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F , ~~ t • f Y .. _ ~ .... r,~ . ~:yif .: GEOTECHNICAL REPORT Prepared For Douglas Moyer December 8, 2004 For the Property Described as Tax #'s 950100314, -326, -327 i , i. },~ ,. if ; ~i ,. ~ ~i ~ , Section 16, Township 27 North, Range 1 East, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com .i l ` ~' ~ NORTHWESTERN TERRITORIES, INC. ~` `~- A JLS GROUP COMPANY 717 SOUTH PEABODV STREET, PORT ANGELES, WA 90362 Engineers ^ Land Surveyors t=i Goologisls NT-~ c i Cons(ruclion lnspeclion ri Malorials Tosliny (JGO) 452-0491 FAX 452-8490 www nli4u com F-Mail inloriVnh4u cone JLS GROUP, INC. Geotechnical Report Tax #'s 950100314, -326, -327 December 8, 2004 Douglas Moyer 7444 Spicewood Dr. Sacramento, CA 95831 Subject: Geotechnical Report for Tax #'s 950100314, -326, -327 Located in Section 16, Township 27 North, Range 1 East, W.M., Jefferson County, WA Dear Mr. Moyer: Background At your request, Bill Payton, Engineering Geologist with NTI Engineering and Surveying (NTI) conducted a bluff stability inspection at the above referenced location on November 10, 2004. The purpose of this inspection was to examine the bluff at the subject site by visual means in order to determine the relative stability of the bluff and make recommendations in regards to the proposed construction of a single family residence in general accordance with Section 3.6.10 of the Jefferson County, WA Unified Development Code (UDC). It is our understanding that a 28' by 56' manufactured home and a 20' by 24' garage are planned for the site, with a septic system to be located on the adjacent lot to the south. Site Description The subject high bluff property, which consists of three adjoining lots, is located at 1431 Thorndyke Road. The property is bounded on the north and south by developed residential property, on the west by Thorndyke Rd., and on the east by Peabody Way, a small road/trail that is located about midway down the bluff (Figures 1 and 2). Current development at the property includes an excavated foundation site, driveway, fence and well. The upland portion of the property slopes slightly towards the street, away from the bluff and is vegetated with sparse grasses and weeds (Photos 1 and 2}. The bluff at the property is about 110 feet high with an average slope angle of about 28 degrees, although the upper bluff is steeper at approximately 38 degrees. The bluff is vegetated with brush and young trees, the large trees having been cut down with the stumps left in place (Photos 3 and 4). Some of the tree trunks havo curvod trunks, indicating that down slope creep of the surface soils is occurring. Occasional old growth stumps were also present on the bluff. The presence of these stumps and the recently removed mature trees on the bluff indicate that the bluff in this location has been relatively stable for many years. The bluff face is relatively planar with no gullies, suggesting that channelized surface water runoff has not been a significant problem at the property. Likewise, there appears to be little or no active erosion of the sparsely vegetated upland. No springs or seeps were noticed on the property, however, the property at the base of the bluff below Peabody Way was wet indicating that groundwater exits the bluff in this area above a silt layer in the soil strata. Also, some of the large trees at the base of the bluff were leaning and others were down. This indicates possible slide activity and/or the effects of windthrow due to the saturated soil conditions. During the exceptionally wet winter of 1996-7, a landslide occurred on a portion of the bluff roughly'h mile north of the subject property. Reportedly, the slide, which was about 3'-8' deep, was triggered by excessive water in the soil (from groundwater and surface water sources). And one of the major contributors was uncontrolled surface runoff from offsite, which jumped a culvert and flooded the property. It is understood that this situation has since been mitigated. Even though the subject property is in close proximity to the slide mentioned above, it appears that the slide was a local event predominantly triggered by uncontrolled offsite runoff. The mature trees on the bluff at the subject property, that have been recently cut down, appear to be more than 10 years old, which suggests that there was no sliding on the bluff at the subject property during the 1996-7 winter. Further, the likelihood of offsite runoff flowing over the bluff at the subject property is low due to the slope of the upland portion of the property towards the road. This also indicates a more favorable condition of the bluff. However, the potential for a slide exists anywhere along the bluff. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas maps the soils in the upland area of the subject property as Vashon lodgment till (Qvt1), and the bluff in the area of the subject property as Undifferentiated stratified sediments older than Vashon lodgement till (Qpf). The Vashon lodgment till is described as consisting mostly a compact mixture of boulder to sand size particles with some silt and clay. The Atlas lists this soil as excellent for foundation stability, good for seismic stability, and states that it stands in steep natural and/or cut slopes for long periods. The Undifferentiated stratified sediments are described as consisting mainly of sand and gravel, but in some areas contain silt, clay, peat, and possibly till. This soil is listed as generally good for foundation and seismic stability within the confines of slope stability considerations, and variable for slope stability stating that this soil may be subject to landsliding. The Atlas maps the stability of the upland as Stable and the bluff as Unstable. 