|See the world (and its fossils) with UCMP's field notes.
|SEARCH | GLOSSARY | SITE MAP|
From previous experience, Caltrans knew that the Fourth Bore would be cutting through rocks that are fractured, unstable, and potentially dangerous; during construction of the Broadway Low Level Tunnels (Bores 1 and 2) in the 1930s, four workers were killed and several others injured in two cave-ins. And not helping matters any, the tunnel was going to be below the water table (scroll down to see the East Bay Hills cross-section below); with water finding its way into the cracks and discontinuities in the rocks, an already dangerous tunneling environment is made even more so. Furthermore, there was the possibility of encountering pockets of methane gas, so workers could not bring anything that might make a spark into the tunnel; this meant no internal combustion engines, cell phones, or cameras.
Faults are at fault
Most East Bay residents are familiar with the Hayward Fault since it gets a lot of press. And rightly so. The Hayward is considered a very dangerous fault since it runs through a densely populated region and has not shown any major movement for nearly 150 years; it has great potential for producing a large earthquake (estimated at magnitude 7.4). The Hayward Fault is located just about a mile southwest of the Caldecott Tunnel's west portal. Although no active faults bisect the Caldecott, and tunnels in general tend to resist deformation when there is strong ground motion, the possibility of a large earthquake had to be taken into account in the design and construction of the Fourth Bore. In fact, State Route 24, which passes through the tunnel, is considered a critical lifeline between Alameda and eastern Contra Costa counties; the tunnel must allow passage of emergency vehicles within 72 hours of a major earthquake. So, despite the engineering challenges presented, Caltrans needed to deliver one tough tunnel!
Data gathered from this preliminary work was used in preparing a Geotechnical Baseline Report (GBR). This 100-page document included "a project description; interpretations of the geological and geotechnical data obtained for Bore No. 4 and cross passages; descriptions of ground classes, which are defined by anticipated ground conditions and behaviors; descriptions of groundwater conditions; construction considerations; and summaries of relevant previous tunneling experience."3 The GBR served as a guide for contractors in determining the appropriate tunnel geometry and linings, as well as the structural supports needed for each rock type. It also provided the basis for formulating realistic cost estimates when bidding on the project.
The formations involved and their characteristics
The GBR divides both the Sobrante and Claremont formations into three distinct subunits (after Page, 19505). Let's take a look at all the individual rock units that crews would encounter along the Fourth Bore alignment.
Sobrante Formation: The first 200 meters of the Fourth Bore cuts through this formation, which is made up of Middle Miocene sandstone and shale deposited on or near the continental shelf prior to the arrival of the San Andreas Fault System.
Claremont Formation: These are 15- to 14-million-year-old alternating beds of shale and chert deposited in a deep coastal basin. Page (1950) also recognizes three subunits of the Claremont:
Note the mention of sandstone dikes. Tunnel crews encountered sandstone dikes in the Claremont Formation during construction of the previous bores. In some cases, when a dike was contacted, sand would flow out like uncompressed beach sand! Current thinking suggests that the sands come from the Sobrante Formation's Portal Sandstone, stratigraphically below the Claremont Shale. The Claremont's chert and shale became brittle and fractured before the sands beneath them hardened into a sedimentary rock. Pressure then forced sand-rich slurries into the fractures, creating the sandstone dikes.
Orinda Formation: The Middle Miocene Orinda is made up of "… interbedded bluish gray and greenish gray conglomerate, sandstone, and siltstone, and grayish red claystone" deposited as an alluvial fan (see photo below) between 10 and 12 million years ago. The fan formed on the east slope of high ground rising about where San Francisco Bay is today. An unconformity (a surface indicating a period of no deposition) separates the Claremont Shale from the Orinda Formation, representing a period of erosion after the sea floor emerged.
Moraga Formation: Beginning around 9.5 million years ago, a period of volcanic activity began with a series of basaltic lava flows and the occasional tuff that buried the Orinda Formation. Clearly there was no break in deposition between the uppermost layer of the Orinda and the lowermost lava flow of the Moraga because that lowermost flow has clearly "baked" the uppermost rocks of the Orinda. The lava flows and igneous intrusions associated with it originated from Round Top, located a little over a mile southeast of the tunnel. As noted earlier, these intrusions had been encountered during construction of the previous three bores but the frequency with which they occur has diminished with each successive bore, probably because each new bore has been farther away from Round Top. The igneous dikes of the first two bores were some of the worst rocks encountered, as they were weak and soft when wet.
