The lining of a shaft performs several functions including stabilising the walls of the shaft, providing structural support to the guides for the conveyancing systems, as well as to optimising air flow for the mine ventilation design. Therefore, selection of the most appropriate type of shaft lining needs to be considered during the design phase. This document provides a high-level overview of the typical applications, construction methods, benefits, and drawbacks of more commonly used shaft linings.
Unlined
Description: Unlined shaft excavations rely on the rock mass strength for structural stability.
Typical Application: Shafts only remain unlined after construction in the most stable ground conditions. It is not recommended that shafts used for access of material hoisting be left unlined due to long term safety and stability concerns. Commonly used for ventilation, ore/rock passes or water conduits.
Construction Method: Most commonly “bottom up” construction in the form of a raisebore/blind bore.
Benefits:
- Faster, simpler and cheaper than alternatives due to the elimination of the lining stage in the construction cycle and the associated material import cost/construction time.
- Environmentally beneficial due to lower material consumption.
- Improved safety in the case of bored shafts.
Drawbacks:
- Stability of the shaft is dependent on the geology and geotechnical conditions present.
- Likelihood of long-term shaft wall deterioration and material infall.
Bolts and Screen
Description: Bolt and screen shaft linings are formed from varying combinations of rock bolts, mesh, and sprayed concrete. The role of this lining is either to provide support and/or reinforcement to the rock mass in order to stabilise it or prevent local infall of ground.
Rock bolts are designed to carry axial forces exerted by rock mass movement (relaxation/failure/creep) and shear loading along failure planes.
Mesh is pinned between bolts to capture loose debris, with the mesh aperture size varied as appropriate to the debris particle size.
Sprayed concrete of nominal thickness (not structural) can also be applied to act as a barrier to debris, bind loose rock together at the face, or seal the face off from exposure to air or moisture which may cause long term deterioration of the face.
Typical Application: Only suitable for ground that is self-standing after excavation with minimal water inflows. This type of lining is also applicable to creeping strata (e.g. salt) due to its ability to deform and the ease of remediation compared with rigid linings.
Construction Method: Typically, a top down construction cycle with personnel required to work in the shaft. Mesh is anchored in place by rockbolt faceplates and where required shotcrete is the final layer.
Benefits:
- Very flexible and repairable, making it suitable for a wide range of conditions.
- Relatively cheap compared with rigid linings.
Drawbacks:
- Safety of personnel working in the shaft.
- Material waste can be high when applying shotcrete.
- Not suitable for situations with high water inflows.
Steel Liner Plates
Description: Liner plates are prefabricated steel elements bolted together to form a complete shaft lining.
Typical Application: Only suitable for temporary ground support to avoid local ground infall. Generally used on shallow shafts less than 40m depth and 10m in diameter.
Construction Method: Usually constructed ‘top down’ with each ring bolted to the ring above. Every 2-3 rings, a formwork is placed on the bottom ring to allow grouting behind the liner plates, which is injected through grout ports in the plate to anchor the plates in place and create an interface for ground load to be transferred to the lining.
Benefits:
- Lightweight and easily handled when installing.
- Relatively low cost.
- Quick installation possible.
Drawbacks:
- Lightweight and can only withstand relatively low ground loading, therefore only suitable for shallow depths (less than 40m).
Precast Segmental Concrete
Description: Precast concrete shaft segments are very similar to those used in tunnelling. Segments are bolted together to form a high strength structural lining ring.
Typical Application: Generally used for medium diameter (10-30m) shallow civil engineering shafts of depths less than 100m.
Construction Method: Usually constructed ‘top down’, using either a caisson or underpin construction methodology.
For the caisson method, a steel cutting edge is installed under one ring of segments and then pushed into the ground using a series of hydraulic jacks that are connected to the collar. The material is excavated from the inside of the shaft using an excavator in the hole or grab crane from surface to remove material before the next ring of segments is added from the top.
For the underpin method, each ring of segments is suspended from the ring above (or from the concrete collar in the case of the top ring) and then grouted in place before excavation of the ground underneath the lining to allow installation of the next ring.
Benefits:
- High strength linings can be rapidly constructed.
- Timber packs can be included to increase deformation capacity if required.
- “Off the shelf” and custom segments available from manufacturers.
Drawbacks:
- Lining system is limited by the practical handling weight of the lining segments.
- Transport of a high number of heavy segments to site.
- High strength concrete requires excellent quality control.
- Expensive.
Nominal Concrete
Description: Nominal linings provide limited structural support to prevent debris from entering the shaft as well as facilitate ventilation via their smooth finish and carry services.
Mass concrete nominal linings may need to be designed to be pressure relieved to prevent the build-up of water pressure loading; to this end, weep holes and or drainage garlands are incorporated.
