Geotechnical Engineering

DHARM

408 GEOTECHNICAL ENGINEERING

(a) Clay (saturated)

( = 0, perfectly elastic)f

(b) Sand (c = 0) (c) c- soilf

Fig. 11.8 Contact pressure distribution under a uniformly loaded rigid foundation
For a rigid foundation, placed at the ground surface on sand (c = 0), the contact pressure
at the edges is zero, since no resistance to shear can be mobilised for want of over-burden
pressure; the pressure distribution is approximately parabolic, as shown in Fig. 11.8(b). The
more the foundation is below the surface of the sand, the more the shear resistance developed
at the edges due to increase in overburden pressure, and as a consequence, the contact pres-
sure distribution tends to be more uniform.
For a general cohesive-frictional soil (c – φ soil) the contact pressure distribution will be
intermediate between the extreme cases of (a) and (b), as shown in Fig. 11.8(c). Also for a
foundation such as a reinforced concrete foundation which is neither perfectly flexible nor
perfectly rigid, the contact pressure distribution depends on the degree of rigidity, and as-
sumes an intermediate pattern for flexible and rigid foundation. However, in most practical
cases the assumption of uniform contact pressure distribution yields sufficiently accurate de-
sign values for moments, shears and vertical stresses, and hence is freely adopted.

11.8.2Active Zone from Pressure Bulb Concept

Terzaghi (1936) related the bulb of pressure with the seat of settlement. Since it is possible to

obtain an infinite variety of pressure bulbs for any applied pressure, one has to refer to an

assumed isobar like that for (1/n)th of the contact pressure, q, as shown in Fig. 11.9.

G.S.

q

B

Unstressed zone

0% isobar

Dn Stressed

zone

-. q-isobar^1 n

Fig. 11.9 The (1/n). (q)-isobar (Jumikis, 1962)