DHARM578 GEOTECHNICAL ENGINEERINGThe results of the load test are first plotted to natural scale and the early straight por-
tion is extended backwards to cut the settlement axis; the settlement at zero loading is known
as the ‘‘zero correction’’ (possibly due to uneven seating of the plate). The zero correction is
applied to all settlement values to get the corrected values of the settlements.
The corrected settlements are plotted on log-log graph against the corresponding pres-
sures. The plot usually consists of two straight lines as shown in Fig. 14.17. The point corre-
sponding to the break gives the failure point and the pressure corresponding to it is taken as
the bearing capacity.
IS: 1888–1971 also recommends this method for use with plate load tests.14.9.3Limitations of Plate Load Tests
Although the plate load test is considered to be an excellent approach to the problem of deter-
mining the bearing capacity by some engineers, it suffers from the following limitations:
(i) Size effects are very important. Since the size of the test plate and the size of the
prototype foundation are very different, the results of a plate load test do not directly reflect
the bearing capacity of the foundation.
The bearing capacity of footings in sands varies with the size of footing; thus, the scale
effect gives rather misleading results in this case. However, this effect is not pronounced in
cohesive soils as the bearing capacity is essentially independent of the size of footing in such
soils.
The settlement versus size relationship is rather complex in the case of cohesionless
soils (Terzaghi and Peck, 1948); however, in the case of cohesive soils, this relation is rather
simple, the settlement being proportional to the size. This should be considered appropriately
in arriving at the bearing capacity based on the settlement criterion.
(ii) Consolidation settlements in cohesive soils, which may take years, cannot be pre-
dicted, as the plate load test is essentially a short-term test. Thus, load tests do not have much
significance in the determination of allowable bearing pressure based on settlement criterion
with respect to cohesive soils.
(iii) Results from plate load test are not recommended to be used for the design of strip
footings, since the test is conducted on a square or circular plate and shape effects enter.
(iv) The load test results reflect the characteristics of the soil located only within a depth
of about twice the width of the plate. This zone of influence in the case of a prototype footing
will be much larger and unless the soil is essentially homogeneous for such a depth and more,
the results could be terribly misleading. For example, if a weak or compressible stratum exists
below the zone of influence of the test plate, but within the zone of influence of the prototype
foundation, the plate test may not record settlements which are sure to occur in the case of the
prototype foundation. This aspect has also been explained in Chapter 11 on ‘‘Settlement Analy-
sis’’, with the aid of the pressure bulb concept.
Perhaps the plate load test is the only good method for the determination of bearing
capacity of gravel deposits; in such cases, bigger size plates are used to minimise the effect of
grain size.
Thus, it may be seen that interpretation and use of the plate load test results requires
great care and judgement, on the part of the foundation engineer.