.... ,.~., ,~, ,~, ,~, ,,~., ,~,....... DEWATERING -AN EFFECTIVE TOOL FOR SLOPE STABILIZATION - Rathin Biswas Associate Manager, Rock Mechanics Cell, Rampura Agucha Mines, Hindustan Zinc Ltd. Abstract Stabilizing a slope is very critical issue for geotechnical engineering. Two of the main four Slope stabilization parameters viz. Rock mass Characteristics and stress on slope are the inherent parameters for a particular slope. Bya limit equilibrium analysis it can be observed that shallower slope cost a lot, thus De-watering is the best way for slope stabilization. INTRODUCTION Slope stability is the most safety concern for a slope (Pit wall of open cast mines, Waste Dump slope, and Tailings Dump slope). Instability cost the man, machinery and money. It depends on the mechanical properties and the design parameters of the slope. The Rock (material) properties, inclination of slope, ground water condition and stress on slope are the main four parameters. Material properties cannot alter for a particular material and stress on it is also not changeable for a particular operation. The things that can be altered are inclination of the slope and ground water condition. For shallower inclination of a slope (Pit wall of open cast mine, Waste dump slope, Tailings Dump slope) a lot of cost is involved on it and for alteration of any slope inclination, permission from the Regulatory body is required. LIMIT EQUILIBRIUM ANALYSIS To analyze the fact one Limit equilibrium analyses was conducted on a model of the slope at the proposed over all Angle of38", for a depth of 250 m. Two types of Mohr Coulomb Rock Zone were considered for the analysis. Zone-l (less competent rock material) is considered for first 30 m from the surface and other rock body deemed comparable more competent (Zone -2). A Limit equilibrium analysis was conducted to determine the Factor of Safety against shear failure in a slope. When the Factor of Safety of a slope is 1, its stability is at border line. A Factor of Safety of 1.2 (minimum) is generally regarded as acceptable for short-ternl stability. For long-ternl stability. which is what is required for waste dumps, a Factor of Safety of 1.5 (minimum) is universally accepted as the standard. The Slope Stability analysis was conducted using the industry standard limit equilibrium software Siope/W (by Geo-Slope International, Canada). Static analysis performed for this analysis. Limit equilibrium analysis (Slope/W) determines the Factor of Safety against shear failure in a slope. Four different methods, namely- Ordinary or Fellenius method, Bishop's method, Janbu Method and Morgenstern-Price (M-P) method are considered for the ilnalysis each of the options. The ordinary or Fellenius method ignores all interslice forces and satisfies only moment equilibrium,Bishop's methorl includes interslice normal force but ignore the interslice shear forces, Janbu's method is similar to Bishop's method only difference is that the Janbu's method satisfies only horizontal force equilibrium as opposed to moment equilibrium and Morgenstern-Price (M-P) method is mathematically more rigorous formulations, which include all interslice forces and satisfYall equations of.statics. Acomparison ofthese four methods is tabulated on the table 1. Table -1: Interstice force characteristics, relationships and equations of statics Satisfied: """~'''''''''~''''''''''~(''''''''~''''''''''~'''''''''''~c''''''''~c'''''''~'c~ 22nd MINES SAFETY WEEK 2008 Moment Force Interslice Interslice Inclination of X/E Method Equilibri Equilibri Normal Shear (X) Result and X-E um urn (E) Relationship Ordinary Yes No No No No Interslice forces Bishop's Yes No Yes No Horizontal Janbu's No Yes Yes No Horizontal Morgenstern-Price Yes Yes Yes Yes Variable; User function