Reinforcing a tunnel face using grouted fiberglass bolts
As part of the extension works for the Underground Research Laboratory (URL) at Bure, the GVA2 gallery, oriented along the minor horizontal stress, is excavated by means of a TBM. Whenever excavation must be stopped for any significant time period (maintenance, scientific investigation, etc.), the face and the rock mass ahead are reinforced using a jet grouted skin and grouted fiberglass bolts. The aims of this work is to study the influence of face reinforcement on the tunnel stability and to check if the creep behavior of the rock may induce a potential jamming of the TBM during the excavation stop.
Two support profiles are checked: one uses classical lining segments protected by a compressible mortar in the annular gap; the other uses compressible lining segments surrounded by a classical mortar layer (Fig.1).
Tunnel excavation is simulated for a distance of 10 radii, with an advance step of 0.8 m. When the excavation stops, the face reinforcement, including 20 cm of shotcrete lining and 14 6m-long grouted fiberglass dowels, is installed. The simulation is then continued for a face stop lasting 16 days.
Due to the anisotropic initial stress state, the ratio of vertical to horizontal convergences is about 2.5.
The concrete lining sustains loading by the ground, with a safety coefficient higher than 1.5.
The fiberglass bolts at the tunnel face do not reach their ultimate strength (245 kN), and face extrusion remains below 9mm after a 16-days excavation stop. Extrusion is slightly higher for a non-reinforced face.
However, the longitudinal convergence profile shows a potential risk of TBM jamming during the face stop.