Keywords: countermeasures of groundwater in deep foundation pit
order
It is generally believed that foundation pit excavation should meet the following necessary conditions: first, keep the foundation pit dry and create an environment conducive to construction; The second is to ensure slope stability and realize safe construction. If these necessary conditions are ignored, the consequences will be very serious. Some foundation pits have accumulated water or the soil is thin and soft, which makes it difficult for workers to stand on their feet and cannot be constructed; Some have "quicksand phenomenon", which leads to slope collapse and geological damage; The displacement of some internal foundation pits is large, which affects the safety of adjacent buildings. These anomalies are all caused by groundwater. Therefore, attention should be paid to the treatment of groundwater in foundation pit construction.
I. Artificial treatment of groundwater
There are many feasible methods for groundwater treatment, which can be divided into water method and drainage method. Water-stopping method is to form a water-stopping curtain around the foundation pit by effective means to keep the groundwater out of the foundation pit, such as open caisson method, grouting method and underground continuous wall. Drainage method is to exclude surface water and groundwater within the scope of foundation pit, such as open ditch drainage and well point dewatering.
The water stop method is relatively expensive and difficult to construct; Well point dewatering is an effective modern construction method, which is simple in construction and easy to master in operation technology, and has been widely used. This paper briefly introduces the well point dewatering method with engineering examples.
Well-point dewatering method, that is, water-permeable well-point pipes are set around the foundation pit of the proposed project, and certain pumping equipment is configured to continuously pump away groundwater, so that the groundwater in the foundation pit can be reduced to the design depth. Well point waterproofing is suitable for foundation pits with different geometric shapes, which can overcome quicksand and stabilize slopes. Because the earthwork in the foundation pit is dry, it is beneficial to mechanized construction, shorten the construction period and ensure the quality and safety of the project.
At present, the commonly used well point dewatering methods in China are light well point, jet well point and electroosmosis well point. In China, the well point dewatering method was gradually developed after the founding of New China. A large number of seepage well point pipes are buried near the foundation pit of this project. At the same time, the pumping pipeline system is assembled on the ground, and the groundwater is continuously pumped through the well group, so that the groundwater level in the foundation pit is reduced to a certain depth below the foundation pit and the foundation pit is kept dry. This method is usually called well point dewatering method.
Well point dewatering method has the following advantages: simple construction and easy operation technology; It has strong adaptability and can be used for foundation pits with different geometric shapes; After precipitation, the soil is dry, which is convenient for mechanized construction and subsequent processes; Under the action of well point, the soil layer is consolidated, the soil layer strength is increased and the slope stability is improved; Groundwater is pumped away by the filter tube, which prevents the harm of quicksand. Save supporting materials and reduce earthwork, etc. Well point dewatering method has become an effective method in water-bearing and permeable soil layer at present.
1. Light well point dewatering method
(1) The light well-point pumping system is vacuum pumping, which depends on pumping equipment to a great extent except pipeline system. At present, the commonly used vacuum pump and diaphragm pump supporting air extraction devices.
Light well point well point pipe, filter pipe, water collecting main pipe, main pipe, valve and so on constitute a pipeline system, which is started by pumping equipment to form a vacuum in the well point system and a vacuum zone around the well point, and the vacuum zone extends to a certain range through the silicon well. Under the action of vacuum force, the groundwater near the well point passes through the manhole and is forcibly sucked into the well point system through the filter, which reduces the groundwater level near the well point. In the process of operation, there is a head difference between the groundwater level near the well point and the groundwater level outside the vacuum area. Under the action of this head difference, the groundwater outside the vacuum zone flows by gravity. Therefore, light well point dewatering is often called vacuum forced pumping method, more accurately, it should be vacuum-gravity pumping method. Only under the action of these two forces, the groundwater in the foundation pit will be reduced and a funnel parabola of precipitation will be formed in a certain range.
The connection between the well point pipe and the main pipe can be made of steel pipe and transparent plastic pipe. Due to the vacuum force, a spring is installed in the plastic pipe to strengthen the external tension and ensure the smooth flow of groundwater.
There are two forms of connection between supervisors: Faranfa and ferrule method.
(2) Problems that should be paid attention to in construction
After lowering the groundwater level, the soil will consolidate, which will also cause ground settlement within the radius affected by pumping, and sometimes bring certain harm to the existing buildings around. In the construction process of lowering groundwater level, in order to avoid excessive settlement of surrounding buildings, it is a powerful measure to adopt reinjection well point. This method is to drill a row of holes near the buildings within the radius affected by pumping water, and before pumping water to reduce the groundwater level, explore the water level in the holes in advance and record it. When pumping water to reduce the groundwater level, in order to prevent the groundwater level from being lower than the existing buildings, water is injected into the borehole at the same time of precipitation, so as to ensure the original groundwater level unchanged and prevent the ground subsidence from bringing harm to the existing buildings.
