PLATE LOAD TEST – A FIELD TEST

The standard method for a field load test is given by the American Society for

Testing and Materials (ASTM) under Designation D-1194 (ASTM, 1991).

Circular steel bearing plates 6 to 30 in (152.4 to 762 mm) in diameter and 1ft

x 1ft (304.8mm x 304.8mm) square plates are used for this type of test.

Procedure:

1) A pit of depth, Df must be excavated. The width of the test pit should

be at least four times the width of the bearing plate to be used for

the test.

2) The bearing plate is placed on the soil at the bottom of the pit, and

an incremental load on the bearing plate is applied.

3) After the application of an incremental load, enough time is allowed

for settlement to occur.

4) When the settlement of the bearing plate becomes negligible,

another incremental load is applied.

5) A load-settlement plot is obtained.

Figure 16. Diagram for plate load test

Figure 17. Typical load-settlement curve obtained from plate load test

For ultimate soil-bearing capacity of actual footings can be approximated as

follows:

For clays:

𝒒𝒖(𝒇𝒐𝒐𝒕𝒊𝒏𝒈) = 𝒒𝒖(𝒑𝒍𝒂𝒕𝒆)

For sandy soils:

𝒒𝒖(𝒇𝒐𝒐𝒕𝒊𝒏𝒈) = 𝒒𝒖(𝒑𝒍𝒂𝒕𝒆)

𝑩(𝒇𝒐𝒐𝒕𝒊𝒏𝒈)

𝑩(𝒑𝒍𝒂𝒕𝒆)

For a given intensity of load, q, the settlement of the actual footing can also

be approximated from the ff. equations:

In clay:

𝑺(𝒇𝒐𝒐𝒕𝒊𝒏𝒈) = 𝑺(𝒑𝒍𝒂𝒕𝒆)

𝑩(𝒇𝒐𝒐𝒕𝒊𝒏𝒈)

𝑩(𝒑𝒍𝒂𝒕𝒆)

In sandy soil:

𝑺(𝒇𝒐𝒐𝒕𝒊𝒏𝒈) = 𝑺(𝒑𝒍𝒂𝒕𝒆) (𝑩(𝒇𝒐𝒐𝒕𝒊𝒏𝒈)

𝑩(𝒑𝒍𝒂𝒕𝒆))𝟐

Housel (1929) also proposed a method for obtaining the soil-bearing capacity

of a footing that rests on cohesive soil for a given settlement, S.

According to this procedure, the total load carried by a footing of area A and

perimeter P can be given by:

𝑄 = 𝐴𝑞 + 𝑃𝑠

Where:

q = compression stress below the footing

s = unit shear stress at the perimeter

Note:

q and s are the two unknown that must be determined from the results of

the field load tests conducted on two different-size plates.

However, Housel’s method is not widely used in practice.

Sample Problems

1) The ultimate bearing capacity of a 30-in diameter plate as determined

from field load tests is 6000 lb/ft2. Estimate the ultimate bearing capacity

of a circular footing with diameter of 4.5 ft. The soil is sandy.

2) The following are the results of two plate load tests on cohesive soil:

Plate Size Settlement (in) Total Load, Q (lb)

1.5ft x 1.5ft 0.5 15,750

2.5ft x 2.5ft 0.5 33,750

If a square footing 5.75ft x 5.75ft is to be constructed and the allowable

settlement is 0.5in, what is the magnitude of the total load that it can carry?