technique and its applications are presented. In ad- tion, the limitations of the axis-translation technique ThesuccessfulimplementationofTerzaghi'seffective and the methods that can be used to alleviate some of stress principle for saturated soils in the engineering these limitations are highlighted. Various parameters practice has led many researchers to attempt to extend thatin?uence theaxis-translationtechniquesuchasair a stress stable variable framework to unsaturated diffusion, water volume change, long term testing and soils. For example Bishop (1959) modi?ed Terzaghi evaporation are brie?y described. effective stress equation with the introduction of a soil parameter, v (which is a function of degree of saturation) to interpret the mechanical behaviour of 3 Hanging Water Column unsaturated soils. Several researchers during the last threedecades haveproposedframeworksforinterpret- Buckingham (1907) was one of the pioneering sci- ing the engineering behavior of unsaturated soils in tists who measured the relationship between capillary termsoftwoormoreindependentstressstatevariables potentialandwatercontent(i.e.,
SWRC)andexpressed over a large suction range (Fredlund and Morgenstern itasacontinuousfunctionusinghangingwatercolumn 1977; Toll 1990; Wheeler and Sivakumar 1995). (i.e., negative water column) technique. This relation- The research direction in recent years has been ship has been considered as an important milestone towards understanding the comprehensive behaviour in the history of soil physics and the mechanics of of unsaturated soils by extending the critical state soil unsaturated soils (Barbour 1998; Narasimhan 2005).