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Slope Stability Analysis at New Roadcut to Mae Ngad - Mae Kuang Water Diversion Tunnel Project, Tambon Luang Nua, Doi Saket District, Chiang Mai Province. Mr.Nuttakit Chaisri (Advisor : Dr.Kannipa Motanated) Department of Geological Sciences, Faculty of Science, ChiangMai University. ABSTRACT

Road construction plan into the control building of Mae Ngad - Mae Kuang water diversion tunnel project was cut through fractured rocks (i.e. sandstone) which have low stability. Deformation and fractures have been developed on the cut slope during blasting, giving rise to serious safety concerns. This research was conducted to study the geological engineering conditions (i.e. rock strength, groundwater, and joints arrangement) of the slope cut. Rock Mass Rating (RMR) shows that fractured rock has fair rank; Geological Strength Index (GSI) shows that slope of this rock mass has medium stability; stereonet shows kinematic possibility of wedge failure. Slope stabilizing system shouldbe installed to prevent future slope failure. The analytic method and procedure used in this research could be useful to similar projects.

RESULT

INTRODUCTION Looking N

1 square meter

Figure 5. Representative rock mass surface (arkosic sandstone) shows joints to classify rank and assess stability

Figure 1. Topographic map of the study area

Figure 2. Slope face in the study area

Study area is in the north of Mae Kuang Udom Thara dam, Tambon Luang Nua, Doi Saket District, Chiang Mai Province. The study area characterized by high mountains surrounding basins. New roadcut scheme to diversion control building used blasting to eliminate surplus rock masses but it cause evoluting of crack or discontinuty and unstability in slope face.

1. Rock mass RMR average value is 51 and has a fair rank. 2. GSI indicates medium stability. o 3. These rock samples have internal friction angle 36 and cohesion of 26.3 MPa. 4. Kinmatic analysis (Figure 6) indicates possible wedge failure.

OBJECTIVE

1. To study physical properties and mechanical properties of rock mass. 2. To analyze the slope stability and kinematic approach.

Direction of movement

Direction of movement

METHODOLOGY

1. Evaluate the rock mass by using geomechanics classification of jointed rock masses (Rock Mass Rating or RMR). 2. Assess stability of slope by using Geological Strength Index (GSI). 3. Triaxial compression test to determine mechan the mecanical properties of rock. 4. Determine the potential modes of rock slope failure by using stereonet method (refer parameters from triaxial compression test). 5. Calculate factor of safety by using Swedge software. Line of intersection between the representative orientations of the two discontinuity sets Great circle of discontinuity Great circle of discontinuity cluster set 1 N cluster set 2

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Figure 3. Mohr-Coulomb failure envelope

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Figure 4. Stereonet plot analysis

Direction of movement

Figure 6. Kinematic possibility of wedge failure

FUTURE STUDY

1. Explain how to reduce/eliminate possibility of failure. 2. Calculate factor of safety (FS) of wedge mass in Swedge software.

REFERENCE

Bieniawski, Z.T., 1989. Engineering rock mass classification. John Wiley & Sons, New York, 251p. Marinos, P., and E. Hoek (2000), GSI: A Geologically Friendly Tool for Rock Mass Strength Estimation, Proceedings of GeoEng2000, Melbourne, Australia, v. 1, pp. 1422-1440. Hoek, E. and Bray, J. W., 1981, Rock Slope Engineering, 3rd ed., The Institute of Mining and Metallurgy, London, England, p.358 Palmström A., 2005. Measurement of and Correlation between Block size and Rock Quality Designation (RQD), Published in Tunnel and Underground Space Technology 20(2005),362-377. Wyllie, C. and Mah, W., 2004, Rock Slope Engineering, Civil and Mining, 4rd ed., 36 p.

Department of Geological Sciences, Faculty of Science, Chiang Mai University 239 Huay Kaew Road, Chiang Mai, Thailand 50200

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