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Home My WebLink About961804001 Geotech Assessment (2007) J ." # .:` ~ ~ ~~~ ~~ ~.:~ ,~ rte. ~~ ~ r , 4 ~ - '~' Yl '-'dart y ~ ~ ~ ~ ~ ~ 5 °a . ~~~}f ~`<'«~#r. ~ ~ .. ~3e ~,. "-arc ~ ~ ~ 9 ~~~ { _ ~F- 4tL=~jg «2 ~~~~ '~ $ sia ~Y GEOTECHNICAL REPORT Prepared For George Hodgdon July 20, 2007 For the Property Described As Tax #'s 957001501, 961804001, 961804003, 962100004 Section 2, Township 29 North, Range 1 West, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 71 ~ S. Peabody Street Port Angeles, Washington 98362 ~x- Phone 360-452-8491 Fax 360-452-8498 ~ ~ ~ ~,4 ~ i ,: ~ ~ ~ ~ Y ~ . • K Web Site www.nti4u.com ~° ii ~'~ ~~ E-mail info@nti4u.com ~ ,~~ ~ ~ ~ ~~~~~ ~"=- NTI ENGINEERING & SURVEYING _ ~l , 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 r`" *~ ,~.~ 6 ~ ~ c ~~ ~~ ~`~~ °~ _ Enginee Construc rs ~ Land Surveyors _ Geologists, ~ ~, ~ tion Inspection `' Materials Testing ~~~~ ~~ ° '~ ' ` ~ :a 7 (3G0) 452-8491 1-80Q-654-5545 FAX 452-8498 E-Mail: info~nti4u com ~ ~ ~p ~ ~ ' ~~ l~ ~ ! ~~~ =~ ~ I _._ , www. nti4u. com av~1~1~ ;~` ~~VEL~P(~~EN~ ..,.t,:~~.~ _ .w..........~.~ ____._..A.._ l 20 J 2007 y u , George Hodgdon 322 Leighland Ave. Port Angeles, WA 98362 Subject: Geotechnical Report f or the Property Described as Tax #'s 957001501, 961804001, 96180400 3, 962100004, Located in Section 2, T29N, R1 W, W.M., Jefferson County, WA D M H d d ear r. o on: g Background At your request, NTI Engineering an d Surveying, Inc. (NTI) conducted a geotechnical inspection of the above-mentioned p roperty on July 5, 2007. The purpose of this inspection was to examine the mari ne bluff at the property by visual means for any obvious signs of geologic concern a nd to make recommendations in regards to the future development of the property. It is our understanding that you plan to reconfigure the parcels and sell the lots. Site Description The subject property is located at 11 0 Maple Street in Irondale, north of Port Hadlock (Figure 1). The property is bounded on the north by vacant wooded land, on the south by residential property, on the west by N Maple Street, and on the east by Port Townsend Bay. There is also reside ntial property northwest of the subject property (Figure 2). There is an existing hous e on the property that was constructed in 1920. The upland portion of the property h as been cleared and is vegetated with grass and brush (Photo 1). The property slope s to the northlnortheast towards the bluff at approximately 10° (18%). The bluff is about 80' high with an average slope angle of approximately 35° (70%) (Figure 3). The bluff is vegetated with predominantly berry bushes and maple trees. Some of th e trees have curved trunks, indicating that down- slope creep of the soil is occurring. There is a large maple tree at the base of the bluff indicating that the bluff has been rel atively stable at this location for many years. There are slide scars on the bluffface in th e vicinity of the property. These slides coincided with areas where seeps were prese nt. No seeps were observed on the bluff at the subject property, however, seeps w ere observed on the bluff north and south of the property. Y } f f 4 „~~ :. a~ 2 ~, .,' ~p~fi a 4 t~ >~~ '~ ~~ ~ ^ ~ 0. ~~ ~~ . .; .A.a~ ~,,,«a".~.. W ~' R;h~n ~ E; ~ 3~ ~., ~" i~ 3"~1 t ~. , 3 S wr ~ i~ Figure 1 Mapped Geology of the Site and Wicinity C, ~ ~ i r ^~ ~5~~~ ga.. ~ N ~ li ~~. w s ~,~,~ ~ ! r F 1 1~~ 4 M ~, ~ ~ ~# The Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975) maps several soil'~~~ types in the vicinity of the property. The Dick loamy sand (DcC) and the Hoypus gravelly sandy loam (HvC}are mapped over most of the upland portion of the property. Thes e soils formed in glacial outwash on planes and terraces, and consists predominantl y of silty sand and gravel. Map unit Cu refers to cut and fill land which is mapped where fill was placed in low, depressional, wet or swampy areas. This unit is mapped near the northeast corner of the upland portion of the property. This area did not stand ou t as being fill during the site visit, although no subsurface exploration was perform ed. There was fill material under the existing house site. Map