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HomeMy WebLinkAbout821224003 Geotech AssessmentGolder Associates Inc. i 18300 NE Union Hill Road, Suite 200 Redmond, WA USA 98052-3333 Telephone (425) 883-0777 Fax (425) 882-5498 `*~~ www.golder,com Apri121, 2008 Alan and Bonnie Fure 2114 NW Far Country Lane Issaquah, WA 98027 Attention: Alan and Bonnie Fure RE: RESULTS OF GEOTECHNICAL RECONNAISSANCE TAX PARCEL 821224003 PORT LUDLOW JEFFERSON COUNTY, WASHINGTON Dear Mr. Fure: ~~ Golder Associates Our ref 083-93241 ~~~~~d~~ MAY -~ 9 200 •~ Golder Associates, Inc. is pleased to present the results of our geotechnical reconnaissance to determine appropriate buffer distances from a mapped landslide hazard zone for a detached garage proposed to be constructed at the parcel number listed above. In preparing this report we reviewed a site plan showing the current property conditions prepared by Triad Associates Inc. dated 1/18/08, and visited the site on March 29, 2008 to perform a geologic reconnaissance. The proposed garage is located adjacent to a steep slope that meets the definition of a landslide hazard under the Jefferson County Critical Areas Ordinance. In accordance with the ordinance a standard buffer of 30 feet is established from the top of landslide hazard areas. We understand that you wish to reduce the buffer to 15 feet as allowed under the ordinance. The purpose of this letter report is to provide our opinion if the reduced buffer will adequately protect the project and the landslide hazard area. SITE AND PROJECT DESCRIPTION The Fure property is located at the west end of W. Boat Drive in Port Ludlow and consists of two rectangular shaped lots accessed from W. Boat Drive along a gravel road. The property is situated on an eastward facing hillside that ranges in elevation from about 120 feet to 420 feet. The property slopes are variable ranging from nearly flat on the east end to as steep as 1H:1V (horizontal to vertical) in portions of the west end. The existing house is located at the west end of the property at about elevation 400 on the relatively flat crest of a ridge that. cuts through the west end of the property (Figure 1). The proposed garage will be constructed on the south side of house in the approximate location shown on Figure 1. It will consist of a one to two story wood frame structure on spread footings. The floor will be slab-on-grade construction. Vehicle entry will be from the driveway on the west. The proposed garage site is flat and .excavation and grading for the footings is expected to be minimal with cuts. and fills less than two feet. There is a steep slope to the south of the proposed garage location. The slope drops down to the south east at about 1H:1V from about elevation 400 to about elevation 300. An abandoned logging road cuts across the middle of the slope. OFFICES ACROSS AFRICA, ASIA, AUSTRALIA, EUROPE, NORTH AMERICA AND SOUTH AMERICA Alan and Bonnie Fure Mr. & Mrs. Fure -2- SITE GEOLOGIC CONDITIONS The purpose of the site visit on March 29, 2008 was to examine the slope and site conditions to assess the relative stability of the slope. The slope south of the garage was thickly vegetated with young to mature second growth fir trees and shrubs. The slope was generally uniform with the exception of the logging road mentioned above. We saw no visible evidence of past or present landslide activity extending down the slope as well as fifty feet east or west of the proposed garage location. The abandoned logging road that cut across the slope about fifty feet below the crest contained near vertical upslope side cuts up to about five feet high. These cuts were stable, showing little signs of sloughing or raveling. There were no signs of seepage or recent erosion on the slope. The geology of the site is mapped on the geologic map of eastern Jefferson County (Jefferson LD.M.S. Revised February 21, 1995) The map indicates the site is underlain by Vachon Lodgement till which is underlain by Vachon Advance Outwash. The site soils were observed in the field in road cuts along the driveway leading up to the house and in driveway cuts just northwest of the proposed garage as well as in hand dug holes on the slope south of the proposed garage. Our observations were in agreement with the geologic map. The ridge crest on which the house and proposed garage are located, is underlain by a dense to very dense Lodgement till, consisting of a mixture of silt, sand, gravel and cobbles. The till is underlain by a dense to very dense advance outwash sand that consists of stratified fine to medium sand with occasional layers or lenses