Measurement of Ko and Ko' during loading and unloading of loose sand

The coefficient of lateral earth pressure at rest in loose sand during virgin loading, Ko, and during unloading, Ko, have been determined from laterally confined load-unload tests. The tests included measurement of lateral pressure with null pressure gauges, side wall friction with newly designed friction meters and applied pressure and base pressure with load cells. The importance of accounting for side-wall friction when evaluating the distribution of vertical pressure over the height of the soil specimen was demonstrated. Relatively uniform friction was observed during loading, but this was not the case during unloading unless friction reduction measures were employed. While the measured value of Ko was found to be close to, if slightly higher than the value commonly estimated on the basis of friction angle, o, the ratio of Ko, to Ko was found to reasonably fit an expression of the form Ko Ko⁄=1+C∙log (OCR), with C equal to 1 in the present tests.