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Report % [ DESIGNk June 28, 2016 Polygon Northwest Company 109 East 13t Street Vancouver, WA 98660 Attention: Chris Walther Revised Geotechnical Subg rade Observation Letter West River Terrace - Lots 90 to 92, 103 to 108, 119 to 128 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 90, 91, 92, 103, 104, 105, 106, 107, 108, 119, 120, 121, 122, 123, 124, 125, 126, 127, and 128 of the West River Terrace development. We 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 90 to 92 required fills of up to approximately 4 feet. Fill placed on Lots 90 to 92 relates to a mechanically stabilized earth (MSE) retaining wall that is constructed along the western extents of the project site. West River Terrace Lot Location Lot# Street Address Lot# Street Address 90 13717 SW 175th Lane 119 17286 SW Jean Louise Road 91 13735 SW 175th Lane 120 17270 SW Jean Louise Road 92 13743 SW 175th Lane 121 17248 SW Jean Louise Road 103 13742 SW 175th Avenue 122 17224 SW Jean Louise Road 104 13728 SW 175th Avenue 123 17210 SW Jean Louise Road 105 13714 SW 175th Avenue 124 17186 SW Jean Louise Road 106 13711 SW 174th Loop 125 17172 SW Jean Louise Road 107 13733 SW 174th Loop 126 17164 SW Jean Louise Road 108 13745 SW 174th Loop 127 17130 SW Jean Louise Road 128 17112 SW Jean Louise Road 1 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 1 Construction for the West River Terrace residential development began in September 2015 and is ongoing. Below is a summary of our site visits related to the grading and foundation subgrade preparation for the mentioned lots: • Lots 103 - 108, 119- 128(lots in cut areas) We observed the site stripping of Lots 103 to 108 on September 25, 2015. We observed the site stripping of Lots 119 to 128 on October 2, 2015. On June 27, 2016 we observed prepared lot subgrades. • Lots 90-92 (lots in fill areas) We observed the site stripping on October 2, 2015. Fill placement on Lots 90 to 92 is associated with a MSE retaining wall built along the western perimeter of the lots. We observed prepared retaining wall fill from April 14, 2016 to April 26, 2016. We used a Troxler 3430 nuclear density gauge to test compaction of the fill. On June 27, 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 90, 91, 92, 103, 104, 105, 106, 107, 108, 119, 120, 121, 122, 123, 124, 125, 126, 127, and 128 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 is expected to be less than 1 inch and differential settlement between adjacent foundation elements is expected to be less than 14 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 G EO DESIGN= 2 Polygon-123-03:062816 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. ♦ ♦ ♦ We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, GeoDesign, Inc. 4.*"iLE0 PROp4.. s Reed S. Kistler, E.I.T C9� (li�G N�F4) #0y Geot- hnical Staff 63114PE 1/11 iv O GON °h 14, Zoo 4 �- n M. Dimke, P.E., G.E. y-4114'N D k44- Principal Engineer !EXPIRES: 12/31/17 cc: Mike Waddell, Polygon Homes(via email only) Mike White, City of Tigard (via email only) RSK:SVM:rc One copy submitted(via email only) Document ID: Polygon-123-03-062816-geol-lot_90-92_103-108_1 19-128.docx ©2016 GeoDesign,Inc. All rights reserved. CWDESIGN? 3 Polygon-123-03:062816