Design, seepage analysis and controls of waste dumps

In recent years, there are numbers of landslide
occur everywhere. Most of the landslide happens
on the cut slopes or embankment along roads,
highway and sometimes within the vicinity of
highly populated residential area especially those
in the highly terrain. Thus, a proper understanding,
monitoring and management of slope stability are
essentially important to minimize the severity or
casualty in any landslide.

1) Hydrogeological and rain water condition in
the dumping area.
2) Load bearing capacity where dump is to be
laid.
3) Geometry and strength of the dump
material.
4) Methods of dumping.
5) Dynamic loading and Ground movement.

1. Natural slopes
2. Man-made slopes
The natural slopes are those that exist in nature
and are formed by natural causes. Such slopes
exist in hilly areas. The sides of cuttings, the
slopes of embankments constructed for taking
roads, railway canals etc. and the slopes of the
earth dams are examples of finite slopes.

1. Gravitational Force
2. Force Due To Seepage of Water
3. Erosion of the Surface of the Slopes Due
To Flowing Water
4. The Sudden Lowering Of Water Due To
a Slope
5. Forces Due To Earthquakes

The slope stability analysis is important owing
to the influence of slope angle upon the design
and economics of opencast mining operation
i.e.,
1) Steep enough to be economically
acceptable.
2) Flat enough to be safe.

1) Collection of the samples from the field.
2) The analysis of these samples to find out
parametric variations affecting the slope stability
as the cohesion, angle of friction, density and
other parameters.
3) Utilisation of the results of sample collected to
predict the stability in terms of safety factor
4) Determination of the factor of safety of the
various slopes using the mine plans and sections
5) Valuation of the safety factors and suggest
corrective measures.

The stability of refuse embankments is usually
solved by limit equilibrium methods of analysis.
These analyses are conducted by calculating
the minimum factor of safety (FS) for a slide
surface through the slope as follows:
FOS = Available shear strength÷ Equilibrium
shear stress
If FOS = 1, a slope is in a state of “just-stable”
limit equilibrium

 A factor of safety is defined as that factor by which
the shear strength of the soil must be reduced in
order to bring the mass of soil into a state of limiting
equilibrium along a selected slip surface.
 For an effective stress analysis, the shear strength
is defined as:
 S=c’+(σn-u)tan Ф’
Where:
 S=shear strength,
 c’=effective cohesion,
 Ф’=effective angle of internal friction,
 σn=total normal stress,
 u=pore water pressure

Where the slip surface under the study for a slope
passes through soil materials whose shear
strength is based upon internal friction and
effective stress, the method of slices is recognized
as a practical means to account for the expected
variation in shearing resistance that develops
along the different portions of the assumed
slippage arc. The procedure is applicable to
slopes in isotropic soils, layered soils and non-
isotropic soils. . The procedure involves dividing a
mass assumed to be involved in a slope slide into
vertical slices.

A method presented by Bishop utilises method of
slices and considers forces to be acting on each
slices. The requirements of equilibrium are applied
to the slices comprising the slope with the factor of
safety against a slope failure then being defined
as the ratio of the maximum shear strength
possessed by the soil on the trial slippage plane (τ
available) to that shearing resistance necessary
for equilibrium (τ mobilised) ,
FS= τ available/ τ mobilised

 Shear strength of the soil is given by
T available = c’+ (σ – μ) tan Φ = c’ + σ’ tan Φ
 Shearing resistance mobilized is given by
T mobilised= [1/ (FS)] [c’+ (σ – μ) tan Φ]

1) Geo-technical parameters of external dump
material.
2) Geo-technical parameters of foundation
strata below external dump.
3) Hydrostatic force of water flowing through
the foundation of dump mass.
4) Depth of soft soil strata below external
dump.
5) Seismic effect on the dump mass.

