Project Sheet: Llechwedd Quarry

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Client

  • Greaves Welsh Slate Ltd

Contractor

  • Greaves Welsh Slate

Works Undertaken

  • Site visit
  • Slope stability assessment
  • Recommendations on working methods

Description of Project

Greaves Slate Ltd proposed to re-start working of the Back, Old and New Veins at the old Votty and Maenofferen quarries (part of the Llechwedd quarry). The proposed changes were as follows:

  • Construct an access road at a high level to gain access to an outcrop of the Back Vein
  • Stabilisation of the slope above and below this track (not allowing the slope angle to exceed 34° - the natural angle of repose)

GroundSolve Ltd was required to prepare a geotechnical assessment for Greaves Slate in accordance with the Quarry Regulation 1999. This involved carrying out an inspection of the quarry and giving recommendations to ensure the excavation can be undertaken in a safe manner.

Past underground works on the slate veins have created large voids beneath the proposed workings which can become unstable and collapse. GroundSolve Ltd suggested addressing these risks in the following ways:

  • Accurate surveys of exposed caverns
  • Drilling from the surface to locate the voids before machinery is used

Once the voids were identified, it was recommended that the roof should be blasted to ensure that it collapses making it safe to work above. These voids were then backfilling with slate waste to create a safe and level working platform.

GroundSolve Ltd was required to inspect the quarry every four months in accordance with the Quarry Regulations.

Initially, the potential failure mechanisms needed to be identified to understand the stability of the rock face and the likelihood of further instability. It was noted that there were two rock types in need of consideration – igneous rocks formed by volcanic eruptions and slate bands, originally deposited as a mudstone. Numerous failure mechanisms were recorded.

Following stability assessments recommendations were given on safe working methods of the slate given the numerous restraints.

Old caverns in the Back Vein at Maenofferen quarry
Old caverns in the Back Vein at Maenofferen quarry