The concept of effective stress in partly saturated soils

The definition of the principle of effective stress and its implications have been examined for saturated and partly saturated soils. The results of a series of oedometer and all round compression tests of partly saturated and fully saturated soils are presented. These results together with additional experimental data indicate that most soils, from sands right through to clays, exhibit behaviour which, below a critical degree of saturation, cannot be accounted for by the effective stress principle. In sands the critioal degree of saturation appears to be below 50%. In clays, however, the critical degree of saturation is upwards of 85%. An explanation for the observed behaviour of partly saturated soils is offered. It is apparent that structural changes resulting from a change of pressure deficiency in a soil are very different from those resulting from an equivalent change in applied stress. The investigation as a whole indicates that, below the critical degree of saturation, the concept of effective stress in a partly saturated soil is not valid. It is suggested that the term 'intergranular stress' is more suitable than the term 'effective stress' since its use does not imply the validity of the principle of effective stress. The practical significance of the investigation is discussed briefly and the lines along which further research would prove profitable are indicated.