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Home My WebLink AboutGeotechnical Assessment for MEP2006-00081 - GEO General - 12/26/2006 RECEIVED GeoResourcesLL C Ph 253-896-1011 FJO 0 4 204 5007 Pacific Hwy. E., Suite 20 Fx 253-896-2633 Fife, Washington 98424 A016 3M cEpA.SIR December 26, 2006 Mr. Phil Bennett PO Box 1863 ?Belfair, Washington 98528 (360) 277-0643 Geotechnical Assessment Proposed Single Family Residence xxxxx North Shore Road Mason County, Washington PN: 322245000032 Job: Bennett.North Shore.L INTRODUCTION AND SCOPE This report summarizes the results of our geotechnical engineering services for the proposed residence to be located on the vacant parcel between the existing residences located at 11001 North Shore Road and 11041 North Shore Drive. GeoResources previously completed a Geotechnical Engineering Report for the parcel at 11301 North Shore Road dated May? 14, 2004. We visited the site on November 24, 2006. The general location of the site is shown on the attached Site Vicinity Map, Figure 1. Our understanding of the project is based on discussions with you and your agent (Mr. Lee Boad), our recent site visit, our past experience on the nearby parcel, and our understanding of the requirements of Mason County Title 8 Critical Areas Ordinance. The site is currently undeveloped residential property. We understand that you wish to construct a new residence on the vacant parcel. The purpose of our services was to evaluate the site and determine whether the findings of our previous report would be similar for the conditions observed on the subject parcel. Specifically, our scope of services for the project included the following: 1. Reviewing available surface and subsurface soil and ground water information, including USGS Geologic Maps, the Mason County Soil Conservation Service soil survey, and the Coastal Zone Atlas for Mason County. 2. Visiting the site to conduct a geologic reconnaissance to assess the site's slope, soil, and surface water conditions. 3. Reviewing the previously written report for the parcel located at 11301 North Shore Road. 4. Preparing a written geotechnical assessment letter for the subject parcel summarizing our site observations. No new subsurface explorations or detailed geotechnical engineering analysis were completed as part of our current scope of work. Bennett.NorthShore.L December 26, 2006 Page 2 SITE CONDITIONS Surface Conditions The site is currently undeveloped, and is bounded by existing residences on the north and south. The site is roughly trapezoidal in shape, encompasses approximately 0.28 acres, and is bisected by North Shore Boulevard. The site has about 160 feet of frontage along North Shore Boulevard, and approximately 100 feet below (east) and 100 feet above (west) the roadway. Access to the site is via the shoulder of North Shore Drive. The site generally slopes down from west to east, with North Shore Boulevard situated on a flat bench or terrace. The slope above the roadway is as steep as 50 percent, and has about 50 feet of relief. The slope below the roadway is on the order of 35 percent, with less than 30 feet of relief. This lower area slopes down to a flat area along the shoreline. The slope is generally undisturbed, and is covered with a combination of fir and deciduous trees with a moderately dense understory. We did not observe any evidence of potential mass soil movement or deep- seated slope instability in the site area at the time of our site visit. No evidence of significant erosion was observed at the time of our site visit. Site Soils A review of the Soil Survey of Mason County (Soil Conservation Survey) indicates the site soils consist of the Everett gravelly sandy loam (Ek) that form on 15 to 30 percent slopes. The Everett soils are generally derived from gravelly glacial outwash and is described as well drained. The Everett soils are listed as having a moderate erosion hazard potential according to the SCS. A copy of the SCS soils map for the site is included as Figure 3. As previously discussed, we observed no evidence of surficial erosion at the time of our site visit. Geologic Conditions The Geologic Map of the Shelton 1:100,000 Quadrangle, Washington by Robert L. Logan (2003) maps the soils in the vicinity of the site consisting of Till (Qgt) in the upper elevations of the site and Advance outwash (Qga) in the lower portions of the site. These glacially derived soils were deposited during the Vashon Stade of the Fraser Glaciation approximately 12,000 to 15,000 years ago and are described as a mixture of clay, silt, sand, gravel and boulders. Near surface soils observed in open road cuts, as well as in our explorations advanced on the parcel located at 11301 North Shore Boulevard generally consisted of dense sands and gravel with variable silt, cobbles and boulders, which appear consistent with the Everett soils. A