In a tight spot

Soil nailing has helped maximise the tight spaces rig crews have to work in on the site of a new supermarket and car park. Alexandra Wynne visits the north London site to see how they coped.

Rig crews are grappling with a site complicated by a railway leading to Rickmansworth train station to the south and a row of protected trees and pedestrian route on an embankment to the north side. Development plans for a supermarket and four-storey car park mean they must find a way to maximise usage of the site.

"Virtually the whole site is being developed so we're left with very little space to work in," says developer Kier project manager, Rob Brown. "That's probably the biggest constraint on this job."

Kier, as main contractor and subcontractor May Gurney, needed to excavate the sloping ground to create a vertical bank that would allow them to build right to the perimeter of the about 35m by 160m site. They had to do so without disturbing the footpath and trees on top of the chalk embankment.

"The building's footprint will sit where the existing bank is," says May Gurney area manager Mike Cowan. "Without reinforcement, the excavation work needed to build there would dislodge the bank and the area above." The entire site consists of between 1m and 5m of made ground that sits on up to 15m of chalk.

"We originally looked at using sheet piles to reinforce the vertical bank following excavation," says Brown. "But the idea of using soil nails and shotcrete developed as we began to realise how much easier the installation would be in such a tight area."

Because sheet piles are not flat, using them to reinforce the vertical bank would take up much needed room that the soil nailing would free up.

Kier, which awarded the £100,000 soil nailing subcontract to May Gurney, were keen to avoid any logistical difficulties when creating an area for the sheet piling. A standard 360º excavator rig with a soil nailing masthead attached at the front can easily fit in a small area that would not accommodate the rig needed for sheet piling.

Now the wall of the car park's exit can sit extremely close to the perimeter of the site once the embankment is complete.

The 86 steel soil nails being used to reinforce the bank are Ischebeck Titan 30/16 hollow bars with a 30mm outside diameter and 16mm inside diameter. Varying lengths from 7m to 9m are drilled at 10 degrees from the horizontal into the excavated vertical bank. They are arranged 1.25m apart in a six row grid, over the approximate 90m2 face adjacent to the car park exit.

Working from the top of the bank downwards, site workers installed soil nails to depth and then pumped grout down the hollow stem. The pressure forces the grout to escape through four holes in the 76mm sacrificial drill bit and to fill the annulus surrounding the outside thread of the nail.

Before the embankment is covered with steel mesh, 25mm thick 250mm steel washer plates are placed around the end of each nail. May Gurney then sprays 75mm to 150mm of concrete over the surface before the process is repeated on the next section of embankment below.

Approximately 400 CFA piles, generally situated under the building's columns, and minipiling work on an existing pedestrian walkway will also be done by May Gurney in a separate contract worth £170,000.

Railway lines leading into nearby Rickmansworth train station to the south have forced engineers to work under safety constraints imposed by London Underground and Metronet. A representative from the train operator must be present to check restrictions on work taking place in an area 19m from the south boundary. Cowan believes that the relatively low headroom of the 16.4m high Mait HR 110 piling rig has helped to comply with the restrictions, but checks are made to see that it sits perpendicular to the railway lines when working in this area.

CFA piles of either 450mm or 600mm diameter (up to maximum 14m depth) will support the steel framed supermarket building to 1480kN compression, 310kN tension and 140kN horizontal loads.

In the final phase of May Gurney's share of the project, due for completion in mid-February, May Gurney will install minipiles to support an existing pedestrian walkway before removing an end section to make room for the new car park to be built.

Cowan explains that by choosing to use the 3.2m high Hutte HBR 202 rig for the 600mm diameter minipiles, May Gurney expects to offset any potential problems from working in a 4.2m headroom restriction beneath the walkway. Kier's share of the project, worth around £19M, is due for completion 12 months after the start of enabling work in November 2006, with the store opening scheduled for this November.

Article supplied courtesy of Ground Engineering magazine.