Geosynthetic-Strip Reinforced MSE Wall for Dam Expansion

Abstract

To significantly increase the water storage capacity of a reservoir, it is necessary to increase the height of the dam itself. This paper describes the design method and construction challenges of a geosynthetic-strip-reinforced mechanically stabilized earth (MSE) wall used for the expansion of Los Vaqueros reservoir dam in Contra Costa County, CA. The site is located in a high seismic area, and with their proven performance under such conditions, an MSE wall with a maximum height of 15m (49 ft) was designed and installed to provide a wider dam crest while being part of the embankment system to increase the dam height by 10.4m (34 ft). Evaluation of the wall was performed using a conventional AASHTO Allowable Strength Design method and takes into account fully submerged and rapid drawdown conditions, as well as high seismic acceleration. The wall section geometry had a trapezoidal distribution using variable lengths of soil reinforcement to avoid conflicts with the dam clay core and the existing rock backface. High tenacity polyester GeoStrap TM (geosynthetic strip) was used for the soil reinforcement for the practical reason of strength in relation to the variable soil reinforcement lengths posed by site space limitations. In order to minimize the hydrostatic pressure applied to the MSE wall, highly permeable gravel backfill was used as select fill in the reinforced volume. This paper also discusses construction challenges associated with the use of gravel backfill, the presence of a rock backface and clay core behind the wall, the nonuniform lengths of soil reinforcement and maintaining a vertical wall facing.