The Massachusetts Water Resources Authority (MWRA) applied hard rock directional drilling technology to delineate structurally complex geologic sections along the 28.3km MetroWest Water Supply Tunnel. Surficial bedrock mapping, vertical and angle borings, and seismic refraction surveys revealed high groundwater inflow and ground support potential at several fracture zones associated with high angle faults and zones of soft clay fault gouge. Two tunnel segments in particular appeared to warrant costly ground improvement programs – one at the eastern end wye, traversing crystalline granite, felsites, healed breccias, and argillite, and another segment at the western end across the Bloody Bluff Fault comprised of sound to highly fractured gneiss, felsite, mylonite, quartzofeldspathic zones, and mafic dike intrusives. Based on the potential variability in rock quality impacting construction risks, the Devico directional drilling system was used to increase confidence in the in-situ conditions at each segment.

The Devico directional drilling system at each segment utilized a flexible NQ size core barrel yielding a BQ size core encased in a clear tube, a programmable survey tool to guide the drill rod to the desired angle or curve, and DEVISOFT software to calculate borehole position and provide adjustments.

The Devico system was used along a 533m section of the eastern segment. The boring was advanced through hard crystalline granite, 18.3m of soft clay gouge within the bottom 5 degrees of the curve (using grout), felsic volcanics, healed breccias, and argillite. The high total recovery (>95%) confirmed a construction risk reduction and allowed a TBM to be considered for the work, at considerable cost savings.

The results of the 580m borehole advanced through the suspected Bloody Bluff Fault revealed 0.3m to 2.4m thick fracture zones separated by 2m to 8m of thick zones of intact rock, along a distance less than projected. The borehole was grouted prior to the arrival of the TBM, and may have contributed to a significant reduction in groundwater control and anticipated ground support.

The Devico directional drilling system added considerable value to cost saving measures and should be considered for use in future projects to reduce construction risks and as a dispute avoidance tool for similar complex geologic zones.