ABSTRACT
The technique is outlined in Figure 1 which shows the stages of construction for a new vehicular underbridge beneath a four track railway.
A reinforced concrete box is cast on a jacking base adjacent to the railway embankment, see Figure 1(a). A purpose designed tunnel shield is provided at its leading end, and thrust jacks are provided at its rear end reacting against the jacking base. Anti-drag systems (ADS) are installed and the box is jacked forward to the embankment. The box is then tunnelled through the ground by carefully excavating 150 mm off the face and jacking the box forward by a corresponding increment, this sequence being repeated many times, see Figure 1(b). When the box has reached its final position, see Figure 1(c), the shield, jacking equipment, etc are dismantled and the portal wing walls and roadway are constructed, see Figure 1(d).
