Time, tunnel length, geology

by Nick Ryland Barton

Time, tunnel length, geology – therefore Qtbm is the title and summarizes the intention. In detail the case records, including world records, all point to the importance of accepting a time-dependent and length-dependent advance rate AR which gives different results compared to the popular tendency to focus on net penetration rate PR prognoses. In the Qtbm concept PR is only a starting point, and (after a learning curve) the AR declines from week to month due to geological challenges such as very hard rock, and/or serious fault zones requiring pre-injection. The standard AR = PR x U equation is recast to a more realistic U = T (total hours, raised to the power m, the negative deceleration gradient). The negative gradient m gets worse in faulted rock (low Q) and worse in extremely hard massive rock (high Q). Numerous prognosis results are given together with Q-logging of exposures and use of Vp velocities to indicate a possible high-Q sub-surface not seen when logging rock exposures.

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