Water control through bulkhead design

Water control remains a fundamentally important aspect of underground coal mine design and operation worldwide.

Flooding of mine workings can put workers at risk with loss of infrastructure and assets.

Flooding can also cause deterioration of roadways, as was evidenced at the Broadmeadow mine in 2008 soon after the water receded due to a sudden reduction or pore water pressure, mobilisation of joints/cleats and swelling of clay layers in coal measures.

Surface flooding due to large rain events can also have an impact on underground operations if there is limited surface storage.

Many types of water holding structures have been constructed in mines, however they fall mainly into two types: bulkheads or plates and plugs which could collectively be described as barriers.

These seals are often required to impound large volumes of water to control the hazard.

The designs of these water retaining structures can be summarised as:

  • Shotcrete bulkheads or slabs (notched or keyed)
  • Plug type structures using a high yield grout such as FB200
  • Polyurethane and aggregate core bulkheads using dry stack concrete block containment walls.
  • Plugs for surface portal sealing using flyash/cement blends
  • Gypsum based plaster (water resistant) bulkheads using a cement such as Sprayplast UW

Bulkhead effectiveness is impacted by the properties and condition of the surrounding strata. Bulkheads can fail if the material that they are anchored into or keyed into is not strong enough to resist the applied pressure and pressure from water seeping around the structure.

When failure has occurred, leakage in bulkheads has generally been through the surrounding strata or along the strata/bulkhead interface.

Preferred practice is to construct bulkheads within a roadway, which will not be affected by changes in vertical stress. However, this is not always possible when sealing longwall gateroads where chain pillars experience increased vertical load from abutments causing further breakage and dilation of surrounding strata.

Sealing of bulkhead sites can be undertaken with cement-based grouts and two-component organic resins such as Polyurethane (PUR), which are used to treat finely fissured rock masses.

PUR acts like a glue resisting any further strata movement whereas injected cements are more of a gap filler used in bulk injection.

In more permeable strata, a useful strategy is to inject a curtain around the immediate roadway with PUR, and then drill through and inject behind this liner with lower cost cement-based grouts.

As PUR sets rapidly, the pressure in the grouted holes does not have to be maintained by stopcocks/valves in the holes as is required with slow setting cement based grouts.

The primary objective of grouting activity is to decrease the flow of water and gases through the excavation damage zone once the bulkheads are in place.

Pre-injection is going to be more effective and far less costly because the joints and fractures are being sealed before being dilated by water pressure, which also increased the potential for grout washout.

For more than 20 years, Minova has manufactured a range of cementitious and steel fabricated systems that are used in the design and construction of underground mine ventilation structures and bulkheads.

The company offers vast experience and capability in engineering, design, product manufacture, fabrication and contract installation of VCDs.

Minova has successfully developed and implemented many specific devices for individual customer requirements.

Visit https://www.minovaglobal.com/apac/products/ventilation-control-devices/ to learn more and download its technical papers.

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