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Report u r /y151 �(tp- X97 NODESIGN May 2, 2016 �\ Polygon Northwest Company 109 East 13th Street Vancouver, WA 98660 Attention: Mr. Chris Walther Geotechnical Subgrade Observation Letter River Terrace Northwest- Lots 201 to 205 Tigard,Oregon GeoDesign Project: Polygon-130-05 This letter provides a summary of our foundation subgrade recommendations and observations for the row homes located on lots 201 through 205 of the River Terrace Northwest development. An initial geotechnical report', dated April 27, 2015, was prepared by Hardman Geotechnical Services, Inc. for the site. We issued a report update'to the initial geotechnical report for the site on June 12, 2015. We also prepared a memorandum'detailing additional geotechnical explorations and our slope evaluation on September 30, 2015. The street addresses and accompanying permits of the aforementioned lots are presented in the table below. Mass grading plans indicate that the cut and fills prepared on lots 201 through 205 were minimal. River Terrace Northwest Lot Location Lot# Street Address Permit# 201 13464 SW Beach Plum Terrace MST2O16-00043 202 13470 SW Beach Plum Terrace MST2O16-00044 203 13478 SW Beach Plum Terrace MST2016-00045 204 13486 SW Beach Plum Terrace MST2016-00046 205 13494 SW Beach Plum Terrace MST2016-00047 Construction for the River Terrace Northwest residential development began in September 2015 and is ongoing. We observed site stripping of lots 201 through 205 on September 11, 2015. On April 8, 2016 we observed the prepared subgrade of lots 201 through 205. 'HGSI. Geotechnical Engineering Report;River Terrace Northwest;Scholls Ferry Road and Roy Rogers Road;Washington County,Oregon,dated April 27,2015. HGSI Project No. 15-1811 GeoDesign Inc. Memorandum,River Terrace Northwest,Geotechnical Report Update,dated June 12,2015. GeoDesign Project: Polygon-130-01 'GeoDesign Inc. Memorandum,River Terrace Northwest,Additional Geotechnical Explorations and Slope Evaluation,dated September 30,2015. GeoDesign Project: Polygon-130-01 15575 SW Sequoia Pkwy,Suite 100 I Portland,OR 97224 1503.968.8787 www.geodesigninc.com • -I 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 201 through 205 at the River Terrace Northwest 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 designed and constructed in accordance with the following recommendations. Footings can be proportioned for an allowable soil bearing pressure of 2,000 pounds per square foot. 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 34 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 EODESIGN= 2 Polygon-130-05:050216 We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, GeoDesign, Inc. CEO PR Op ��G1NFR,p`ro Reed Kistler, E.I.T. 63114PE Geote - ical taff 1i ,�OIGONo/ ( 0 'eh 14. 2� (G tigwN D tM4' S /M. Dimke, P.E., G.E. EXPIRES: 12/31/17 Principal Engineer cc: Mr. Mike Waddell, Polygon Homes (via email only) Mr. Mike White, City of Tigard (via email only) RSK:SMD:rc One copy submitted(via email only) Document ID:Polygon-130-05-050216-geol_lot_201-205.docx ©2016 GeoDesign,Inc. All rights reserved. ill G EODESIGN_U 3 Polygon-130-05:050216