Completed in 1910, Crane Valley Dam is a hydraulic fill structure, 145 high and with a crest length of approximately 1,900 feet. In response to a request from the Division of Safety of Dams (DSOD), the owner, Pacific Gas and Electric, undertook a stability analysis which indicated that the dam would be susceptible to liquefaction in the event of a significant seismic event. PG&E, in close cooperation with DSOD, developed a retrofit plan that included installation of a rock-fill buttress at the western section of the dam to increase its factor of safety.
Construction of the buttress would require excavation of the residual soil, fill and weathered rock over a 500-foot length of dam toe. Piezometers installed to monitor groundwater levels had indicated artesian conditions within the weathered rock which had resulted in very saturated soils at the toe of the lower level embankment. Underlying artesian conditions could result in upward seepage of groundwater that could threaten the stability of the downstream embankment and potentially lead to failure. Dewatering was therefore required to ensure the continued integrity of the embankment during the work.
Site soils consisted of 1 to 9 feet of saturated fill material above predominantly low-permeability clayey silt overburden that varied in thickness with the slope of the dam. Within the zone of excavation, the overburden ranged from 20 feet to 45 feet. Weathered rock was present beneath this stratum.
System Design and Installation
Mueser Rutledge Consulting Engineers modeled the dewatering system using finite element methods to determine depth of well installation and pumping rates required to maintain drawdown a minimum of 3 feet below excavation subgrade. Moretrench worked closely with Mueser Rutledge to design the most fail-safe system to achieve the design intent. The dewatering depth, together with the low permeability soils, made an ejector system the best choice for the work.
Moretrench employed cased hole drilling methods which included duplex and drill and drive techniques to advance a steel casing through the overburden and seat it up to five feet into the weathered rock, followed by air rotary drilling to clear the casing. A down-the-hole hammer was used to drill the bedrock. Installation depths ranged from 20 to 70 feet below working grade.
A total of 136 ejector wells were installed, with 117 around the perimeter of the excavation and two interior rows of wells installed to increase system efficiency as well as delineate three separate work areas for the general contractor. Moretrench’s scope of work also included installation of five deep wells, extending 50 feet into sound rock, to alleviate artesian pressure and four piezometers to 13 feet below subgrade to monitor drawdown. At project completion, the ejector wells were grouted in place and abandoned.