Quarry development requires the balancing of constraints which often compete or conflict with one another. Boral?s Redlynch Quarry is a site at which many constraints are exaggerated because of the physical characteristics of the site, the unique tropical environment of Cairns, Queensland?s regulatory conditions, recent urban encroachment and the upfront cost of quarry development.
At Redlynch, the meta-greywacke resource is on a very steep mountain ridge. The current workings are restricted to the mountain?s base. However, over 50 years of reserves can be released by developing from the top of the 280m high ridge. The ability to quarry this material is limited by complex geotechnical conditions and potential impacts on new residential developments.
Boral is advanced in Redlynch?s long term development. The project has identified ways to plan for and manage constraints at Boral?s other sites.
DEVELOPMENT PLANNING
The long term extraction footprint was created by a process of reduction from the maximum possible area. First, the site?s geology restricts reserves to the ridge. The potential footprint was then reduced by cadastral standoffs, buffers from watercourses and other approval conditions. An assessment determined the likely impacts of quarrying on the nearest neighbours.
The noise and visual impact studies indicated that it could be challenging and impractical to meet environmental limits at neighbouring houses. This is because recent urban encroachment has resulted in houses being built within 400m of the quarry and because environmental limits have reduced since quarrying commenced in 1990.
The potential quarry footprint was systematically decreased until modelling indicated noise limits would be practically achieved at the nearest houses. This reduction set the crest line along the spine of the ridge rather than daylighting the top of the hill, even though the larger footprintis allowed within the existing development approval.
Having established the lateral extents of the quarry, the bench configuration was designed. This process was complicated by the complex geotechnical conditions of the site, eg:
? The intense jointing and slippery backs.
? The geometry of the pit being parallel to the main joint set.
? The presence of at least three geotechnical domains.
? The back-breaking of blasts.
Shallow batters and wide benches normally offset the geotechnical complexities of the deposit. However, a more aggressive batter angle is required to achieve a satisfactory yield at Redlynch. This is because the steepness of the ridge provides limited width to extract material.
The final mine planning parameters were established by geotechnical engineers who determined that the site could be safely worked using a combination of 15m faces and 5.7m benches. To reduce the slope angle and improve geotechnical stability, face angles were reduced from 75? to 65?. In addition, wider catch benches were introduced every six benches to reduce the slope angle. This concept could be effectively implemented at many other steep quarries.
Access to the upper benches required a new 2.2km haul road to be constructed. Different load and haul fleets were assessed in respect to capacity, haul road gradient and running width. This was an iterative process with consideration of haul road design requirements such as gradient, width, cut and fill volumes, and emergency stopping.
It was determined that a fleet of articulated trucks could be safely operated on a narrower, slightly steeper road than rigid trucks. The haul road was then designed to gain elevation across the flanks of the hill between five switchbacks. The road will generally be one lane, but will have several long passing lanes.
The haul road design also incorporates safety measures to arrest a runaway vehicle. This will be achieved by constructing escape ramps, and also by building emergency collision bunds. These methods have since been implemented at other Boral sites with downhill hauls.
The pit design has an excellent yield of just two megatonnes of overburden to more than 22Mt of resource. However, the upper benches have poor stripping ratios, which is a commercial constraint. The long term project is sensitive to upfront costs associated with haul road construction and upfront stripping.
In response, stage plans were developed to buffer these costs. This will be achieved by the accelerated development of an 80m high section at the base of the ridge adjacent to the workings. The lower development area will be stripped before the top of the ridge, providing cheap, reliable raw feed near the plant while the top is developed. The internal fleet will alternate between campaigning raw feed from the base and stripping the hill top.
QUARRY DEVELOPMENT
Boral commenced the first major phase of quarry development in 2009 by stripping the section at the base of the hill. Challenges were encountered which should be planned for in similar stripping programmes:
? Overburden was variable in thickness. In thick areas, some overburden should be left in place for drill access. Large in-situ outcrop will inhibit stripping, so access roads for blasting should be constructed early.
? There is a balance between temporary access ramp and long term haul road designs. It was not possible to mechanically rip the ramp to long term grade. Therefore, risk assessments on short term ramps should consider emergency stops, additional bunds, vehicle passing, drainage and surfacing.
Operation within the new development area has also created blasting challenges. Blast performance must consider environmental impacts, wall stability and fragmentation. These criteria are critical, having the potential to be a show stopper for long term projects. Blasting must also deliver the mine plans precisely to preserve the design?s integrity.
The major environmental challenge at Redlynch is blast overpressure, particularly due to recent residential developments close to the quarry site. The quarry is situated above a valley in which surrounding ridges create an amphitheatre, and air blast is often amplified due to clouds.
The intense jointing and unfavourable face orientation at Redlynch often results in blast damage to working faces. Back-break, slippery-backs, toe and overhang reduce the stability of faces and also reduce design efficiency for subsequent blasts.
Boral has worked closely with Orica Quarry Services for the past year to improve blast performance. Numerous methods have been trialled to target specific performance criteria. Results indicate that slight compromises are required in each performance criteria in order to achieve an overall improvement, and that prioritising one criterion over another is not possible.
Redlynch has historically created large oversize quantities, so most work on blast fragmentation has targeted oversize reduction. Crushing plant belt weigher traces have been examined for different blast techniques to compare primary throughput, a practical measurement of feed grade. Surprisingly, techniques which reduce oversize by five per cent resulted in a 20 tph drop in production rate. This is because oversize is generated at the front and back of the blast, which is largely independent of fragmentation within the bulk of the blast.
LESSONS LEARNED
Long term quarry development planning is an iterative process which must account for competing constraints from different disciplines. Prioritising one constraint above another may be impossible, impractical or unfeasible. This is demonstrated by examples of conflicting constraints at Redlynch, eg:
? Wide benches are ideal for geotechnical stability but result in low yield.
? Developing the hill top first is the ideal mine plan but results in low initial yield.
? Pre-splitting is ideal for wall stability but increases overpressure.
? Upfront stripping is the ideal mine plan but requires excessive upfront cost.
? Large rigid trucks are efficient but require impossible haul road parameters.
Underpinning all quarry development projects is the need to balance physical and regulatory constraints while remaining commercially viable.
Travis Potts is a project development manager for Boral Resources (Queensland).