Strength parameters Dump material Foundation material
Cohesion(kN/m²) 125 65
Angle of internal friction(°) 16.5 14
Bulk density(kN/m³) 19.8 23.7

Height of dump Slope angle Ordinary Bishop Janbu M. price
60m 26° 1.311 1.384 1.289 1.379
50m 29° 1.297 1.38 1.281 1.375
40m 36° 1.293 1.336 1.28 1.331
30m 40° 1.275 1.33 1.261 1.326

 1 Geo-technical parameters of internal dump
 2 Geo-technical parameters of interface
material.
 3Hydrostatic pressure i.e. upward thrust of
water exerted on the dump by water table
within dump mass.
 4 Seepage force of water flowing through the
dump mass.
 5 Seismic effect on dump mass.

Strength parameters Dump material Foundation material
Cohesion(kN/m²) 110 55
Angle of internal friction(°) 17 19
Bulk density(kN/m³) 19.3 20.8

Height of dump Slope angle Ordinary Bishop Janbu M. price
60m 25° 1.287 1.359 1.268 1.354
50m 27° 1.275 1.359 1.262 1.355
40m 35° 1.253 1.33 1.242 1.324
30m 38° 1.276 1.339 1.266 1.336

1) The height of the external dump is varying between 30 to
60m which is considered in the stability analysis.
2) The slope angles for external dump is varying between 40
to 26.
3) As per hydro-geological study of the mine, the foundation
of the external dump is submerged with ground table. Hence
the foundation of the external dump is considered to be in
submerged condition for stability analysis.
4) The maximum depth of the soil strata is 20m. Hence the
external dump may experience base failure through the 20m
thick soil strata leading to floor having near toe of the external
dump.

1) The height of the internal dump varies from
30 to 60m for stability analysis.
2) The slope angles for internal dump is
varying between 38 to 25.
3) A seismic force equivalent to horizontal load
0.04*dead load of dump mass is considered
here for stability calculation as the mine is
situated in zone-3 of seismic zone map of
India.

The stable slopes are essential for safety of
men and machine. Also vast amount of land
and money can be saved by optimizing slope
geometry. It is, therefore, technical and
economic necessity that the efficient slope
geometry be achieved by optimizing the slope
of the dumps.

Slope/W is the leading software product that used
for computing the factor of safety of earth and rock
slopes. With Slope/W, engineer can analyze both
simple and complex problems for a variety of
slope surface shape, pore-water pressure
conditions, soil properties, analysis methods and
loading conditions. Besides that, by using limit
equilibrium, Slope/W can model heterogeneous
soil types, complex stratigraphic and slip surface
geometry, and variable pore-water pressure
condition using a large selection of soil models.

1) Bishop, A.W. (1955) "The Use of the Slip Circle in the Stability
Analysis of Slopes," Geotechnique, Great Britain, Vol. 5, No. 1, Mar.,
pp. 7-17.
2) Bishop, A.W. and Morgenstern, Norbert (1960) "Stability Coefficients
for Earth Slopes," Geotechnique, Vol. 10, No. 4, December, pp. 129-
150.
3) Casagrande, A. (1960) "An Unsolved Problem of Embankment
Stability on Soft Ground," Proceedings, First Pan-American Conf. on
Soil Mech. and Found. Engrg., Vol. 2, pp. 721-746.
4) Cousins, Brian F., "Stability Charts for Simple Earth Slopes," Journal
of the Geotechnical Engineering Division, ASCE, Vol. 104, No. GT2,
Feb., 1978, pp. 267-279.
5) Crawford, C.B. and Eden,W.J. (1967) "Stability of Natural Slopes in
Sensitive Clay," Journal of the Soil Mechanics and Foundations
Division, ASCE, Vol. 93, No. SM4, July, pp. 419-436.
6) Hoek, E. and J. W. Bray, Rock Slope Engineering, Third Edition, The
Institution of Mining and Metallurgy, London, 1981.

Design, seepage analysis and controls of waste dumps

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Design, seepage analysis and controls of waste dumps