Cost: $6.3 million
The Los Angeles County Metropolitan Transit Authority’s Gold Line Eastside Extension is approximately six miles (9.66 kilometers) 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 (2.9 kilometer) long tunnel alignment is located under 1st Street with two cut-and-cover stations at Soto and Boyle streets. The tunnel route intersects commercial and residential districts that required building protection against settlement.
McMillen Jacobs Associates 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. 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. Compensation grouting was deemed the most flexible building protection method based on geotechnical conditions. It was used as needed during tunnel construction. This project utilized a combination of design-build and design-bid-build contract delivery methods.