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Home My WebLink AboutGeo-Tech Assessment Review for BLD2004-00688 - BLD Engineering / Geo-tech Reports - 6/7/2004 MASON COUNTY TO BE KEPT IN THE s PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER PARCEL FILE 0 N Shelton,Washington 98584 Ny DATE: June 7`h, 2004 INTER-DEPARTMENTAL COMMUNICATIONS TO: Grace Miller, DCD - Planner FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WO#PLG-04 SUBJ: Geo-Tech Assessment Review NAME: Irwin SFR @ Cranberry Creek BLD2004-008ifr 688 Rick, The geotechnical assessment prepared for the proposed Brent Irwin Single Family Residence (SFR)to be constructed on Mason County Tax Parcel No. 32135-23-90001, has been received and reviewed by Public Works. The report appears to satisfactorily address County requirements for geotechnical assessments. The assessment's author indicates that, in his opinion, the proposed development of the property will not cause stability problems for either the subject property or neighboring properties, and that a more detailed investigation(report) is not warranted. From the contents of the assessment, I recommend accepting the report as satisfying the County's requirement(s) for stability investigation and geotechnical reporting. Recommendations contained in the assessment should be incorporated into the site's development and made conditions for permit issuance. Adequate erosion and sediment control features need to be implemented during land disturbing activities to protect neighboring properties and State waters from adverse stormwater runoff impacts. The migration or release of silty water or mud from the applicant's property will be considered a violation of County and State water quality protection regulations. In summary, the geotechnical assessment appears acceptable, and the proposed residential development of the property should be allowed to proceed, subject to the engineering geologist's recommendations. Please feel free to contact me at County extension 461 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. Sin erely, AL�4. I Al An A. Tahja File: H:\WP\GEO\Assessments\Irwin.doe Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. TO p� KEPT PO Box 12267, Olympia WA 98508-2267 D E Sr- Phone 360-357-7883 • FAX 360-754-4240 or 867-9307 �� TF�C e-mail: bradleynoble@juno.com PARCEL FILE 26 March 2004 PLANNING Mrs . Janien M. Lind-Irwin 4771 East State Route 3 Shelton, Washington 98584 Subject: Geotechnical assessment of the slope stability and erosion potential at your proposed building site on the south side of Cranberry Creek, Tax Parcel Number 321352390001, Mason County, Washington. Dear Mrs . Lind-Irwin: As requested, we visited the above property in order to develop an opinion on slope stability and erosion potential of the site and their probable long-term influence on your proposed construction of a new residence and detached garage. Our scope of work consisted of a site visit, review of available geologic information, and preparation of this report based on our observations, research, and discussion with Brent Irwin about the proposed location and type of structures proposed. The site is located south of Cranberry Creek in a Short Subdivision for Gene Nye. This lot is about five acres in size with access by a private road. Two streams are found on the property. Crossing the southern area is a small class 3 stream. On the north side and flowing around the property to the east is Cranberry Creek, a class 1 stream. These streams have a 150-foot setback plus a 15-foot building setback from the edge of buffer. The access road to the property crosses through the buffers but was most likely constructed prior to the adoption of the Critical Areas Ordinances and would be considered a "grandfathered" road. With the mandated buffers and setbacks, there is a building site in the center section of the property. This is a nearly level area with natural moderate 04032001 Page 1 of 3 i TO BE KEPT IN TF 01 Page 2 of 3 PARCEL FILE Page 2 slopes to the north, south, and east. This site appears to have been used at some time in the past for recreational property, as we found evidence of former habitation in this area. We found the slopes in the area surrounding the building site to vary in slope angles . Average slope angles are less than 40% or less than 22 degrees . These slopes are significantly less than the expected angles of internal friction for these soils . With slopes less than the angle of internal friction, slope stability is inferred. There is an erosion potential at the northwest corner of the property where Cranberry creek changes direction from a southern flow to an eastern flow. At the setback of the structure, we do not expect that stream bank erosion will be a hazard to the new residence for at least the next 100 years at observed present erosion rates . Soils exposed in the area are Vashon recessional outwash sands and gravels over Vashon subglacial till. We expect that the outwash soils mantle the till and are typically thin in this area, less than ten feet in average thickness . Slope erosional processes have mantled the exposures of the subglacial till . We did observe till cropping out in the stream bank of Cranberry creek. We expect that the subglacial till, due to its cementing, limits the ability of Cranberry creek to rapidly deepen its channel or expand its flood plain. This interpretation of site geology is consistent with Plate 1 of the Water-Supply Bulletin 29 for Southeastern Mason County, Washington by John B. Noble and Dee Molenaar. To verify that our interpretation of general slope stability in this area is stable, we reviewed Relative Slope Stability of the Southern Hood Canal Area, Washington by Mackey Smith and R.J. Carson published by the Washington Department of Natural Resources 1977 . We find that this area is considered to be stable because it is underlain by subglacial till . Subglacial till has many engineering properties similar to those of lean concrete and can stand steeply with apparent stability and only slow erosion occurring due to the weak cementing of the soils . TO BE KE''T IN TF L 04032001 PARCEL FILE Page 3 of 3 With granular soils overlying the subglacial till, and the relatively shallow slope angles at this site, we expect that soil creep, the slow downslope movement of the near-surface active soil zone is the only major erosional process working on the natural slopes. Oversteepened road cuts leading to the site are prone to ravelling and sloughing. We do not expect that major failures of the oversteepened cut face would close the access road. It is more of a on-going maintance issue to remove soils that have fallen to the roadway. Laying the slopes back to a 2: 1 (horizontal to vertical) slope would minimize future erosion. We do recommend that the slope be protected from concentrated flows of storm water from impervious surfaces . Concentrated flows can create areas of localized saturation and induce slope instability or rapid erosion of the slope face. We recommend that all water from roofs and impervious surface be collected into a tightline collection system with the water carried to the toe of slope for infiltration. Based on our field observations and research, it is our opinion that a residence may be placed on this site at the location indicated with no risk of slope failure or erosion of the slope face affecting the foundation support. We do not consider the preparation of a geotechnical report appropriate for this site. The recommendations presented in this report must be incorporated in the site development work to ensure long-term protection of the slopes from concentrated storm water flow. If you have any questions, or if we may be of additional service to you, please contact us at our Olympia office. Cordially, c� Washy BRADLEY-NOBLE GEOTECHNICAL SERVICES �9f0 David C. Strong, L.E.G. <'c`°� 1129 Sed Geo % DAVID C. S700-!G