Mowlem Johnston Microtunnelling (MJM) has just completed the scheme's 520m tunnel section using a remotely operated Herrenknecht mini-tunneller 22m below ground level. Although its tender price was higher than that of competitors, the firm was selected because its unmanned machine eliminated the risks associated with working in tunnels, reports major projects manager Andy Crawford.
The ground profile is made up of chalk with underlying strata of weathered chalk.
'It's sound, stable, and good for boring, ' Crawford says.
'We hit a few flint cobbles, maximum size 600mm diameter, soon after starting, but the mini-tunneller easily coped with them. The rest was plain sailing.' Spoil was removed in slurry form. The minitunneller's ideal working conditions are in wet ground - up to 30m head of water - in which slurry is created automatically by water ingress at the face. At Piddletrenthide the contractor was forced to bowser in water to keep spoil fluid.
Arisings were treated by screens, and cyclonic and centrifugal processes to separate solids from water.
Tunnelling started from a launch pit at the scheme's downstream end, with 800t hydraulic rams driving forward 2.5m lengths of pipe and the mini-tunneller.
As a precaution against excessive friction MJM created four 'interjacking' points, but in the event only needed to use two.
The annulus between the concrete pipe and minitunneller overream was filled by remotely injecting bentonite grout.