Ground pressure causes gradual heaving of the tunnel base in mountain tunnels. This can influence train operations, and form an obstacle to the track clearance gauge. A design method is proposed to counter heaving of the tunnel base through the installation of rock bolts.
Model tests and numerical analyses were performed by means of different reinforcement patterns (the number and length of bolts), in order to clarify the most effective configuration and the working mechanism of roadbed rock bolts. Results showed that in the case of double track tunnel in standard condition, about four 5 meter-long rock bolts should be inserted at each section at a 1 m standard installation interval, along the longitudinal axis of the track (Fig 1).
In addition to model tests, a numerical analysis method was developed to express heaving behaviour in existing tunnels over time (Fig 2). The developed method represents heaving of the tunnel base with the assumption that natural ground strength around the tunnel gradually reduces the underlying force, therefore, it can evaluate/predict the effectiveness of roadbed rock bolts in terms of scale of upheaval (mm) and speed of heaving (mm/yr). This method may therefore be employed to set the level of reinforcement with rock bolts required to retain heaving of tunnel base below the desired level (Fig 3).
The results of this research and outcome of a case study form the basis of the “Manual for roadbed rock bolt design”. This manual facilitates design work by providing standard rock bolt patterns according to heaving magnitude.