Southsea Coastal Defence Scheme -Frontage 1
With a value worth around £180m, the Southsea Coastal Scheme is the UK’s largest local authority-led coastal defence project, stretching for 4.5km from Old Portsmouth to Eastney. The scheme will help to reduce the risk of flooding to more than 10,000 homes and 700 businesses.
The project, split into six sections known as frontages, is phased in such a way to minimise its impact on the public, with the most challenging sections being tackled first.
Following emergency repairs, completed by the team in December 2019, VolkerGround Engineering was awarded a project by VSBW, a joint venture between VolkerStevin and Boskalis Westminster, to construct over 10,000m2 of steel sheet piled flood defence walls. This was part of the wider coastal defence project, replacing various existing flood defences dating back to the 1700s; the existing dock walls were commissioned by Henry VIII!
In November 2020, at the height of the Covid-19 pandemic, the team mobilised VolkerGround Engineering’s StillWorker silent press to begin piling along frontage 1, which covers a 615m section from Long Curtain Moat to Clarence Pier. This was a challenging section with various heritage considerations such as piling close to the spur redoubt, Kings Bastion, Clarence Pier and previously unchartered military defences. The original design called for pre-augering and silent piling but due to the unforeseen changing ground conditions and obstructions, an alternative solution was needed to achieve design levels. VolkerGround Engineering worked with VolkerStevin and Historic England, to monitor vibration levels and trialled its ABI TM22 leader rig. As the levels were within advised limits of under 5mm/s, use of the leader rig was permitted.
Work on frontage 1 was broken into two parts. The new sea wall for the western end required two parallel walls, seven metres apart. Whilst the historic military nature of the section was already known, more historic walls were found during the work, as such the pile line was adjusted to avoid the historic structures.
The more straight forward eastern end comprised of a single steel sheet piled wall.
VolkerGround Engineering planned its programme of works carefully, avoiding high tide conditions to ensure the safety of operatives. The team overcame a number of challenges including, extreme high winds, high tides that breached the existing sea defences, unexpected obstructions and Covid-19 pandemic restrictions. The team completed the programme of works to the full satisfaction of the client.
