15 ATTACHMENT 9 - Laserfiche WebLink

Page 3 1696-16 Our review of historic aerial photographs (1949 black-and-white, USDA series GS -JL aerial photographs 2-57, 2-58, 2-74, 2-75) confirms the presence of a landslide complex. Although a Terraseach, Inc. California Certified Engineering Geologist examined two stereo pairs of aerial photographs to evaluate the presence or absence of landsliding on the subject property (page 8), the photos used in the examination are not provided. Information on the vintage and scale of the aerial photographs exanuned also is not presented in the report. It is customary, standard of practice, to provide this information with the cited references. More importantly, the location of the landslide mapped by Nilsen should be shown on the site geologic map and positive evidence for the presence or absence of landsliding should be presented. Our field examination of the site identified a broad swale along the southeastern margin of the property, roughly coincident with the margin of the larger landslide mapped by Nilsen. A stock pond is located at the head of the swale, upslope of the proposed limit of grading for Lot 6. The source of the water for the stock pond is not apparent and may be a spring. Because uncontrolled water seepage or flow is one of the major causes of soil movement, it is important that the source of the water be identified. No information is provided in the report on what will be done with the stock pond and possible water seepage uphill of Lot 6. The presence or absence of a landslide margin, colluvial- related slope creep, and/or seepage from a spring in the vicinity of Lot 6 should be addressed. We recommend that the source of the water in the stock pond be identified, any seepage controlled by adequate drainage, and the colluvial swale be cleaned to a firm soil or rock base prior to placement of fill. Subsurface exploration studies on the adjacent Heinz property indicates the large landslide feature is likely much smaller than the outline shown by Nilsen. However, Terrasearch should confum that this feature does not exist on the Sarich parcel by supplemental subsurface exploration. In addition, the smaller landslide feature should also be evaluated unless it is concluded it is an active feature. Appropriate, specific mitigation measures should be recommended for identified concerns, including seepage in the vicinity of proposed Lot 6. 2. Estate lot cut slope -Proposed grading for the hilltop estate lot consists of a significant cut up to 40 feet high. As stated on page 17 of the report, "the subsurface material that will form the SO foot cut slope planned for the estate lot is not known, but is anticipated to be Livermore Gravel bedrock material and the cut slope is expected to be preliminarily stable at an inclination of 2.75:1. This will be confirmed during grading or prior to grading when access for a drill rig is available" (emphasis added). Elsewhere grading is described as consisting of "cuts to 40 feet at the estate lot" (page 6) and, based on our examination of the preliminary grading plans, we believe that the proposed cut slope will be about 40 feet high. However, we believe that more information is necessary on the height and possible stability of the proposed cut slope. In particular, we are concerned about the absence of information on the dip of bedding in the vicinity of the proposed estate development. Our concern is that the Livermore gravels are mapped (Barlock, 1988; Majmundar, 1991) with a gentle (2-28 degrees) northeasterly dip, roughly slope parallel. No subsurface information is provided for the proposed estate lot although an additional boring is proposed at a later time. More subsurface information is required for adequate characterization of the estate lot and the height and possible stability of the proposed cut slope. We recommend that the bedding orientation within the Livermore Gravel (assuming that bedrock at the estate site is Livermore Gravel) be more fully characterized and that a boring be completed at the site to a depth greater than the proposed cut prior to final grading design. sa t_ ~~ y ~< ~ ~ ~ ~ x .,.~~,~~,.,~ Wis. ,~~;c.