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HomeMy WebLinkAboutGeoTech Assessment Review for SPL2003-00005 - SPL Inspections - 8/31/2004 s TO BE KEPT IN THE MASON COUNTPARCEL FILE PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton,Washington 98584 DATE: December 5', 2003 INTER-DEPARTMENTAL COMMUNICATIONS TO: Diane Marcus-Jones, DCD -Planner FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. WO# PLG-03 SUBJ: Geo-Tech Assessment Review NAME: Fiorini SFR(a),E Mason Lk Drive SPL2003-00005 Diane, The geotechnical assessment prepared for the proposed Julio&Bonnie Fiorini at 3021 E Mason. Lake Drive East has been received and reviewed by Public Works. The assessment appears to dispel the need for further geotechnical investigation, conforms to standards of engineering principles and practices for this time and place, and appears to conform to the minimum requirements of DCD for geotechnical assessment reporting. The evaluation raises no issues or concerns, and appears acceptable for the level of reporting required. Recommendations contained in the report should be incorporated into the site's development. The escape of silt or mud from the subject property constitutes a violation of both State and County stormwater quality protection regulations, and could subject violators to fines and penalties. 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, Al A. Tahja File:H.IWMGEOIAssessmentsIFiorinLdoc l WORK ORDER S�- UBLIC W©� RKS DEPT. �ZG2� ' � R V4 Date: �c1 ,. 5 %� 3 (E) Work Order PERMIT#: 3F, J- Number p "� :Requested by:. « ( '' Authorized by: Type of Work: ` Date: CHARGE_TO: NAME01, AGENCY/COMPANY SILUNG ADDRESS PHONE Pub.Works Person in Charge: `r f: }ACOPCOMOG<-0b �'.NS' iryg0i}:a4%':+.:y:..•...: (c) Project Time Line: (from-to dates) TO Protect Statt date: J` 03 Estimated Fkgsh Date: Awrc mate houn: U q Z ESTIMATED TOTAL i'S: �! COST ESTIMATE (D) Jwl EMP1101?M �B3t! H4SdLt $!C[ .3( Edna* TOTAL= !1� E3AI>< TOTAL IPS EQUIPMENT USED., MATERIAL USED: (F) Actual Cost $ BARS: PROJ t DATEWORKEDY� =— t Hours SAIVOW Edwe TOTAL$ EQUIPMENT USED: Rate TOTAL VS MATERIAL.USED: --- TOTAL ALL (a) BILLED DATE IM/0 PAID DATE RECI CKO r RECEIVE ® Bradley-Noble Geotechnical Services j�� A Division of The Bradley Group, Inc. OCT Z 7 ZOO •#� A PO Box 12267, Olympia WA 98508-2267 G Phone 360-357-7883 • FAX 360-867-9307 FOSTER& WILLIAMS ARCHITECTS 22 October 2003 Julio & Bonnie Fiorini 30330 9th Avenue South Federal Way, Washington 98003 Subject: Geotechnical assessment of slope stability and erosion potential at your residence site 3021 East Mason Lake Drive East, Tax Parcel Number 22233 52 00087, Mason Lake Area of Mason County, Washington. Dear Mr. & Mrs . Fiorini: As requested by your project architect, Mr. Grant Foster of Foster and Williams Architects, we visited the above site in order to observe site conditions , review the proposed reconstruction for geotechnical issues, and prepare a report addressing slope stability and its probable long-term effect on the proposed reconstruction. Our work consisted of a site visit, review of proposed reconstruction of the residence, research into the area geology and slope stability history, and preparation of this report. The property is located on the west-facing slope above Mason Lake. We found a low slope up from East Mason Lake Drive East to an upland area where the garage was located. The slope increased down to the west towards Mason Lake from the upland area. A two-story residence in poor repair is found between the lake and the upland area. This residence was constructed with a daylight basement and second level above the basement. Two shallow test pits were open between the residence and lake. The existing residential structure will be razed as part of the proposed site development and a new residence also incorporating a daylight basement with a larger building footprint will be constructed on the lot. Additional excavation into the slope will be 03101301 Page 1 of 4 a ro w 03101301 Page 2 of 4 required as part of the new construction. We understand that the finish floor of the new residence will be founded at the same general elevation as the existing basement floor level . To bring the second level to the same general elevation as the existing upland elevation, foundation walls on the order of 9 to 10 feet will be constructed. Area geology controls slope stability and erosion rates in this area. We do not consider wave action or high water level from Mason Lake to pose any risk to the project at the indicated elevation of the basement floor. Examination of road cuts and cut slopes around existing structures in the general project area indicate that two soil units are found in this area. We also found similar soils exposed in the two shallow test pits to the west of the residence. The near-surface soil unit is the gravelly sands and sandy gravels of the Vashon recessional outwash (Qvr) . This soil unit is thin in this area, typically less than four feet thick. Underlying the outwash soils is the Vashon subglacial till (Qvt) . Based on exposures, the Vashon subglacial till underlies the site. Subglacial till is a mixture of silt, sand, and small gravels . It was deposited at the base of the advancing Vashon continental glacier as it moved into the Puget Sound Lowland about 14,000 years to 19 , 000 years ago. 