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Report (32) filsj(ofn-" aav' -- NUDES IGN= May 9, 2016 Polygon Northwest Company 109 East 13'h Street Vancouver, WA 98660 Attention: Mr. Chris Walther Geotechnical Subgrade Observation Letter Polygon at Bull Mountain - Lot 52 1 5402 SW Seine Court Tigard, Oregon GeoDesign Project: Polygon-122-04 City of Tigard Permit No. MST2016-00025 This letter provides a summary of our geotechnical subgrade observations for Lot 52 of the Polygon at Bull Mountain development located at 15402 SW Seine Court in Tigard, Oregon. We prepared a geotechnical engineering report', dated July 2, 2014, providing geotechnical recommendations for the site. Mass grading plans indicate fills of over 8 feet were required for Lot 52. Construction for the Polygon at Bull Mountain residential development began in August 201 5 and is on-going. The following is a summary of the site visits related to the grading and subgrade preparation for Lot 52. We observed the site stripping on September 1, 2016. We observed fill material placement from September 4 through September 24, 2016. We used a Troxler 3430 nuclear density gauge to test compaction of the fills. On May 6, 2016 we observed the prepared foundation subgrade for Lot 52. As of our May 6, 2016 site visit, fills of up to 2 feet may be placed to reach finish grade for the non-building area of the lower bench for Lot 52. Members of our geotechnical staff observed geotechnical-related tasks identified above on an intermittent and on-call basis. The results of our site visits were summarized in daily field reports, copies of which have been provided to Polygon Northwest, Pacific Community Design, the City of Tigard, and members of the design and construction team. 'GeoDesign,Inc., 2014. Report of Geotechnical Engineering Services;Southeast River Terrace;SW 150'h Avenue and SW Hawk Ridge Road;Tigard,Oregon,dated July 2,2014. GeoDesign Project:Polygon-122-01 15575 SW Sequoia Pkwy,Suite 100 I Portland,OR 97224 1503.968.8787 www.geodesigninc.com To the extent observed and tested,the construction outlined above for the Lot 52 foundation subgrade preparation at the Polygon at Bull Mountain residential development has been completed in general conformance with the intent of the project plans, specifications, and our geotechnical recommendations. We understand additional fill is required to embed some of the lower exterior footings the minimum required. Footings for the residence on Lot 52 at the site can be designed and constructed in accordance with the following recommendations from our July 2, 2014 geotechnical report. Footings can be proportioned for an allowable soil bearing pressure of 2,500 pounds per square foot. A higher value may be possible, depending on the grading plan. The allowable bearing pressure is a net bearing pressure;the weight of the footing and overlying backfill can be ignored in calculating footing size. The allowable bearing pressure applies to the total of dead plus long-term live loads and may be doubled for short-term loads such as those resulting from wind or seismic forces. Continuous wall and spread footings should be at least 18 and 24 inches wide, respectively. The bottom of exterior footings should be at least 18 inches below the lowest adjacent final grade. The bottom of interior footings should be placed at least 12 inches below the base of the floor slab. For foundations designed in accordance with the recommendations provided above, total post- construction settlement is expected to be less than 1 inch and differential settlement between adjacent foundation elements is expected to be less than %z inch given the anticipated building loads for wood-framed residences. In wet weather, we recommend placing a sufficient amount of crushed rock (typically 2 to 4 inches)to prevent disturbance to the foundation subgrades. The contractor is responsible for the construction sequencing and methodology for footing excavation and construction. Consequently, the actual amount of rock placed to protect foundation subgrades from disturbance in wet weather should be selected by the contractor. Rock used to protect the subgrades during wet weather should cover the foundation-bearing surfaces and be compacted until "well keyed." Any foundation subgrade soil that is disturbed should be removed prior to the placement of crushed rock and/or pouring of the foundations. Lateral loads on footings can be resisted by passive earth pressure on the sides of the structures and by friction on the base of the footings. Our analysis indicates that the available passive earth pressure for footings confined by structural fill or footings constructed in direct contact with the undisturbed native soil or structural fill is 350 pounds per cubic foot(pcf). Typically, the movement required to develop the available passive resistance may be relatively large; therefore, we recommend using a reduced passive pressure of 250 pcf. Adjacent floor slabs, pavements, or the upper 12-inch depth of adjacent unpaved areas should not be considered when calculating passive resistance. A coefficient of friction equal to 0.35 may be used when calculating resistance to sliding. ♦ ♦ ♦ G EODES IGNz 2 Polygon-122-04:050916 We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, GeoDesign, Inc. EO PR opF 63114PE Reed Kistler, E.I.T. ,, c Geo hnical Staff �. O GOIN c oo� nia. 2 1WN D n M. Dimke, P.E., G.E. EXPIRES: 12/31/17 Principal Engineer cc: Mr.Jason Canfield, Polygon Northwest Company(via email only) Mr. Mike White, City of Tigard (via email only) RSK:SMD:rc Attachments One copy submitted(via email only) Document ID: BeavSchool-48-03-050916-geop-rev.docx ©2016 GeoDesign, Inc. All rights reserved. G EODESIGN= 3 Polygon-122-04:050916