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The Cut and Cover Tunneling Method for the Mid-Coast Trolley Underpass

Tunneling Through to Make Way for a New Line

The Mid-Coast Trolley project extends new trolley line service from Santa Fe Depot in Downtown San Diego to the University City community. At La Jolla Colony Drive, the line will pass under the street – where the Mid-Coast Trolley Underpass will be constructed through the cut and cover tunneling method. This is the only 0.03 miles of the 10.92 miles of new trolley line that are underground.

Preliminary construction for this underpass, including underground utilities relocation and excavation, began in 2016. Major construction activity is scheduled to begin March 18, 2019.

Cut-and-cover tunneling is a type of tunnel construction and the oldest method of tunneling, historically known to be used 4,000 years ago in Ancient Babylon. This technique is used typically for relatively shallow tunnels at depths of 60 feet and rarely exceeds 100 feet. The method is determined by several factors, including geology, cost, and potential disruption of other activities.

The cut-and-cover tunneling will involve excavating a trench with slurry and reinforced concrete walls to act as support from softened earth and water. Concrete roofing provides deck and roadway support on the surface. As drilling, excavation, and utility relocation takes place, the roofing will prevent excess noise from escaping the tunnel.

As construction ramps up for the Mid-Coast Trolley Underpass, detour routes and signage will be in place for commuters and residents. Here’s what to expect: 

  • Northbound La Jolla Colony Drive will be re-striped to accommodate both southbound and northbound traffic from east of the Interstate 5 on- and off-ramps to Rosenda Court. Southbound traffic will be shifted to the northbound lane. 
  • Freeway ramps will remain open.

Some full weeknight and weekend closures may be required. During these intermittent closures, there will be no access to La Jolla Colony Drive from east of the Interstate 5 on- and off-ramps to Rosenda Court. Details of each closure will be included in future construction updates. 

Refer to the Mid-Coast Trolley Underpass construction notice for even more specific information on what to expect, construction impacts, and a detour map.

Project activities include drilling, road widening, excavation, removal of dirt, and concrete construction. Construction is expected to be complete by late 2019 and Trolley service utilizing the tunnel is anticipated to begin in late 2021.


For more information about all Mid-Coast projects, please visit or contact Shift by phone at 844-SHIFT-SD or email us at Learn more about these projects and stay up-to-date with the latest timely construction schedules by following @ShiftSanDiego on Twitter.