FABRIC OF META-STABLE SOILS AND ITS INFLUENCE ON HYPOTHESIZED FAILURE MECHANISMS

A geotechnical investigation was undertaken by the Bureau of Reclamation to evaluate the potential for hydro-compaction of a proposed canal alignment in southwest Colorado. Soils underlying the canal varied in composition from lean clay (CL) to silty sand (SM/ML) with depths up to 80 feet. Laboratory tests of samples indicate dry unit weights range between 82 to 106 lb/ft3 (1.043 < e < 0.652). Fifty-seven one-dimensional consolidation tests were used on a variety of soil types yielding percent strain (with wetting at maximum load) that ranged between 3.2 to 14.2 percent. One depositional process of the foundation includes the formation of alluvial fans by way of sheetflow and mud/debris flow of sediments from adjacent mesas with collapsible soils that occur in the interfan areas.

Over the past 60 years, some simple methods have been proposed to identify and quantify meta-stable soils. One such method that uses dry density or void ratio versus liquid limit reflects a soils microscopic structure through basic engineering and physical soil properties and indicates its propensity to strain when moisture contents approach saturation. Scanning electron microscope (SEM) images of undisturbed soil samples are employed to illustrate the micro-structure and fabric of collapsible soils. Such structure is linked to hypothesized potential failure mechanisms (i.e., shear strength loss during wetting of clay bridges or buttresses and the solutioning of cementing agents). The findings of this research will be presented based on work compiled and published in partial fulfillment of graduate work at the University of Colorado, Boulder.