Loading...
HomeMy WebLinkAboutGeotechnical Assessment - BLD Engineering / Geo-tech Reports - 10/18/2000 { Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. 2401 Bristol Court SW, Olympia, WA 98502 Phone 360-357-7883 PO Box 12267, Olympia WA 98508-2267 FAX 360-754-4240 e-mail: bradleynoble@juno.com 18 October 2000 Mr. Paul Ouellette URS Greiner Woodward Clyde Federal Services 2401 4th Avenue, Suite 808 Seattle, Washington 98121-1459 Subject: Geotechnical assessment of slope stability at the Lilliwaup Post Office site. Dear Mr. Ouellette: As requested, we are preparing a specific discussion of the site geology and presenting an opinion of present slope stability and future slope stability after construction of the new facility. Many of these issues are presented in the geotechnical report we prepared for this project. Three geologic reference maps were used in preparing this report. The Coastal Zone Atlas for Mason County is a compilation of known geologic information. It relies heavily on the work of the next two references . The Geologic Map of the Olympic Peninsulas, Washington by R. W. Tabor and W. M. Cady 1978 published by the Department of the Interior, U. S. Geologic Survey in the Miscellaneous Investigation Series Map I-994 (East Half) . A more detailed mapping of the geology of the area is presented in the Geologic Map of the North Central Mason County, Washington by R. J. Carson, 1976 . In the general area, mapped Quaternary and Holocene landslides are found. Recent landslides to the north of this area have blocked Highway 101. The instability of the slopes can be attributed to area geology. We have found permeable sands and gravels over impermeable glacial lacustrine deposits where ground water perched on the lacustrine deposits cause slope instability. The second major cause of slope instability in the area are the oversteepened slopes 00071603 Page 1 of 3 ����� 00071603 Page 2 of 3 that have not yet reached equilibrium since the retreat of the Vashon continental glacier from the Puget Sound Lowland, about 11,000 years ago. We find major current active landsliding along the slopes of the Hood Canal because of the oversteepened slopes . In all three references we find mapped landslides to the project south and along the north side of Lilliwaup Bay. The project site is located in mapped deposits associated with delta and alluvial fans of the recessional outwash of the Vashon glacier. This is consistent with our interpretation of the site geology in the project area. We have found no indication that at this site or in the general area, recent or active landsliding is occurring. The site soils of delta and alluvial outwash can stand steeply with apparent stability. This is because of the grain size distribution and consolidation of the material. We expect that high natural angles of internal friction and high coefficients of friction are found in these soils . These contribute to the stability of the slopes in the project area. In our exploration of the site, we did not encounter ground water. In Test Pit Four, we did encounter soils that we associate with either beach or bar deposits . During periods of high tide, it may be found that these soils are hydraulically connected to the Hood Canal if they are continuous to the north. The soils forming the slopes and explored by the first three test pits in our report are considered to be both permeable and porous. We do not expect that ground water would be a destabilizing influence at this site as the soils are expected to drain rapidly. The previous use of this property as a borrow pit has created steep cut face walls in the west area adjacent to the driveway serving the residence to the northwest. Examination of these faces during our site work suggest that the site soils can stand steeply for some time with apparent stability. This is consistent with our interpretation of the site geology and engineering geology interpretation of the soils . We do not consider that the site will be destabilized by the proposed development. Based on our understanding of the project, unrestricted slopes will .r 00071603 Page 3 of 3 be constructed per our recommendations in the geotechnical report prepared for this project. The project plans now include the construction of retaining walls . We have presented design recommendations for these walls. In our opinion, the site development will not affect slope stability, and, in fact, the proposed construction will stabilize the present oversteepened slopes created by the former gravel extraction at this site. If you have any questions, or if we may be of additional service to you, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES C ` David C. trong Engineering Geologist r V .7661 L a I 29 24I . Is 7 r BNC469 '\ O BM 5464 I 9.8 I I i I BM A464 55 a 1 — �F 'BMD1 25.S I j BMZ4b3 42.7 l(11 R sci I 6. I � 179-186- I y I � � _iw Figure 1. Proposed Project Vicinity,Lilliwaup,Washington.