Settlements Due to Arid Collapsible Soils

The arid alluvial deposits common to the western United States frequently undergo moderate to severe collapse when wetted that can threaten or destroy the stability of structures that are founded on such soils. Additionally, in some areas collapse has been known to occur due to overburden stresses alone. As development expands in these areas a method to estimate the degree of this collapse is essential. Current settlement methods are reviewed and evaluated as they apply to collapsible soils, providing a basis for the development of a proposed settlement method for determining collapse. To aid in the determining the basis for the proposed settlement method finite element analyses are conducted to determine if the collapsible silts found near Nephi, Utah are cohesive or frictional in their behavior. The role of the pressuremeter (PMT), an in situ testing device, and its value in determining relevant soil properties for collapse prediction is discussed. Testing methods are presented for dry, wet, and dry/wet combination conditions in the soil. Finite element analyses are conducted to determine the influence zone about the pressuremeter probe and the effects of non-coalescing moisture patterns on PMT test results. A comprehensive evaluation of a field test conducted near Nephi, Utah provides necessary data for the development of the proposed settlement method. A component of settlement particular to collapsible soils is identified and factors that contribute to this component are discussed. The collapse strain, a property of metastable soils that is essential to the proposed settlement method, is defined and methods of determining its value are presented. Making use of the yield strain, loading conditions, and limit pressures of the soil in the dry and wet state methods for determining collapse strain are proposed. Within the collapsible layers zones of collapse that are dependent upon the loading conditions are identified and a method of estimating the height of these zones is proposed. The values determined for collapse strain and the height of the collapse zone are then used to predict the collapse portion of settlement. For the silts tested near Nephi, Utah the proposed method gives results within 5% of the measured average settlements for the loading conditions of overburden alone, existing structures, and structures on wetted collapsible soil. Extrapolation of the data provides a method of determining collapse for embankment loading. The proposed method for predicting average collapse is applied in an example problem making use of a PMT test pair from a debris earth dam in Colorado