      A STUDY OF DEFORMATION BEHAVIOUR OF AN INSTRUMENTED SLOPE SUBJECT TO RAINFALL NEAR THADAN DAM THAILAND Apiniti Jotisankasa 1 Akihiro Takahashi 2 Tomohide Takeyama 2 Warakorn Mairaing 1 1 Department of Civil Engineering, Faculty of Engineering, Kasetsart University, Thailand fengatj@ku.ac.th 2 Department of Civil Engineering, Tokyo Institute of Technology Japan ABSTRACT : A methodology has been developed using DACSAR Finite Element programme to predict the deformation of soil slope subjected to known pore water pressure distributions. Field monitoring results of pore water pressure and shear strains within the soil slope near Thadan dam, Nakornnayok Thailand during May 2008 until October 2008 have been used to validate such method. In the simulation, Drucker-Prager constitutive model is assumed for the slope material based on compression and shearing behaviour from consolidated drained direct shear tests. Several assumptions have been made in order to specify the hydraulic conditions for all elements at each step. Firstly, only positive pore water pressure is used and negative pore pressure is neglected in the analysis (pore water pressure assumed to be zero if negative). Secondly, the pore water coefficient, u r is used to generalize spatial distribution of the pore water pressure from point-wise measurement. The Young’s modulus, Poisson ratio, and initial coefficient of earth pressure have been estimated based on the results of direct shear tests and some empirical correlations. The sudden surge of shear strain (1.2%), due to heavy rainfall of September 2008, observed in the slope can be satisfactorily simulated using the FE programme, though the simulated shear strain is about half the measured value. Finally the programme was used to estimate the value of shear strain at the measurement point which is expected to occur when the slope starts to undergo overall failure. This value of shear strain (4-4.5 %) can be used as an approximate threshold value of shear strain for early warning system at the studied slope near Thadan dam site. KEYWORDS : Landslide, Slope stability, Numerical analysis, Warning system, Deformation 1. INTRODUCTION Problems of slope instability due to heavy rainfall in Thailand have become more serious since the last decade, especially in cut/fill slopes along highways in steep areas. A number of studies have been carried out in Thailand in order to mitigate such threats to the infrastructure such as hazard zonation, slope stabilization, development of early warning system (Mairaing, 2008, Taesiri and Yuwathanon, 2005, Jamnongpipatkul et al., 2008, Soralump & Bunpoat, 2006, Jotisankasa & Vathananukij, 2008). The major mechanism of slope instability in Thailand is related to rain infiltration into soil slope which leads to increase in pore water pressure and consequently reduction in effective stress and shear strength. In order to gain more in-depth understanding of the actual slope behaviour subjected to rainfall, Jotisankasa & Porlila (2008) developed a slope monitoring system, consisting of MEMs tensiometer/piezometer for pore water pressure measurement (Jotisankasa et al., 2007) and MEMs inclinometer for shear strains. The prototype monitoring system has been installed since May 2007 in a slope of failed soil mass that had been re-graded in 2004 (Figure 1). Slope failure in 2004 was triggered by an intense rainstorm which amounted to about 300mm in three days. The slope is situated in Nakornnayok province, east of the central region, where the geology consists of undifferentiated Permo-triassic volcanics rocks, including rhyolite, andesite, tuffs, and agglomerate (Royal Irrigation Deparment, 2004). The material on the slope is classified as medium plasticity silts (MH/ML) with basic properties summarized in Table 1. Jotisankasa (2008) carried out detailed studies of the material properties including, shear strength-suction relationships, soil-water characteristic curves and field infiltrability. Table 1 Basic properties of the material at monitored site Liquid Limit Plasticity Index % gravel % sand % silt % clay Soil Type 46- 51 6-18 0.5- 5.5 13.7- 18.9 38.9- 53.9 31.9- 36.7 Silts MH/ ML : 141 :