The Pen y Clip Tunnel

The main feature of the scheme at Pen y Clip is the 1.0km hard rock tunnel. The existing headland road has been upgraded to become the eastbound carriageway and Telford’s old road has been re-established for non tunnel traffic. Where the dual carriageway splits to two levels to the west of the Pen-y-Clip tunnel, an intricate arrangement of massive retaining walls was needed.

The retaining wall between the two carriageways was anchored into the hillside, and props the upper wall. The lower wall retains the fill which carries the eastbound carriageway, and its deep foundations intercept a potential slip plane.

The first of the walls was the central one. Its completion allowed contractor Laing Civil Engineering to work below to build the 12m high wall at the bottom of the slope.

Construction of the lower wall required a novel approach. Space was limited, and with the railway only a metre away, it was impractical to excavate massive foundations.

Instead of having a continuous base, the wall is supported on underground posts, or barrettes, and hung from counterforts which are extensions to these. The barettes, by Bachy UK, are 6m by 1m in plan, and up to 25m deep. They are at 5m centres, so there are two beneath each 10m length of wall. Excavations were made using a grab, and supported by bentonite slurry. Huge reinforcement cages were lowered in, ready for the barrettes to be concreted.

Tunnelling subcontractor Trafalgar House Construction Tunnelling found poor quality rock until it reached the main outcrop of Microdiorite. The ground in the outer thirds of the tunnel was described as “rubbish” – wide open joints, infilled with clay and what looked alarmingly like top soil. Support in the jointed zones is by steel ribs, with a concrete lining in front. Drainage systems are concealed behind ceramic coated steel cladding.

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