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Cross purpose

Supermarket giant Tesco is having another go at building a new store over the railway at Gerrards Cross after the newly built tunnel it was to sit on partially collapsed first time around.

It was never going to be an easy sell. Supermarket giant Tesco had set its sights on the quaint and affluent Buckinghamshire village of Gerrards Cross, just west of London.

For one, squeezing a new grocery superstore into the area would be a problem, packed as it is with bakeries, florists and the usual village shops and lacking in developable space. Tesco was undeterred and came up with a plan to buy a plot of land from Network Rail and create 1.82ha of new space above a portion of the twotrack Chiltern railway, which runs through a cutting on the outskirts of the village.

It was not necessarily the easiest of options – the scheme involved erecting a precast, reinforced concrete vault that would form a tunnel along 320m of track. The structure was to be backfilled to create part of the site for the store.

The structure would have to knit into the Packhorse Road bridge – a late 19th century three span brick structure – at its north east point, stopping just short of Marsham Lane bridge at the south east end. But the retailer was to discover just how difficult a job it would become on 30 June 2005, when at 7.34pm a 30m long tunnel section collapsed onto the railway.

Just before the collapse, the tunnel had been completed and most of the backfill had been placed, typically using incinerated bottom ash aggregate (IBAA). Work on the store structure had also begun. A 65m long stretch surrounding the collapse site had to be cleared of debris and the project was halted, leaving its long-term prospects hanging in the balance.

This month, the appearance of a 280t crawler crane on site signalled the start of work to rebuild the tunnel using the same type of precast units that fell down well over three years ago. However, this time around the units will no longer perform a structural role. Instead they will be used as permanent formwork for a cast insitu concrete lining. This in turn will be beefed up with extra concrete and surrounding retaining structures. Lighter fill was used to reduce loads.

It’s a very complex piece of civil engineering because of how the structure behaves. Thermal movement of 3mm to 4mm is typical

Martin Baughurst, Costain  

Before the collapse, Jackson Civil Engineering was main contractor and White Young Green the consultant on a project that was to cost £20.3M. But in 2006 Tesco opted to change the team with the appointment of Costain as main design and build contractor along with consultant Scott Wilson as lead designer.

The crawler crane now on site has been installing Macretemanufactured segments to replace those that formed the original tunnel vault, and which collapsed. Each is a 1.8m wide, 23t halfspan that is staggered to meet joints in two opposite units at the top of the tunnel crown. A male to female steel joint – much like tongue and groove – ensures that the 300mm thick half-spans lock together to form a 25m wide total span.

Part of the planning process meant allowing enough space for future rail expansion and the size of this span would allow room for a four-track line and higher carriages, giving the line high-speed potential.

At its base, each segment is wedged into recesses in concrete trenches with timber and then grouted into place. These trenches were excavated during the earlier project – supported by 3,000 minipiles – and are in need of only minor repairs. “It’s a very complex piece of civil engineering because of how the structure behaves,” says Costain project manager Martin Baughurst. Alignment at the top is critical, he adds, and near to the base two tie bars per panel will be installed to connect the new arch to the insitu concrete surrounding it.

Baughurst adds that from thermal movement alone, 3mm to 4mm of expansion is quite typical. But added to that are expectations of around 20mm of movement when construction activities surrounding the structure pick up. Most notably, this will likely occur when original backfill is removed from above and during the time when new backfill is placed.

But limits have been set to ensure these do not exceed 60mm. Soldata’s Cyclops monitoring system, comprising three mirrors on alternating units – inherited after Network Rail checked stability after the collapse – is keeping a check on the tunnel’s movement around the clock.

However, the structure was always designed to be flexible and a nominal 20mm gap separates each unit to allow for this. The newly replaced section will supplement the stretch of tunnel that was left in place after the collapse and will be connected to it by a half unit prestressed joint. “They [original units] have been here for three years,” says Baughurst. “We can’t just carry on with the new units because there will be a difference in how that [old structure] and the new one moves.”

Rather than simply continuing the scheme with the precast concrete arch, this time around the design is more complicated. Baughurst is keen to stress that, despite being permanent, the original arch is structurally redundant. Its only destiny now is to behave as formwork for the more robust 600mm thick reinforced concrete arch – one continuous structure – that will support the store above.

The Health & Safety Executive is still investigating the cause of the 2005 incident, but it has highlighted the method used to place infill material around the concrete arch as a contributing factor. And the increase in civils works on what had been originally planned for the scheme before the collapse points to a more cautious approach, whatever the cause of the earlier accident. “We’re taking no chances,” says Baughurst.

Since last November, Martello Piling has been carrying out substantial work to install contiguous piled retaining walls on either side of the tunnel. These will retain the existing fill and create a robust working space for heavy plant. These retaining walls comprise 26m deep structures on either side of the railway and 19m deep structures further back.

