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HomeMy WebLinkAboutBLD2004-00430 Geotechnical Report • STRATUM GROUP 1451 Grant Street,Bellingham,WA 98225 i • Phone(360)714-9409 '�� March 5, 2004 -P1. Of 1 t 5 + uLfrELOPI r' DBHI LLC 2331 Fairhaven Avenue East Seattle,WA 98102 v Re: Geology Evaluation McCurdy Point Property Jefferson County, Washington We are pleased to present the results of our geology evaluation of the above referenced property on a shoreline bluff. 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 bluff. This evaluation was limited to a visual inspection of the property and vicinity,a visual inspection of the bluff face, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the bluff face slope conditions,shoreline conditions,and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the bluff face and on the uplands above the bluff 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 shoreline bluff stability,recommendations for site development, and recommendations for further investigation,if necessary. SITE 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. ;Amu aLIJ • 0 4 - 043q. • March 5,2004 JEFFERSON COUNTY ICO McCurdy Point Property,Jefferson County,WA Geology Evaluation,Shoreline Bluff -Pi ul ` The Surficial Geologic Map of the Port Townsend 30 by 60 Quadrangle,Puget Sound Region, Washington(Pessl, Dethier,Booth and Minard, 1989)and the Geologic Map of Northeastern Jefferson County, Washington(Gayer, 1976)indicate the subject property is underlain by Advance Outwash Deposits deposits. These deposits consist of sand, gravel, silt and clay deposited by meltwater flowing from the advancing ice margin of the Puget lobe of the Vashon age glaciation. Observations on the uplands and bluff face on the subject property and on the bluff face in the vicinity of the property are partially consistent with the mapping described above. The uppermost part of the bluff all along the shoreline of the subject property and at least portions of the upland portions of the property are underlain by glacial till consisting of silty sand and gravel in a very compact condition. The till is on the order of a few feet to 15 feet thick. The thickest portion of the till is near the northern tip of the property at McCurdy Point itself. The till thins to the southeast and southwest of the point. Portions of the upland are not underlain by till. These areas are either underlain by a thin covering of recessional gravel or gravel deposited by wasting ice as the glacier retreated from the area or by advance outwash deposits where the till layer is not present. Several boulders are scattered on the ground surface of the upland consistent with an ice wasting deposit. - Advance outwash sand, silt and gravel is present beneath the till all along the bluff face. The advance outwash varies in composition along the bluff face.Near the point there is very little sand.Towards the southeast the sand unit immediately beneath the till increases in thickness. This sand unit contains very little silt or clay and therefore has very little cohesion. Towards the southwest there is very little cohesionless sand.Below the sand the bluff is underlain by silt, clay, sand and gravel units with silt and clay being the predominant units. These units are either advance outwash deposits or preglacial deposits.In either case the units have been very compacted by the overriding glacial ice and along the bluff southwest of the point the sedimentary layers are very deformed. This deformation is consistent with deformation caused by overriding glacial ice. A glacial drift layer consisting of clayey silt with pebbles,cobbles and boulders embedded into the fine grained matrix is present along the base of the bluff at the point.This is the only area along the shore that this unit is present. A result of this unit is that the beach at Point McCurdy consists of cobbles and boulders whereas elsewhere along this stretch of shoreline the beach consists primarily of sand with a few gravel sections. This also is the likely reason for the existence of the point, as the very compact drift unit at the base of the point and the cobbles and boulders within the drift are much more resistant to erosion that the bluff materials southeast and southeast of the point. Stratum Group File:2.25.04 2 - • PERMIT # OLD ry l 4 04 ' 0430 March 5,2004 l': McCurdy Point Property,Jefferson Coun y,WA'S 0 rV COUNTY D C D Geology Evaluation,Shoreline Bluff SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The property is approximately 33 acres in size and consists of an upland area bounded on the northwest and northeast by a very steep northwest and northeast facing shoreline bluff. Except for the shoreline bluffs on the outer edges of the property,the upland portion slopes gently towards the northwest. The shoreline bluffs on theproperty range in height from approximately 60 feet-on the northwest bluff to 70 feet at the point on the northern end of the property to approximately 100 feet on the northeast bluff. The northwest facing bluff is very steep with much of the bluff being nearly vertical.The northeast facing bluff is vertical on the upper 10 to 15 feet and the slopes at approximately 40 degrees for 10 to 40 feet in height and then steepens down the shore at an angle typically on the order of 65 degrees or steeper. The upland portion of the property is covered primarily with second growth Douglas fir with an understory of salal and evergreen huckleberry and other brush. The northwest facing bluff is mostly devoid of vegetation with the exception of a few areas of grass and wild rose.The northeast facing bluff has stands of trees on portions of the slope consisting of Douglas fir,red alder and western red cedar as well as areas of brush and no vegetation.None of