Effect of degree of saturation on the unconfined compressive strength of natural stiff clays with consideration of air entry value

When extracting stiff clays at a certain depth, negative water pressure/suction builds up inside the samples. If the generated suction is high enough to desaturate the largest pores, the soil sample will be desaturated. From a practical point of view, it is essential to investigate the effect of degree of saturation Sr on the soil mechanical behaviour. In this study, nine boreholes of stiff clays were taken from different depths. The unconfined compressive strength (UCS) was determined for soils at different depths. A first analysis showed that there is no well-established relationship between UCS and Sr. To understand this observation, the water retention property and compression behaviour were further investigated to determine the main air entry value (AEV) and the over consolidation ratio (OCR). Soil microstructures were observed through mercury intrusion porosimetry (MIP). Based on the results, the sampling-induced suction, the void ratio of air at main air entry value eM-AEV, the void ratio of water at main air entry value em-AEV, the water ratio ew and the air ratio ea were estimated. The eM-AEV (or ea) and the em-AEV (or ew) were selected to analyze the results of UCS. It appears that UCS is controlled by em-AEV or ew when Sr is higher than the one corresponding to the main air entry value. By contrast, in case of lower Sr, UCS is rather controlled by eM-AEV or ea. This suggests that Sr has a significant effect on UCS only when the soil suction is beyond the main air entry value, UCS being rather controlled by soil dry density at lower suctions. This finding provides useful information for analyzing the mechanical behavior of natural soils in geotechnical practice.