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HomeMy WebLinkAboutGeoTech Assessment Review for BLD2004-01266 - BLD Engineering / Geo-tech Reports - 9/10/2004 STATE°� Nt MASON COUNTY p 7 A o S i N PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER z Shelton,Washington 98584 �0 1864 DATE: September IOth, 2004 INTER-DEPARTMENTAL COMMUNICATIONS TO: Joanne Long-Wood, DCD -Planner FROM: Alan A. Tahja,P/W- Co. Hydr. Engr. WO# PLG-04 SUBJ: Geo-Tech Assessment Review NAME: Evans SFR @ I I I E Lund Drive BLD2004-01266 Joanne, The geotechnical assessment prepared for the proposed Angel and David Evans' Single Family Residence(SFR)to be constructed at 111 E Lund Drive,has been received and reviewed by Public Works. The report appears to satisfactorily fulfill County requirements for geotechnical assessment reporting. The assessment's author indicates that, in his opinion,the completed 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. The engineering geologist has indicated that the existing fill slope needs to be stabilized, and that erosion control may be required by the County in the toe area. 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 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. Sin erely, � z Alan A. Tahja"----- File: H: \WP\GEO\Assessments\Evans.doc 1 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 30 August 2004 4? s�p0 'e1�0� 0 Angel and David Evans 111 E. Lund Drive Shelton, Washington 98584 Subject: Geotechnical assessment of the slope stability and erosion potential of the site soils at your building site, 111 E. Lund Drive, Mason County Tax Parcel Number 321307590181 in Mason County, Washington. Dear Mr. & Mrs . Evans: 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 development of this lot. At the time of our site visit, the area of proposed construction had been cleared. Underground utility trench was being excavated from Lund Drive onto the site for power and phone lines . A domestic tube well had been installed in the southern area of the clearing. We observed loose fill slopes along the south edge of the clearing work. From our telephone conversation with you, we understand that you are proposing to place the residence in the lower area of the southeast portion of the lot. This area was staked at the time of our site visit. The graded slope to the north and northwest of the lower building site will be a landscaped area. Typical soil profile for this area is exposed in the utility trench in the upland area. In this area we find a thin layer of Vashon recessional outwash (Qvr) over Vashon subglacial till (Qvt) . The outwash sands and gravels are thin, typically less than four feet in thickness in this area. Underlying the outwash soils 04081501 Page 1 of 4 Qom. .. 04081501 Page 2 of 4 is the very dense and highly overconsolidated Vashon subglacial till . We noted that subglacial till was also cropping out in areas of the cut slope in the landscape area. 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 Wisconsian 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 it 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 . Your site development work has created a modified slope and reduced the natural slope angle on this property. The grading work has leveled the area of proposed construction of your new residence. Fill material appears to have been placed to the south of the building site with thickness up to six to eight feet at the edge. Since this fill section was not placed as a controlled structural fill section, you should not construct buildings on this fill section without preparation of a geotechnical report. To improve the stability of the fill slope during the coming winter season, we recommend that the slope be graded to a maximum slope angle of 2 : 1 (horizontal to vertical) as required in Section J106 of the 2003 International Building Code ( IBC) . The face should be track rolled by a dozer to consolidate the face of the fill and to imprint the surface with grouser marks . The surface should be seeded with a fast growing grass to provide temporary protection from sheet washing. The grouser marks will aid in holding the seed to the surface. As part of this work, the top of the slope should be bermed to prevent water from the site flowing over the fill face and eroding or gullying the face. Erosion control may be required by the county in the toe area to prevent silt-laden water from leaving the site. 04081501 Page 3 of 4 With the underlying subglacial till being for all practical purposes impermeable, control of surface storm water will be required to ensure dry crawl spaces . We expect that during periods of heavy or prolonged rainfall, flows of storm water will develop on the surface of the subglacial till. Saturation of the outwash soils may extend to the surface. Construction of drainage swales to direct surface flow of water away from the building site is recommended. French drains on the upslope side of the building site may be required to intercept flows of ground water flowing on the subglacial till. Good perimeter drainage is also required around the foundation to prevent ponding of water. Sloping of the backfill against the foundation should conform to Section 1803. 3 of the IBC, a 5% minimum slope away from the foundation for 10 feet to prevent ponding of surface water against the foundation. We consider the natural slopes in the project area to be stable. Natural and cut slopes in the project are less than the angle of internal friction of the material forming the slope, and slope stability is inferred. During our site inspection, with the exception of the fill slope, we did not observe any indication of recent or past mass wasting processes on this or adjacent properties . We did observe that the slope does steepen down to the south, south of the toe of the fill slope. This slope is also at slope angles less than the angle of internal friction of the soils and is stable. To verify our interpretation of the site geology, we referred to Plate 1 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) . The Geologic Map of the Shelton 1: 100,000 Quadrangle, Washington by Robert Logan, 2003, Washington Division of Geology and Earth Resources, Open File Report 2003-15 maps this area as Qgt, Till, late Wisconsian deposited by the glacial ice of the Puget lobe. To verify our opinion on the general site stability of the area, we reviewed Relative Slope Stability of the Southern Hood Canal by Mackey Smith and R.J. Carson published by the Washington Department of Natural 04081501 Page 4 of 4 Resources, Division of Geology and Earth Resources 1977 . At the scale of the mapping, only general opinion is presented and this map is not site specific . We find this area meets the criteria for a Class 1. Slopes typically less than 15% but may be greater locally. Believed to be stable as they are underlain by very stable material such as till. We concur with this opinion for your property. Based on our field observations and research, it is our opinion that the residence can be safely construction on this lot at the location indicated without risk of loss due to soil movements or accelerated erosion. All footings for the residence must be founded on native undisturbed soils or on structural fills meeting the minimum requirements of the IBC. With the site being underlain at shallow depth by highly overconsolidated soils, 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, BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. Strong, L.E.G. 9fo 1129 0 used Geo� DAVID C. S7RONG PLo-r L4,J FOR. S iTE f;-DDtcrSS �A�rz � R+�G�I EvANS X X X X E . JvoNppR, ' SC) C. SYC4 MlOTLV- CT S 14ELTor4 , WA '8 y-8Ll Plot Ma Drawn S 4 E L To n1 r WA. 9 6 S 8 y Hi Plotine Homes To Meet y - 7 s 9 o pecifications. Any Revisions To Be Made T. i# 3�1 3 0 7 S9 o 18 1 S«t 1 E By The Home Owner. i N 0Z7-71 c9 i 74 r: t i / t e;, Way w ov4Frz tso' �,z1• ° uV Date: VVVKK ORDER - PUBLIC WORKS DEPT. PERMIT M _ Work Order Requested by:. �`, �, ,a Number. �r�f Authorized by: Type of Work; S �.� a � �`Dale: CHARGE TO: NAME AGENCY/COMPANY 131LUNG ADDRESS PHONE n Pub. Works Person in Charge:c .:»cs......•+..... (c) Project Time Line: (from-to dates } Pro)eot Start slate: To Es&Wed Fkf Data: AWwdrr>ata hours: ESTIMATEDTOTAL (D) VAL% COST ESTIMATE Empror« B31i Hours $u .• TOTAL EMPMERT USED: ?O MATERIAL USED: ---------------- (F) AchW CostBAF 2= DATES' PROD P. Nam LEAtt Hours • WWII TOTAL$ EQUIPMENT USED: --- � MATERIAL,USED: ta) BILLED DATE TOTAL ALL -----�" -----IN PAID DATE R E c l------_______C 0 CL Ua �5