How excavation is done for foundation in water logged sites
Excavation of Foundation in Water Logged Sites: Excavation of foundation in water logged sites causes a serious issue for the site engineer. To resolve the issue, there exist different methods which are based on the depth of excavation, depth of water table and many other factors. The methods given below are mostly accepted at the time of digging foundation trenches in water-logged sites.
(1) By building drains: This method is normally applied in shallow foundations in water-logged ground. Under this method, the drains with perfect sizes are built by the sides of the foundation trench.
The drains are filled with sub-soil water from the sides and the surrounded area and deliver it into a shallow pit or sump well. From the sump, the water is incessantly discharged or pumped out. This is the most economical method of draining excavated area and any unskilled labor will be able to maintain it easily with simple equipment.
(2) By building up deep wells: Sometimes, 30 to 60cm diameter wells are erected at 6 to 15 m centres all round the site in coarse soils or porous rock where it is essential for draining of large quantity of sub-soil water for temporary drainage of the ground. The water preserved in the wells is pumped out incessantly. This method is suitable for depths of excavation up to 24 m.
(3) Freezing process: This process is mostly recommended for excavations in water-logged soils like sand, gravel and silt. It is beneficially applied for deep excavation like foundation for bridges etc. specifically when excavation should be arranged alongside a subsisting structure or near some waterways. The process comprises of developing a sort of coffer darn by freezing the soil around the area that should be excavated.
Freezing pipes containing smaller diameter inner pipes are submerged about one metre centre to centre along the boundary of the area to be excavated. The layout of the pipes should desirably be such that the area covered is circular in plan. Freezing liquid is then delivered to the freezing pipes by refrigeration plant. This makes the ground around the pipes to freeze and produce a thick wall of frozen earth around the area to be excavated. This process can be applied for up to 30 m depth of excavation.
(4) By chemical consolidation of soil: Under this method, the soft water-logged soil is changed into a semi-solid mass with enforcing chemicals like silicates of soda and calcium chloride into the soil. This method is suitable for small works.
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