Project At-a-Glance
- New 12-ft wide by 14-ft high modified horseshoe-shaped tunnel
- Almost 3.6 miles long
- Excavated by primarily road header, drill-and-blast, and hand mining techniques
- Deepest application of unreinforced secant piles in North America, at the time of construction, to support the 115-ft deep Vargas Shaft
Project Narrative
The New Irvington Tunnel (NIT) is a critical redundancy component of the Hetch Hetchy water system that is operated by the San Francisco Public Utilities Commission (SFPUC) and provides drinking water to San Francisco. Located east of Fremont, the project involves the construction of a new tunnel, connections to existing facilities at each end of the new and existing tunnels, and other ancillary infrastructure. Almost 3.6 miles long, the NIT is roughly parallel to the Existing Irvington Tunnel (EIT), which was constructed over 80 years ago. This new 12-ft wide by 14-ft high modified horseshoe-shaped tunnel was excavated by primarily road header. Also, drill-and-blast and hand mining techniques with, the latter implementing sequential excavation of top heading and bench with breasting and spiling in the worst ground conditions, have been deployed.
Construction of the NIT encountered challenging and highly variable ground conditions ranging from massive, strong sandstone that was difficult to excavate which required the use of drill-and-blast methods; to highly sheared and faulted shale and clay gouge where very careful hand mining with pre-support and grouting to reduce groundwater inflows was the technology of choice. Conditions such as running, caving, flowing, raveling, and squeezing ground were also encountered in a number of areas along the alignment.
A critical component of this project is the Vargas Shaft a 41-ft diameter and 115-ft deep temporary shaft that was mined to allow tunnel drives in two directions from an intermediate location. Brierley personnel were called upon by the shaft sub-contractor, Malcolm Drilling Company, Inc., to develop an alternative and constructible design for shaft support. We modified the shaft support design to extend secant piles into competent bedrock, essentially drilling to beyond the full depth of the shaft. The resultant use of unreinforced secant piles as a stand-alone compression ring shoring system for a deeper than 100-ft excavation was unprecedented at the time.
Brierley Associates also served as Tunnel Contractor’s Engineer. Engineering services included: pre-bid evaluations and bid preparation assistance; design and submittal preparation for tunnel excavation and initial support, and dewatering; and full-time on-site engineering geologic mapping for determination of Ground Class and Support Category selection and documentation.
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