Leading geotechnical specialist, Bachy Soletanche Limited (BSL) has recently completed work on a
circular secant shaft near Ammanford, South Wales. The shaft will assist in the construction works
of the Felindre to Tirley natural gas pipeline, transporting natural gas from Tirley to the two new terminals under construction at Milford Haven in Pembrokeshire.

Barhale Construction plc awarded BSL the circular shaft contract on the 90kilometre stretch from
Felindre to Brecon. However, the essential pipeline works - in its entirety - will run from Felindre to
Tirley, covering a total of 196kilometres. The two sections are to be built by two separate
contractors.

National Grid has estimated that more than 20 percent of UK gas requirements will run through the
natural gas pipeline, helping to meet the UK’s growing demands and address the decline in North
Sea reserves*.

The shaft is vital in the preparation works for the project, as a tunnel-boring machine (TBM) will be
lowered 22metres below ground to begin the tunnelling works for a section of the pipeline. Steve
Mallinson, contract manager for Bachy Soletanche commented:
“BSL designed the shaft around the ground conditions and the required depth. Our design was for a
hard/hard secant circular shaft using large diameter auger (LDA) piles.”

“The original design was for a traditional Caisson Shaft, but due to large boulders found in the
ground we came up with a piled solution as an alternative.”

The hard/hard secant piling method requires the use of reinforced concrete piles for permanent
retention. Secant walls are constructed in two stages with the primary, female piles installed first.
Secondary, male piles are then cut into the primary piles.

BSL’s design was to create a circular shaft using the secant piling method that would provide a shaft
large enough to accommodate the TBM.

In order to create the circular shaft, 32 rotary piles of 1200mm diameters are bored to a depth of
24metres. The piles are bored through a ground profile including one metre of soft clay, dense
gravel with large boulders to 15.5 metres, and weak to strong mudstone down to 25metres.

A Bauer BG 24 was utilised as the ground conditions required heavy-duty drilling equipment.
However, even this powerful rig still required additional torque, and a torque multiplier was employed to increase the rig’s torque from 24Tm to 48Tm. Steve Mallinson continued:
“Our rig needed enough power to drill through the large boulders and screw the casing down to
24.0m”

The piles are reinforced with steel, yet carbon fibre bars are used for piles in the area of the
tunnel drive. This creates a “soft eye” at the start of the tunnel - allowing the tunnelling to begin with ease.”

Bachy Soletanche began work on site in late March and completed the project at the end of May. Further works on the project will continue throughout the year.

Email: fiona@manifestcomms.co.uk