Probe drilling ahead of TBMs in difficult ground conditions: Fid-Bau Portal

Probe drilling ahead of TBMs in difficult ground conditions

Bilgin, Nuh / Copur, Hanifi / Balci, Cemal

This chapter summarizes the results of probe drilling carried out ahead of TBMs in Melen and Kargi projects in a very complex geology. In summary it may be concluded that normalized penetration rate of a probe drill is a good indicator for detecting weak zones ahead of a tunnel. Probe drilling rate changed with changing geological formations and showed sharp increases and decreases in transition zones. Probe drilling rate is also very much affected in fault zones. In some cases, probe drilling rate increased within fault zones due to swelling and squeezing characteristics of the gauge materials.

The method of analysis of probe drilling rate data in the Melen project permitted the defining of a critical normalized probe drilling value. Values higher than the predetermined critical value point out critical geological zones susceptible to pressurized water ingress.

TBM thrust and torque values increased with decreasing probe drill penetration rates in competent hard rock formations in the Kargi energy tunnel. However, this study introduced a new concept of TBM thrust/probe drilling penetration rate and TBM torque/probe drilling penetration rate ratios explaining the variation of penetration rates in a statistically more reliable manner. This concept showed that by observing closely the variation of TBM thrust and torque values it is possible to predict the transient and fault zones, dictating the carrying out of umbrella arching, which will strengthen the weak zone in front of the tunnel.