Lessons Learned from a Reduced-Size Exploratory Shaft for the Los Angeles Metro D-Line (Purple Line) in Tar-Impacted Soils

Los Angeles Metro Rail system includes an expanding underground transportation network with over 20 mi of subway in service and another 10 mi currently in construction. An extensively monitored exploratory shaft was excavated in tar-impacted soils next to a future Los Angeles Metro station to assess the excavation performance and associated ground deformations in tar soils. The exploratory shaft was excavated to dimensions of 5.5 m in width, 11 m in length, and 22.6 m in depth. It served as a "small-scale" test bed for the future, full-size Metro station that was approximately 15 m wide, 360 m long, and 23 m deep. Data from inclinometers, surface settlement markers, piezometers, and strain gauges provided in situ performance indicators during and after the shaft excavation. Maximum lateral movement of 24.8 mm was recorded near the mid-span of the excavation, equivalent to about 0.1% of the excavation depth. Two-dimensional numerical modeling, using the software package PLAXIS, was performed to simulate the excavation performance during various construction stages at the excavation mid-span. The simulations were performed with linear elastic (LE) and hardening soil (HS) constitutive soil models using parameters estimated from in situ and laboratory testing. The HS model predicted lateral movements that compared favorably with the measured in situ wall deformations.