Plate Sinkage versus Confined Compression Tests for In Situ Soil Compressibility Studies

The objective of this study was to evaluate two methods [plate sinkage test (PST) and confined compression test (CCT)] of predicting the compressibility of the in situ soil. The study also considered the effects of internal stresses on soil pre-compaction stress image. The experiment was conducted on a Typic Haplargids soil. A quasi-static loading apparatus was used to run the in situ PST and CCT. The compressibility tests were performed at different water contents after the second irrigation on the ploughed topsoil and subsoil. The results showed that the values of image measured in the PST were higher than the ones measured in the CCT. This might be related to distortion of stress–strain curve measured in the CCT due to sampling disturbance and the boundary conditions. A strong relationship between image and water content was observed for the topsoil. The compressibility of subsoil was lower than that of topsoil owing to fine texture, higher bulk density and more stable structure of the subsoil. The pre-compaction stress and the effective stress generated by soil matric suction had linear relationship for the topsoil. The image of the topsoil approaches very high values (i.e. 500 kPa) upon drying. This implies the dominant effect of effective stress generated by matric suction on the bearing capacity of the topsoil. However, image of subsoil slightly changed with the effective stress. A significant relationship was found between image and cone penetration resistance (Cp) of the topsoil. Slope of the fitted line for the PST was two times of that for the CCT. The relationship between image and Cp was not significant for subsoil. The PST was able to determine image better than the CCT. Internal stresses due to matric suction are important with respect to compaction of the unstable soils in central Iran. Cone penetrometry could be used as a quick method for prediction of pre-compaction stress of the topsoil.