The Hidden Hazard: Ground Support Design Non-Compliance Creep Underground

It might be that time of year when the underground geotechnical risk assessment gets a refresh. The “pedestrian struck by rockfall from decline wall” risk needs addressing and comes up for discussion. It’s the perennial grey swan in the room. There’s an awkward silence and gazes are averted, someone sighs. What is the risk of this low probability, high consequence event occurring based on a documented incident? The incident presented in this article is particularly interesting because there were two key contributory factors that aren’t considered nearly as much as they should be in underground geotechnical risk assessments. These are, firstly, hung services in underground development concealing rockfall risks, and (I’m coining this term) ‘ground support design non-compliance creep’. I’ll go into these in more detail further on.

In Smith v MMG Golden Grove Pty Ltd [2020] WADC 103, a 2013 underground rockfall incident sheds light on subtle yet critical factors contributing to workplace injuries. While the case primarily focused on an insurer's liability for a settlement paid to an injured worker, the background details reveal a concept I call "Ground Support Design Non-Compliance Creep."

The incident involved an experienced plant operator struck by a 15 kg rock dislodged from a decline wall. The ground support in this area was installed four years prior. An ICAM report highlighted several issues: two fault lines in the wet wall, the mesh being installed 4.1 metres above the floor instead of the required 3.5 metres, and reduced visibility of loose rocks due to a water deflector strung on the wall. The worker also reported having to use excessive force to operate a water header, potentially dislodging the rock.

It's plausible that continuous water flow led to gradual lowering of the decline's floor through deliberate grading over time. This "cambering" to manage water inadvertently increased the distance between the floor and the installed mesh; making it appear non-compliant with the 1999 Code of Practice (which stated mesh should start at 3.5 m from the floor). This subtle, ongoing change, rather than initial faulty installation, likely contributed to the incident.

Additionally; services hung on walls; like the water deflector, can conceal loose rock, masking hazards during geotechnical inspections. This "creep" in ground support effectiveness and hidden risks emphasise the ongoing need for rigorous geotechnical programs, including thorough scaling and inspections that account for dynamic underground conditions and concealed areas.