spt n value to soil bearing capacity
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N Allowable Bearing Pressure forFoundations on Non-Cohesive Soil
of topsoil and silty clay overlying a considerable depth ofmedium dense sand. Average SPT values for the top metreof sand are in the range N = 22 41; a conservative averagevalue of N = 25 will be assumed for determining the allow-able bearing pressure.
A square base of 2 m 2 m is initially assumed. From Fig. N.1, the allowable bearing pressure is na = 280 kN. This gives a capacity of
Pa = na BL= 280 2.0 2.0= 1120 kN
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360 Appendices
700
max. settlement 25 mm
600
500
400
Allo
wab
le b
earin
g pr
essu
re (k
N/m2
)
stan
dard
pen
etra
tion
resis
tanc
e
300
200
100
0 1 2 3Width of footing: B (m)
4 5 6
N = 50
N = 40
N = 30
N = 20
N = 10
N = 5
Fig. N.1 Allowable bearing pressure on sands (Reproduced from Terzaghi, K. & Peck, R.B. (1996) Soil Mechanics inEngineering Practice, 3rd edn, by permission of John Wiley & Sons, Inc.).
is critical see sections 2.3.2 and 2.3.4.) Nevertheless asquare column base should be preferred to a rectangularone; mass concrete square bases will require a lesser thick-ness for load dispersion, while reinforced square bases will have lower bending moments and thus require lessreinforcement.
A = BL= 2.0 2.8 = 5.6 m2
It will be noted that the rectangular base results in a slightlylower area than the square base. (The opposite is true forcohesive soils where bearing capacity rather than settlement
SFDD01 1/8/06 11:23 AM Page 360
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