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Diaphragm walls: Diminutive diameters

grab 2

Work on what may be the UK’s smallest diameter diaphragm wall shaft has just been completed next to one of the deepest diaphragm wall shafts recently constructed in London.

While much media focus is on the large tunnel boring machines(TBMs) starting to arrive in central London for the main Thames Tideway tunnels, to the east a smaller – but still critical – tunnel is under construction.

The shafts required to launch and receive the TBM undertaking the Thames Tunnel Modification Works tunnel scheme may be smaller in diameter that those further west but the technical challenge is just as great.

Work on the new 850m long tunnel is about to start (see below). It will connect the Beckton sewage treatment works – Europe’s largest – to the existing Lee Tunnel and ultimately to the Thames Tideway Tunnel. The overall scheme is being delivered by Thames Water’s AMP6 alliance partner Eight20 – a joint venture between Costain, Atkins and Black & Veatch –with specialist input from Spie Fondations and Joseph Gallagher for the shaft construction.

“The new tunnel will play a crucial role in relieving the future ThamesTideway Tunnel of sewage and rainwater at times of extreme high flows, preventing damage to the environment,” says Eight20 senior project manager Mark Morrison.

The 9m diameter inlet shaft at the northern end, is where the TBM is being launched. It was constructed using the wet caisson method.

Contractor Gallagher undertook the task using precast concrete rings sunk into position as excavation progressed thanks to predominantly clay ground conditions at that location. Work on the launch shaft started in January and was completed in October 2017.

grab 4

Tideway Beckton

The shaft is formed by five panels

However, the reception shaft close to the 20m diameter, 74.5m deep LeeTunnel shaft required a more technical solution because ground conditions were more complex. Spie has used diaphragm wall methods to construct the 8m diameter shaft through made ground with significant obstructions, alluvium, River Terrace Gravels, the Upnor Formation of the Lambeth Group, the Thanet Sands and Chalk.

Work on this structure started in July and was on target to be completed in early December 2017.“It is the smallest diameter diaphragm wall that Spie has undertaken and I think it may be the smallest in the UK too,” says Spie UK business development manager Adrian Mercer.

The size of the shaft was not the only challenge – around a third of itis located on top of backfilled Victorian sludge beds. The brick-built structures extended to 12m below ground level and had been backfilled with foamed concrete.

Spie developed a 1.6m diameter secant coring approach to effectively pre-bore the shaft. Casing was used to ensure the verticality of the bore through the variable structure and the cores were backfilled with a very weak grout

“We cored down to the River Terrace Gravels,” says Mercer. “That operation went well, but it took slightly longer than anticipated.

“The original design for the shaft would have been wholly over the sludge beds but the location was moved to reduce the coring programme and save time.”

In total, 14 cores were bored through the structure. With that work completed in September, the guide wall put in for the secant coring was removed and the guide wall for the diaphragm wall was constructed by L&A.

“The diaphragm wall is formed by five panels, each with three bites, so the guide wall appears very faceted in plan and was complicated to construct,” explains Mercer.

The other challenge for the guidewall was that it had to be strong enough to take weight of the diaphragm wall rig because the small scale of the shaft – and space available on site – meant that the guide wall would be trafficked frequently. The size of the shaft also meant that the work had to be carefully planned as only one operation – digging, de-sanding of the bentonite support fluid, cage placement or concreting – could be undertaken at a time rather than several being carried out at once.

stop end template

stop end template tideway

Honeycombe fill panels were used with the angled stop ends

The diaphragm walls for the shaf extend to 44m below ground level, although the TBM will enter at 20m depth.

“The shaft will be excavated to 24m and the extended length of the diaphragm walls was to create a water-tight cut off within the Chalk,” says Mercer.

“However, once work got underway it was clear that the Chalk we were toeing into was not as intact as anticipated, so permeation grouting will be carried out later to form a 10m thick grouted plug below the base slab of the shaft.

“We had access to the records from construction of the shaft for the Lee Tunnel during the design phase, which was very useful.

”The original plan for the shaft was to form the diaphragm wall panels using a cutter to deal with the ground conditions and meet the 1 in 400 verticality that has been specified across the Tideway project.

However, this verticality requirement was relaxed to 1 in 150 which made it viable to undertake the work using a grab and reduce the cost of the work.

“We are using a 15t grab for this project, which is quite large for the size of the shaft but is needed for the hard digging in the Thanet Sands,” says Mercer. The grab has been used with a new Liebherr HS8100 HD machine and there has been a spare grab on site throughout to ensure work was not delayed should there be a breakdown.