review of the Coastal Zone Atlas for Mason County indicates that the site areas is situated in an area mapped as "I" for intermediate stability, with localized areas of "U" for unstable areas. However, no areas of unstable old slides (Uos) or unstable recent slides (Urs) are mapped in the immediate vicinity of the site. Landslide Hazard Indicators According to the Mason County Code, Section 8.52.140, the purpose of the landslide hazard section is to identify areas that present potential dangers to public health and safety, to prevent the acceleration of natural geological hazards, to address off site environmental impacts, and to minimize the risk to the property owner or adjacent property owners from development activities. This code defines a landslide hazard area as: Bennett.NorthShore.L December 26, 2006 Page 3 (A) Areas with any indications of earth movement such as debris slides, earthflows, slumps and rock falls (see figure F.100 attached to the ordinance codified in this chapter); (B) Areas with artificial oversteepened or unengineered slopes, i.e. cuts or fills; (C) Areas with slopes containing soft or potentially liquefiable soils; (D) Areas oversteepened or otherwise unstable as a result of stream incision, stream bank erosion, and undercutting by wave action; (E) Slopes greater than fifteen percent (eight and one-half degrees) and having the following: (i) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock (e.g. sand overlying clay); and (ii) Springs or groundwater seepage. (F) Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least ten feet of vertical relief." In addition, the following information may be used as a guide by the County to indicate areas that have a higher likelihood of meeting the above criterion. (2) The following information may be used as a guide by the county to indicate areas that have a higher likelihood of meeting the classification criteria above: (A) The areas identified on the Mason County soil survey map as having slopes greater than fifteen percent. (B) The areas identified on the Coastal Zone Atlas, Volume 9, of Mason County, Washington as: (i) Unstable - "U" (ii) Unstable Old Slides - "UOS" (iii) Unstable Recent Slides - "URS" (iv) Intermediate Slopes - "I" (v) Modified Slopes - "M" (C) The areas identified as Class 2, 3, 4, or 5 of the maps: "Relative Slope Stability of the Southern Hood Canal Area, Washington," by M. Smith and R.J. Carson, Washington State Department of Natural Resources, Division of Earth Resources, 1,977 and `The Geological Map of North Central Mason County, Washington," by R.J. Carson, 1,976, U.S. Geologic Survey OFR 76-2. A review of the Coastal Zone Atlas for Mason County does indicate that the site is located in an areas mapped as being Intermediate (1) for slope stability. A copy of the Coastal Zone Atlas is attached as Figure 4. Additionally, according to Mason County, the site is classified as having landslide hazard areas due to the site having areas with slopes of forty percent or steeper with a vertical relief of ten or more feet. Erosion Hazard Indicators According to the Mason County Code, Section 8.52.160, the purpose of the erosion hazard section is to identify areas that present potential dangers to public health and safety, and to prevent the acceleration of natural geological hazards, and to neutralize the risk to the property owner from development activities. This code defines an erosion hazard area as areas in Mason County underlain by soils which are subject to severe erosion when disturbed. Such soils include, but are not limited to, those for which potential for erosion is identified in the Soil Survey of Mason County, USDA Soil Conservation Service, 1960, or any subsequent revisions or addition to this source. Bennett.North Shore.L December 26, 2006 Page 4 These soils include, but are not limited to, any occurrence of River Wash ("Ra") or Coastal Beaches ("Cg") and the following when they occur on slopes fifteen percent or steeper: (1) Alderwood gravelly sandy loam ("Ac" and "Ad"); (2) Cloquallum silt loam ("Cd"); (3) Harstine gravelly sandy loam ("Hb"); (4) Kitsap silt loam ("Kc"). According to Mason County, the site is classified as having erosion hazard areas due to it being underlain with Alderwood soils (Ad). CONCLUSIONS AND RECOMMENDATIONS Based on the results of our field visit observations, the findings our 2004 report for the nearby parcel is consistent with the conditions observed on the subject parcel. Landslide Hazards Portions of the site above North Shore Boulevard meet the technical criteria of a Landslide Hazard area (slopes steeper than 40 percent). However, the portion of the parcel below North Shore Boulevard do not have any of the primary indicators as listed in Mason County Code, Section 8.52.140 and therefore should be the preferred building location. The steeper portion of the slope below