Question 27 of 27

Question 6.2 deep well dewatering (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.

6.2.1 — finding transmissibility (T) and storage (S). A controlled pumping (aquifer) test is run at a known constant rate Q from one well while drawdown is logged with time at that well and at nearby observation wells. The Theis (1935) transient solution relates drawdown s to Q, T, S via the well function: $$s = \dfrac{Q}{4\pi T}W(u), \qquad u=\dfrac{r^2S}{4Tt}$$ Jacob’s (1946) straight-line simplification, valid once u is small, linearises this: plotting drawdown against the LOG of time gives a straight line whose slope directly gives $$T = \dfrac{2.30\,Q}{4\pi\,\Delta s}$$ (Δs = drawdown per log cycle) and whose time-axis zero-drawdown intercept t₀ gives $$S = \dfrac{2.25\,T\,t_0}{r^2}$$ This semi-log graphical method is the standard field technique for characterising pit-wall hydraulic properties before a deep-well system is designed.

6.2.2 — layout and depth. Submersible borehole (or line-shaft turbine) pumps are set in wells drilled and screened into the water-bearing rock, laid out in PLAN as a ring (or partial ring) around the pit crest with spacing chosen so neighbouring wells’ drawdown cones overlap enough to intercept the full inflow before it reaches the wall; additional wells are added on ramps/benches INSIDE the pit wherever the perimeter ring alone cannot depress the water table below the advancing floor (matching this paper’s own note that wells may sit “on the ramp inside the pit”). Pump-SET DEPTH is determined from the predicted drawdown cone (using T and S from 6.2.1) plus a safety margin below the lowest anticipated pumping water level in that well, so the pump is never left running dry as the cone deepens with continued pumping.

6.2.3 — feasibility and cost. Feasibility depends on the rock mass being transmissive enough (from 6.2.1’s test) that a PRACTICAL number of wells can achieve the needed drawdown, and on drill access from stable ground outside the advancing wall. Cost is estimated by summing well construction (drilling, casing, screen, gravel pack), pump/motor and controls per well, times the number of wells the layout requires (from 6.2.2), plus a perimeter power-distribution and discharge-pipeline network — typically a HIGHER capital cost up front than the in-pit sump-pump system of Question 6.1, because deep wells must be drilled and equipped well ahead of mining rather than simply relocated as the pit deepens.

6.2.4 — operational advantages. Because deep wells depress the water table AHEAD of mining rather than collecting water after it has already entered the pit, wall rock stays drier and more stable (higher effective friction, lower pore pressure/uplift, directly improving slope factor of safety), floor and haul-road conditions are drier (less equipment downtime, better tyre life, fewer mud-related productivity losses versus the in-pit sump system), blasting is more reliable in dry holes, and pumping is continuous and predictable rather than being disrupted every time an in-pit sump (like the one in Question 6.1) has to be relocated as the pit advances or deepens.