Eastern Transportation Corridor

In the rugged hillside terrain of northbound State Route 241 (SR-241) in the northernmost part of Irvine, California a critical slope—designated as Slope 9C-470-R (AKA Loma Ridge)—experienced slope cracking and fissuring on a 300-foot tall earthen buttressed slope. Historic heavy rains that occurred in the winter of 2004/2005 caused infiltration into the subsurface creating increased pore pressure along clay beds leading to a translational block glide landslide. This natural disaster posed a significant threat to SR-241 and demanded urgent attention. 

Our geotechnical team stepped in promptly to address and evaluate the situation through detailed subsurface exploration, installation and monitoring of a network of slope inclinometers.  We ultimately provided recommendations to immediately unload significant quantities of earthen fill to temporarily stabilize the slope allowing for geotechnical investigations and rigorous slope analyses to be conducted in support of a permanent correction. 

In 2009, the slope was repaired through construction of a 180-foot wide, 900-foot long and 300-foot tall, drained stability buttress shear key. The earthwork construction project required detailed slope mapping during excavation of several million yards of Puente Formation bedrock over a 9-month period.  Our team successfully managed contractor activities and performed continuous geologic mapping of field exposures daily, 6 days a week during excavation, in order to provide real time data to the geotechnical engineers to evaluate slope stability modelling. Field activities consisted of mapping bedrock exposures, confirming elimination of disturbed bedrock and landslide rupture surfaces, and approving subgrade for placement of structurally controlled fill.  Additionally, our team provided oversight during installation of a multiple-tiered subdrainage and back-drainage systems to provide adequate passive dewatering of the mechanically buttressed 300-foot-high hillside along northbound SR-241.

During grading, previously unknown geologic conditions were encountered that required extensive review as they related to the remedial plan. Back-calculations were performed using state-of-the-art limit equilibrium and finite element modeling software to study failure kinematics and factors of safety. Reports and analyses were reviewed by Caltrans District 12 and comments from Caltrans were implemented into the mitigation plan.

Communication was key in the project success as the at-risk partners included the original grading contractor, Foothill Transportation Corridor and the Eastern Transportation Corridor Agencies, all requiring daily debriefing with contractors and engineers from multiple municipalities and State agencies in order to provide updated as-built geologic cross-sections and supporting slope stability analyses to support ultimate turnover to the State of California (Caltrans).