STRUCTURAL GEOLOGY AND THE DIP-SLOPE FAILURE ISSUE <br />As discussed above, under the previous section, the applicant’s geotechnical consultants misidentify <br />the rock type underlying the PUD, assuming that the ridge is underlain by igneous rock, when, in <br />realityit is underlain by sedimentary rock. However, this is a very important distinction. The <br />sedimentary rock making up the ridge consists of the Eugene Formation which is a shallow marine <br />deposit made up not only of sandstone, but a complex rock unit consisting of interbedded sandstone, <br />siltstone, claystone, and weathered volcanic ash beds. Throughout the Eugene/Springfield area, <br />these bedded sedimentaryrocks dip generally to (are inclined toward) the east. Based on the <br />observations in the front yard of 2808 Capital Drive, the easterly dip is true in this vicinity also. The <br />result is similar to a stack of cards tilted in an eastward direction. Planes of weakness caused either <br />by the bedding in therock or by weak beds themselves, such as clayeydecomposed ash layers, are <br />much more likely to slide in a direction parallel to the eastward-inclined bedding than westward, <br />where failures are forced to break through the package of bedded rocks (seefigure on the next page). <br />This is reflected in the DOGAMI SLIDO map of the larger area, where clearly the majority of the <br />mapped slides originate on east-facing slopes. <br />This propensity for east-facingslopes to be less stable than west-facingslopes is caused not only by <br />inclined planes of weakness, but also by groundwater flow and discharge areas. Precipitation falling <br />onto the entire ridge partially sinks into the ground and some of it infiltrates into more permeable <br />beds in the rock (e.g. sandstone), flowing initially downward, until it encounters an impermeable <br />stratum (e.g. siltstone or claystone). Because this stratum will be following the general eastward- <br />dipping trend, the water then “runs down-dip” on top of the impermeable unit. In this manner, not <br />only water fallingon the east-facing slope of the ridge will be transmitted to base of that slope, but <br />also some water fallingon the west-facing slope of the ridge. Where this water exits the rock <br />underlying the slope on the east-facing slope, it emerges as seeps. If the rock in that vicinityis <br />covered with an impermeable clayey soil layer, the water will build up in the rock until the pressure <br />is sufficient to either break through the soil layer or dislodge portions of the soil layer by reducing <br />the friction holding them in place in manner very similar to the oil pressure in a car’s engine. This <br />results in sliding of portions of the slope. Such “excess pore pressure” is very likely responsible for <br />most of the multitude of slope movements on east-facing slopes of the hills in the Eugene- <br />Springfield area. <br />The applicant’s geotechnical assessment either does not recognize this fact (as indicated bythe <br />remark regardingthe presence of “stable volcanic rock”) or ignores the presence of the dip slope and <br />fails to investigate the portion of the PUD most sensitive to dip slope failures in an area where <br />groundwater could develop significant pore pressures. Instead, the subsurface exploration was <br />limited to the low-sloping area at the top of the ridge. <br />7 <br />Capital Hill PUD Geotechnical Review, GeoScience, Inc. 3/7/18 <br /> <br />