In fact, when the amount of pumped water is greater than the amount of groundwater recharge, the scope of influence will continue to expand to the boundary of the recharge area as time goes on because the groundwater is in an unstable state. However, when the pumping amount is less than the recharge amount, the groundwater will gradually stabilize, and the influence radius is smaller than the boundary of the recharge area. The analysis depth is 90 meters, divided into 12 engineering geological layers.

The plane of the modeling range is shown in Figure 2, and the three-dimensional finite element grid of the foundation pit is shown in Figure 3:

The well is divided into two sections at the phreatic and confined aquifer, and the controllable water stop valve is installed and the aquitard. The underground water table of the unconfined aquifer is buried at a depth of 1.2 meters, and the water head of the confined aquifer is 4.35 meters and 7.5 meters. At the boundary, the groundwater head is maintained at the initial water level.

Close the controllable water stop valve, and enter the QDX3-35-2.2 type submersible pump to drain and dive. Calculated by applying the above-mentioned finite element model, it takes about 15 days to drain the water in the foundation pit, and the water inflow from a single well is 21m ³ /day. At this time, due to the water retaining effect of the underground diaphragm wall, no settlement occurs outside the foundation pit.

Using the above finite element model to calculate and determine, the water output of a single well is 80m ³ /day when the confined water is pumped, and the water inflow of the foundation pit is 3000m ³ /day. In this way, about 36 stratified and controllable combined dewatering wells need to be set in the foundation pit. It can also be determined through the calculation of the finite element model that when the foundation pit is excavated to 10 meters, piping may occur, and it is necessary to depressurize the confined water. At this time, the original submersible pump is first taken out, the controllable water stop valve is opened, and the QJ type submersible deep well pump is lowered to extract pressurized groundwater.

When the controllable water stop valve is opened, the pressurization and dewatering of the pressurized water will be carried out. According to the calculated pumping volume, the groundwater level will be controlled by stopping and starting the pump, so that the head of the pressurized water will be kept at the excavation surface of the foundation pit. at a depth of 1 meter below. On the 72nd day, excavation reached the bottom of the foundation pit. On the 89th day, the construction of the floor structure was completed and reached the strength requirements. The well was closed and the dewatering task was completed. Observations show that the water head drop of the confined aquifer around the foundation pit ranges from 200 to 300 meters, but the water head drop near the pit outside the pit is only -10.2 meters, which is much smaller than the water level drop inside the pit (-24 meters). This shows that although the underground diaphragm wall does not penetrate the confined aquifer, its water-retaining effect is still obvious.

Using the above-mentioned finite element model analysis, during the 75 days of pumping the confined water, the surface settlement around the foundation pit was basically controlled within the range of 10 to 20 meters from the wall, and the maximum settlement outside the wall was about 30mm, while 20 meters outside the foundation pit The settlement is small, only 5mm in 90 days. However, the traditional method of simultaneously lowering the groundwater and the groundwater under pressure will cause the groundwater level outside the foundation pit to drop by 400 meters, and the ground subsidence range outside the foundation pit will reach 25-30 meters after pumping for 90 days, and the edge of the pit will be the largest. Settlement up to 60 ~ 80mm.

As can be seen from the results of this embodiment, the precipitation scheme for the precipitation problem in the excavation of the foundation pit with a confined aquifer below can save 30-50% of the cost compared with the traditional foundation pit precipitation scheme; the impact of the precipitation process on the surrounding environment It is also smaller than the traditional foundation pit dewatering scheme, which makes the foundation pit dewatering scheme more reasonable and economical.

Claims (6)

1, the method for a kind of foundation ditch diving and artesian aquifer one well layering associating precipitation is characterized in that, may further comprise the steps:

(1) the precipitation well adopts a well, and the degree of depth reaches artesian aquifer, and the filter pipe of precipitation well divides the two sections design, last filter pipe is positioned at phreatic aquifer, drainage diving, and following filter pipe is positioned at artesian aquifer, reduce pressure-bearing and contain water pressure, water barrier adopts well casing to connect filter pipe up and down;

Close controlled sealing valve when (2) drainage is dived under water, to keep the hydraulic pressure in the artesian aquifer;

(3) beat the several mouthfuls of complete penetration of wells that pass water table aquifer in planning to build the place, carry out open-air pumping test, pump-out when determining the phreatic well drainage and influence basin are determined by three-dimensional Analysis of Groundwater Seepage Flow finite element, and the diving pump-out and the radius of influence to determine;

(4) be deep into water barrier when excavation, the thickness of water barrier from the excavation face to the artesian water top board is not enough to resist the artesian aquifer head and exists prominent when gushing danger, open controlled sealing valve, will put into hypomere precipitation well under the vacuum pump, the water in the extraction artesian aquifer is to reduce hydraulic pressure;

(5) according to the buried depth of the planar dimension of foundation ditch, cutting depth, subterranean wall, the aquifer structural environment; Bore, quantity, the radius of influence of hydraulic slope and the surface settlement on every side of the cutting depth the when method of the FEM (finite element) calculation that application is determined based on three-dimensional seepage action of ground water and surface settlement is determined dewatering confined water, pump-out, well.

2, the method for foundation ditch diving according to claim 1 and artesian aquifer one well layering associating precipitation is characterized in that the water barrier in well casing is installed controlled sealing valve, is used for isolating diving and artesian water at a well.

3, the method for foundation ditch diving according to claim 1 and artesian aquifer one well layering associating precipitation is characterized in that, described controlled sealing valve cuts out the pressure in the controlled sealing valve maintenance artesian water when drainage is dived under water; When the needs dewatering confined water, open controlled sealing valve.

4, the method for foundation ditch diving according to claim 1 and artesian aquifer one well layering associating precipitation is characterized in that, the FEM (finite element) calculation that described three-dimensional seepage action of ground water and surface settlement are determined, and step is as follows:

1) modeling of finite element: cross-directional length is greater than (Ll+2R) rice, width is greater than (Lw+2R) rice, vertical direction should be greater than (Hcp+Hcd1+Hcf+Hcd2) rice, wherein be L1 foundation ditch length, Lw is the foundation ditch width, and R is the radius of influence, Hcp is the thickness of phreatic aquifer, Hcd1 is the thickness of first water barrier, and Hcf is the thickness of artesian aquifer, and Hcd2 is the thickness of second water barrier;

2) primary condition is set: the underground water level of phreatic aquifer is set at initial head, and the pressure head of each artesian aquifer is the initial head of each artesian aquifer; Initial head in the water barrier is identical with the head of position artesian aquifer thereon;

3) fringe conditions is set in the calculating: constant head is adopted in the periphery and the bottom of calculating the field;

4) grid dividing: on the plane grid dividing should be around the foundation ditch grid in 20 meters scopes, outwards amplify gradually;

5) dry point of Ji Suaning: the amount that underground water is taked in the calculating is respectively by the distribution of grid cell node;

6) with drawdown scope and the scope of sedimentation and deformation, the bore of the cutting depth when determining dewatering confined water, pump-out, well and the pipe well quantity dewatering construction scheme on stratum on every side in the above-mentioned model calculating groundwater mining process.

5, the method for foundation ditch diving according to claim 4 and artesian aquifer one well layering associating precipitation is characterized in that, described distribution of pressing the grid cell node, and when drainage was dived under water, the no water yield was distributed on the node of artesian aquifer; During the artesian water step-down, close the water yield node in the water table aquifer.

6, the method for foundation ditch diving according to claim 4 and artesian aquifer one well layering associating precipitation is characterized in that, described grid, and vertical division should be even.

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