sentences:  As built, the tunnel was approximately long, curving slightly from the southern portal in Skanda Vale, running straight for most of its length and then curving for the final third into Dolgran.The digging would require two construction shafts which could then be used for ventilation.Construction started in the spring of 1857 to bore the tunnel at the same time as work began on the first stage of the railway line in the town of Carmarthen.