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G E'O DESIGN April 16, 2010 Centex Homes 11241 Slater Avenue NE, Suite 100 Kirkland, WA 98033 Attention: Mr. Tom Brown Report of Geotechnical Engineering- Services and.Reliance Letter Proposed Tigard Residential Development Tigard, Oregon GeoDesign Project: Centex -16 -01 INTRODUCTION This report , presents the results of our,geotechnical engineering.servicesfor the.proposed residential development located at the southwest corner of the intersection of SW- Scholls Ferry Road;and SE 135'h Avenue in Tigard, Oregon. We,understand.the site encompasses approximately 8:5 acres. and includes 88 individual lots. The-lots are currently vacant are developed with utilities, curbs, and paved. roadways for future residential development: • GeoDesign assumed the role.of geotechnical engineer of record for the-project and provided construction observation services to Integrity Development during. earthwork; our involvement began in July 2007 and continued until March 2008. A geotechnical. report for the•property was completed by GeoPacific Engineering, Inc. on January 13, 2006 entitled Preliminary Geotechnical Engineering Report,. Scholls Ferry Townhomes, SW Barrows. and SW Scholls Ferry Road, Washington County, Oregon. In addition, GeoDesign previously prepared supplemental recommendations for use in,design of gravity retaining walls in a letter entitled, Geotechnical Engineering Services, The Village at Summer Creek, SW Barrows Road and SWScholls Ferry Road, Tigard, Oregon, dated June 6, 2007. The current plan is to construct residential structures on the 88 lots that have been, prepared. Foundation loads were unknown at the time of this report; we have assumed that they will - be typical of one- to four -frame residential structures. We further understand that some site,grading will take place to accommodate design elevations. Cuts and fills are not expected to exceed 5 feet, respectively. A site plan showing existing improvements and our hand -auger explorations is shown on Figure 1. SCOPE OF SERVICES The purpose of our work was to confirm that our previous work is in conformance with the geotechnical engineering report and to provide supplemental recommendations for use in design and construction of the proposed development. Specifically, we completed the following scope of services: • Reviewed in -house files for existing information on subsurface conditions in the site vicinity, prior earthwork, and the previous geotechnical report for the site. • Visited the site and completed a visual surface reconnaissance to confirm that no additional earthwork has been conducted since completion of the project. • Completed eight hand augers to a maximum depth of 7 feet below ground surface (BGS). • Prepared this report, which also serves as a reliance letter, that presents our findings and provides updates to the conclusions and recommendations provided in the previous geotechnical report. SITE CONDITIONS SURFACE CONDITIONS We completed a visual surface reconnaissance at the site. During our visit, we observed that the ground surface is slightly sloped down towards the south from the north. As discussed above, the lots have been developed with associated utilities, sidewalks, curbs, retaining walls, and asphalt -paved roadways to allow access to the lots. The lots are covered by grass, with small patches of blackberry brush near the proposed playground area at the east portion of the property. Areas of ponded water were observed on Lots 5, 9, 1 0, 35, and 30 through 32. While on site, we observed the cracks in the asphalt roadway on SW Coriander lane. As noted in our field reports, the asphalt was reheated during placement using propane torches, which may have contributed to the cracking. SUBSURFACE CONDITIONS We completed eight shallow hand -auger borings (HA -1 through HA -8) at selected locations across the site. The approximate boring locations are shown on Figure 1. Based on our explorations, the shallow subsurface conditions in the area generally consist of compacted fill that is medium stiff to stiff silt with varying amounts of clay, sand, and gravel. The fill generally ranges in thickness from 0.2 foot to 2 feet BGS. The fill was encountered in most of the borings except for HA -5 and HA -6, which were completed near the proposed playground area at the east portion of