PLATE LOAD TEST

This is a field test for determining the ultimate bearing capacity of soil. The test consists of loading a steel plate placed at the foundation level and recording the settlements corresponding to each load increment. This test load is gradually increased till the plate starts to sink at a rapid rate. The total value of load on the plate in such a stage divided by the area of the steel plate gives the value of the ultimate bearing capacity of soil. The ultimate bearing capacity is divided by suitable factor of safety (which varies from 2 to 3) gives the value of safe bearing capacity of soil.

Figure 1. Vertical Section of Plate Load Test

Procedure of plate load test as follows: 

1. A pit is dug at site up to the depth at which the foundation is proposed to be laid. The width of the pit should be at least 5 times the width of the test plate. At the centre of the pit a small square depression or hole is made whose size is equal to the size of the test plate and bottom level of which corresponds to the level of actual foundation. The mild steel plate (also known as bearing plate) used in the test should not be less than 25 mm in thickness and its size may vary from 300 to 750 mm.

2. The load is applied to the test plate through a centrally placed column. The loading to the test plate is applied with the help of a hydraulic jack. The reaction of the hydraulic jack is borne either by the gravity loading platform or by the reaction truss method. Figure 1 shows gravity loading platform. In gravity loading method, a platform constructed over a vertical column resting on the platform. The loading is done with the help of sand bags, stones or concrete blocks. In the case of reaction truss method, the truss is usually made of mild steel sections, is held to the ground through soil anchors. The lateral stability of truss is achieved by the guy ropes.

3. The load is applied in convenient increments say of about one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity.

The settlement of plate is noted by means of dial gauges mounted on independent datum bar. The load is directly recorded from the pressure gauge of the hydraulic jacks. The observations are properly recorded and load settlement graph is plotted as shown in the figure 2. At the points where graph sharply takes turn is noted. The point gives the ultimate load intensity (bearing capacity).

Figure 2. Typical load settlement curve for different soils

The bearing capacity and the safe bearing capacity of the soil are calculated as:

Following are the limitations of the plate load test:

1. The test duration is short and hence does not give the ultimate settlement particularly in case of cohesive soils.

2. The test results reflect the character of soil located within the depth less than twice the width of bearing plate. It corresponds of a pressure bulb of one-tenth of the loading intensity at the test plate. The foundations of structures are generally larger and hence the settlement and resistance against the shear failure will depend on the properties of a much thicker stratum of the soil. 

3. For clayey soils, the ultimate pressure for a large foundation is nearly the same as that on the test plate. But for dense sandy soils, the bearing capacity increases with the size of the foundation and hence the results obtained on the small size bearing plates are found to give conservative values.