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HomeMy WebLinkAbout901263014 Geotech Assessment~. PO Box July 30, 2007 Peter Rust 3117 Sheridan Street Port Townsend, Washington 98368 Re: Geology Hazard Evaluation Tax Parce1901263014 Center Road Chimacum, WA Dear Mr. Rust: Stratum Group herein presents the resu referenced property. The purpose of thi of slope failures that could potentially i subject property. ATUM GROUP Bellingham, Washington 98227 one (360) 714-9409 is of our geology hazard evaluation of the above. geology evaluation was to qualitatively evaluate the risk npact the access driveway and building pad areas on the The property is comprised of one tax p rcel that totals almost 6 acres. The site is located approximately two miles south of Chi acttm in Jefferson County, Washington. The location of the property is indicated on Figure 1. he property is located on a west facing slope and is currently forested .and undeveloped. ~I A shallow landslide occurred along th existing road approximately 10 years drive. Our assessment of the geology be rebuilt across the slide deposit witl failures at the former slide site is min: been. No slope stability issues were r Additional recommendations are desc The scope of our services included the western boundary of the subject property. just above the ;o. The slide covered a portion of the existing access nd geologic process indicate that the access driveway can ut increasing the risk of slope failures and that the risk of gal as long as the slope is not cut into as it previously had ted in the areas proposed for a residence and shop. bed in the conclusions section of this report. PE OF SERVICES 1) Conducted a site visit tc~ visually inspect the subject property including the slope conditions and relevantconditions in the vicinity of the property. 2) Observed surface soil .slopes on and in the v itions on the slopes and on the uplands above the y of the property. July 30, 2007 Center Road Properly, Tax Parcel 90123014, Geology Hazard Assessment 3) Reviewed geologic subject property 4) Prepared this report sun feasibility of building a slope stability, recomm~ further investigation, if G County, WA slope stability maps, and imagery available for the izing our findings, including an evaluation of the fence on the subject property, an evaluation of the :ions for site development, and recommendations for GEOLOGY Northwestern Washington has been ocG pied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 1 ~ ,000 years ago). During these glacial and accompanying interglacial periods, the underlying bed ock was eroded and a relatively thick layer of glacial related and interglacial fluvial sedimen s were deposited over the underlying bedrock in the vicinity of the subject property. The Geologic Map of (Yount, Minard, and Dembroff, 1993) i undifferentiated Pleistocene deposits at as a thin strip along the western bound of poorly exposed gravel, sand, silt, anc on the remaining portion of the subject non-stratified clay, silt, sand, gravel, ar. by the glacier and subsequently compa~ portions of the area and also drapes dis Northeastern Jefferson County, Washir subject property and vicinity are underl Observations on the subject property iY brown silt. The upper portions of the s: glacial till that is comprised of compac weathered siltstone/sandstone on a ver If this exposure is bedrock, it is consis limited basis to the west of the subject The site is not mapped as an erosion Jefferson CountYMap (Feb 1995). " Areas -Seismic Hazard, Eastern Jef in the Seattle 30' by 60' Quadrangle, W asnm;;ton dicates the subject property is underlain by 1 Vashon till. The undifferentiated deposits are mapped y of the subject property and are described as consisting clay of uncertain provenance. The Vashon till is mapped -roperty and is described as light to dark gray, unsorted, l boulder sized material.. The till was deposited directly ed by the overriding glacier. The till mantles the upland ~ntinuously down valley walls. The Geologic Map of rt, on (Gayer, 1976) indicates the upper portions of the in by Vashon lodgement till. licate that the western portion of the site is underlain by match the mapped description and are underlain by silt, sand, and gravel. There is potentially an outcrop of limited exposure on the lower central portion of the site. nt with the Twin Rivers formation which is exposed on a and area on the Critical Areas -Erosion Hazard, Eastern site is listed as a seismic hazard area on the Critical on County Map (Feb 1995). Stratum Group File: 6.12.07 July 30, 2007 Center Road Property, Tax Parcel 901263014, Geology Hazard Assessment SITE County, WA The subject property is comprised of on tax parcel that is approximately 6 acres. The site is located in a rural residential area of Jef rson County. Center Road bounds the subject property to-the west. Farm fields are located in t e valley west of Center Road. Forested properties bound the subject property to the north, east, d south. The site is located in northeast quarter North, and Range 1 West of the Willan a ridge that is located between West V~ south of Chimacum, Washington. The elevation from approximately 380 feet The location of the site is indicated on provided in Figure 2 and provides perti the southwest quarter of Section 26, Township 29_ tte Meridian. The property is located on the west side of ey and Chimacum Valley, approximately two miles to is located on a west facing slope and ranges in the east to approximately 165 feet in the west. Figure 1 -Site Vicinity Map. A site sketch is ~t information concerning the site. SITE OBSERVATIONS A dirt road access road enters the west ', ide of the property from Center Road. The road traverses up the slope to the southeast onto the a joining property to the south and then switches back to the northeast and re-enters the upper p '',rtion of the subject property. It is my understanding that. a small l~ distance up the road shortly after the The landslide appears to have been the part of the road. The cut slope itself do above failed as a small slab and slid ov slide area slopes up at an angle of appr~ convergent topography. The material ~ brown silt. This brown silt material w~ north and south. Some weathered, bro~ hillside. The exposure was limited and siltstones mapped in the vicinity as Tw nettles, blackberries, and small alder