Triaxial testing of unsaturated samples of undisturbed residual soil from Singapore

A series of multi-stage controlled suction triaxial tests has been carried out on undisturbed samples of residual soil from the Bukit Timah and Jurong formations in Singapore. The Bukit Timah formation is granitic in origin while the Jurong formation is sedimentary. Tests were performed at three different constant net confining pressures and for various matric suctions. The angle of friction (φ′) for the Bukit Timah soil was found to be 32°. The highest φ^b angle for the undisturbed residual soil from the Bukit Timah soil was determined to be 28° at matric suctions lower than 25 kPa. Beyond this matric suction value, the φ^b angle decreased and dropped to almost zero for matric suctions above 150 kPa. The results for unsaturated tests on the undisturbed residual soil from the Jurong Sedimentary Formation suggested an angle of friction (φ′) of 51°. This very high friction angle could be attributed to the high density and cementation of the specimens. However, an alternative interpretation is also provided based on the results of saturated tests which showed an average φ′ of 36°. This interpretation indicates high effective cohesion values between 91 and 173 kPa, consistent with saturated tests. In this case φ^b values of 10-22° are indicated for the Jurong soil.