Geotechnical Engineering

DHARM

BEARING CAPACITY 597

Pressure kN/m^250100200300400500600
Settlement mm 1.5 2.0 4.0 7.5 12.5 20.0 40.0
The size of the plate was 750 mm × 750 mm and that of the pit 3.75 m × 3.75 m × 1.5 m.
(i) Plot the pressure-settlement curve and determine the failure stress.
(ii) A square footing, 2m × 2 m, is to be founded at 1.5 m depth in this soil.
Assuming the factor of safety against shear failure as 3 and the maximum permissible
settlement as 40 mm, determine the allowable bearing pressure.

(iii) Design of footing for a load of 2,000 kN, if the water table is at a great depth.
(i) The pressure-settlement curve is shown in Fig. 14.23
The failure point is obtained as the point corresponding to the intersection of the initial
and final tangents. In this case, the failure stress is 500 kN/m^2.

∴ qult = 500 kN/m^2

(ii) The value of qult here is given by

1

2

γbp Nγ.

bp, the size of test plate = 0.75 m

100 200 300400 500 600 700 800 900 1000

0

5

10

15

20

25

30

35

40

45

50

Settlement, mm

Failure point

Pressure, kN/m^2

qult

Pressure

settlement curve

Fig. 14.23 Pressure-settlement curve (Ex. 14.22)

Assuming γ = 20 kN/m^3 ,

500 =

1

2 × 20 × 0.75 Nγ

∴ Nγ = 500/7.5 ≈ 6.7

φ = 38°

∴ Nq ≈ 50 from Terzaghi’s charts.

For square footing of size 2 m and Df = 1.5 m,

qnet ult = 0.4 γ b Nγ + γDf (Nq – 1)

= 0.4 × 20 × 2 × 67 + 20 × 1.5 × 49 = 2,542 kN/m^2