15 X 15 m 5 m Sand 90 = 75 kPa p = 1.9 Mg/m³ 1 m 10 m Clay Sand A Psat = 1.8 Mg/m³ W = 42% P sat = 1.9 Mg/m³
A mat foundation, 15 m x 15 m, is made of reinforced concrete and to be supported by
a three-layer soil profile, as shown.
The mat is 1 m thick, and the average stress on the surface of the slab assessed from the
structural engineering analysis is 75 kPa. (Unit weight of concrete = 23.58 kN/m^3)
The 5-m thick sand layer immediately below the mat foundation has been compacted to
standard Proctor specifications, most likely to optimum moisture content, which is why
its moist density is given.
(A) Determine the pre-construction effective stress at Point A (bottom of the clay layer).
This is the in situ effective stress (overburden pressure) measured from the ground
surface prior to the placement of the mat foundation.
(B) Determine the vertical stress increase induced by the mat foundation at Point A
using the “Influence Chart,” commonly referred to as the “Spider Web.”
(C) Determine the vertical stress increase induced by the mat foundation at Point A
using the “Stress Isobars.”
(D) Determine the vertical stress increase induced by the mat foundation at Point A
using the original “Boussinesq’s Solution.”
(E) Using results from question (D) above, determine the post-construction effective
stress at Point A, that is, the cumulative stress after placement and loading of the
mat foundation.
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