units Co and Ro refer to c oastal beaches and bluffs. The beaches consist of sandy and gravelly soil, while there '~ ~s no soil description associated with the Ro unit. The Washington State Department of Ecology's Coastal Zone Atlas -1918, maps the property and vicinity as Vashon recessional outwash (Qvr1 }. This soil is described as consisting of sand and gravel deposited by meltwater streams from the retreating Vashion glacier. South of the subject property on the bluff, this unit is mapped as being 30' thick with a 50' thick layer of glacial till below it, followed by 45' of Vashon advance ~~~ ~~~ ,~ ~~ A ~~ outwash. The Atlas maps the stability of the upland as Stable and the duff as Unstablek~~~ n G~ ~,.~, old slide. ., ...~-: _ r, _...~~ . ~~ ..v~L~~~~~E~~ The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" -1981, also maps the property as Recessional outwash and ice-contact stratified drift. This unit consists primarily of unconsolidated gravel, with some sand, silt, and clay. Visual observations made at the site are generally consistent with the above descriptions. Surface exposures on the upland and on the bluff revealed predominantly sandy gravelly soil. _~ ' Bluff Recession Figure2 As with most of the shoreline of wes undergoing bluff recession. The blu tern Washington, the bluff at the subject property is ff has been formed by marine erosion and undercutting along the shoreline, re sulting in oversteepening and subsequent periodic slides on the bluff face. Typically, th outer few feet of bluff soil. This ong ese periodic slides are shallow, and only involve the oing bluff recession results in a slow landward migration of the bluff. While no rece nt obvious slides were observed on the bluff at the subject property, there are recent slides north and south of the property, both of which are in areas where seeps were pres ent. ~ __._ ~ ~1 ~~ .. ~~q pp ~a } ~~ ~ 1 ~~;~a~ "~.J~r iN~' a~~~a~r~~N~ ~ ~ ~. F I ~~~ ~ R ~ ~ ~ ~ .;w ~~ . ~~~ R~~ It is very difficult, if not impossible, t o predict rates of bluff recession or if or when a landslide may occur. Bluff recession can occur in several ways and is generally cyclic in nature. The bluff mayweather slowl y overtime and recede very little for several years. Then, a portion of the bluff may slou gh off suddenly. Average rates of bluff recession in the greater Puget Sound area have been reported to be on the order of an inch ortwo a year, but in some areas may averag e as much as 6 inches per year. At any particular location, landslides typically occur in frequently, often decades apart. The location of the property in Port Townsend Bay is relatively well protected from large storm waves that can erode the toe of the bluff such as along the Strait of Juan de Fuca. This is evidenced by the vege tation that exists along the base of the bluff, including the large maple tree menti oned above (Photo 2). The relatively protected location, along with the apparent lac k of springs on the bluff at the subject property suggests that the average rate of bl uff recession here may be towards the low end of the scale. Photo 1: View of property looking southwest towards existing house from northeast corner ~-~ LI ~~ Figure 3 Conclusions and Recommendations ~1 ~, ~. ,~ ~~ ~s $d . ~~ ~~ As mentioned above, the bluff at th e subject property is in a relatively protected location, and slide activity appears to be infr equent, however, the bluff at the subject property is undergoing bluff recession, and periodic slides should be expected. The average bluff slope angle of 35° is within the typi cal "angle of repose", which is defined as the maximum slope or angle at which lo ose, cohesionless (also unsaturated) material remains stable, and commonly rang es between 33 and 37 degrees on natural slopes. This also contributes to the relative stability of the bluff. Given the above mentioned assum ptions regarding rates of bluff recession and a 30 year economic life of a house, the b luff could recede anywhere from 2.5' to 15' by the end of the house's economic life. F or an 80' high bluff, the International Building Code (IBC} mandates a 27' footing setba ck from the face of the slope. If we assume the median