SR 06:201 - Laserfiche WebLink

Page 2 P-6748 PROPOSED CONSTRUCTION The Site Plan in the BGC report indicates the site will be subdivided to create seven new lots, a water tank property, a new street, and individual driveways. Grading operations will be performed to develop the new street and driveways, building pads, the water tank area and rebuilding portions of a swa1e on Lot 6. SITE CONDITIONS The site conditions as described by the BGC report generally consist of shallow clayey soils over Livermore Gravel bedrock. Thicker soils were encountered in the major swales and lower slopes. Various small landslides were mapped by BGC and loose fill was encountered in the proposed building pad area for Lot 7. SITE OBSERVATIONS During our site visit, we noted the following key areas of concern: I. The apparent youthful nature of a large colluvial swale that underlies the northeastern portion of Lot 6, including hummocky topography and an apparent landslide 'toe', suggests the presence of a possible landslide. Minor downslope deflection of tree trunks may be consistent with active slope movement (creep). These observations are consistent with the log of test pit TP-5 that shows a 'fairly sharp basal contact' with a 150 degree dip (downslope?). Although not documented in adjacent test pits, the description of the basal contact and colluvial material is consistent with that documented within the landslide debris exposed in TP-3 below the water tank on Lot 7. 2. BGC notes the presence of a landslide on the south side of the proposed water tank site, exposed in TP-3. The assumption is that the landslide will be removed during grading for the tank (page 3, BGC report). However, the stability of the colluvial swale, and colluvial deposits, downslope of the landslide is not discussed by BGC. Our concern includes the presence of arrow within the colluvial deposits as mapped on the Site Plan, suggesting downslope movement. Our field observations confirm the presence of potentially unstable colluvial deposits within the two landslides, and perhaps in downslope colluvial deposits on Lot 7. 3. We noted a small possible landslide, including a 2-ft scarp, on the northwest side of the main access road below the tree marked' 110' on Lot 4 (?). This possible landslide is located across the ravine from the mapped questioned landslide on Lot 4. Minor cracking in the road suggests that the slide may be active although it likely is not a major feature and may not pose a direct risk to proposed building pads, depending on where the pads are located. 4. During our field reconnaissance of the site, we noted several deeply incised gullies on slopes and upstream of drainages. Specifically, a small gully is located downslope of the existing house on Lot 5 and at the head of a north flowing drainage on the northeast portion of Lot 6. Although minor, these deeply incised features suggest that localized erosion is occurring near the heads of active drainages and on sleep slopes. 5. Landslide areas are immediately adjacent to the building pads for Lots 5 and 7. No discussion is provided by BGC regarding isolating the building areas from potential upslope migration of the landslides in the future. ALAN KROPP & ASSOCIATES, INC.