Excavating in loose sand, under a tidal river, and out to sea would be expected to present a number of known tunnelling and shaft-sinking problems, but the Costain–Maunsell design-and-build joint venture contracted to carry out this work on the Great Yarmouth Power Project were faced with both easier and more difficult ground than expected.

Working for main project contractor Bechtel, Costain was contracted to sink two shafts and drive two tunnels for the project; one for a high-voltage power cable duct and the other for a cooling water outfall into the North Sea.

After commencing work they won additional work for a short third drive, carried by pipe jacking, as a cooling water intake from the River Yare.

The site for the gas-fired power station is situated on a narrow spit of beach sand between the North Sea to the east, and the south-flowing River Yare to the west. It is being constructed on the site of an old power station, which also had cooling water outfall, the end of which is much nearer the beach than the new one.

Cable passage

Underground excavations for the HT cable passage comprise a 9m id (10.5m od) shaft formed by wet caisson sinking, and a 2.07m id segment-lined tunnel 150m long. Tunnelling was chosen as a convenient way of crossing the river and also because Norfolk has a planning restriction on future overhead power cable pylons.

Costain bored the tunnel on a slight upgrade using a Lovat M100 earth-pressure balance TBM named ‘Victoria’. This was part of Costain’s own fleet and was modified and refurbished by Specialist Plant Associates for the project. On the west bank of the River Yare Costain sunk a 7m id shaft to receive and remove the TBM.

Costain-Maunsell chose wet caisson sinking for the shaft construction as the water table is only 2.5m down, and it would avoid a large dewatering programme. The procedure, also employed for the seal well shaft (see below) consists of forming a cofferdam to a depth of 8m using sheet piles. The ground inside is then excavated to the water-table. The caisson cutting shoe and first rings are assembled within the cofferdam, with polystyrene sheets put around the caisson to later form an annular gap for a bentonite wall. Next, the shaft collar is cast using plastic-fibre-reinforced concrete, at the same time installing the bentonite and flushing pipes. The bentonite wall is formed when the concrete has cured, and caisson jacking begins. Excavation from within the caisson is by crane grab.

Sinking the launch shaft was one of the major problems faced by Costain-Maunsell to keep on schedule. Costain first employed a set of six hydraulic jacks, each of 50tf, in order to drive the caisson down 26.5m to concrete base slab level. The level was determined by the need to avoid the tunnel disturbing the sheet piles on either bank of the Yare. The caisson, formed of 275mm-thick Charcon rings, was equipped with a steel cutting shoe at the base. Ground was expected to be mainly sand, silt and some clay with no more than loose cementing. However, at a depth of around 9m a major barrier to progress was encountered.

Construction manager Paul Guider explained that as a natural geological obstruction was totally unexpected, Costain enlisted divers to inspect the base of the caisson to check if some man-made obstruction such as harbour structure or a sunken ship had been hit. Nothing was found except hard rock. Samples extracted by breaker showed that the ground was a band of strongly cemented sand, like nodular ironstone. This is known locally as Norfolk Crag and was present at a much higher horizon than anticipated.

Despite adding another four jacks and using the maximum jack force available together with 300t of kentledge, very little progress was made until the Norfolk Crag was broken through with hydraulic breakers.

Groundwater

Costain installed a special composite seal arrangement over a total length of approximately two metres to withstand the expected groundwater pressures as the TBM broke out from the shaft. First, a standard rubber lip seal is encountered in the shaft, followed by a double-lip seal held in a bolted steel ring. Next came two inflatable seals, or different construction, supplied by Associated Polymer Services. The first type is designed to close around the TBM shield and the second to close around the erected segment rings. As an additional precaution, six deep dewatering wells were installed by WJ Groundwater to reduce the water pressure on the tunnel to around one bar.

The entire TBM drive was in the expected, predominantly sandy ground. Only limited conditioning agent was used to improve the properties of the excavated ground at the face, but generally the wet sand performed well in the EPB screw. Tunnel cover under the dredged channel of the Yare is approximately 12m. A segment erector mounted in the TBM tail can was used to build 6-piece rings of Butane trapezoidal segments.

Twelve 132-kV cables, installed by Visser & Smit Hanab, now enter the shaft from a reinstated open-cut trench leading from the power station. In the shaft, during construction, they were protected from accidental damage by retaining the scaffolding structure. At the base of the shaft they turn through another 90o bend to run in the tunnel invert under the River Yare to the reception shaft.

Each of the four land-based shafts in the project is to be capped with 700mm thick concrete slabs pre-cast on site. Due to site space, limitations to slab for the cable shaft had to be cast on nearby Great Yarmouth Port Authority land. A mobile port crane, specially designed for handling heavy containers with relatively long outreach, was borrowed to carry the completed slab to the construction site. This, and other slabs, could then be simply pushed over the completed shaft top.

Wet-caisson sinking to 9m diameter also formed the shaft for the cooling water outfall, known on the project as the seal well. Costain installed 32 caisson rings given a depth of approximately 20m to the base slab.

