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 arc the inherent parameters for a particular slope. By a limit equilibrium analysis it can be observed that shallower slope cost a lot, thus De-watering is the best way for slope stabi Iization.

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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:
Method
Moment
Equilibri
um
Force
Equilibri
urn
Interslice
Normal
(E)
Interslice
Shear (X)
Inclination of X/E
Result and X-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
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22nd MINES
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Input Parameters
The rock mass strength considered for all the Options is as in the table 2.
Table -2: Rock mass characteristic
-------
--~----
Unit
Weight
(KN/m3)
Rock mass
Cohesion (KPa)
Rock mass
Friction Angle (°)
Zone 1- (down to 30 m from
the surface)
25
250
25
Zone 2- (Below 30 m from the
surface)
--------
27
280
30
Zone
_J_-
------
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 shown
on Figures Jb, 2b & 3b respectively.
Table 3: Minimum Factor of Safety for different Conditions
Ideal condition
Method
Ordinary
Bishop
Janbu
M-P
Normal Condition
Desired Condition
Moment
1.251
1.312
Force
-
Moment
0.999
1.117
Force
-
1.241
-
1.304
1.299
1.109
1.014
1.111
-
Moment
1.192
1.315
Force
-
1.193
1.308
1.309
-
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 Crore
additional cost encountered for a Slope having I Km length (taking an excavation cost Rs 100 per m3). To achieve the same
stability the Over all Slope angle for Normal Condition may be shallower by 4 degree to get same stability of the Ideal Slope (for
this condition).
But the same stability can be achieved by lowering ground water table by approximately 60m. [t is assumed that three sets of sub
horizontal hole of each IOOmsection @IOOm length and one set of submersible pump at each 100m section of 200m depth may be
able to lower down 'the water table (a normal hydrological condition) and from a general cost calculation it is found that
approximately 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 be
concluded that dewatering is the cost effective tool for slope stabilization and that can be done by pumping out the water from
upper levelor by means of sub horizontal hole at lower level ofthe slope.
REFERENCES
KrahnJohn 2004 Stability Modeling with SLOPE/W, Oeo-Slope [nternational Ltd, Canada
OeoStudio Tutorials 2004, Oeo-Slope International Ltd, Canada
Hoek E. & Bray 1.W. 1981,Rock Slope Engineering; Institute of Mining and Metallurgy.
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