North Shore Road includes the roadway embankment, which should be an engineered slope. Also, the Coastal Zone Designations of "I" and "U" are likely attributed to the height and inclination of the slopes, since no historic or recent landslides were shown on maps in the vicinity of the site. Proper planning, design, drainage and construction techniques can further reduce the risk of significant erosion and slope instability. Erosion Hazards As previously stated, the Everett series soils underlying the site have moderate erosion hazard. Therefore, it is our opinion that the potential erosion of the site is not a limiting factor for the proposed development. Development at the site will be restricted to the lower portion of the lot. Erosion control measures may include, but not necessarily limited to, berms and swales with check dams to channel surface water runoff, ground cover/protection in exposed areas and silt fences. Graded areas should be shaped or otherwise protected to avoid concentrations of runoff onto cut or fill slopes, natural slopes or other erosion-sensitive areas. Temporary ground cover/protection such as jute matting, excelsior matting, wood chips or clear plastic sheeting should be used until permanent erosion protection is established. All disturbed areas represent an increased risk for erosion. We therefore recommend that temporary and permanent erosion control measures be installed and maintained during construction and following, until permanent erosion control or landscaping is in place. Collected stormwater from the addition's roof should be collected and be discharged directly to Hood Canal. LIMITATIONS We have prepared this letter for use by Mr. Phil Bennett and members of the design team, for use in the design of a portion of this project. The data used in preparing this report and this report should be provided to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations are based on data from others and limited site reconnaissance, and should not be construed as a warranty of the subsurface conditions. Bennett.NorthShorel December 26, 2006 Page 5 When the design is finalized, we recommend that the design and specifications be reviewed by our firm to see that our recommendations have been interpreted and implemented as intended. The scope of our services does not include services related to environmental remediation and construction safety precautions. Our recommendations are not intended to direct the contractor's methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. Variations in subsurface conditions are possible between the explorations and may also occur with time. Sufficient monitoring, testing and consultation should be provided by our firm during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those antici- pated, and to evaluate whether earthwork and foundation installation activities comply with contract plans and specifications. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No other conditions, expressed or implied, should be understood. Q A ➢ We have appreciated the opportunity to be of service to you on this project. If you have any additional questions or comments, please do not hesitate to call us at your earliest convenience. Respectfully submitted, GeoResources, LLC Residential Engineering Services, PLLC Keith S. Schembs, LEG Glen Coad, PE Pri o Was Owner time h��f GI_p 44 9 g A 925 KEITH SCOTT SCHEMES EXPIRES 10 / 31 / O WGC:KSS:kss Doc ID:Bennett.North Shore.L Attachments: Figure 1 —Site Vicinity Map Figure 2—Site Plan Figure 3—SCS Soils Map L Hood Canal s T�1►uy7 x Ne Shore Rd OWN Altterbroo,4 Golf$ Yacht Club Approximate Site Location q wv s Not to Scale GeoResources, LLC Site Location Map 5007 Pacific Highway East, Suite 20 Proposed Residence Fife, Washington 98424 North Shore Boulevard Phone: 253-896-1011 Mason Count Washington Fax: 253-896-2633 y� File: Bennett.NorthShore.SVM December 2006 Figure 1 r-- 'r W 4 322240080000 �; 32224500003A 322245000030 low Y, 22450a0031 322245000035 ' 04 45000037 , 32223 $0000 322248888838 "� 0 4'o , HOOD CANAL 322245000040 } 245000042 3222422222045 z2 r 322238888888 Approximate Site Location GeoResources, LLC FIGURE 2 - Site Exhibit Project: Bennett Residence 5007 Pacific Highway East, Suite 20 Location: xxxxx—North Shore Drive Fife, Washington 98424 Mason County, Washington Ph: 253-896-1011 Fax: 253-896-2633 Client: Mr. Phillip Bennett Date: December 2006 Job #: Allsop.19thKPN.SP Oh f tot 'Its, ``�a1•� _- —'�'~yT a p F Hwy A may,. w r, " 1r ly'►a �' • � �., ,� s - v 4 a it • i Washington j t w,.ley;��, '' "" _ ••�,' ' Approximate Site Location Not to Scale GeoResources, LLC NRCS SCS Soils Map 5007 Pacific Highway East, Suite 20 Proposed Residence Fife, Washington 98424 North Shore Boulevard Phone: 253-896-1011 Fax: 253-896-2633 Mason County, Washington File: Bennett.NorthShore.SCS December 2006 Figure 3 I