Transverse longhole stoping
"Everything in underground mining is a compromise between what looks good on paper and what an orebody will allow one to do"
Transverse Longhole Stoping is a popular large scale underground mining method known for its ability to simplify stope sequencing. It is an open stoping method and is very similar to sub-level open stoping, Longitudinal longhole retreat, and blast hole stoping.
- 1 Description
- 2 Selection Considerations
- 3 Ground Support
- 4 Drilling, Blasting, and Equipment
- 5 Safety
- 6 Case Study
- 7 References
Transverse longhole stoping is a bulk mining method in which the long axis of the stope and access drifts are perpendicular to the strike of the orebody. Typically drawpoints are located in under-cut access drifts which extend from the footwall, and the free face is mined in a horizontal retreat from the hanging wall to the footwall. In general, transverse longhole stoping is used where the rock mass quality of the hanging wall limits the length of the open mining span. This methodology requires more footwall waste development (for footwall drifts and drawpoints), however, since each stope has an independent access, it has more flexibility with regards to sequencing and scheduling. A standard transverse layout can be seen below.
- High tonnage bulk mining method
- Facilitates sequencing and allows for flexibility in planning and mining
- Large stope sizes can result in high productivity and lowered drilling costs
- Easily mechanized
- Multiple stopes can be mined simultaneously
- Repetitive techniques help to facilitate planning, training, mining, and safety
- Relatively high recovery
- High development requirements result in high capital cost
- Development primarily in waste
- Poor selectivity
- Poorly suited to single narrow vein structures
- Moderate dilution, especially when using backfill
- Considerable ventilation needs due to active equipment on top and bottom sills
Variation: Longitudinal Longhole
Transverse longhole stoping is often used in conjunction with longitudinal longhole stoping methods. Longitudinal longhole methods operate along the or parallel to the strike of the orebody. Longitudinal longhole stoping is more favourable to transverse stoping in areas where ore thickness is diminished, or where narrow veins of ore are present. The orientation of longitudinal mining methods means that the hanging wall and footwall of the orebody will most likely form the sidewalls of the stope. In general, longitudinal methods are used where the rock mass quality of the hanging wall rock is competent enough to allow the development of a substantial opening in the hanging wall or footwall. Longitudinal longhole methods are very well suited to retreat mining, and can be planned such that development occurs within the ore itself, reducing development costs, and generating revenue in the process. While longitudinal longhole stoping methods are fairly limited in their ability to depart from a set mining sequence – mining operations start at one end of a mining block, and work sequentially to the other end of the mining block. A typical longitudinal layout can be found in the image below.
Differentiation between longitudinal and transverse methods
Below are definitions for general terms for the transverse longhole mining method, shown schematically in figure below.
• Span: Length of stope along the strike.
• Width: Perpendicular distance between footwall and hanging wall.
• Height: Distance along exposed hanging wall not the vertical height between levels.
• Longitudinal pillar: Pillar aligned along strike of the stope.
• Rib pillar: Pillar aligned transverse of stope, perpendicular to strike.
• Sill pillar: Horizontal pillars that separate levels or stopes.
• Dilution: Reduction of ore grade due to mixing of ore with barren rock.
• Internal dilution: Rock that must be mined due to geometry of ore body and the requirement to mine rectangular areas. The term is synonymous with planned dilution.
• External dilution: Dilution caused by sloughing or failure of stope walls and back, is outside blasted stope boundary. External dilution is defined as external waste tonnage divided by ore tonnage. The term is synonymous with unplanned dilution.
Several factors must be considered when selecting Transverse Longhole stoping as a mining method. Orebody geometry has a large impact on the suitability and effectiveness of transverse methods. The width of the orebody in question must be thick enough to justify the cost of access drift and cross-cut development along the top and bottom sill of the mining horizon. In-situ stress conditions, as well as mining induced stresses, will determine ground support requirements. Pillars may or may not be needed depending on the competency of the ore and host rock, as well as the planned stope dimensions. When considering transverse methods the mining widths, as well as stope height, should be analyzed with respect to stability conditions in order to determine maximum stope sizes. The availability of backfill, in conjunction with maximum allowable mining span (as determined by stability conditions) will determine the suitability of transverse methods. Ventilation requirements for transverse longhole stoping are generally considerable, as ventilation must be provided to both the top and bottom sill where heavy mechanization increases the required cubic feet per minute (cfm).
When considering transverse longhole stoping, all of the above factors much be considered, and it should be determined whether the mine site can meet all of the aforementioned requirements.
Similar to transverse methods, longitudinal methods require a steeply dipping orebody in order to avoid hangups in blasted stopes, and to facilitate drilling and mucking.
Transverse longhole stoping requires a relatively wide mining width in order to minimize development costs and to maximize stope productivity. In Canada, there are no instances of transverse methods utilizing mining widths less than 15 metres (18 metres is acknowledged as an acceptable mining width to use with transverse methods). The ideal orebody will have a dip greater than the angle of repose of the ore, or just steep enough to allow ore to fall easily into the open stope. Typically the targeted dip for this method is between 60 and 90 degrees although any angle over 45 degrees would suffice. Steeper dipping orebodies help to minimize dilution in blasthole rings which intersect the hangingwall and footwall. Transverse longhole methods are utilized in orebodies with rock mass qualities that vary from poor to very good as seen in the figure to the left. Mining widths vary drastically, but are generally wider than 15 metres.
