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Home My WebLink AboutBLD2007-00906 Project Document Review - BLD Engineering / Geo-tech Reports - 2/24/2009 -'s,V _actor � E N 61 N E E R I N 6 1 N f 309 WASHINGTON STREET NE OLYMPIA WA 98501 TEL:360 352-2477 9 FAX: 360 352-0179 www.vectorengineeringinc.com February 24, 2009 Earl Iddings POBox 2755 Belfair,WA 98258 Via e-mail: earlj@dewatto.com Re: Project Document Review_Griffiths Property Location: 791 NE Dewatto Beach Drive Building Permit: 2007-0906(Mason County Data) Dear Earl, The following is based on research of the available public documentation, interviews with you, interviews with Mason County staff, a site visit on the county Right-of-Way, and our professional opinion on this matter. We have reviewed 10 individual documents formulating the permit documents on the county record for this project. They are: (1) Geotechnical Report Dated May 16, 2006 by Geotechnical Testing Laboratories (GTL) (2) Site plan and ecology block wall design dated May, 07 for the driveway by Tajha Engineering, Inc. (TEI) (3) Addendum letter to the Geotechnical Report(GTL) received at Mason County May 24, 2007 (4) SEPA comments dated July 17, 2007 (5) Geotechnical Report Review, by Alkai Consultants dated August 6, 2007 (6) Geotechnical Report Revision by GTL dated August 23, 2007 (7) Alkai Geotechnical Report Revision review dated September 5, 2007 (8) Grading Plan Addendum (TEI) dated October 29, 2007 (9) Approval of Geotechnical report and grading addendums,Alkai consultants dated November 15, 2007 (10) Bird Impact Letter (TEI) dated April 11, 2008. Our site visit was conducted on February 18, 2009.The weather was cold and clear. Locally, between January 10, 2009 and February 10 2009 ambient weather had been dryer than average per NOAA records. X:\9000 Flies\9004 Iddings(Wall_Deslgn_Check)\Communication\Serial Letters\9004_dsn_rev1ew.doc EXHIBIT A-3 It is our opinion that the site plans and accompanying wall design should not be approved with only the documentation provided above. Supplemental information should be required by the jurisdiction prior to approval any building or grading permits. We are of the opinion that the design schema is adequate generally, but the proponent and county should address the following items: 1. Stormwater site plans. Mason County Code (MCC) chapter 14 states: Development of individual,detached,single-family residences and duplexes exceeding two thousand square feet of impermeable surfacing must have a small parcel erosion and sediment control plan, and a residential site improvement plan(which includes a written description of the proposed work and development) prepared and submitted for county review and approval. The site plans provided show the road and building structure, which by scale is roughly 3300 square feet of proposed impervious area. There is some data provided with these plans regarding the permanent drainage facilities for the site, but there is no titled"small parcel erosion and sediment control plan". There Is Information throughout the package alluding to this from both GTL and TEI.There should be a drawing made addressing all necessary plans and details to preclude damage to any downstream or neighboring properties during construction. Further, the site plan does not include any parking or turning areas at the terminus of the driveway. The means of accessing the building should be shown on the plans, so that a correct and final determination of impermeable areas and their stormwater controls and design can be - made. 2. Site plans and Right-of-Way determination. The plan drawings show a county right of way of 30'which based on Mason County Geodata appears to be 45 feet (Geodata is not survey accurate). According to local residents,the Right-of-Way is 60 feet. The site is currently under survey, when the record of survey is complete, it will clearly reflect legal documents and Right-of-Way dilineation for the site. This dimension is critical because it determines how far back from the existing edge of pavement the wall will be built.The design assumes that the Right of Way is 30'. It appears that there is a 15 to 30 foot difference between the design documents and recorded documents, if this is true then the following is applicable to the documents noted above: • The"Bird Impact Letter"(10) states that the fill will be used along the western portion of the driveway with minimal cut to the vertical slope, but If the right of way is moved to the south, more substantial cut will be required including removal of the nesting holes. • A more substantial cut and fill quantity will be required for the grading permit. • The property slopes beyond the top of the bluff, if the driveway is moved to the south, then the elevation gain required from the county road to the top of bank will be increased. The proposed driveway slope is currently about 25% (161'H: 40'V).The fire department and local building department should review this driveway 3. Groundwater and Erosion: During our site visit we noticed two individual seep locations on the vertical face at the right of way. One was visible towards the east property line (see Figure 1), and the other near the west property line.The soil material on site is correctly noted by GTL as Alderwood gravelly sandy loam.This material appears to be easily eroded by concentrated runoff as shown in Figure 1. Our observations of the bluff show that there are stratified, highly cohesive soils in small horizontal layers in the sandy loam. It appears that the groundwater follows these layers downslope to where