UNIQUE FEATURES OF GEOTECHNICAL PROGRAMS FOR THE METROWEST WATER SUPPLY TUNNEL

For most of it’s length, the MWRA’s MetroWest Water Supply Tunnel crosses geologic terrain comprised of complexly inter-related, highly metamorphosed rocks and two regional fault zones. Geotechnical challenges have included: engineering characterization of the complex lithologies; pre-construction evaluation of Tunnel Boring Machine (TBM) performance; evaluation for rock conditions especially at regional faults; and prediction of groundwater infiltration. The geotechnial work was presented in Geotechnical Data Reports (GDR), project memoranda and Geotechnical Design Summary Reports (GDSR).

Unique elements of the 6-year long geotechnical program included: extensive contributions by several consulting geologists to help systematize characterization of the complex lithologies for over 22,000 feet of deep core holes; specialized rock core testing in Norway, Switzerland and the U.S. for TBM performance evaluations; the unprecedented drilling of two directionally drilled horizontal core holes for distances up to 1,900-ft to characterize the Basin and Bloody Bluff Fault Zones.

CONCLUSIONS Geotechnical programs for tunnels are as much about risk management as they are about design. On this project, ground related construction claims have been modest and all have been resolved expeditiously. This is the result of the combined efforts of an informed owner, appropriate engineering designs, an effective and thorough geotechnical program, good construction management, and skilled tunnel contractors. For example, pre-construction, TBM boreability was regarded as a major cost risk due to the hard and abrasive nature of the rock. The tunnels have been excavated without any boreability claims, which are attributed to the thorough characterization of rock conditions as well as an effective TBM specification. Overall rock support quantities have been less than predicted, however differing site conditions have been recognized in a few instances. This is attributed to the evaluation of ground conditions and appropriate design judgements. Finally groundwater infiltration quantities have been slightly less than the baseline predictions. The predictions were based on a statistical analysis of packer data and groundwater modeling tempered by judgements based on experience.