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Report s/ c/9 73 © 5L) ve4 ivv L9 WCG/ West Coast Geotech, Inc. GEOTECHNICAL CONSULTANTS June 21, 2004 W -1878 Diversified Construction 12439 SW 22 Avenue Lake Oswego, OR 97035 Attn: Mr. Tom Clarke GEOTECHNICAL SERVICES (BOULDER WALL DESIGN REPORT) BEVLAND OFFICE BUILDING WEST TIGARD, OREGON Gentlemen, In general accordance with our proposal of June 17, 2004, and your authorization of the same day, West Coast Geotech, Inc., is pleased to provide you with geotechnical design recommendations concerning the proposed boulder retaining walls for the above - referenced project that is generally located northwest of the intersection of SW Beveland Road and SW 72 Avenue in Tigard, Oregon. Any discussion of subsurface soil conditions that may be contained herein is not based on any actual site - specific test pits or borings since neither test pits nor borings were conducted for this project. Hence, we will make assumptions about the subsurface soil conditions in order to provide you with geotechnical design recommendations for the boulder wall design only. The subsurface soil conditions will need to be verified during construction in order to validate our geotechnical design recommendations. This report was prepared for your use in the design of the subject facility and should be made available for information on factual data only. This report should not be used for contractual purposes as a warranty of interpreted subsurface conditions discussed herein, if any. SITE AND PROJECT INFORMATION We understand that the proposed project consists of the design and construction of two boulder walls (more or less, on the periphery of the property with an overall maximum height of 8 feet, more or less) in order to create a more level lot. One of the boulder walls will retain native cut P.O. Box 388 West Linn, Oregon 97068 503/655 -2347 FAX 503/655 -0642 Diversified Construction June 21, 2004 Page 2 while the other boulder wall will retain fill for a level parking lot, based on our telephone conversation with your Excavator on June 1, 2004. If our understanding of the project is substantially different than what we have presented in the "previous- paragraph, please bring the new information to our attention as soon as possible so that we can determine if our recommendations require modification. BOULDER WALL DESIGN RECOMMENDATIONS General We have not conducted any surveying or determined any elevations; hence, we will rely on you and/or your Excavator to provide the control necessary in determining elevations, boulder wall heights and slopes. If any of these variables are significantly different (as measured by you and/or your Excavator) than what is presented herein within this report, then it is your responsibility to bring the matter(s) to our attention. Boulder Walls We recommend that 1 -1/2 ton angular boulders with a typical size of 3 to 4 feet in diameter, more or less, be used for the boulder wall planned for this project and should be selectively placed to fit "snugly" with surrounding, adjacent boulders. The attached figure(s) at the end of this report shall be used in constructing the boulder walls for this project and shall be used in conjuction with this report and not as a separate item(s): • Figure 1 for the 0 to 6 -foot tall, more or less, boulder wall (and, includes a level backslope behind the boulder wall), • Figure 2 for the 6 to 8 -foot tall, more or less, boulder wall (and, includes a level backslope behind the boulder wall). If grading plans change and/or if the boulder wall height changes, we recommend that we be allowed to review the changes and modify any or all recommendations contained within this report. Diversified Construction June 21, 2004 Page 3 A copy of our boulder wall design calculations is provided in Appendix A for the fires provided. In general, we attempted to rely on the following factors of safety, more or less, in our design calculations: • Factor of Safety Against Sliding (Primary and Most Important Variable, in our • - _opinions for boulder wall design) = 1.4, minimum, 1.5 or greater, preferred, • Factor of Safety Against Overturning (Secondary) — 2, minimum • Factor of Safety for Average Allowable Bearing Pressure (Secondary) — 3, minimum, • We also attempted to check the maximum toe pressure, more or less, to determine if the maximum toe pressure exceeded what we believe was the ultimate bearing pressure (although, this check is not determined to be a critical factor for boulder wall design as this check is for critical for concrete wall design, in our opinion, because the stones do yield since the stones are essentially blocks), • The same can be said for determining the presence of negative contact pressure at the toe. Stones do yield, hence the check for determining the presence of negative contact pressure at the toe is not critically important, in our opinion, for boulder wall design. Hence, we may allow negative toe pressure to be present in our boulder wall design and do not allow this check to govern the design of the boulder wall. Our design also assume that a spoil's trench does not exist in front of the boulder wall. If your Excavator plans to borrow inorganic soil and replace the over - excavation with spoil's /organic soils in the general proximity of the proposed boulder wall locations, we recommend that we be allowed to evaluate the effect on the design of the boulder walls due to such grading/borrowing. Our design assumes that the native soils adjacent to the boulder walls generally consist of firm, approved, fine- grained silts capable of supporting an allowable bearing pressure of 2,000 psf (average). If any fill is placed in the vicinity of the boulder walls, we assume that the fills have been or will be placed, compacted and tested in lifts to dry densities of at least 95 percent of the standard Proctor maximum dry density (ASTM D698) beginning upon an approved, firm, native subgrade (reasonably level and properly benched, as necessary) capable of supporting an allowable bearing pressure of 2,000 psf (average). The fill shall consist of approved, inorganic onsite fine - grained soils that does not contain organic or deleterious debris nor substantially containing clay or over -sized material such that compaction cannot be adequately achieved and/or tested using a nuclear densometer. Our design assumes that the excavated soils at the native, cut slopes generally consist of firm, inorganic silt/clayey silts with an approximate unit weight of 120 pcf and an internal friction Diversified Construction June 21, 2004 Page 4 angle of 34 degrees and an allowable bearing pressure of 2,000 psf (average) with a friction factor of 0.5. Site visit(s) during excavation and construction of the boulder walls are strongly advised to confirm the subsurface soil conditions