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HomeMy WebLinkAboutGeological Assessment Review for BLD2005-02048 - BLD Engineering / Geo-tech Reports - 12/27/2005 PEON.STA rF OT MASON COUNTY o = PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton, Washington 98584 OJ �a 1864 DATE: December 27, 2005 INTER-DEPARTMENTAL COMMUNICATIONS TO: Kell McAboy PARCEL#32130-75-00010 FROM: Patricia C. — PW BUILDING PERMIT NUMBER: BLD2005-02048 SUBJECT: Geological Assessment Review NAME: Greg Dillon Hi Kel; The Geological Assessment Review prepared for the proposed Single Family residence at 185 East Capitol Peak Road has been received and reviewed by Public Works. The assessment appears to satisfactorily address County requirements for Geological assessment. The assessment's author indicates that, in his opinion, that a residence incorporating a daylight basement in the design solution is feasible at this site at the location indicated and may be constructed without risk of loss or damage due topsoil movements. Understanding that a wetland area appears seasonally to occupy much of the northern area of your property, recommendations contained in the assessment should be incorporated into the site's development and made conditions for permit issuance The engineer suggested roof leader system should be a tightline system isolated from the footing drain system. This system should be constructed using rigid plastic pipe with watertight glued joints. The water collected from the roof should be carried to a common discharge line that also collects water from the footing drain and underslab drain system. The collected water should be discharged to the north of the drain field to protect the drain field area from excess storm water. With all the other recommendations of the engineer incorporated into the site's development, stability issues appear to have been adequately addressed. 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 now and in the future. 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 now and in the future. Please feel free to contact me at 427-9670 or Bob Thuring ext. 452 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. Sincerely, Patricia Carroll WORK ORDER • PUBLIC WORKS DEPT, µl. dote: ``.,.1°S.r..wI Work Order PERMITC, s -v 20 -►£s Numliera R Ant wed l[ ' Date; Auftdzed W Type of Works CHARGE TO: NAMB AGENCY10OMPANY 60.UNG ADDRE88 ' PHONE �c�?'',..,.�.,.,,,..,,,,;. p� a e:Vmm•to da esTO • Iron EiTMAATE0I�OTILL s lW !fit! 6e�ployw lfMl[iAAM Mal i .. ti f ldlQ gm. AtWd Cost elARts PRatNti ��„�� W►R 6mploris COMMENT USED: ltDlt ram' MATawAr.,u�au� . .TOTAL ALL _.. a)' elueD DATE�,,,, .„INY r PAID•DATa�,..,�... Aw awe 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 42 O�© 26 January 2005 � Mr. Greg Dillon (p��� 328 Wyandotte Avenue Shelton, Washington 98584 Subject: Geotechnical assessment of slope stability and erosion potential at your proposed building site for a new single-family residence at 185 East Capital Peak Road, Mason County Tax Parcel 32130-75-00010. Dear Mr. Dillon: 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. The project area is located in a large lot subdivision with typical lot sizes of five acres. At this site, you own two adjacent five-acre parcels. On Tax Parcel 32130-75-00020, the parcel to the east of the parcel where construction is proposed, there is an existing single-wide modular residence. While during our field discussion, you did not indicate any plans for this property, the opinions and recommendations contained in this report le to an proposed construction on this adjacent pa rcel as the would be applicable p 1 PP Y P topography and geology are similar. The site is located on a north-facing slope. From the top of slope to the toe, we found a uniform slope of 13 degrees or 24 percent slope. The slope from the top where single-family residences have been constructed to the toe area on your property has been cleared and the surface maintained as grass. At the toe of slope on your properties, we find the area to the north to flatten in grade. There is a buffer area of trees between the cleared area of your properties and a wetland area which appears seasonally to occupy much of the northern area of your properties. The area of proposed construction will be on the slope, with development occurring in the southern about 200 feet of the property, the area of dry ground. The new residence is indicated as being constructed in the southern area of the property at the end of the easement road. 05010801 Page I of 4 �� 05010801 Page 2 of 4 From our field discussion, we understand that you are proposing to construct the new residence using a daylight basement in order to take advantage of the slope at this site. We agree that this is a good design approach for this site. Typical soil profiles in this area are exposed in road cuts of this development. In this area there is a thin layer of Vashon recessional outwash (Qvr) over Vashon subglacial till (Qvt). The outwash sands and gravels in the upland are thin, typically less than four to six feet in thickness. Underlying the outwash soils is the very dense and highly overconsolidated Vashon subglacial till. 