Kishorn Dock, Wester Ross, Kishorn Dock, Wester Ross, ScotlandScotland

Presented by Mark Day & Richard CollumbinePresented by Mark Day & Richard Collumbine

Geological ContextGeological Context Torridonian sedimentary Torridonian sedimentary

rocksrocks 20 – 170 MNm20 – 170 MNm-2-2

Arkose bands (gritstone Arkose bands (gritstone group) unalteredgroup) unaltered

Shaly layers contain Shaly layers contain chlorite produced by chlorite produced by thermal metamorphismthermal metamorphism

Geological ContextGeological Context

Discontinuity dataDiscontinuity data

1. Bedding planes (131. Bedding planes (13°° – 21 – 21° SW° SW))- Spacing variable- Spacing variable- Tight discontinuity, rough irregular surface/no water - Tight discontinuity, rough irregular surface/no water

flowflow2. Set of parallel faults (602. Set of parallel faults (60°° – 80 – 80°° NW) NW)

- Spaced at 18m- Spaced at 18m- Open discontinuity 10mm, smooth with - Open discontinuity 10mm, smooth with

slickenside's/no water flowslickenside's/no water flow3. Joint set discontinuities (403. Joint set discontinuities (40°° S) S)

- Spaced at 18m- Spaced at 18m- Open discontinuity, smooth surface/no water flow- Open discontinuity, smooth surface/no water flow

Plan view of Kishorn DockPlan view of Kishorn Dock

ProblemProblem

Rock mass inhomogeneousRock mass inhomogeneous Potentially unstable in vertical cutsPotentially unstable in vertical cuts Awkward to excavateAwkward to excavate Difficult to blast clean angled benches on Difficult to blast clean angled benches on

excavated faceexcavated face

ProblemProblem

When blasting took place When blasting took place • Nearest fault plane to face openedNearest fault plane to face opened• Wedge of rock between plane and faceWedge of rock between plane and face• SpallingSpalling• Dock face left with dangerous overhangDock face left with dangerous overhang

Cross Section BlastingCross Section Blasting

Cross Section BlastiingCross Section Blastiing

Cross Section BlastingCross Section Blasting

Question raised: what angle Question raised: what angle of slope was needed to of slope was needed to

make it safe?make it safe?

Possible SolutionPossible Solution

• A slope at right angles to the angle of dip A slope at right angles to the angle of dip of the reverse faults (~30of the reverse faults (~30°) °)

• Angle would fit well with friction angle of Angle would fit well with friction angle of sandstone (25° - 35°)sandstone (25° - 35°)

- not feasible because of cost- not feasible because of cost

- insufficient free ground at end of - insufficient free ground at end of excavationexcavation

Actual SolutionActual Solution

Slope angle of main bench reduced from vertical to Slope angle of main bench reduced from vertical to 7575°°

Chain mesh was hung on face to minimise risk from Chain mesh was hung on face to minimise risk from rock fallrock fall

Solution proved to be adequate and in the first 2 year Solution proved to be adequate and in the first 2 year period no rock fellperiod no rock fell

THE ENDTHE END