in order to validate the boulder wall designs contained herein. Our design also assumes that the boulder retaining wall will not be surcharged by the presence of nearby elevated footings and/or concrete slabs /Sport's Courts. If any of these type structures fall within the zone of influence behind the boulder wall(s), say, within an imaginary slope of 1H:1 V from the inside bottom edge of the boulder wall excavation to the outside edge of any footings and/or concrete slabs /Sport's Courts, then we recommend that we be allowed to consider the effect of surcharges on the boulder wall. In addition, a nonwoven geotextile may also be recommended to separate the fine - grained soils from the drainrock on the faces of the temporary cut/fill slopes for those critical areas where footings and/or concrete slabs /Sport's Courts may be present within the boulder wall's zone of influence. We recommend that we be allowed to consider these aspects in detail for any footings /slabs in near proximity to the boulder walls. The same recommendations in the previous paragraph should be considered for any traffic surcharges that may be present above the boulder retaining wall if trucks /cars are present within the boulder wall's zone of influence. Boulder Row Construction The bottom row of stones should be embedded approximately 1 foot, more or less, below lowest adjacent grade which should be taken as the final grade on the outside portion of the wall. The subgrade should be excavated using a smooth bucket trackhoe, if at all possible unless hard rock conditions prevail at subgrade levels, and should be sloped inward and downward slightly (say, less than five degrees from horizontal) in order to aid to the overall stability of the boulder wall. The lowest level of stones should be founded on firm, native, approved, inorganic, suitable subgrade after all the unsuitable fill/topsoil, if any, has been satisfactorily removed or upon engineered fill that has been satisfactorily placed, compacted and tested in lifts beginning on an approved native subgrade. Bench cuts should be excavated in terraced areas where fills will be placed. The purpose of bench cuts is to "key -in" the new engineered fill to the native soils in order to promote stability. A relatively clean, uniform crushed drain -type rock (on the order of 2 to 4- inches in typical diameter with little, if any, fines) should be used to help "seat" the boulder stones in place and to backfill between the existing boulder wall and the reinforced section of new boulders. Where Diversified Construction 4 Pale topsoil is to be placed over the drainrock, if any such topsoil is planned, we recommend that a nonwoven geotextile such as Mirali I 40N. or equivalent, he placed to separate the drainrock from the topsoil. We also recommend that a perforated drain (4 -inch ADS Drainguard or similar drainpipe) be considered for embedment within the drainrock near the bottom of the drain hit ct behind the walls. The purpose of the drainpipe is to collect any intercepted surface and /or shallow groundwater flow and transport the collected water to a suitable stormwater disposal area. The drainpipe should be properly sloped to allow for the flow of the collected water. Otherwise, water seepage may flow through the boulder N. all and saturate the lower elevations on the downhill side of ihe boulder walls. The Owners /Contractor should be made aware that although boulders are often used to construct walls to support slopes and retain fill, there still is some risk associated with these type walls. if the boulders do not fit %yell (or "knit" together) or stand too vertical at slope_ there will he a risk that a boulder(s) can dislodge and fill off the wall and cause damage downhill of the boulder wall. The better that the boulders fit with each other (and incorporating an inward slope of the boulder wall during construction). the lower the danger; however. there will always be sonic risk of falling boulders no matter how well the boulders fit (especially during record earthquakes). LIMITATIONS It is recommended that close quality control be exercised during the preparation and construction of boulder walls and boulder wall terraces. Boulder walls should be constructed by experienced Boulder Nall Subcontractors. We strongly recommend that site visits be conducted by the Geotechnical Engineer, on a prat- tinge, as- called basis, to observe the condition of the subgn at the boulder wall location after exca ation and also during boulder vtall construction. A final site visit should he conducted alter completion of the boulder vwall. Othermm ise. without the site visits as recoinniended, our design may be invalid /negated and /or we will not be able to write a final letter that addresses compliance with our recommendations, ifso required by the City. If there is a substantial lapse of time between the submission of ibis report and the start of work at the site. if conditions have changed due to natural causes of construction operations at or adjacent to the site, or if the basic project scheme is significantly modified from that assumed, it is recommended than. this report be re■ icvved to determine the applicability of die conclusions and recommendations considering the changed conditions Enid time lapse. • Diversified Construction June 21, 2004 Page 6 Unanticipated soil conditions are commonly encountered and cannot be fully determined merely by producing a boulder wall design report without any field explorations. The actual subsurface conditions may be significantly different than what is assumed herein in this letter report. Such unexpected conditions frequently require that additional expenditures be made to attain a = properly constructed project: Therefore, - a= contingency°fund is recommended to' accommodate such potential extra cost. Be advised that the Local Governing Agency may sometimes require additional geotechnical or other studies in order to approve the project as part of the planning approval/building permitting process. Our Boulder Wall Design Report(s) does not guarantee that the project will be approved by the Local Governing Agency without these additional studies, if required by the Local Governing Agency, being performed. Expenses incurred in reliance upon our Report(s) prior to final approval of the Local Governing Agency are the exclusive responsibility of the Client. In no event shall West Coast Geotech, Inc., be responsible for any delays in approval which are not exclusively caused by West Coast Geotech, Inc.. We trust that this letter -report is sufficient to meet your current needs. If you have any questions, please call at your convenience. Sincerely, 4% PRO, A. WEST COAST GEOTECH, C. � A s . DIN 1 By 1 " ! V ., t ON Michael F. Schrieber, P.E. o 20, Nq � @. Geotechnical Engineer • &CF: , 3 President C:W1878.DOC