rate of bluff recession of 0.3 feet per year, and apply a factor of safety of 1.5, then in 30 years, the bluff may have receeded about 14'. Using this assumption, if the initial building setback from the bluf f was 40', then the house would be approximately 27' from the bluff in 30 years. This still satisfies the IBC setback requirement. Based upon our inspection and the above assumptions, we recommend a minimum building setback of 40 feet from the foundation of permanent structures to the top of the bluff, not including decks or other lig htweight temporary structures such as gazebos. Since there is no way to accurately predict the bluff erosion rate, an increased setback ~_ ~` .._ s.r. ..rn+- •,.. w.n vnn orv~.b.,m.rn~ ~.._........ _..__lf y °.~ a y n &9"s. m ~~ 7 ~, ~E 4v ~, i ~ .,~, ,. "! °---~~ 4 y ` E ~~' f s } C may be more beneficial for long term asset planning. Septic systems, drainfields, and X .. d ells should also be behind the 40' buildin setback. We also recommend that the ~`~~ ~'~ ~ 9 ~~ ~~-~~~~u~~~ti~~~r Landslide Hazard Vegetative Buffer be reduced to 15 feet, measured from the top of the ~~ ~ ~ ~ ~ u ~~ ~ ..~ . bluff. This buffer is within the 40' building setback, not in addition to it. The following recommendations should also be considered with regards to development of the subject property: 1. It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that d evelop should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. 2. Heavy irrigation or other activ ities that would contribute large quantities of water to the soil should be avoided . One cause of slope instability is the presence of excessive groundwater. ~, . ~, .~ Photo 2 View of Bluff from Beach J i ~ .~ . ,~.,.... td S #R ~ is ~::.` ~,"' ~ 3. Surface runoff from hard surfaces such as roofs, driveways, walkvays and patios should be controlled and routed to a drainage control system such that surface water discharge to adjacent properties does not exceed predevelo~pment conditions. From a bluff stability standpoint, the best option would be to route the drainage to the base of the bluff via tightline drain. If on-site infiltration is used, drywells ordispersion trenches should be constructed behind the 40' building setback. The drainage control system should comply with all applicable regulations. 4. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not exceed predevelopment conditions. 5. All drainage control devices should be maintained in good working order and inspected at least once a year. 6. The existing house was constructed at least partially on fill, and settlement of the chimney was noticed. If this house is removed and a new house built in its place, the existing fill may need to b~e removed, or otherwise stabilized in orderto provide a stable foundation. ,~, ~~.Y ~~ ~'~ ~i~~,4Fi~~ ~.M~..~.~.u__..~_.___ For further information please review the three enclosed publications published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", ``Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These publications can also be viewed at the DOE website at: http:l/www.ecy.wa.gov/bibliolsea.htrnl under the 1993 and 1994 year heading. The DOE website also contains more useful information regarding slope stability and site development; this reference is highly recommended. '~~~ Limitations This report has been prepared exclusivelyfor use byyou and your agents in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. Copies of this report should be provided to future owners of the property. Others may use it only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no subsurface exploration or laboratory testing was performed. Soil and geologic conditions can vary significantly between test holes andlor surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, '' NTI ENGINEERING &SURVEYING Robert A. Leach, P.E., MBA Principal Engineer Bill Payton, L.E.G. Engineering Geologist ~ a ~,~ '~ a y ~ r' ~ Jti xf~ ~,:::~ ~ i t „, 1 4 y1 ! S :; ~ c 9 i~ R~ ~.~, ,. ~ i ,~s'~~~ ..~a.... u.: R1 A. j o~~ o~~_w~s..~q~ ~;,~~~ ~~~ H~~~~ y ~ 1 OJ v/•' I ~~ ~ ,, ~n %1~~ ~ 4 ~A 1371 0 ~~~ 'I EXPIRES 12/3D/2006 ', ~~ ~~ ~ G:IGen1Bi111ReportslHODG0701.2(29-1 ).bluff stability.lrondale.doc