I don’t think there’s a single pile performing a structural function for the permanent job. It’s a big piling job, but at the end of the works, the piles will all be redundant

Martin Baughurst, Costain  

 

To create these, rig operators are installing 660 large diameter piles. Large diameter piling typically requires tall rigs but that has not been possible because of Network Rail height restrictions. Fortunately, Martello has manufactured low-headroom rigs – three of which are at work at Gerrards Cross – that comply with Network Rail restrictions while still powering the auger to drill 900mm diameter piles to depth. It is working at a rate of 21 installed piles per week to meet its July deadline. However, the job has been complicated by the presence of made ground from the first stab at the project, mainly comprising IBAA.

To combat the poorer drilling conditions through the overburden, 10m temporary casings support the surrounding soil through the made ground, gravel and clays overlying chalk. Each pile has heavy duty, asymmetrical reinforcing bars of between 14mm and 60mm diameter, running their full length. These retaining walls are only being installed to support the cutting slopes during the civils works; they will have no structural function when the project is complete. “I don’t think there’s a single pile performing a structural function for the permanent job,” Baughurst says. “It’s a big piling job but at the end of the works the piles will all be redundant.”

Before the tunnel arch segments could be lifted in, site workers had also been busy building the platform that will support the crane which is lifting in the segments on the north side of the tracks. This involved extending an area of the cutting slope to create a 10m wide, 65m long working platform.

To create this, contractors replaced a timber king post wall with vertical cast insitu concrete slabs. Above this, site workers installed a Phi Group Textomur facing system comprising geogrids and geotextiles to form an 8m high reinforced earth slope. The geogrid and geotextile units are built up with 125mm layers of compacted fill and Systems Geotechnique finished the whole thing off with 160, 16.5m long Dywidag hollow soil nails drilled and grouted into the slope in two layers to spread the load through the new king post concrete wall.

With the platform in place the crane has been lifting the arch segments in at a rate of two to three per day – or rather night, as crane work over the railway has to be carried out exclusively during night-time possessions between 12.30am and 5.30am.

The aim is to complete installation of all 63 segments by 10 April to allow follow on work for the 7.85m high permanent arch to be cast and the entire tunnel to be backfi lled up to the new ground level. Baughurst says this part of the work will be carefully sequenced with concrete cast and backfilling done symmetrically on both sides of the railway to ensure loads are distributed as evenly as possible.

Around 70,000t of 6N/mm2 structural fill and IBAA will be used for the lower section of backfill, topped off with 25,000m3 of the lightweight foamed concrete. This is another move away from the previous design and using foamed concrete will exert less of a load on the tunnel than a solution involving the use of just IBAA. Baughurst describes the foamed concrete as “pumice stone-like” and says its 0.4kg/ m3 weight is dramatically lighter than ordinary concrete that weighs in at 2.5kg/m3.

Assuming there are no further setbacks, the assortment of civils works on the project is due for completion by next summer. Although Tesco is holding back on revealing today’s price tag, in its 2007 annual report it revealed a write off equating to £35M for the scheme, and that was before it decided to go ahead with the rebuild. If all does go to plan, from 15 November 2010 the residents of Gerrards Cross and its surrounds will be able to buy their groceries from one of the most talked about Tesco stores in England.  

TUNNEL TROUBLE

At 7.34pm on 30 June 2005 the driver of a Chiltern Railways Stratford-on-Avon to London Marylebone train reported the partial collapse of tunnel construction works over the railway at Gerrards Cross. A train had passed through only a minute before and another was stopped just after the alert – halting 300m away from the obstruction.

About 30m of the 320m long concrete tunnel had collapsed – with 29 reinforced concrete tunnel segments and up to 6m of infill material blocking the line. A further 16 adjacent segments were subsequently identified as having been damaged or dislodged in the collapse and had to be dismantled.

The cause of the accident is still subject to a Health & Safety Executive investigation, but initial indications suggest that the sequence for placing fill on the tunnel was the main contributing factor.

At the time of the collapse engineers told NCE that it was possible that an imbalanced placement of fill on either side of the tunnel, along with too high a load on the crown, could have caused the collapse (NCE 7 July 2005). About 25,000t of earth debris had to be removed from the track before repairs could begin, and the line was closed for a month causing severe disruption to commuters.

In 2006 Tesco appointed contractor Costain and consultant Scott Wilson, who produced a revised concept for the tunnel design, and the supermarket announced its intention to rebuild the store to the new design.   

GERRARDS CROSS SITE TIMELINE

June 2005 Partial collapse of tunnel construction works. All construction halted

March 2006 Costain replaces Jackson Civil Engineering on the project. Scott Wilson brought in to review design

November 2008 Work restarted. Existing steel frame store dismantled and temporary bridge installed to allow plant across the railway line. Crane platform works commence. Contiguous piled wall construction works commence. Trackside works commence. Network Rail Form B Approval for Permanent Work complete

February /March2009 Tunnel unit installation works to collapse zone commence. East and West portal works commence, bulk earthworks commence and ramp/ crane platform works complete

April 2009 Tunnel arch reinforced concrete surround works commence. Tunnel unit installation works to collapse zone complete

July 2009 Contiguous piled wall construction works complete

September 2009 Tunnel foam concrete works Commence   

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