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HomeMy WebLinkAbout001061006 Geotech Assessment (2007)STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360)714-9409 February 26, 2007 Amy Williams Bohlin Cywinski Jackson 1932 First Avenue Suite 916 Seattle, WA 98101 Re: Geology Evaluation, Shoreline Bluff Parcel 001061006. Jefferson County, Washington Dear Ms Williams: Stratum Group is 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. I previously completed a report on the site dated October 18, 2002. This report provides a more detailed evaluation of the steep shoreline bluff geologic units. This evaluation was done to better define the scale and mechanisms of failures that may take at the subject property. 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. February 26, 2007 - Parcel Number 001061006, Middle Point, Jefferson County, WA Geology Evaluation, Shoreline Bluff 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 Geolo i~c Map of the Port Townsend 30 by 60 Quadran lg a Puaet Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) and Geologic Map of Northeastern Jefferson County, Washington (Gayer, 1976) indicate the steep shoreline bluff exposed on the subject property is underlain pre-Fraser glaciation deposits. Gayer (1976) provides several section interpretations of the bluff in the vicinity. Near the subject property Gayer describes the base of the bluff as being. underline by Possession Till overlain by Kitsap Formation, overlain by Vashon advance outwash, overlain by Vashon Till. Observations on the bluff face on the subject property and on the bluff face in the vicinity of the property are generally consistent with the mapping described above. A photo of the bluff face with geology units indicated is provided in Figure 1. The steep bluff face is underlain by very compact silts sand and gravel. The base of the slope is underlain by very compact sandy silt. This unit is overlain by a very compact sand, a glacial till layer, thinly bedded compact sand and gravel and then another glacial till unit at the top of the bluff. The upland portion of the property is underlain by compact sandy gravel with some silt. This unit appears to be a thin layer of recessional glacial sand and gravel or reworked glacial till. Stratum Group File:10.6.0211 2 Fib 26, 2t~7 Parted ~t OtD l Ob 100{, a ~~~, ~~ ~ ~€ testy, ~A , _ A. ^, G Evalaa~tdom, ~t~ores~e ~[ttff _ ~.. Gd '~' - ~ ~;~.* 7;11 ~~ sire ~~f ;--` - . L..O.kq~~GT .~eqE-f ~Y ~ •~y ~ r a te ~~ _ s - .. .. =:~. - Figure 1. Face of bluff on the subject property. Bluff is approximately 140 feet high. sue, ~ F~~: io.~.o2~ 3 February 26, 2007 Parcel Number 001061006, Middle Point, Jefferson County, WA Geology Evaluation, Shoreline Bluff SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 2 and a general site plan sketch is provided on Figure 3. The property is approximately 5.5 acres in size and consists of an upland area bounded on the west by a very steep northwest facing shoreline bluff. The upland portion slopes very gently towards the west, and is covered primarily with second growth Douglas fir, western red cedar, grass and brush. The overall slope of the steep shoreline bluff averages approximately 65 degrees; however, portions of the bluff are vertical or nearly vertical. The total height of the steep portion of the bluff is approximately 140 feet. On vertical portions of the bluff soils on the bluff face are exposed. Alder and cherry trees with an undergrowth of brush and grasses cover the less steep portions of the bluff. No mature trees are present on the bluff slope or on the bluff slopes in the vicinity of the subject property. The bluff is 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. Very little landslide debris is present at the base of the bluff. 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. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the northeast by wave action. The beach at the base of the bluff consists of a sand and cobble beach with areas of sand and a few scattered boulders. It has been my observation that waves reach the toe of the slope along this section of shoreline on a fairly regular basis during periods of higher tides. Wave action on the lower part of the bluff has been causing the bluff to become over steep triggering periodic landslides. Shallow soil landslides are evident all along this section of shoreline. Where the bluff is underlain by more cohesive sediments, the bluff is very nearly vertical. The predominance sand and gravel units with little silt and clay and hence lower overall cohesion has led to the shoreline another sections to be less steep. The bluff at the subject property is located in a geology transition zone with some units of high cohesion and other units with less cohesion. Of note is that the base of the bluff at the subject property is underlain by very cohesive hard silt. The till layer at the top of the bluff and the till layer in the mid section of the bluff are thicker here than elsewhere along the bluff. Slope failures of the bluff have occurred within the past few years on portions of the bluff face on Stratum Group File:10.6.021( 4 February 26, 2007 Parcel Number 001061006, Middle Point, Jefferson County, WA Geology Evaluation, Shoreline Bluff the subject property. Routine slope failures are apparent all along the bluff to the northeast of the property. Northeast of the subject property there is very little bluff vegetation and the bluff is generally very steep. Southwest of the subject property the bluff is more vegetated and, although still very steep, the bluff is not as steep as it is to the northeast. This difference is the result of a several variables between the bluff southwest of the subject property and to northeast. The differences are that to the southwest the bluff is higher, some