PLATE LOAD TEST - FOUNDATION SITE EXPLORATION.

Plate Load Test is a field test to determine the ultimate bearing capacity of soil, and the probable settlement under a given loading. The test esssentially consists in loading a rigid plate (usually of steel) at the foundation level, and determining the settlements corresponding to each load increment. The ultimate bearing capacity is then taken as the load at which the plate starts sinking at a rapid rate. The method assumes that down to the depth of influence of stresses, the soil strata is reasonably uniform.

FIG.  2.21  TEST PIT.

The beanng plate is square, of minimum recommended size 30 cm square, and maximum size, 75 cm square. The plate is machined on sidcs and edges, and should have a thickness sufficient to withstand effectively any bending stresses that would be caused by the maximum anticipated load. The thickness of steel plate should not be less than 25 mm.

FIG. 2.22 PLATE LOAD TEST: REACTION BY GRAVITY LOADING.

The test pit width is made five times the width of plate (Bp). At the centre of the pit, a small square hole is dug whose size is equal to the size of the plate and the bottom level of which corresponds to the level of actual foundation (Fig. 2.21). The depth Dp, of the hole should be such that

The loading to the test plate may be applied with the help of a hydraulic jack. The reaction of the hydraulic jack may be borne by either of the following two methods:

(a) gravity loading platform method,
(b) reaction truss method.

In case of gravity loading method a platform is constructed over a vertical column resting on the plat form, and the loading is done with the help of sand bags, stones or concrete blocks. The general arrangement of test set-up for this method as shown in Fig. 2.22. When load is applied to the test plate, it sinks or settles. The settlement of the plate is measured with the help of sensitive dial gauges,. For square plate, two dial gauges are used. The dial gauges are mounted on independently supported datum bar. As the plate settles, the ram of the dial gauge moves down and settlement is recorded. The load is indicated on the load-gauge of the hydraulic jack.


Fig. 2.23 shows the arrangement when the reaction of the jack is borne by a reaction truss. The truss is held to the ground through soil anchors. These anchors are firmly driven in the soil with the help of hammers. The reaction truss is usually made of mild steel sections. Guy ropes are used for the lateral stability of the truss.

Note. In olden days, the loading on the plate was made with the help of gravity loading consisting of weighed sand bags on a platform constructed over the central loading column. The settlement of the plate was measured with the help of a dumpy level. Such an arrangement is crude since the settlements are not measured upto the desired accuracy and the arrangement gets disturbed during the incremental loading. Certain mishaps have also been reported due to the tilting of the loading platform (Fig.  2.22) or by reaction truss (Fig. 2.23).

FIG.  2.23  PLATE LOAD TEST: REACTION BY TRUSS.

⇒ Test procedure.
⇒ Safe bearing pressure on permissible settlement.
⇒ Limitations of plate load test.