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HomeMy WebLinkAboutGeo-Tech Assessment Review for BLD2004-01094 - BLD Engineering / Geo-tech Reports - 7/16/2004 M � MASON COUNTY �Q- C � o` s°o PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Y O T Shelton,Washington 98584 r2 N Y Y O 1864 DATE: July 16th, 2004 INTER-DEPARTMENTAL COMMUNICATIONS TO: Joanne Long-Wood, DCD - Planner FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. WO#PL -04 SUBJ: Geo-Tech Assessment Review NAME: Lessard SFR k Mason Lake BLD2004-01095 r Bl.>Zal- o toque{ Joanne, T �f The geotechnical assessment prepared for the proposed Keith Lessard Single Family Residence (SFR) to be constructed at 2931 Mason Lake Drive East, 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. Si cerel y,, y Alan A. Tahja File: H:\WP\GEO\Assessments\Lessard.doc WORK ORDER - PUBLIC WORKS DEPT. N►) Date: � / 9) Work Order PERMIT#: o - . © -Ol�I 5l` Number. Requested by: d Authorized by: o Type of Work: Date: D � �-u- c3-t�1it� C cC� CHARGE TO: RECoEIVENAME AGENCY/COMPANY JUL _004 BILLINGADDRESS HONE MASON .;Ii. Pu WOTOM&onin Cha e: Io) Protect Time Line: (from-to dates).. ............ x :;:,: ;. <.:;:.;;..::,>:,.»;:::_:.:::.::.;:<;•::;.;::,:.:. F Project Sort date: Estknated FtNeh Date: TO Apprmdmate horx*: LZ ESTIMATED TOTAL$-S: �) PJ34 �Ployee COST ESTIMATE . IZ Rate � Subtotal Edna*% TOTAL$ 7 6� 3 Z z -ZZ 42, EQUIPMENT USED: MATERIAL USED: (') Actual Cost s F ..f.•- +�r`.Y�. BARS: PROJ e: DATE Yee 1t1IQ Name jEt3>11t{ Hour Subtotal TOTAL= EQUIPMENT USED: DateS O TOTALS S MATERIAL USED: (G) BILLED DATE TOTAL ALL INV/ PAID DATE REC.N CKN l Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 29 May 2004 Mr. Keith Lessard 2803 NE 187th Avenue Vancouver, Washington 98682 Subject: Geotechnical assessment of the slope stability between Mason Lake Drive East and Mason Lake at 2931 Mason Lake Drive East, Mason County Tax Parcel Number 22233-52-00080 . Dear Mr. Lessard: As requested, we visited the above site in order to observe site soil conditions and slope angles and so prepare a geotechnical assessment for this project. Our work consisted of a site visit, review of available geologic information, review of the site plan prepared by Pioneer Digging Inc . , and preparation of this report. At the time of our site visit, the lot had been cleared and mineral soil exposed. Using a Brunton Pocket Transit, we measured slope angles between the road and the lake. We found the site to be of low relief with average slope angles at or slightly above 15% . The slope is down from the road to the northeast to the edge of Mason Lake. The site plan indicates the new residence and future garage will be constructed in the northeast area of the lot. At this indicated location, the residence will be at the same general setback as are adjacent structures . We observed area soil conditions both on the site and in available exposures . p Two soil units are found in this area. The upper soils are sands and gravels associated with fluvial deposition onto the outwash plain of the retreating Vashon lobe of the Cordilleran glacier as it retreated from the Puget Sound Lowland, about 11, 000 years ago. These soils in this area are thin, typically less than three to four feet in thickness and are mapped as Qvr. The outwash soils 04052501 Page 1 of 3 04052501 Page 2 of 3 overlie the Vashon subglacial till (Qvt) . Subglacial till is a mixture of silt, sand, and small gravel . It was deposited at the base of the advancing ice sheet and overridden. The grain size distribution and loading by the ice sheet have created a soil unit that has many engineering properties similar to those of lean concrete. It is for all practical purposes impermeable. Site geology is presented in Plate 1 of Water-Supply Bulletin 29 , Geology of Southeastern Mason County, Washington, by John B. Noble and Dee Molenaar. This area is mapped as Qvt, Vashon subglacial till . We observed outcrops of subglacial till in road cuts in the general area and these road cuts stand steeply with apparent stability and only slow erosion. Relative Slope Stability of the Southern Hood Canal Area, Washington by Mackey Smith and R.J. Carson, 1977 published by the Division of Geology and Earth Resources of the Washington Department of Natural Resources identifies the area as a Class 2 area which is believed to be stable under normal conditions . Based on our field observations, slope angles in the project area, and site geology, this site should be in a Class 1 . The scale of mapping does not allow for site specific classification. We believe the slopes between the road and Mason Lake are stable. You should expect excavation for footings and crawl spaces to encounter subglacial till. Due to cementing and overconsolidation, excavation can be difficult, especially if small equipment is used. We expect that flows of ground water will develop on the surface of the subglacial till during periods of heavy or prolonged rainfall. To ensure dry crawl spaces and slabs, we do recommend that a footing drain system be installed. The roof rain leaders should be placed into a separate tightline system. Water from the footing drain and roof rain leaders are to be carried to the toe of the slope for infiltration. No uncontrolled water from impervious surfaces is to be directed over the slope. Concentrated flows can create areas of localized saturation and induce accelerated erosion. At this site, silt-laden water entering the lake must be avoided by good storm water practices both during construction and as part of the final drainage plan. 04052501 Page 3 of 3 To control surface sheet washing of the exposed mineral soils outside of the building area, we recommend that these soils be replanted to allow vegetation to become re-established. Use of native, drought-resistant planting is recommended. Based on our site observations and research we do not consider this site to be a landslide hazard or erosion hazard area. Because of the gentle slopes on this site and the fact that the site is underlain by a soil unit that has many engineering properties similar to those of lean concrete, slope failure is unlikely. Control of storm water from impervious surfaces must be addressed in the site development plans . We do not consider that this site would require a geotechnical report in order to evaluate the site stability. If you have any questions, or if we may be of additional service to you on this project, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES of W a s hi 06. David C . Strong, L.E.G. u 1129 O d Ge°\ DAVID C. STRONG