the trees on the northeast-facing slope are mature. Slope failures of the bluff have occurred within the past few years on portions of the northwest facing bluff on the subject property. Routine slope failures are apparent all along the northwest facing bluff.This entire section of the bluff has very little bluff vegetation and the bluff is generally very steep. The southeast facing bluff is more vegetated and, although still very steep, the bluff is not as steep as the northwest facing bluff. Vegetation is better established on this bluff slope,but there is ample of evidence of relatively recent slides on portions of the slope on the subject property and on slopes to the southeast of the subject property. Very little landslide debris is present at the base of the either the northwest facing bluff or the northeast facing bluff. The bluff slopes on the subject property are eroded primarily by wave action that undermines the base of the bluff. The shoreline at the site is exposed to open waters of the Straight of Juan de Fuca. Large waves are common at the site and during storms the shore and the base of the bluff is exposed to very large waves. The northwest facing bluff receives more frequent large wave action than the northeast facing bluff. However,even the northeast facing bluff will receive large waves because of the open nature of the shore in this area. Because of the very compact nature of the bluff soils,the slope failures appear to consist of relatively narrow slab type failures. Ravelling of material off the exposed soils on the unvegetated portions of the bluff,and periodic shallow topsoil failures also play a minor role. Stratum Group File:2.25.04 3 - PERMIT # 0 4 - 0430 � . McCurdy Point Property,Jefferson County,WA JEFFERSON COUNTY D C D Geology Evaluation,Shoreline Bluff The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the east by wave action. 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. The lack of large mature trees along the bluff on the subject property and adjoining property bluff slopes is indicative of relatively rapid erosion rates that do not allow enough time to for trees to be become established. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity,we conclude that a residence can be located on the upland portion of the property such that bluff erosion or landsliding would not threaten the structure within the expected life of the structure(75 years)as long as our recommendations are followed. The residence should be located at least 150 feet from the top of the northwest shoreline bluff and at least 75 feet from the top of the northeast shoreline bluff as indicated in the Site Sketch Map(Figure 2).This distance will be adequate to provide protection for a residence for at least 75 years if an erosion rate of 6 inches per year is used for the northwest facing bluff and 3 inches per year for the northeast facing bluff. It is our opinion that these rates of erosion are very conservative and the erosion rate is likely less rapid. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes or the shoreline bluff as long as our recommendations are followed. Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However,the eroding bluff acts as a feeder bluff(erosion of the bluff provides sediment)for the beaches northeast of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction.There are currently no shoreline-armored areas along this stretch of coastline. Given the size of the waves that the shoreline in this area is exposed to shoreline armoring will also have the effect of significantly lowering the beach due to the subsequent downward scouring armoring would trigger. Stratum Group File:2.25.04 4 PERMIT BID 04 - 04330 March 5,2004 JEFFERSON COUNTY nCD r { ;;F McCurdy Point Property,Jefferson County,WA Geology Evaluation,Shoreline Bluff Soils on the upland portion of the property appear to be relatively well drained, and therefore we 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 roof drains and any other drainage be discharged to a water infiltration system that will distribute water into the ground at the site. Site grading soils or debris,landscape debris,or any other material should not be disposed of over the bluff face or placed at the top of steep bluff. These materials will form a thick mat that will likely slide more readily and damage stabilizing vegetation on the slope. Trees and brush within the building setback buffer and on the bluff face may be removed without posing a risk of increasing bluff erosion or instability. Bluff erosion and stability is overwhelming dominated by shoreline process.If not for the erosion at the base of the bluff, the bluff would be relatively stable. In addition, the high-energy wave action at the shore is such that no landslide material remains at the beach level so the role of vegetation providing natural shoreline armoring is not an issue at this site. However,the top of the bluff is within the Jefferson County Shoreline Management Program and any clearing or vegetation alteration will need to be consistent with the Shoreline Management Program. The septic drain field should not cause problems for slope stability and may be located within 50 feet of the top of the bluff. 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 {1111ag `•0 h�4 \ cO Dan McShane, .Sc.,L.E.G. Licensed Engineering Geologist 41.:' ' ` `R' 00 1376 % k. .‘° Daniel McShane Stratum Group File:2.25.04 5 iC•rS' Y�• r" 4,fit I: �l N . - le) _1 to l$1..' -4N: Q D 0 ; ; , a) - ' - y _ / ry / V I ,4 / p 4 , f • ..&'-.V / s -� e c v n" ''.1I •. -► o ( , , y am1Ill I s 2 , --c i € ' '111. Ij -. - • �- a �.o Y I •u i VInv 1f ' r li 11 11j.Is S/ \ �� f r l�j�j� _ 4 X �.. P .., V ......_ 1 �0 •J y 0 ../..N, N oO 0 �� `+ -° N \ '\ N �� r% P"i >d i 6' "-` I.)w - yl n