Design Approaches for Swamps
Compared with ordinary soil, for construction of roads in swamps, special treatment and techniques are to be adopted depending on the characteristics of the underlying soil.
- Partial or total removal of undesirable material
- Stage construction and surcharge fill
- Use of sand drains
- Use of light weight material
Partial or total removal of swampy soil
The undesirable material is excavated and removed. Bog blasting is a method in which a layer of sand is deposited over the undesirable soil and the soil below is blasted. The blast disperses the clay sideways and the resultant cavity is occupied by the overburden sand.
Stage construction and surcharge fill
As consolidation takes long time, system of stage construction is adopted so that embankment settlement and further construction proceed side by side. Alternatively, the embankment is constructed to an elevation higher than the final requirement. The surcharge so placed accelerates the settlement.
Use of vertical sand drains
Vertical sand drains consist of bore holes of 0.4 to 0.6 m diameter, dug through the swamps and filled with sand. The depth of the borehole can be 10m to 20m.
The drains are spaced 2 m to 6 m centre to centre both transversely and longitudinally. A sand blanket layer of 1 to 1.5 m is placed on top of the vertical sand drains. The sand drains facilitate quick removal of water from the soft strata as the embankment load squeezes the water from the pores. The drainage of water helps in the following ways: Quick settlement of the bank takes place and shear strength of soil is increased.
Use of lightweight materials
Settlement can be minimized by reducing the weight of the embankment. For this, light materials such as cinder, fly ash and saw dust can be used in embankment.
The soil properties can be changed either by addition or by mechanical blending of different soil types. The additives normally used are lime, cement, sodium silicate, calcium chloride and bituminous materials. In mechanical stabilization, the stability of the soil is improved by blending the available soil with imported soil or aggregate.
Use of geo textiles
Geo textiles are synthetic fabric made out of nylon, polyester, and poly propylene. They are non- decomposable. They are used for the following purposes: ensureing physical separation of layers, acting as a filtering medium, working as a reinforcing layer permitting heavy loads on weak soils.
Use of geo grids
Geo grids are high strength plastic grids which are interposed either at the bottom of a layer or in a layer itself to increase the strength. When used at the bottom of a granular layer and above the sub grade, the grid increases the load bearing capacity of the soil.