735 Unsaturated Soils: Theory and Pracice 2011 Joisankasa, Sawangsuriya, Soralump and Mairaing (Editors) Kasetsart University, Thailand, ISBN 978-616-7522-77-7 1 INTRODUCTION The mechanics of landslide is very complex since it involves mechanics of unsaturated soils. Soil shear strength is the prime factor that governs the landslide behaviour. When the soil is saturated, there is only one stress state variable that influence the soil shear strength which is effective stress (Terzaghi, 1936). However when the soil is partially saturated there are two independent stress state variables that governed the soil shear strength which are namely net stress and suction (Fredlund et al., 1978). This makes the mechanics of shallow rainfall induced failure which is also commonly known as localize failure very com- plex to understand. It is very difficult to back analyse such failure with factor of safety less than unity de- spite applying the soil properties and the shear strength as obtained from the laboratory tests on un- disturbed soil samples. The application of the linear variation of shear strength relative to effective stress is a simplification of a more complex behaviour. However this simplification must be avoided in order to understand a complex behaviour and the true genuine behaviour need to be considered. Thence, it is anticipated that the interpretation of soil shear strength at low stress levels need to be further deli- brated. At this low stress levels, the application of linear type of shear strength envelope most of the time cannot fit exactly the Mohr circles obtained from the laboratory tests. Besides, there have been many reports that the shear strength behaviour of various soils is non-linear at low stress levels like the reports of Bishop (1966), Charles and Watts (1980), Fukushima and Tatsuoka (1984), Md Noor and Anderson (2006) and Md.Noor et al. (2009). Moreover, there is something which is still puz- zled about the effect of antecedent rainfall. Essen- tially the effect of infiltration which reduces the suc- tion and subsequently reduces shear strength is taken into account to explain the phenomenon. However when some failure occurred some times after rainfall has stopped, it demands a further explanation besides the macro effect of infiltration on shear strength. This type of failure is also commonly known as “delayed failure” (Hudson and Hencher, 1984; Franks et al., 1996). And this is anticipated to be influenced by the effect of changes within the clay micro-structure when it is in a prolong contact with water where the clay platelets become fatter and softer when water is absorbed (Penumadu and Dean, 2000). This hy- pothesis is based on the fact that clay becomes less plastic and softer to knead when water is added. It is commonly understood that its physical properties changes from high plasticity towards low plasticity and finally reaches liquid state when moisture content continuously increases. This paper will quantify the state of strength reduction when the clay micro- structure changes with increasing moisture content. The shear strength of the test soil is quantified through the consolidated drained triaxial tests (Head, 1981). Typically the soil specimens are subjected to three stages in the test which are namely; saturation, consolidation and shearing. However in the soaking tests, the soil is soaked prior to the saturation stage by subjecting it to certain pore water and confining pressures to simulate the condition in the field. The shear strength envelopes when the soil is un-soaked and soaked for a different period were determined. The application of curvi-linear shear strength enve- lope of Md.Noor and Anderson (2006) was applied ABSTRACT: Granitic residual soil grade VI has a proportion of clay in its matrix. The clay acts as the binder to the coarser constituents and contributes towards the overall strength of the matrix. Its strength would be af- fected when the soil absorbs water which softened the clay minerals like during a long rainy spell or effect from continuous antecedent rainfall. A study on the effect of a different period of soaking on the strength of this type of soil has been conducted. The undisturbed soil specimens were soaked in a triaxial cell prior to the con- solidated drained tests. The curvi-linear shear strength envelopes for different period of soaking were obtained. The soaking shows the reduction in the shear strength as the curvi-linear shear strength envelope is lowered when the period of soaking increases. Slope stability analysis conducted on a cut slope of gradient 1:1 shows that the slope is stable for soaking up to 3 and 7 days but fails within the wetted zone when the soaking reaches 10 days. The underlying unsaturated layer acts like a hard layer which bounded the limit of failure. This ex- plains why such slope which has been standing for quite some time may suddenly fail. KEYWORDS: Soaking, curvi-linear envelope, shear strength, slope, stability Effect of soaking on stability of cut slope in granitic residual soil M.J. Md.Noor & B. Hadi Institute for Infrastructure Engineering and Sustainable Management, Faculty of Civil Engineering, Universiti Tekologi MARA, Shah Alam, Malaysia, mohdjamaludinmdnoor@yahoo.com