Rockmass Behavioural Uncertainty: Implications for Hard Rock Geotechnical Baseline Reports

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van der Pouw Kraan, Michelle
Geotechnical Baseline Reports , Tunneling , Rockmass Characterization
Geotechnical Baseline Reports (GBRs) have become a prevalent risk sharing mechanism on North American tunneling projects as they are based on the following risk allocation concept: the subsurface ground conditions described within the GBR are the financial responsibility of the Contractor, whereas encountered conditions which exceed those described belong to the Owner. This interpretation is intended to reduce project bid prices due to subsurface ground conditions uncertainty and the geotechnical rationale for a differing site conditions claim. However, recent tunnel project case studies have used the GBR as a risk transfer mechanism by presenting a conservative and/or limited interpretation of the expected ground conditions. In particular, the expected ground conditions are described with a summary of the intact and rockmass properties and empirical rockmass classification systems. This research has shown that the application of intact rock properties and rockmass classification systems to describe the various rockmasses along the tunnel alignment leads to rockmass behavioural uncertainty. Empirical rockmass classification systems are not able to adequately capture the effects of geologic uncertainty and the collective impact of the individual controls on rockmass behaviour. A new rock engineering design tool was developed which utilized geologic uncertainty and the capabilities of numerical modelling methods to predict and quantify rockmass behaviours. The 3D Rockmass Behaviour Map reduces subsurface ground conditions uncertainty as the range of possible rockmass behaviours is presented as a function of the three critical geomechanics parameters. Quantifying rockmass behaviours per tunnel domain demonstrates the effects of geologic uncertainty with rockmass behaviour mode switching. GBRs should include this 3D Rockmass Behaviour Map and quantified rockmass behaviours as these tools reduce uncertainty in the expected ground conditions and provide a greater understanding of the anticipated rockmass behaviours. Rather than using a conservative GBR which shifts subsurface ground conditions risk to a Contractor, this improved prediction of the expected ground conditions may result in better subsurface risk allocation, reduced construction contingencies, aid excavation means and methods selection, reduced geotechnical basis for a differing site condition claim, and provide greater certainty in the final project price and schedule.
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