wave energy protection is provided to the bluff southwest of the property by Protection Island, and most importantly, the bluff to the southwest is underlain by thicker till units that are more resistant to slope failures and provide coarser material (boulders) top the beach. The geologic units underlying the bluff at the subject property consist of very compact glacial till at the top of the. bluff underlain by compact sand and gravel, then an older glacial till, then more compact layers of sand and gravel and at the base of the bluff very compact silt. No seeps or springs were observed anywhere on the bluff slope on the subject property. The till unit on the mid portion of the bluff thins towards the northeast and is not present on the bluff face northeast of the subject property except at McCurdy Point approximately one mile to the northeast. Rotational failures have taken place in the past on the shoreline bluff at least at two locations southwest of the subject property and one location northeast of the subject property. Although these types of failures were not identified on the subject property, I have spent some time evaluating the rotational slides because of the proximity of those slides to the subject property and the height of the bluff. The scale of any potential rotational slides on the subject property is greatly constrained by the layering of various units. The locations along the bluff where past rotational failures have taken place each had fairly thick and uniform geologic units creating conditions for fairly large rotational blocks (on the order of 40 feet). I 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 presence of straight mature Douglas firs on the nearby steep bluff face indicates that the erosion rate on the steep slope must be relatively slow. This is consistent with the very compact nature of the soils on the bluff face, which are resistant to erosion. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I 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 (100 years) as long as my recommendations are followed. Stratum Group File:10.6.0211 5 4 _..: ,. February 26, 2007 Parcel Number 001061006, Middle Point, Jefferson County, WA Geology Evaluation, Shoreline Bluff _ The residence should be located at least 75 feet from the tap of the shoreline bluff as indicated in the Site Sketch Map (Figure 3). This distance will be adequate to provide protection for a residence for at least 150 years if an erosion rate of 6 inches per year is used. It is my opinion that this rate of erosion is very conservative and is likely less rapid. Erosion rates along the eastern end of the Strait of Juan de Fuca are estimated to be on the order of 6 inches per year. Because of the protection afforded by Protection Island and the height of the bluff (the higher bluff height means a greater volume of soil would need to be eroded), and the presence of an erosion resistant silt unit at the base of the bluff and two thick till units on the upper bluff slopes, I estimate that erosion rates along this section of coast to be somewhat less - on the order of 4 inches per year. The top of the bluff will not systematically retreat 4 inches per year, but the bluff will progressively get steeper and mid slope failures will take place and eventually another failure will take place at the top of the bluff. Based on site observations on the bluff, the size of top of bluff failures should be expected to be on the order of 5 feet. The recurrence interval of top of bluff failures that send material to the base of the bluff appears to be on the order of 25 to 30 years based on the age. of tree stands along the steep shoreline slopes. This estimate is complicated by a several factors: 1) the bluff height varies, 2) clearing of trees on the slope and at the top of the slope varies, 3) drainage and slope management on the developed properties and undeveloped properties likely has varied, 4) variability in the underlying geologic units causes some slopes to have more frequent small slides and other slopes to have larger less frequent failures, and 5) the variable steepness and underlying geology play a significant role in the establishment of trees on the slope after a failure takes place (For example, the nearly vertical slopes on the bluff to the northeast preclude the establishment of trees.). Overall though the bluff is very steep and erosion at the toe of the bluff should be expected to continue. As the very steep slope weathers shallow slab type failures are likely to occur. Based on my observations along the top of the bluff at the subject property and all along the top of the bluff northeast and southwest of the bluff, it is my opinion that these slab type failures will be on the order of a 5 feet thick. 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. Stratum Group File:10.6.021I 6 February 26, 2007 Parcel Number 001061006, Middle Point, Jefferson County, WA Geology Evaluation, Shoreline Bluff I 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 the recommendations are followed. Soils on the upland portion of the property appear to be relatively well drained, and therefore I 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. I recommend that roof drains and any other drainage be discharged to a dispersion/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. 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. 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, St m Group 7 an McShane, M.S., L.E.G. Licensed Engineering Geologist Stratum Group .' ® ~. _ - b~' ,,, f r y ~ ~: ~~ 4 / 4 1 ,~ , r.a ~~`c~~ -E,, File:10.6.0211 ~~ 7 ., t l C c C F~~are ~. ~itg L~+~atiQn 4B°0?.4'33' i+l, 122°SI.~i~1' 'U~r V'+~~5~4' 122°52.U~.,O' ',fV 122°51.dU0' 1N 4~f~;S~4~ 122°C*C~.OJO° W ,~ _.,_ r ~. 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