Groundforce Shorco’s new MP375 hydraulic props are playing an important supporting role at one of the UK’s most ambitious civil engineering projects.

The new Luton DART (Direct Air-Rail Transit) project will replace a slow and inefficient bus service to provide a direct link between Luton Airport Parkway station and London Luton airport 2.3km away, cutting the journey time to less than four minutes.

Open-cut tunnel

Much of the railway is located underground and Groundforce Shorco’s £1.2m contract involves ground support for four locations: the viaduct piers; an open-cut tunnel known as the Trough; a cut-and-cover tunnel, known simply as the Tunnel; and Central Station, an underground terminus and maintenance area at the airport.

Poor ground conditions mean that extensive support is required within the excavations: propping is split into nine zones and each zone is between 50m and 100m in length.

In total, Groundforce will be supplying approximately 140 props to support excavations ranging in width from 11m to 21m.

“The ground conditions are very poor,” said Daniel Hobson, section manager for the main contractor, Volker Fitzpatrick/Kier joint venture. “It consists of structureless chalk and flinty clay in complex bands and pockets,” he said.

The chalk in particular is difficult to work with as it has a tendency to dissolve into a slurry when wet. “It’s weird stuff. When we were drilling boreholes we literally couldn’t retrieve any cores in some places; once it gets wet it turns to toothpaste,” said Hobson.

“Added to that, this is a very complicated engineering job. We’re doing a mixture of top-down, bottom-up and cut-and-cover tunnelling and building a 50m viaduct.”

One of the most demanding elements is the Central Station maintenance area where an open excavation has been avoided by installing a roof slab over the underground facility before commencing excavation.

Power-floated surface blinding layer

“We prepared the ground, power-floated a surface blinding layer and applied a wax sealer and release agent and then installed the slab and capping beam in one go,” said Hobson.

Excavation then commenced at one end of the structure and progressed along a sequence of headers and benches dug out from underneath the slab.

This excavation required lateral support at a depth of around 4.5m – but with the roof slab already in place the props couldn’t be simply lifted into the excavation as usual. Instead, two apertures were created in the slab to allow equipment to be brought in and excavated material taken out.

“The props for the central station maintenance area had to be split into 4m-long sections allowing them to be dropped through the voids in the slab,” said Groundforce major project manager Andy Simms.

The Luton DART project is also the first outing for Groundforce Shorco’s new MP375 prop – a higher-capacity version of the MP250 unit.

Loads are high on this project – so much so that Groundforce is supplying around 60 of its highest-capacity MP750 props in addition to 21 of its MP150s and 38 MP250s.

The MP375 – with a nominal load capacity of 375 tonnes – sits between the MP250 and the MP750 (250 tonnes and 750 tonnes respectively). Its primary advantage is that it can support high loads in corners, spanning at 45o.

“The minute you put a prop at an angle, the load increases. Across a corner the load, imposed at an angle of 45o, goes up by around 40%. Once these loads get close to the capacity of the MP250 you need a larger prop, ” said Simms.

“Specified loads are getting bigger as projects increase in size, which is why we’ve developed the MP375. With the new prop we can do higher loadings and still maintain plenty of working space within the excavation.”

One of the largest ground support contracts

The project – one of the largest ground support contracts Groundforce has ever delivered – is further complicated by the need to work around a live airport.

Co-ordination between the client, main contractor, specialist contractors and engineering consultants Tony Gee & Partners (tunnels) and Hewsons Consulting Engineers (Central Station) has been facilitated by the extensive use of 3D modelling.

The use of modular hydraulic propping equipment has helped streamline the programme, said Hobson. “It means we don’t have to cut and weld steel because it’s completely adjustable. This and the availability of Groundforce’s equipment has saved us a shed-load of time. The Groundforce technical team has been really helpful, too.”