Abstract:
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.
