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Engineered for Value

Woolwich very nearly didn’t get a Crossrail station but thanks to some careful negotiation and construction sequencing the ground engineering for the new station box is almost complete. Claire Symes reports.

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Value engineering to reduce the cost of delivering Crossrail has meant changes to the design. At Woolwich in south-east London, original plans for a station were almost scrapped with the borough missing out on up to £80M a year of benefits from a Crossrail connection.

But by carrying out a major value engineering exercise of its own, Berkeley Homes has helped to more than halve the cost of the station’s underground box and work on site is now underway.

Berkeley has been developing the former Woolwich Arsenal site since 2000 and has already built 1,500 new homes there, with plans for 2,700 more. Because of this, the station was a key piece of infrastructure for the area.

Berkeley started negotiations to save the station in 2005 but it wasn’t until February last year that an agreement was drawn up to enable the project to go ahead. With less than 30 months before the tunnel boring machines (TBMs) were due to arrive in the box, work has been fast tracked and ground engineering work is now nearing completion.

Timing of the work was critical as delays could impact on the construction of the twin bored tunnels that will be driven from Plumstead to the east, into the station box and then under the Thames. Berkeley has to hand over the site to Crossrail’s tunnelling contractor Hochtief/Murphy in July next year so the housebuilder reviewed the construction sequencing to cut nine months off the programme.

The result is that some of the piling work was carried out by Cementation Skanska ahead of the main works. The rest of the piling for the over site development and diaphragm walling work for the box itself is being carried out by Balfour Beatty Ground Engineering (BBGE), while O’Keefe is moving excavated material, temporary works and reprofiling the site. Byrne Brothers has already started building on top of the diaphragm walls.

“Design of the box started in 2009, essentially at risk until the development was signed”

Berkeley is effectively the design and build contractor for the scheme although it is part funding the work (see box). Waterman has provided design advice, while Berkeley has subcontracted specialist companies for each element of the work.

“Design work on the box started in 2009, essentially at risk until the development agreement was signed,” says Berkeley Homes project director Mike Woolliscroft.

The final design for the box is 18m deep, 26.6m wide eastern and western boxes, with a central 15m deep, 20.4m wide section, that will complete a 256m long station box. Walls are formed by diaphragm walling installed by BBGE while cast insitu floor and roof slabs are formed progressively by Byrne Bros as earthworks contractor O’Keefe excavates the box and places temporary supports.

“The eastern end will be where Crossrail will build the station entrance and the western end will house the plant and equipment for the station,” says Waterman associate director Peter Knight.

The real milestone for the scheme after moving onto site in March 2011, was the start of construction in early 2012. BBGE started work on diaphragm walling in February but before this could get underway the contractor had to carry out grouting work to ensure the diaphragm walling could be carried out.

The geology at the site comprises 3m of made ground and alluvium over around 11m of Thanet Sands with a 1m thick layer of the silty, flinty Bullhead Beds over the Chalk. The design called for the 25m deep diaphragm wall to found in the Chalk but the overlying Bullhead Beds created a high potential for loss of bentonite so grouting was essential to ensure the work could be carried out.

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“We installed over 1,200 grouting treatment holes,” says BBGE engineering manager Mark Pennington. “Grouting for this problem has never been done before so there was no previous design to guide us and it was very much trial and error. Once we had completed a block of treatment, we carried out falling head tests to check the design.”

All seemed to be well but when BBGE excavated the first diaphragm wall panel there was rapid and significant loss of bentonite. The panel was quickly backfilled and further grouting was carried out. So far this additional work appears to have paid dividends as no problems have been experienced.

The panels, which are being excavated by clam shell grabs mounted on Casagrande C600 and B250 rigs, vary in width from 3.1m up to 7.2m and are 1m deep.
After the panels are excavated, reinforcing cages are placed and the bentonite is replaced with C32/40 concrete.

Even this is not simple as the reinforcing cages feature connections for the floor and roof slab, so almost every cage is different.

The other major variation in reinforcement design is located at either end of the box where the TBMs will break through and exit.

