Dear reader we have discussed few posts about contact pressure distribution beneath foundation considering either rigid or flexible foundation supported on cohesive or cohesionless soil. Here we also discuss same thing but we are considering raft foundation. We have already clarified raft foundation and mat foundations are identical foundation system.
In discussing combined footing our aim is to coincide center of gravity of foundation with center of resultant load, to have uniform pressure distribution; to achieve these, we have introduced foundation like trapezoidal footing. But we have to know-does actually the distribution is uniform throughout the foundation?
Say contact pressure is not uniform, it is maximum at some point exceeding bearing pressures and in some location far below bearing pressure, the foundation is not only inefficient-uneconomical but also it is unsafe considering settlement, shear failure etc. When we consider internal stability that is the foundation itself safe structurally-say thickness, reinforcement and serviceability are not served, the structure will also collapsed.
So in designing raft foundation, one of the most important parameter to measure is contact pressure distribution. Unfortunately many theoretical approaches are found in many research papers but very few experimental results are available. The relationship among contact pressure, soil characteristics and settlement of foundation are very much inter-related and complex. Soil-structure or soil-foundation interaction even now not can clearly be understood. Considering above aspects we found following variables influence contact pressure distribution beneath raft foundation:
a. Nature of soil beneath raft foundation
-Homogeneous or layered formation
-Thickness of individual layers
-Relative locations of layers
b. Properties of soil
c. Nature of foundation
d. Rigidity of super-structure
e. Intensity of loads and distribution
f. Size of the raft
g. Duration of measuring pressure
h. Existence of adjacent foundation