LONGITUDINAL CRACKING OF FLOOD RETARDING STRUCTURES RELATED TO COLLAPSIBLE SOILS

This presentation describes the techniques used and the results of recent evaluations of four flood retarding structures operated and maintained by the Flood Control District of Maricopa County (District) in Arizona. Evaluations of Vineyard FRS and Saddleback FRS were prompted by the discovery of longitudinal cracking on the upstream face or toe of the structures after rainfall events. Evaluations of Buckeye FRS No. 1 and Spook Hill FRS were completed as part of the District's dam safety program, and encountered vertical longitudinal cracking near the centerline of the structures. At Buckeye FRS No. 1 the crack formed at a depth of about 16 feet, below the bottom of the central filter/drain of the structure.

It has long been known that cracking of such structures typically occurs as a result of desiccation, localized differential foundation settlement and regional subsidence related to aquifer depletion. The four structures typically are about 25 to 30 feet high and were constructed with broad foundation cutoffs extending through the surface fine-grained Holocene soils to underlying competent Pleistocene soils. Detailed evaluations of the structures have included sampling within borings extending into foundation soils, test pits and test trenches in conjunction with detailed mapping of the cracking observed, laboratory evaluations of consolidation-collapse potential, and analyses of the potential for differential movements to have occurred, resulting in horizontal strains of sufficient magnitude to cause cracking.

The evaluations concluded that the most probable mechanism for the development of longitudinal cracks is partial wetting of the collapsible foundation soils beneath the upstream sections of the structures. The vertical, centerline cracks in two of the structures were previously unknown, but can be explained by partial wetting as a result of very infrequent storage events and the desiccated brittle characteristics of the embankment soils. Details of the embankment configurations, likely storm events and seepage and other analyses completed to support the mechanism for cracking are presented.