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Plate Load test

Plate Load Test

The load carrying capacity and extent to which a soil undergoes settlement under a given load must be known beforehand in order to design a foundation for the structures. In the field the ultimate bearing capacity and corresponding settlements can be determined with the help of plate load tests.

The plate load test, which is used to determine bearing capacity and settlement, may be carried out by either using a gravity loading or truss loading method. A test pit of the same depth as the foundation and width equal to five times the size of bearing plate is dug in the ground. A bearing plate (usually 30 cm square or diameter and 25 mm thick) is then placed in the test pit. The size of the bearing plate chosen for a test depends on the type of soil. A square plate of 45 cm size may be adopted for clayey or silty soil and also for loose to medium-dense sandy soils having a standard penetration resistance number < 15. A bearing plate of 30–75 cm may be adopted for carrying out the test in dense sandy or gravelly soil the standard penetration resistance number of which is between 15 and 30. The size of test plate further depends on the maximum size of grains.

The test plate is loaded with the help of the hydraulic jack, using either a gravity loading frame or truss loading frame to bear the reaction. The gravity loading method of the plate load test is shown in Fig. 9.18A and B. The loading frame rests on the columns built on the sides of the test pit. The loading frame is loaded with sand bags, rocks, or concrete blocks. The applied load is transmitted to the ground using an extension pipe and through the bearing plate. Four dial gauges are placed diagonally on each corner of the bearing plate to measure the settlement. The applied load is controlled using a hydraulic jack placed between the loading frame and extension pipe.

The test plate is placed in such a way that the centre of test plate, hydraulic jack, and the loading frame coincide with each other. A seating pressure of about 7 kN/m2 is applied before starting the test. The load on the soil is then increased in increments (20% of the estimated safe load or 1/10th of the ultimate load). The settlement of the bearing plate is measured at intervals of 1, 5, 10, 20, 40, 60 min, etc., until no significant change in settlement is observed. Once the rate of settlement is observed to be < 0.2 mm/min, the next load increment is applied, and the settlement observation is started again. The load increment is continued till failure or until a settlement of 25 mm is achieved under normal conditions; however, in special cases the test may be continued till a settlement of 50 mm is reached.

 

The typical load settlement curve for the plate load test in different types of soil is shown in Fig. zero correction to the load settlement curve may be required sometimes prior to the calculation of the bearing capacity of the soil. It is done by drawing a straight line intersecting the zero load line from the early straight line of the curve, which is then subtracted from the settlement readings.

The limitations of the plate load test are as follows:

  • As the width of the bearing plate is very small compared with the actual foundation, it only provides an estimate of the bearing capacity up to a depth of twice the width of the bearing plate.
  • Since the plate load test is conducted over a very short duration, the settlement measured during plate load should not be considered as the ultimate settlement. This is particularly relevant to clayey soils.
  • The plate load test underestimates the bearing capacity of dense sandy soils, because the bearing capacity of dense sandy soils increase with an increase in the size of the footing.
  • The failure load is often not well defined in the load settlement curves obtained from the test. Hence, errors may arise based upon personal interpretation.
  • The effect of the water table may not be taken into account properly in the test. It is advised to lower the water level by pumping, if it is encountered at the testing depth.