Foundation, Concrete and Earthquake Engineering

What is the Thickness-Settlement Relationship of Mat or Raft Foundation?

When a differential settlement of greater degree is suspected for shallow foundation, raft or mat foundations are used. In case of soil having loose and soft soil pockets, the mat foundations are adopted to minimize differential settlement. Mat foundation generally covers area equal to base area of structure or greater and it continues in both directions.

Foundation engineers often change the thickness of mat foundation to provide economical solution of costly mat foundation and from analysis it is found that the thickened portion of mat should be provided below columns. One observation shows that mat thickness can be reduced up to 40% away from respective column faces. But this thickening required careful observation considering all other factors geotechnical as well as stresses that can be concentrated at sensitive locations.

A 1.5m Thick Raft or Mat foundation-reinforcement arranging before concreting
Though thickening of mat render a greater bending moment capacity, it provides greater punching shear as well beam shear capacity to the mat. Hence a uncracked section is achieved. But when economy is concerned, the foundation engineers pay attention to thickening in right location where stress concentration is much more resulting a overall thin but localized thick, economical mat foundation. Here in this post we will try to learn the settlement-thickness relationship.

It is found that greater mat thickness produce positive bending moments whereas it simultaneously produces a less negative bending moment. The maximum settlement of a structure founded with mat foundation is not influenced greatly by the mat thickness but it provides a significant reduction in differential settlement

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