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 Method 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. Under the action of gravity, the water from the slopes will flow to the sumps. The sumps collect the water and is later pumped out.
Significant 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 method used for dewatering shallow excavations that have coarse grained soils or the soils that have permeability that is greater than 10-3 cm/sec.
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Suitability of Sumps and Ditches for Dewatering of Excavations
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 method 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.
Use of Sumps and Ditches in Cofferdams
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. Then the bracing is installed and proceeds with the excavation of earth. Later the water that seeps into the cofferdam area is pumped out.
The use of sumps and ditches in the dewatering of sheet excavation face the limitation similar to that of open excavations. 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. This can help in the reduction of seepage into the bottom of the excavation.
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 process 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.
Advantages of Sumps and Ditches
The advantages of Sumps and Ditches are:
- The method is widely used. It is appropriate for small depth lowering.
- This method is found to be most economical one among dewatering systems while considering the installation and the maintenance procedures
- This method can be applied for most of the soil and rock conditions.
- The site is mostly recommended where boulders or massive obstructions are met within the ground.
- The greatest depth up to which the water table can be lowered by this method is 8m
Disadvantages of Open Sump and Ditches
The disadvantages of this method are:
- In areas where there is high heads or steep slopes, the method is not demanded. This method will bring collapse of the slopes and cause dangerous problems
- The use of sumps and ditches in open or timbered excavation will bring risk in the stability of the base.