Shear and soil-water retention behaviour of a variably saturated residual soil and its implication on slope stability

Numerous shallow slope failures took place in residual soils derived from sedimentary rock formation of Uttaradit province, Northern Thailand in 2005, due to prolonged and intense rainfall. Shear behaviour as well as water retention behaviour of this material has thus been investigated in details in order to investigate the slope failure mechanism. Fully saturated consolidated-drained (CD) as well as suctionmonitored direct shear tests have been performed on undisturbed samples collected from depths of 0.3-1 m. A miniature tensiometer has been used for suction measurement during these tests. In addition, influence of number of drying/wetting cycles on saturated shear strength is investigated. The results from a simple infinite slope analysis suggest that the major slope destabilization mechanism is a combination of material degradation and pore water pressure increase. denced by Collins & Znidarcic (2004) and Godt et al., (2009), slopes with a relatively steep gradients (e.g. > 45deg.) might fail when the pore water pressure becomes close to zero and not yet highly positive. The main objective of this study is therefore to investigate the shear and soil-water retention behaviour of the residual soils from the Uttaradit landslide area both in saturated and unsaturated conditions. A typical slope within the area has also been instrumented in order to obtain a better picture of the pore water pressure variation in the field. In addition, the influence of wetting/drying cycles on shear behaviour of the soil will be studied, together with their implications on slope stability. 2 INSTRUMENTED SLOPE 2.1 Site description The studied site is situated near the foothill in the agricultural area with a variety of fruit plantations and an average slope angle of about 26deg. The character of this site is considered representative of the major landuse type in the hilly area of Uttaradit. Figure 2 shows the photo of the site and lay out of instruments. Five stations of tensiometers, and inclinometers were installed at five elevations. The tensiometers have been developed by Jotisankasa et al. (2007) and manufactured at Kasetsart University using commercial miniature MEMs pressure sensors and standard 1bar AEV porous stone. The devices, capable of measuring pore water pressure ranging from -80 to 600kPa, were installed at depths of 0.5m and 1m. One automatic tipping bucket raingauge was also installed at the toe of slope. Inclinometers installed were based on MEMs accelerometer attached to a PVC tube buried to a depth of relatively competent bedrock at about 1m. The readings from the sensors were recorded with a logging system that can wirelessly transfer the data to mobile phones. Figure 2. Photo of the studied site and its cross-section In addition, a test pit was also excavated to a depth of about 1m for visual inspection (Fig.3), thinwall tube sampling, and double-ring infiltration tests. Falling head tests were performed within lined boreholes to determine the so-called saturated permeability at depths of about 2 and 3m. The interpretation of the falling head test was based on Hvorslev (1951) and Garga & Blight (1997). As shown in Figure 4 and Table 1, at greater depth, the soil becomes coarser, denser and of greater value of specific gravity. The deeper completely weathered rock was with more visible feature of cementation. The upper material likely to be involved in slope failure is classified as low plasticity silt. Figure 3. Typical soil profile of the studied area, based on Little (1969) interpretation. Table 1. Basic soil properties and classification __________________________________________________ Depth Atterberg’s limits, % Void USCS __________________ (metre) LL PL PI Gs ratio, e __________________________________________________ Top soil 48.2 32.4 15.7 2.616 1.090±0.05 ML 0.3 40.5 29.4 11.1 2.724 0.963±0.05 ML 0.8 40.8 29.6 11.2 2.728 0.703±0.04 ML 1.0 39.9 28.1 11.8 2.746 0.736±0.07 SM __________________________________________________ Figure 5 shows the decrease in permeability with depth. The value of permeability reduces, by about three to four orders of magnitude, from the upper 1m of the soil to the bedrock at the depth of 2-3m. Large scatter in the permeability data is evident especially for the ground at depth less than 1m, which is probably attributed to the heterogeneity of the residual soil. As also noticed by Vaughan (1985) and Gerscovich et al. (2006), the accentuated variability in permeability can lead to strongly non-uniform distribution of pore water pressure and sometimes highly positive pore water due to impeded seepage. Figure 4. Grain size distribution curves Top Soil