The company’s Bull Mountains mine no. 1 in the US state of Montana recently achieved a first in underground coal mining by successfully holing a longwall into a 12.8m-wide pre-driven recovery room.

Previous conventional and pre-driven longwall recoveries at the mine took up to three months to complete as adverse ground conditions were encountered.

Although located at a shallow depth of between 61m and 79m, those ground conditions were attributed to weak roof competency and regional jointing aligned at a shallow and therefore unfavourable angle to the longwall face.

The difficulties encountered during the recoveries included poor roof conditions during longwall shield recovery and adverse weighting during longwall holing.

Golder Associates was brought in to design the ground support required for the pre-driven roadway.

Unlike most other pre-driven recovery rooms attempted, the roadway was driven in two 6.4m-wide passes to a final width of 12.8m so the longwall did not have to stop and install the bolt-up mesh prior to holing.

Golder Associates’ worldwide longwall database indicates that prior to this case, the widest pre-driven recovery room in which a longwall successfully holed was 11m at a US mine.

The database provided a valuable insight to minimum practical design standards.

In addition to this, consideration was given to depth of cover, the density of rib reinforcement, coal mine roof rating, the width of the roadway and the presence of any significant geological structure.

It was concluded that the standing support needed to be greater than or equal to 1.2 megapascals and the density of roof reinforcement greater than or equal to 0.9 meganewtons per metre.

For the recovery room, several significant variations to the previously used designs for longwall holing and recovery were implemented, namely the introduction of Australian ground support design principles and hardware.

In regard to the ground support, the mine opted to completely backfill the roadway with cellular concrete.

The roof was reinforced with 7.6m long, 63 tonne capacity cables and the inbye fender rib was reinforced with 3.6m long, 30t capacity fibreglass dowels.

Following development of each phase, the ground support was installed and the roadway backfilled.

To check the adequacy of the support design, borehole pressure cells were installed in the fender and outbye pillar and linear extensometers were placed in the roadway prior to backfilling.

During the holing process, it was critical to monitor both the position inbye of the holing point where the fender started to yield and the load-bearing ability of the backfill.

The results of the monitoring data suggested a favourable and controlled loading environment during longwall holing.

In using Golder’s geotechnical design principals, the 377m-wide longwall holed into and mined through the roadway without incident.

In addition to the numerous firsts achieved by the mine, the successful holing resulted in the mine saving approximately nine weeks during the longwall move.

Ry Stone is a principal geotechnical engineer with Golder Associates.