Plate load test (PLT)

Essential aspects of the equipment and test procedure

The plate load test consists of the incremental loading of a circular steel plate, placed at the surface of the ground to be tested, by measuring the resulting settlement. It can thus be considered to be a simulation, on a reduced scale, of a shallow foundation. Notwithstanding this fact, as will be seen below, its purpose is not necessarily related to the design of such foundations.

Figure 1.33a shows a simplified schematic of the plate load test. The photo in Figure 1.33b illustrates a test setup in which the hydraulic jack acts against the rear axle of a loaded

Plate load test

Figure 1.33 Plate load test: a) simplified schematic; b) test setup where the hydraulic jack acts against the rear axle of a truck, also showing the settlement measuring system (photo: Carlos Rodrigues).

truck (with cement bags, concrete blocks or other material) in order to increase the available weight, hence the maximum reaction. In the figure, the system for settlement measurement is also visible: deflectometers (in this case, three) are installed at the jack and connected to a steel beam positioned sufficiently far from the plate so that it can be considered fixed.

It is known that the depth of the soil mass beneath the loaded area, that influences the measured deformation response, depends on the dimensions of such area, namely the diameter of the plate. Hence, the diameter of the plate should be as large as possible because: i) for the prediction of the behavior of shallow foundations, given their dimensions, a substantial thickness of the soil mass will be involved; and ii) the quality of the estimates improves with the increase in the tested volume of soil, especially in heterogeneous soil masses.[1]

However, it must be noted that the use of large diameter plates for application of stresses similar to those transmitted by the structure under study will normally require very high loads. The implementation of a system capable of providing a reaction to the jack under such circumstances generally entails very high costs and extended deadlines. In view of that, it is common to adopt stresses equal to or greater than those expected for the structure under study and, for these, to select the appropriate plate diameter, considering the maximum available reaction. The most common diameters generally range between 0.30 m and 0.80 m, and the respective thickness is large enough for the plate to behave rigidly.

  • [1] In particular, at a depth twice the plate diameter, the incremental stresses in the ground correspond to a minimal fraction (about 10%) of the applied pressure at the surface (see Figure A6.2.3).
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