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. The existence of Mason Lake is due to a combination of erosion by the glacier creating the topographic low occupied by the lake and the subglacial till which prevents infiltration. We expect that excavation for the new structure will also encounter subglacial till. Excavation of subglacial till can be slow, especially if small equipment is used. The subglacial till and outwash sands and gravels offer good support for standard spread foundations . Since the subglacial till is for all practical purposes impermeable,it does require attention to drainage to ensure dry daylight basements . 03101301 Page 3 of 4 To verify our interpretation of site geology and apparent stability, we referred to two documents . 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 . 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 this area as being a Class 2 area which is believed to be stable under normal conditions . Based on our field observations and our work in the area, we believe that the slopes surrounding this project are stable between Mason Lake and the roadway. This opinion is based on the geology and the fact that slope angles are less than the angle of internal friction of the soils forming the slope. When slope angles are less than the angle of internal friction, slope stability is inferred. Removal of the exiting structure and excavation to extend the daylight basement is expected to expose cohesionless sands and gravels at the top of the cut face and subglacial till in the lower face of the cut. During periods of heavy or prolonged rainfall, seepage may develop at the contact of the permeable sands and gravels and the impervious subglacial till . You need to be aware that the void between the cut slope and forms or foundation walls after casting, once the forms or wall exceed four feet in height, are considered to be a trench by the Department of Labor and Industries . The sands and gravels of the recessional outwash control the sloping and shoring requirements . All trenching and excavation must conform to Chapter 296-155 WAC Construction Work, Part N, Excavation, Trenching and Shoring for a soil type C . We expect that the foundation walls of the daylight basement will be braced at the toe by the concrete slab of the basement floor and at the top by the second level floor framing and floor diaphragm. To prevent bowing of the wall, we recommend that these structural elements be installed prior to backfill with free-draining material . The braced foundation walls should be designed to resist an active soil pressure of at least 40 pounds per cubic foot. r 03101301 Page 4 of 4 Good drainage practices are required to ensure a dry basement. A perimeter footing drain must be installed at the base of the foundation walls . Storm water from roofs and impervious surface should be collected into a separate tightline system with the water carried to the west of the structure for infiltration or direct discharge at the back of the beach. This will prevent saturation of the soils in the backfill from storm water during periods of heavy or prolonged rainfall . Backfill area of the basement walls must maintain a positive slope of at least 2% away from the foundation for a minimum of five feet. This will prevent ponding of surface water against the foundation walls and aid in ensuring a dry basement. Based on our field observations and research, it is our opinion that the residence may be constructed on this property at the location indicated on the site plan provided to us with no risk of slope failure or erosion of the slope affecting the foundation system if the recommendations of this report are incorporated in the design solution. Examination of the existing residence did not indicate that it had suffered distress due to sloe soil movement. To ensure dry basement slabs, attention to collection and disposal `' of storm water from impervious surfaces must be addressed in the design. The year around ground water level or' water Mason Lake will not have any effect on the project. a " We do not consider the preparation of a geotechnical report appropriate for this site based on moderate slopes and overconsolidated soils underlying the project area. If you have any questions, or if we may be of further assistance to you on this project, please contact us at our Olympia office. Cordially, o� W a s h�� BRADLEY-NOBLE GEOTECHNICAL SERVICES vj 7 David C. Str ng, L.E .G. � 1 �yp±Yn]vEJti• cc : Grant Foster (two copies) . nSMd Ge° DAVID C. STR; i�� hV � EXIST.GRADE TO BE REVISED APPROX.HOUSE SECTION = 0 cn — EXISTING GRADE TO LAKEti ii n i u EXISTING RETAINING WALL TO REMAIN ° 0 it