You are a mine planning engineer at a medium sized operation producing 1.5 mtpa of coper ore at 3% Cu. The mine was originally designed as an open cut using a bench height of 15m. Footwall benches were 15 m wide to give a slope angle of 45°, but on the hanging wall side alternate benches are only 10m wide to produce an angle of about 50°.

Underground mining was commenced using a footwall decline from the 15m bench to access 2 lifts of open stoping down to the -210m level. The first lift of stopes are completely mined out, an extraction of the second lift is 50% complete. A 6m diameter shaft has been sunk to a depth of 700m from a collar at +55m for the long term. Mining below the -210m level is to be by sub-level caving. A transport level is to be developed at the -510 level to transport all ore from levels above via ore passes to a crushing station at the -610m level and then to a loading station at the -630m level.

You are required to design a drainage and pumping system to deal with the mine water for the next 10-15 years based on the data in the following sections. As the mine grade is low and power costs are high every effort should be made to minimise the operating costs of the pumping system. Describe your proposed drainage and pumping system with the aid of diagrams where required and explain the general philosophy behind the system. Detail and assumptions made and make comments and conclusions as required.

Data

Ground water: currently averages 1,000,000 litres/day and is expected to increase at a rate of 10% per year as the mine deepens. Flows are higher in winter up to double the average for 3-4 months. The main source is the fissured footwall rock.

Mine water: averages 70,000 I/day with large variations in use on an hour by hour basis. When sub level caving commences the usage is likely to double but usage is also expected to be more uniform.

Surface runoff: Currently this is contained in the open cut and is pumped from a sump in the pit bottom. Once crown pillars are blasted and caving is initiated this surface water will find its way into the cave and eventually into the mine. Any future estimates need to be based on expanding the catchment area and on the following rainfall data.

Rainfall: the average annual rainfall s 740 mm, the average monthly figures are as follows

January                                                         10mm

February                                                        10mm

March                                                            40mm

April                                                              60mm

May                                                              80mm

June                                                              100mm

July                                                               110mm

August                                                          120mm

September                                                     90mm

October                                                         60mm

November                                                      40mm

December                                                      20mm

Based on measurements in the locality the maximum rainfall intensity recorded over the last 50 years is as indicated below and needs to be accounted for:

Water Quality: Run of mine water is expected to be slightly acidic due to the formation of sulphuric acid from sulphides within the rock. Likely pH is in the range 5.5-6.5.

Cave angles: Expected cave angles are 60° in both footwall and hanging wall and 75°at ends of the ore body

Rock properties: SG of 2.7 for all rocks. Hanging wall and ore have moderate strength, footwall is very strong.