The application of sumps and ditches within an excavation is one of the elementary method of dewatering employed in construction. The water entering these installed units can be pumped out.
The general procedure of dewatering with sumps and ditches is depicted in the figure-1.
Fig.1. Dewatering approach by the Installation of Sumps and Ditches
The sump is located below the ground level of the excavation as shown in figure-1, at one or more corners or the sides. The procedure involves the cutting of a small ditch around the bottom of the excavation, that is falling towards the sump.
The sumps is the name given for the shallow pits that are dug along the periphery of the excavation or the drainage area, which is named as ditches. The sumps collect the water and is later pumped out. Under the action of gravity, the water from the slopes will flow to the sumps.
substantial amount of seepage can result in raveling or sloughing or softening of the slope in the lower part. The slump bottom may also be subjected to piping.
The above problems can be solved by the use of inverted filter that is of many layers. These have coarser material in successive layers from the bottom of the sump pit to the upward direction.
This is a simple approach used for dewatering shallow excavations that have coarse grained soils or the soils that have permeability that is greater than 10 -3 cm/sec.
If the construction demands for lowering the water table or the ground water head of the area to a depth greater than 1 feet, the approach of sumps and ditches is not suitable.
If sumps and ditches are employed for greater depth lowering, seepage will be prominent that will result in the instability of the excavation slopes. This wrong decision will also bring effects that are detrimental for the integrity of the foundation soils of the area.
In order to overcome the problems that arise due to minor raveling and to support the collection of seepage water, it is recommended to employ filter blankets or drains in the sump and ditch system installed.
Then the bracing is installed and proceeds with the excavation of earth. Later the water that seeps into the cofferdam area is pumped out. In areas that are confined, the common method of excavation that is followed is the driving of a sheet pile that is either wood or steel, below the subgrade elevation.
The use of sumps and ditches in the dewatering of sheet excavation face the limitation similar to that of open excavations. This can help in the reduction of seepage into the bottom of the excavation. The formation of hydraulic heave at the bottom of the excavation which is found to be very dangerous can be reduced by the driving of sheeting into the impermeable strata that is underlying.
Those excavation carried out below the water table can be effectively conducted with the help of sheeting and sump. This is merely dependent on the site conditions. The hydrostatic pressure and toe support are the two factors that is to be considered important while designing the sheeting and the bracing.
The construction procedure and the pumping out activities can be conducted smoothly by covering the bottom of the excavation by means of a inverted sand and gravel filter blanket.