1. BANK HEIGHT CONSIDERATIONS IN KOTA STONE QUARRIES
A. Hussain
Slope stability and permissible bank height are interlinked and are most important part of
the basic design of opencast mining operations. In Indian mining scenario the number of
operating opencast mines is steadily increasing due to higher productivity, lower
gestation period and extremely high productivity, higher targets which can not be met out
by the conventional underground technology. However, inspite of its known advantages,
the technique is not fully safe and healthy leaving aside involved land degradation,
environmental concerns, socioeconomical problems. The higher output considerations
have necessitated deployment of higher capacity equiepments which in trun demanded
high pit geometery including increased depth and higher bench heights. This factor
invited concerns towards operating slopes stability.
In general steep pit slope angles helps in reduction of overburden stripping and
thus improves operation’s economy, but Jeopardize the safety of the working persons
and deployed equipments on the other hand, mild slopes ensures complete safety, but at
the other hand increases cost of operations. As far as design of ultimate pit slopes or
bench height considerations are finalized the process do not depend upon these factors
alone but important factors such as material’s Compressive strength, Tensile strength,
Shear strength and angle of internal friction etc. plays vital roles
An exercise was carried out to understand evaluation of the stability of the high
overburden benches in the Kota Stone quarries, where current practice is to maintain 25.0
to 30.0 meter high vertical benches in basalt or limestone overburden masses.
Kota and Jhalawar districts of Rajasthan are blessed with large deposits of
dimensional limestone commonly known as Kota Stone. It is a cheap elegant and quick
fixing media for floor construction works, at the same time it has developed a great
architectural trend by interwoven with white marble or other decorative stones to give
pleasing floor patterns. The stone is available in blue, green, brown or other
combinations. There is hardly any substitute of Kota Stone floors for economical and
functional building Extraction of this stone has not only fulfilled the need of the local
area, state and country but mining and allied operations have provided and still
continuing bread and butter to a very large number of families. Perhaps lime stone
industry is the largest industry in this region. This industry has also shared in the national
development as government earn large revenue under different heads. Kota stone, lately
has also established a sizable export market, which provides a good foreign exchange to
contribute towards our national economy.
Geology
The Kota stone deposits are located between latitndes 24’ 45’-24’ 48’ and
longitudes 75’ 45’ – 76 5’ and area is covered on toposheets in 45/0/113, 45/0/114,
54/D/1 and 54/D/2.
Kota Stone geologically speaking is a part of semi-series of lower VINDHYAN
group. The local nomenclatures are will understood and prevailing from the inception of
the mining in this area. The regional stratographic sequence of deposits are given as
under
A. UPPER VINDHYAN
1

2. 1.Bhandar series
2. Rewa series
3. Kaimur series
4. Semi series
B. LOWER VINDHYAN
1. Suket series
2. Nimbahera series
3. Jhalraputan series
The workable limestone beds are located at a depth of 25-30 mtrs from surface
and dip gently at 10 percent towards south-east. A representative stratographic
sequence of these deposits are given as under.
1. Top soil- 0.00-2.50 mtrs thick
2.Mixed calcareous rocks- 10.00-25.00 mtrs
3.Lime stone sacks - 10.00-14.00 mtrs thick
To examine the stability the main criteria anchored was factor of safety which is a
ratio of the available shear strength to that required to keep the bank stable. Slope
stability vis-à-vis factor of safety are given as under
FOS less than 1 Unsafe
FOS between 1.00 – 1.25 Questionable safety
FOS between 1.25- 1.40 Satisfactory,limited
More than 1.40 Satisfactory for all purposes
An exercise was carried out to understand the stability status of vertical banks
maintained in overburden benches primarily composed of bedded high silisic
limestone. The various parameters observed are as under-
1. Specific gravity 2.46 mt/cu mtr
2. Bulk Density 1.70mt/cu mtr
3. Uniaxial compressive strength – 1200 kg/sq.cm
4. Tensile Strength – 180 kg/sq.cm
5. Shear Strength – 180 kg/sq.cm
6. Angle of internal frication – 26 degree
7. Coefficient of internal friction – 0.7
8. Porosity – 11 percent
9. Cohesion strength – 200 kg/sq.cm
If it is assured that face stability is maintained solely by the force of cohesion, the
force of frictional resistance may be neglected, then the maximum height can be
derived from the following equation (N. Melnikov and M. Chesnokov – safety in
opencast mining)
L= 4C
M sin 2 µ
Where L= Vertical face height in mtrs
2

3. C = Specific force of cohesion T/Sq.M
M = Volume weight of Material, T/Cu.M
µ = Slope angle of slip plane, in Degrees
At the greatest possible value of Sin 2 µ i.e. at µ = 45 degree, the maximum
height of the vertical face may be
L = 4C
M
Since a cohesive mass of material separates from the parent material, not along a
plane but along a cylindrical surface, a correction factor is introduced in the form
of Fellenious formula i.e.
L = 0.958 x 4C
M
= 0.958 X 4 X 19.986
2.46 = 31.13 mtr
In actual practice, the strata is non uniform having local weakness, which are also
to be accounted for. Further the force of cohesion and frictional resistance are also
inconstant in value and depend upon deposit depth, water content, fractures,
Joints and other geological structural components. Taking into account these all
factors, the safest vertical height should not exceed 30.0 mtrs.
This status can also be verified by studying analytical system of rock
strength, where rock strength (S) can be interlinked with normal compressive
strength on the failure surface MN/Sq.Mtr (Gong Benyis of central coal mining
Research Institute, china.)
S= C+P Tan Ө
S = Rock strength in MN/Sq.Mtr
Where C = Cohesive force in MN/Sq.Mtr
P = Normal compressive strength on the failure surface in MN/Sqm
Tan Ө – Coefficient of internal friction
Ө - Angle of friction in degrees
The values of strength cab be found at various slope angles from the strength
diagram. As per the chinese model, the figures for lime stone are given as under
Uniaxial Tensile strength Rt = 14.71 MN/Sq.M
Uniaxial Compressive strength Rc = 148 MN/Sqm
Ratio of Rc = 10
Rp
3

4. The coefficient of internal friction equals to 0.6 to 0.7 and angle of friction is
approximately 26 Degree. Mohar circle showed the cohesion as 0.196 MN/Sq.m at
internal friction angle of 26 Degree
This is equivalent to 196 kpa or 19.60 mt/ square meter
The factor of safety for above 70 Degree slope angle comes to about 1.23 which is almost
at satisfactory level.
Benches in the area are well stable and there is not a single case of bank failure from the
time of inception, which proves that empirical safe height considerations
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