Dr Brendan O’Kelly, Trinity College Dublin, and Pat Naughton, National University of Ireland, Galway. This note was first published in GE’s July 2003 edition.
Accurate determination of the mechanical and pore-pressure responses of the ground to applied loads is fundamental to soil mechanics design. However, laboratory-measured stiffness values are often inconsistent with the values obtained from back-analysis of the ground performance, principally due to limitations of standard test apparatus (which have limited sample loading and boundary conditions) and sample disturbance effects. For example, the Bishop and Wesley stress path and triaxial apparatus can only subject a test specimen to axi-symmetric loading conditions.
Most ground engineering problems, however, involve multi-directional loading that invariably causes rotations of the principal stress axes and changes in the relative magnitude of the intermediate principal stress in the ground.
The more sophisticated hollow cylinder apparatus (HCA) allows independent control of the magnitude of the three principal stresses and rotation of the major-minor principal stress axes making more generalised stress path testing possible. Although such equipment is still rare, development and use has steadily increased, principally at leading research establishments (Hight et al (1983); Ampadu and Tatsuoka (1993);and Richardson et al (1996)).
A new automated HCA (Figure 1) was recently developed at the Department of Civil Engineering, University College Dublin (Ground Engineering April 2003). The new apparatus allows accurate measurement of the mechanical and pore-pressure responses from very small strains (of the order of 10’train) to sample failure, following generalised stress paths.