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Home My WebLink About977100421 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 January 3, 2005 Jim Jordan 64 Discovery Crest Place Port Townsend, WA 48268 Re: Geology Hazard Evaluation, Shoreline Bluff Lot 5, Discovery Crest Place Port Townsend, WA Deaz Mr. Jordan: f~ i FEB 1 71005 t j DEPT. Of COi~AfiUh+Ty QEV lE oPW1ENr Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property on a shoreline bluff. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed residence on the subject property and provide general site development and maintenance recommendations. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of .the shoreline at the base of the bluff. ~ The properly is located on an approximately 350-foot high bluff on the east side of Discovery Bay. The location of the property is indicated on Figure 1. The property is located in the southeast quarter of Section 24, Township 30N, Range 2 West 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 the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed. surface soil conditions on the slope and on the uplands above the slope. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. January 3, 2005 Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluation of Shoreline BInB GENERAL GEOLOGY ;` i to =,~ .L,.,i a ~F ii ~? r i f'i :.. ;w r E 8 1 1 2005 ~~ ~-_, ' ~ a' ~t rF~RC ?IV Crlr~~~1 ~ 3 ___-.___.._.~ .......~_____ 3 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' bY60' Quadran e, Puget Sound Reuaon Washin on (Pessl, Dethier, Booth and Minazd, 1989) and the Geologic Map of Northeastern Jefferson CountX, Washington (Gayer, 1976) indicates the bluff is underlain by Vashon advance outwash deposits and glacial and nonglacial sedimentary deposits of pre Fraser Glaciation age. The advance outwash deposits consist of clay to gravel stratified deposits deposited by meltwater from the advancing glaciers at the start of the ice age. The pre Fraser Glaciation deposits are a broad term applied to deposits that were laid down prior to the last ice age. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property aze consistent with the mapping described above. The upper slopes of the property in the vicinity of the existing building are underlain by sand and gravel in a compact condition consistent with the site being underlain by compacted advance outwash sand and gravel. Exposures of stratified sediments on the very steep bluff slopes on the lower portions of the bluff consisted primarily of silts and clay with a few organic rich zones. These deposits are also in a very compact condition. The Slope Stability Map of Northeastern Jefferson County, Washington (Gayer, 1976) indicates that the bluff slopes in the vicinity of the subject property are both active and former landslide areas and known areas of poor natural stability. Observations confirm that there are potential unstable slopes on the subject property and the vicinity. The shoreline at the base of the bluff is erosional and thus the slopes above the shore are azeas where old slides and recent slides have taken place. The slides along the shoreline consist of relatively shallow landslides associated with shoreline erosion at the base of the bluff over steepening the slopes above. The areas mapped as being of poor natural stability is a broad overview due to the potential for the advance outwash sediments to become saturated and the lack of cohesion that is sometimes present within the advance out wash sediments combined with the generally steep slopes in the areas. Conditions that would lead to saturation and azeas of cohesionless outwash sediments do not appeaz to be present on the subject property and immediate vicinity. Stratum Group File: 11.1.04A 2 January 3, 2005 Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluatan of Shoreline Bluff w,.a ~ ' -. i F E B 1 7 2005 s~~'# i ?~ _ ' __ , JFF~'cRSC~ C~ L'NTv _ ~3E:rt')T i~,r;~`~i1;~;_'Yi;E°~`EL+?r'ti/~C~!f SPECIFIC SITE OBSERVATIONS The subject property is located on a high shoreline bluff on the northwest shore of Discovery Bay. The property is accessed via a gravel driveway from Discovery Crest Place. The location of the property is shown on Figure 1. A sketch map of the property with pertinent features indicated is provided in Figure 2 The property slopes down from Discovery Crest Place down to the shore of Discovery Bay. Discovery Crest Place is approximately 400 feet in elevation above Discovery Bay. A steep sided ravine is located on the upland portion of the property. The ravine begins on the upland area just north of the property and curves