Los Angeles, CA

Gold Line Eastside Extension

Los Angeles County Metropolitan Transit Authority (LA Metro)


The Los Angeles County Metropolitan Transit Authority’s Gold Line Eastside Extension is approximately 6 miles (9.7 km) long, with eight new stations extending from Union Station in downtown Los Angeles east along 1st and 3rd Streets through various East Los Angeles communities. While much of the route is at-grade, a twin-bore, 1.8‑mile-long (2.9 km) tunnel alignment is located under 1st Street with two cut-and-cover stations at Soto and Boyle Streets.

Fast Facts
  • 6 miles (9.7 km) of light rail
  • Crossing of five major Caltrans-owned freeways
  • Eight stations
  • 1.8 miles (2.9 km) in twin-bore soft-ground tunnels

Scope of Work

Delve Underground provided geotechnical, structural, and tunnel engineering services in support of preliminary and final design. This work included building assessment, analysis of settlement due to tunneling, and design of protection for at-risk buildings above the various underground transit structures.

Challenges & Innovations

The tunnel route intersects commercial and residential districts that required building protection against settlement. Earth pressure balance tunnel boring machines were chosen to reduce potential surface impacts, including surface settlement, and to eliminate the need to dewater during tunneling. During construction, the buildings were monitored for movement to determine if mitigation measures needed to be implemented. Used on an as-needed basis during the tunnel construction, compensation grouting was deemed to be the most flexible building protection method based on geotechnical conditions.