Caldecott Tunnel Geology
Ivy Morrison and Chris Risden, California Department of Transportation—November 2, 2012
By Cinda MacKinnon, from the January 2013 newsletter
On November 2nd approximately 40 participants split into two Caltrans field trip sessions to learn about the new fourth bore of the Caldecott Tunnels and the geology of the East Bay Hills. We stopped on both sides of the tunnel to examine steeply dipping outcrops (from oldest to youngest, west to east) of the marine sandstone and shale of the Sobrante Formation, Claremont Chert and Shale, nonmarine conglomerate and sandstones of the Orinda Formation, and the Moraga volcanics.
History: The original Kennedy Tunnel first connected Alameda and Contra Costa Counties in 1903. It measured 1040 feet in length by 17 feet wide and was lined with timber. It provided limited access between the counties due to its steep approaches and narrow width. In 1928, George Posey, known for the Posey Tube connecting the cities of Oakland and Alameda, supervised the planning of two new lower tunnels and served as Chief Engineer. Professor George Louderback evaluated the site geology. Work was completed in 1937.
By the late 1950s, daily vehicle counts through the Caldecott Tunnels reached levels well beyond their capacity. A third bore started in 1960 was a near duplicate of the first two bores, except for its larger profile. The third bore introduced the “pop-up” lane change system that redirects traffic in the second bore depending on the time of day.
Engineering: Our first stop was the Caltrans construction office in Lafayette for a brief overview by Ivy Morrison, Public Information Officer and Chris Risden, Engineering Geologist for Caltrans. They discussed the New Austrian Tunneling Method (NATM) as well as the need for the fourth bore. The NATM (also known as the Sequential Excavation Method) is a widely recognized tunneling technique developed in Europe in the 1960s that uses the surrounding rock mass as one of the main strength elements. The flexibility provided by this method allows engineers to adjust certain reinforcing elements based on observed rock behavior, while constructing a tunnel at a reasonable cost and without sacrificing safety to workers or drivers.
NATM deploys two types of support: an initial lining of sprayed fiber-reinforced concrete (known as shotcrete), rock bolts, and lattice girders; and a final lining consisting of traditional reinforced concrete. The initial lining is somewhat flexible and allows a controlled deformation of the rock to achieve equilibrium. By controlling the deformation of the rock using various initial lining elements, tunnel engineers can maximize the strength of the rock mass and reduce the stresses placed on the final lining.
Rock data was collected by drilling several cores along the proposed alignment. Laboratory tests performed on the rock cores provided engineers with the data necessary to categorize the various rock types that would be encountered during tunnel construction. Engineers determined the strength of each excavated face as geologists identified the rock types and assessed the behavior of the rock mass immediately after each round (8-12 feet) of excavation. Fourteen rock types were recognized, requiring seven initial lining designs for support.
Because the size of the fourth bore (total excavated dimensions are approximately 50 feet wide x 36-41 feet high), engineers designed the excavation to occur in stages, starting with a top heading excavation consisting of roughly half the opening followed by the bottom half, called the bench. The tunnel was surveyed to monitor deformation of the initial lining and behavior of the excavation; additional support measures were then installed as needed. While the site is not bisected by an active fault, the Hayward fault runs perpendicular to the tunnel and very close by. Thus the tunnel is designed to withstand strong ground shaking.
Following completion of excavation and installation of the initial lining, the next step is construction of the final lining and roadway and installation of rock monitoring systems.
Cross section from UCMP
Geology: The geology is characterized by near vertical, marine and nonmarine sedimentary rocks of middle to late Miocene age (see cross section). Marine and nonmarine strata, as well as volcanic rocks, record the change in regime from a convergent plate margin to a transform plate margin.
An activity called “probing” using a drill jumbo was done once every several rounds of excavation to help assess conditions immediately ahead and to gradually release trace levels of gases and groundwater.
Near the west portal of the tunnels, we viewed shaley marine sandstone typical of what is exposed throughout the western 200 meters of the fourth bore. These rocks are thought to be the Sobrante Formation and are the oldest rocks in the section.
Along Claremont Avenue we examined the Claremont Chert and Shale, also known as the Monterey Formation in other parts of the state; it is a petroleum source rock. The entire unit is overturned and dips steeply to the west.
Chert beds of the Claremont Shale on Claremont Avenue. Andrew Alden photo
These rocks represent deposition in a much deeper ocean basin than the other formations and characterize sedimentation in a forearc basin of the convergent environment. The migration of the Mendocino triple junction marked the cessation of subduction and closing of the basin. The Claremont is present in the west and middle section of the tunnel alignment. Traces of petroleum in these rock spoils required that they be disposed of at a hazardous waste site.
Igneous dike (lighter colored rock) intruding the Claremont Shale within the tunnel. Note the initial lining protecting workers.
The eastern end of the tunnel penetrated nonmarine shale, sandstone, and conglomerate of the upper Miocene Orinda Formation. An angular unconformity between the Orinda and underlying Claremont documents the transition from a convergent plate margin to a transform plate margin. The rocks of the Orinda Formation were most likely deposited as part of an alluvial system during uplift of the San Francisco Bay block. Provenance studies show reworked Claremont chert clasts, but a majority of the sediments are derived from Franciscan rocks. Spectacular exposures of the Orinda are found on the east side of the tunnel. The lower part of the formation contains estuarine and nearshore marine mollusk fossils, while the upper part of the formation has produced an assemblage of land mammals that includes horses, rhinoceroses, camels, pronghorns, oreodonts, and gomphotheres as well as significant terrestrial plant fossils. Excavation required full-time paleontology monitors as part of the environmental mitigation.
Oreodont tooth from the Orinda Formation. Andrew Alden photo
Basaltic to andesitic rocks of the Moraga Formation, also known as the Grizzly Peak volcanics (9.5 Ma), interfinger with fluvial sediments. Their origin is attributed to mantle upwelling into the space formerly occupied by the subducted slab as the transform plate boundary evolved. The contacts are frequently marked by reddish bake zones where the volcanics flowed over the Orinda Formation. Locally, flows erupted from Round Top in Sibley Volcanic Regional Preserve just south of the freeway. Within the tunnel, volcanic dikes were encountered which weather to clay and hence weaken rock properties.
Crews are installing the electrical, drainage and ventilation systems, including 19 jet fans. Following construction of the final concrete lining, the roadway, and testing of the monitoring systems, the fourth bore of the Caldecott Tunnel is expected to open to traffic in late 2013.
Many thanks to Ivy Morrison and Chris Risden of Caltrans for leading this well organized and informative field trip. Also thanks to our field trip director Tridib Guha for another great NCGS day.