through the mid-portion of the property before abruptly ending at the top of a steep slope. The steepest side slopes of the ravine are on the order of 30 to 40 degrees. The ravine is well vegetated with mature Douglas fir and cedars and thick underbrush. With exception of water run off from the road immediately in the vicinity of the road, there is no evidence of surface water flow in the bottom of the ravine. The ravine appears to be the result of no longer active stream flow and probably formed as glacial ice was melting in the area. The side slopes of the ravine appear stable with straight trees and only minor surface soil creep. The upland portion of the property south of Discovery Crest Place and west of the ravine is accessed by a gravel driveway. The driveway crosses the upper part of the ravine to the building area where there is an existing dwelling. The upland area is located west of the ravine and north of the steep slope. The upland area slopes gently towazds the top of the steep edge of the shoreline bluff and consists of a planaz slope of 10 degrees or less. The soils underlying the upland area as well as the slopes above are underlain by very compact sand and gravel. A second upland area on the property is located east of the ravine. This upland area slopes gently down to the south towazds the ravine at an angle of approximately 10 degrees and then gradually steepens further down the ravine slope to a maximum slope angle of 30 degrees. This upland azea is tree-covered primarily with Douglas fir with a few madrone trees and a thick understory of salal and other brush. Approximately 320 feet south of Discovery Crest Place there is a sharp slope break. Portions of the slope break are vertical and as high at 15 feet. Below the initially very steep slope the slope continues down at an angle of approximately 40 degrees for approximately 300 feet. This slope is planer and is grass-covered with scattered mature Douglas fir and madrone trees on the upper part of the slope and thicker stands of trees and brush on the lower slope. The slope is underlain by sand and gravel colluvium. Except for minor soil creep there is no evidence of recent slope movement on the slope and all the trees observed are straight. Stratum Group File: 11.1.04A 3 January 3, 2005 Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluation of Shoreline Bluff ~# FEB 1 7 ~~~ 2005 ~ ~ JEFFERSON COUNTY DEPT. OF COMNtUNITY DEVE~DPMENT The steep 40 degree slope ends on a nearly level bench area. The bench is between 50 to 100 feet wide and is located approximately 100 feet above the shoreline. This bench continues to •the northwest and southeast above the shoreline at approximately the same elevation for at least one- halfmile. A final nearly very steep slope that is vertical in places slopes down to the shoreline itself from the bench area. The very steep slope down to the beach is approximately 100 feet high and is underlain by alternating layers of silt, clay and fine sand. The sediments are very compact and stand vertical. The very steep slope is the result of shoreline erosion at base of the bluff. Slope failures on the shoreline bluff of the subject property and adjoining shoreline properties consists of undermined slabs of bluff breaking off on a periodic basis. Most of the steep shoreline bluff just above the beach for a half mile to the southeast and at least a half mile to the northwest of the subject property consists of the same fairly uniform very steep silt and clay bluff. One exception is the shoreline slope immediately to the southeast of the subject property. The shoreline bluff for approximately 200 feet consists of a very old landslide deposit of poorly sorted sand and gravel and clay. The shoreline bluff at this section of shoreline is somewhat less steep. The landslide appears to consist of a combination of sand and gravel from the upper slope mixed with silt and clay layers from the lower portion of the slope. The sides of the landslide are overlying the silt and clay unit, that otherwise makes up the lower part of the bluff. This slide southeast of the subject property is a very old feature as it is covered with a stand of mature trees. The slide appears to have been the result of a water seep located at the top of the silt clay units beneath the sand and gravel units above. The slope above the slide area is the one area where the bench is narrower. The slopes above the slide appear to be stable. I have walked most of this section of shoreline and it is the only area where a spring of water is present. No springs or seeps or vegetation indicating seasonal springs or seeps were