Table of Contents
Mine planners and engineers are often faced with the decision of choosing which method of underground access is best suited for their particular requirement. Underground access can be provided by a shaft, ramp, adit, or slope for either exploration and/or actual production operations.
The selection of the type of opening required to explore a new underground ore body is a complex engineering problem. The optimum solution requires an accurate evaluation of all factors such as ground conditions, potential water inflow, vertical depth, capital costs, and construction schedule. For the purpose of this study, a shaft and ramp access are the only two access methods evaluated.
The shaft used for this evaluation consists of a circular shaft, monolithically concrete-lined to a finished diameter of 4.5 m (14.75 ft). The ramp used for this evaluation consists of a 3.7 m by 3.7 m (12 ft by 12 ft) ramp driven at a grade of minus 15%. This grade is based on recent trends in the western mining districts for an access ramp.
Ground Conditions
Typical ground support for a mine or exploration access opening consists of either a single support system of a combination system depending on the conditions encountered. Basic ground support systems include: (1) concrete, (2) timber sets, (3) steel sets, (4) rockbolts, shotcrete, and (6) steel liner. For the purpose of
comparing the cost associated with constructing a shaft or ramp for exploration access, ground support as required for each category of ground conditions is uniformly applied to the full length of the access opening. In reality, ground conditions would likely be a combination of conditions requiring a variety of ground support combinations.
Hydrological Conditions
The exploration access opening is assumed to be either a dry or wet access. For the purpose of this evaluation, the hydrological conditions have been defined as:
- Wet Uncontrolled water inflows impact development productivities, thus requiring a grout cover to reduce inflows.
- Dry Water inflows are minimal and grouting is not required.
The maximum water inflow of .006 m³/sec (100 gpm) for a shaft and .005 m}/sec (75 gpm) for a ramp is based on previous development experience. Inflows greater than these will impact the development productivities.
To minimize the water inflow to the working face, a combination of a grout cover and water rings/sumps have been utilized for this evaluation.
A major assumption made in regard to potential water inflows is that a source for continuous recharge of the water is present, thus excluding the possibility of dewatering the area around the access openings prior to the start of excavation activities. The rate of recharge is varied according to the ground conditions of the assumed fracture size and spacing.
Cost Estimate General Criteria and Assumptions
The analysis of the capital cost and development schedule to construct a ramp and shaft is based on set configurations with variable depths, ground conditions, and hydrogeological conditions. To provide an accurate cost comparison between the two access methods, the basic requirements for each access are the same.
To arrive at the proper conclusion as to the appropriate access type for a particular application, the following areas must be optimized:
- Lowest capital cost
- Shortest construction period
The development of the shaft and ramp is based on all of the work being done by a contractor for the following reasons:
- Minimum construction schedule is preferred
- Special construction techniques and equipment are required
- Special supervisory and hourly labor are required
Cost estimates include labor, permanent materials, direct equipment costs, equipment rentals, equipment operating costs, electrical power, subcontractor and contractor bond/margin/insurance. Specific rates applied to each cost estimate are:
- Electrical power cost $0.045 /kwh
- Sales tax 6.5%
- Insurance and contractor bond 2.2%
- Contractor margin 10%
A total of six basic cost estimates are produced for sinking a 4.5 m (14.75 ft) diameter shaft to a depth of 240 m (787 ft). The estimation of the shaft construction including ground support, water control, and utilities are broken down into 70 m (230 ft) increments so that comparisons with the decline can be made on equal vertical increments. The six cost estimates consist of:
Ramp Construction Cost Estimate
A total of six basic cost estimates are produced for driving a 3.7 m x 3.7 m (12 ft x 12 ft) ramp at 15%, for 1,600 m (5,249 ft) which will result in a vertical depth of 240 m (787 ft).
The six cost estimates consist of:
- Good Ground Conditions – Dry
- Good Ground Conditions – Wet (Grouting)
- Moderate Ground Conditions – Dry
- Moderate Ground Conditions – Wet (Grouting)
- Poor Ground Conditions – Dry
- Poor Ground Conditions – Wet (Grouting)
The ramp is used for exploration activities, therefore, no permanent equipping is required. Utilities consist of what is required for development and exploration activities only, which include a 150 mm (6-inch) compressed air pipe, 50 mm (2-inch) service water pipe, two 250 mm (10- inch) water discharge pipes, and both power and communication cables.