the property. A thin layer of loose gravel with silt and sand was encountered at the surface in HA -6. Native alluvium consists of medium stiff silt with some clay and trace organics. In HA -2, we encountered loose sand with minor gravel and encountered refusal on concrete at approximately 6 inches BGS. We drilled four more borings within a 10 -foot radius of HA -2 and encountered concrete at 3 to 6 inches BGS. The area of the sand fill over concrete appears to extend over Lots 47 through 49. The approximate area is shown on Figure 1. . GEODESIGNZ 2 Centex -16- 01:041610 Based on our explorations, the average depth of stripping in lightly vegetated areas will be approximately 1 to 2 inches. Stripping activities should be completed ;as recommended in the previous geotechnical report. • CONCLUSIONS AND RECOMMENDATIONS In general, we anticipate that the geotechnical recommendations provided -in the previous geotechnical report are still applicable to the site with the, exception of the additional 1 to 2 inches of stripping anticipated at the site and.our revised shallow foundation recommendations. WET WEATHER/WET SOIL GRADING The silty soils at the site are easily disturbed during the wet season and when they are moist. If not carefully executed, site- preparation, utility trench work, and roadway,excavation can create extensive soft areas and. significant subgrade- repair costs can result. If construction -is planned when the surficial soils are wet or may become wet, the construction methods and schedule should -be carefully considered with respect to protecting the subgrade to reduce the need -to over.- excavate disturbed or softened -soil. --The project budget should reflect-the recommendations below-if-construction is planned during wet weather or when the surficial soils are wet. If construction occurs when silty, wet soils are present, site- preparation activities may need to be accomplished- using- track-mounted excavating equipment that loads removed ,material into trucks •supported on.granular.haul roads. The thickness of,the material for haul - roads and staging areas will depend on the amount and type of construction traffic. Generally, a 12- to 1.8.:inch= thick: mat :of imported granular material -is sufficient for light staging; areas and.the basic building pad but is:generally.not expected-to be-adequate to support heavy equipment or truck traffic. The granular mat for haul roads and areas with repeated,, heavy construction traffic typically needs to be increased to° between 18 to 24 inches. The actual thickness of haul roads and staging areas should be based on the contractor's approach to site development-and the amount and type•of construction traffic. The imported granular mater.ial:should , be placed•in one lift over the prepared, undisturbed.subgrade and - compacted using a smooth -drum, non - vibratory roller. In addition, ageotextile fabric may be required as a barrier between the subgrade and. imported granular material in areas of repeated construction traffic. The imported granular material should -be pit- or quarry-run rock, _crushed rock, or crushed gravel and sand and should meet -the requirements set forth in the 2008. Oregon. Standard Specifications for Construction (OSSC) 00330.14 (Selected Granular Backfill) and OSSC,00330.1 S (Selected Stone Backfill). The placement of the imported granular fill should be done in conformance with the - specifications provided in OSSC 00331 (Subgrade Stabilization). The geotextile should meet the specifications provided in OSSC 02320.20 (Geotextile Property Values) for soil separation. The geotextile should be installed in conformance -with the specifications provided in OSSC 00350 - (Geosynthetic Installation) material should be fairly well - graded between coarse and.fine material and have less than 5 percent by dry weight passing the U.S. Standard No. 200 Sieve. GEO DESI G N? 3 Centex -16- 01:041610 FOUNDATION SUPPORT Dimensions and Capacities - Continuous footings for stud bearing walls should be at least 15 inches wide. The bottom of exterior footings should be at least 18 inches below the lowest adjacent exterior grade. The bottom of interior footings should be established at least 12 inches below grade. Footings' bearing on-subgrade prepared as recommended above-should be sized based on-an allowable bearing'pressure 2;500 pounds per square