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

interest. Specifically, the peak bedrock acceleration having a 10% probability of exceedance in
50 years (i.e., 475 year return period) is approximately 0.2 g along this portion of the Columbia
River. These strong ground motions, modified to account for site effects, are clearly capable of
initiating liquefaction in the sandy soils that are prevalent along the riverfront.
The acceptable seismic performance of earth structures built adjacent to the river will reflect the
sensitivity of bridge structures and ancillary components to the magnitude and pattern of
potential soil deformations (e.g.; shallow sloughing, deep-seated failure). Anticipated repair costs
for potential failure modes, as well as the consequence of loss of serviceability of key
transportation lifelines in the event of a major failure are also primary concerns. Pertinent issues
for this example problem are shown below.
The influence of river stage on the static pore pressures in riverfront embankments and
foundation soils, and the variation in seismic performance of the levee as a function of
the river level. The static stability of the levee is evaluated at three pertinent river stages
(i.e., the water level at the crest of the levee, the 100-year flood elevation, and the
summertime, low flow condition).
The excess pore pressure generation and post-liquefaction behavior of both sandy and
silty foundation soils.
The potential for large permanent deformations or flow failures of the Columbia River
sediments.
The potential for excessive settlement of earth structures due to volumetric changes in the
soil following cyclic loading.
The applicability of the standard sliding block-type analyses presented in this report for
estimating earthquake-induced deformations of riverfront embankments.
The application of a 2D numerical dynamic effective stress model for estimating the
seismic performance of riverfront embankments
The effectiveness of soil improvement for mitigating liquefaction hazards and
minimizing earthquake-induced deformations.
8.1.1 Geotechnical Site Characterization
Soils reports provided by the Portland District of the Corps of Engineers and the Port of Portland
were reviewed along with information provided by local engineering consulting firms, the
Oregon Department of Geology and Mineral Industries (DOGAMI), and the technical literature.
This information was supplemented with the results of a more rigorous, site-specific
investigation (Dickenson et al. 2000). A total of four exploratory mud-rotary borings, 12 cone
penetration (CPT) soundings, and one dilatometer (DMT) sounding were carried out in this
complementary investigation. This field investigation provided requisite engineering properties
for the geotechnical analyses outlined here. The field investigation was augmented with data
from a comprehensive geotechnical laboratory investigation. High quality, thin-walled tube
samples and the disturbed split-spoon samples obtained during the field investigation were
transported to the Geotechnical Engineering Laboratory at Oregon State University. The
following standard laboratory tests were performed: moisture/density, gradation, Atterberg
limits, consolidation, direct shear, and triaxial compression tests (TXUU, TXCU). Also, a suite
of more sophisticated cyclic triaxial tests were completed.
140