Module 1 : Site Exploration and Geotechnical Investigation
Lecture 4 : In-situ tests [ Section 4.2 : Pressuremeter test, Vane shear test, Plate load test ]
   
  Plate load test
 

The plate load test is a semi-direct method to measure the allowable pressure of soil to induce a given amount of settlement. Plates, round or square, varying in sizes, from 30 to 60 cm and thickness of about 2.5 cm are employed for the test.

The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a cross beam or a steel truss anchored suitably at both the ends. The settlement of the plate is measured by a set of three dial gauges of sensitivity 0.02mm placed at 120º apart. The dial gauges are fixed to independent supports which do not get disturbed during the test. Fig shows the arrangement for the plate load test.

 

Procedure:

  The method of performing the test is essentially as follows:
Excavate a pit of size not less than 5 times the size of the plate. The bottom of the pit coincides with level of the foundation.
 
If water table is above the level of foundation, pump out the water carefully and it should be kept just at the level of the foundation.
 
A suitable size of the plate is selected for the test. Normally a plate of size 30cm is used in sandy soils and bigger size in clay soils. The ground should be leveled and the plate is seated over the ground.
 
A seating load of about is first placed and released after sometime. 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 0.25mm per hour. Load increments shall be approximately one-fifth of the estimated safe bearing capacity of the soil. The average of the settlements recorded by 2 or 3 dial gauges taken as the settlements of the plate for each of the load increment.
The test should continue until a total settlement of 2.5cm or the settlement at which the soil fails, whichever is earlier, is obtained. After the load is increased, the elastic rebound of the soil should be recorded.
 
 
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