Under unfavorable ground conditions, when the groundwater level is too high, or when there is a threat of flooding the construction site with surface groundwater, it becomes necessary to organize dewatering and drainage.
To artificially lower the level of groundwater in construction, the following methods are used:
- open dewatering;
- wellpoint drainage system;
- electroosmotic drainage;
- vacuum drainage;
- open dewatering wells.
Open drainage is used in clay and sandy soils with good water permeability. This method is the simplest and most economical, but at the same time leads to soil liquefaction, the removal of part of the soil by filtering water, resulting in a decrease in the natural strength of the base.
The main and most common method of artificial dewatering is wellpoint. It is based on the use of wellpoint installations, consisting of steel pipes with a filter element in the lower part (wellpoint), a water collector on the surface of the earth, and a vortex pump.
Wellpoints are immersed in the ground in one or two rows along the perimeter of the pit or trench, depending on the type of soil, the distance between the needles can be from 0.75 to 1.5 meters. With the help of wellpoint drainage, it is possible to lower the groundwater level by 4 – 5, and with a two-tier arrangement of wellpoints – by 7 – 9 meters.
The method of draining soils using the phenomenon of electroosmosis is used in poorly permeable soils with a filtration coefficient of less than 0.05 meters per day with a pit width of up to 40 meters. The electroosmosis method is based on the ability of water to move in the pores of the soil under the influence of a direct electric current from the anode to the cathode. At the same time, along with well points, steel electrodes are immersed in the ground at a distance of 0.5 – 1 m from them towards the pit. Wellpoints are connected to the negative pole (cathode), and electrodes to the positive (anode).