DHARMBEARING CAPACITY 581Thus, the net ultimate bearing capacity depends upon
(i) the unit weight of soil, γ, and
(ii) the angle of shearing resistance, φ (since Nγ and Nq depend upon φ), besides the size,
b, and depth, Df , of the footing.
An appropriate value of γ must be used depending upon the condition of the soil with
regard to water content and its location relative to ground water table.
The angle of shearing resistance of a cohesionless soil or sand is known to be dependent
upon the density index ; the density index is correlated to the penetration resistance value, or
the standard penetration number, N. Peck, Hanson and Thornburn (1953) have provided a
chart for evaluating φ, and the bearing capacity factors of Terzaghi, Nq and Nγ, from the
standard penetration number, N. They have also given a simple chart relating N to φ. This is
shown in Fig. 14.18 and the former in Fig. 14.19. If settlement is of no consequence, it is
possible to think in terms of ultimate bearing capacity according to Terzaghi’s formula by
using these charts. But this procedure is not popular.
The necessary corrections for the observed value of N are to be applied before use in
conjunction with these charts. These corrections, required for the grained soils like silts below
the water table and for the effect of overburden pressure, are dealt with in Chapter 18.
The charts of Figs. 14.18 and 14.19 do not apply to gravels or those soils containing a
large percentage of gravels.
90
80
70
60
50
40
30
20
10(^0) 28 30 32 34 36 38 40 42 44 46
Angle of internal friction, °f
SPT value, N
Medium Dense denseVer y
Ver yloose Loose
Fig. 14.18 Approximate correlation between N-value and φ for
granular soils (After Peck, Hanson and Thornburn).
If settlement criterion governs the allowable bearing pressure, as it invariably does in
the case of footings in sands, the design charts given by Terzaghi and Peck (1948) or that given
by Peck, Hanson and Thornburn (1953) may be used for the determination of allowable bear-
ing pressure for a specific allowable settlement of 25 mm or 40 mm, as the case may be. These
are shown in Figs. 14.20 to 14.22.