LOCATION:
The construction of a 750m long new erosion control rock revetment, three new flood bunds and demolition of Great Salterns Quay.
The Milton Common and Great Salterns Quay scheme in Southsea represents Phase 2 of the wider North Portsea Island Coastal Defence Scheme, which covers 8.4km of the Portsea Island coastline and is being delivered in phases over a 7 – 10 year period.
- Construction of a 750m long erosion control rock revetment comprising 6,000 tonnes of rock ranging in size from 60kg to 300kg along the coastal frontage of Milton Common.
- Construction of three flood bunds set back in Milton Common varying in height from 0.2m to 1.5m.
- Demolition and removal of the 50 year old Great Salterns Quay.
- Construction of a sheet piled sea wall.
- Footpath construction and reinstatement and landscaping works comprising the planting of specially selected trees, shrubs and grasses to increase the biodiversity of the area.
Challenges
Significant stretches of the scheme frontage are designated as a SPA, SAC, RAMSAR site and a SSSI.
We had propsed building the haul route along the foreshore, however this had to be reconsidered post contract award due to the presence of protected sea snails.
Material excavated from the revetment didn’t meet the requirements to be spread over mudflats as we had proposed.
Solutions
We constructed a 600m haul route on the flood plains. To reduce costs we agreed with Eastern Solent Coastal Partnership to temporarily dress over the haul route for reuse during Phase 4, saving muck away costs on Phase 2 and import costs on the following phase along with associated plant and labour costs. This realised a cost saving of £240k.
The three new bunds were formed using 11,000m³ of wet, non-hazardous chalk recovered from the demolition of Great Salterns Quay, realising a cost saving of £1m. The process involved spreading the chalk across the site boundary and turning it regularly for drying before it reached the optimum moisture content for compaction.
The 3000m³ of excess non-hazardous material we were unable to spread on the mudflats was reclassified. By segregating the concrete and brick waste on site, the majority of surplus material was reclassified as inert and transferred to a local recycling facility. The remaining surplus material was used to backfill the revetment and raise footpath levels behind it by 100mm, which produced a cost saving of £360k.
Outcome
The project won the London & South East Constructing Excellence Project of the Year Award.
We were subsequently awarded Phase 3 and more recently the final phase in this whole scheme.
