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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

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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

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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

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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