2. Deep well point
Deep well point dewatering is to bury well pipes deeper than the basement around the deep foundation pit, and rely on deep well pump or deep well submersible pump to lift the groundwater in the deep well to the ground and discharge it, so that the groundwater level drops below the pit bottom.
Deep well point dewatering has the advantages of large displacement, deep dewatering, unlimited suction and large well spacing. However, it has a large one-time investment and high hole-forming quality. Deep well point dewatering is suitable for large permeability coefficient (10 ~ 250 m/d); The soil is sand and gravel; Rich groundwater, deep precipitation (10~50m) and large area.
(1) construction technology specification
(2) Matters needing attention in well point setting and use stage
① Pore formation
(2) Install the well tube and fill it with filter material.
③ well washing
④ Install the water pump.
⑤ Matters needing attention in the use stage
Ⅰ. Well points in the foundation pit should be pumped at the same time, so that the water level difference can be controlled within the required range.
Ⅱ. Strengthen water level monitoring, especially for deep well points near existing buildings. Observation wells should be set near buildings (structures). When the water level difference is too large, remedial measures should be taken immediately, such as setting up recharge well points.
Ⅲ. Prevent the discharged groundwater from seeping back into the foundation pit.
Ⅳ. When the submersible pump is running, check whether the cable collides with the shaft wall, so as to prevent water from penetrating into the motor along the cable core after abrasion.
ⅴ. For the deep well point located in the foundation pit, due to the long well pipe, after digging to a certain depth, the well pipe should be connected and fixed with the supporting structure or column nearby.
ⅵ. When there is an impermeable layer at the bottom of the foundation pit, in order to remove the upper groundwater, sand drain can be used to cooperate with deep well dewatering.
ⅶ. Pull out the well pipe after use.
Second, the engineering example
The project of Shandong Animal Husbandry and Veterinary Science and Technology Service Center has 13 floors above ground and 1 floor underground, with a height of 39m and a building area of 13000m2. The foundation is a bored pile with a pile diameter of 0.6m, the design value of single pile bearing capacity 1600KN, and the depth of the pile tip embedded in moderately weathered rock stratum is not high.
1 engineering geological conditions
From top to bottom, the strata of the site are: ① miscellaneous fill; ② Silty clay; ③ Residual sandy (gravel) clayey soil, the upper part of which is mostly clayey soil with strong viscosity; The lower part is mostly silty sand, which is easy to soften when it meets water and is liquefied by shaking, so it is rich in water. (4) Strongly weathered granite, with medium-coarse grain and broken block structure, developed cracks and large water inflow; ⑤ Moderately weathered granite; ⑥ Weathered granite.
2 well point dewatering method
According to the geological data, the hydrogeological conditions of the proposed site are relatively simple, and the groundwater in the site belongs to Quaternary pore diving, and the main recharge source is atmospheric precipitation, which is greatly affected by seasonality. The groundwater level in the site is between1.2-1.73m, and the water level is very high. Its aquifer is mainly the third gravel layer, the second and fourth layers are weakly permeable, and its permeability coefficient is (0.72~45.52)* 10-6cm/s, and the fifth layer is the water-resisting layer. In this project, large-diameter deep wells are used for dewatering, 15 dewatering wells are set around the proposed building, and deep-water electric pumps are used for pumping water. Considering the deep precipitation and wide influence radius, if the precipitation is extracted for a long time, it will inevitably have an impact on the buildings near the venue. In order to enhance the precipitation effect and shorten the pumping time, intermittent pumping is adopted to reduce the external influence area and set up settlement observation points. The diameter of dewatering well is greater than 600mm, the hole depth is 15.0m, the diameter of wall-protecting casing is 600mm, and the casing is covered with two layers of nylon mesh cloth; Gravel with a particle size of 0.5 ~ 2.0 cm is filled around the casing as a filter layer. The filter layer should be filled to the original groundwater level, and the upper part should be backfilled with clay and compacted. In this project, the tube well is buried in strict accordance with the requirements of the construction specification of dewatering well, and the hole is formed by mud wall protection drilling method. After drilling, clean the drilling hole, and then install the well pipe.
Construction achievements
The well point dewatering of this project was successful, the water level was controlled, and only a small amount of mud and sand flow appeared, which improved the construction conditions, ensured the quality of the structure below 0.000 and completed it on time, and achieved good economic results.
put on file