Aspec Engineering helped design the shaft liner and supporting structures at an underground coal mine for Redpath Australia. Grace Go and Richard Morgan explain how the company tackled the design and installation process.
Redpath Australia, a mining company based in Eagle Farm, Queensland, was contracted to a mine owner to build a shaft at an underground coal mine. Aspec Engineering was contracted to aid in the design and installation of the shaft, which would channel fresh air down to a depth of 318 metres.
The Redpath Downcast Vent Shaft Project involved sinking a shaft into an active underground coal mine. Aspec designed the concrete slab and piles required to be placed on site to support the Redbore 90 raise drill machine so that the 5.3-metre-diameter raise bore hole could be reamed through from underground.
After drilling and reaming out the shaft excavation, a steel jacking frame was installed on top of the concrete pad to jack down the steel liner in 100 segments, each of 3.2 metres in height. The segments were bolted and formed a continuous liner. The pressure of groundwater to the liner was relieved by having 16 drain holes in each segment. Geofabric was installed at the location of the drain holes to collect and direct the groundwater to the holes. The gap between the steel liner and the surrounding earth wall of approximately 150 millimetres was filled with cementitious grout and no fines concrete at designated sections to position the liner in a stable manner and to drain the groundwater to prevent hydrostatic pressure forming on the liner.
Raisebore support concrete pad and piles
The concrete pad and piles were designed to accommodate the raise bore machine, following the parameters in the Redpath technical specification.
To ensure that any water on the ground surface surrounding the shaft would freely drain away and not soak into the ground around the shaft collar, the top of concrete for the shaft collar slab was to be at RL 345.5 metres, which is one metre above natural ground level.
The pile reinforcement and the connection to the shaft collar had to be capable of transferring the maximum vertical thrust and torque generated by the Redbore 90 raise borer to the piles without exceeding the design vertical load and horizontal shear stress limits of the pile to collar connection and the piles.
This first phase of work consisted of piling (22 x 25 metres each at 1050 millimetres in diameter) to a depth of 25 metres to provide a support for the 5.3-metre-diameter raise bore hole to be pulled through from underground at a depth of 318 metres. The centre of piles was excavated to four metres from surface (RL 339.5) to enable 40MPa concrete ring beam to be constructed to support the piles midway between the collar and the low strength sandstone at 8.5 metres.
Shaft lining design
The shaft liner was to provide continuous support to the shaft walls over the full length of the shaft. The following requirements were to be met:
- The shaft liner needed to be permeable (nonhydrostatic) as required to prevent ground water accumulating behind the lining and imposing hydrostatic loads on the lining in excess of its load bearing capacity
- The shaft liner needed to have a design working life of 25 years.
- The shaft liner needed to be backfilled with cementitious grout or concrete with a 28-day compressive strength of 40 MPa
- The steel cylinder needed to be strong enough to withstand the maximum hydrostatic load that could develop at the base of any grout or concrete column if water was to penetrate down behind the liner between the steel cylinder and the grout or concrete fill.
A single ring of the shaft liner is 5 metres in diameter and is 3.2 metres long. Therefore, 100 rings were installed to cover the full length of 318 metres.
Each ring was fabricated in four identical quadrant segments for maximum stacking density for transport. Rings were assembled on site by bolting. Each alternate ring was initially to be fitted with eight drain points at the lower section of the liner to limit hydrostatic loading from ground water.
Hydrostatic pressure build-up due to groundwater was managed by provision of a geofabric drainage product. This was detailed for all liner segments with drain holes (16 in total) backed by strip drains, including drain holes in the circumferential stiffeners and segment flanges.
The grout mix was to meet the strength requirement of 40MPa and fill the 150 millimetre gap between the liner and the surrounding rock wall. The mix consisted of water, sand and cement with inclusion of a superplasticiser. The conflicting requirements were that it was important to ensure the that both strength and flowability could be achieved without generating excessive heat. The sand content was therefore maximised to minimise heat generation.
To achieve 40MPa, the grout mix needed to be 1:1 sand (dry) to cement by volume with a water-cement ratio of 0.48 by weight of cement. A superplasticizer admixture such as Sika ViscoCrete PC HRF-2 was required to achieve this water-cement ratio. Aspec recommended that trial mixes be prepared and tested to optimise the grout mix before use in the project.
Grout strength of 40 MPa was achieved at 7-day based on tests by Redpath. Less water was used originally but later increased to improve flowability. The following grout mix design was employed on site.
Per cubic metre of grout;
1440kg Cement Per m3 x 0.65 m3 = 936 kg
1500kg Sand Per m3 X 0.58 m3 = 870 kg
Water = 320 L
Plasticiser = 8 L
Jacking System
The jacking system concept used four jacks to lower the shaft liner segments into the raise bore hole. Aspec finalised this concept and provided detailed design drawings and specifications for fabrication.
The full length of liners was supported by the jacks / chair pins (around 320 metres long and 600 tonne mass). The continuous length of steel liners was created from the top of the bore, by adding liners one by one on top of each other as they are lowered. Each liner was supported by the liner above it until the length of liners contacted the bottom.