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. The site lends itself to construction of a daylight basement at the location indicated to us in the field. You need to be aware that excavation of subglacial till can be difficult due to the overconsolidation and cementing of this soil unit. If small equipment is used for the excavation, work will be slow. Subglacial till is highly abrasive to excavation equipment. The excavated subglacial till is marginal for use as structural fill or general fill as these soils contain plastic fines and are moisture sensitive. If moisture contents can be controlled, then this soil unit may be used for fill. Good compaction control is required to ensure that any fill placed meets the minimum requirements of the International Building Code (IBC). You will also have to include erosion control in your site work to ensure that silt-laden water does not flow off the construction area. Excavation to construct a daylight basement will result in cut slopes exceeding four feet in height. Once foundation walls or form work exceed four feet in height, the void between the wall or forms and the cut slope are considered to be a trench under Department of Labor and Industries Standards. All trenching and excavation must conform to Chapter 296-155 WAC, Construction Work, Part N, Excavation, Trenching and Shoring. We recommend a soil type C for the outwash sands and gravels and soil type A for the subglacial till. To ensure uniform soil support for the structure, we recommend that all footings be founded on the subglacial till unit. Safe bearing capacity of these soils is in excess of 5000 p.s.f. To ensure dry daylight basements, attention to drainage must be included in the site plans. It is less expensive to address drainage and water-proofing during the 05010801 Page 3 of 4 construction phase than to have to attempt to retrofit drainage work after construction or occupancy. 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. At the base of the foundation walls, a perimeter footing drain system must be installed. We recommend the use of rigid plastic pipe for the drain system as it resists crushing better than Flexible ADS type of pipe and positive slope to drain is easier to maintain. To ensure drainage of the backfill against the foundation walls, we do not recommend the use of excavated subglacial till as it contains fines. We recommend that the backfill be clean, free-draining gravel or sandy gravel. Use of asphalt emulsion for damp-proofing the foundation walls is not recommended. There are products on the market that are sprayed on water-proof membrane that will provide better protection to prevent water infiltration through the foundation walls. Positive drainage must be provided under the basement slab. Use of perforated drain pipes extending under the slab backfill to collect any water flowing into the backfill will aid in ensuring a dry basement slab. The collected water may be connected to the footing drain discharge system. The roof rain leader system should be a separate tightline system isolated from the footing drain system. We recommend that this system be constructed using rigid plastic pipe with watertight glued joints. The water collected fro the roof should be carried to a common discharge line that also collects water from the footing drain and underslab drain system. The collected water should be discharge to the north of the drain field to protect the drain field area from excess storm water. The foundation walls for the basement will be braced at the toe by the basement slab and the top by the second level floor framing and floor diaphragm. We recommend that these structural elements be installed prior to placement of backfill. Sloping of the backfill against the foundation should conform to Section 1803.3 of the IBC. This will prevent ponding of water against the foundation walls. 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. 05010801 Page 4 of 4 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 M olenaar. The r 'p olect is mapped as Vashon subglacial till (Qvt). 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 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. This 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 incorporating a daylight basement in the design solution is feasible at this site at the location indicated to us in the field and may be constructed without risk of loss or damage due to soil movements. This opinion is based on the requirements presented in this report being incorporated in the design and construction. 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 his 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 �ee Wash��9 David C. Strong, L.E.G. 1129 �7Sed Goo�o DAVID C. STRONG