Loading...
HomeMy WebLinkAboutBLD2006-00387 - Geotechnical Report • • G E Deotechnical Engineers,Geologists Group Northwest, Inc.Inc• &Environmental Scientists February 22, 2006 G-2140 Dr. Tom Hamilton 8230 SE 59th Street Mercer Island, WA 98040 Subject: Geotechnical Evaluation Single Family Residence 41 Madrona Beach Drive Port Townsend, WA 98368-9767 Dear Dr. Hamilton: At your request, Mr. Bill Chang from GEO Group Northwest, Inc., visited the site on November 18, 2005 to perform a geotechnical evaluation for the subject site. The purpose of our work was to perform a pre-purchase evaluation of the property, including a review of the soils and geology. Property Description The property consists of a 3/4 acre high bank waterfront lot on a private drive at the west side of Discovery Bay on the Quimper Peninsula. The property is located approximately 6 miles south of Port Townsend on the west side of State Route 20 in Jefferson County, Washington. The cabin consists of a 876 square foot wood frame building supported by timber posts on pier blocks. A newer addition is located at the SE corner of the cabin and appears to be supported on a spread footing foundation. Wood decks are present at the north and west sides of the cabin. The decks are also supported on pier blocks (small precast footings sold at building materials stores) The property is roughly 116 feet in width and 274 feet in length. The cabin is located approximately 40 feet above the mean sea level. The steep slope forming the bank is approximately 45 degrees to the horizontal and is vegetated mostly with blackberry vines, sword ferns, horsetails, grasses, ivy. Big leaf maple and alder saplings were also observed. 13240 NE 20th Street, Suite 10 • Bellevue, Washington 98005 Phone 425/649-8757 • FAX 425/649-8758 • • February 22, 2006 G-2140 Geotechnical Evaluation Page 2 Geology The geologic map for the area, Geologic Map of the Olympic Peninsula. Washington by R.W. Tabor and W. M. Cady published by the United States Geological Survey in 1978. indicates the site is underlain by terciary marine sandstone - Quimper Sandstone of Durham (Tq) consisting of medium to coarse grained feldspathic sandstone; thick bedded to massive. Layers of fossil debris and concretions common. The tertiary Quimper formation (Tq) was deposited roughly 20 to 40 million years before the present time in a marine environment. These deposits were added to the North American coastline by the subduction of the Pacific Plate underneath the North American Plate. The soils present at the surface of the site are due to the weathering and decomposition of the sandstone into soils. Soils transported from uplands due to erosion processes also form a thin blanket at the surface. Site Visit Observations During my site visit on November 12, 2005, we met with you and your realtor. The following observations were noted: 1. The cabin was built in the 1950's and consists of a wood frame building supported on precast pier block foundations. Measurements of interior surfaces indicate that the cabin has a high spot neat the middle of the living room and the floor slopes down toward the west, north, and south. A maximum of 6 inches of settlement were noted from the living room to the bathroom floor to the south. Other areas had 2 to 3 inches of differential settlement. 2. Wood decks located at the north and west sides of the house were also supported with pier blocks placed directly on the ground surface. The decks also exhibited settlements of 1 inch in 4 feet. 3. The steep slope below the cabin is approximately 45 degrees to the horizontal and is vegetated mostly with blackberry vines, sword ferns, horsetails, grasses, ivy. Big leaf maple and alder saplings were also observed. Many of the big leaf maple saplings had been cut and were disposed of on the slope or at the base of the slope. 4. A timber stairway is located on the steep slope at the west side of the cabin that provides access to the beach. The timber stairway is in poor condition and needs to be replaced. GEO Group Northwest, Inc. i February 22, 2006 G-2140 Geotechnical Evaluation Page 3 5. Water seepage was observed at the base of the slope in an area covered with horse tails (hydrophitic vegetation). Some erosion was observed, although the water seepage areas had an abundance of small gravel. 6. The top of the slope at the north end of the cabin had more visible erosion and bank recession. This is probably due to the removal of vegetation and the existence of softer soils in this area. CONCLUSIONS AND RECOMMENDATIONS Based on our observations at the site, geology review, and our experience, we have made the following conclusions: A. The slope at the site is founded on sandstone bedrock and should be stable. The surface of the slope is subject to weathering and erosion, although the potential of slope recession appears to be limited due to the sandstone underlying the surficial soils. B. The cause of settlements at the cabin are due to the poor construction and poor foundation preparation at the time that the cabin was built, and the settlement likely took place within 10 years of the cabins's construction. C. Small diameter pipe piles can be used to underpin the existing cabin and deck. They are recommended if a new cabin is planned. Specific recommendations and design of the pipe pile support system is not within the scope of this evaluation. D. The timber stairway that provides access to the beach is in poor condition and needs to be replaced. Due to the steepness of the slope, we recommend that the stairway be supported on small diameter pipe piles that are driven into the dense soils. This will provide proper support for the stairway and not impact the stability of the slope, since no excavations are needed for the pin pile supports. E. The steep slope should be vegetated with native species of shrubs and trees such as salal, Oregon grape, sword ferns, junipers, cedars and other appropriate ground cover to minimize erosion. GEO Group Northwest, Inc. i • February 22, 2006 G-2140 Geotechnical Evaluation Page 4 If you have any questions, please call. Sincerely, GEO Group Northwest, Inc. ZM Cit s.y. vyo�.W .94 William Chang, P.E. Principal Engineer ., eV� SSI°NAL EXPIRES: 2/19/ Ora GEO Group Northwest, Inc. ~AR~'HRISS /7, ENGINEERING ~I NBC. Mr. Greg Ballard Associate Planner Jefferson County Dept. of Community Development 621 Sheridan St. Port Townsend, WA., 98368 Re: Tom Hamilton Beach Access Stairs 41 Madrona Beach Dr. Port Townsend, WA 98368 Subject: Jefferson County Flood Damage Prevention Ordinance Deaz Greg, '~arco.,1 9 0- 081 p i t 'C3 La Db-oo3$-1 SDP oc~-ood32 P.O. Box 5537 Rremertori, WA 98312 P: 360 - 792.- 2421 F: 360 - 792 - 2949 harry.williams@marchriss.com ~~~~~ ~~~~a~~ SEP 21 200 ~~~~~~5~ ~~U~~ ~~~ Per our telephone discussion, and after reviewing the Jefferson Flood Map you sent me, the established flood surge elevation had not been established for the location of Mr. Hamilton's stair. However, you had mentioned that a 3' high surge above the high water line would be a safe assumption. The platform is approximately 7' above the high water line, therefore, the platform would still be about 4' above the surge elevation. This means that the 8x8 posts would be partially submerged. After reviewing the stair design, including foundation and framing connections, I have concluded that the stair platform can safely resist the potential hydrodynamic forces from potential flooding. In addition, based on DOE's requirement for reducing the platform size from 7'x10' to 4'x4', I have revised the calculation package that was originally submitted to reflect those changes. It is my understanding that Mr. Hamilton will resubmit the revised calculations. Please call me if you have any questions. /rti W f ~~ Sincerely, ~~I ~'v ~ - Harry Williams P.E. r0 ~ae'° 5~ ~ ~F • R -,v ti•:AL ~:, /' tEXPIRES OS/13/ r i RESIDENTIAL • COMMERCIAL • INDUSTRIAL ~~._/ % ~'3~' /