Effect of nose blade angle on face stability of jacked box tunnelling

A typical jacked box tunnel usually consists of a precast concrete box tunnel and a steel nose blade installed in the box front. Two of many design factors for this tunnel type are the face stability and the face resistance; an increase in face stability is typically accompanied by an unintended consequence of an increase in face resistance. A series of numerical analyses was performed to explore the interaction between the face stability and the face resistance. Three design parameters were considered, namely amount of unexcavated soil inside box tunnel, nose blade angle and soil depth. Plane strain finite element models were used to simulate the tunnel jacking process; the software used was Plaxis 2D. The face stability represented by safety factor is affected mainly in a decreasing order by the amount of unexcavated soil inside box tunnel and the nose blade angle. However, if a minimum jacking force is required, an optimal combination of the two design parameters should be adopted.