Safety aspects of large dragline- operated opencast mines – An overview

Synopsis The Jayant opencast operation is one of the largest opencast coal mines in India. Prior to 2008 the mine experienced a number of dragline dump failures, which was a major hindrance in sustaining production. Northern Coalfields Limited (NCL) and the mine management engaged several design, research, and academic institutions to carry out dump slope stability studies, particularly of dragline dumps. Birla Institute of Technology prepared a report on the investigations in May 2009. In this paper we review the findings of the report and the measures taken to tackle the safety aspects of dragline dumps.


VOLUME 122
The Journal of the Southern African Institute of Mining and Metallurgy [1] where P a = Height of seepage at the toe of the dump. L or β = Overall angle of slope of dump. D p = Behind the toe of dump i.e., 60 m.
By applying Casagrande's equation, at a distance of 300 m from the toe of the dump the height of the phreatic surface within the dump is calculated as tabulated in Table I. With this height of water table, the seepage and hydrostatic forces are calculated and considered in the stability calculation.
The phreatic surface (DPY, DPX) ( Figure 2) is also evaluated through relevant condition: The height of the water table is calculated as 25 and 36 m ( Table I).
The upward thrust of the water can be defined as the product of the unit weight of the water and the volume of the dragline dump submerged under the water table within the failure mass (Roy, 2016) The seepage force is calculated as the product of the upward thrust and the sine of the gradient of the horizontal phreatic surface (Murthy, 2002;Sengupta and Roy, 2015) (Figure 4).

Seismicity and blast vibrations
Seismic forces are regarded according to the Indian Standard criteria for earthquake-resistant structural design (5 th edn) IS 1893:2002 (IS-1893 (part 1), 2002). The horizontal seismic coefficient (A h ) design for the Jayant dragline dump is determined by the following expression (Sengupta and Roy, 2015): where Z = Zone factor (study area is located in zone III according to IS 1893:2000) I = Importance factor R = Response reduction factor Sa/g = Average response acceleration coefficient of dump mass.
According to the Indian seismic map, the project is located in zone III, with the horizontal seismic coefficient of 0.02 m/s 2 , as per the IS code considered here. The blast vibration coefficient on the dump mass due to ongoing blasting was estimated such that the horizontal coefficient of 0.04 m/s 2 will include both seismicity and blasting (Mosinets and Shemyakin, 1974).  The height of seepage face is shown in Figure 2 The Journal of the Southern African Institute of Mining and Metallurgy VOLUME 122 JANUARY 2022

Dump floor inclination
The mine floor inclination varies from 2° to 4° (CMPDI, 2018). For stability calculations a dump floor inclination of 3° is considered here (Sengupta and Roy, 2015).

Dragline dump height
The dragline dump height, which varies between 60 to 100 m, and the surcharge load of the shovel dump on the dragline dump are also considered in the stability analysis (Mosinets and Shemyakin, 1974;Zaitseva and Zaitsev, 2009;Sengupta, Sharma, and Roy, 2014).

Coal rib
According to existing practice in this mine, a coal rib of 7 m base width and 3 m top width with average Turra seam thickness of 19 m, as shown in Figure 3, is considered as a resisting force against dump failure (Roy, 2003). The coal rib left at the toe of the dump acts as a retaining wall and reduces dragline dump rehandling to some extent (Colwell and Mark, 2003;Besimbaeva et al., 2018).

Laboratory tests for the generation of geotechnical information
Samples of the dump material as well as the interface material were collected and transported to BIT Mesra for determination of the strength parameters (Ranjan et al., 2017) (Table II).

