The Pacific Highway Upgrade between Tintenbar to Ewingsdale near Byron Bay, NSW includes a tunnel beneath St Helena Hill. The permanent tunnel lining was required to be impermeable to prevent dewatering of the surrounding groundwater system, which is located within a series of Neogene basalt lava flows. The tunnel extends through two primary geological units including weathered low to medium strength basalt and high to very high strength jointed basalt, with the latter being the most prevalent geology. Hydraulic testing of this geology indicated that the average percentile permeability approximated 4.3E-2 m/day for the medium strength rock and 1.0E-1 m/day for the high to very high strength rock.
Initial hydrogeological modelling was undertaken to assess the potential effect of changes in rock mass permeability due to blasting damage (i.e. within the "Excavation Disturbance Zone or EDZ"), the risk of exceedance of the specified groundwater drawdown criteria and whether post-excavation grouting would then be required.
Numerical simulations of blasting induced vibrations were used to estimate that the area inside the 750 mm/s peak particle velocity contour (extending approximately 0.7m beyond the tunnel excavation), would represent the zone where significant permeability changes could occur. A range of permeability changes were assessed in the modelling, which indicated that groundwater elevation changes were indeed sensitive to changes in the permeability of the EDZ.
To clarify the requirements for any remedial grouting, 21 shallow packer tests were completed behind the temporary shotcrete lining of the tunnel during construction. The post-excavation packer testing was compared with 93 pre-construction packer tests and a single "constant discharge" pumping test. Statistical analyses of these data indicated that the increase in permeability of the EDZ was less than 10 times pre-excavation permeability of the basalt rock mass. Revised hydrogeological modelling based on these measured permeability changes in the EDZ indicated that the risk of significant dewatering of the aquifer system was low and that remedial grouting would not be required.
This paper provides quantitative data on the impact of drill and blast excavation on the permeability of the EDZ; data on the link between blast velocities and permeability changes; and, demonstrates that shallow packer testing coupled with hydrogeological modelling can be used as a rational basis to decide whether remedial post-excavation grouting is required to meet tight groundwater drawdown criteria.