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, there is a need to organize water depletion and water drainage.
For the artificial lowering of the groundwater table in building, the following methods are used:
– open water drainage;
– wellpoint filtration;
– electroosmotic water drainage;
– vacuum water drainage;
– open water-reducing 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 the liquefaction of the soil, the removal of part of the soil by filtering water, resulting in a decrease in the natural strength of the substrate.
The main and most common method of artificial water depreciation is acupuncture. It is based on the use of needle-filtration plants consisting of steel pipes with a filter link in the lower part (wellpoint), a catchment collector on the ground surface and a vortex pump. The filters are immersed in the soil in one or two rows along the perimeter of the excavation 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 a needlefiltered water outflow it is possible to lower the groundwater table by 4 to 5, and with a two-level arrangement of the needlift filters – by 7 to 9 meters.
The method of draining soils using the phenomenon of electroosmosis is used in weakly permeable soils with a filtration coefficient of less than 0.05 meters per day with a pit width of up to 40 meters. The method of electroosmosis is based on the ability of water to move in the pores of the soil under the influence of a constant electric current from the anode to the cathode. In addition to the wellpoints, steel electrodes are immersed in the ground at a distance of 0.5-1 m from them in the direction of the excavation. The filters are connected to the negative pole (cathode), the electrodes to the positive (anode). Electrodes are placed relative to each other in staggered order with a step in the row of 0.75 – 1.5 meters, under the influence of electric current, water contained in the pores of the soil moves to the side of the needle filters, while the filtration factor increases by 5 -25 times.
The vacuum method of water depletion is used in soils with a filtration coefficient of 0.01 – 3 meters per day. At the same time, vacuum dewatering installations with light needlepoints equipped with air tubes are used. During the operation of the unit, a vacuum is created and groundwater is sucked, after which the pump picks out excess water and merges. With this method, the lowering of groundwater reaches 20-22 meters.
Open water-lowering wells equipped with pumps are used in those cases when the groundwater table must be lowered to a great depth and when the use of wellpoints is difficult because of the large inflow of water, the need to reduce the large area and constraint of the territory. The main working element of the system is a filter column, consisting of a filter, a settler, over-filter tubes inside which a pump is placed, usually artesian.