SA1 Swansea Waterfront site report

With 360 manhole rings to be lifted into position, Morrison needed to explore ways of raising the efficiency of the procedure. The contractor opted for a manhole ring lifter launched last summer by Probst. The device is lowered into the ring by the excavator driver and then, through the movement of a lever, expands to grip the inside of the ring and affect the lift. Traditionally, manhole rings are lifted from the outside using a combination of pins inserted into the ring and hooking chains.

Morrison Construction, part of Galliford Try, has brought its water frameworks experience to bear on a civils project at Swansea Docks, using novel techniques resulting in budget and programme benefits.

The Swansea-based contractor is involved on behalf of the Welsh Assembly government in developing part of the infrastructure for the regionally significant SA1 Swansea Waterfront site for mixed-use redevelopment. An element of the programme was to build a new retaining wall around an area of the former docks, including a slipway to create a development platform above the 1-in-1,000 years flood level in readiness for the proposed construction of a luxury five-star hotel.

"At the moment there's no confirmed time frame for the proposed hotel development," says Morrison project manager Mark Tyrell. "As a result, we've had to make the wall robust enough to withstand a severe exposed marine environment, but sufficiently flexible to allow its easy removal should such future development plans require it."

The options

The most obvious approach would have been to build a conventional sheet piled wall or modular reinforced concrete structure, but this was ruled out partly as the piles would need to be designed to meet the, as yet unknown, requirements of the hotel developer, potentially including under-croft car parking. The use of piles long enough to enable a future car park to be built would have resulted in potentially abortive expense to the client, with no guarantee that the piles would be used in the final design.

Tyrell adds: "The other option would have been to build a reinforced concrete wall, but this again may have compromised the permanent form of the future planned hotel development by dictating the line of the wall and making it harder for a subsequent developer to adapt.

"In the end, we hit on the idea of using manhole rings - which, after all, are essentially small caisson units and, as a result, provide greater flexibility. If the hotel developer doesn't need an under-croft car park, then it can leave the manhole rings as they are and fill in the area that they enclose with a concrete slab as a base for the building. But if the developer needs extra depth below the main structure, then the manhole rings are relatively straight-forward to remove," says Tyrell.

Morrison is a regular user of manhole rings under its long-standing framework to carry out maintenance and capital works for Dwr Cymru Welsh Water.

The rings

A total of 360 rings of 1,050mm in diameter were placed, two or three on top of each other to suit existing ground elevation and founded at a depth of one metre, to form a retaining wall 154m long. The rings were filled with compacted material - including recycled crushed concrete from the old dock - and topped with dressed coping stones salvaged from the old dock walls. Other site-won concrete block stones salvaged from a decommissioned lock were placed for added sea defence protection in front of the manhole ring wall at the top of the former slipway, building up the profile of the new revetment. The slope was then dressed with 350mm to 750mm imported stone rip-rap.

Sustainability has been a watchword of the SA1 project in many respects. For aesthetic purposes, the manhole rings have been faced with plastic sheet piles made from recycled PVC windows by HL Plastics. "They are cost effective, yet still provide the durability that's required until further development starts, and match the finishes used elsewhere within SA1," explains Tyrell.

Morrison has been reappointed by the Welsh Assembly to construct other elements of the SA1 project that will reshape the land profile at the south-west corner of the Prince of Wales Dock and prepare it for its final redevelopment phase.

Part of an abandoned in-filled dry dock is being excavated to create the main channel into the Prince of Wales Dock, which is to be redeveloped as a five-star marina next year. Meanwhile, the former lock channel entrance to the original port of Swansea is being excavated and blocked off at the northern end to create an artificial water feature as part of planned environmental mitigation features within SA1.

Dredging

Morrison has already completed revetment works on the western and eastern banks of the former tidal basin leading into the two channels, as well as dredged the basin and approach channel as part of the advance works for the marina and lock development. Some 90,000m3 of marine silt has been dredged under Department for Environment, Food and Rural Affairs licence to reduce the silt level within the basin by a metre and up to 6m in the approach channel.

Under the latest contract, Morrison is working with engineer White Young Green to investigate means of stabilising the residual silt via either a wick-drained system with stone or sand columns, or cement stabilisation. Eventually, the basin will be enclosed to the south by a new lock. Morrison's task here is to create the land platform for the lock with 75,000m3 of site-won fill.

"We're building the platform in four layers," explains Tyrell. "We're forming the working platform immediately above the soft compressible marine silts with approximately 14,000m3 of imported clean granular material to allow access for wick-drain installation plant. Further layers consist of 34,000m3 of site-won material from plots within SA1. Finally, we're placing 23,000m3 of material excavated from the old dry dock and 6,000m3 excavated from the former lock channel."

Enhanced mooring facilities within the tidal basin will require construction of locally steepened (1:1) slopes. Again, the contractor has come up with an innovative solution, using concrete-filled Neoweb-specialised geofabric, which was developed for desert highway construction in the first Gulf War.

It has all made for a very busy winter and spring of earthmoving, with the carrot of possible further work in the next phase of constructing the lock structure itself.

Project: SA1 Swansea Waterfront infrastructure works

Client: Welsh Assembly government

Value: £2m

Civils contractor: Morrison Construction

Consulting engineer: White Young Green

Schedule: began in March 2006, completed June 2007

Principal subcontractor to date: UK Dredging