According to Mercer, this spare proved vital when large timbers were encountered in the made ground. “These became entangled in the rope and pulley system, so the spare grab was put into action while repairs were undertaken,” he says. “We had to dig out the timber, so some overbreak is expected in the panels in that section of the shaft.”

Other obstructions at even greater depth saw Spie looking to its French parent company for help. This which came in the form of 2.8m wide claw on a 14m long tube and is known on site as the super chisel.

super chisel 2

super chisel 2 tideway

A super chisel was used to deal with obstructions

“There were concerns that the obstruction – we think it was concrete – was causing deflection on the panel so we needed to be sure it was removed before continuing work.”

There are three bites per panel with a 2.8m wide bite at either end and a 600mm wide plug in the middle.“The small diameter of the shaft meant that we had to use the stop ends at an angle, which created a void which we wanted to avoid filling with concrete, so we developed a sacrificial honeycomb plastic fill that was tied tothe back of the stop end,” explains Mercer.

“Using the stop ends at such an angle and to 44m depth was a challenge.”

Spie used its bespoke hydraulically released stop ends for the project, which Mercer believes is a first for the UK and aided the removal process. A full depth water bar was also used between the panels.

Explaining the process on site, Mercer says: “We started with either the left or right panel and took the grab down the back of the stop end to clean and remove the plastic void former to get continuity between the panels.

The shaft is designed in hoop stress so it not heavily reinforced. The reinforcement cages, which have been made by Express in South Wales, extend to 29m and are made in one piece with a rectangular design for each end bite and a trapezoidal-shaped cage for the central plug.

The steel reinforcement has been replaced by glass fibre reinforced polymer(GFRP) in two places: where the TBM will break through at 22m, and closer to ground level where the shaft will connect to the Lee Tunnel shaft via a culvert.

“The GFRP sections created a challenge for lifting the cages,” says Mercer. “We worked with Byrne Looby to design lifting steels to support these sections while they are positioned.”

GFRP has been used in three of the cages. Construction of each panel takes four to five days. “Once excavation is complete, we move the stop end, clean the bentonite support fluid, place the cage and then concrete,” explains Mercer.

“We are de-sanding and de-silting the bentonite due to the presence of Chalk and the process takes up to six hours.”

“The small diameter of the shaft meant that we had to use the stop ends at an angle, which created a void which we wanted to avoid filling with concrete”

The concrete mix design also presented some challenges as Spie specified a mix it commonly uses in France, which is a higher standard than is conventionally used in the UK. “We wanted a 600 flow after six hours to maximise the working life,” says Mercer.“It takes up eight hours per pour. ”Spie carried out trials with a number of suppliers, but London Concrete in Bow proved successful in meeting the tight specification, which was only achieved by importing innovative additives from France.

Work on site has gone well and the final panel was due to be completed in early December. Work to create the grouted plug will start soon – possibly before the end of the year – and be followed by excavation and casting of the base slab to ensure the shaft is ready to receive the TBM, which is expected to break through in mid-2018.

The arrival of the TBM does not mark the end of the project, as the shaft still has to be lined.“

The diaphragm wall is designed to support the long term earth pressures but not the long term water pressures from its operational use, so a secondary lining will be needed,” says Mercer.

According to Mercer, similar shaft construction techniques will be used on the same scale by Spie on the Grand Paris Metro, so experience gained at Beckton will soon be put into practice elsewhere.

Time to tunnel

TBM Tideway

TBM Tideway

The Lovatt earth pressure balance TBM that will drive the 850m long, 3m diameter tunnel between the two shafts at Beckton was in the final stages of assembly when GE visited the site. It was expected to have started tunnelling as this issue went to press.

“The team deserves huge credit, it’s taken them two years to get to this point,” says Morrison.The TBM has been named “Susie” after the wife of long-serving Thames Water employee Alan Crump who joined the Thames Water Authority in 1972.

The cutterhead has been lowered into the shaft and the rest of the TBM and conveyors will be sequentially lowered and launched as the tunnelling work starts to progress. Precast concrete tunnel lining segments are currently stacked up around the shaft site but will be installed as the TBM moves forward. It is expected that “Susie”will take six months to complete the bore and emerge into the reception shaft constructed by Spie.

Once the TBM is removed, work will start on installing a secondary concrete lining in both of the shafts and the tunnel to provide waterproofing for the structure.

 

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