DHARM580 GEOTECHNICAL ENGINEERINGwherein x represents the perimeter-area ratio, P/A. Housel assumes that σ and m are con-
stant for different loading tests on the same soil for a specific settlement, which would be
tolerated by the prototype foundation. Hence, he suggested that σ and m be determined by
conducting small-scale model tests by loading two or more test plates or model footings which
have different areas and different perimeters and measuring the total load required to pro-
duce the specified allowable settlement in each case, at the proposed level of the foundation.
This gives two or more simultaneous equations from which σ and m may be determined.
Then the bearing capacity of the proposed prototype foundation may be calculated from
Eq. 14.115, by substituting for x, the perimeter-area ratio of the proposed foundation. Thus
this procedure involves a kind of extrapolation from models to the prototype.
The method is commonly known as ‘‘Housel’s Perimeter Shear method’’ or ‘‘Housel’s
Perimeter-Area Ratio method’’.
14.12 BEARING CAPACITY FROM LABORATORY TESTS
The bearing capacity of a cohesive soil can also be evaluated from the unconfined compression
strength. From the concept of shearing strength, the bearing capacity of a cohesive soil is the
value of the major principal stress at failure in shear. This stress at failure is called the
unconfined compression strength, qu:
σ 1 = qu = 2c tan (45° + φ/2) ...(Eq. 14.116)
When φ = 0°, for a purely cohesive soil,
qu = 2c ...(Eq. 14.117)
This applies at the ground surface, i.e., when Df = 0. The ultimate bearing capacity may
be divided by a suitable factor of safety, say 3, to give the safe bearing capacity.
Casagrande and Fadum, (1944) suggest this procedure as an indirect check of the ulti-
mate bearing capacity of cohesive soil, since the allowable soil pressures commonly specified
in building codes are conservative from the standpoint of safety against rupture of the clay.
Further, bearing capacity problems may be studied experimentally from the shape of
the rupture surface developed in the soil at failure. Experimental results may be translated to
prototype structures by means of the theory of similitude, or modelling. (Jumikis, 1956 and
1961).
14.13 BEARING CAPACITY OF SANDS
The net ultimate bearing capacity of a footing in sand which is required in the proportioning of
the size, is given by:
qnet-ult = α γ b. Nγ + γDf(Nq – 1) ...(Eq. 14.118)
where α = Shape factor, which is given as
0.5 for continuous footing of width b,
0.4 for square footing of side b, and
0.3 for circular footing of diameter b.