What are the methods of foundation pit dewatering?

The methods of foundation pit dewatering mainly include: open ditch plus collecting well dewatering, light well point dewatering, jet well point dewatering, electro-osmosis well point dewatering, deep well point dewatering and so on.

(1) Dewatering in open ditches and collecting wells

Dewatering of open ditch water collecting well is a kind of artificial drainage method. It has the characteristics of convenient construction, simple equipment and low cost, and is widely used in construction sites. This method is often used as an auxiliary dewatering measure of plugging method or other dewatering methods in foundation pit slope support projects in high water level areas, which mainly excludes underground diving, construction water and rainwater in the sky.

In areas with abundant groundwater, if only this method is used for dewatering, it will be more difficult to spray concrete when bolting and shotcreting nets support, and sometimes it will be difficult to add drainage pipes, and the working face will be muddy, which will hinder the construction work.

Therefore, this dewatering method is generally not used alone for foundation pit slope support in high water level areas, but can be used alone in low water level areas or projects with small permeability coefficient of soil layer and allowing slope release.

(2) Light well point dewatering

Light well point dewatering (first-class light well point) is a widely used dewatering method in China. Compared with other well point systems, the construction is simple, safe and economical, and it is especially suitable for occasions with small foundation pit area and low water level.

Generally speaking, this method reduces the water level by 3-6 meters. If the depth of precipitation is required to be greater than 6m, in theory, a multi-level well point system can be adopted, but enough space is needed around the foundation pit to facilitate sloping or trenching, and the foundation pit supporting project with limited space is generally not allowed, so the first-level light well point system is generally adopted.

The suitable permeability coefficient of soil layer for light well point is 0.1-50m/d. When the permeability coefficient of soil layer is small, it is necessary to seal the top of well point pipe with clay to ensure the air tightness of each connection part of well point system, so as to improve the vacuum degree of the whole well point system and achieve good results.

(3) Jet well point dewatering

The recharge well point system can generate a vacuum of 250mm Hg at the bottom of the well point and reduce the water level in depth, generally within the range of 8-20 m, and its applicable soil permeability coefficient is the same as that of the light well point, generally 0. 1-50m/d, but its pumping system and recharge well pipe are very complicated, with high operation failure rate and large energy loss, and the required cost is higher than other well point methods.

(4) Electro-osmotic well point precipitation

Electroosmosis well point is suitable for fine-grained soil with small permeability coefficient, such as clay, loam, silt and muddy clay. The permeability coefficient of these soils is less than 0. 1m/d, so it is difficult to achieve the purpose of precipitation with general well points. ?

Water in fine-grained soil can be effectively pumped out by electroosmosis. It is necessary to combine light well points or injection well points, and the depth of lowering water level depends on light well points or injection well points. In the process of electro-osmosis well point precipitation, it is complicated to measure and make necessary adjustments to voltage, current density and power consumption, and make records.

(5) tube well point precipitation

Tube well point is suitable for gravel layer with large permeability coefficient, stratum rich in groundwater and occasions where light well point is difficult to solve. The outlet flow of each tube well can reach 50-1100m3/h, the soil permeability coefficient is in the range of 20-200m/d, and the buried depth of groundwater level is reduced by about 3-5m. This method is generally used for groundwater precipitation.

(6) Deep well point dewatering

Deep well point dewatering is a widely used dewatering method in foundation pit support, which has the advantages of large displacement, large dewatering depth and large dewatering range.

Extended data:

Factors to be considered in foundation pit dewatering

When dewatering the foundation pit with the above treatment method, the following factors should also be considered in the selected dewatering method:

(a) the site conditions, design and construction data of the building

Site conditions restrict the formulation of precipitation scheme, mainly including the height, distribution and structure of existing buildings around the site and the distance from the proposed project; Underground facilities around the foundation (including water supply and drainage pipelines, optical cables, gas supply pipelines, etc.). ); External pumping and drainage channels and power supply, etc. Relevant design and construction data include the scale and distribution of foundation pit excavation; Relevant requirements of underground construction, etc.

These conditions determine the dewatering method and specific design and construction scheme, and also determine the specific implementation measures to ensure the safety of surrounding buildings and underground facilities.

(2) Geological conditions

Understand the layered geological histogram and geological profile of foundation soil, physical and mechanical properties of each layer of rock and soil, groundwater type and burial, hydrological address, water quality analysis results, especially the permeability of soil layer. The permeability coefficient of soil depends on formation conditions, particle size distribution, colloidal particle content and soil structure, so the permeability coefficient of soil layers in different depths and directions is different.

