Concrete focus: new pierhead landing stages in Liverpool

When Liverpool City Council first mooted new landing facilities for tourist and cruise ships, the idea was to build them in steel. After all, pontoons are something like ships and so floating boxes in steel formed the reference design from Mouchel Parkman.

But Balfour Beatty and its consultant engineer, Gifford, proposed an alternative concrete construction, once they had won the £18.2m design and build contract. A steel bridge will still be used to link to shore, however, to bring luxury coaches alongside the ships.

"The problem was the volatility of the steel price on the world markets at the time of the bid in 2003 and even now," explains Paul Brown, project manager for Balfour Beatty Civil Engineering. "Concrete for the pontoons looked like a more economic alternative." The contract was awarded finally last year.

As it happens, concrete also suited Lloyds of London, which has to certify marine designs of this kind, and especially roll-on roll-off structures. "There are questions of long-term durability for objects immersed in seawater," says project engineer Nick Clarke from Gifford, "and they have a preference for concrete."

So concrete it was agreed to be, at least for the four main pontoons that will make up the landing stage on the new pier head. It will make little difference to the hoped-for thousands of Americans and other tourists who will moor alongside in their cruise ships for disembarkation, especially as the pontoon superstructure, comprising of reception and pilot's buildings, will continue to be in steel and glass. Their eyes will almost certainly be on the huge Liver building and its famous bird statues above. The new facility sits right underneath the river frontage of the building and its two neighbours making up "The Three Graces".

Just upstream is the pierhead for the famous "ferry 'cross the Mersey" and alongside is the landing stage for the Isle of Man Steam Packet Company. "It is an iconic location," says John Williams for client 2020 Liverpool, the joint venture of the City Council with Mouchel Parkman that is overseeing the city's long-term development and regeneration.

Surges and floods

The new facility will extend the existing landing stage by 258m. It comprises four 64m-long pontoons, which will float up and down on the enormous tides in the Mersey estuary. One of the largest tidal ranges in the world, it means accommodating a 13m variation in tidal depth, once surges and flood effects are added in.

The pontoons will be restrained by enormous piles, one at each end. A large steel collar projection from the pontoon fits around each of these 2.5m diameter giants, with four bearing faces, to keep them in position as the water rises and falls.

The eight piles socket deep into the Sherwood Sandstone underneath the estuary. Subcontractor Commercial Marine & Piling first of all vibrated steel casings down to the rockhead - "there is just a little silt and some 3m of weathered layers above the firm rock," says Clarke - and then drilled out a 3m-diameter hole 11m down. That left plenty of adjustment as the awkward 98t steel tubes were positioned and the outside annulus was grouted tight in the rock.

Two barges, a jack-up and a spud leg floating barge were used for the work, which meant bringing a special drill to site, hired in from German specialist Bauer, and a large crane to lift the huge tubes. But while this was a significant job, the main work for the project has been creating the four pontoons. Just how to do that was an issue in itself proposals included making them on a shipbuilders' slipway and launching them into the water. "But we could not get a really definite commitment that that would work," says Brown. "The problem is that the loads on the structure could cause distortions when it was jacked up for sliding, and when it hit the water. There were too many uncertainties for our taste," he says. The main option, therefore, was to build them in a drydock. But while the huge port has many drydocks, there are not many big enough for four pontoons, one pair 19m wide and the other pair 26m wide, and with a total length over 260m. One dock that was located was busy and could not clear the minimum 10 months required.

There was one big dock empty, however: Canada Drydock, which was used, among other things, for work on the Lusitania. But it had not been used recently and owner Mersey Docks & Harbours said it would have to be taken "as seen". Inspection showed that the steel gates were still intact, although leaking, and that it should be usable. "The first challenge was to close the dock gates," says Brown. Divers were used to inspect them and make repairs.

The next issue was the pumps. The dock's own system was defunct and therefore some dozen big external pumps had to be brought in both to dewater the dock and to maintain it empty afterwards for the works. "There were moments when as much seemed to be coming in as we were pumping out," he recalls.

The client had warned them of an unusual step in the floor running the length of the dock, and there was also the keel block to deal with. Both were buried under 900mm of silt that needed cleaning out. Then a layer of 6F2 fill was used to build up the floor to a level and above that a concrete construction floor was put down.

