ENGINEERING GEOLOGY AND SITE CHARACTERIZATION FOR STATE ROUTE 710 TUNNEL TECHNICAL STUDY

摘要

The extension of State Route (SR) 710 to alleviate traffic congestion and to improve air quality in the Los Angeles basin has been in the planning stages for over 40 years. Several options are being evaluated to alleviate the traffic congestion by the California Department of Transportation (Caltrans) and Los Angeles County Metropolitan Authority (Metro). One of the proposed options is constructing tunnels to extend SR 710. The proposed extension that uses the tunnel will be one of the longest and the largest highway tunnels in the world, at over 50 feet (15.2 meters) in diameter and lengths ranging from 4.5 (7.2 kilometers [km]) to 11(17.7 km) miles. A geotechnical study was implemented by Caltrans and Metro to evaluate the subsurface conditions along five zones proposed for the tunnel option. The purpose of the geotechnical study was to determine the geologic, groundwater, and seismic conditions within the selected study zones to identify factors that affect the geotechnical feasibility of designing and constructing the proposed tunnel, and to provide a basis for a comparison of the geologic conditions with respect to tunneling design and construction. Challenging geotechnical conditions were identified in the study area, including variable ground conditions, high groundwater, active faults, high seismicity, contaminated soils and groundwater associated with superfund site, and naturally occurring gas (methane and/or hydrogen sulfide). This paper summarizes the approach implemented for the geologic and geotechnical characterization of the study area and discusses the potential challenges for tunneling.