Through observation of the graph to the right, it can be seen that longitudinal mining is well suited to orebodies with mining width less than or equal to 20m, except in a few rare cases. This relationship can be seen in the figure to the right. Orebodies ideal for this method are very tall and have medium to narrow width and an extensive length.
Top Sill (Drilling and Loading)
Top sill development is conducted in order to provide a platform from which blasthole drilling and loading can be effectively conducted. The top sill layout is generally very similar to that of the bottom sill, as stopes are drilled from the top sill with the aim of being mucked out from the corresponding access drift on the bottom sill.
Bottom Sill (Mucking)
Bottom sill development is conducted in order to provide access to Load-Haul-Dump (LHD) vehicles or other forms of mucking equipment. Mucking equipment operating on the bottom sill utilizes the access drifts to reach the bottom of blasted stopes, from which they muck out the blasted ore. As previously mentioned, the top and bottom sill layouts are generally very similar, as bottom sill access drifts are used to muck out the blasted rock drilled from their corresponding top sill drift.
Transverse longhole stoping generally requires that vertical rings of blastholes are used in order to extract stopes. When extracting the stope closest to the hanging wall (typically this is the first stope extracted) it is necessary to create a slot to allow for the expansion of blasted rock. Slot raises can be developed in several ways, the most common being raise boring (since development is already in place at the top and bottom sill). Following the initial extraction of the first stope, rock can be blasted into the void created by the previous stope, however, in some cases where backfill is used to completely fill in the previous stope, slot raises must be created at every stope.
Slot raises can also be used to diminish the major stress acting on stopes if the slots are of sufficient size. Diminishing the major principal stress by utilizing slot raises can reduce the bracing that holds key blocks in place. While access drifts are driven in the same direction as major induced stress, providing shielding from in-situ stresses. This orientation of stopes and drifts results in long faces being more exposed to stress making them relatively unstable.
The orientation and management of stresses means that a transverse mining approach will result in greater yielding within development when compared to longitudinal methods. Transverse methods will also result in higher dilution from the hanging wall if the hanging wall is composed of strong material. Clamping stress from the strength of the hanging wall will not be retained during mining, resulting in longer, less stable extraction faces. The transverse layout generally has more production potential and shorter ventilation raise lengths.
The need for access to the top and bottom sills of each mining level requires that ramp or shaft access be available in order for equipment to access both sills of each level. Inter-level ramping can provide the mobility needed for equipment to transfer between levels. Ramps can also provide a means of ventilation, while manways and ventilation raises can also be constructed to connect top and bottom sills, as well as other mining levels.
Under certain stress conditions, the use of pillars may be necessary when utilizing transverse longhole stoping methods. While it is possible to mine stopes adjacent to one another when using backfill, stress conditions can sometimes require that pillars be left in place to protect nearby mine workings. Rib pillars can be used to separate horizontally adjacent stopes, and in the case of transverse longhole stoping, can be created by leaving narrow stopes un-mined in between stopes which would otherwise be adjacent to one another. Sill pillars can be used to separate vertically adjacent stopes. Rib and sill pillars can prove useful in controlling stress conditions in adjacent workings, and preventing hazardous rock behaviours such as rock bursting and falls of ground.
Crown pillars should be used when mining activity approaches the ground surface in order to prevent surface subsidence.
Post-pillars are an effective method of providing additional support in adjacent stopes during transverse longhole stoping, however they require careful planning and mining to maintain pillar shape and effectiness.
Where it has been determined that backfill should be used, stopes are mined and mucked, and subsequently filled with consolidated fill that is typically comprised of hydraulic fill, paste fill, or cemented rock fill depending on available materials and rock characteristics. Unconsolidated fill is not usually considered for transverse stoping operations as the fill would not provide sufficient support, and a dramatic increase in dilution would be noticed. It is imperative to note that backfill requirements are not tailored to each transverse mining layout and the information below is only a brief overview of many possibile solutions.
Typically, once the primary stopes are mucked and filled, mining of secondary stopes can after curing of fill in the primary stopes achieves a strength that will generate minimal dilution. Generally, the binder:content ratio of backfill is 30:1 to 20:1 (fill:cement by volume). Alternatively, a permanent pillar can be left behind to confine the unconsolidated fill with only the primary stopes being extracted along the strike. With this variation, secondary stopes are in actuality narrow pillars left behind (approximately 3–5 m). A disadvantage of this method is the inability to follow variations of an irregular hanging wall dip.
