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HomeMy WebLinkAboutPettygrove's 1st Block 18 Lot 1 - Soil Evaluation - 2007.05.17=I II SI{ANNON AWLSON, INC. -llt cEorEcHNrcAL AND ENVTRoNMENTAL coNsuLTANTs ALASKA COLORADO FLOFIIDA MISSOURI OREGON WASHINGTON -6cp o(o -O= t May 17,2007 Mr. Brian Sorensen 1109 Maple Street Port Townsend, WA 98368 RE: EVALUATION OF SOILS BEHIND ROCKERY AT 1109 MAPLE STREET, PORT TOWNSEND, WASHINGTON Dear Mr. Sorensen: This letter presents our observations and conclusions regarding the soils behind the rockery immediately upslope of the residence at the address referenced above. We understand that the City of Port Townsend requested this evaluation before they permit occupancy of the residence. The purpose of our services is to provide the evaluation requested by the city. Our work was authorized by you on April 27 ,2007 . The observations and conclusions in this letter are based on our observations at the site on April27 ,2007 . ROCKERY DESCRIPTION The rockery is approximately 42 feet long and reaches a maximum height of about 12 feet in front of the residence. The residence is located about 4 to 6 feet in front of the rockery. A gravel drive is located behind the rockery on the uphill side. The exposed bottom 4 feet of the rockery is near vertical, inclined slightly back up the slope. The upper 8 feet slopes up at about 45 degrees in a series of approximately 2-foot-wide by 2-foot-vertical steps. These inclinations and dimensions are illustrated in the generalized subsurface profile through the rockery in Figure 1. The rock is angular basalt and is typically about 2 to 2Yzby 2lrby 2%-to 4 feet in dimension. The rock was typically placed so that each row of rocks is about 2 feethigh with the longest rock dimension orientated parullel to the face of the slope and rockery. We observed chinking between the large rocks consisting of approximately 2- to 4-inch-diameter angular basalt quaffy spalls. 4OO NORTH 34TH STREET'SUITE lOO P.O. BOX 300303 SEATTLE, WASHINGTON 981 03206.632.8020 FAX 206.695'6777 TDD: 1.800'833.6388 www-shannonwilson.com i, ' ll4AY 21 7fl17 2t-1-20735-00r Mr. Brian Sorensen May 17,2007 Page2 SHANNON EWISON,INC. We met with the contractor who constructed the wall, Lou Slagle of Elite Trucking, during our site visit. He reported thaI" a4-inch-diameter slotted polyvinyl chloride (PVC) drain pipe was placed at the back of the rockery along the base and bedded in lYr,-inch-minus clean crushed rock. While we did not observe the drain pipe or bedding behind the rockery, we did observe the drain line clean out about 2 feetin front of the rockery, between the rockery and the residence. SUBSURFACE CONDITIONS Published geologic maps of the areaindicate that the site is underlain by Pleistocene Vashon Lodgment Till. Vashon Lodgment Till is typically an unsorted mixture of clay, silt, sand, and gravel with occasional cobbles and boulders that was deposited directly beneath the Vashon Stade ice sheet as it advanced from Canada across the Puget Sound region approximately 17,000 years ago. The ice sheet that overrode the till is estimated to have been a few thousand feet thick. Consequently, the till has been compacted to a very dense state due to the weight of the overriding ice and typically has relatively high shear strengths. A test pit was excavated about 3% feetbehind the face of the rockery at a location near its maximum vertical height during our site visit on Aprrl2T by your contractor, Elite Trucking. We observed and logged the soils exposed in this test pit. The test pit confirmed the presence of Vashon Lodgment Till at depths between about 2%to 3 feet below the existing ground surface, overlain by densely compacted qualry spalls, sand backfill, and crushed rock driveway surfacing material. More detailed descriptions of the soils observed in the test pit are provided in Figure 1. Till was also observed in side hill cuts for driveways and parking areas elsewhere on this site. CONCLUSIONS We understand that the City of Port Townsend is concerned that this rockery may retain significant quantities of fiIl. We appreciate the City's concem regarding the potential presence of significant quantities of fill behind this rockery because, in our opinion, it would not be prudent to use a rockery as steep as the one at the site to retain a l2-foot-high fill slope. Rockeries are generally used as an erosion cover on slopes too steep to maintain vegetation but 2l -l -207 3 5 -00l.Ll /wp/LKD 2t-t-20735-001 ) SHANNON AWLSON, INC.Mr. Brian Sorensen May 17,2007 Page 3 that are inherently stable, and not as filI retaining walls. Consequently, rockeries are commonly used to cover cuts in Vashon Lodgment Till (the till as a relatively high shear strength and may stand in near vertical cuts several feet high over a number of years). Based on the information provided to us by the contractor who built the rockery and the subsurface conditions we observed in the test pit during our site visit, it appears that the rockery between the residence and drive does not retain significant fill quantities. The rockery appears to be primarily a cover for relatively stable slopes underlain by till, which, in our opinion, is an appropriate use ofa rockery. Please note that there is some risk of future instability present on all hillsides, which the owner must be prepared to accept. Such instability could occur because of future water line breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events on a slope thatmay cause sliding. LIMITATIONS The conclusions in this letter report are based on site conditions as they presently exist and assume that the explorations are representative of the subsurface conditions throughout the site; i.e., the subsurface conditions behind the entire rockery are not significantly different from those encountered in the test pit. If, during subsequent site activities (e.g., construction), subsurface conditions different from those encountered in the explorations are observed or appear to be present, we should be advised at once so that we can review those conditions and reconsider our conclusions where necessary. Within the limitations of scope, schedule, and budget, the conclusions presented in