it exits the bluff at the northern property edge. This proposal does not address how stormwater from the bulk of the site will be handled. It does clearly illustrate how the stormwater from the steep portion of the driveway will be handled. If infiltration will be proposed for rooftop drainage control, the proponent should show that injection into site soils in concentrated areas, will not pipe downslope and out of the bluff creating further erosion of the bluff like that shown in Figure 1. -2- XA9000 Files\90041ddings(Wall_Design Check)\Communication\Serial Letters\9004_dsn_review_R.Ldoc 4. Soil Cohesion values. On sheet 13 of the GTL report (1) it states that the site was modeled using a gravelly sandy loam with cohesion of 200 psf. On the first sheet of the Driveway Stability Analysis portion of item (8) a cohesion value of 700 psf is used. The first sheet of item (8) states that"The attached analysis employs the soil characteristics provided in GEOTECHNICAL TESTING LABORATORY'S geotechnical report. . "which is item (1). This discrepancy is never rectified in other attached documents. It is our opinion that the cohesion value picked by GTL is more correct for the design. This should be addressed prior to approval of any building permits for the wall. The report never specifies whether a clay formation is present and if so at what depth below visible soil. This is a crucial question as are the questions of groundwater and seepage characteristics of the bluff. Significant competent geotechnical exploration and analysis is required to speak to these issues. Strength and stability analysis without this data seems rather fatuous. This soil does not appear to classify as dense till with a substantial fines fraction, which would result in a substantial reliable cohesion value. The soil formation at the right of way showing in the attached figures may possibly express a small apparent cohesion, less than the espoused 200 psf, and certainly well below 700psf as used in the stability calculations. This property, apparent cohesion, is uncertain in nature and ought not be considered in calculations; except as it may act adversely. To sum up predicting that this bluff will stand near vertically throughout its acceptable life cycle with only the lab and field data taken thus far, is inappropriate for a design of this nature. The soil parameters have a substantial affect on the final design and analysis. The end result of the construction performed here should be safer than the existing bluff as it currently stands. The current bluff is a landslide hazard area (see Figure 2). County maintenance staff should have records or at least anecdotal data regarding removal of slide debris from the right of way after the December 2007 slide. Preclusion of another land slide is the paramount consideration of this design, as landslides in this location can cause damage to neighboring properties, and blockage of the county right-of-way. 5. Design Review: It is our opinion that the wall structure being proposed is significant in it's proximity to the Right of Way, and failure of the structure would cause adverse conditions for all residents living beyond it on the private extension of Dewatto Bay drive. A complete structural design (full plans, connections, chosen materials, specifications and all details) should be submitted for the design package. The jurisdiction should require and organize a 3`d party review of the design and accompanying documentation. - 3 - XA9000 Files\9004 Iddings(Wall_Dcsign Chcck)\Communication\Serial Letters\9004_dsn_review.doc In summary, it is our opinion that the site should be examined globally from a drainage standpoint to ensure that further bluff damage is not incurred during or after construction. In fact due to the somewhat tenuous nature of the design it may well be appropriate to develop far more complete engineering than is customary for the environmental permitting effort. Further subterranean exploration should be required in this area to make these findings. Before any building permits are issued all new impervious areas and their stormwater controls for the site should be designed and approved per MCC chapter 14. Any further wall designs should note the data discrepancy in line item 4 above, and have the driveway steepness approved by the jurisdiction. A proper structural design with complete details and specifications should be made and reviewed by a third party reviewer. Prior to any other work a proper survey should be made regarding the actual location of the wall in relation to the topography and existing roadway. Should you have any questions or concerns, please do not hesitate to call or email. Very Truly Yours, i R an re, E of ct Manager Attachments: Figures 11LMOO WAS z 4 04 OL EXPIRES Y 08130 ' -4 - X:\9000 Files\9004 iddings(Wall_Design Check)\Communication\Serial Letters\9004_dsn review.doc ve_ cf.. or E N 6 1 N E E R I N G` I N E 309 WASHINGTON STREET NE OLYMPIA WA 98501 TEL:360 352-2477• FAX: 360 352-0179 www.vectorengineeringinc.com Figures. a Sceo Locatioll n F I! otit0 1 of erosion' from seep i Figure 1. Seep area towards east property line. X:\9000 Flles\9004 Iddings(Wall_Design_Check)\Communication\Serial Letters\9004_dsn_revlew.doc :. ju J.� Y t a -, �N.: $.-3shz`..J^��_• __ --��'.:.. :�.�T."-».,:e �5:._�-a.2.+? 31...:.�wsJ..�.,'Su�'��".;Stt,+44�:�.�;....�. ._�i_�.��''a�a��u.�'u.��>;.'. Figure 2 Landslide at site, into Right-of-Way. December 5, 2007 Photo courtesy Earl Iddings. - 6 - X:\9000 Files\9004 Iddings(Wall_Design_Check)\Communication\Serial Letters\9004_dsn_review.doc