The North Hollywood Project, Los Angeles

The North Hollywood Project is a 10 km twin tunnel extension to the Los Angeles Metro Red Line valued at US$1·3 billion. The extension includes three stations constructed by cut and cover methods, together with the rail track, the necessary support systems, and rolling stock. This paper covers the main civil engineering construction of the project. The construction of the cut and cover stations is described. A number of interesting features of the tunnel construction are described, including the boring of the main running tunnels using tunnel boring machines (TBMs), the construction of a special seismic section of oversize tunnel through a major fault using shotcrete lining, track level rooms, starter tunnels and cross passages between running tunnels. An important emphasis of the paper is on the rock tunnels driven by the TBMs. The difficulties caused by severe geological conditions include dealing with water at high pressure and fissure grouting to reduce water inflow through the permanent lining. Both the temporary and the permanent tunnel lining are described.