foot. This-is a net bearing pressure; the weight of the footing and overlying'backfill'can be ignored in calculating footing sizes. The recommended allowable bearing pressure applies to the total of dead plus long - term-live loads and can be increased by one -third for short -term loads (such as those resulting from wind or seismic forces). Based on our analysis'arid experience with similar soils, total post - construction settlement should be less than 1 inch,'with differential settlement'of less •than % inch over a 50-foot span. Resistance to Sliding 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 thatthe-available• passive earth pressure for footings confined by on -site soils and structural fills is 350 pounds per cubic foot, modeled, as, an equivalent fluid ,pressure. Adjacent concrete slabs, pavements, or the upper 12 -inch ° depth 'of - adjacent, unpaved areas should not be considered when calculating passive resistance.- In addition, in orderto rely'upon passive ^resistance, a minimum of 10 feet of horizontal-clearance must exist between the face of the footings and adjacent down slopes. For footings`in contact with the on -site native material, a coefficient'of friction equal to•0 :30 may be used-when calculating resistance to sliding. This value should be increased to.0.40'for crushed rock or imported granular fill. Foundation Drains Foundation drains should-be considered on the outside of the perimeter footings of all buildings and•routed to 'a suitable`discharge because of the potential for shallow groundwater. The foundation drains should consistof 4- inch = diameter; perforated drainpipe embedded in a minimum 2- foot- wide-zone of- drai'n rock. Drain rock should of angular,'granular material with a maximum particle size of 2 inches and should meet OSSC 00430.11 (Granular Drain Backfill Material). The material should be free of roots, organic matter,-and other unsuitable materials; have less than 2 percent by dry weight passing the U.S. Standard No. 200 -Sieve (washed - analysis); and'have at.least at least two mechanically fractured faces. Drain rock should be wrapped in.a geotextilefabric that meets-the specifications provided in OSSC 00350 (Geosynthetic Installation) and OSSC 02320 (Geosynthetics) for drainage geotextiles. CONSTRUCTION CONSIDERATIONS All footing andlloor subgrades- shbuld be evaluated by the project geotechnical engineer or their representative to confirm suitable bearing conditions. Observations should also confirm that all • GEO DESIGN? 4 Centex -16- 01:041610 loose or soft material, organics, unsuitable fill, prior topsoil zones, and softened subgrades (if present) have been removed. Localized deepening of footing excavations may be required to penetrate deleterious materials. If footing excavations are conducted during wet weather conditions, we recommend that a minimum of 3 inches of granular material be placed and compacted until well -keyed at the base of the excavations. The granular material reduces subgrade disturbance during placement of forms and reinforcement and provides clean conditions for the reinforcing steel. 4♦♦ We appreciate the opportunity to be of continued service to you. Please call if you have questions concerning this report or if we can provide additional services. Sincerely, GeoDesign, Inc. tg& PROF Viola C. Lai, P.E., G.E. / /�`/ � Project Engineer � \ 4SREG J 6) �C Y 11 , 26') C� Brett A. Shipton, P.E., G.E. I T T A SN1Q't Principal Engineer EXPIRES:6 - 24/ VCL: BAS: kt Attachments Two copies submitted Document ID: Centex- 16 -01- 041610 - geolr.doc © 2010 GeoDesign, Inc. All rights reserved. G EO DES IG N= 5 Centex -16- 01:041610 • FIGURES vi I —O Printed By. cdavis I Print Date. 4/16/2010 1:55:19 PM File Name: \ \geodesign. local \ Files \lobs \A�D \Centex \Centex- I6 \Center1601 \ Figures \CAD \Centel- 16.01.5P01.dreg I layout: FIGURE I 1, i i f } 2 \ . P , \ i //,' ! 