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HomeMy WebLinkAbout990600360 Geotech AssessmentRug 22 06 01:44p Pat Cooper 1-360-437-7914 p.8 rik&1Nei:, SHAW NON ~a WI LSU1V INC. ~~` i `~ ® ~ GEOTECHNfCAL ANO ENV!RONMENTA:._ CON~U A T y'j ~''~"cE °~ lT N 5 ~~ivr~cu~:: August 12, 1998 , . ~,~ ~ ~ .,,a E,.~~ . ~-~ ~ ,. ~~~ _i~~~~il~~ l~g,l ~~~~ Capt. UT_P. Cooper, USN (ret.) PSC 464, Box 20R FPQ AP 965.14 RE: GEOTECHNICAL PRE-PURCHASE EVALL'ATTON OF SO FOSTER LANE, PORT LUDLOW, WASHINGTON Dear Capt. Cooper; This letter summarizes my observations, conclusions and recommendations regarding the stability afthe site and residence at the above-referenced address. These conclusions and recommendations are based on observations made during my visit to the site on 3une 30, 1998, area geologic maps and a review of building plans dated December 2, 1977. Preliminary observations, conclusions and recommendations were provided to you orally upon completion of the site visit. Our work has been conducted in accordance with our written agreement dated July 7, 1998. SITE DESCRIPTION The property is located on the east-southeast side of the cul-de-sac at the end of Foster Lane in Port Ludlow, Washington. The site is located between about 200 and 250 feet above sea level, on the side of a large hill that slopes down to the east to Puget Sound, generally between about 10 and 20 degrees. Across the site, the topography consists of a relatively flat building/driveway area on the west and a slope on the east that extends down to the east between about 20 and 30 degrees'and locally is as steep as about 38 degrees. The height of the slope is about 30 feet. The residence was reportedly constructed in 1979 and is a two-story structure that includes a basement that daylights to the east and south. A portion of the upper story of the residence is cantilevered out over the slope to the east and. is supported on large timber posts which are in turn supported on isolated concrete footings. The remainder of the residence is reportedly founded on continuous spread footings. The building plans indicate. that the footings that support the timber posts are 3-feet square and are located at least i .5 feet below the ground surface. AOC N4RTrl .4Tf-~ STfiiEET • SUt TE 10~ P.O. B~JX 30x303 W-8415-©1 SEATTLE, YVASHINaTOiy 98Y03 e0E•832.802C FAX 206.633-6777 TDC: !•8:1G•833•f~388 Rug zz 06 01:45p Pat Cooper 1-360-437-7914 p.9 ~ II fo Capt. W.P. Cooper USN ~et ~.;, ~~~,~°~`~T^~ ~ ~ :y~ s~ ~. a August I2, 1998 ~: >, Page 2 Significant signs of settlement or lateral movement were not observed in the exposed foundations. (i.e., in the concrete basement walls; exterior Foundation walls or the timber posts and concrete foundations beneath the cantilevered portion of the residence} nor in the basement concrete floor slab. The slope beneath the. cantilevered portion of the residence is about 10 feet high and extends down to the east between about 14 and 20 degrees. Beyond the cantilever, the slopes steepens to about 30 degrees, except at the northeast corner of the residence where the Iower slope steepens to about 38 degrees for a vertical height of about 10 feet, below which it flattens-out to about 30 degrees. A small slide scarp (about 6 feet wide by 3 fEet deep) was observed an this steepened portion of the slope near the northeast .corner. of the residence. This slide. reportedly occurred as a result of heavy snowmen, precipitation and runofl'from the street and property over the slope on about January 1, 199'. We note that heavy snowmen and precipitation was a relatively rare occurrence and resulted in numerous slides in the Puget Sound area at that time. Vegetation an the slope along the east side of the residence includes fir and cedar trees (up to 21/z feet in diameter). Some of these trees were slightly bowed down hill, which is indicative of soil creep. Soil creep, which occurs on most slopes, is the irr-perceptible down slope movement of soil due to the force of gravity. Undergrowth at the site includes saIal, which is indicative of relatively well-drained near surfaced soils. GEOLOGIC CONDITIONS Geologic maps of the area indicate that the site is underlain by Pleistocene-age (13,500 to 1?,900 years. old) Vachon Till ancUor Vachon Advance Outwash, ~Vashon Advance Outwash typically consists of sand with lesser amounts of silt and gravel, The advance outwash was deposited on the pre-existing land surface, in front of the continental Vachon Stade ice sheet that advanced from Canada across the Puget Sound region approximately 17,000 years ago Vachon Till is typically an unsorted mixture o£clay, silt, sand and gravel with occasional cobbles and boulders that was deposited directly beneath the ice sheet as the glacier advanced over the area. Till was hat deposited uniformly beneath the glacier and is often absent. The ice sheet that overrode the *-ill and the underlying advance outwash was estimated to be an the order of weals-ai.Lrx.aa~r~~A}g W-84I S-O1 ., ~, ., -r ~ ~ - r p l 'r Fa . 