To perform plate load test to obtain a load-settlement curve of a soil at a particular depth so as to estimate the ultimate bearing capacity a foundation.
Test plate of square size (300 mm or 450 mm or 600 mm or 750 mm)
Hydraulic jack (50 T capacity)
Hydraulic pump (50 T capacity)
Pressure gauge (to read 50 T load with a sensitivity of 0.5 T)
Proving ring or load cell (50 T capacity)
4 no of dial gauges (with sensitivity of 0.01 mm and a travel of 50 mm)
Dial gauge supporting channels
Dial gauge stands
Magnetic bases for dial gauges
Loading platform equipment (girders, plates, sand bags etc) or Truss with anchor rods
Tripod (to erect the platform or truss frame)
Pulley block (to lift the girders or truss frame)
Excavate the test pit. The test pit size should be 5 times the size of test plate (Bp) and depth equal to the depth of foundation.
The loading platform should be erected over the test pit such that, the vertical line drawn through the center of gravity of the load passes through the centroid of the base of the test pit. This can be achieved by positioning the reaction girder (truss) with the help of the plum bob and spirit level. The load over the platform should be at least 50% more than the anticipated load to be transferred on the plate. This will ensure the safety of the platform for any small shift of the CG of the loading platform from the vertical axis of the pit.
Position the test plate centrally at the base of the pit. Ensure that the ground surface below the test plate is perfectly horizontal and level so that no stress concentration takes place below the plate during loading. If the ground surface is slightly uneven a thin layer of sand is spread underneath the test plate.
Position the hydraulic jack over the plate so that when hydraulic pressure is built up the jack pushes against the loading platform. If require place a loading column between the test plate and the jack to ensure the jack reach the loading platform.
Position minimum of two dial gauges diagonally at the corners (preferably 4 at all corners of the test plate) to record its settlement of the plate. The dial gauges should be supported carefully over a stable base which does not settle with the plate. This can be achieved by placing two cross beams at ground level over which the dial gauges are supported with the help of magnetic bases. The settlement of the plate is transmitted to the dial gauges through dial gauge stands.
The dial gauges should be positioned such that the plunger of the dial gauges is at its beginning of rebound (i.e. the plunger is initially pushed up) so that it will get released as the plate settles and the reading changes and the difference in reading provided the settlement of the plate.(See fig-1 for plate load setup)
The load is applied on the plate by pumping the hydraulic pressure into the jack. The upward movement of the plunger of the jack tries to push the loading platform up. Since the pressure built up in the jack (say 1 or 2 T) is much less than the load on the platform, the pressure in the jack will push the test plate down by taking the loading platform (say 30 to 40 T) as rigid support. This is a case similar to the expansion of a system between two supports. Due to the expansion the weaker support (ground) yields.
Apply a seating load of 0.7 T/m2 which is released before the actual loading is started.
Note the initial readings of the dial gauges.
The load is applied through the hydraulic jack in convenient increments. The load increments may be one fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity or any other smaller loads. The applied load is read either from the pressure gauge fitted to the hydraulic pump or from the proving ring fixed between the jack and the reaction platform.
Settlement of the plate is observed from the dial gauge readings. Settlement should be observed for each increment of load after 1, 4, 10, 20, 40 and 60 minutes and thereafter at hourly interval until the rate of settlement becomes less than about 0.02 mm per hour. Enter the readings in the tabular form. (See fig-2)
After completing the recording of settlement readings under a load, the next load increment is applied and the dial gauges readings are noted under the new load.
The loading increments and the recording of the settlements under each load are continued until the maximum load is reached. The maximum load that is to be applied corresponds to 1.5 times the estimated ultimate load or to 3 times the proposed allowable bearing pressure.
Observations and Calculations
The load intensity and settlement observation of the plate load test are plotted. The figure shown below (Fig-3) shows a set of typical load settlement curves. The ultimate bearing capacity is taken as the load at which the plate starts sinking at a rapid rate, i.e. when the curve drops down to a vertical line. From figure 3 it can be observed that in dense sand or stiff clays the failure is not pronounced. In such cases a plot of load and settlement, both being taken in logarithmic scales, gives two straight lines. The intersection of these lines is taken as the ultimate bearing capacity of soil (see fig.4).