Question 5 of 11

Question 1.5 (9 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1.5.1 — finding transmissibility (T) and storage (S) by pump test. A pumping (aquifer) test is run: one well is pumped at a known, constant rate Q while drawdown is logged with time in that well and in one or more nearby observation wells. The Theis (1935) non-equilibrium solution relates drawdown s to Q, T and S through the well function W(u), s = (Q/4πT)·W(u), u = r²S/4Tt. Jacob’s straight-line simplification (valid once u is small, i.e. for times not too soon after pumping starts) linearises this: plotting drawdown against the LOG of time (or of r²/t for multiple observation wells) gives a straight line whose slope directly yields T = 2.30Q/4πΔs (Δs = drawdown per log cycle) and whose time-axis intercept t₀ yields S = 2.25Tt₀/r². This graphical (semi-log) method is the standard field technique for characterising the hydraulic properties of the pit-wall rock mass before designing any dewatering system.

1.5.2 — pump types, layout, and depth. Deep-well dewatering uses vertical line-shaft turbine pumps or, more commonly today, submersible electric borehole pumps set well below the anticipated drawdown cone in wells drilled and screened into the water-bearing rock. Wells are laid out in plan as a ring (or partial ring) around the pit crest, spaced so adjacent wells’ drawdown cones overlap enough to intercept the full inflow before it reaches the wall, with additional wells added on benches/ramps inside the pit where the perimeter ring alone cannot depress the water table below the advancing floor. Pump-set depth is determined from the predicted drawdown cone (using the T and S found in 1.5.1) plus a safety margin below the lowest anticipated pumping water level in that well, so the pump never runs dry as mining deepens.

1.5.3 — feasibility and cost. A perimeter deep-well system is feasible where the rock mass is reasonably transmissive and reachable by drilling from stable ground outside the advancing wall, and it is generally MORE capital-intensive up front (drilling, casing, screening and equipping many wells, plus a perimeter power/pipeline network) than an in-pit sump pump, but it pre-drains the wall in advance of mining, which the sump-pump alternative cannot do. Feasibility depends on drill access, rock transmissivity being high enough that a practical number of wells can achieve the needed drawdown, and the value of the operational benefits in 1.5.4 outweighing the higher capital cost.

1.5.4 — operational advantages. Because the wells depress the water table AHEAD of mining rather than collecting water that has already entered the pit, wall rock is drier and more stable (higher effective friction, reduced uplift — directly improving the factor of safety computed in Question 4), floor and haul-road conditions are drier (less equipment downtime, better tyre/traction life, fewer mud-related productivity losses), blasting is more reliable in dry holes, and pumping is continuous and predictable rather than being disrupted every time an in-pit sump is relocated as the pit deepens or a working face advances past it.