The excavation for the foundation has to be done with care. The sides of the excavation should suitably be supported by sheet piling, soldier piles and timber or some other standard method.
Dewatering will be necessary when excavation has to be taken below the water table level. Care has
to be taken to see that the adjoining structures are not affected due to the lowering of the water table.
3. Critical depth
In Type 2 foundations the shear strength of the soil is low and there is a theoretical limit to the depth to which an excavation can be made. Terzaghi (1943) has proposed the following equation for computing the critical depth Dc,
Skempton (1951) proposes the following equation for Dc, which is based on actual failures in excavations
or the factor of safety Fs against bottom failure for an excavation of depth D is
4. Bottom heave
Excavation for foundations reduces the pressure in the soil below the founding depth which results in the heaving of the bottom of the excavation. Any heave which occurs will be reversed and appear as settlement during the construction of the foundation and the building. Though heaving of the bottom of the excavation cannot be avoided it can be minimized to a certain extent. There are three possible causes of heave:
1. Elastic movement of the soil as the existing overburden pressure is removed.
2. A gradual swelling of soil due to the intake of water if there is some delay for placing the foundation on the excavated bottom of the foundation.
3. Plastic inward movement of the surrounding soil.
The last movement of the soil can be avoided by providing proper lateral support to the excavated sides of the trench.
Heaving can be minimized by phasing out excavation in narrow trenches and placing the foundation soon after excavation. It can be minimized by lowering the water table during the excavation process. Friction piles can also be used to minimize the heave. The piles are driven either before excavation commences or when the excavation is at half depth and the pile tops are pushed down to below foundation level. As excavation proceeds, the soil starts to expand but this movement is resisted by the upper part of the piles which go into tension. This heave is prevented or very much reduced.
It is only a practical and pragmatic approach that would lead to a safe and sound settlement free floating (or partly floating) foundation.