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HomeMy WebLinkAboutBLD2009-00367 GEO TECH • STRATUM GROUP P.O.Box 2546,Bellingham, WA 98227 Phone(360)714-9409 September 14, 2009 Loren Kreutner 134 Bishop Road Port Angeles, WA 98363 Re: Geology Hazard Assessment 244 Lee Way, Brinnon, WA Township 26N, Range 2W, Section 23 Jefferson County, Washington Dear Mr. Kreutner: Stratum Group is pleased to present the results of our geology hazard assessment of the above referenced property. The purpose of this geology evaluation was to evaluate geology hazards impacting a proposed building site and provide general site maintenance recommendations for the development of the property. No landslide hazards are present on the subject property. A potential erosion hazard is present on the site due to soil conditions and periodic perched groundwater. Drainage installed at the site has appropriate addressed this issue. This evaluation included a visual inspection of the property and vicinity, hand dug test pits on the building site and in the vicinity and review of available geologic mapping in the area. The property is located on the east side of the Olympic Peninsula approximately 1000 feet west of Dabob Bay. The location of the property is indicated on Figure 1. The property is located in the northeast '/4 of Section 23, Township 26N, Range 2W W.M. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface and subsurface soil conditions on the slopes and on the uplands above the slopes. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the stability in regards to the proposed garage and residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. GENERAL GEOLOGY • • September 14,2009 244 Lee Way,Brinnon,WA Geology Hazard Assessment 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 deeply eroded. Glacial related deposits were deposited over the bedrock in various places on and in the vicinity of the subject property. The Bedrock Geologic Map of the Seattle 30' by 60' Quadrangle, Washington(Yount and Gower, 1991) indicates the subject property and vicinity is underlain by basalt of the lower Eocene Crescent Formation. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Menard and Dembroff, 1993) indicates that the subject property and vicinity is underlain by Tertiary bedrock and glacial till. Observations on the subject property and vicinity are consistent with the mapping described above. Bedrock exposed on the site and at shallow depths on the site consists of massive brown to black basalt that has been very slightly metamorphosed. Glacial sediment, where present, consists of very compact glacial till. The till is silty to clayey with angular rock fragments consisting primarily of the underlying basalt; however, granitic rocks are present as well consistent with the till being derived from the continental ice sheet. The proposed building site was created by partially cutting into thr' slope such that the cut slope consists of bedrock overlain by glacial till with the uppermost till being weathered to a depth of approximately 2 feet. The uppermost weathered till horizon is very permeable and readily transmits perched ground water above the till and bedrock. SPECIFIC SITE OBSERVATIONS The proposed building site is at an elevation of approximately 280 feet. The site is located on an eastward-facing slope above Dabob Bay. The overall slope between the building site and the base of the slope at Dabob Bay is approximately 20 degrees. The slope in the immediate vicinity of the building site is approximately 20 degrees as well. The slope gradually steepens to rocky outcrops approximately 1,200 feet west of the subject property as the slope becomes part of a high steep mountainside with a the summit of a 2,208. The slope in the vicinity of the proposed building site is underlain by bedrock and glacial till. The proposed building site is an approximately 165 feet by 55 feet level area prepared by cutting into the hill slope and placing the cut slope material on the slope below the cut. Hence, the uphill side of the level building area is a cut slope of with a maximum height of approximately 10 feet with most of the cut slope being significantly less steep. Except for the uppermost two feet of the cut slope which consists of weathered glacial till and top soil, the entire cut slope consists of bedrock and very compact glacial till. The slope above the cut slope slopes upward from the proposed building site at an angle of approximately 18 to 20 degrees. Vegetation on this slope consists of a Stratum Group File 8.25.09 2 • • September 14,2009 244 Lee Way,Brinnon,WA Geology Hazard Assessment mix of western red cedar, Douglas fir, red alder, big leaf maple, madrone trees and brush consisting primarily of salal, evergreen huckleberry, bracken ferns and salmon berry. The fill slope below the level building site area slopes downward at an angle of approximately 38 degrees. This slope consists of brush covered areas over rocky soils and boulders. The slope below the fill soil slope is approximately 20 degrees except where crossed by the building site access road. The access road to the building site is a paved driveway which switch backs up the slope one time from Lee Way located at the east end of the property. Due to the well drained top soil and weathered till rainwater readily infiltrates into the ground on the slopes above the subject property. The impervious glacial till and bedrock at a depth of two feet blocks further infiltration creating the potential for perched water on top of the till and bedrock. Cuts slopes in these settings have the potential to be locations of seasonal ground water seeps particularly during very wet weather periods. This type of drainage problems has been observed on cut slopes during very wet weather periods in the vicinity of the property. A shallow ditch to convey water around the building pad area and to spread the potential water has been excavated at the base of the cut slope. Evidence of periodic flow is present in the ditch. The outlets of the ditch show no evidence of surface water flow and water appears to readily infiltrate back into the ground at these locations. The proposed septic drain field is located to the east of the proposed building site. The drain field area is on a slope of approximately 15 degrees. The slopes below the drain field range from 15 to 20 degrees. No unstable areas are present on the site or in the vicinity. CONCLUSIONS AND RECOMMENDATIONS It is my opinion that the proposed building site is not located in a landslide hazard area and none of the slopes on the subject property are at risk of landslides. There is a risk of erosion from concentrated groundwater flow from intercepted groundwater on the property if intercepted groundwater is misdirected. The drainage constructed at the base of the cut slope at the building pad addresses this issue and the drainage poses no increased risk of erosion on the subject property or off of the subject property. The cut slope is stable because of the very compact nature of the glacial till and bedrock within the cut slope and the maximum height of 10 feet. However, we recommend that the residence be set back from the base of the cut slope a distance at least one-half the height of the cut slope. This Stratum Group File:8.25.09 3 • • September 14,2009 244 Lee Way,Brinnon,WA Geology Hazard Assessment distance would mean that the residence should be located no closer than 5 feet from the base of the highest portion of the cut slope, which is approximately 10 feet high. The building pad area is level and compact and there is no indication of settlement on the outer edge of the building pad or on the slope of the building pad. Because of the risk of differential settlement on the outer edge of the building pad area due to foundation loads, I recommend that the home foundation be founded on native or well compacted soils and the base of the foundation should be located at least 7 feet horizontally from the slope. It is acceptable to place the deck closer to the edge of the building pad as long as the deck is not structurally part of the main home. The footprint of the proposed home was staked out on the ground at the time of my site visit in August 2009. The home placement is consistent with my recommendations. The potential for perched ground water seeping and flowing across building pad area poses an erosion risk to the building pad area. This issue has been addressed by the ditch along the base of the cut slope that would direct any perched water seeps. The ditch directs water to the forest floor to the north and south of the building pad area. Stormwater runoff from the roof should directed to the ditch at the base of the cut slope or directed to the ditch outlets and onto the forest floor north and south of the building pad area. I do not anticipate that site development will cause any significant changes to the stability of the slopes in the area as long as our recommendations are followed. Stratum Group appreciates the opportunity to be of service to you. 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