On straight tunnels with diameter between 1200 and 3500 mm and length exceeding
200 250 metres, the pipe jacking system is generally used.
Here the tunnel lining is constructed outside in prefabricated rings from 1500 to 2500 mm
The main jacking station inside the well acts, by thrust jacks, on the last ring lowered
from above, gradually driving the entire lining, and thus the machine, along the tunnel.
The friction developed between the lining and the tunnel surface limits the length of
lining which can be installed using a jacking station to 40 - 70 metres.
Therfore, in tunnels exceeding 40 - 70 metres of length, intermediate jacking stations are
These consist of segmented steel rings and a set of hydraulic rams feeded from outside.