Mining: Meshblock Seals Under Highly Convergent Conditions

Meshblock Concrete Seals Under Highly Convergent Conditions

Location: Queensland, Australia

The NUGX longwall blocks were started in 2004 and the coal seam in this area was located in a bowl with developing workings sometimes being flooded from above during Summer monsoonal rain events. There was a genesis of ventilation seal development with the mine sealing initially with polyurethane and aggregate core seals followed by Minova in 2009 with FB200 high yield grout plugs and finally in more demanding strata conditions Meshblock high strength concrete seals. Panels P12 to P08 were extracted down the dip due to constraints on development drivage with P08 being extracted in 2016. In the lower sections where water pooled 30 metre water capacity concrete bulkheads were constructed with 50 psi, 10 metre bulkheads required in P12. Floor heave had started in P10.

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Site

Newlands Underground Extension, a longwall mine located in the northern Bowen Basin and part of the Glencore Group.

The Situation

The NUGX longwall blocks were started in 2004 and the coal seam in this area was located in a bowl with developing workings sometimes being flooded from above during Summer monsoonal rain events. There was a genesis of ventilation seal development with the mine sealing initially with polyurethane and aggregate core seals followed by Minova in 2009 with FB200 high yield grout plugs and finally in more demanding strata conditions Meshblock high strength concrete seals. Panels P12 to P08 were extracted down the dip due to constraints on development drivage with P08 being extracted in 2016. In the lower sections where water pooled 30 metre water capacity concrete bulkheads were constructed with 50 psi, 10 metre bulkheads required in P12. Floor heave had started in P10.

When P9 longwall was in line with P10 installation road the stress notch formed and second abutment accelerated floor heave. 600 mm Meshblock high strength concrete seals examined in MG9 were experiencing 2 metres of heave in a 4 metre height roadway.

Geotechnical Setting

The seals examined in P9 had a cover depth of 350-370 metres and a strong face cleat orientation (E-W) is parellel and poorly aligned to the cut-throughs with blocks P8-P12 in a syncline.  Maximum horizontal stress (Figure 2) is also typically aligned between 15 and 30° and perpendicular to cut-throughs.

In this southern area the roof was of reasonable quality with the Upper Newlands seam (UNS) uniaxial compressive strength in the range 20-25 MPa and typically 6 metres thickness.  Seam grades within the western limb examined were 10-12%.

In P9 the Lower Newlands Seam and UNS had 1.5 metres of carbonaceous and silty interburden associated with softer floor conditons. When a floor strength index (FSI) was less than 2.0, heave was experienced.  These areas had an FSI of 1.0-1.5  and basically the conditions experienced by the seals were a result of a foundation failure causing pillar punching and flow of floor material around the seals.

Solution

Previous seal designs had experience damage due to high stresses and floor water inflows. It was decided that the 50 psi Meshblock concrete seal design would be installed in the gateroads.

600 mm thickness seals are constructed from S50 grade concrete that is typically 40 MPa in 7 days and 60 MPa at 28 days, significantly stronger than the floor (<10 MPa) , coal (22 MPa) and similar to the immediate roof Siltstone/Sandstone.  The MeshblockTM formwork method is illustrated in Figure 4 below.

The seal is poured using surface batched concrete which is quality control tested with prepared cylinder moulds. Concrete is designed to be pumped and sprayed and can be put to sleep if necessary and reactivated.  The concrete is pumped into the formwork using an air-powered S4200 Jacon concrete pump after being transported in 2 cubic metre QDS kibbles from the surface.

Results

Working with the customer, Minova was able to provide a ventilation seal system that was “fit for purpose” to successfully seal the longwall blocks under highly convergent conditions due to floor heave.  The core of these seals was of sufficient strength to resist the stresses from longwall induced abutment loads and foundation failure without suffering damage that enabled the passage of air.