of silt. The only location where significant seepage was observed was in a ravine bottom crossed by the gravel driveway on the north side of the property. CONCLUSION The site geologic conditions at the proposed garage location and on the adjacent slope to the south appeared stable and are suitable for supporting the intended garage construction. In our opinion, the proposed garage as well as the adjacent slope will be adequately protected with the proposed 15 foot buffer from the crest of the slope provided the following recommendations are followed. No fill material from site grading shall be dumped over or onto the slope south of the proposed garage. No fill shall be placed within the 15 foot buffer zone and the buffer zone shall be maintained in a vegetated condition after construction. . Roof gutter downspout water from the garage should be collected and directed away from the crest of the adjacent slope. Apri121, 2008 083-93241 We trust this letter meets your needs. If you have any questions please give us a call. Sincerely, ~~~ ~~~~ ~. ~..~ GOLDER ASSOCIATES, INC. amen G. Johnson, L.G. L.E.G. Principal Attachments: Figure 1 JGJ/SB James Ca. 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JEFFERSON COUNTY, WASHfNC±T UN ~ G~ f"c'1 ._.~""1 ~,"""1 ~ . s. : } . .. ., r,.,. : t ; ~ ~~ Golder Associates Inc. 18300 NE Union Hill Road, Suite 200 Redmond, WA USA 98052-3333 Telephone (425) 883-0777 Fax (425) 882-5498 www.golder.com Septem ber 2, 2008 Alan and Bonnie Fure 2114 NW Far Country Lane Issaquah, Washington 98027 Golder Associates ~~~ ®3 2Q~~ Our Ref.: 083-93241.000 ~E~~~~~ cou~~ oco RE: ADDENDUM TO RESULTS OF GEOTECHNICAL INVESTIGATION TAX PARCEL 821224003 PORT LUDLOW, WASHINGTON Dear Mr. Fure: Golder Associates Inc. (Golder) is submitting this addendum to our geotechnical steep slope reconnaissance report for your proposed garage construction in Port Ludlow. The report was dated April 21, 2008. The purpose of this report addendum is to address comments received from the Jefferson County Department of Community Development, dated August 28, 2008. There were four comments listed on the August 2008 letter. Two comments, numbers 2 and 3, pertained to a landslide hazard area to the northwest of the proposed garage, and potential impacts from the presence of an existing trench drain within the footprint of the proposed garage. Comment numbers 1 and 4 are being addressed by the applicant. Golder's response to comments number 2 and 3 are presented below. Comment 2 The landslide hazard area referred to being northwest of the garage is a small hill, about ten feet in height, west of the entrance driveway. The steep portion of the hill facing the proposed garage was created through excavation for the driveway. The vegetated hill is underlain by dense, Lodgment till, with no water. seepage and does not pose a landside risk. We recommend, in accordance with Section 18.22.170 (6) & (7) of the Jefferson County Critical Areas Ordinance, that the buffer from the toe of the slope be reduced from the standard 30 feet to the minimum allowable of 15 feet. This will provide the same protection from landslide hazard as the standard buffer. Comment 3 The location of the trench drain is shown on the attachments provided by you. I observed this drain at the ground surface when I made my site visit on March 29, 2008. The drain consists of an excavated trench backfilled with rounded washed gravel. There was no observed flow from this drain during my site visit and you have communicated to me that you have never observed flow from the end of the drain implying the water collected by the drain likely infiltrates. We evaluated several potential impacts this drain might have on the proposed construction and steep slope setback as proposed in our April 2008 letter. Our comments are as follows: 090208jgj 1_Fure Addendum LettecDocx OFFICES ACROSS AFRICA, ASIA, AUSTRALIA, EUROPE, NORTH AMERICA AND SOUTH AMERICA September 2, 2008 Al and Bonnie Fure -2- 083-93241.000 • The presence of the surface drain has no affect on the slope setback distance provided in our Apri12008 report. • The foundation for the garage can be constructed over the gravel drain. Care should be taken during construction not to block the drain. • The gravel drain could be used to infiltrate/disperse roof gutter downspout water from the proposed garage without adverse impact to the landslide hazard area. We trust this addendum meets your needs. If you have any questions, please give us a call. Sincerely, GOLDER ASSOCIATES INC. James G. Johnson, L.G., L.E.G. 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