Triaxial behaviour of partially saturated granular soils at low stress levels
Granular soil is used as a backfilling material in trenches that are prepared for the installation, repair and replacement of service pipes. The soil is likely to be partially saturated and subjected to low stress levels (<100 kPa), as it exists at shallow depths (about 5m below the ground). A new double-wall triaxial cell has been designed for testing partially saturated granular specimens with height/diameter equal to 375mm/150mm, at low stress levels. The cell is able to make separate measurements and control of the pore air and pore water pressures. It was designed to make specimen stress and strain measurements internally. A conventional triaxial cell was modified to carry out tests on saturated granular specimens of the same dimensions at low stress levels. Two gradings of Limestone, fine and coarse, with maximum particle sizes of 5mm and 20mm respectively are used in the triaxial tests. The experimental results showed that the suction in the range 25 kPa to 75 kPa can have an effect on the volume, stiffness and shear strength of partially saturated granular soils at low stress levels. The results also showed that there is a linear relationship between q and (p-u_a) for the unsaturated soils or (p-u_w) for the saturated soils. And Fredlund's equation cannot be used to predict the shear strength of partially saturated granular soils at low stress levels, if the value of φ'is taken from tests carried out on saturated specimens. The results also showed that there was a relationship between q,(p-u_a), suction, specific volume and water content, all at failure.