Jacked box tunnelling represents a non-intrusive method for constructing underbridges beneath existing infrastructure, write J.W.T. Ropkins and D. Allenby. Jacked box tunnelling provides the engineer with a non-intrusive method for constructing a new underbridge, culvert or subway beneath existing infrastructure, for example highways and railways. Figure 1 shows the construction stages 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, Figure 1a. A purpose designed tunnelling shield is provided at its leading end and thrust jacks are provided at its rear end reacting against the jacking base. Anti-drag systems are installed and the box is jacked forward to the embankment. The box is then tunnelled through the ground by carefully excavating 150mm off the face and jacking the box forward by a corresponding increment, this sequence being repeated many times, Figure 1b. When the box has reached its final position, Figure 1c, the box/ground interfaces are grouted, the shield and jacking equipment dismantled, and the portal wing walls and road pavement constructed, Figure 1d. The method can be used in a variety of ground types and is effective by virtue of the techniques applied to control ground movements during tunnelling.
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