Here the steel reinforcing has been replaced with glass fibre reinforcement to create “soft eyes” for the TBM to cut into. In the cage, the glass fibre has the same tensile strength as the steel so has not impacted on the wall construction but its presence will minimise the risk of damaging the TBM cutter heads.

In addition to the reinforced diaphragm wall, BBGE will also construct a twin lean mix guide wall at the eastern end, where the TBMs will be received into the box, and a single lean mix guide wall at the western exit end.

“BBGE has put a lot of effort into creating work zones to separate people and plant”

BBGE is completing around 1,000m3 of diaphragm wall a week with a two-day turnaround between primary and secondary panels. The company’s work doesn’t conclude with the construction of the panel though - BBGE is breaking down the top of the panel to the construction level to help speed up the follow on work by Byrne Bros.

Dewatering is also underway following construction of a slurry cut off wall midway along the box to prevent it impacting on the ongoing diaphragm wall construction.

According to Woolliscroft, flow levels of up to 250l/minute are expected, hence why he is keen to advance the dewatering work. When GE visited site, BBGE had
completed over 50% of the 88 panels and BBGE project manager Donna Fowler expected to be finished in July.

In addition to the CFA piles being installed within the station box that are critical to anchor the box floor slab against buoyancy, BBGE is also installing 327, 750mm diameter piles to the south of the box to support the rest of the over site development.

Most of these are being constructed to depths of between 26m and 31.5m but 24 are being constructed to 33.5m - the deepest CFA piles ever undertaken by BBGE. “The original plan was to use a mix of CFA and bored piles because of some of the high loadings, but given the level of activity on the site, it made more sense to carry out the work using one technique,” says Pennington.

An extra section of auger was attached to the rig on site to achieve the design depths.

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The southern boundary of the over site development is formed by a 260m long contiguous piled wall. Due to the constraints within the site and the tight programme,
construction of part of the wall was let by Berkeley as a separate early package of works. Most of the 298, 750mm diameter piles were constructed by Cementation Skanska to depths of up to 34m late last year with the final 48 piles for the wall, which are being constructed to depths of up to 28m, being built by BBGE under the current phase of work.

When asked why Berkeley chose to work with a different ground engineering contractor for the main works, Woolliscroft says that the decision was partly down to the local experience of BBGE’s staff and its cooperation on planning.

“Some of the BBGE team worked for Amec during construction of the nearby Woolwich DLR station so were familiar with the ground conditions,” he says. “BBGE also helped us to find better ways of carrying out the build, such as scraping the top of the diaphragm wall to avoid the need for breakdown by the groundworks contractor. This reduces the time involved and is also quieter than using breakers.”

Another area where Woolliscroft has been impressed by BBGE’s approach has been in terms of safety. “Some of the main issues on this site are linked to logistics and the number of activities and companies working so closely together,” he says. “BBGE has put a lot of effort into creating work zones to separate people and plant where ever possible.”

Although the end date for Berkely Homes is officially July next year, Woolliscroft is aiming to have the work completed by May 2013. Nonetheless, it will be another four years after that before the expected 8,000 passengers during peak hours will be able to use the new station.

Construction costs

Funding for the scheme came from the savings made for Crossrail in reducing the length of bored tunnels from construction of the station box. The rest was met by Berkeley Homes itself. The total cost of the station box is £70M.

Value engineering has resulted in the cost of the work being more than halved from an initial estimate of £150M.

The savings have been delivered by raising the levels of the tunnels to reduce the depth of the box which also enabled the length of the box to be reduced as the shallower depth reduces the ventilation demands so less space for equipment is needed.

Berkeley also changed its designs for the over site development to reduce the costs. “The original plan was for the flats over the station to be bridged across the box but now the loads are directly transferred onto it,” says Berkeley Homes project director Mike Woolliscroft.

“By the end of 2010 the changes to the superstructure suggested by Waterman enabled us to get the scheme within fundable limits and the development agreement was signed in February 2011.”

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