ducks guts, page-6

Thanks for your comments Bob77. I understand your point and I think we would be in agreement.

My understanding from the reports of the Co is that they have 3 or 4 depressurisation bores for geotechnical/safety reasons (your analogy of the retaining wall is perfect) and 2 dewatering bore to intersect an aquifer in an unknown location (but I suspect they are at the southern end of the pit based on sketch plans they have presented). Intersect is the important word here because invariably not all water is able to be removed from an aquifer. I suspect the Co worked this out with their consultants during the feasibility studies. If they need more depressurisation bores so be it and I will beg forgiveness for my comment.

The perimeter based dewatering system (and not a depressurisation system) I think the Co is alluding to, requires close spaced bore holes to ensure the cone of depression around each bore intersects. Given they want to drive the pit much deeper, such a system around a pit of say 1.8km in circumference would require a lot of holes to achieve their objective and may not pass muster with the environmental authorities. I’ve seen this done before but wow it could mean 30 to 40 holes at say $0.5 million EACH to set up. Costs of running the system would be high. Using this system means that none of the rainfall is handled (must be about 100 to 150 litres per second on average 24/7 based on the average rainfall) so a dedicated in-pit pumping system must be maintained. The Co has reported they have intersected some underground workings with a pilot hole that could IMO provide some temporary relief (the description of the water table dropping, however, sounded quite bizarre). The reason I mentioned this in the first place is that the Co has not provided a summary of any consultant’s report that suggests they need to do something dramatically different. I will apologise now but it looks like they are ‘winging’ it.

Dewatering the mining face ahead of the mining schedule is often completed with in-pit sump holes for exactly the reason that the cone of depression is at the location where the water is needed to be removed (in the middle of the pit). This reduces the amount of water needed to be pumped. Coupled with depressurisation bore holes those purpose was so eloquently described by Bob77, provides the most effective and safest method of water management in these conditions.

Occam's razor: entia non sunt multiplicanda praeter necessitatem (entities must not be multiplied beyond necessity)…. In other words "simpler hypotheses are generally better than the complex ones"