Estimating underground excavation costs

LARGE excavations are required in underground mines for facilities such as crushing stations, loading stations, workshops and pump stations. This note may be of interest to engineers who need to estimate the cost of such a large excavation.

LARGE excavations are required in underground mines for facilities such as crushing stations, loading stations, workshops and pump stations. This note may be of interest to engineers who need to estimate the cost of such a large excavation.

In a Scoping Study (accuracy +/- 30-50%), factored costs of similar facilities, including the excavation costs, are appropriate.

Alternatively an excavation cost per cubic metre and a support cost per square metre can be used, together with a factored lump sum for the installed cost of the equipment.

Costs should be factored from a site having similar rock conditions.In a Prefeasibility Study (accuracy +/- 20-25%) a general layout of the facility should be prepared at least in plan view, and a preliminary assessment of ground support requirements made.

Sufficient thought should be given to excavation techniques to be able to estimate the volume of rock to be broken by each of hand-held, jumbo or longhole drilling, and the method of rock removal for each.

Excavation, loading and haulage costs per cubic metre for each method can then be applied.

Ground support costs can be estimated per rockbolt in backs and walls, plus per metre of cable bolt where required.

Mesh and shotcrete (or fibrecrete) costs can be estimated per square metre applied, with shotcrete typically at 75mm or 100mm thickness.

If estimating shotcrete costs volumetrically, allow 10-20% rebound losses for wet-mix fibrecrete, with a further 10-20% roughness factor.

After allowing these factors and wastage (spillage, left in agitator bowl or dumped) the total shotcrete useage will be 1.8 to 2.5 times that for a theoretical case using the design thickness of shotcrete applied to a design excavation.

Overbreak, actual excavation roughness, rework, temporary support and specification of a minimum design shotcrete thickness are some of the other causes besides rebound and wastage.

The specification of 50mm minimum thickness can result in an average actual thickness of 75mm, depending on the roughness of the actual excavation.

In large excavations there is a greater possibility of overbreak. Depending on its purpose, the excavation cost may include the cost of a concrete floor.

In poor ground, particularly where well-developed structures are present, substantial overbreak may occur, for example around the crusher pit and crushed ore pass below the crusher. This needs to be considered when estimating concrete volumes.

Blinding concrete on final excavation floors in a particular area can reduce overbreak from subsequent adjacent excavations below this level.

Blinding concrete helps prevent chamfers (overbreak) being created at the intersection between a horizontal surface (a permanent floor) and a vertical face subsequently excavated adjacent to this permanent floor.

In a Feasibility Study (accuracy +/- 10-25%) a detailed design of the excavation should be prepared showing how the major mechanical components fit into the excavation.

Allowance needs to be made also for the installation of the mechanical components including mobile crane access, conveyor cable reels etc.

Sufficient geotechnical work, possibly including stress measurement and 3D modelling, should have been done to enable the orientation to be optimised and the permanent ground support to be specified in detail.

A Construction Method Statement should be prepared, explaining the method and sequence of excavation, temporary ventilation, how the broken rock will be removed, any need for temporary ground support, and the method of installing permanent ground support.

In many tall excavations a “top down” method is used, allowing cable dowels and permanent back support to be installed prior to taking out the bulk of the chamber.

Typical industry practice is to complete mining and support of the chamber before handing it over for civil and mechanical construction.

An alternative is an integrated mining and construction schedule, with concrete and steel work constructed at the top of the chamber off a solid rock floor, eliminating the need for extensive scaffolding and working at heights.

Careful blasting techniques will then be required for later stages of excavation to prevent damage, but the overall cost and / or duration of the project may be reduced.

Large excavations are now more likely to be made using development jumbos than longhole methods.

Access by rubber tyred vehicles allows shotcreting and cablebolting to be safely and efficiently performed. Longhole methods need to take account of more skilled and labour intensive installation of ground support, and the potential for ground movement or relaxation while support is being installed.

Mobile cranes, specially constructed ladderways and systems for working at heights may also be necessary. Longhole methods may also result in overbreak in poorer ground, and efforts to reduce overbreak can result in underbreak with associated time consuming survey and minor stripping.

Temporary ground support is likely to be required in one (temporary) wall, while excavating and installing cablebolts in the backs of a large chamber and in subsequent excavation lifts below this level.

Good geotechnical knowledge of crusher chambers is required as there is less flexibility in locating crusher chambers, particularly in relation to a block cave or long conveyors.

Conveyor transfer points require some thought in scheduling of excavation sequences, particularly where access is required for two or perhaps even three separate excavation lifts.

Good QA and survey systems or generous (0.3 to 0.5m) tolerances are required to ensure that steelwork and infrastructure components fit in the completed excavations.

Excavation crews need to understand that large excavations are civil engineering projects and not a mining job such as advancing a decline for truck haulage.

A dedicated crew for large excavations may have quality and schedule benefits although initially appearing more expensive.

The interface between a mining contractor and infrastructure contractor also needs consideration.

Schedule delays on the excavation can result in significant costs to the infrastructure contractor and the principal.

All of the points mentioned have direct or indirect costs, which are probably not all captured in most costing systems.

Actual costs are therefore typically higher than those shown in cost reports.

Cost estimation should be based on contractor’s tender or at least a firm contractor’s schedule of rates based on a detailed scope of work for the specific project.

Alternatively the cost may be built up from first principles using a “notional gang” mining crew with hourly labour and on-costs, together with current itemised costs for drilling equipment and consumables, explosives, loading and haulage, ground support, concrete, ventilation, temporary pumping, maintenance, power supply and supervision.

Peter McCarthy

Managing Director

AMC Consultants

pmccarthy@amcconsultants.com.au

Tony Weston

Senior Mining Engineer

AMC Consultants

tweston@amcconsultants.com.au

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