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Home My WebLink AboutMEP2008-00344 BLD2008-00344 - MEP Letters / Memos - 5/8/2008 PMVII8A Jerome W.Morrissette&Associates Inc., P.S. Civil•Municipal•Geotechnical Engineering and Planning 1700 Cooper Point Road SW,#B-2, Olympia,WA 98502-1110 (360)352-°456 / FAX (360)352-9990 05 May 2008 ^ " ;,----� .-.--..-7 Mason County—Department of Community Development- Planning `'v , ,i " 1 PO Box 279 ---_. Shelton, Washington 98584 I �.�IV ATTENTION: Mr. Robert Fink, Planning Manager SUBJECT: Consultant Review—Swanson Geotechnical Report for BLD 2008-00344— Tahaj Engineering(JWMA N07123-028) Dear Mr. Fink: Pursuant to Mason County Geo Tech Work Order MEP 2008-00344,the associated Geotechnical Report has been reviewed for compliance with Mason County Resource Ordinance— 17.01.100 Landslide Hazard Areas. As part of this review, it has been assumed that Mason county Planning Department staff has visited the project to confirm that the basic site data provided in this report is a true rendition of the in situ site conditions. A brief site visit was performed for this site as a preliminary review of the site conditions indicated potential impacts on nearby existing residential properties. On this basis,my comments are as follows: • 17.01.100.E.5(2) Site Plan—The site plan is required to show important development features. Although the plan submitted shows site specific features,no information is given with regard to the surrounding context of the development. Further mapping should be provided to show that the project site is located in a residential subdivision comprised of less that 0.25 acre lots. The site plan also needs to provide details of property perimeter and downstream drainage. • 017.01.100.E.5(3)Exploratory Holes and Probes—No soil test holes,water well logs or test pit data have been provided for this site. The existing hazard slope is covered with vegetation and plant litter with no clearly exposed surface, except in a 2 ft vertical height surrounding the newly installed water meter box. As such,there does not appear to be reasonable information available to assess the subsurface conditions adequately. Site specific subgrade soil information is required to a depth of at least 15 ft for this site. • 017.01.100.E.5(4) Geologic Map with Boundaries of Hazards,Associated Buffers& Setback—The report does not include the required map with boundaries,buffers and setbacks. Slope hazard areas and the slope gradients are not shown on any of the mapping provided. Neither the required 50 ft vegetation buffer nor its recommended reduced buffer are shown. The Mason County Building Code required slope construction setbacks are not shown. • 17.01.100.E.5(5) Site Cross Section—The site cross section provided shows the ground surface along the profile line,but does not provide any subsurface characterization(ie soil strata,types, strength • parameters, etc)to connect the site conditions defined for the site with the slope stability evaluations performed. • 17.01.100.E(6) Slope Stability An.1 , ses—Soil strata and strength characteristics are not defined via soil test holes,probes,test pits or i;sh exposed slope faces. Supporting data needs to be developed in the sections of the report preceding ese analyses to substantiate the analyses performed. As such,the analyses and safety factors listed di not appear to be representative of the site conditions. • 17.01.100.E(7)Restrictions—Drai• ;ge, Septic,Fill, Footing Features o Drainage -Recommendation. in should be revised to be consistent with 17.01.100 D(3)of the Landslide Hazard Ordinance hich requires that site drainage be conveyed via tight pipeline to the toe of the hazard slope release via an energy dissipating device at the toe. Top of slope infiltration and spreader tren es are not permitted on Landslide Hazard Sites. Splash blocks are not permitted on Landslill - Hazard Sites. The perimeter swale that is located along the southeast and westerly site • • daries show instability with surficial erosion along steeper segments of the swales. The est side swale appears to end between the project site and adjacent property to the no • In the vicinity of the terminal point there appears to be a soft undermined area with no evi I ence of a culvert or pipeline as continuation of the flow path. The soft area appears to be a sinkhole. The proposed site development will generate additional surface runoff adding to the Swale erosion conditions and potential sinkhole development. These conditions should be investigated and addressed. o Septic/Primary Drainfield—The proposed primary drainfield is shown to extend along the face of a retaining wall and beyond the slope break toward the hazard area. The site has been characterized as having glacial till (hardpan)within 3 ft of the ground surface. This drainfield configuration would potentially short circuit to the face of the slope, creating a