Loading...
Report J.H. SIGMUND CONSULTING ENGINEER, INC. S IT(999 - 0 ©O2$ CIVIL ENGINEERING /f8.69$ r .� 7q4 V _ LAND SURVEYING July 6, 1999 ROBERT POSKIN, CBO Senior Plans Examiner City of Tigard 13125 S.W. Hall Boulevard Tigard, Oregon 97223 Re: Fanno Creek Business Park Half Street Improvements City of Tigard File No. DR 15 -98 Dear Mr. Poskin, This letter is in response to your Site Plan Review letter dated June 17,1999. The items as noted are: Accessibility: 1. The accessible parking stalls layout has been modified to locate the aisles on the passenger sides. Site Work: 1. Written confirmation of compliance with the approved plans regarding the Water Quality Facility will be made to the City of Tigard upon the completion of the project. 2. The required soils report will be submitted separately by Carlson Testing. Engi g: he completed forms and a legal description of the dedication are enclosed for the ' stys acceptance Hirlirvt.d Nov j k cvi If you have any questions, or need additional information, please call. Sincerely, .H. Sigmu PE, PLS 8835 SW Canyon Lane, Suite 402 Phone: 503 -292 -5825 Portland, Oregon 97225 Fax: 503 -291 -1613 r''- 0:/22/99 TUE 08:19 FAX 503 684 0954 CARLSON TESTING _ _ lj002 Main Office Branch Office P.O. Box 23814 4060 Hudson Ave., NE Tigard, Oregon 97281 Salem. OR 97301 Carlson Testing, Inc. Phone (503) 684 -3460 Phone (503) 589 -1252 FAX (503) 684 -0954 FAX (503) 589 -1309 PROJECT FAX MEMORANDUM TO: Bill Pease, P.E. FROM: Alan P. Bean P.E. BMGP Engineers Fax # 399 -8259 PAGES: 1 DATE: June 3, 1999 RE: Geotechnical Investigation Results CC: Bud Horton Excavating Nelco Flexspace Tigard, Oregon We have completed our explorations, testing and preliminary engineering studies associated with the above referenced project. The purpose of this memorandum is to present findings to date and provide preliminary design proposals. The two major issues facing the site development are variable fill consistencies (7.5 to 10 feet thick), and loose liquefiable soils (see UBC Sect. 18..) present below elevation 128. The presence of the liquefiable soils when combined with the grade differences between the creek and fill pad, result in a lateral spreading (slope stability) hazard during a design level seismic event. Displacements of 1 -3 feet laterally can be anticipated during a significant event; actual displacement will depend largely on earthquake magnitude (or number of significant cycles). • Others, in state agency studies, have put the risk of a triggering earthquake (acct. — .23g) at approximately 10% in 50 years, although some experts feel it may be less than 5% - albeit based on the brief recorded seismic history. While there are numerous methods of mitigating this hazard (structural mat, systematic pile reinforcement), few are economically feasible for a project of this magnitude. As a result of prior studies performed on similar sites, we have developed a method of mitigating or diminishing the effects of the hazard too, in our opinion, more acceptable limits. Conceptually, the intent is to provide structural mat type integrity, at a fraction of the pile or mat expense, by relying on the combination of compacted soil cohesion, a high tensile strength geotextile, and thickened edge type footings, that together are capable of redistributing concentrated loading /stress to broader areas. In any event, it should be assumed that the structure would need substantial repairs or be non - repairable following a major event (earthquake insurance). Our proposed mitigation is outlined below, and we recommend that an estimate for the work be developed prior to proceeding further with the project to determine economic impacts. It is important to clarify that the presence of the liquefaction is not, in itself, a life safety (UBC) issue; however, our studies to date indicate potential resulting lateral spreading of the fill pad probably is. The proposed mitigation is designed to diminish both the lateral spreading hazard as well as the differential settlement risk associated with the variable fill consistency. • 03/22/99 TUE 08:20 FAX 503 684 0954 CARLSON TESTING U003 Page 2/2 June 3, 1998 Based on our understanding of the site elevations and structure, we propose the following methodology be considered for mitigating the hazard. • Cut the site, either in one or two phases, down to elevation 138 • Rip and compact subgrade • Place high tensile strength woven geotextile (staked taunt $1.60 sq. yd.) • Replace 12" stockpiled soil (compacted to 95% of T -180) • Repeat Geotextile at elevation 139 • Place 12" stockpiled soil • Repeat Geotextile at elevation 140 • Place 12" stockpiled soil • Repeat Geotextile at elevation 141 • Fill to 142, add 6 inches of permeable slab base aggregate, and excavate footings (-18" below finish floor) Based on prior earthwork projects and relative quantity, cut/fill onsite prices can be anticipated to vary between $3.00 to $7.00 per cubic yard. Competitive geotextile cost and placement may be anticipated to be on the order of $1.60 per square yard of geotextile. Footings as described above may be designed for an allowable bearing pressure of 2250 psf, and may be increased to 3200 psf for seismic loading. The bearing pressure is based on an allowable static settlement of 1 inch, maximum column loads of 50 kips, and maximum continuos footing loads of 4 kips /If. The site coefficient may be assumed to be SE,, and the Zone 3 factor of Z =0.3 is acceptable. In our opinion the Ca coefficient may be limited to 0.36, as the loose soils at depth are incapable of propagating accelerations greater than 0.36g. The base sliding coefficient of friction may be assumed to be 0.40. Modulus of subgrade reaction may be taken as 250 psi /i. If you have, any questions regarding this information, I can be reached at (503) 684 -3460