Prediction of SWCC for Coarse Soils Considering Pore Size Changes

Summary

The soil water characteristic curve (SWCC) of a soil is often predicted from the pore size distribution (PSD) of the soil. Experiments show that the PSD may change in the drying or wetting process. It is necessary to consider the pore-size distribution changes in predicting SWCCs.

In this research, different grain-size proportions of completely decomposed granite were separated and then mixed artificially to obtain five soil specimens of different grain-size distributions. These specimens vary from gravel to sand, and silty clay. The SWCCs of these specimens were measured using the axis-translation technique and a Dewpoint psychrometer; and the PSDs of these soil specimens were measured using a mercury intrusion porosimetry (MIP) method.

The PSDs obtained from the MIP tests were used to predict the SWCCs, which were compared with the experimental results further. The PSDs of each soil sample at full saturation and at the end of the SWCC drying test were compared. The PSDs showed a considerable pore-size reduction after soil drying. This phenomenon is referred to as ‘pore shrinkage’. A model proposed by Simms and Yanful (2001) was adopted to consider the shrinkage influence in the prediction of the drying SWCC from the PSDs of saturated and dried soil samples. After applying this model, the predicted SWCCs were closer to the experimental SWCCs, especially for fine soils.

For coarse soils, the pore volume with pore diameter larger than a specific value that cannot be measured by MIP tests is large. This volume was estimated and used to correct the predicted SWCC for each coarse soil. With this correction, the predicted SWCCs for coarse soils were also close to the experimental SWCCs.