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Plans /, //t p i jt f IDES I GN? N. August 9, 2016 „ Polygon Northwest Company 109 East 13th Street Vancouver, WA 98660 Attention: Chris Walther Geotechnical Subgrade Observation Letter Polygon at Bull Mountain - Lots 47 and 48 Tigard, Oregon GeoDesign Project: Polygon-122-04 This letter provides a summary of our geotechnical subgrade observations for Lots 47 and 48 of the Polygon at Bull Mountain development in Tigard, Oregon. The City of Tigard permit numbers for the lots are presented in the table below. We prepared a geotechnical engineering report' dated July 2, 2014 providing geotechnical recommendations for the site. Mass grading plans indicate fills of over 12 feet were required for Lots 47 and 48. Polygon at Bull Mountain Lot Location Lot# Permit Number 47 MST2016-00207 48 MST2016-00208 Construction for the Polygon at Bull Mountain residential development began in August 2015 and is ongoing. The following is a summary of the site visits related to the grading and subgrade preparation for Lots 47 and 48. We observed the site stripping of Lots 47 and 48 on September 1, 201 5. We observed fill material placement on Lots 47 and 48 from September 2 through September 24, 2015 and on July 7, 2016. We used a Troxler 3430 nuclear density gauge to test compaction of the fills and/or proof rolling if density testing was not capable due to the fill's grain size. On August 9, 2016 we observed the prepared subgrade of Lots 47 and 48. 'GeoDesign,Inc.,2014. Report of Geotechnical Engineering Services;Southeast River Terrace;SW 150`h Avenue and SW Hawk Ridge Road;Tigard,Oregon,dated July 2,2014. GeoDesign Project:Polygon-122-01 15575 SW Sequoia Pkwy,Suite 100 I Portland,OR 97224 1503.968.8787 www.geodesigninc.com 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, the City of Tigard, and members of the design and construction team. To the extent observed, the construction outlined above for subgrade preparation of Lots 47 and 48 at the Polygon at Bull Mountain development has been completed in general conformance with the intent of the project plans, specifications, and our geotechnical recommendations. Footings for the residences on the mentioned lots can be designed and constructed in accordance with the following recommendations from our July 2, 2014 geotechnical report. Footings can be proportioned for an allowable soil bearing pressure of 2,500 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. ♦ ♦ ♦ G EODESIGNZ 2 Polygon-122-04:080916 V We appreciate the opportunity to work with you on this project. If you have questions concerning the information provided, please call. Sincerely, GeoDesign, Inc. ��tO PROp % ��G N t�,,P s16 Reed S. Kistler, P.E. (Oregon) 63114PE r Staff ngineer t� �,/1,./ (.., r OREGON 0 / /Li 7 ( 6'6,414, 2 S,,;„t.: M. Dimke, P.E., G.E. Principal Engineer JEXPIRES: 12131117 I cc: Jason Canfield, Polygon Northwest Company(via email only) Mike White, City of Tigard (via email only) RSK:SVM:rc One copy submitted(via email only) Document ID: Polygon-122-04-080916-geol-lot-47_48.docx ©2016 GeoDesign,Inc. All rights reserved. [DESIGN= 3 Polygon-122-04:080916