Drainage of excavations.
Drainage of the trench: a) arrangement of grooves, b) cross-section; 1 - groove, 2 - sump, 3 - gravel, 4 - plank cover.
Construction excavations should be protected from rainwater runoff and groundwater inflow, if the bottom of the trench is below the groundwater level. If excavations are made in cohesive and impermeable soils (cops, clay), then rainwater from outside the trench should be drained using drainage ditches made outside the trench outline.
Rainwater from the bottom of the trench and water flowing from the ground above the bottom, it is discharged through drainage grooves or through drainage - whether temporarily or permanently - to the collecting sump, from which it is selected by vessels or a pump. The grooves in the trench or the drainage should be arranged in this way, so that the ground under the foundations is not disturbed, and therefore it did not cause additional subsidence of the foundations.
If the excavation is carried out in non-cohesive soil (sands, gravel) and the bottom of the trench is below the water table, it is then used to lower the groundwater level through depression. Lowering the water level can be achieved by using tubular wells with a diameter from 15 do 50 cm with the filter at the bottom, in which the suction pipe is placed, or wellpoints, i.e.. small diameter wells from 38 do 80 mm. The capacity of the wells is much greater than that of wellpoints, moreover, the water is taken from a greater depth. Needle filters can be used to lower the groundwater level by approx. 1,5—2.0 m with wellpoint spacing not greater than 2,0 mm.
Lowering the water level in dusty soils with the use of wells or needle filters may cause the soil to be washed out from under the buildings in the vicinity of the excavation. If such a phenomenon occurs, existing buildings may be damaged (scratching) due to uneven settlement.