tr my site visit in July 2007. The slopes to the south and north of approximately 18 degrees. de covered the lowermost portion of the road a short was widened approximately 10 years ago. esult of the cutting of the slope for widening the lower s not appear to have failed; however, the steep slope r the cut slope onto the road surface. The slope above the ximately 37 degrees and is located in an area of slightly thin the landslide debris is comprised of fairly uniform also observed in the vicinity of the slide area to the n siltstone-sandstone bedrock was observed in the nay be simply extremely hard silt, but is consistent with n Rivers Formation. The slide area is covered with es. The soil at the base of the slide was wet at the time of slide area are significantly less steep with slope angles of File: 6.12.07 Stratum Group July 30, 2007 Center Road Property, Tax. Parce1901263014, J Geology Hazard Assessment .,. _~_ ~ County, WA No other areas of slope movement are p went on the slope along the dirt access road. Cut slopes further up the road are typically a few f 'et to a maximum of five feet high and appear to be stable with the exception of minor ravelling o surface material. The remainder of the subject property i ', almost entirely forested with alder, big leaf maple, Douglas fir and some cedar trees. An ~ ea has been cleared of trees in the east-central portion of the property at the end of the dirt road a d is primarily covered with grasses. Old logging skid roads extend through the property and t e proposed building area may have been a small staging area for previous logging events in the ~ icinity as areas of rotting wood are still present in areas. This area is a potential building site for a home and shop. No indications of landslides or unstable slopes were observed anywhere. in the vicinity of the proposed building area and slopes above and below the proposed building', area are relatively gentle with no indication of slope movement. The portions of the prope ~ proposed for building is a planar slope that slopes to the west at approximately 10-15 degrees. I, OF GEOLOGIC RISK The small landslide that impacted the l in an area where there is a slight conve slopes. In addition the area is underlai>! The slight convergent topography crea ground water flow. The landslide was with the cutting into the steep slope fo lower end of the dirt road was subsequ constructed. ewer end of the dirt road on the subject property occurred gent topography with steeper slopes than the surrounding by weathered bedrock which precludes water infiltration. ;s a preferential pathway for surface and/or shallow ikely triggered by an extremely wet winter that coincided the access driveway approximately 10 years ago. The ntly covered by the slide material shortly after it was It is my opinion constructed the dirt ro d across the landslide deposit will not increase the risk of future failures as long as the slope abo e the road at this location is left undisturbed and no further cutting into the slope takes plac .Reestablishing the access driveway in approximately the same location with allow the acces driveway to reconnected to the remainder of the driveway. It is my opinion that managing the wat' r in the vicinity of the slide area will greatly reduce the potential for the failed soil to reactivat in this area. The drainage management can be achieved by building the north end of the access driveway on a well drained gravel base, the addition of culverts, and diverting the runoff from'the upper portions of the access driveway away from the slide area. File: 6.12.07 Stratum Group July 30, 2007 Center Road Property, Tax Parcel 901263014, J Geology Hazard Assessment Except for the small failure just above indications of slope. movement are pre CONCLUSI A home or other structure built in the as long as the recommendations prese • Roof run off and any other on-site soils. The run off s dispersed. old road cut on the lower part of the dirt road, no other anywhere else on the site AND RECOMMENDATIONS r portion of the site will not be at risk of slope failures below are followed: water run off should be allowed to infiltrate into the not be concentrated into one area, but fully • Trees can be removed to cre to building pads for the construction of the home and shop; however, evergreen tr' es should be retained wherever possible in the vicinity of the building area and on the slopes below the building areas. Evergreen trees can be limbed and alder trees can b removed for creating a view corridor. • No debris (construction math rials, tree limbs, plant clippings, etc) should be placed on slopes greater than 20 degre' s. Dri • The access driveway shouldllbe excavated approximately one foot below grade and filled with drainage gravel (~-4 inch) • Runoff from the driveway a ove the landslide area should be tightlined through the landslide area or should be irected into a relief culvert prior to reaching the old slide area. This water should be d~''scharged into adispersion/infiltration trench on the slope below • A drainage ditch should be slide to intercept water befi the road bed. The ditch sho • No tree removal should tak :avated along the east side of the road as it crosses the it reaches the road bed to minimize water getting into be excavated to a depth of 1.5 feet. place on the slope above the former failed slope. Stratum Group File: 6.12.07 July 30, 2007 Center Road Property, Tax Parcel 901263014, Je Geology Hazard Assessment CLOSURE Please note that there are inherent risks',, ssociated with lots near or adjacent to steep slopes. These are risks that the building owner ' hould recognize and be willing to accept. If conditions appear different than those described inthis report, or other concerns arise, we request that we be notified so we can review those areas d modify our recommendations as required. We appreciate the opportunity to be of ervice to you. Should you have any questions regarding please contact our office at (360) 714-9 09. Sincerely yours, Stratum Group ~ ~ ~~ Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group i1 File: 6.12.07 ~, t $ ' .~ ¢ ~ ~~ ~ ~~ ' f44~.' r' n • ~ ~ "g.,~ ~ x a ~ ~ ~°~~.' •i .. . q is =~ x,y' 1. Sate kip' ~_ y`1 ~- ~ m l O iJ _~tiw a Z _0 a O n Z 0 '. ~ ~.. ~ h ia-- ~yi ~T_r G n... ~" ~K ~ r C! 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