Tunneling beneath a metropolitan area is bound to lead to technical challenges as well as turn up some unexpected discoveries.

Builder Skanska Traylor Shea Joint Venture (JV) unearthed the skeleton of a mastodon as its crew worked on the 3.9-mi. extension of the Los Angeles County Metropolitan Transportation Authority's (Metro) D Line.

Called phase one of a three-phase, $9.7 billion expansion of the Metro's D Line, the section, including three underground stations (see sidebar), is expected to open in early 2026.

Running beneath Wilshire Boulevard in Mid L.A., the project is notable for its use of equipment to reduce its carbon footprint as well as its encounter with prehistoric Los Angeles at the famed La Brea Tar Pits, said Tim Lindholm, Metro's chief program management officer.

Alexandra Garcia of LA Metro photo

"We knew there was a strong possibility of uncovering fossils. So, we had to operate our machinery with extra caution to avoid damaging anything buried beneath the surface," Lindholm said. "During construction, more than 500 fossils were unearthed, including some dating back to the Ice Age."

Skanska, Traylor and J.F. Shea brought "a talented and experienced team," to the project, Lindholm said. They previously worked together on "large, complex infrastructure projects across the country." The builders maintained traffic, utilities and business access in the tight urban environment.

Anomalies, Safety Measures

Lindholm noted the difficult ground conditions the builders faced — tar sands, methane zones and undocumented oil-field remnants. The tunnels were classified as "gassy." In a first, the team used horizontal directional drilling and magnetometer surveys to identify hazards before discovering them during tunneling,

Additional safety measures included "vapor-extraction systems, targeted dewatering to manage groundwater and continuous settlement-monitoring along the corridor," Lindholm said.

"We've encountered a few surprises during construction, with the tunnel boring machines [TBMs] specifically when we were looking for oil wells that may have been within the tunnel alignment," he added.

One surprise was the steel anomalies found at tunnel depth at two locations.

"At the east end, directional drilling and a magnetometer located a potential casing within the tunnel alignment; this anomaly was avoided by steering the TBMs around it," he said.

"The anomalies at the west end proved to be more of a challenge. These anomalies, at the intersection of Wilshire and San Vicente, were too close to the Wilshire/La Cienega station to steer around and needed to be removed."

Working with Metro, Skanska Traylor Shea Joint Venture JV installed chemical grout blocks around the anomalies to stabilize the soil. Then, at 1-ft. increments, hand mined in front of the TBMs to locate the anomalies.

"The anomalies, while not oil wells, were steel beams that would have severely damaged the TBMs," Lindholm said. That never happened. The anomalies were removed and tunneling to La Cienega was completed.

Zero Emissions

The construction of the first phase of the D Line extension was an opportunity to test innovative equipment that would decrease the carbon footprint on construction sites, Lindholm said.

Alexandra Garcia of LA Metro photo

Because the tunnels were classified as gassy, electrical equipment underground is required to be Class 1 Division 2 (explosion proof).

"We utilized a highly specialized fleet of equipment consisting of small excavators, track loaders and Gradalls, all designed to run at lower operating temperatures than traditional equipment," he said.

The team piloted a zero-emission Volvo EC230 Electric excavator at the La Brea station to test the capabilities of a battery-powered excavator to perform excavation work.

"We also piloted a zero-emissions electric compaction roller at the Fairfax station for the compaction of subgrade and crushed aggregates," Lindholm said.

For the tunneling, other electrical equipment with zero emissions include: backfill grout batching plant, conveyor systems, ventilation systems (tunnel and stations), cooling plant and compressed air plant.

At one point, the team needed a specialized ventilation system operating 24 hours a day to remove contaminated air and ensure safety for workers.

Avoiding Tar Pits

The La Brea Tar Pits (actually, it's asphalt that oozes up from pools) is located near Wilshire Boulevard and the Section 1 route. According to Lindholm, tunneling along the alignment presented a unique challenge.

"Historic open-face tunneling machines could not safely be used due to the gases present in the ground. As a result, Metro required a pressurized face TBM," either Slurry Pressure Balance or Earth Pressure Balance.

Back in 2015, STS undertook laboratory testing for both SPB and EPB TBMs, Lindholm explained.

Alexandra Garcia of LA Metro photo

"The results showed that, with an SPB, separating the tar from the bentonite slurry would be nearly impossible, but the tar impacted material could be transformed into an earth paste for use with an EPB TBM using specially modified soil conditioners — engineered by Traylor's in-house soil conditioning lab, Boraid," he said. "These tests, along with geotechnical information, led to the selection of an EPB TBM and the use of conveyors for muck extraction."

The second and third phases of the D Line extensions are under construction. Work began on the second phase in 2017 and is 85 percent complete. The third phase extension began in 2018 and is 75 percent complete. Both are scheduled to open in 2027 in time for the 2028 Olympic and Paralympic Games.

According to Lindholm, remaining work on the second phase of the D Line extension includes completing the Century City and Beverly Drive Stations, as well as track installation and operational systems testing. For the third phase of the project, completion of construction of the two underground stations in West LA and at the VA hospital are still under way as is trackwork and systems installation, testing and commissioning. CEG