Graduate Thesis Or Dissertation
 

Soil engineering properties and vegetative characteristics for headwall slope stability analysis in the Oregon coast range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/b8515t67r

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  • Six headwalls in the central Oregon coast range were selected for study of soil engineering properties and vegetativecharacteristics important for analysis of slope stability. The headwalls were considered representative of those which would be candidates for timber leave areas due to geomorphic and topographic features, including steepness of slope and presence of a well defined constriction point on the downslope end. Of the six, only one headwall had evidence of recent debris-avalanche activity. No standing overstory species were found on any of the headwall blocks, the area of the headwall thought most susceptible to failure. Douglas-fir (Pseudotsuga menziesii),maple (Acer sp.), and red alder (Alnus rubra) were common on the headwall perimeters. Salmonberry (Rubus spectabilis) and swordfern (Polystichum munitum) dominate the understory plant community on the headwalls. Subsoils from two profiles on each headwall were sampled for index and classification properties. The soils are typically shallow, averaging under one meter in depth. They are silt-sand-gravel mixtures, with non-plastic fines. The soils all have exceptionally low density, and correspondingly high void ratio.Consolidated-undrained triaxial tests with pore pressure measurements were conducted using relatively undisturbed samples to determine the effective soil strengthparameters, angle of internal friction, ø', and cohesion, c'. Low consolidation pressures were chosen to model field overburden conditions. The arithmetic mean ø' was 31.1°.All the soils are considered to be slightly- to non-cohesive. There are no apparent correlations between index or classification properties and strength parameters. Variability of index properties and strength parameters appears to be as great within a particular headwall as between the six study sites. Variability in the results of the strength tests are thought to reflect natural variability resulting from colluvial soil formation processes, as well as difficultiesinherent in collecting and testing relatively undisturbed soil samples. Further investigations of strength parameters designed to include root contribution to strength are needed in order to more fully define stability characteristics of headwalls.
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