Pont Cwm Pydew Landslide |
April 2009, |
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Client: |
Cyngor Sir Ddinbych / Denbighshire
County Council |
Contractor: |
Colin Jones (Rock Engineering)
Ltd |
Design
Engineers: |
Groundsolve Ltd.
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Description of
Project: |
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Following a landslide in
2001, the B4391 road to the south of Bala was at risk of being
undermined by future failures. The road cuts though a steep valley
side, and is supported locally by masonry retaining walls. Downslope of
the landslide there is a hairpin bend, under which a culverted mountain
stream flows. The bend collected water and was hazardous in freezing
conditions.
In 2008 GroundSolve Ltd. were appointed to carry out an investigation
of the landslide and design a cost effective, long term solution.
Rotary percussive probing was carried out using rope access techniques
to define the rockhead profile along the failure. A soil nailed
solution was then designed, with an anchored reinforced concrete beam
at the crest of the slope to provide further protection to the
carriageway. In addition to the reinstatement works on the landslide,
the following improvements were also incorporated into the works to
minimise future maintenance and repairs:
- Reinforcement of failing masonry retaining walls
withanchored, masonry clad reinforced concrete buttresses.
- Formation of a pre-cast drainage channel to collect
groundwater which discharged from the upslope heathland;
- Repairs to the existing culvert headwalls Site works
commenced in April 2009, and were completed on schedule in August 2009.
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Value Engineering |
GroundSolve Ltd.
were responsible for the project management and site supervision of the
works. The supervision was carried out on a visiting basis to minimise
costs. However, regular site visits and constant updating of the ground
model and measure allowed the works to be optimised and enabled
additional works to be incorporated into the works while staying on
budget and on time. The following additional works were carried out
within the original budget for the works:
- Increase in soil nailed area by 80% to cover recent
landslide;
- Increase length of anchored beam at crest of slope by
50% to cover recent landslide;
- Replacement of soft verges with aggregate;
- Formation of an earth safety bund;
- Installation of drainage gullies to remove standing
water from carriageway.
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Environmental and Sustainability Issues |
The site was
bounded by a SSSI, and CCW permission was required for aspects of the
works. Due to the sensitive environmental nature of the site the
following measures were implemented:
- The concrete buttresses were clad with local masonry,
recovered by the contractor from a disused quarry in the forest
downstream of the site;
- Excavated soil from the landslide was used to
reinstate the drainage works, ensuring no foreign seeds were introduced
to the SSSI;
- Excess arisings from the works were used to landscape
a local disused quarry in the ownership of CCW;
- Vegetation on the landslide was left in place to
encourage rapid growth that will rapidly camouflage the rock mesh and
base plates for the soil nails.
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