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Report 13753 (7SILA MDESIGNu May 13, 2016 Polygon Northwest Company 109 East 13th Street Vancouver, WA 98660 Attention: Mr. Chris Walther Revised Geotechnical Subg rade Observation Letter West River Terrace - Lots 98 to 101 and 110 to 118 Tigard, Oregon GeoDesign Project: Polygon-123-03 This letter provides a summary of our foundation subgrade observations for the single-family home Lots 98, 99, 100, 101, 110, 111, 112, 1 1 3, 114, 115, 116, 117, and 118 of the West River Terrace development. A revision has been made to our recommended minimum footing width, decreasing the minimum continuous footing width from 18 to 16 inches. GeoDesign, Inc. prepared a geotechnical engineering report', dated December 5, 2014, providing geotechnical recommendations for the site. The street addresses of the lots are presented in the table below. Mass grading plans indicate that the majority of the lots are located within cut areas; however, Lots 114, 1 1 5, 116, 117, and 118 required fills of up to approximately 3.5 feet. West River Terrace Lot Location Lot# Street Address Lot# Street Address 98 13840 SW 175th Avenue 113 13837 SW 174th Avenue 99 13850 SW 175th Avenue 114 17372 SWJean Louise Road 100 13790 SW 175th Avenue 1 1 5 17368 SWJean Louise Road 101 13776 SW 175th Avenue 116 17344 SW Jean Louise Road 110 13769 SW 174th Loop _ 117 17328 SW Jean Louise Road 1 1 1 13787 SW 174th Loop 118 17302 SWJean Louise Road 1 12 13853 SW 174th Avenue Construction for the West River Terrace residential development began in September 2015 and is on-going. Below is a summary of our site visits related to the grading and foundation subgrade preparation for the mentioned lots: '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 • Lots 98-101 and 110-113 (lots in cut areas) We observed the site stripping on October 2, 2015. On April 26, 2016 we observed prepared lot subgrades. • Lots 114-118 (lots in fill areas) We observed the site stripping on October 2, 2015. On October 7 and 8, 201 5 we observed prepared fill material. We used a Troxler 3430 nuclear density gauge to test compaction of the fill. On April 25, 2016 we observed prepared lot subgrades. 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 98, 99, 100, 101, 110, 1 1 1, 1 12, 1 13, 1 14, 1 1 5, 1 16, 1 17, and 118 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 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 o i s expected to be less than 1 inch and differential settlement between adjacent foundation elements is expected to be less than Y2 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 G EODESIGNY 2 Polygon-123-03:051316 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. ♦ ♦ ♦ We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, ��ikt.0 PRQ, . \g �G 1 N�� s�o GeoDesgn, Inc. C9 �A y 71 , 63114PE ( OREGON/ 44 r 6lit:41 04' t4. 2d Sha Dimke, P.E., G.E. S Q, Principal Engineer gl'VN 0\�` EXPIRES: 12/31/17 RSK:SMD:rc One copy submitted(via email only) Document ID: Polygon-123-03-051316-geol-lot_98-101_110.118-rev.docx ©2016 GeoDesign,Inc. All rights reserved. G EODESIGN'Y 3 Polygon-123-03:051316