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

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

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