The proposed tunnel alignments cross geologically complex conditions, requiring a detailed investigation and instrumentation plan. The potentially active San Gabriel Fault is part of the larger San Andreas Fault system, and borings had to be drilled at inclined angles to intersect and gather information about the dominant fault traces and associated groundwater. Extensive testing was performed in each boring, including packer testing, groundwater sampling, and hydraulic jacking at pressures as high as 10,000psi (69MPa) for in-situ rock-stress measurements. Additional geophysical testing was performed, measuring in-situ fracture systems and conductivity properties of the surrounding ground. In total, Crux drilled five borings to depths ranging from 900ft to 2,700ft. To measure groundwater pressures and their potential effect on tunnel design, construction and lining performance, vibrating wire piezometers were installed and tailored to the rock conditions. As anticipated, faults were encountered in multiple borings. The combination of weakened ground near the faults and the depth of the boreholes made it difficult to use typical installation methods. To mediate this, Crux developed an installation solution that bundled piezometers and tremie pipes into a grouting shoe at preselected depths. The grouting shoe was then lowered to its final depth using a sacrificial wireline. Implementing this system allowed for successful piezometer installation in deep borings under very challenging subsurface conditions.
In total, Crux drilled five borings to depths ranging from 900ft to 2,700ft.
The Angeles National Forest is subject to reoccurring forest fires, and part of the drilling programme was scheduled to take place during peak fire season. All project staff underwent detailed training and mitigation courses related to exposure and emergency evacuation. The project experienced a handful of wildland fire warnings, and one active fire within extremely close proximity to a drill site. Crews were evacuated according to procedure and, upon return, reviewed fire safety and evacuation plans to ensure all project protocols were efficient and effective.
Completing multiple deep cores within a federally protected area required innovative techniques to minimise effects and maximise efficiency. All drilling and instrumentation was successfully completed and will assist in determining the feasibility of the rail tunnelling through the San Gabriel Mountains. The Palmdale-to-Burbank project section is part of phase one of the high-speed rail system, which will connect San Francisco to the Los Angeles Basin. Construction on this phase began in 2015 and is expected to wrap up by 2029, with more than 119 miles under construction to date. The rail system will eventually extend to Sacramento and San Diego, totalling 800 miles and as many as 24 stations, providing an efficient and economic option for California travellers.
The Palmdale-to-Burbank project section is part of phase one of the high-speed rail system, which will connect San Francisco to the Los Angeles Basin.
Technical paper published by GeoDrilling International, October 2017 DOWNLOAD