Jack-up barge

The target for the tunnel drive was an 8m diameter caisson sunk shaft constructed from a jack-up barge by specialist subcontractor Seacore. Originally it was intended that a specially designed reversed circulation drill would carry out most of the excavation once an airlift dredge had removed the seabed deposits. As work progressed, however, it was found that the underlying deposits, again chiefly sands with silt and some clay, could be extracted by the airlift.

The offshore work began by loading the jacking barge with a special 9m high caisson cast off the jetty of the River Yare. This was skid-loaded onto the rear of the barge using lateral rollers and the barge strand-jack system. At 4m in height, this component weighed 320t and was the largest single load handled in the construction of the diffuser shaft. It was designed as a stable platform and lead section to stabilise subsequent caisson jacking. The barge was then floated out to the planned shaft position.

Seacore’s Peter Clutterbuck explained that the barge is first secured with a 4-point mooring system. The jacklegs are then lowered, pre-loaded and secured. Four strand jacks are then employed to lift and position the caisson ring before lowering it to the seabed. One benefit of using the jack-up barge is that it offered a rudimentary steering capability not available to the shore-based operations.

The first caissons and cutting shoe were lubricated with polymer mud, as used in offshore drilling. It was also possible to use jetting to aid the progress of the cutting shoe, but this was seldom required. Further caisson rings were added and pushed down using the strand jacks, with the inner spoil removed by air-lift. In all, three 1m deep caisson rings and five 700mm deep were installed in addition to the large first ring. A final temporary steel section maintained the lip of the shaft above seal level until fitted out. Having got the caisson down to the required level, Seacore watered the shaft and installed a reception plug for the TBM breakthrough.

Each ring is secured to the next with Macalloy bars to ensure a secure structure.

Outfall drive

Another modified and refurbished Lovat EPB TBM, this one a model M131called “Heidi” after the contractor’s site office manager, was launched from the seal well shaft to head out to sea. The 2.87m i.d. drive is formed from 158 rings of Charcon trapezoidal segments, and ends at the target shaft with a cover of 7.5m to the seabed.

Bulkhead air locks were installed in the tunnel 90m in from the well seal shaft, as well as all the necessary surface compressed air working facilities from Specialist Plant. This served as a precaution against undue leakage but the set-up was only used to allow inspection of the TBM cutterhead for wear before breakthrough to the shaft. Tony Ridley Hyperbaric Associates provided consultancy and operators for the compressed air equipment.

Following the installation of the temporary steel ring on the top of the diffuser shaft, the TBM was then driven through the shaft wall through a rubber ring seal. After excavating around the shield machine Seacore lifted it out onto the barge in two sections. Seacore then positioned the 2m diameter grp riser pipe in the shaft, and reflooded it with the bulkheads protecting fitting out work in the seal well shaft.

The seal well features a concrete weir over which excess cooling water spills over to the diffuser tunnel.

The diffuser cap is a specialist designed hexagonal dome with a grp liner from Johnston Pipes. The 150-tonne component was pre-cast on a jetty at the nearby River Yare and was transferred onto the jack-up barge in the same manner as the caisson when the barge retruned to the quay to offload the TBM. The cap has a rubber seal to ensure a perfect fit to the shaft that is essential for adequate operation and security. It carries four ‘spouts’ that face out to sea. Seacore lifted the cap into position using the strand jacks, bolted it in position and grouted the annulus around the riser.

Divers working on the diffuser installation had a substantially limited period, the length of which was dictated by the tidal movements rather than air availability. Despite having anchor points around the diffuser to tether themselves to, such is the force and variability of tidal currents along this coast, and especially near the River Yare, that divers often had only half-hour working periods. Barge operations were also restricted. Lifting and installation activities had to be planned for slack water in the neap tide period, with wave height at a maximum of half a metre. The schedule required working to continue through the winter and sometimes bad weather resulted in work crews having to remain in the barge’s accommodation for up to two days.

The normal working period was a seven-to-seven day shift for shaft sinking and two 12-hour shifts for the tunnelling. The slow progress on cable-shaft sinking also necessitated a 12-hour night shift.

Once the diffuser cap was installed the head ports were plugged and tunnel pumped dry again. At the time of T&T I’ s visit the bulkheads were being removed and tunnel cleaned for operational readiness. All that remained to be done at the diffuser was the setting of riprap scour protection around the outlet.

Costain-Maunsell was awarded the contract for the inflow tunnel after starting work on the main contract. This involves a pipe jack of 62m length using the M100 Lovat TBM. It had originally been a pipe jack machine but was converted to segmental lining operation for the cable tunnel drive. It was then reconverted for pipe jacking by Specialist Plant to install the 2.1m id (nominal) Buchan concrete pipes under the quayside road to the river. The pipe jack work was manned by a Joseph Gallagher crew and completed in six weeks through grey silty sand with some clay. The drive is not yet connected to the river but subcontractor May Gurney has completed the reception pit and riverside intake structure.

Working together

The Great Yarmouth Power Station Project is an example of the integration of underground construction activity into a system utilising many other disciplines and contractors who have all had to work in harmony. Both the natural difficulties of underground and offshore construction, as well as the problems of integrating multiple activities have been overcome. The Costain-Maunsell work was due to be completed last month (July).

Related Files
Sketch plan