Bearing Capacity and Shallow Foundations - Elements of Soil Mechanics - page 288

Example 9.9: Allowable bearing pressure from SPT results

Refer to Example 6.2. A strip footing, 3 m wide, is to be founded at a depth of 3 m.
Assuming that the strength characteristics of the sand are constant with depth, deter-
mine the allowable bearing pressure.

Solution:

From Example 6.2, (N1) 60 = 26
From Fig. 9.14, for a corrected N = 26 and B = 3 m:
Allowable bearing pressure = 250 kPa

But this value is for dry soil and the sand below the foundation is also below groundwater
level and is therefore submerged. Therefore,

D
D B

=

1 5
3

.

=

w

q = × × + q 0 5 . 1 250 0 5 × × + . 1 156 kPa a

+ + 3

Exercises

Exercise 9.1

A strip footing 3 m wide is to be founded at a depth of 2 m in a saturated soil of
unit weight 19 kN/m 3 . The soil has an angle of shearing resistance, φ ′ of 28° and an
effective cohesion, c ′ , of 5 kPa. Groundwater level is at a depth of 4 m. Adopting
a factor of safety of 3, determine a value for the safe bearing capacity of the
foundation.
If the groundwater level was to rise to the ground surface, determine the new
value of safe bearing capacity.

Answer 871 kPa; 248 kPa

Exercise 9.2

A soil layer has the following properties:

′ = ′ = ° =

c 7 kPa ; φ 20 ; γ 20 kN m /

3

.

A footing of dimensions 7.5 m × 2.5 m is to be founded at a depth of 1.5 m into
this layer. Determine the ultimate bearing capacity of the soil.

Answer 488 kPa

Exercise 9.3

A continuous concrete footing ( γ c = 24 kN/m 3 ) of breadth 2.0 m and thickness 0.5 m
is to be founded in a clay soil (c u = 22 kPa; γ = 19 kN/m 3 ) at a depth of 1.0 m. The