Wiltshire
Longleat Safari Park
The ornamental Half Mile Pond on the Longleat Estate was designed by world-renowned landscape architect Lancelot ‘Capability’ Brown in the mid-1700’s. Its capacity 163,000 cubic metres of water is currently home to exotic wildlife including hippopotamus, sea lions and even a small monkey island.
This certainly wasn't the average job for Evolve with the residents remaining in situ for the duration.
Wear your hard hat. Wear your high vis jacket. Watch out for the hippopotamus!
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Challenge
Despite being called a pond, this particular body of water is officially classified as a reservoir. Following a formal inspection and subsequent report, certain improvements were deemed necessary in order to maintain the safety of the reservoir, both for its exotic inhabitants and for the many visitors taking regular pleasure boat rides to view the animals close up at the famous safari park.
The first recommendation was for a sheet pile wall to be installed along the western embankment to stabilise and strengthen the dam crest. This would also increase its height to prevent widespread and uneven water overtopping during extreme flood events. Existing leakages were to be plugged and sealed along the embankment.
It was also noted that the Estate's popular light gauge railway had been installed too close to the water and would need to be taken up and re-laid.
Of course being a popular tourist destination, the works needed to be completed to a very short programme during the winter 'off-season’ while the park was closed. This would help to minimise the disruption to the light gauge railway operation and negate any ill effects on the general visitor experience to the Longleat Estate.
Solution
Evolve worked closely with geotechnical specialists and undertook site investigation work to understand the ground conditions to ensure the sheet piled wall design would be the most effective solution. Ground anchoring was required at certain points along the proposed wall length to ensure the new structure could withstand the water pressure from the pond. The sheet piled wall was to be located within the pond itself, meaning the existing embankment retaining structure could be left in place, forming part of the long-term waterproofing measure.
The proposed wall tied into two high spots along the 500m run and was also prepared for any future flood protection works.
A mixture of CCTV surveying and historical records were carefully examined, while the ability to model three dimensionally ensured that the appropriate proposed sheet pile lengths were used in the correct locations by the main contractor. Critically, this ensured that an existing brick and pipe culvert was not breached.
The need for addressing the leakages highlighted in the Reservoir Engineers Report became readily apparent as the project progressed, as a significant ground settlement issue occurred where a section of the embankment sank approximately 100mm in the space of a few weeks. This area was treated with 3m deep 'columns' of remedial strengthening material along a 32m stretch prior to the backfilled earth and the railway being re-laid.
Further sections of embankment were found to be very wet during the construction process and additional remedial drainage solutions were installed prior to the track being re-laid to ensure that these areas could sufficiently drain and prevent any ground water build up in future.
Sheet piles with clasp sealant were installed for the new wall via a vibratory device fixed to an excavator which sat on a pontoon within the pond. Great care was taken to ensure the safety of staff working on the water, as well as the animals residing in the pond. The sheet piles were installed with a minimum 500mm embedment within the clay bed level to ensure the designs were watertight. Timber cladding work to dress the sheet piles was scheduled for a later date to give the finished product a more natural 'safari' feel.
To ensure the scheme could take place, a section of the light gauge railway was lifted prior to any work commencing to allow clear access to the embankment, with the aid of a specialist railway contractor. Evolve then provided setting out coordinates and levels for the new track position to ensure its re-laid position met the minimum safe distance from water requirements. A new track high point occurred where the sheet piling wall branched inland to meet the topographical profile so coordination with the specialist railway contractor was critical to ensure that British Rail standards were adhered to.
In addition to the original scope of works, the eastern 'Hippo Field' harbour side was similarly strengthened to ensure the Longleat boats could continue to be safely lifted out of the water for maintenance and repairs.
