Caldecott Tunnel Fourth Bore, United States of America

The Caldecott Tunnel Fourth Bore project includes the construction of a two-lane tunnel bore. It is being built north of the three existing Caldecott tunnels in California, USA. The proposed tunnel will be located along State Route 24 between Oakland and Orinda. It will be 9.7m high and 12.58m wide.

The route – connecting Alameda and Contra Costa counties – carries about 160,000 vehicles per day, passing through the three bores of Caldecott tunnel and remains congested during peak hours. The proposed fourth bore will ease the congestion and when completed, there will be two separate bores for westbound traffic and eastbound traffic. It will also improve mobility for motorists and reduce delays in travel times.

The $420m Caldecott improvement project is being jointly developed by the the Department of Transportation (Caltrans), the Contra Costa Transportation Authority and the Alameda County Congestion Management Agency.

The Caldecott Improvement project began in 2000 with the funding approved in 2004. The environment study of the fourth bore tunnel was completed in 2007, while the technical study did not start before the same year. The final design of the project was completed in 2008-2009. The ground breaking for the project was carried on 21 January 2010. The tunnel is slated for completion in winter 2013-2014.

Project contractors

The main contractor of the project is Tutor-Saliba Corporation of Sylmar, California. The preliminary and final designs have been provided by Jacobs Associates, as per the contract awarded in 2004. The scope of contract involved providing details of geo-technical characterisation too.

Caldecott tunnel fourth bore project

The horse-shaped mined tunnel is 1,036m long and features two 12ft lanes. The tunnel will have a 10ft shoulder on the right, and a 2ft shoulder and 3ft walkway on the left. There will be seven cross-passages, which will connect the third and fourth bores and serve as emergency exits. The existing maintenance building will be demolished and replaced by a new two-storey operations and control building.

The project will also include the construction of retaining walls near the east and west sides of the tunnel, and permanent soundwalls on the west side. Short sections of the cut-and-cover tunnel at each portal and other additional roadway improvements are also included in the project. After the completion of the tunnel, the staging areas will be landscaped.

The tunnel design incorporates various safety and emergency measures such as the installation of a jet fan ventilation system, a wet standpipe fire protection system, and other operation and control systems such as CCTV monitoring, heat and pollutant sensors and traffic monitoring systems.

Two other ancillary small projects also feature in the Caldecott improvement programme. The Kay Street project will install a traffic signal at the intersection of Kay Street and Broadway, and will also modify the intersection of Kay Street and Caldecott Lane to improve traffic flow. The other project constitutes the modification of a freeway connector between westbound Route 24 and northbound Highway 13, and will receive the traffic coming from the third and fourth bores of the Caldecott tunnel.

Tunnel fourth bore design and construction

The design and construction of the fourth bore tunnel was dependent on the sequential excavation method (SEM) or the New Austrian Tunnelling Method (NATM). The tunnel mining design should face challenges from the weak rock and unstable fault zones.

For the design, rock and soil samples were collected from the proposed alignment to review and analyse surface geological formations, faults and groundwater conditions. The proposed alignment was also refined and probable impact on the third bore was taken into consideration.

With NATM construction methodology, the length of each excavated section is dependent on the surrounding geological conditions. The fourth bore tunnel will be mined in stages from top to bottom so that the smaller areas can be supported with sprayed-on-concrete.

This means that about 7ft can be tunnelled through hard, unfractured rock at one go. A roadheader is likely to be used to excavate the tunnel, using pre-support measures. The initial support will be a mix of lattice girders, reinforced concrete (RC) and rock bolts. The final lining of RC, cast-in-site, will follow this.

After some of the section is excavated, shotcrete will be applied to walls and steel rods will be installed around the perimeter of the excavation. The shotcrete and steel rod will provide temporary support during construction. The entire length of the tunnel will be gradually covered with shotcrete.

When completed, the final concrete and waterproof lining will continue. Drainage fleece will also be placed along with tiles on the lower portion of the walls. Other tunnel utilities such as lighting, ventilation, cross-passages and final pavement surface will be carried on at the last stage of tunnel construction.

Caldecott tunnel fourth bore financing

The project has been jointly funded by federal, state and regional sources. APRA Funds is financing the major part of the project. Other financiers include Contra Costa County, Regional Measure 2, State Transportation Improvement Program, Corridor Mobility Improvement Account and Regional Measure 2.