This month US contractor Affholder is scheduled to break through on the 1.8km long, 2.4m diameter South Mountain Water Transmission Line Tunnel in Phoenix, Arizona. The event will bring the project in on time and within the $11M budget.
Affholder, who was awarded the contract as the second lowest bidder in July 2000, is using a significantly modified Robbins TBM to drive the tunnel. The modifications were deemed necessary after a subsurface exploration programme revealed primarily hard rock conditions, but with two sections in the middle of the tunnel through soil and mixed face conditions. Project design and construction phase consultant Haley & Aldrich specified a TBM that featured dual propulsion systems, shielding, recessed cutters, specific torque requirements and the ability to reverse the rotation direction. Grippers allowed forward thrust through rock whilst a push ring against steel ribs and wood lagging was used for thrust through soils.
The contractor began excavation in January 2001and reached the first soils and mixed ground section (Reach II) in March that year prompting a switch from split set stabilisers to steel ribs and wood lagging. The second mixed ground and soil section (Reach IV) was reached some months later after a 360m drive through the jointed gneiss of Reach III. The final section, Reach V has been some 675m through granite. The contractor passed through the zones without significant problems.
The tunnel for client, the City of Phoenix, forms part of the 27km long South Mountain Water Facilities Pipeline, scheduled for service in August 2002. It will supply the city’s growing population with an additional 178m litres/day of drinking water. A full report will appear in T&TI’s December Focus on the US.