2 The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps the soils in the upland area of the subject property as Lodgment till (Qvt): boulders, cobbles and pebbles in a matrix of sand, silt and clay; a compact and unsorted mixture. The bluff is mapped as Undifferentiated glacial, fluvial, glaciofluvial, lacustrine, and glaciolacustrine deposits (Qu). According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the upland portion of the subject property is in an area mapped as the Indianola sandy loam {ioC). This sandy soil formed in glacial outwash sediments. The bluff soils at the subject property are mapped as the Cassolary sandy loam (CfE). This predominantly silty sand soil is found on canyon slopes and coastal bluffs. This soil formed in reworked glacial and marine sediments. Visual observations made at the site, including shallow septic design test pits, indicate that the upland and bluff are composed predominantly of sand and pebbly sand, with silt at the base of the bluff below Peabody Way. Conclusions and Recommendations While this report cannot guarantee that a slide will not occur at the subject property, the bluff at the subject property appears to be grossly stable at present with no evidence of recent slide activity. The bluff is vegetated with young trees and brush. Occasional old growth stumps on the bluff suggest that the bluff has been stable for a long time. Based on the bluff vegetation, it appears that there was no sliding on the bluff during the wet 1996-7 seasons that saw so many slides in the Puget Sound area. Also, the average bluff slope is at or below the "angle of repose" which is defined as the maximum slope or angle at which loose, cohesionless material remains stable, and commonly ranges between 33 and 37 degrees on natural slopes. The County mandates a 30-foot setback from the bluff due to the presence of the landslide hazard area. This setback may be reduced with the submittal and approval of a geotechnical report. The International Building Code (IBC) regulations for steep bluffs also affect construction at the subject property. In this case, the IBC requires that the face of the footing of the house be at least 37 feet from the face of the bluff (See Figure 1805.3.1). Based upon our investigation, we recommend that the landslide hazard buffer be set at 27 from the top of the bluff, and that the proposed modular home be set at least 37 feet from the top of the bluff. This will satisfy the IBC requirement as we-I as provide a 10' space between the landslide hazard buffer and the modular home for a deck if desired. We also recommend that the septic drainfield on the adjacent lot be no closer than 30 feet from the top of the bluff. The following recommendations should also be considered with regards to the proposal: It will be necessary to maintain ground cover in order to reduce erosion from surface runoff. Any bare areas that develop in the future, on the upland or bluff, should be revegetated. Native deep-rooted vegetation that requires little or no 3 ._ .l €7 L.~~J J irrigation would be the most beneficial. However, the grass that is currently growing at the top of the bluff appears to be preventing erosion and should be adequate unless there is renewed erosion. Please consult the enclosed publications for further information. 2. Vegetation on the bluff face provides stabilization to the bluff face soils and helps remove water from the soil. Existing vegetation should be left undisturbed, and the bluff should be allowed to naturally revegetate in trees. If a better view is desired as the trees mature, minor limbing and pruning should be done in such a way that minimizes disturbance to the soil and root zone and that insures the continued health of the vegetation. Trees should not be topped. If a tree is in danger of falling over, it should be cut down, leaving the root and stump in place. A tree expert should be consulted in this matter. 3. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 4. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. From a bluff stability standpoint, the best option appears to be controlled release into the roadside ditch. An engineered drainage and erosion control plan should be developed for this property to address these issues and that conforms to all county, state, etc. regulations. 5. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 6. Drainage control devices should be maintained in good working order and inspected at least once a year. Based on the findings, recommendations and limitations of this report: There should be minimal landslide hazard as suggested by a tack of evidence of recent landslide activity on site in the past. 2. Observations of slope stability indicate that the proposal should not be subject to risk of landslide under the current conditions at the site. 3. The proposal should not significantly increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. 4. The proposal should not decrease slope stability on adjacent properties. 5. The proposal should be stable under normal geologic conditions. >> _. . 4 For further information please review the three publications (included with the original of this report) 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 on the DOE website at: http://www,ecY wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains additional useful information regarding slope stability and site development; this reference is highly recommended. Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. It may be used for other purposes 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 laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or 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, NORTHWESTERN TERRITORIES, INC. Robert A. Leach, P.E., MBA Principal Engineer Bill Payton, L.E.G. Engineering Geologist t G:\Gen\Bill\Reports\MOYD0401.16(27-1 E).Thorndyk~F~doG~ ~ ~~~5 ... i _ .:_ --- _5. , ~O ~Ro - wasy ~Cy ~SS~~hIA L -ENG\a r 7 O~/~~ EXPIRES 12/30/2004 w`~~ f d;- , ~~.- ~' 4 ~~/~ 1~ f , cn9ru~rr+a a ~ / 'o~ 191 ~~$~ $~~o Wllilam C. E'ayton Jr. Expires 11/06/05 Appendix ~E~ 1 6 2aa~ ` I --~- _ i.__ ..-- - I _ `~ ~ ~ -yr---- - -` W __ -, --.~T r --~- ,, r~ II` Subject Property '~I ~ 'li ~ j I I I I~ ~ 1 I __ _ 1 __~ ~ ~ 11 - I- - -- - i ~ ~ ~r ~1 ~ I'I II i I I' ~ 1 ~ I I I I _. _~._ _.._ i I I I I III i ' I II t'I ~. `.. i i '' I I 1 i , II I ~ i 't 1 I. ~ ,; 1 I ._--~~._ _ ___ -_.~T - --_~ ~ --; s` I 1 ' I , 1' I I I'' ~ ~ 1 I II - ~t~i.~.r ~~h' .~F~~c'A'l'*n iVF.~ .:1'S' 1V _''1'.~-_ L`Er~':4:Ci ~~5~ I fl Ii 111 ~I'I . ~ ~ ~ 1 ~ 7.00 F E E', 1 6 2i'~l".~ _ ~ _ .. F ~ 0 1 6 2005 C B 1 6 ?00'~ 2003 International Building Code FACE OF FOOTING 70P OF SLOPE FACEOF E '__.. _...; JI .__.. _...i STRUCTURE `~~\// TOEOF FH3 BUT NEED N07 N SLOPE EXCEED 40 FT. MAN. ~-i H2 BUT NEED NOT IXCEED T5 FT. MAX. For SI: I foot = 303.8 mnt. FIGURE 1805.3.1 FOUNDATION CLEARANCES FROM SLOPES j L l.) ~ ~ l~~~