The mechanical behavior of normally consolidated soils as a function of pore fluid salinity

Abstract

Pore fluid salinities in the Gulf of Mexico area can reach levels of 250 grams of salt per liter of pore fluid (g/1). It is now necessary to determine the effect that this salinity level can play on the mechanical behaviors of marine sediments. An extensive laboratory testing program involving Constant Rate of Strain (CRS) and Ko consolidated undrained shear in compression triaxial testing (CKoUC) was undertaken. Soil specimens reached axial effective stresses of 10 MPa with a select few reaching 40 MPa in a modified CRS device. All triaxial tests were performed in a low pressure triaxial apparatus. The shear behavior of all soils was obtained in the normally consolidated region. Several reverse leaching tests were performed in the CRS cell whereby high salinity pore water was flushed through a low salinity soil while consolidation was halted. The majority of testing was performed on resedimented specimens using several different soils from around the world. These include Boston Blue Clay (BBC), London Clay, Gulf of Mexico soil and sodium montmorillonite. Several derivatives of BBC were resedimented including BBC which had some of its natural salt removed via leaching, and also BBC which had its fabric completely dispersed using sodium hexametaphosphate. Pore fluid salinities ranging from 0 (distilled water) to 256 g/l were used when resedimenting these soils. The resedimented BBC test results were compared to intact BBC soil which was recovered from below the MIT campus. The results show that BBC and London Clay are relatively insensitive to increases in pore fluid salinities up to 256 g/l. This sensitivity can be increased by leaching some of the natural salt from the soil and then resedimenting to different pore fluid salinities. The strength characteristics of BBC over this large salinity range also remain fairly constant with the only differences being observed in leached BBC. It was also seen that the strain to failure for resedimented BBC is half that which is required for intact soil with all other measured parameters being similar. Interesting observations were seen in relation to how the fabric of BBC evolves with an increase in stress level and a theory of how floc breakage occurs at a given stress level is proposed. A potential crude method of quantifying the contribution of electro-chemical forces to a soils strength is also suggested