Excavation of the South Mountain water tunnel in Phoenix, Arizona was successfully completed on 23 August 2001. Site work is continuing with installation of the water pipe, backfilling and contact grouting the tunnel and pipe annulus.

Tunnel work is expected to be complete by 5 January 2002 with the pipeline placed into service in August 2002. Haley & Aldrich, in association with Stanley Consultants and HDR Engineering provided full design and construction management services for the tunnel. The tunnel is on schedule and within the $11M budget in spite of the difficult ground conditions. A thorough exploration program, a detailed Geotechnical Baseline Report, special design measures, and good construction methods were keys to the successful project.

As part of the 27.3km South Mountain Water Facilities pipeline, the tunnel will supply the growing population of the City of Phoenix with an additional 178 million litres per day of drinking water. The Ahwatukee Foothills, which this pipeline will serve, has experienced 145 percent population growth over the past decade, and is the only part of Phoenix that depends on a single waterline. The tunnel is located beneath Phoenix’s South Mountain Park and was driven beneath two scenic ridges. Preliminary studies selected a tunnel from several shallow alternatives because of hydraulic performance, economic viability, and reduced environmental and visual impacts. The tunnel is 1.85km long, has an excavated diameter of 2.4m, and a 1.22m diameter prestressed concrete cylinder pipe (PCCP) pressure pipeline will be installed as the final lining.

Explorations highlight challenges

A thorough exploration program was crucial to the success of this project. Preliminary studies indicated the tunnel would be completely in rock due to the presence of rock ridges at both portals and a rock outcrop in the valley between them. Exploratory borings in the valley, however, revealed two reaches of soil at the tunnel horizon. Subsequent explorations confirmed these findings and refined the limits of the soil reaches. Rock consisting of Estrella Gneiss and Komatke Granite, characterised as hard, strong, massive, and coarse-grained with foliation and jointing, was discovered. The soils detected consisted primarily of mixed grain alluvium with minor deposits of colluvium. Some soil lenses exhibiting secondary calcification were also encountered. Locally known as caliche, these soil lenses can exhibit behaviour similar to rock.

Although the presence of the soil within the tunnel horizon was a challenge, prior knowledge allowed for a suitable design to be implemented that substantially decreased the probability of large delays, cost overruns, and claims. To address these challenges, Haley & Aldrich lowered the tunnel elevation to reduce the length of tunnel in soil. Also, two types of initial support were specified-rock bolts in good rock, and steel ribs with lagging in soil and poor rock. In addition, a TBM with special features was specified, including dual propulsion systems, shielding, recessed cutters, specific torque requirements, and the ability to reverse the rotation direction. The contractor, Affholder Inc., provided a refurbished Robbins TBM adapted for both rock and soil, and excavated the tunnel.

Construction

Tunnel excavation lasted approximately eight months from 19 December 2000 until 23 August 2001 when the TBM “holed through” on the northwestern portal. The first soils and mixed ground section (Reach II) was encountered after only a few months in March 2001, prompting a switch from split set stabilisers to steel ribs and wood lagging as support. The second mixed ground and soil section (Reach IV) was reached some months later after a drive through 360m of jointed gneiss (Reach III) that separated the two soil zones. The final reach, through some 675m of granite was excavated without significant problems.

Work was conducted in 9-hour shifts with a total of 221 shifts (8.8m per shift) required to complete the excavation. The maximum advancement was 26m in a single shift. During mining, the TBM averaged approximately 1.5m per hour, while overall production, including down time, averaged 0.9m per hour. Down time was primarily due to cutter changes and electrical and mechanical repairs.

Pipe installation and backfilling of the tunnel and pipe annulus was completed using innovative methods to avoid injection holes (grout ports) in the pressure pipeline. Although common methods utilise holes in the pipe to inject backfill into the annulus, the City of Phoenix required an alternative method to preserve the structural integrity of the pipeline. The backfill was delivered from both portals through a series of sacrificial delivery pipes using measured injection volumes. The procedure went as planned and the methods were successful. Special attention was paid to the backfill material mix design to minimise the heat of hydration. Cellular concrete comprised of cement (35%), fly ash (65%), water, and a foaming agent with a density of 65pcf and strength of 500psi was used. Contact grouting is following to fill any remaining voids at the tunnel crown.

Geotechnical baseline report

A key to the success of the project was the use of a Geotechnical Baseline Report (GBR) prepared by Haley & Aldrich. This document provided a contractually binding baseline of subsurface conditions for the tunnel, and facilitated communication between the design team, the contractor, and the City of Phoenix. Additionally, the report assisted bidders and the chosen contractor in project planning and evaluation of tunnel excavation methods and support requirements. By providing all project team members with an established baseline of ground conditions and behaviour, the GBR was instrumental in avoiding potential claims.

Summary

The South Mountain Tunnel is an example of a successful project that utilised a combination of explorations, design, and good communication between project participants to overcome extremely difficult ground conditions. With a different, less careful approach, the project could have been an example of lessons learned the hard way. The project further demonstrates the versatility of TBM technology. When variable ground conditions are known in advance, the TBM can be specifically designed for those conditions.

Related Files
Geological cross section of the South Mountain alignment