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HomeMy WebLinkAboutBLD2006-00430 Geotechnical Report s • ®'II SHANNON 6WILSON, INC. ALASKA CALIFORNIA CLORADO MISSOURI AND ENVIRONMENTAL CONSULTANTSFLORIDA OREGON WASHINGTON February 4, 2008 Emmet and Judith Wolfe 363 Porter Lane Port Townsend, WA 98368 RE: GEOTECHNICAL EVALUATION OF CONSTRUCTION, WOLFE RESIDENCE, 363 PORTER LANE, PORT TOWNSEND, WASHINGTON, JEFFERSON COUNTY BUILDING PERMIT NO. BLD06-00429 Dear Mr. and Mrs. Wolfe: As you requested, we have prepared this letter to provide a summary geotechnical evaluation of the construction of the above-noted residence located in Port Townsend, Washington. We understand that as part of the requirements for your building permit, the Jefferson County Building Department requires a final summation letter prepared by a geotechnical engineer certifying that the construction was done in accordance with the recommendations presented in the geotechnical pre-construction report prepared for the site by Shannon &Wilson, Inc. dated March 21, 2006. Our scope of work consisted of a site visit on January 21, 2008, to observe the constructed residence and preparation of this letter. Our geotechnical report prepared for the site provided recommendations for the following: ► Building and septic drain field setback from the top of slope ► Foundation bearing soils ► Site drainage ► Construction of rock walls SETBACKS We observed that the main part of the building, excluding the deck, is within about 100 feet of the top of slope. The septic drain field was constructed about 15 feet northeast of the northeast 400 NORTH 34TH STREET•SUITE 100 21-1-20457-002 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.695.6777 TDD: 1.800.833.6388 www.shannonwilson.com • • Emmet and Judith SHANNON F�WILSON,INC. Wolfe February 4, 2008 Page 2 corner of the building, to within about 120 feet of the top of slope. We have reviewed the site conditions and it is our opinion that the setbacks are adequate for this project. The owner needs to be aware that typical bluff slopes in the Puget Sound region continually retreat over time. An appropriate building setback is a function of the rate of slope regression, the design life of the structure, and the risk the owner of the structure is willing to assume. The regression rate for this specific slope is unknown. A reduced setback distance can result in a shorter design life and/or a higher risk of damage to the structure due to slope retreat. In our opinion, based on the design life of the structure, the risk of damage to the structure would be low. BEARING SOILS AND SITE DRAINAGE In our geotechnical report, we recommended that we observe the site development to confirm that the geotechnical recommendations in our report were followed. However, we were not contracted to perform these services and did not observe the site development. We have reviewed photographs taken by Zimmerman Architecture during the foundation construction work, which appear to show that the building foundations were placed on the recommended bearing soils. We assume that all loose, soft, and disturbed soils were removed from the foundation areas prior to placing reinforced concrete. During our site visit, the building foundation had been backfilled to the ground surface. We observed the portions of the foundation walls and pier supports exposed above the ground surface and the concrete sidewalks at the residence. Although we observed hairline cracks in the east foundation wall, it is our opinion that these cracks may be from shrinkage. The foundation walls, pier supports, and sidewalks did not show any evidence of significant settling, and in our opinion, the building foundation appears to be performing as designed. We discussed the site construction with you and understand that roof drains, foundation drains, and landscaping drain systems were installed at the site. We understand that the drains collect in F E.t 0 8 2008 21-1-20457-002-L1.doc/wp/LKD 4 t''4 v . „ILA/yin 21-1-20457-002 • • Emmet and Judith Wolfe SHANNON&WILSON,INC. February 4, 2008 Page 3 a perimeter system that directs the water eventually to a flow diffuser on the beach below. The drain system at the structure was backfilled to the ground elevation at the time of our visit. We observed that roof downspouts were directed into tightline piping, and that a series of catch basins appeared to capture and direct water to a 6-inch-diameter,high density polyethylene (HDPE)pipeline located at the top of the bluff slope north of the residence. The HDPE pipeline then transmits the water to the beach below. The HDPE piping is exposed where it extends from the top of the bluff slope to the beach. No photographs were taken during the installation of the drain lines at the structure, and as such, we cannot confirm that the drain system installed at the residence meets the recommendations in our geotechnical report. A review of photographs taken by Browne Engineering, after the installation of the HDPE outlet piping at the slope, shows that the HDPE outlet piping was generally installed as designed, except as noted below. We observed that where the drain line drops down over the top of the bluff slope, the pipeline is exposed, and the site soils are not vegetated. We recommend covering the piping, as shown in the plans, and re-establishing vegetation at this location to minimize soil erosion and bluff retreat. We recommend the use of jute mat to hold the surface soils in place during revegetation and diverting surface water runoff away from this area. ROCK WALLS The building plans for the site showed that rock walls up to 6 feet high would be constructed along the south side of the residence. Rock walls in excess of 4 feet in height generally require geotechnical recommendations for construction. However, we understand that you changed this during construction, and the rock walls were replaced with scattered or terraced landscaped round rock between 2 and 4 feet in diameter placed on an approximately 2 Horizontal to 1 Vertical (2H:1 V) slope, which do not require geotechnical recommendations for construction. The round rocks did not appear to be keyed into the slope, so some downslope movement of the rocks may occur over time. 21-1-20457-002-LI.doc/wp/LKD 21-1-20457-002 • • Emmet and Judith Wolfe SHANNON&WILSON,INC. February 4, 2008 Page 4 We appreciate this opportunity to be of service to you and look forward to working with you in the future. Please contact me at(206) 695-6801 if you have any questions or need additional information and services. Sincerely, SHANNON & WILSON, INC. rhai 4s Go* A1M ,Iit.47 1' ' p 16945 01 NAL [�i_ I Thomas M. Gurtowski, P.E. Vice President DPO:TMG/dpo 21-1-20457-002-Ll.doc/wp/LKD 2 1-1-2045 7-002