"That was a no-fines layer," says Brown, "which meant it was porous and would allow the water through when it came to floating off the structures, hopefully preventing them from sticking."

The structures are relatively straightforward, he says. Essentially each pontoon is a series of cells up to 4.7m deep, made up from a 9m by 9m grid over a base slab and with a soffit above. Not every single cell wall is built up some had just a ground beam along the side.

"The structure has an outer wall and then varying sized cells inside with the main internal structure forming an inner wall," says Clarke. The idea is that if there should be a ship collision or other impact and the walls are breached, there remains an inner watertight section that would help prevent sinking. The complicated aspect is that much of the structure is thin, especially the walls, to keep the weight down for buoyancy purposes. The base slab is 200mm thick, poured in sections and the walls 230mm thick, with a top soffit of 250mm.

That means reinforcement must be heavy. "There are three layers of up to 32mm bar in the walls, which required some juggling to place properly," says Brown. A self-compacting concrete was used for the walls, which helped enormously, says Brown. It also saved time and meant work crews could be reduced because there was no need for vibration.

Cost saving

The cost saving from using fewer man hours and a reduction of up to three weeks on the schedule was enough to cover the expense of the additives, says Brown. "But only because we have our own particular supplier for those." A Fosroc material was used.

Usually a ready mixed company will want to supply its own self-compacting mix, he says, and that can cost up to £20 or more per m3 on the normal price. In Balfour Beatty's case the cost was considerably less, although he is reluctant to say exactly what. Tarmac was the supplier from a batching plant just 1.5km away.

Concrete for the base and the top slab was a normal mix placed by pumping. For each pontoon these were done in quarters or thirds, with around 140m3 being the maximum. Walls were done with skips, he says, mainly because subcontractor Gormley Construction had some spare capacity on the two crawler cranes used to lift formwork and equipment 10m down into the dock.

Formwork was partly proprietary from RMD Kwikform, but also involved the use of a fair amount of timber. The timber could be broken down into relatively small pieces after it was struck, says construction superintendent Jez Arthur, necessary in order to bring it out through the small manholes left in the top of the pontoons, once the slab was complete. These were just 600mm by 900mm.

"People might ask why we didn't use permanent formwork," says Brown. "We looked at it, but it added additional weight and you have to do everything to maintain maximum buoyancy."

The float out of the finished units in mid-June went well and they were brought round to a nearby quayside for fitting out. Railings, deck buildings, glazing and M&E is required to be mostly complete before they are tugged further up the estuary, to sit in their final location. Huge composite material fender units are also being fitted on the sides of the structure.

Once in place, there is further work necessary. The collars have to be fitted around the piles. The piles in the meantime have been fitted with the concrete exterior that forms four bearing faces the design of these was left until the piles were in place to allow for final adjustments to the "as built" positions of the piles.

The four pontoons are to be linked together as well, with structural connection flanges that lock them vertically so that they rise and fall as a single unit. These are steel projections the width of each unit, which fit into "female" slots in the pontoon ends. The steel projections are sitting loose in the slots at present and will be jacked into position and bolted tight once the units are on site.

There is other work to be done. A steel bridge unit has to be added to carry cars and coaches on and off the jetty. This is fairly big in itself, some 80m long and weighing 400t. It has been fabricated in Italy by Cimolai, which also made the big steel tubular piles, and is being delivered at present in four main pieces. These will be delivered from elsewhere in the port and will then be assembled on shore. Multiple axle heavy loaders will "walk" the bridge to its final position with a small floating crane and another on the pontoons, to lift it in.

The bridge connects to the existing pier head pontoons because of the position of the roadways onshore and will connect to the new section by a steel bridging unit that links the old and new. It will have its own steel flotation tank at the end, 10m by 10m and 6m deep, which will take its deadload and therefore avoid adding to the weight on the old jetty.

An emergency pedestrian escape bridge at the other end will complete the connections. This will also have service links for water, electricity and so forth.

The facility is to be complete by early autumn in time for the 40th anniversary celebrations of the QE2. From then onwards, Liverpool City hopes the new facility will boost visitor numbers, and their spending dollars, into the city, and the region around.

Finding a drydock big enough to build the four pontoons was tricky, but eventually the Canada drydock was adapted for the construction.