Geotechnical Engineering

DHARM

576 GEOTECHNICAL ENGINEERING

Information regarding the bearing capacity may be obtained from the pressure-settle-
ment curves obtained as a result of plate bearing test in the field; however, great care should
be exercised in the interpretation of the results. Typical curves are shown in Fig. 14.16.
Curve I is typical of dense sand or gravel or stiff clay, wherein general shear failure
occurs. The point corresponding to failure is obtained by extrapolating backwards (as shown in
the figure), as a pronounced departure from the straight line relationship that applies to the
initial stages of loading is observed. (This coincides approximately with the point up to which
the range of proportionality extends).
Curve II is typical of loose sand or soft clay, wherein local shear failure occurs. Continu-
ous steepening of the curve is observed and it is rather difficult to pinpoint failure; however,
the point where the curve becomes suddenly steep is located and treated as that corresponding
to failure.
Curve III is typical of many c – φ soils which exhibit characteristics intermediate be-
tween the above two. Here also the failure point is not easy to locate and the same criterion as
in the case of Curve II is applied.

Thus, it is seen that, except in a few cases, arbitrary location of failure point becomes
inevitable in the interpretation of load test results.

0 100 200 300 400 500 600

10

20

30

40

50

60

I

III

II

Approximate range

(Elastic compression)

of proportionality

local

cracking Shear failure

Settlement mm

Pressure kN/m^2

Fig. 14.16 Typical load-settlement curves from plate load tests

Determination of bearing capacity from plate load test

The size effect has been empirically evolved in the form of the following equation (Terzaghi

and Peck, 1948):

S

Sp

=

bb

bb

p

p

(.)

(.)

+

+

L

N

M

M

O

Q

P

P

03

03

2

...(Eq. 14.110)