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Report MS7-?l(o- 00170 379 (7c C DESIGN= May 13, 2016 Polygon Northwest Company 109 East 13`h Street Vancouver, WA 98660 Attention: Mr. Chris Walther Revised Geotechnical Construction Observations and Subgrade Recommendations West River Terrace - Lots 73 to 82, 93 to 95, 102, and 109 Tigard, Oregon GeoDesign Project: Polygon-123-03 This letter provides a revised summary of our foundation subgrade recommendations and geotechnical construction observations for single-family Lots 73 to 82, 93 to 95, 102, and 109 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. We prepared a geotechnical engineering report', dated December 5, 2014, providing geotechnical recommendations for the site. The street addresses of the aforementioned lots are presented in the table below. Mass grading plans indicate that the majority of the lots are located within cut areas; however, Lots 78 to 82 required fills up to approximately 6 feet. West River Terrace Lot Location Lot# Street Address Lot# Street Address 73 17307 SW Jean Louise Road 81 13758 SW 174`h Loop 74 17331 SW Jean Louise Road 82 1 3766 SW 174`h Loop 75 17349 SW Jean Louise Road 93 13749 SW 175`h Avenue 76 17363 SW Jean Louise Road 94 13765 SW 175`h Avenue 77 17387 SW Jean Louise Road 95 13793 SW 175`h Avenue 78 13726 SW 174`h Loop 102 13754 SW 175`h Avenue 79 13738 SW 174`h Loop 109 13751 SW 174`h Loop 80 13752 SW 174`h Loop .. r•,,< ` 1P-W11,914', 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: 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 • • Lots 73 - 77,93-95, 102, 109(lots in cut areas) We observed the site stripping on September 25, 2015. On May 4, 2016 we observed prepared lot subgrades. • Lots 78-82 (lots in fill areas) We observed the site stripping on September 25, 2015. On October 20, 21, and 22, 2015 we observed prepared fill material. We used a Troxler 3430 nuclear density gauge to test compaction of the fill. Prepared fill material, approximately 2 feet below finished grade, observed on our October 22, 2015 site visit was below 92%of maximum dry density as determined by ASTM D1557. On May 4, 2016 we observed the prepared lot subgrades to be soft to medium stiff. Recommendations for the lot subgrade are presented below. 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, except for the shallower low fill densities indicated for Lots 78 to 82, the construction outlined above for subgrade preparation of Lots 73 to 82, 93 to 95, 102, and 109 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. Based on the low test densities for Lots 78 to 82, the upper two to three feet of fill in this area should be moisture conditioned and compacted as structural fill. Alternately, granular pads could be installed to prepare the footing subgrades for the lots. We anticipate 12-inch-thick granular pads would be required below the footings; however, if granular pads are used, we recommend a member of our geotechnical staff observe the footing subgrade preparation to evaluate required granular pad thicknesses. The granular pads should consist of imported granular material, as defined in the"Structural Fill" section of our geotechnical report. Footings for the residences at the aforementioned lots 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 %z inch, given the anticipated building loads for wood-framed residences. G EODESIGNY 2 Polygon-123-03:051 316 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(pct). 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. ,c3 -G NFF ss, Reed Kistler, E.I.T. rt• ,r 63114PE Geotechnical Staff OaEGON o r S M. Dimke, P.E., G.E. EXPIRES: 12/31/17 Principal Engineer RSK:SMD:rc One copy submitted(via email only) Document ID:Polygon-123-03.051316-geol-lot_73.82_93.95_102_109-rev.docx ©2016 GeoDesign,Inc. All rights reserved. G EO DESIGNY 3 Polygon-123-03:051316