observed anywhere on the steep shoreline slope on the property. The beach conditions along the section of shoreline are in a fairly natural condition. No shoreline structures that interfere with beach sediment transport or erosion rates are present along this shore area. All the slides appear to be primarily tbe result of erosion at the base of the bluff from wave action. As the bluff becomes oversteepened the lower slopes fail first and overtime the slope failure progresses up the slope to the top of the bluff. All the landslides observed appear to be shallow slab type failures involving only a few feet thick sections of soil. The toe of the bluff is underlain by very compact silts and clays that are fairly resistant to erosion. Wave erosion at the toe of the slope is limited to periods of very high tide combined with storm events. Wave size is Stratum Group File: 11. I .lk1A 4 ~ T t ~ ~' ' .... r...... , .. ~ ~.-..... ~'; ?i~ ~1 January 3, coos ~ ~ _; ~ F E 8 i 7 2445 ~ t ~%,;~ Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluation of Slwreline Bluff " , ~:• ~ _-~-• - -_ ._ _ F ~3~ t,,*•,,' c: ~. fairly limited on this section of coast as the site is in a fairly narrow area of grater wttl3in -•-- -.~ Discovery. Hence, the shore in not open to any long fetch that would generate large waves. Less steep slopes along the shoreline where landslide debris has accumulated supports tress indicating that the wave action takes a fair bit of time to erode away bluff slope failures. Most of the bluff is simply too steep and the soils to compact to support tree growth. CONCLUSIONS AND RECOMMENDATIONS Building Site Recommendations Based on my visual inspection of the subject property and vicinity, I conclude that a residence can be located on the gentle sloping upland areas as indicated in the site plan sketch. Two potential building areas are present on the property. One is located on the upland west of the dry ravine and the other is located on the upland east of the ravine. The acceptable building areas are indicated on the attached Figure 2 Site Sketch Plan. West Building Site For the west building site, the building should be setback from the top of the steep southwest slope a minimum of 50 feet. A deck may be constructed within the setback from the top of the steep slope as long as it is not an integral part of the structure of the residence. That is if a slope failure occurs, the deck will detach from the house. A 5-foot building setback should be maintained from the top edge of the ravine on the east side of the property. This setback is to avoid differential settlement along the top of the slopes of the ravine if standard foundation footings are used. Other foundation methods may be used if the house extends onto or above the ravine slopes, but the foundation design should be reviewed by a geotechnical engineer. Alternatively, it would be acceptable to excavate downward to create a level building area that would daylight toward the ravine, but no fill soils should be placed on the side slopes of the ravine. East Building Site For the east building site, the building should be setback from any slope greater than 25 degrees at least 5 feet, and should be setback from the top of the steep southwest slope a minimum of 50 feet. The house may be extended onto or above the steeper ravine slopes, but if the house extends beyond the setback from the 25-degree slopes, the foundation design should be reviewed by a geotechnical engineer. Stratwn Group File: 1 I .l .04A 5 ,. y . January 3, 2005 Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluation of Shoreline Biuff General Site Development Recommendations ~~~ jc FE8 1 7 205 ~~~;~~ ~ ..._. JEFFERSON r~fJ~?T'Y `~ DEQT. OF CO+~i~VtiJi~ilr'r t'~~iE~t?F!4r,r.;r'~ No debris should be placed on the top of the bluff. Thinning and trimming of trees on the bluff slope and within the setback area should be limited to topping and limbing for view corridors. Thinning should leave at least one tree on a 30-foot center. Any debris from thinning on the steep slope itself should be removed from the slope. Soils on the upland portion of the property appeared to well drained. Storm water run off from the roof gutters should not be routed towards the bluff and should be infiltrated into the ground in a dispersed manner. I recommend that any storm water drainage be directed into a level spreader towards the bottom the ravine. The septic drain field should be located at least 30 feet back from the top edge of the bluff. The addition of water at this location does not pose a risk of causing slope stability problems. There is an existing route