Hydrogeological Characterization in the Development of Underground Structures, Los Angeles Basin, California

Abstract

Underground construction offers economic benefit to urban developments where land values demand maximizing the potential of available resources, and the vertical nature of urban development requires thorough characterization of the hydrogeological as well as the geotechnical properties of sites. This unique set of challenges, if ignored, can result in engineering complications and economic disadvantages for urban development projects. Urban hydrogeology has often been studied in its relationship to water-resources management and large-scale trends, rather than the site-specific testing and analysis required for dewatering during building construction. Aquifer pumping tests were performed at two sites in the Hollywood Basin in Los Angeles, California, where there are ongoing subsurface construction and dewatering operations. Step drawdown and constant-rate pumping tests were performed at each site, and data were collected from both pumping and observation wells screened in sand and gravel aquifer units. Time-drawdown curves were analyzed via well-known analytical solutions for drawdown in confined and leaky aquifers. While one site responded to pumping in accordance to traditional analytical models, the other exhibited evidence of secondary recharge to the aquifer from local underground construction features. As a result of these findings, construction and dewatering plans at each site were altered in ways that deviated significantly from preconceived estimates. These case studies demonstrate the need for rigorous aquifer testing and analysis at urban construction sites undergoing dewatering, and they show the pitfalls that can be avoided through the application of such methods.