Penmaenbach Head Railway Line (pre A55 Expressway)

Scope and Content

The photograph of the railway line around Penmaenbach Head was taken before the Tunnel and A55 Road were built. The Penmaenbach Tunnel provided a new carriageway for westbound traffic on the A55 North Wales coast road through the Penmaenbach headland, west of Conwy. The headland is a rhyolitic intrusion of Ordovician Age. The eastern slopes are covered by glacial deposits: west of the headland where the Tunnel will emerge is covered by a rhyolitic scree. The total length of the Tunnel is 640 metres: from the east portal, soft ground tunnelling techniques were expected to be used for 80 metres through Aeolian sand and moraine. At the west portal, the overlying scree was removed. The very high strength of the rhyolite required drill and blast methods to drive the tunnel: care was needed to control the blast induced vibrations as the generally unlined railway tunnel constructed in 1840 was only about 60 metres away from the new tunnel. The tunnel has a permanent concrete lining: primary support, provided by sprayed concrete and rock bolts with steel ribs at the transition from moraine to rock and at the west portal. From the evidence of the railway tunnel tests the inflow of water was expected to be small. The tunnel cross section provided for a 7.3 metre wide carriageway with a 0.6 metre verge on the offside and a 1 metre wide refuge on the nearside with a minimum vertical clearance of 5.1 metres. Axial flow booster fans provide longitudinal ventilation. The total cost of the tunnel was estimated at £10 million (November 1981 prices). Construction was started in 1984 and was expected to take three years.

Original Index No. K0084.