PILE LOAD TEST

The most reliable method for determining the load carrying capacity of a pile is the pile load test. The set-up generally consists of two anchor piles provided with an anchor girder or a reaction girder at their top (Figure 1). The test pile is installed between the anchor piles in the manner in which the foundation piles are to be installed. The test pile should be at least 3B or 2.5 m clear from the anchor piles.

Figure 1. Pile Load Test

The load is applied through a hydraulic jack resting on the reaction girder. The measurements of pile movement are taken with respect to a fixed reference mark. The test is conducted after a rest period of 3 days after the installation in sandy soils and a period of one month in silts and soft clays. The load is applied in equal increment of about 20% of the allowable load. Settlements should be recorded with three dial gauges. Each stage of the loading is maintained till the rate of movement of the pile top is not more than 0.1 mm per hour in sandy soils and 0.02 mm per hour in case of clayey soils or a maximum of 2 hours (IS: 2911-1979). Under each load increment, settlements are observed at 0.5, 1, 2, 4, 8 12, 16, 20, 60 minutes. The loading should be continued upto twice the safe load or the load at which the total settlements reaches a specified value. The load is removed in the same decrements at 1-hour interval and the final rebound is recorded 24 hours after the entire load has been removed.

Figure 2 shows a typical load-settlement curve (firm line) for loading as well as unloading obtained from a pile load test. For any given load, the net pile settlement (S_n) is given by

S_n = S_t – S_e

Where S_t = total settlement (gross settlement); S_e = elastic settlement (rebound)

Figure 2 also shows the net settlement (chain dotted line).

Figure 2. Load Settlement Curve

Figure 3 shows two load-net settlement curves obtained from pile load tests on two different soils. At the ultimate load (Q_u), the load-net settlement curve becomes either linear as curve (2) or there is a sharp break as in the curve (1), as shown in the figure. The safe load is usually taken as one-half of the ultimate load.

According to IS: 2911, the safe load is taken as one-half of the load at which the total settlement is equal to 10 percent of the pile diameter (7.5 percent in case of under-reamed piles) or two-thirds of the final load at which the load settlement is 12 mm, whichever is less. According to another criterion, the safe load is taken as one-half to two-thirds of the load which gives a net settlement of 6 mm.

The limiting settlement criteria are also sometimes specified. Under the load twice the safe load, the net settlement should not be more than 20 mm or the gross settlement should not be more than 25 mm.