The Metro’s Exposition Light Rail Transit travels along an 80-year old storm drain tunnel under Colorado Avenue between 6th Street and 17th Street in Santa Monica. The facility is a United States Army Corp of Engineers (USACE)-owned storm drain, maintained by the Los Angeles County Flood Control District (LACFCD), known as the Kenter Canyon Storm Drain. The storm drain comprises of a 10’-3” three-layer brick arch tunnel with a reinforced concrete invert. A finite difference soil-structure interaction modeling program Fast Lagrangian Analysis of Continua (FLAC) was performed to determine the construction and future impacts of the Light Rail Vehicle (LRV) loading against the capacity of the underground tunnel with an overburden weight of the soil ranging between 5.5 feet to 10.5 feet. The FLAC program uses a finite difference explicit method that translates a set of differential equations into finite difference equations for each zone, relating forces at nodes to displacements at nodes.
The study concluded that the 80-year old tunnel at its present condition has performed adequately for the past decades and demonstrated numerically in the analysis that the structure should perform competently under the future combined LRV and HL-93 loading.