Investigation for Safety of Final Quarry Bench During Mine Closure Stage – a Case Study

The closure of opencast coal mining operation in Balanda mine will be a source of danger for inhabitants or any other infrastructure near the surface of ultimate quarry bench from safety point of view. In view of the above, MCL (Mahanadi Coaleld Limited) and the management of the Balanda opencast mine conducted a variety of design research and academic institutions to conduct a slope stability analysis of Balanda ocp (opencast project). It is also important here to note that the Birla Institute of Technology, Mesra, Ranchi, has prepared a study report on the above. This paper deals with design of ultimate 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 coal eld in Angul district of Orissa, India and falls under the administrative control of Mahanadi Coal elds 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 metre and overburden rock above coal seam is 65-70 metre. The present working is in western part near dip side lease hold boundary of the mine close to fault F 2 -F 2 (Fig 1 & 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 (Srivastava, Jha, and Shandilya 2015).

Objective Of Mine Closure Planning
To allow a productive & sustainable after use of the site which 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, shery, etc.
To protect public health and safety.
To eliminate environmental damages and thereby encourage environmental sustainability.
To minimise adverse socio-economic impacts.

Stability Analysis Design Parameters
The parameters regarding geo-engineering, which are analysed are as follows (Sharma, Sengupta, and  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 insigni cant 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 In this case, I =1, α 1 = Basic horizontal seismic co-e cient.
In this case, α 1 = 0.04 as this project falls under zone -III.
Hence, design value of horizontal seismic coe cient = 0.04 The effect of blasting in the quarry benches is measured and a blasting co-e cient of 0.04 has been taken into consideration in the stability calculations (Singh et al. 2012). Summation of seismic and blasting co-e cient (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
Slope stability analysis has been done for a factor of safety of 1.2 by both Fellinius method and Bishop's simpli ed method (Roy 2008). Later acknowledging all the approved factor of safety recommended by various organisation/bureaus such as " National coal board, U.K.", "United states D'Appolonia consulting engineers", "Mines branch Canada" ,"Stability of pit slopes and dumps by G.L Fiesenko, Russia for opencast mine slope design", a factor of safety of 1. With the above geo-engineering parameters, stability analysis was done with the help of Fellinius and Bishop's simpli ed method. The recommended safe slope angle is obtained in the range of 47° to 57°( table -2) corresponding to overall rock strata height of 80 to 60 metre respectively for a factor of safety of 1.2 (Sengupta, Sharma, and Roy 2014). and Design Institute, Ranchi. The mine has been worked out by both dragline and shovel-dumper combination ( Fig. 1 & 2). The shovel-dumper benches have already been reached to a nal position. All the shovels and dumpers deployed in over-burden removal have been diverted to some other mines of Mahanadi Coal eld limited. Further movement of these top benches is not feasible at present in absence of any shovel-dumper. Hence, the existing position of the top benches is at their nal position (Fig. 2).
Presently, the 30-40 metre height of bench is being worked out with the help of 2 draglines (one of 20/90 & other of 10/60). The average height of dragline bench is about 35 metre. Out of this, the top 10 metre strip of overburden rock is being de-capped with the help of 10/60 dragline and the bottom 25metre 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 nal quarry batter as per recommendation of the study report [2] will vary between 32° to 43° which will be atter than the maximum allowable slope angle of 47° to 57° with a factor of safety of 1.2. Hence the closed mine will not pose any safety problem regarding stability of nal benches.
The views expressed in this paper are those of authors and not of the organization they belong. Figure 1 Surface plan of Balanda ocp. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
Mohr's diagram from compressive strengths of drilled core