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HomeMy WebLinkAbout901135002 Geotech Assessment (2003)November 7, 2003 A1 Scalf Jefferson County DCD, Director 621 Sheridan Street Port Townserld, WA 98368 RE: Geology Revaluation Lots 1-4 Sugar Hill Estates Terry McHugh John £.$coff REAL ESTATE THIS OFFICE INDEPENDENTLY 0WNE0 & OPERATED John L Scott P~i'o Port Townsend Office llOV 10 2003 Dear Al, As you can see I'm still trying to get some relief from the unnecessary set-backs up at Sugar Hill. Before Jerry Smith retired he was working on this case and giving me feedback from you. He mentioned to me that you were not inclined to accept the Stratum Group's Geo-Tech report dated November 8, 2002 because you did not believe it to be as thorough as the Shannon / Wilson report that was recorded with the plat. Due to that comment I asked Dan McShane of the Stratum Group to review the S&W report more thoroughly and comment on it as it relates to the actual site conditions. As you can see by his enclosed letter he feels strongly that S&W report does not accurately describe the conditions and the slope location itself. I realize that the UDC states a minimum of 15' as a set-back and the Stratum Group recommends a 10' set-back. Harry Properties Inc.(the current owner) would be satisfied with the UDC 15' set- back and feels that is necessary for the end user to be able to use that additional 15' to ensure a more build able and enjoyable parcel. Thank you for taking the time to review this new information. We look forward to your prompt response. ~ [J~ ~ ~ Sincerely, ~ ~,L B: t~, 7, Terry McHugh Cc: Harry Properties Encl. Office: (360) 379-4570 Toll Free: {800) 869-7834 Fax (360) 385-4196 E-Mail: tmch@olypen.com Address: Port Townsend Office · 2219 West Sims Wag ° Port Townsend, WA 98368 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 November 8, 2002 Ron Simpson Covenant Mortgage Corporation 9725 SE 36th Suite 304 Mercer Island, WA 98040-3840 Re: Geology Evaluation Lot 3, Sugar Hill Estates Section 13, Township 29 N, Range 1 W Jefferson County, Washington Dear Mr. Simpson: We are pleased to present the results of our geology evaluation of the above referenced property. The western portion of the lot consists of a steep potentially unstable slope. The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to a potentially unstable slope. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the steep slopes on the property and in the vicinity of the property, and review of available geologic mapping in the area. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the steep slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the steep slope and on the uplands above thc slope by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the slope stability, recommendations for site development, and recommendations for further investigation, if necessary. November 8, 2002 Lot 3, Sugar Hill Estates Geology Hazard Evaluation GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial Geologic Map of the Port Townsend 30- by 60-Minute Quadran~e, Puget Sound Region, Washington (Pessl, Deither, Booth and 1V~ard, 1989) indicates the subject property is underlain by two units: 1) Glacial and nonglacial sedimentary deposits of Pre-Fraser Glaciation age and 2) glacial till. The Geologic Map Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, gfmard, and Dembroff, 1993) indicates the subject property is underlain by glacial till and the Pre-Frasser Glaciation sediments consist of undifferentiated Pleistocene deposits and Possession Till. The pre-Fraser glaciation deposits are described as consisting ofinterbedded oxidized brown, red- brown, and gray gravel, sand, silt, and day. The layers are moderately to well bedded and the unit contains minor amounts of ice-contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant peat and woody debris. The two glacial tills consist of a poorly sorted mixture of rock fragments ranging in size from clay, silt, sand and gravel and cobbles deposited directly by glacial ice. Observations on the steep slope and upland area of the subject property are consistent with the mapping described above except that a relatively thin deposit of loose glacial recession sand and gravel is present on the uppermost slopes and upland area of the subject property. The steep slope is for the most part underlain by very compact silt (Pre-Fraser deposits) and glacial till. Glacial till was observed near the top of the slope and near the base of the slope. The till near the base of the slope is likely the Possession Till. The property is located on the east side of Chimacum Valley. The valley is a glacial valley created dmht8 the last ice age. The valley was formed by the erosion of pre-Fraser deposits by the Vashon ice sheet and/or water flowing underneath the glacier. SPECI17IC SITE OBSERVATIONS The site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The