Dealing with the geology: Excavation and support
From the graphic we can see that the Orinda Formation was expected to have the strongest rocks, with the Claremont Formation a close second. But still, because of the fractured, unstable nature of the East Bay Hill's rocks, the GBR specified that the Sequential Excavation Method (SEM) also known as the New Austrian Tunneling Method (NATM), a system developed in Europe in the early 1960s be employed. Instead of utilizing stiff supports to bear the ceiling load, the SEM uses the inherent strength of the surrounding rock as part of the support system, thereby keeping materials and costs to a minimum. The tunnel design and specifications outlined in the Fourth Bore's GBR were all based on the SEM philosophy.
The SEM is a very flexible approach to tunneling. Based on rock strength and behaviors, practically all aspects of the process can be modified for the conditions, whether it be round length (distance of each new advance in the excavation), the type of support to be installed and the timing of its installation, the need for additional support, or the subdivision of the excavation into multiple stages of excavation in which the tunnel is gradually enlarged.
The GBR specified that the tunnel be excavated in two stages: first, the top heading, followed by the bench and invert. This two-stage process is done to maintain tunnel stability and to control ground movement.
The GBR is the contractor's blueprint for the tunnel, providing information on where a specific kind of rock will be encountered, its condition and possible behavior, and what kind of support is appropriate. However, an accurate assessment can only be made during actual excavation. So after each advance or round generally 8 to 12 feet supports are tailored to the rock conditions.
In general, the steps taken after each round of excavation of the top heading, are as follows:
The Fourth Bore was excavated from both the Orinda and Oakland sides, with the excavation on the westbound side getting underway in August of 2010. About three quarters of the tunnel was excavated from the Orinda side because the Orinda and Claremont formations were more stable than the Sobrante. The eastbound, Oakland side excavation began in March of 2011. The breakthrough of the top heading excavations took place on November 29, 2011, about 200 meters in from the Oakland side.
By the time of the breakthrough, all of the top heading and most of the bench had been excavated in the relatively short stretch of tunnel that had begun on the Oakland side. At this point, the roadheader reversed direction and was used to remove the bench in the long stretch back to the Orinda portal. A smaller roadheader was used to excavate seven cross-passages essentially emergency exits connecting the Fourth Bore to the Third Bore.
With the digging portion of the project about done, the next step was to install a waterproof membrane on the initial shotcrete coating. Gantries were constructed outside the west portal; these mobile rigs moved westward through the tunnel on rails, enabling crews to install both the membrane and the final, two-foot-thick concrete coating poured over a network of reinforcing bar. Where the rock at the base of the tunnel (the invert) was competent and not prone to swelling or expansion, it was not necessary to pour a concrete floor (see graphic below), but where the rock was questionable, a concrete invert slab was installed. The first 50 meters of the Sobrante Formation and about 300 meters at the eastern end in the Orinda Formation received the invert slab treatment.
The SEM proved to be the perfect excavation approach for the Fourth Bore. The contractor for the project, Tutor-Saliba Corporation of Sylmar, California, (cue the trumpets!) completed the job with an excellent safety record and no cave-ins.
However, even with the final tunnel coating installed, there were still many months of work remaining that involved installation of operations, communications, ventilation, and emergency systems. More can be learned about this stage on the Fourth Bore website. Just under four years since the groundbreaking ceremony (January 2010), the Fourth Bore was opened to traffic on November 16, 2013.
Next, we turn our attention to the fossils recovered during tunnel construction.
1 Page 69, Caldecott Improvement Project: Geotechnical Baseline Report, prepared by Jacobs Associates, June 2009. 100 pp.
2 Page 6, Field Trip Guide "Tunneling Through a Miocene Plate Boundary" by Chris Risden and Ivy Morrison. Courtesy of Chris Risden, Senior Engineering Geologist, Caltrans' Office of Geotechnical Design West.
3 Page 5, Caldecott Improvement Project: Geotechnical Baseline Report.
4 Page 40, Sloan, D. 2005. Geology of the San Francisco Bay Region. University of California Press. 335 pp.
5 Page, B.M. 1950. Geology of the Broadway Tunnel, Berkeley Hills, California. Economic Geology 45(2):142-166. DOI: 10.2113/gsecongeo.45.2.142
6 Quoted excerpts are from Caldecott Improvement Project: Geotechnical Baseline Report. Prepared by Jacobs Associates. June 2009. 100 pp.
HOME | SEARCH | GLOSSARY | SITE MAP | FREQUENTLY-ASKED QUESTIONS