1 U Capt. ~.P. Cooper, USN (ret,) 1 August 12, 1998 Page 3 3,000 to 4,G00 feet thick in this area. Consequently the till and the underlying advance outwash have been glacially consolidated to a very dense or hard state. Since the retreat of the glaciers, the upper few feet of the very dense,'hazd soil has loosened and weathered, and topsoil has developed at the ground surface. No subsurface explorations were performed at this site; however the vegetation is indicative of relatively well drained subsurface conditions, which is consistent with advance outwash underlying the site. As previously noted, thc~topography near the northeast corner of the residence is relatively steep, and a small slide scarp is present there. The soil exposed in the slide scarp is typically a medium dense, slightly silty to silty sand with wood debris in the soils toward the top of the scarp. It is likely that the soils at this location consist of fill that vvas pushed down the slope during the construction of the residence, and has resulted in the locally steepened slope. SLOPE STABILITY Based on our observations ofthe site, it appears that risk ofdeep-seated slope movements affecting the site and residence is relatively low, in our opinion. Furthermore, it is our opinion that the shallow surficial soils on the slope are the most susceptible to movement and are presently creeping down the slope. In general, the risk of soil movement on a slope can be reduced by not over-steepening the slope (e•g, do not excavate the toe of the slope), not increasing the weight on the slope (e.g., do not place yard debris or fall at the crest of the slope}, maintaining the slope as dry as possible {e.g., route roof downspouts and yard drains in tightlines to the base of the slope or storm drain system, minimize the arnountofsurface water that could flow down the face ofthe slope}, and maintain a vegetative covea• over the surface of the slope.. At this site in particular, a 6-inch diameter, perforated drain pipe was observed at the ground surface directly above the small slide scarp near the northeast corner of the residence. The perforated pipe at this location could potentially dischazge water onto the surface of the slope through the perforations and thereby increase the risk of slope movement at this location, Therefore, we recommend that the portion of this pipe above the slide scarp be re lac . with- W84a5-Da.LTR I I ~~~~~~~ ~~ ~~ .doc'peclnjg v~-s~ls-al ~ ~ ~d 'ic __ v ~! i$, Fn.n - Y ,a hug ~~ D6 01:47p Pat Cooper 1-360-437-7914 p.ll ,~ '~ t, ~~ Capt. Ut~.P. Cooper, LTSN {ret.) - ~, . ~ ,~;,, ~ ~~ ~~~~ a .- t, .~ ~ ,.~. ~a August 12, 199$ o. ~, 5 ~. - ~i'~,,~ _ ~ €~~~s"~~ ~ii'~ y'. z~ ;~ Page 4 ,~ ~. ._ 3 'F non-perforated pipe (tightline). The perforated pipe should be followed up slope to the [ocation where it emerges from beneath the ground. The location were the perforated pipe enters the ground surface should be excavated a few feet farther, back into the slope, following the perforated pipe. The perforated pipe should be cut a few feet beneath the ground surface, a.nd the non-perforated pipe should be attached at this point. Tn addition, a ditch dam should placed around this joint to force water that may be in the trench with perforated pipe into the non- perforated pipe. A ditch dam can be constructed by cutting and placing a few sacks of concrete mix below, alongside, and above the pipe joint and moistening the cut concrete sacks. The non- perforated pipe should extend tt} the base of the slope. , Pleas note that there is some risk of future instability (shallow or deep-seated) present on all hillsides, which the owner must be prepared to accept. Such instability could occur because of future water line breaks/Ieaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events vn a slope that may cause sliding. i LIMITATIONS The conclusions and recommendations in this letter are based an site conditions observed during our site visit and inferred from geologic maps and assume that observed conditions are representative ofthe subsurface conditions throughout the site; i. e., the subsurface conditions are not significantly different from those inferred from the site visit or indicated on geologic maps. If, durung subsequent site acti~~ities (e.g., construction), subsurface conditions different from those inferred. in this letter are observed or appear to be present, we should be advised at once so that we can review those .conditions and reconsider cur recornmendationswhcre necessary. If there is a substantial lapse of time between the submission of this report and the start of any construction,. pr if conditions have changed due to construction operations at or near the'ste, it is recommended that this report be reviewed to determine the applicability of the conclusions and reeomrnendations considering the changed conditions and time lapse. Within the limitations of scope, schedule, and budget, the conclusions and recommendations presented in this letter were prepared in accordance with gereratly accepted professional geotechnical engineering principles and practices in this area. ai the time this letter was prepared. We make no other warranty, either expressed or implied. wsais-oi_LTx.~a~~„~g W-8415-01 Copt: W.P. Cooper, USN (ret.) r ~ August I2, I9~8 ~~~ Page 5 ~~ :. _ This Ietter was prepared for the use of lbfr. Cooper in the pre-purchase evaluation of the stability of this site and residence. With respect to possible future construction, it should be made available to prospective contractors for information on factual data only and not as a warranty of subsurface conditions, such as those interpreted from the site visit and discussion of geologic conditions included in this letter. Please note that the scope of our services did not include any environmental assessments or evaluations regarding the presence or absence of wetlands or toxic substances in the soil, surface water, groundwater, or air on or below or around this site. We~are able to provide these services and would be ha - . .,__. . ppy to discuss these with you if the need arises. Shannon & Wilson has prepared the attached, "Important Information About Your Geatechnical Report," to assist you in understanding the use and limitations of our report. If you have any questions regarding our observations, conclusions ar recommendations contained in this letter, please coil me. Sincerely, SHANNON & WII.SON, INC. .~~Gpti~J ~-~oF~ss~ ~L / OREGON ~ ~' Ji .A J. Pt'Fi ! ' r- • l No. 50 5~2~~ Gr CEO l t/ G lS~ ~. ,r.° ,. ~, . ;~~~ ~~~~ ~a, ;~ ;~~~,,~ ~~9~~~ a ,. .~~ tl William J. Perkins, R.P.G. Principal Geologic Engineer WJP:HHD/wjp Enclosure: Important Tnforrnatian About Your Geotechnical Report i-JOU-'TJ f-IJ1'T P. 1C W8415-0i.LTR.doc/pecinjg W-8415-01