The stabilization of the excavation face of a shallow tunnel in difficult ground conditions

The stability of the excavation face of shallow tunnels excavated in difficult ground conditions is at present a relevant problem in the tunnelling sector. Even though face reinforcement interventions with fibreglass dowels have proved to be efficient, there is still no reliable analysis and dimensioning method available. A new calculation procedure is illustrated in this paper for the analysis of the face static condition in shallow tunnels and also for when reinforcement interventions with fibreglass dowels are used. The procedure is based on the limit equilibrium method applied to the ground core ahead of the face. The main result of the calculation is that the safety factor of the excavation face is also obtained in the presence of reinforcements and from this it is then possible to proceed with the dimensioning of the intervention. The procedure has been applied to two real cases and satisfactory results have been obtained.