Production of aggregate of acceptable quality, at the required rate of production and at economic cost, depends on the selection and utilization of the most suitable equipment to match the conditions. Achieving these objectives is a crucial task of the quarry engineer and also requires careful planning and scheduling of operations. Each phase of the extraction plan will be presented in a series of mine plans and schedules showing the progressive extraction of the payrock minerals and, in most cases, the designed backfill and restoration as well. In the same way the type and numbers of mobile equipment will need to be programmed and scheduled. Planning for equipment replacement is largely a question of the investment policies dependent on the capital depreciation set by the operating company.
However, during the life of a quarry or pit the rate of output, the ratios of waste to payrock and the dimensions of production faces are all likely to vary and consequently the type, numbers and capacity of all excavation equipment will need to be adjusted to suit the particular requirements.
The optimum number of each machine and their deployment can be determined by consideration of the required rate of production, the geological conditions and performance data for a single unit obtained from comparative experience or the manufacturer.
Selection of the drilling equipment and method takes into account a number of factors which may vary within a single quarry and will almost certainly vary during the life of a quarry. The rock type and rock mass classification will influence the performance of any drill rig in terms of rates of penetration and wear on rods and bits. Typically a DTH rotary percussion rig will achieve 20m/h in a soft limestone but only 10-12m/h in a hard igneous rock or a sandstone. Hydraulic rig penetration rates can vary between 15 and 25 m/h.
The selected drilling pattern of burden x spacing x face height will enable the yield per hole in cubic metres to be calculated. Based on the drill performance, after making allowances for availability and utilization, the yields per blast and the number of drills to achieve the production targets throughout the development can be determined.
The principal factors which need to be considered in selecting the loading equipment and scheduling its deployment throughout the quarry or pit life are: (i) the daily and hourly production rates; (ii) the configuration of quarry faces and the number to be worked; (iii) the expected fragmentation and the maximum lump size; (iv) the capacity and type of haulage trucks; (v) the digging rates, cycle times and efficiencies of each type of excavating machine.
As production rates vary throughout the life of a quarry as may the rock conditions, so the number and type of machine need to be scheduled to ensure cost effectiveness of the operation.
The numbers and capacity of dump trucks will generally be determined by the required production rate, but there are various factors which often influence selection. The type of excavation and ground conditions including manoeuvring space, gradients and running surfaces will decide whether rigid or articulated dumpers are appropriate.
From the mine plan the haulage distances and cycle times can be calculated at any stage and thus indicate the numbers needed. Careful analysis of haulage costs can assist in deciding if alternative methods of moving materials from the excavations to the plant should be adopted. The alternatives might include load-hauldump using a single machine, a front end wheeled loader in the case of rock quarries, or motorized scrapers in the case of sand and gravel pits.