this letter report were prepared in accordance with generally accepted professional geologic/geotechnical engineering principles and practices in this area at the time this letter report was prepared. We make no other wananty, either expressed or implied. This letter report was prepared for Mr. Sorensen in the evaluation of the soils behind the rockery upslope of the residence. With respect to possible future construction, it should be made 2l -l -207 3 5 -001.L1lwp/LKD 2t-t-20735-001 SHANNON EWLSON,INC. Mr. Brian Sorensen May 77,2007 Page 4 available for information on factual data only and not as a warranty of subsurface conditions, such as those interpreted from the test pit and discussion of subsurface conditions included in this letter report. Please note that the scope of our services did not include environmental assessments or evaluations for the presence or absence of wetlands orhazardous 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 pleased to discuss these with you as the need arises. Shannon & Wilson has prepared the enclosed "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our letter report. Please call if you have any questions regarding this letter report or our recommendations' Sincerely, SHANNON & WILSON,INC. slt /o+ William J. Perkins, L.E.G Associate WJP:TMG/wjp Enclosures Figure I - Generalized Subsurface Profile Through the Rockery Important Information About Your Geotechnical Report iam Perkins 21 -l -207 3 5 -001.L1lwp/LKD 2t-r-20735-00r Filename: J:\211\2073$001\21-1-2073il01 fig 1.dwg Date: 0$17-2007 Login: CNTSoil Layer Contacts (Typ.)(Dashed Where lnfened)Test PitTEST PIT SOIL DESCRIPTIONSDense to very dense, dark brown, slighfly sandyGRAVEL; moist; (Crushed Rock).Medium dense to dense, light brown, fine tomedium SAND; moist; (Fill).Very dense, dark brown, trace to slightly sandyGRAVEL with cobbles; moist; (2" to 4" QuarrySpalls).Dense, becoming very dense with depth, brownwith dark brown and orange, moftling, slightlygravelly, silty SAND; moist; (Till).Approx. 2.5' x 2.5' Basalt Rock (Iyp.)1 (rvp.)NOTEThe topographic and geologicconditions are based on fieldobservations. Variations betweenthe conditions shown on this profileand actual site conditions are likelyooLo=tt,ot2420--.,)Scale in FeetHorizontal = VerticalFIG. 1G EN ERALIZED S U BSU RFACEPROFILE THROUGH THE ROCKERYMay 2007 21-1-20735-001SHANNON & WILSON, INC.Geotechnical and Envircnmental Consultants1109 Maple StreetPort Townsend, Washington.ttoI =lll SHANNON & WILSON, INC. Geotechnical and Environmental Consultants Attachment to and part of Report 2l-1-20735-001 Date: May 17,2007 To: Mr. Brian Sorensen Port Townsend. Washington I wr pORIRNT I N FORMATION AeOUr YOU n G eOreC H N ICAL/E t tVl ROtrt M ENTAL RepoRr CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. Consultantspreparereportstomeetthespecificneedsofspecificindividuals. Areportpreparedforacivilengineermaynotbeadequatefor a construction contraator or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended pu{pose without first conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first conferring with the consultant. THE CONSULTANT'S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS. A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set ofproject-specific factors. Depending on the project, these may include: the general nature of the structure and property involved; its size and configuration; its hisiorical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations' Unless your consultant indicates otherwise, your report should not be used: (l) when the nature ofthe proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration ofthe proposed project is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors which were considered in the development of the report have changed. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnicaVenvironmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnicaVenvironmental report. The consultant should be kept apprised of any such events, and should be consulted to determine if additional tests are necessary. MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in your report. While nothing can be done to prevent such sifuations, you and your consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect' Page I of2 U2007 A REPORT'S CONCLUSIONS ARE PRELIMINARY. The conclusions contained in your consultant's report are preliminary because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions canbe discemed only Juring earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consultant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations if another party is retained to observe construction. THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can ogcur when other design professionals develop their plans based on misinterpretation of a geotechnicaVenvironmental report. -To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy oftheirplans and specifications relative to these issues. BORING LOGSAND/OR MONITORING WELL DATASHOULD NOT BE SEPARATED FROM THE REPORT. Final boring logs developed by the consultant are based upon interpretation offield logs (assembled by site personnel), field test results, and laboratory-undlo, office evaluation of field samples and data. Only final boring logs and data are customarily included in geotechnical/environmental reports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit effors or omissions in the transfer process. To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepared for !orr, yor should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whom the i"poit *u, prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While a contractor may gainimportant knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnicaVenvironmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem, conzultants have developed a number of clauses for use in their contracts, reports and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identifu where the consulta.rt's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give fulI and frank answers to your questions. The preceding paragraphs are based on information provided by the ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland Page2 of2 1/2007