Limitations of plate load test

Limitations of plate load test

(i) In no case shall a test plate smaller in width than 30 cm be used, because experimental evidence has indicated that the load settlement behavior of the soil is qualitatively different for smaller widths of the test plates compared to that of larger widths.

(ii) Settlement of a foundation cannot exceed about four times the settlement of a plate of 30 cm width, howsoever large its width may be.

(iii) If the soil plate load test may lead to misleading results. Consider for example, the subsoil conditions illustrated. Where the upper stratum is strong soil like dense sand and the lower stratum is a weak soil like soft clay. In such a situation, the load test reflects the load settlement characteristics of the stronger stratum but does not give any indication of the settlement behavior of the poorer soil below. The foundation being of a much larger width its bulb of pressure of the same load intensity as that on the test plate, will extent into the weaker stratum. Hence, the extrapolated settlement of the foundation will be much smaller than the actual settlement, leading to unsafe design.

(iv) The effect of capillarity in a sand bed is to increase its effective vertical stress or its stiffness. A test plate resting on a capillary sand bed undergoes smaller settlement than a plate resting on dry or submerged sand bed.

(v) A plate load tset is of short duration. The settlement measured is only immediate settlement. In granular soils, immediate settlement can be taken as the total settlement, while in cohesive soils, consolidation settlement, which constitutes most part of the total settlement, cannot be predicted through this test. Hence the plate load test is not of much relevance in clayey soils for which the settlement criterion is very important in the determination of the allowable bearing pressure of a foundation.



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