Forest soils with low bulk densities are often considered less
susceptible to compaction than soils with higher bulk densities. The
objective of this study was to determine if soil strength controlled the
compression of soils with low bulk density. Four soils were selected
for this evaluation. Three of these were andic soils with low bulk
density and the fourth soil was a more dense, cohesive soil.
Undisturbed samples of saturated and partly saturated soil were
compressed in a one-dimensional consolidation test apparatus.
Measurements with separate samples were at one of 7 normal stresses
between 0.033 and 1.96 MPa. Shear strength of saturated soil was
measured in direct shear tests. Primary consolidation of saturated
soil was completed in less than one minute at all normal stresses.
Shear stress and bulk density increased continuously during shear
strain. The compression index of the cohesive soil was significantly
larger (p<0.05) than that of the andic soils. The shear strength of
andic soils (average cohesion intercept of 0.016 MPa and friction angle
of 33.3°) was significantly higher (p<0.05) than the cohesive soil
(cohesion intercept of 0.028 MPa and friction angle of 28.9°). When
saturated, the cohesive soil was more compressible than the andic
soils because of lower soil strength. A nonlinear model of soil
compression was developed that accurately predicted the compressed
density of saturated and partly saturated soil as a function of normal
stress, initial bulk density of undisturbed samples, and degree of
saturation. As degree of saturation decreased, the compressibility of
the cohesive soil decreased more rapidly than it did for the andic soils.
As a result, bulk density of dry cohesive soil increased less than it did
for dry andic soils. Differences in the compressibility of soils were
attributed to texture and clay mineralogy. The differences in the
compressibility of these soils were much smaller than were the
differences in bulk density. Decreasing water content affected the
compressibility of the cohesive soil more than it affected the andic
soils. Because soil strength controls the compressibility of these
forest soils regardless of bulk density, it will also determine the
susceptibility of soils to compaction by machines. / Graduation date: 1991
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37486 |
| Date | 02 April 1990 |
| Creators | McNabb, David H. |
| Contributors | Boersma, Larry |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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