Recommendations
Considering the above parameters and by applying both Fellenius and Bishop's simplified method (Moosavi, Shirinabadi, and Gholinejad, 2016), the slope angles for the dragline dump are calculated (Table III) and recommended for a minimum factor of safety of 1.20 for different heights of the seepage face (Besimbaeva et al., 2018).  The results are documented for different dragline dump geometries in Table III.
The angle of repose of the dragline dump is the overall angle with respect to the horizontal plane over which it is standing. The dragline dump is considered to be cohesionless for the purpose of determining the angle of repose, although in reality the dump has an emerging water table, an inclined floor, pore pressure within the dump material, and is affected by blast vibrations and pore pressure within the dump material. Also, cohesion is generated due to the compaction of the dump material under its own weight. Hence the angle of repose of the dragline dump is 37° in the ideal case, but in actual site conditions owing to the above prevailing geo-engineering considerations the overall slope of the dump will differ.
The above recommended slopes of the dragline dump are maintained by optimizing the following parameters of the dump profile as shown in Figure 3. ➤ At the mining level of the dragline -berm width ➤ Berm width present at coal rib/barrier ➤ Angle of slope below mining level of the dragline.
It is recommended that truck and shovel dumps overlying the dragline dump are formed 120 to 150 m away from the toe of the dragline dump (Sharma and Roy, 2015), (i.e. the interval between the toe of the dump formed by the shovel dump and dragline dump should be at least two cut widths 120-150 m). In this case, the dragline dump will act as a foundation for the shovel dump. Hence, the geotechnical properties are considered to be the same for both the shovel dump and its foundation. Accordingly, the following combinations of shovel dump are calculated and recommended (Government of India, 2017; Directorate General of Mine Safety, 2008) in Table IV and shown in Figure 3.
The recommended overall slope angle of the shovel dump can be maintained by adjusting the berm width at the coal rib roof level.

Precautionary measures
In addition to managing the slope, several proposals are suggested and implemented to ensure the stability of internal dumps as well as dragline dumps (Sharma and Roy, 2015): i. Topsoil is dumped separately away from the existing internal overburden dump. ii. To form a foundation for the dragline dump, no surfacesoil may be dumped at the level from where coal has been extracted. iii. By ensuring normal gravitational seepage of water in the direction of the sump area, nominal collection of water takes place where coal has been extracted. iv. The dragline dump receives sufficient time to settle, followed by supplementary truck dumping, therefore the distance between the toe of shovel dump and the dragline dump is between 120 and 150 m, i.e. two cuts beyond the toe of the dragline dump (Sharma and Roy, 2015). v. The voids in the dragline dumps are consolidated with the help of dozers. vi. Some coal is left at the toe of the dump to act as a barrier (coal rib). It is designed in such a way that the overburden dump should cover up the coal rib/barrier as much as possible, and that the coal rib/barrier is likely to burn naturally. vii. Efforts are made to extract coal from the coal rib without blasting, at regular intervals of 200 to 250 m along the strike length of the pit, so that there is no accumulation of water against the coal rib. viii. Before dumping by dragline, the interface layer is cleaned from where coal has been extracted to as great an extent as possible (Singh et al., 2012). If possible, fragmented overburden rock is dumped to cover the slushy floor at the base of the dragline dump to increase the coefficleint of friction at the dump floor. ix. As per the recommendations of BIT Mesra, Ranchi (Roy, 2016): a) If possible, the mine floor (foundation of internal dump) may be ripped or blasted at intervals to a depth of 1 to 2 m, thereby increasing the coefficient of friction prior to dumping by dragline (Government of India, 2019). It is also recommended that minor blasts to promote the flow of water to the sandstone beds below the open pit floor should be carried out to limiting water retention at the base of the dump. b) Regular monitoring of the dumps through a non-contact survey using a laser profiler or laser scanner is proposed to detect any movement of overburden dumps or dump faces that will indicate a potential dump failure. As the dragline dumps are inaccessible, a reflectorless instrument based on laser technology is recommended for surveying the   60  33  31  70  32  30  80  31  29  90  30  28  100 28 26 displacement of the dump face between the crest and the toe of the dump. c) It is proposed that monitoring of the dump should be carried out and recorded at seven-day intervals during the monsoon and post-monsoon seasons (July to November), whereas in the dry season (December to June) monitoring should be done at 15-day intervals. In the case of any movement of the overburden dump, the de-coaled floor near the toe of the dump is declared as a hazard zone with removal of men and machinery from the hazard zone. d) The seepage of water from the face of the dragline dump is to be monitored when the coal rib has been breached at 7-day intervals from July to November and at 15-day intervals for rest of the period.

Conclusion
Based on the recommendation of the Birla Institute of Technology Mesra, the Jayant opencast project has maintained the dump profiles by adjusting the following parameters as shown in Figure 3a) Berm width at the coal rib roof level b) Angle between the coal rib roof and the dragline mining level c) Berm width at the dragline mining level d) Slope angle above the dragline mining level. The abovementioned measures have successfully prevented any major failure of backfilled dumps in spite of the huge volumes of waste rock (around 40 million m 3 per year loose volume) handled and dumped within the de-coaled area.