Friday, October 26, 2012


The plate is firmly seated in the hole, and if the ground is slightiy uneven a thin layer of sand is spread underneath the plate. The load is applied with the help of a hydraulic jack (preferably with the remote control pumping unit), in convenient increments, say of about one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity. Settlement of the plate is observed by 2 dial gauges fixed at diametrically opposite ends and supported on a suitable datum bar. The dial gauges should have a sensitivity of 0.02 mm. Settlement should be observed for each increment of load after an interval of 1, 4, 10, 20, 40 and 60 minutes and thereafter at hourly intervals until the rate of settlement becomes less than 0.02 mm per hour. After this, next load increment is applied. The maximum load that is to be applied corresponds to 1 1/2 times the eslimated ultimate load or to 3 times the proposed allowable bearing pressure.

The water table has marked influence on the bearing capacity of sandy or gravelly soil. If the water table is already above the, level of footing, it should be lowered by pumping and the bearing plate seated after the water table has been lowered just below the footing level. Even if the water table is located above 1 m below the base level of the footing, the load test should be made at the level of water table itself.

The load intensity and settlement observations of the plate load are plotted as shown in Fig. 2.24 (a). Curve I corresponds to general shear failure and curve II corresponds to local shear failure. Curve III is a typical of dense cohesionless soils which do not show any marked shear failure under the loading intensities of the test.
When the load settlement curve [Fig. 2.24 (a)] does not indicate any marked breaking point, failure may alternatively be assumed corresponding to a settlement equal to one-fifth of the width of the test plate. In order to determine the safe bearing capacity it would be normally sufficient to use a factor of safety of 2 or 2.5 on utlimate bearing capacity.


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