Geotechnical Investigation <br />Capital Hill PUD <br />Eugene, Oregon <br />The Willamette River Valley in the area of the subject site is believed to be underlain by undifferentiated sedimentary <br />rock, tuffs and basalt from the Miocene and Oligocene epochs (approximately 15 to 35 million years ago). Deposits of <br />silt and clay from fluvial and lacustrine environments covered the bedrock to various depths during the presence of low <br />energy streams and lakes in the mid-Willamette Valley. Subsequent compression forces and uplifting of the Cascade <br />and Coast Range Mountains depressed the Willamette River Valley. The rapid uplift of the Cascade and Coast Range <br />mountains steepened stream gradients causing increased erosion of the mountains and resulting deposition of thick <br />gravel layers incised within the fluvial and lacustrine deposits. <br />Approximately 13,500 years ago the Willamette Valley was cyclically flooded by catastrophic breaks in the ice dams <br />of Lake Missoula. These flood events filled the valley to an elevation of 350 to 400- feet before retreating, causing <br />sequences of upward fining deposits of silt and clay that may or may not still be present in areas depending on erosion <br />by subsequent creek and river actions. <br />3.2 Site Geology <br />The hill-crest and relatively-gentle upper slopes underlying the project site are classified in the geologic literature as <br />middle Miocene to upper Eocene-age, mafic to intermediate-composition intrusive volcanic dikes and sills, described <br />as composed of basalt, basaltic andesite, andesite, gabbro and diabase that are erosionally-resistant, with common <br />landsliding along the high-angle margins of the intrusions. The steeper eastern slopes are classified as lower <br />Oligocene/middle Eocene-age Eugene Formation, described in OFR-06-12 as: shallow marine sandstone and siltstone <br />with local/minor thin conglomerate beds; predominantly thick-bedded to massive arkosic sandstone with glauconite <br />and some mica, some is tuffaceous with matrix composed of glass shards; typically tan to brown, locally weathered or <br />hydrothermally altered to a soft massive white sandstone composed entirely of quartz, white clay and mica. Bedding <br />mapped in the Eugene Formation in the site vicinity is generally northeast at approximately 10 degrees. OFR-10-03 <br />and the DOGAMI HazVu and SLIDO websites map landslides offsite near the southeast and northeast corners of the <br />site below elevations of approximately 700 feet and two smaller slides above the Ribbon Trail below lots 19 & 20 and <br />below lot 17 (See Figure-3). <br />The website for the U.S. Dept. of Agriculture Natural Resource Conservation Service Pacific Northwest Soils website <br />(attached) maps surface soils on the hillcrest and upper slopes as Philomath cobbly silty clay on 3-12% slopes, described <br />as clay based colluvium and residuum from basic igneous rock. The typical profile is cobbly silty clay over weathered <br />bedrock at a depth of 24-inches. Steiwer Loam on 20 to 50% slopes is mapped on the steeper slopes to the east and is <br />described as silt (ML) colluvium and residuum from basic igneous rocks and sedimentary rocks. Our subsurface <br />observations were consistent with mapped units. <br />3.3 Slope Stability and Geologic Hazards <br />The DOGAMI HazVu website maps two small landslides on site, one just above the Ribbon Trail below lots 19 and 20 <br />and the other at slightly higher elevation but still within the wooded buffer area below lot 17. Both were observed <br />during the reconnaissance and appear to be relatively minor surficial slumps. Another bowl-shaped scarp possibly <br />related to slumping was observed near the middle of lot 17 but was obscured by a debris pile, and over-steepened and <br />irregular slopes were observed adjacent to the proposed road extension from lot 20 at Capital Drive to the primitive <br />loop road at lot 17. At test pit T-5 (lots 12 and 13), residual slide debris was noted within the top 2.5-feet of material. <br />These irregularities may be the result of grading and erosion associated with logging operations. Larger landslides are <br />mapped on neighboring properties below the northeast and southeast corners (See Figure-3). Geologic literature states <br />Eugene Formation materials on ridgetops adjacent to volcanic intrusions, such as mapped at the subject site, are <br />susceptible to landsliding. <br />Branch Engineering, Inc. <br />6 <br />