subject property consists of an upland area bounded on the west by the steep slope of Chimacum Valley. The elevation of the upland is approximately 380 feet and the base of the steep Stratum Group File: 10.6.02B November 8, 2002 Lot 3, Sugar Hill Estates Geology Hazard Evaluation slope is at an elevation of approximately 150 feet. A relatively level, cleared building area is located on the upland portion of the property at the top of the steep slope. The steep slope slopes downward from the upland area at an angle of approximately 15 degrees for approximately 30 feet. The slope angle lessens to an average slope of approximately 35 degrees further down the slope. The slope is well vegetated with a mix of trees and thick brush. Portions of the slope are steeper because a number of old logging skid roads have been cut into and switch back and forth up the slope. Except for minor sloughing of cut slopes and topsoil creep, no evidence of slope failures was observed on the steep slope. The entire steep slope is underlain by very compact silt, sand, and gravel deposits and glacial till. Based on the age of the trees on the slope, we estimate that the slope was clear cut approximately 40 to 50 years ago. The upland area of the property consists of relatively gentle slopes. Except for the building area just above the steep slope, the upland portion of the property slopes gently to the east away from the steep slope. No water seeps were observed on the steep slope and no seasonal wet areas are present. We did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplffi of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property, we conclude that the steep slope on the west side of the property is relatively stable and is only subject to minor topsoil and slope sloughing. A residence can be located on the relatively level upland portion of the property as long as it is located 10 feet back from the top of the steep slope. A residence can be built within the area indicated on the Figure 2 Site Sketch Map. It is our opinion that a residepc~ iocated wSthin the area indicated on Figure 2 will bc at minimal risk of being impacted by landslides. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope as long as our recommendations are followed. We recommend that any storm water generated be discharged to the east of the building away from the steep slope. Stratum Group File: 10.6.02B November 8, 2002 Lot 3, Sugar Hill Estates Geology Hazard Evaluation Site grading soils or debris, landscape debris, or any other material should not be disposed of over the steep slope or placed at the top of steep slope. Trees on the steep slope may be limbed for view purposes, but no trees should be removed unless a thinning or harvest plan has been reviewed by a geologist familiar with slope stability and tree removal. Although the steep slope has previously been logged without causing obvious landsliding, the slope stability may respond differently to a second cut because of different forest cover. The proposed septic drain field is part of a community drain field to be used by several properties and is located 300 feet northeast of the proposed building area. The septic drain field should not cause problems for slope stability on the subject property or on any other properties as it is located such that it will not effect slope hydrology. Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Str~um Group Da cS ane, M.S., P.E.G. Professional Engineering Geologist _ _ [ ._Daniel McShane, ! Stratum Group File:10.6.0'2B NAD2'/12~43'0~" W STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 October 31, 2003 Terry McHugh John L. Scott Real Estate 2219 West Simms Way Port Townsend, WA 98368 Re~ Geology Evaluation Sugar Hill Estates Section 13, Township 29 N, Range 1 W Jefferson County, Washington Dear Mr. McHugh: As per our conversation I have reviewed the past site specific geology reports I have written for Sugar Hills Estates Lots 1 through 4 dated November 4, 2002. I have also reviewed the Geotechnical Report Sugar Hills Estates Chimacum, Washington prepared by Shannon & Wilson, Inc., dated February 1996. It is my understanding that Jefferson County has indicated that the setback from the top of a steep slope on Lots 1 through 4 should be as indicated on a previously prepared plat map. It is also my understanding that Jefferson County has sited the Shannon & Wilson report as justification for the set back distance. The setback distance indicated on the map was prepared by a survey as the setback line is drawn through several survey marked locations. The setback line was marked on the ground assuming a 30-foot distance from the top edge of the steep slope. The map refers to the setback line as 30' Landslide Area Buffer. The steep slope is shaded with the upper edge of the shaded area demarcated with a light dashed line. The 3 O-foot setback for the lots