As with backfill, ground support techniques are similar for all types of open stoping. Most transverse methods systematically use cable bolt support for stope backs, with most stope overcut development supported by a two metre by two metre pattern of six to eight metre long, twin-strand cablebolts. Cable bolting of stope hanging walls is also quite common in practice. These bolts are installed at the top drilling sublevel of the open stope, their purpose not being solely to provide stability for the hanging wall, but more so to avert any instability from propagating to the stope up dip.  A good example of cable bolt use in a transverse layout can be seen in the image below.
the sub-level open stoping page for ground support
the Queen's MineWiki page for ground support
As with all underground mining methods, the stoping sequence for transverse longhole stoping is driven primarily by ore grade, operational constraints (i.e. access, ventilation, backfill, etc.), and rock mechanic considerations. By producing the best possible grade early in the project along with a constant flow of materials to the mill, mining sequence can provide an excellent base for mine success. Consistent tonnage may only be achieved through rigorous management of geotechnical risk and ground conditions. Stope sequencing can be effectively utilized to mitigate the effects of mining induced stress by creating an active stress shadow, which can be managed and manipulated to shelter existing and future excavations.
Most open stope mines, including mines which practice transverse longhole stoping, sequence their stopes based on the high stress conditions of underground mining. One popular method of mine sequencing known as the 1-5-9 sequence is outlined in the image to the left of the page.
Drilling, Blasting, and Equipment
Transverse longhole stoping typically utilizes In-The-Hole (ITH) or top-hammer drills capable of creating holes of 75mm-200mm diameter (3-8 inches). Drilling is generally carried out from the top sill in a ring drilling or parallel drilling pattern, drilling towards the bottom sill access. Both ring drilling and parallel drilling require that drillholes be aligned in a planar orientation in order to effectively separate rock and ore from the active face. Drilling deviation should be minimized due to the nature of transverse stoping as stope heights are usually short enough to minimize the effects of drill deviation.
Slot raises are often created by use of a raise bore, which requires a pilot hole to be drilled, generally from the top sill down to the bottom sill. Slot raises are used to provide space for rock expansion during blasting.
For more drilling techniques, refer to the sub-level open stoping page:
Blasting requirements such as the powder factor required to achieve adequate rock breakage and fragmentation heavily influence the layout of the required drilling pattern. Explosive type is usually dictated by overall economics rather than by mining method, although different types of explosives will have different drilling requirements. Loading of explosives is most easily conducted from the top sill, as explosives can be gravity fed into the drillholes.
Ring blasts require that every hole within a specific ring be set off at once in order to propagate fracturing and the force necessary to blast the specified panel of ore away from the working face of the stope. When using slot raises, sequencing should be conducted so that blastholes nearest to the slot are detonated first, enlarging the available opening for subsequent holes.
For more blasting techniques, refer to the sub-level open stoping page
Transverse longhole stoping provides the opportunity for a high degree of mechanization to be used in development and mining operations. Development generally requires that drill-jumbos in conjunction with ANFO loaders, rock bolters, and LHD’s be used in order to drive development and access drifts. Stope drilling is usually conducted by ITH or top-hammer drills, while loading of blastholes can be done by hand or with specific loading equipment.
Mucking is typically carried out with the use of LHD’s. The size of mucking equipment depends upon the desired production rate in that specific area of the mine, and also upon the size of openings through which the equipment has to pass. Remote control operation of LHD’s or other mucking equipment is often required as personnel should not be exposed to open stopes. LHD’s can either transport muck to orepasses or to haul trucks.
For equipment selection, refer to the sub-level open stoping page
Transverse longhole stoping presents a set of risks similar to those present in other open stoping methods.
Drilling from the top sill requires that no workers be present in the corresponding bottom sill access drift, as drill steel can generate excessive loose material when penetrating the back of the bottom sill access drifts. Remote drilling is sometimes required when the potential exists for drill steel intersecting rock bolts or mesh, in addition to previously drilled and loaded holes.
Poor blasting can create additional hazards in the bottom sill near the entry to the stope, as the quantity of loose material can be increased through poor blasting. Due to the dangers of loose material in open stopes, remote control mucking is employed in transverse longhole stoping.
- Hartman, Howard L. SME Mining Engineering Handbook. 2nd ed. Littleton, Colorado: Society for Mining, Metallurgy, and Exploration, 1992. Print.
- Hustrulid, W. Underground Mining Methods. Littleton, Colorado: Society for Mining, Metallurgy, and Exploration, 1998. Print.
- Hudyma, M. Open Stoping Mining in Canada. AUSIMM, 2000. Print.
- Bronkhorst, D. Innovative Mine Design for the 21st Century. Rotterdam, 1992. Print.
- Nickson, S. Cable Support Guidelines for Underground Hard Rock Mine Operations. Unpublished MASc Thesis. University of British Columbia, 1992. Print.
- Bawden, W. Stope Sequencing at the Golden Giant Mine. 9th Underground Operator’s Conference. Ontario. 1989. Print.
This article was designed and created by Group 2 (Mark Wheeler, Christopher Hey, Devon Coburn, Ignacio Garcia, and Ariana Van Laren) as part of the MINE 448 Underground Mine Design Course taught by Professor Stephen McKinnon at Queen's University.