Our Client proposed to upgrade the western retaining wall of the Half Mile Pond reservoir at Longleat Safari Park by raising the crest level to account for overtopping in a 1:10,000 year event and to address some deterioration in the existing masonry and sheet pile walls.
The proposed new sheet piled wall would be constructed over a length of approximately 600m to depths of between 3m and 7 m depending on ground conditions. The Half Mile Pond is home to a population of sealions and hippos. Our team was appointed to provide ground investigation services within the reservoir and to undertake the design of the sheet-piled wall to ensure its stability.
Contract Challenges
The reservoir is believed to have been created in 1857 by Capability Brown, when an earthfill embankment was constructed approximately at 5m above natural ground. Our team was initially appointed to assess and investigate a small area of subsidence which had formed behind the existing retaining wall in the embankment supporting the reservoir. The investigation indicated the embankment had locally softened due to water ingress from the reservoir, which increased the need for the new retaining wall to be constructed. Our team then undertook a larger ground investigation along the full length of the western boundary of the Half Mile Pond to inform design parameters for the construction of the new sheet pile wall. The overwater investigation was undertaken using a drilling rig mounted on a platform supported by an excavator located on the crest of the embankment, with minimal disruption to the wildlife in the pond.
Following the completion of the ground investigation works, our geoenvironmental team, in conjunction with the structural engineer for the scheme, undertook a series of geotechnical designs for the proposed sheet pile wall to determine its stability and deflections post-construction accounting for the operation of the miniature railway along the crest of the embankment. Where necessary, the sheet pile wall was supported by ground anchors to minimise post-construction deflections.
Our Solution
Due to our swift response to undertaking the ground investigation, interpretation of the ground investigation and geotechnical design support:
- Areas of soft ground within the embankment were identified for further stabilisation measures,
- The construction of the new sheet pile wall was able to be commenced expediently,
- A reduction in the overall number of ground anchors was possible with associated cost and programme savings
