Investigation for Safety of Final Quarry Bench During Mine Closure Stage: A Case Study

Mine closure includes the rehabilitation process, which is a continuous programme aimed at restoring physical, chemical, and biological quality that has been harmed by mining to a level that is acceptable to all individuals involved. BIT Mesra was assigned a R&D project on development of dragline dump profile which includes both on-going and closed mines. As a part of the study, the stability of both highwall and dump was investigated for this mine. The stability of the final quarry batter of this mine was studied considering the formation of Planar, Wedge, and Circular failure. Due to favorable position, inclination and orientation of fault planes, i.e., F2-F2 and other nearby fault planes, the probability of formation of Plane and Wedge failure was minimal. There was no presence of unidentified fault planes intersecting the final quarry batter leading to planar and wedge failure. Hence, circular failure involving minor structural discontinuities was investigated with help of the computerized model based on Fellenius and Bishop’s method. Accordingly final quarry batter was designed with optimum combination of safety and maximum extraction of coal. It has also been ensured that a part of OB dump was back filled on the floor of the exhausted mine in such a way that the final depth is not more than 60 m, for the purpose of rainwater harvesting and pisciculture. This paper deals with design of final quarry bench and discusses various geo-technical parameters for carrying out stability analysis.


Introduction
Balanda opencast mine is located in the south-eastern part of Talcher coalfield in Angul district of Orissa, India, and falls under the administrative control of Mahanadi Coalfields Limited (MCL). The mine was sanctioned for an annual rated capacity of 1.0 Million ton in 1959 and the project is in operation since then. The mine has come on the verge of closure. There is only one coal seam dipping at 2°-4°with thickness as 13-14 m and overburden rock above coal seam is 65-70 m. The present working is in western part near dip side lease hold boundary of the mine close to fault F 2 -F 2 (Figs. 1 and 2). To delineate the location of fault F 2 -F 2 and geo-technical properties of rock strata, four boreholes were drilled near the present working face [1].

Importance of Mine Closure Planning for a Mining Project
• To allow a productive and sustainable after use of the site, this is acceptable to the mine management and regulatory authority. • To bring back to its original shape of the land as far as practicable for use of land for agriculture, fishery

Stability Analysis Design Parameters
The parameters regarding geo-engineering, which are analyzed are as follows [2]: • Geo-technical parameters • Hydro-geological parameters • Seismic and blasting effect

Geo-Technical Parameters
The main geo-technical parameters which are used in the calculation of slope angles are [3]: 1. Cohesion and angle of internal friction of rocks mass in natural conditions. 2. 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 uniaxial compressive strength determined in the BIT, Mesra laboratory, cohesion and angle of internal friction are  [4,5].
where r 1 = Major Principal Stress, r 3 = 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).

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  Table 1 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 have not been considered in the slope stability analysis [3, 6 7, 8].

Seismic and 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 2002) [9]. 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, a is computed as given value (Eq. 2) [10]: b = Co-efficient depending upon the foundation strength.
In this case, b = 1.0. I = Factor depending upon the importance of the structure. In this case, I = 1, a 1 = Basic horizontal seismic coefficient.
In this case, a 1 = 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 [11]. 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.

Stability Analysis
The stability of the final quarry batter of this mine was studied considering the formation of Planar, Wedge and Circular failure. There are different modes of failure in rock slope, i.e.,

Plane Failure
It occurs when a geological discontinuity such as major fault plane, bedding plane, minor faults strikes parallel to the slope face at flatter angle than highwall bench as shown in (Fig. 4). The weight of the sliding mass is calculated from the geometry of the slope and the failure plane. A tension crack running parallel to the crest of the slope can also be included in the calculation.

Wedge Failure
When two discontinuities strike obliquely across the slope face and their line of intersection daylights in the slope face, the wedge of rock resting on these discontinuities will slide down the line of intersection. The calculation of the factor of safety is more complicated than that for plane failure since the base areas of both failure planes and the normal forces on these planes must be considered for stability calculation (Fig. 5).

Circular Failure
When the material is very weak, as in a soil slope, or when the rock mass is heavy jointed or broken, as in a waste rock dump, the failure will be defined by a single discontinuity surface but will tend to follow a circular failure path. Such type of failure is very common in case of soil strata in the highwall or the waste rock dump backfilled or dumped outside the quarry (Fig. 6).
Due to favorable position and inclination of fault lane F 2 -F 2 , the probability of formation of Plane and Wedge failure was minimal. There was no presence of unidentified fault planes intersecting the final quarry batter leading to planar and wedge failure. Hence, circular failure was investigated with help of the computerized model based on Fellenius and Bishop's method. Accordingly final quarry batter was designed with optimum combination of safety and maximum extraction of coal. It has also been ensured that a part of OB dump was back filled on the floor of the exhausted mine in such a way that the final depth is not more than 60 m, considering the rainwater harvesting and pisciculture. Method used for calculation of factor of safety in this study is given below.

Fellenius method [7]
Factor of Safety ¼ Frictional force þ Cohesive Force Disturbing Force Bishop's Simplified Method The above two method were used for soft rock other than planar and wedge failure analysis. Slope stability analysis has been done for a factor of safety of 1.2 by both Fellenius method and Bishop's simplified method. Later acknowledging all the approved factor of safety   With the above geo-engineering parameters, stability analysis was done with the help of Fellenius and Bishop's simplified method.
The recommended safe slope angle is obtained in the range of 47°-57° (Table 2) corresponding to overall rock strata height of 80-60 m, respectively, for a factor of safety of 1.2 [14].

Conclusion
This paper presents a geo-technical approach to deal with quarry bench design during mine closure stage based on the study carried out by Birla Institute of Technology, Mesra, Ranchi. The mine has been worked out by both dragline and shovel-dumper combination (Figs. 1 and 2). The shovel-dumper benches have already been reached to a final position. All the shovels and dumpers deployed in overburden removal have been diverted to some other mines of Mahanadi Coalfield limited. Further movement of these top benches is not feasible at present in absence of any shoveldumper. Hence, the existing position of the top benches is at their final position (Fig. 2). Presently, the 30-40 m height of bench is being worked out with the help of two draglines (one of 20/90 and other of 10/60). The average height of dragline bench is about 35 m. Out of this, the top 10 m strip of overburden rock is being de-capped with the help of 10/60 dragline and the bottom 25 m overburden bench is being excavated by 20/90 dragline, which is following the other dragline along the strike length of the mine. Thus, the above two draglines are working in horizontal tandem manner. After mining of these benches, the final quarry batter as per recommendation of the study report [15,16] (Roy, Sharma, and Shrivastava 2013) slope angle will vary between 32°to 43°which will be flatter than the maximum allowable slope angle of 47°-57°with a factor of safety of 1.2. Hence, the closed mine will not pose any safety problem regarding stability of final benches.
After the design of final quarry batter, various closure evaluations have been made as follows- • Water quality check to prevent acid mining.
• Abandoned mines, i.e., in decoaled areas has been turned up for pisciculture ( Fig. 7) (fish farming) • Overburden tops have been reclaimed for further agricultural use / eco-forestation (Fig. 8) • Successful growth of re-vegetation according to physical, chemical and biological condition of reclaimed dump by qualified personnel.
The views expressed in this paper are those of authors and not of the organization they belong.