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HomeMy WebLinkAboutGeoTech Assessment Review for BLD2005-00239 - BLD Engineering / Geo-tech Reports - 3/22/2005 °N STAr � F°� MASON COUNTY P l LL 04 tii u) PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER tiI = Shelton,Washington 98584 � 0 O 1864 a DATE: March 22Id 2005 INTER-DEPARTMENTAL COMMUNICATIONS TO: Tammi Clark, DCD -Planner FROM: Alan A. Tahja,P/W - Co. Hydr. Engr. WO# PLG-05 SUBJ: Geo-Tech Assessment Review NAME: Snyder SFR @ Jensen Road BLD2005-00239 Tammi, The geotechnical assessment prepared for the proposed William&Tina Snyder Single Family Residence (SFR)to be constructed at 1600 E. Jensen Road,has been received and reviewed by Public Works. The report appears to satisfactorily fulfill County requirements for geotechnical assessment reporting. 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(E&S) 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 prepared for the proposed Single Family Residence and detached garage appears acceptable, and stability issues may be considered to have been adequately investigated and addressed. 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. Sincerely, flan A. Tahjn File: H: \WP\GEO\Assessments\Snyder.doc r WORK ORDER - PUBLIC WORKS DEPT. {At Date: PERMIT : r,L� �s_6}J,_"! Number. (E) Work Order Requested by: 1 , r Authorized by: � Type of Work: Date: ECEPWGETO: NAME AGENCY/COMPANY n�c� M AIRL�i'' BILLING ADDRESS ASON COUPHONE X-1 s t�n�.`1��ics Person in Cha e: 6 luh j�h • J.AfY.ri'::.. !v. .� }% xq}]G}}.:vY.:A794 XLSb..+:'vY.:.•... J:Y 'n4Y4f MC? J,4:n' .`t+.tW..¢:t::i::,•::.::.:e.:'..i" (c) Protect Time Une: (from-to da TO Pto)ect Start dada: Eetlmsted FlMah Dab: Apprmdn�te hours: ESTIMATED TOTAL$,a: COST'imSTIMATE (D) Dal Employee �$� HUM Subtotal' TOTAL A 3 zz >E smla t� EQWMENT USED: • 1 MATERIAL USED: -- tFl Ad"Cost = BARS: PROD N: DATE EmplT" 16L4B5!<Q � fiaars FrInpe% TOTAL$ EQUIPMENT USED: ! not TOE MATERIAL USED: TOTAL ALL —�— (0) BILLED DATE INV/y_ pA10 DATE REC.aK--��CKM�._ ' 5 =ER 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 15 March 2005 William & Tina Snyder P.O. Box 1334 Shelton, Washington 98585 Subject: Geotechnical assessment of slope stability and erosion potential at your proposed building site for a new single-family residence at 1600 E.Jensen Road, Mason County Tax Parcel Number 320065400008. Dear Mr. & Mrs. Snyder: As requested, we visited the above property in order to develop an opinion on slope stability and erosion potential of the site soils. Our work consisted of a site visit, review of available geologic information, and preparation of this report based on our field observations, research, and understanding of the proposed construction for this lot. At the time of our site visit, the lot had been cleared back to the top of the ravine slope with mineral soils exposed on the surface. The building pad had been excavated for the foundation. Test pits were open for drain field soil information in the southwest area of the lot. We found the area of proposed construction of both the residence and drain field to be of low relief with a gentle slope down to the south. About 150 feet south of the south wall line of the residence, there is a shallow slope down into a ravine. Slopes in this area are still vegetated, and slope angles are estimated to be less than 22 degrees or 40% of slope. We did observe at various areas along the top of the ravine that material from the clearing had been dozed over the slope. From our field discussion, we understand that your proposed development for this lot will be in two phases. The first phase is the construction of a residence with a drain field. The second phase will be the construction of a detached garage. Based on our site observations, we suggest that this future garage be constructed on the west property line area between the drain field and reserve drain field area and the residence. The "dig out" for the foundation and exposures in the test pits show typical soil profiles for this area: a thin layer of Vashon recessional outwash (Qvr) over Vashon subglacial till (Qvt). The outwash sands and gravels in the upland area of this development are typically thin, less than four feet in thickness. Underlying the outwash soils ri3'th ye.''C D 05030701 MAR 16 2005 Page I of 3 426 We CEDAR ST: I J 05030701 Page 2 of 3 dense and highly overconsolidated Vashon subglacial till. We observed that in the building "dig out" the excavation had extended to the till in the northern area of the building pad. The outwash soils thicken to the south in the area of the drain field. We expect that the outwash soils become thin on the slopes of the ravine. Subglacial till is a mixture of silt, sand, and small gravel. It was deposited at the base of the Vashon lobe of the Fraser glaciation in late Wisconsinan time as it advanced into the Puget Sound Lowland. The grain size distribution and loading by the ice