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DEWATERING -AN EFFECTIVE TOOL FORSLOPE STABILIZATION

-Rathin BiswasAssociate 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 itcan beobserved 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 ofslope, ground water condition and stress on slope are the main four parameters. Material properties cannot alter for a particularmaterial and stress on it is also not changeable for a particular operation. The things that can be altered are inclination of the slopeand ground water condition. For shallower inclination of a slope (Pit wall of open cast mine, Waste dump slope, Tailings Dumpslope) 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", fora depth of 250 m. Two types of Mohr Coulomb Rock Zone were considered for the analysis. Zone-l (less competent rockmaterial) isconsidered 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 ofSafety of a slope is 1, its stability is at border line. A Factor of Safety of 1.2(minimum) is generally regarded as acceptable forshort-ternl stability. For long-ternl stability. which is what is required for waste dumps, a Factor of Safety of 1.5 (minimum) isuniversallyaccepted as the standard.

The Slope Stability analysis was conducted using the industry standard limit equilibrium software Siope/W (by Geo-SlopeInternational,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 forthe ilnalysis each of the options. The ordinary or Fellenius method ignores all interslice forces and satisfies only momentequilibrium,Bishop's methorl includes interslice normal force but ignore the interslice shear forces, Janbu's method is similar toBishop's method only difference is that the Janbu's method satisfies only horizontal force equilibrium as opposed to momentequilibrium and Morgenstern-Price (M-P) method is mathematically more rigorous formulations, which include all intersliceforces 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:

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Moment Force IntersliceInterslice

Inclination of X/EMethod 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

~ >-

Input ParametersThe rock mass strength considered for all the Options is as in the table 2.

Table -2: Rock mass characteristic------- --~----

Zone Unit

Weight(KN/m3)

25

Rock mass

Cohesion (KPa)

Rock mass

Friction Angle (°)

Zone 1- (down to 30 m fromthe surface)

Zone 2- (Below 30 m from thesurface)

250 25

27

_J_-

280 30

-------- ------

Three different options considered for this analysis. Option I: No piezometeric line i.e. no water present in the rock mass(Ideal condition) (Figure Ia); Option 2: Piezometric plane with in the Slip surface (Nonnal Condition) (Figure 2a):Option 3: Piezometric Plane at lower level of slip surface (Desired Condition) (Figure 3a).

Result

Limit equilibrium analysis carried out for the three conditions. The results are stated in table 3 and the contours are shownon Figures Jb, 2b & 3b respectively.

Table 3: Minimum Factor of Safety for different Conditions

COST ANAL YS[S :

From a general cost calculation it is found that for one degree shallower of the over all slope angle approximately Rs IS Croreadditional cost encountered for a Slope having I Km length (taking an excavation cost Rs 100 per m3). To achieve the samestability the Over all Slope angle for Normal Condition may be shallower by 4 degree to get same stability of the Ideal Slope (forthis condition).

But the same stability can be achieved by lowering ground water table by approximately 60m. [t is assumed that three sets of subhorizontal hole of each IOOmsection @IOOm length and one set of submersible pump at each 100m section of 200m depth may beable to lower down 'the water table (a normal hydrological condition) and from a general cost calculation it is found thatapproximately Rs 5.0Lacs may be required for the dewatering operation and the water may be used for other purpose.

CONCLUSION

From the analysis it can easily found that the dewatering have a clear-cut edge over the shallowing ofthe slope angle thus it can beconcluded that dewatering is the cost effective tool for slope stabilization and that can be done by pumping out the water fromupper levelor bymeans of sub horizontal hole at lower levelofthe slope.REFERENCES

KrahnJohn 2004 Stability Modeling with SLOPE/W, Oeo-Slope [nternational Ltd, Canada

OeoStudio Tutorials 2004, Oeo-Slope International Ltd, Canada

Hoek E. & Bray1.W. 1981,Rock Slope Engineering; Institute of Mining and Metallurgy.

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Ideal condition Normal Condition Desired Condition

Method Moment Force Moment Force Moment Force

Ordinary 1.251 - 0.999 - 1.192 -

Bishop 1.312 - 1.117 - 1.315 -

Janbu - 1.241 - 1.014 - 1.193

M-P 1.304 1.299 1.109 1.111 1.308 1.309

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Figure: 1a: Design Ideal Condition Figure: 1b: Contour of Ideal Slope

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