On what Basis Depth of Foundation depends?
First step in designing the foundation for a particular structure is soil investigation. The soil consultant should interact with other members of the construction team, namely architect & structural engineer to determine the type and extent of tests to be conducted for a particular structure. To facilitate the structural engineer in selecting a suitable foundation type/ system, the soil investigation report should essentially give the following data:-
a) Soil Profile/Bore log, along with soil identifation and classification
b) Location of Water Table, if encountered.
c) Proposed Foundation Depth w.r.t NGL
d) Proposed finished Ground Level
e) Proposed Plinth level of the Buildings
f) The net Bearing Capacity of various types of Foundations based on suitable field/ laboratory test and detailed calculations.
The field test for the determination of net safe bearing capacity generally consist of any one or a combination of the following:-
a) Plate load test
b) Standard Penetration Test- SPT
c) Dynamic Cone Penetration Test- DCPT
The net safe bearing capacity of open foundations is calculated from two considerations:-
First, there must be adequate factor of safety against the total bearing capacity failure (total collapse).
Second, the settlement which the structure will develop when loaded will have to be within the permissible limits so as not to endanger it's stability.