A better understanding of long-term mechanical behavior of over-consolidated clay (OC) is important to numerous geotechnical problems. Most previous experimental studies, however, have used sand or normally consolidated clay (NC); only limited reports of the drained true biaxial test of OC are available. In this paper, an electro-pneumatic (E/P) regulator based automatic control true triaxial apparatus is developed. A mixed boundary loading device designed by Nakai Research Group is used in the apparatus. Unlike other loading devices, the membrane covers not only the vertical but also the horizontal rigid platens, which can apply arbitrary three different principal stresses to the block specimen. The loading pattern that can reduce the comer effect and the interference problem is also analyzed. The method to increase the pressure control precision of the E/P regulator as well as the automatic control system is presented. An extra volume change due to water-absorption of membrane during long time drained test is discussed and a simple solution is proposed. The apparatus is used to study the mechanical properties of NC and OC clays under drained condition. The test results show that the initial stiffness, shear strength and dilatancy are largely influenced by the Lode angle. Furthermore, both the NC and OC clays obey the Matsuoka-Nakai criteria well.
