The tunnelling was complicated by the alternation of pozzolanic earth with tufa necessitating the elaborate lining of most of the tunnel with stonework of opus reticulatum and then with vaulting on top of these walls. Work progressed at 5–7 m per day as indicated by the joints between sections. It was not perfectly straight in plan but included small deviations near the centre where the tunnellers from each end met after remarkably small inaccuracies of alignment given the techniques of the time.