Report - Oregon State Library: State Employee Information Center ...

Tacoma, Washington (Dickenson and Brown 1997b). Additional data has been
gleaned from laboratory testing of reconstituted specimens at Oregon State
University (Brown, in preparation). The geotechnical characterization for these
sites included mud rotary borings, thin-wall tube samples in the low- to highplasticity
silts, penetration resistances provided by SPT and CPT, as well as
limited shear wave velocity and pore pressure data (excess pore pressures during
cone advance and pore pressure dissipation in the fine sands and silts).
Conventional cyclic triaxial and simple shear equipment was employed and
modified with Bender elements for obtaining the shear wave velocity of the
triaxial specimens, and with enhanced volume change instrumentation.
It is well known that the results of cyclic triaxial data are not strictly
representative of field behavior due to testing limitations, such as boundary
conditions, stress paths, and uniform uni-directional loading. Additionally, the
low to non-plastic nature of these soils makes undisturbed sampling very difficult.
Any technique short of controlled ground freezing and coring would be expected
to disturb the soil fabric and densify the material, thereby altering its behavior and
its tendency to generate pore pressure. Determining the magnitude of such
alterations is arduous. However, the qualitative nature of the effect is more
predictable. For loose, saturated deposits with very low to no cohesion, the effects
of sample disturbance densifies the samples and increases their liquefaction
resistance during laboratory testing. For this reason, laboratory tests would likely
overestimate the liquefaction resistance of the in situ soil to some degree. The
cyclic testing does, however, yield useful data on the liquefaction resistance of
predominantly silty soils relative to sandy soils tested using the same equipment.
This data is considered particularly useful for demonstrating how the liquefaction
behavior of the silts varies from that of sand, for which current empirical
procedures for assessing liquefaction hazards are well established.
The results of cyclic triaxial tests on silty soils from various sites in the Pacific
Northwest are presented in Figure 3.11. Cyclic triaxial tests conducted on soils
from the Puyallup River near Tacoma, Columbia River near Portland, and Fern
Hill, Oregon are presented to demonstrate the cyclic resistance curves as a
function of OCR. The data in Figure 3.11 clearly demonstrates the influence of
stress history on the soil’s liquefaction resistance. This effect should be accounted
for in hazard analyses involving overconsolidated soils. The geotechnical
properties of the samples are provided in Table 3.8. The data for relatively
undisturbed specimens of silt soils from the Pacific Northwest provided in Table
3.8 are augmented with test results for reconstituted specimens, as well as the
results of similar testing performed on other low plasticity silts (i.e., Bonnie silt
and a silt from France). This data highlights the range in cyclic resistance
obtained for the various silts using cyclic triaxial equipment. The data provided in
Table 3.8 also demonstrates the influence of sample preparation on the measured
behavior of the specimens. For example, the cyclic resistance of he Tacoma silt
represent tests performed on high quality specimens from the field and on
specimens reconstituted from a slurry and consolidated to stresses equivalent to
those in the field. Although the specimens were consolidated to the same stresses
67