at Sugar Hill Estates is inconsistent with the Conclusions and Recommendations of the Shannon & Willson report. Shannon & Wilson states "Construction on or within 25feet of slopes of 2H.'lV or steeper should be avoided. No fill should be placed on these slopes. Alternatively, additional geotechnical explorations and resulting studies may be 'per_formed_for spec!ftc structures planned on slopes 2H: l V or steeper." I was retained to conduct site specific geology evaluations on Lots 1 through 4 consistent with the recommendations of Shannon & Wilson. Based on my site-specific observations of the slopes and soils on the four lots, I recommended setbacks from the top of the steep slope of 34 feet for Lot 1 and 10 feet for Lots 2, 3 and 4. Detailed observations of slope and geology conditions at each lot are provided in the site specific geology reports I have previously provided. November 1,2003 Sugar Hill Estates Setbacks Geology Hazard Evaluation The greater setback for Lot 1 was based on the sharp slope break at that site, presence of sandy soil on the upper portion of the slope and setting the slope of the outer edge of the fill pad soils. The setback recommendation for Lot 2 is 10 feet. This setback was based on the observation that the entire slope at this site was underlain by very compact silt and clay. In addition, no evidence of slope instability except for minor topsoil layer soil creep is present on the slope. Similarly to Lot 2, Lots 3 and 4 were entirely underlain by compact silt and clay. In addition, the upper slopes of Lots 3 and 4 are relatively gentle. That is the top portion of the slope is less than 2H'IV. In conclusion, my recommended setback distances from the top of the steep slope are not inconsistent with Shannon & Wilson's recommendations on Lots 3 and 4. For Lot 1, I have recommended a larger setback distance based on the site specific slope and geology conditions and for Lot 2 1 have recommended a smaller setback based on site specific slope and geology conditions. I would further note that the plat map setback line is arbitrary and inconsistent with actual slope conditions and the recommendations of Shannon & Wilson. At the time of my site in October 2002 visit I observed that the flagged survey points for the setback were generally more than 30 feet from the top of the 2H: 1V slope beginning. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group ~ Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group File: 10.15.03E Stratum Group 1451 Grant Street Bellingham, WA 98225 (360) 714-9409 November 3, 2003 Client ID: SugarHill Project #: 10.15.03E Geology evaluation of Sugar Hills setback line. Professional Services $160.00 Invoice Total $160.00 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 November 4, 2002 Ron Simpson Covenant Mortgage Corporation 9725 SE 36t~ SuRe 304 Mercer Island, WA 98040-g~,4~ Re: Geology Evaluation Lot 1, Sugar I-Fill Estates Section 13, Township 29 N, Range 1 W Jefferson County, Washington Dear Mr. Simpson: We are pleased to present the results of our geology evaluation of the above referenced property. The western portion of the lot consists of a steep potentially unstable slope. The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to a potentially unstable slope. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the steep slopes on the property and in the vicinity of the property, and review of available geologic mapping in the area. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the steep slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the steep slope and on the uplands above the slope by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the slope stability, recommendations for site development, and recommendations for further investigation, if necessary. November 1, 2002 Lot 1, Sugar Hill Estates Geology Hazard Evaluation GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the x4cinity of the subject property. The Surficial Geologic Map of the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region,_ Washington (Pessl, Deither, Booth and 5/[mard, 1989) indicates the subject property is underlain by two units: 1) Glacial and nonglacial sedimentary deposits of Pre-Fraser Glaciation age and 2) glacial till. The Geologic Map Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard, and Dembroff, 1993) indicates the subject property is underlain by glacial till and the Pre-Frasser Glaciation sediments consist of undifferentiated Pleistocene deposits and Possession Till. The pre-Fraser glaciation deposits are described as consisting ofinterbedded oxidized brown, red- brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice-contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant peat and woody debris. The two glacial tills consist of a poorly sorted mixture of rock fragments ranging in size from clay, silt, sand and gravel and cobbles deposited directly