ABSTRACT

The Willamette Valley is the population corridor of Oregon. The Cascadia Subduction Zone, which can create long-duration earthquakes with magnitudes of approximately 9.0, is a primary seismic hazard for the Willamette Valley. Previous investigations and experience show that Willamette Valley Silt, which is a dominate soil type throughout the Willamette Valley, is prone to liquefaction or strength loss during earthquake motion, because it is a relatively low-plasticity silt. Many of Oregon's transportation lifelines, bridges, and emergency roadways are founded on Willamette Valley Silt; therefore, understanding the liquefaction response of soils is critical. Herein, the cyclic response of undisturbed and remolded samples of Willamette Valley Silt are determined via strain-controlled cyclic simple shear tests. Long-duration motions were used to simulate the expected subduction zone earthquake motions. For each sample of tested Willamette Valley Silt, the Atterberg limits and plasticity indices were found using both the fall cone penetrometer method and the Casagrande method. The results of each method have been compared. The grain size distribution of the soil samples were performed using wet sieve analysis. The index properties were used to apply predefined liquefaction criteria from previous researchers. The importance of the mean grain diameter ratio with respect to liquefaction potential was also investigated. The results showed that liquefaction susceptibility decreases by the increase of the ratio of coarse grain mean grain size to fine grain mean grain size (D50C/D50f). The results also showed that in general, the disturbed samples are more prone to liquefaction compared with undisturbed samples.

INTRODUCTION

During the last Ice Age, Washington, Idaho, and Montana were covered by ice sheets. Every few decades, a very large "Missoula Lake" was created by an ice dam on the Clark Fork River in the Idaho Panhandle. Several very large catastrophic and periodic floods, called Missoula Floods, occurred when the ice dam failed. The floods carried soils and boulders from North America and Canada to the Willamette Valley. At least forty floods transported large amounts of fine sand and silts to the Willamette Valley (Benito and Connor 2003). As a result, the Northern Willamette Valley and the Portland Basin have thick Missoula Flood deposits (more than 35 m), and the Southern Willamette Valley has thinner Missoula Flood deposits (less than 10 m). The Missoula Flood deposits "pinch out" around Eugene, Oregon.

This content is only available via PDF.
You can access this article if you purchase or spend a download.