EXCAVATIONS: Vacuum method: Forced flow.

The above methods are effectíve only in coarse grained soils. For fine grained solis, the welI point system can be extended by the vacuum method. For successful dewatering in the fine, non-cohesive soils, such as silty sands and other fine sands, it is neccesary to apply a suction head to the dewatenng system. Both the well point system and deep well system can be adopted for dewatering such soils by maintaining a vacuum in the well with the use of air tight seals for all points. A hole of about 25 cm diameter is created around the well point and the rise pipe by jetting water under sufficient pressure. While the jetting water is still flowing, medium to coarse sand is rapidly shovelled into the hole to fill it upto about 0.75 or 1 m from the top. The top portion of the hole is then sealed up by tamping bentonite, soil cement or clay (Fig. 2.40). Vacuum pumps are used to create vacuum in yhe sand filling. When the vacuum is drawn on the well point, the ground surface is subjected to unbalanced atmospheric pressure. Although the quantity of water drawn out does not increase much, the unbalanced atmospheric pressure acting on the ground surface consolidates the sub-soil which becomes stiff enough for carring out excavations.

 FIG. 2.40 VACCUM METHOD.