The minimum thickness is governed by practical considerations during casting, specifically related to the stiffness of the kerb ring during handling and placing of the concrete; 300 - 450mm thick nominal concrete linings are typical.
Typical Application: These linings are usually situated within strong rock strata with manageable water inflows (low flow or low pressure that can be sealed using grouting) with diameters of less than 10m. These linings have been in depths over 2km in South Africa.
Construction Method: Installed in either by a ‘top down’ or ‘bottom up’ method. ‘Top down’ construction is preferable as the excavation is supported incrementally as it is progressed. Vertical loads from the lining are transferred locally into the rock by mechanical interlock and shear. Cast in place or Shotcrete Applications of lining are possible.
Benefits:
- Minimises concrete usage, whilst providing a fully serviced shaft environment.
- Addition of steel fibres can increase strength.
- Extensively used with widely available expertise.
Drawbacks:
- Pressure relief measures are equired where hydrostatic pressure exceeds design strength capability to prevent inadvertent water pressure loading,
- Issues can arise with achieving heat of hydration in freezing conditions.
Hydrostatic Concrete
Description: Hydrostatic pressure resisting concrete linings are designed to resist the full hydrostatic pressure at depth. Whilst the concrete may be used to resist water pressures, it is not capable of providing a 100% totally dry watertight environment.
Mass concrete hydrostatic linings can however be constructed to provide a nominally dry lining, with water makes of less than 5gpm (23 litres/minute).
Thickness varies with depth, typically 300-1350mm thickness.
Typical Application: These linings are used where high-pressure water is expected at depth. The practical limit for application of a full hydrostatic liner is around 5-600m as deeper than this would require linings in excess of 2m thickness, diameters are typically less than 10m.
Construction Method: Installed in either by a ‘top down’ or ‘bottom up’ method. ‘Top down’ construction usually preferable as the excavation is supported incrementally as it is progresses, minimising construction time and materials. Vertical loads from the lining are transferred locally into the rock by mechanical interlock and shear. Cast in place concrete is conventional but Shotcrete lining may be possible with rigorous QA/QC.
Benefits:
- Creates a stable shaft environment where water inflows are present.
- A shotcrete lining is possible.
Drawbacks:
- Where hydrostatic pressure at depth is too high, an impractical thickness of concrete lining is required.
- Cannot provide a totally dry shaft.
Cast Iron Tubbing
Description: Tubbing is a heavy-duty version of a lining system often formed from spheroidal graphite cast iron, and capable of resisting large pressures. Tubbing is installed in segments and bolted along vertical flanges to form a tubbing ring; the rings are then bolted along the horizontal flanges to form the vertical shaft lining. Tubbing is used to resist very high water and or ground pressures, the joints between the tubbing sections usually being sealed with lead gaskets and washers to provide water tightness. Tubbing is, however, not 100% watertight and maintenance is required on the caulked joints.
Typical Application: These linings are typically used in high loading conditions such as loose, unstable ground with a high-water pressure, diameters are typically less than 10m.
Construction Method: Tubbing can be installed ‘top down’ or ‘bottom up’. For the ‘top down’ method, no temporary liner is required with new tubbing rings bolted to the ring above, then grout is pumped in behind the tubbing through grout ports to form a bond between the tubbing and ground which holds lining in place via mechanical interlock and shear.
For the ‘bottom up’ method, temporary support is required for the shaft prior to construction of each tubbing ring, with concrete poured behind. Note, if water tightness is required, access to the back of the liner to hammer in lead caulking is required, this requires stable over excavation.
Benefits:
- Heavy duty liner capable of resisting high pressures.
- Minimal maintenance required.
- Suitable for frozen shafts.
Drawbacks:
- Expensive, but cheaper than composite liners.
- Can reduce sinking rate when compared with other liner options.
Composite Steel and Concrete Lining
Description: Composite steel and concrete linings employ a mass concrete lining sandwiched between two layers of steel plate. The outer steel plate primarily acts as a waterproof membrane, whilst the inner steel plates are required for structural strength. The inner steel plates are under the highest stress and unconfined. Consequently, they are at risk of buckling off the face of the concrete, so they must be tied back to the concrete at regular intervals. Composite linings can be considered fully watertight, provided the outer steel lining is fully welded during construction.
Typical Application: These linings are typically used for large, high tonnage mines where the reduction of the down time of shaft hoisting is a key requirement, diameters are typically less than 10m.
Construction Method: It is only possible to install this composite lining in a ‘bottom up’ fashion, so an additional temporary support system is required to the excavation during sinking. For a fully waterproof lining, full welding of all outer steel plates is required.
Benefits:
- Reduced lining maintenance requirements.
- High strength, stable shaft lining.
- Steel lining provide smooth surface for ventilation.
- Very good resistance to shaft movement due to mining activities.
Drawbacks:
- Very expensive type of shaft lining.
- Very difficult and time-consuming to install.
