The objective of a swelling pressure test on soil is to determine the swelling pressure of expansive soil when it is not allowed to undergo any volume change. The volume change is arrested or the soil is not allowed to swell in order to test this.
The expansive clays increase in their volume when they come in contact with water owing to surface properties of these clay types. The pressure which the expansive soil exerts , if it is not allowed to swell or the volume change of the soil is arrested , is known as Swelling Pressure of Soil. The swelling pressure is dependent upon several factors namely (a) the type and amount of clay in the soil and the nature of the clay mineral, (b) the initial water content and dry density, (c) the nature of pore fluid, (d) the stress history of the soil including the confining pressure and (e) drying and wetting cycles to which the soils have been subjected to.
A soil with high swell pressure is considered “unsuitable” for use as embankment fill material and in case the sub-soil is having high swell pressure then suitable “ground improvement measures” may be needed before constructing embankment on such soil. One of the methods for constructing embankments on expansive soils is to lay a CNS (non-swelling clay) layer at the base of the embankment, to isolate the sub-soil from the moisture (mainly rain water) coming in vertically downwards direction. For any clay to be classified as “CNS (non-swelling clay)”, it should not have swell pressure more than the specified value (normally 0.5 kg/m2).
In laboratory, one dimensional swelling pressure test is conducted using either fixed or the floating rings on both undisturbed and re-moulded soils in the partially saturated condition. Two methods are employed, namely, “consolidometer method” in which the volume change of the soil is permitted and the corresponding pressure required to bring back the soil to its original volume is measured and “constant volume method” in which the volume change is prevented and the consequent pressure is measured.