The authenticity of the calculation results of permeability coefficient will directly affect the choice of precipitation scheme. Due to the complexity of influencing factors of permeability coefficient, most of the numerical values provided by general survey reports are indoor test data, and the errors are often large, so they can only be used as reference for precipitation design. Important projects should be determined by field pumping test.

(III) Groundwater situation of the site

Groundwater diving and confined water. The phreatic water is stored between the surface and the first impermeable layer, which is pressureless gravity water and can permeate around. From the engineering practice, most of the diving comes from atmospheric precipitation and the leakage of underground water supply and drainage pipelines, which are mainly accumulated in miscellaneous fill below the surface and the foundation hollowed out by old buildings. Confined water is stored in an aquifer between two impermeable layers. If the aquifer is filled with water, the water will have pressure.

Therefore, it is necessary to find out the distribution thickness and variation of permeable layer and impermeable layer along the depth according to geological and hydrological data; Master the buried depth, annual variation range and water level elevation of confined static water level and mixed static water level in all parts of the site; Find out the location and distance of groundwater recharge source in the site and its connection with permeable layer.

Find out whether the groundwater layer is connected with infinite water sources such as rivers, lakes and seas; Whether diving or confined water is connected with infinite water source, it will make precipitation difficult or even ineffective.

To sum up, there are many defects in foundation pit dewatering, such as uneven settlement of adjacent buildings, and measures should be taken to prevent uneven settlement during construction; According to the site conditions and the design and construction data of the building; Geological conditions; According to the groundwater situation of the site, select the appropriate dewatering method to reduce the accidents in foundation pit construction.

Emergency measures:

1, recharge measures to prevent settlement

Before the implementation of the dewatering project, a settlement monitoring network should be established according to the design requirements. Through settlement monitoring, it is found that some buildings have reached the dangerous level, and recharge measures should be taken in time. When building a recharge well in the subsidence area, the distance between the recharge well and the dewatering well must be >: 5.0m, and the design of the recharge well point should be based on the specific building conditions of the subsidence area.

2, standby power supply measures

In order to ensure the continuous pumping operation during the precipitation period and prevent the construction from being affected by the rising water level caused by power failure, the following measures are taken during the precipitation period:

(1) On the original power supply system, it is used as emergency standby power supply for the second power supply system, and is equipped with automatic switching device.

(2) If the second power supply system cannot be implemented on site, 1 ~ 2 generators will be equipped as emergency standby power supply according to the pumping power consumption, and automatic switching devices will be equipped.

3. Establish an emergency plan

The emergency plan mainly considers five aspects: well tube protection emergency plan, drainage system emergency plan, circuit system emergency plan, precipitation equipment emergency plan and personnel organization emergency plan.

(1) personnel organize emergency plans.

Emergency precipitation operators, such as emergency project leaders, emergency electricians, emergency generator operators and emergency precipitation operators, are on standby 24 hours a day to participate in emergency rescue and eliminate dangerous situations.

(2) Emergency plan for well tube protection and dewatering equipment

Sufficient well pipe materials and dewatering equipment should be prepared to provide a small amount of dangerous goods at any time, and a large number of dangerous goods can be supplied by the manufacturer in time within 5 hours.

(3) Emergency plan for circuit system

In order to prevent sudden large-scale power outage and field circuit system failure, double power supply guarantee measures must be provided in the whole process of dewatering well operation. When there is normal industrial electricity, a diesel generator is equipped at the same time, and the circuit design adopts two-way knife to ensure the free switching of industrial electricity and diesel generator power supply.

In the case of providing dual power supply guarantee, the dual power supply circuit should be carefully laid out. Circuit layout mainly considers two main aspects: line load and falling water box load. The wires used in each level of circuit must meet the load requirements, and the electric box must also meet the load requirements. At the same time, the electric box must be used as a special electric box for precipitation, and other electrical equipment cannot be connected at will.

(4) Emergency plan for drainage system

Whether emergency drainage is normal or not will directly affect the normal operation of precipitation. Under normal circumstances, drainage can be designed, but for emergencies (sudden rainstorm, flood, etc. ), the pumping volume is quite large, the precipitation is quite large, and the water output is very large every day. The drainage system is very strict, which requires a special emergency drainage planning line to be discharged into surface water bodies (such as open channels and canals) nearby.

References:

Baidu encyclopedia-foundation pit dewatering