Soil and hydrologic factors affecting stability of natural slopes in the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5h73q1340

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  • This study was conducted to examine certain soil and hydrologic properties of two major cohesionless soils Occupying 55% of the central portion of the Oregon Coast Range. Knowledge of these properties was desired to determine the role each played in the stability of slopes in this region. Bohannon and Klickitat soils often occupy the steep midslopes where the greatest potential for stability problems exists. The Bohannon series is derived from Tyee sandstone and the Klickitat series is derived from intrusive, igneous parent material. Soil samples were obtained from four widely separated sites, two for each of the soil series and were examined for particle-size distribution, bulk density, porosity, pore-size distribution, aggregate stability, saturated and unsaturated hydraulic conductivity, and shear strength. A 1. 15 ha study site was instrumented with a recording raingage, 78 piezometers, and four tensiometers placed at varying depths in the soil profile. Field measurements were made t subsurface water movement in the Klickitat soil during the 1973-74 water year, one of the wettest on record for this area. An intensive subsurface geologic survey of this study site was also made. Both soils, although derived from very different parent materials, exhibited nearly identical ranges of values for soil and hydrologic properties. Both were found to be extremely porous, highly permeable, very well aggregated and graded, sandy to gravelly cohesionless soils. From engineering and hydrology standpoints, the two soil series can be considered as one. In spite of low bulk densities and high porosities, the dry effective angle of internal friction, [phi], was found to be unusually large in both soils. For the Bohannon and Klickitat soils, [phi] was 40[degree] and 41[degree], respectively. Such large [phi] values for such loosely packed soils were attributed to the high aggregation in both soils. Pseudomorphs were stable enough to function as primary particles and possessed increased surface roughness, angularity, and effective size over what they would have had as discrete particles. The effect of water on [phi] was found to be atypical for both soils. Reductions in [phi] of 9.5[degree] and 11[degree] were noted when the two soils were rested in a drained, saturated state. The severe reductions in [phi] were attributed to aggregate disintegration under direct wetting conditions. A decrease in aggregate content of 29% in the Bohannon soils was accompanied by a 28% decrease in [phi]. For the Klickitat soils, the 26% decrease in aggregate content was accompanied by a 23% decrease in [phi]. Aggregate destruction by direct wetting is a possible mechanism for some slope failures near roads. Movement of subsurface water was predominantly by unsaturated flow. While saturated flow was observed in fractured bedrock near the sedimentary-igneous contact, only one instance of saturated flow in the soil profile was noted. Tenisiometry indicated that minimum capillary pressures of 5-10 cm of water existed during storm events. Analysis of pore-size data and moisture-tension relationships substantiated the effectiveness and adequacy of unsaturated flow as the prime mechanism of water transmission in these soils. Both soils were able to transmit water rapidly and at large fluxes even under unsaturated conditions. Large scale saturated subsurface flow is unncessary for dispersing the low intensity, long duration rainfall found in this region.
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