down to the shoreline from the upland area on the property. The trail follows the ridgeline between the ravine and the steep upper slope down to approximately where the ravine ends. The trail switchbacks down the steep slope to the bench area below and then follows the bench off the property to the old landslide where a trail switchbacks down to the shoreline. There is no other practical route to the shore. No other trails or grading should take place on the very steep shoreline bluff above the beach unless the route is planned and reviewed by a geologist. Conclusions Regarding Slope Features It is my opinion based on my site inspection and geology interpretation that the slope features on the subject property and along the shoreline bluff of this area are very old features. There are numerous ravines much like the one on the subject property located on the slopes southeast of the subject property. I have over the past few years inspected four of these ravines and all of them are dry ravines with no evidence of water flow. All the ravines inspected as well as others abruptly end along the top of the steep slope break that runs for some distance to the southeast of the subject property. I have interpreted these ravines as being a remnant land feature created at the end of the last glacial period. The abrupt slope break is not unusual for high bluff slopes in the Puget Sound region. However, the presence of the rather continuous bench area part way down the slope is fairly unique. Benches can form on slope in a variety of ways such as a string of water seeps undermining the slope above or the result of adeep-seated landslide where the upper part of the bluff slid down as a uniform coherent block. The field evidence described above is contrary to either of those interpretations. Stratum Group File: 11.1.04A 6 `, January 3,2005 Lot 5, Discovery Crest Place, Jefferson County, WA Geologic Evaluation of Shoreline Blnff ..~ ~ ~ ~ r---~, ~~~ ~,~ F E B 1 7 2(?05 ~ ~.:~ .J trt,. f(~~ J~E+E/~"rERSGtd COUNT`! I~`r T• J~ LV;lRlif tl lVi~Y 1~~4~Lj..Gt"iYtEliF I have two theories as to the how the steep slope and bench may have been created. One is that the steep bluff may have been created by glacial ice neaz the end of the last glacial period. The steep slope formed by a remnant glacial lobe within what is now Discovery Bay. Streams flowing from melt water from remnant glacial ice on the uplands above carved the ravines down to the top of the ice and deposited the outwash from the streams onto the glacier. Another possible explanation is that relative sea-level at the end of the last glacial period was briefly higher than it currently is today because the land in the area had been depressed by the Puget and Juan de Fuca ice sheets. In northern Puget Sound relative sea level was as much as 650 feet higher than current sea levels at the end of the last glacial period. The steep slope already existed from glacial erosion. The ravines were created by glacial melt water from the retreating ice sheet and the ravines flowed into an eazly Discovery Bay that was higher on the bluff slope than it is today. As the land rebounded the source of glacial meltwater from the retreating ice ended because the glacial sheet retreated from the area. The bench area represents an erosional terrace created by the waves more readily eroding the sands and gravels than the underlying very hard and compact silt and clay layers. Regardless of the correct interpretation, there is no evidence that the slope instability on the subject property will pose a risk to structures on the upland portion of the property as long as the above site development recommendations are followed. CLOSURE Please note that there aze inherent risks associated with lots neaz or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appeaz 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 asrequired. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 7149409. Sincerely yours, Stratum Group an McShane, M.Sc., L.E.G. Licensed Engineering Geologist Strum Group File: 11.1.04A 7 ,. 1 N• ( ~i t~ , r7~ ~x C t~ i~ u ~ `max ~ ~ ~ « ''~` b~'i - V~r i£, . ~° ~~~~ S F E B 1 7 2G05 • ~r ~,~ • ~ 4 ' JEFfERSGN CG'UNTY j DEPT. OF COtJiiviUltiiV~'Y DEVELt?FNtEN7 _; /~' ~ Vyi/tS~' ,rgre~ 5 +bA~4 r ~ y,,.. --'I d ~~„" ~~` r`'~~ A~ o• f ~` .~'/ ~Y s~ . y .~ ~ ., ~. ~J ;~ ~.t as ~ ~~ ,, ~~ ~~ ;~ r~~ ~Cp S~ ~7. t~ ~~ K av Easy ' Bu;,, ~~S r9i,~a ' 5,~~~ip SJO --_ ~'e ~Ow ~C., rw ~ \ C r~ s 4. ~~ ~ ? t4 ~ S~ y ~l e~p ~ /6~ ~ l~ - s c oue~•' y spy ~•..dJew''ielr Sep -sI ~pG a~ r....~ ~, ~..,.. Sr+Orl~cw*°S 4Mald,~s ~~-s Strata Group Figure 2 Site Sketch Plan