sheet, thought to have been about one-half to one mile thick in this area, have created a soil unit that has many engineering properties similar to those of lean concrete. Subglacial till is for all practical purposes impermeable. In this area, the subglacial till is at least thirty feet thick and overlies overconsolidated advance outwash soils. The stability of the slopes in this area is due to being underlain by subglacial till. We expect that during periods of heavy or prolonged rainfall, flows of ground water will develop at the contact between the permeable outwash soils and the impermeable subglacial till. The flows may be large during heavy rain events. To ensure a dry crawl space attention to perimeter drainage is required. At the base of the foundation, we recommend that rigid plastic pipe be used for the footing drain. This pipe is more resistant to crushing during backfilling than flexible ADS type of pipe. Rigid pipe can more easily be set to grade to maintain a positive slope to drain. The roof rain leaders should be collected into a separate tightline collection system isolated from the footing drain. We recommend that rigid pipe with glued joints be used for this collection system. We suggest that the collected water be carried to the south and discharged into the flow line of the ravine in an approved manner. The footing drain and roof rain leader collection system may be connected to a common discharge line at an elevation such that back-up into the footing zone cannot occur. Sloping of backfill against the foundation must conform to Section 1803.3 of the 2003 International Building Code, This will prevent ponding of water against the foundation walls. We recommend that a minimum vertical distance of three inches be maintained between bottom of vents and top of landscape soil to ensure air flow. Natural slopes on this property are less than the angle of internal friction of the material forming the slopes. With the angle of internal friction less than the slope angle, slope stability is inferred. During our inspection of this property and adjacent properties, we did not observe any indication of recent or past mass wasting events. We expect that soil creep, the slow downslope movement of the near-surface active soil zone due to freeze-thaw cycles and saturation, is slow in this area. The side cast clearing debris that has been placed over the top area of the ravine slope consists of unstable soil masses that are prone to failure and rapid erosion. This material must be pulled back off the slope and disposed of in the area of low relief. The 05030701 Page 3 of 3 disturbed areas on the slope will need to be stabilized. We recommend that you contact the Mason Conservation District for recommended planting to restore slope protection. With the upland area being cleared of vegetation, you will need to address stabilization of this area to prevent silt-laden water from flowing off of your property. Placement of silt fences or straw bales along the top of the ravine slope may be required to confine silt-laden water to your property until vegetation cover can be reestablished. We recommend that this work be completed as rapidly as possible before rains return to this area. To verify our interpretation of the site geology, we referred to Plate I of the Water- Supply Bulletin 29, Geology of Southeastern Mason County, Washington by John B. Noble and Dee Molenaar. The project area is mapped as Vashon subglacial till (Qvt). To verify our opinion on the general site stability of the area, we reviewed Relative Slope n published b the Smith and R.J. Carson,Southern Hood Canal b Macke S Y Stability o f the Sout y Y J Washington Department of Natural Resources, Division of Geology and Earth Resources 1977. At the scale of the mapping, only general opinion is presented as this map is not site-specific. The project site meets the criteria for a Class 2, as the area is believed to be stable due to being underlain by subglacial till. Based on our field observations and research, it is our opinion that a residence and future garage may be constructed at locations indicated without risk of loss due to mass wasting events or erosional processes. Site stabilization is required to prevent fluvial surface erosion of the exposed mineral soils from the clearing work and removal of side cast material from the ravine slope. With the project area being underlain at shallow depth by highly overconsolidated soils, and the lack of indication of slope instability in the project area, and slope angles at less than the angle of internal friction of the soils forming the slope, we do not consider that the preparation of a geotechnical report is appropriate or warranted at this site. If you have any questions, or if we may be of additional service to you, please contact us at our Olympia office. Cordially, of Washin BRADLEY-NOBLE GEOTECHNICAL SERVICES y � David C. Strong, L.E.G. 'C' 1129 Sed GO DAVID C. S1A4N1C