by glacial ice. Observations on the steep slope and upland area of the subject property are consistent with the mapping described above except that a relatively thin deposit of loose glacial recession sand and gravel is present on the uppermost slopes and upland area of the subject property. The steep slope is for the most part underlain by very compact silt (Pre-Fraser deposits) and glacial till. Glacial till was observed near the top of the slope and near the base of the slope. The till near the base of the slope is likely the Possession Till. The property is located on the east side of Chimacum Valley. The valley is a glacial valley created during the last ice age. The valley was formed by the erosion of pre-Fraser deposits by the Vashon ice sheet and/or water flowing underneath the glacier. SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The subject property consists of an upland area bounded on the west by the steep slope of Chimacum Valley. The elevation of the upland is approximately 380 feet and the base of the steep Stratum Group File: 10.6.02B November 1, 2002 Lot 1, Sugar Hill Estates Geology Hazard Evaluation slope is at an elevation of approximately 150 feet. A level building pad has been constructed on the upland of the property near the top of the steep slope. The west edge of the building pad is located 34 feet from the top of the steep slope. The steep slope slopes downward from the upland area at an angle of approhrnately 3 5 degrees. The slope is well vegetated with a mix of trees and thick brush. Portions of the slope are steeper because a numl:,er of old logging skid roads have been cut into and sv,4tch back and forth up thc slope. Except for minor sloughing of cut slopes and topsoil creep, no evidence of slope failures was observed on the steep slope. The entire steep slope is underlain by very compact silt, sand, and gravel deposits and glacial till. Based on the age of the trees on the slope, we estimate that the slope was clear cut approximately 40 to 50 years ago. The upland area of the property consists of relatively gentle slopes with one south slope created by the cutting of an access road through a small ridge located near the top of the steep slope. Except for the building pad area and the area in the immediately in the vicinity of the proposed building area, the upland portion of the property slopes gently to the east away from the steep slope. No water seeps were observed on the steep slope and no seasonal wet areas are present. We did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property, we conclude that the steep slope on the west side of the property is relatively stable and is only subject to minor topsoil and slope sloughing. A residence can be located on the proposed building pad as long as it is located 5 feet back from the west edge of the building pad. A residence can be built within the area indicated on the Figure 2 Site Sketch Map. It is our opinion that a residence located within the area indicated on Figure 2 will be at minimal risk of being impacted by landslides. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope as long as our recommendations are followed. Soils on the upland portion of the property appear to be relatively well drained, and therefore we Stratum Group File: 10.6.02B November 1, 2002 Lot 1, Sugar Hill Estates Geology Hazard Evaluation- do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water catch basins must not be introduced into the perimeter footing drain. We recommend that any storm water generated be discharged into a rigid perforated dispersion pipe or dispersed on the forest floor to the south of the proposed building site and access road. If a dispersion pipe is used, it should be placed in a level infiltration trench excavated perpendicular to the slope and should be located at least 30 feet from the top of the steep slope. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the steep slope or placed at the top of steep slope. Trees on the steep slope may be limbed for view purposes, but no trees should be removed unless a thinning or harvest plan has been reviewed by a geologist familiar with slope stability and tree removal. Although the steep slope has previously been logged without causing obvious landsliding, the slope stability may respond differently to second cut because of different forest cover. The proposed septic drain field is part of a community drain field to be used by several properties and is located 400 feet northeast of the proposed building area. The septic drain field should not cause problems for slope stability on the subject property or on any other properties as it is located such that it will not effect slope hydrology. Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Professional Engineering Geologist Stratum Group File: 10.6.02B / 122~4o~33" ~? NAD27 12~43~" W ~--';.-: :::;:,-'~ m: ., : :.- NAD27 122~43q~" W