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Plans 1 r C' DES1GNN August 16, 2016 Polygon Northwest Company 109 East 13`h Street Vancouver, WA 98660 Attention: Mike Waddell Revised Geotechnical Subgrade Observation Letter West River Terrace- Lots 10 through 19 and 29 through 37 Tigard, Oregon GeoDesign Project: Polygon-123-03 This letter provides a summary of our foundation subgrade observations for the planned single- family homes located on Lots 10 through 19 and 29 through 37 of the West River Terrace development. We prepared a geotechnical engineering report', dated December 5, 2014, providing geotechnical recommendations for the site. Mass grading plans indicate the lots were very close to original grades, generally requiring cuts and fills of less than approximately 1 foot each. Construction for the West River Terrace residential development began in September 2015 and is ongoing. We observed stripping of Lots 10 through 19 and 29 through 37 on October 1, 2015. We observed the subgrades for Lots 34 through 37 on August 10, 2016, the subgrade for Lots 29 through 33 on August 11, 2016, and the subgrades for Lots 10 through 19 on August 12, 2016. 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, and members of the design.and construction team. To the extent observed, the construction outlined above for subgrade preparation of Lots 10 through 19 and 29 through 37 at the West River Terrace residential development has been completed in general conformance with the intent of the project plans, specifications, and our geotechnical recommendations. Footings for the residences on all the lots at the site can be 'GeoDesign,Inc. Report of Geotechnical Engineering Services;West River Terrace;SW Roy Rogers Road;Tigard,Oregon, dated December 5,2014. GeoDesign Project: Polygon-123-01 15575 SW Sequoia Pkwy,Suite 100 I Portland,OR 97224 1503.968.8787 www.geodesigninc.com designed and constructed in accordance with the following recommendations from our December 5, 2014 geotechnical report. Footings can be proportioned for an allowable soil bearing pressure of 2,000 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 16 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%2 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. ♦ ♦ ♦ MIDESIGN= 2 Polygon-123-03:081616 f We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, "S(°O QROFFs tu GeoD-sign, Inc. xt. 63114PE iOR4GON S 14, (G .+N n M. Dimke, P.E., G.E. y'41'iiAi 0 k°4'. Principal Engineer J EXPIRES: 12/31/17 cc: Chris Walther, Polygon Homes (via email only) Mike White, City of Tigard (via email only) SMD:rc One copy submitted(via email only) Document ID:Polygon-123-03-081616-geol-lot_10-19_29-37.dooc ©2016 GeoDesign,Inc. All rights reserved. CDESIGNZ 3 Polygon-123-03:081616