release of wastewater and localized destabilization of the hazard slope. o Septic/Reserve Drainfield—The reserve drainfield is located within 1.0 ft of the proposed dwelling. The dwelling is described as having a basement. As such, an exterior basement wall drain would be required in titre reported glacial till layer. This would position a drain down slope of the reserved drainfield,which would require a 35 ft minimum setback. As such,the reserve drainfield area can not meet the requirements of the Mason County Health Code. • 17.01.100.E(8)Recommendations/Clearing&Grading—Even though the upper site terrace has been cleared and is in use as a campsite,the remaining site areas support existing and emergent vegetation. The proposed building foot print shows encroachment on the adjacent slope. As such, clearly defined clearing and grading limits with limit fencing should be shown on the plan and recommended in the report as a guide for the site developer. • 17.01.100.E(9)Recommendations for Temporary Erosion Control, Buffers& Width—The proposed dwelling foot print encroaches on the adjacent hazard slope on a terrace reported to be underlain by glacial till. As such, cutting the upper slope will tend to direct significant disturbed surface runoff area toward the slope face. Existing vegetation and the hazard slope face are not suitable for surface dispersion. Formal temporary erosion control must be employed and site specific recommendations must be included in the Geotechnical Report. • 17.01.100.E(10) Onsite/Offsite Impacts—see Drainage Impacts described under Item 17.01.100.E(7) Restrictions/Drainage and Septic above. Offsite impacts have not been adequately identified, evaluated or mitigated, and must be covered as part of the Geotechnical Report. • 17.01.100.E(11) Specifications for Vegetation Management, Drainage, Erosion Control and Buffer Widths- Proposed development will have a significant impact on the project site, resulting in a major change in site function and perfo ance. As such,the disturbed areas and remaining undisturbed areas will need to be addressed wit respect to extra surface drainage and interflow loading along with construction disturbance remediati n. Specific site surface improvement recommendations must be provided. • 17.01.100.E(11) Recommendations/Structural Mitigation—The proposal to install a rockery retaining wall on the hazard slope outboard of the proposed dwelling constitutes a structural improvement on the project and will require a Mason County Building Permit. Specific recommendations for founding and insuring the stability of this improvement must be provided along with additional slope stability analyses that include the impacts of the dwelling, wall and fill required by this proposal. Based on the foregoing, I recommend that the subject report be returned to the proponent for revision to achieve compliance with the referenced ordinance. Should you have any further questions or require clarification,kindly contact me at(360)352-9456. Yours very truly, J. W. MORRISSETTE & ASSOCIATES INC. P.S. t Z EXPIRES 09/11/2009 j Edward A. Wiltsie, P.E. Sr. Geotechnical Engineer/Associate ismimumummmmwimimmimimimmmNNNNENNmiiismmiiimMIMMMMMMMMMMMI f oN-STATE MASON COUNTY o Py A o N DEPARTMENT OF COMMUNITY DEVELOPMENT S N Planning Division Y ► y P 0 Box 279, Shelton,WA 98584 of �,o (360)427-9670 /864 REQUEST FOR ADDITIONAL INFORMATION May 07, 2008 BYRON SWANSON 317 NW OVERLOOK DR VANCOUVER WA 98665 Parcel No.: 423295000002 Project Description: SFR Dear Applicant: You have submitted a permit application (case no. BLD2008-00344) for proposed construction or development in the county. Upon review of your application, I require additional information to complete the permit review process. Therefore, review of your application will not proceed until the necessary information .- is provided (see the comment section of this letter for details.) Once the information is submitted and the application is complete, I will continue to process your application accordingly. If the additional information is not provided to the County within 180 days of this request, the application shall expire and no further action on the proposed development shall take place.. Please contact me at (360) 427-9670, ext. 295 if you have questions. Sincerely, Ja1M11 (VALfJ Tammi Wright Land Use Planner Mason County Planning Department 5/7/2008 Page 1 of 2 BLD2008-00344 REQUEST FOR ADDITIONAL INFORMATION 5/7/2008 Case No.: BLD2008-00344 Comments: Enclosed are th results of the review of the Geological Report for Mr. Byron Swanson, for the proposed building site at 451 Wynoochee Drive, Hoodsp , WA. There are a couple of items that Ed Wiltsie, P.E. has listed s needing to be addressed. Please send the required information to: Tammi Wright, !Planner Mason County Planning Department PO Box 279 Shelton, WA 94.584 cc: Alan Tahja, Tahja Engineering, Inc. 5/7/2008 Page 2 of 2 BLD2008-00344