A plate load test involves applying incremental loads to a bearing plate placed on the ground surface and measuring the resulting settlements. The test is used to estimate the settlement of a footing under working loads. A seating load is first applied and removed, then higher loads are placed and settlements are recorded until the rate of settlement decreases. Load-settlement curves are plotted from the results. The test gives the immediate settlement but not long-term consolidation settlement, so it is not very useful for predicting behavior in clay soils. The test also may not be representative if the soil is not homogeneous to a depth of 1.5-2 times the prototype footing width.

1. In situ TestsIn situ TestsIn situ TestsIn situ Tests

2. PLATE LOAD TESTPLATE LOAD TESTPLATE LOAD TESTPLATE LOAD TEST

10. Plate load test
can be used to directly estimate the
settlement of the footing.
performed using standard test procedures such
as outlined in ASTM D 1194-94
Bearing plate , round or square, varying in size,
from 30 to 60 cm and thickness of about 2.5
cm are employed for the test.
Normally a plate of size 30 cm is used in sandy
soils and a larger size in clay soils.

11. A seating load of about 70 gm/cm2 is first
applied and released after some time.
A higher load is next placed on the plate
and settlements are recorded by means of
the dial gauges.
Observations on every load increment
shall be taken until the rate of settlement
is less than 0.25 mm per hour.
Load increments shall be approximately
one-fifth of the estimated safe bearing
capacity of the soil.

12. The average of the settlements
recorded by 2 or 3 dial gauges shall be
taken as the settlement of the plate for
each of the load increments.
The test should continue until a total
settlement of 2.5 cm or the settlement
at which the soil fails, whichever is
earlier, is obtained.
After the load is released, the elastic
rebound of the soil should be
recorded.

13. From the test results, a load-settlement
curve should be plotted as shown in Fig.

14. The allowable pressure on a prototype
foundation for an assumed settlement
may be found by making use of the
following equations suggested by
Terzaghi and Peck (1948) for square footings
in granular soils.

16. B is in meters and settlement in mm
Square Foundations

18. Limitations
Since a load test is of short duration, consolidation
settlements cannot be predicted. The test gives
the value of immediate settlement only.
If the underlying soil is sandy in nature immediate
settlement may be taken as the total settlement.
If the soil is a clayey type, the immediate
settlement is only a fraction of the total
settlement. Load tests, therefore, do not have
much significance in clayey soils to determine
allowable pressure on the basis of a settlement
criterion.

19. Plate load tests should be used with
caution and the present practice is not to
rely too much on this test. If the soil is not
homogeneous to a great depth, plate load
tests give very misleading results.
.

21. A plate load test is not recommended in
soils which are not homogeneous at least
to a depth equal to 1 ½ to 2 times the
width of the prototype foundation.
Plate load tests should not be relied on to
determine the ultimate bearing capacity
of sandy soils as the scale effect gives very
misleading results.

22. However, when the tests are carried on
clay soils, the ultimate bearing capacity as
determined by the test may be taken as
equal to that of the foundation since the
bearing capacity of clay is essentially
independent of the footing size.

25. Housel (1929) technique for determining
load bearing capacity of shallow
foundations on settlement consideration

26. Procedure
1. Conduct two plate load tests with different plate sizes
2. Obtain Load settlement curve for each test
3. Obtain allowable load for allowable settlement from
each test
4. Using following equation find value of m and n
nPmAQ
nPmAQ
pp
pp
222
111
+=
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