The parameters regarding geo-engineering, which are analysed are as follows (Sharma, Sengupta, and Roy 2015): -
3.1 Geo-technical parameters—The main geo-technical parameters which are used in the calculation of slope angles are (Murthy 2002):
-
Cohesion and angle of internal friction of rocks mass in natural conditions.
-
Bulk density of dump materials.
The shear strength parameters i.e. cohesion and angle of internal friction of rock strata have been determined in the laboratory of BIT, Mesra (Birla Institute of Technology, Mesra) by testing drilled cores from all the four boreholes (table - 1). Core logging was done to ascertain the litho-logical composition of the rock strata. From the uni-axial compressive strength determined in the BIT, Mesra laboratory, cohesion and angle of internal friction are estimated by ISRM method (International Society of Rock Mechanics) in the following manner (eqn. 1) (ISRM 1977) (Agustawijaya 2007).
σ1 = C x A [σ3 / C + T/C] n………………………………………….. (1)
Where,
σ1 = Major Principal Stress,
σ3 = Minor principal stress i.e. confining pressure,
T = Tensile strength,
C = Compressive strength,
‘A’ and ‘n’ are coefficients and indicates for different types of rocks. The cohesion and angle of internal friction for the above rock properties are determined from major principal and confining stress by drawing Mohr’s envelop (Fig. 3).
Table 1: Geo-technical properties of rock strata
Rock Strata
|
Depth in metre
|
Bulk Density (kN/m3)
|
Compressive Strength (MPa)
|
Angle of internal friction (deg)
|
Cohesion (MPa)
|
Sand stone
|
0-8.00
|
22.5
|
4.83
|
39.4
|
3.1
|
Sand stone
|
8.00-10.5
|
19.9
|
9.89
|
33.0
|
2.4
|
Sand stone
|
10.5-14.77
|
22.3
|
4.2
|
39.1
|
3.0
|
Sand stone
|
14.80-21.45
|
19.8
|
5.74
|
33.2
|
2.4
|
Sand stone
|
21.45-25.60
|
21.9
|
5.5
|
36.8
|
2.9
|
Sand stone
|
25.60-28.30
|
20.6
|
4.84
|
32.1
|
2.8
|
Sand stone
|
28.30-32.16
|
21.8
|
8.33
|
36.4
|
2.9
|
Sand stone
|
32.16-36.48
|
15.8
|
1.96
|
29.7
|
1.9
|
Sand stone
|
36.48-40.78
|
20.1
|
2.24
|
32.9
|
2.8
|
Sand stone
|
40.78-47.45
|
14.0
|
2.38
|
28.2
|
1.6
|
Sand stone
|
47.45-51.30
|
20.5
|
1.12
|
34.3
|
2.9
|
Sand stone
|
51.30-60.00
|
19.5
|
1.5
|
30.5
|
2.4
|
Sand stone
|
60.00-70.00
|
20.0
|
2.0
|
32.7
|
2.8
|
Coal
|
70.00-83.00
|
18.0
|
1.4
|
30.2
|
1.7
|
3.2. Hydro Geological Parameters — As per the hydro-geological study carried out, annual runoff estimation based in the rainfall runoff relationship established is 558 mm. The approximate area of the colliery, which is considered for estimation of surface runoff, is 4.8 sq. kms (48,00,000 sq. m), thus the estimated total surface runoff generated annually, is 26,78,400 M.cum. As per the hydro-geological study, 23% of total rainfall goes to the ground water table, 32.5% goes back to the atmosphere as evaporation and rest 44% of the rainfall goes as surface runoff to nearby water course. Hence, very negligible amount of rainwater is stored in the surface of the mine. Also due to very low permeability of rocks constituting Balanda mine, there is no existence of seepage line within the whole rock mass except in the bedding plane, which is insignificant in the stability analysis. Due to above reasons, the hydrostatic pressure as well as seepage force will have negligible effect on the rock strata constituting the quarry batter. The chances of slope failure due to these reasons are envisaged as negligible and has not been considered in the slope stability analysis (Murthy 2002) (Moosavi, Shirinabadi, and Gholinejad 2016) (Sengupta and Roy 2015) (Besimbaeva et al. 2018)
3.3. Seismic & blasting effect-
Ground vibration on account of earthquake causes immense damage to quarry batter. The seismic effect has been considered as per latest “Indian standard criteria for earthquake resistant structural design (fifth edition) IS: 1893-2002” (IS 1893 (part 1) 2002). The project falls under seismic zone-III and the basic horizontal seismic coefficient is 0.04 for zone-III as per Indian standard code. An equivalent static approach employing use of a seismic co-efficient is adopted here. In seismic co-efficient method, the design value of horizontal seismic co-efficient, α is computed as given value (eqn. 2) (Mosinets and Shemyakin 1974):
α = β x I x α1………………………………………... (2)
β = Co-efficient depending upon the foundation strength. In this case β =1.0.
I = Factor depending upon the importance of the structure.
In this case, I =1,
α1 = Basic horizontal seismic co-efficient.
In this case, α1 = 0.04 as this project falls under zone - III.
Hence, design value of horizontal seismic coefficient = 0.04
The effect of blasting in the quarry benches is measured and a blasting co-efficient of 0.04 has been taken into consideration in the stability calculations (Singh et al. 2012). Summation of seismic and blasting co-efficient (0.04 + 0.04) is multiplied with dead load of potential failure block of the quarry for taking into account the seismic and blasting effect on the quarry slope.