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We obtained representative samples the various soils encountered in the exploration. Classifications and sampling intervals are presented on the exploration logs included in this attachment. The approxirnate locati'ons.of our explorations are shown on Figure 1. The locations of the explorations were determined in thefield by pacing from existing site features. This information should be considered accurate only to the degree implied by the methods used. SOIL CLASSIFICATION • The.soil classified in accordance with•the "Explorationi*ey" (fable A- 1) "Soil Classification System " A -2); which are included in this attachment. The explorationiogs indicate the depths at which the soils or their characteristics change, although the change could . • be gradual. A horizontal line between soil types indicates an observed (visual or drill action) change. If the change occurred between sample locations and was not observed or obvious, the depth was 'interpreted and the change is indicated using a dashed line Classifications and sampling intervals are presented on the exploration logs includedlin this attachment. • • • G EO DESI G N= A -1 Centex -16- 01:041610 SYMBOL SAMPLING DESCRIPTION Location of sample obtained in general accordance with ASTM D 1586 Standard Penetration Test with recovery Location of sample obtained using thin -wall Shelby tube or Geoprobe® sampler in general accordance with ASTM D 1587 with recovery Location of sample obtained using Dames & Moore sampler and 300 -pound hammer or pushed with recovery Location of sample obtained using Dames & Moore or 3- inch -O.D. split -spoon sampler and 140 - pound hammer or pushed with recovery Graphic Log of Soil and Rock Types Location of grab sample Observed contact between soil or rock units (at depth indicated) Rock coring interval Inferred contact between soil or rock units SZ Water level during drilling (at approximate depths ; — — - indicated) t. Water level taken on date shown GEOTECHNICAL TESTING EXPLANATIONS ATT Atterberg Limits P Pushed Sample CBR California Bearing Ratio PP Pocket Penetrometer CON Consolidation P200 Percent Passing U.S. Standard No. 200 Sieve DD Dry Density RES Resilient Modulus DS Direct Shear SIEV Sieve Gradation HYD Hydrometer Gradation TOR Torvane MC Moisture Content UC Unconfined Compressive Strength MD Moisture - Density Relationship VS Vane Shear OC Organic Content kPa Kilopascal ENVIRONMENTAL TESTING EXPLANATIONS CA Sample Submitted for Chemical Analysis ND Not Detected P Pushed Sample NS No Visible Sheen PID Photoionization Detector Headspace SS Slight Sheen Analysis MS Moderate Sheen ppm Parts per Million HS Heavy Sheen G EO DESIGN? EXPLORATION KEY TABLE A -1 15575 SW Sequoia Parkway -Suite 100 Portland OR 97224 Off 503.968.8787 Fax 503.968.3068 RELATIVE DENSITY - COARSE - GRAINED SOILS Relative Density Standard Penetration Dames & Moore Sampler Dames & Moore Sampler Resi (140- pound'hammer) '(300 -pound hammer) Very Loose 0 -4 0 -11 0 -4 Loose 4 -10 11 -26 4 -10 Medium Dense 10 - 30 26 - 74 10 - 30 , Dense 30 - 50 74 -- 120 30 - 47 Very Dense More than 50 More than 120 More than 47 CONSISTENCY - FINE- GRAINED SOILS Consistency Standard Penetration Dames & Moore Sampler Dames. &.Moore Sampler _ Unconfi!nediCompressive Resistance (140 -pound hammer) (300 -pound hammer) • Strength (tsf) Very Soft Less than 2 Less than 3 Less than 2 :Less-than 0.25 Soft 2 -4 3 -6 2 -5 0.25 -0.50 • Medium Stiff . 4 - 8 6 - 12 . 5 - 9 0.50 - 1.0 Stiff 8 -15- 12 -25 9 -19 • 1.0 -2.0 Very Stiff 15' ='30 25 - 65 .19: - :31 . 2.0 - 4.0 Hard More than 30 More than 65 More than 31 More than 4.0 PRIMARY SOIL DIVISIONS GROUP SYMBOL GROUP NAME CLEAN GRAVELS GW or GP GRAVEL • GRAVEL (< 5% fines) (more than 50% of GRAVEL WITH FINES GW -GM or GP -GM GRAVEL with silt • coarse ara5tion (Z 5% - and 5 12% fines) GW -GC or GP -GC GRAVEL with clay COARSE GRAINED retained on GRAVELS WITH FINES GM silty GRAVEL 'SOILS No. 4 sieve) (> 12% fines) GC clayey GRAVEL GC -GM silty, clayey GRAVEL (more than 50% CLEAN SANDS retained on SW or SP SAND No. 200 sieve) SAND (<5% fines) (50% or more of > SANDS WITH FINES SW -SM SP -SM •• ' - SAND with.silt • coarse fraction (. 5% and 5 12% fines) • . SW -SC ,or SP -SC _ • . SAND.with,clay • 'Passing SANDS WITH FINES SM - silty SAND • No.4sieve) (> 12 %fines) SC clayeySAND SC -SM silty, clayey SAND ML SILT • FINE- GRAINED CL CLAY SOILS Liquid limit less than 50 CL ML silty CLAY • (50% or more SILT AND CLAY OL ORGANIC SILT or ORGANIC CLAY passing MH SILT No. 200 sieve) Liquid limit 50 or CH CLAY greater OH ORGANIC SILT or ORGANIC CLAY HIGHLY ORGANIC SOILS PT PEAT MOISTURE ADDITIONAL CONSTITUENTS CLASSIFICATION Secondary granular components or other materials Term Field Test such as organics, man -made debris, etc. Silt and Clay In: Sand and Gravel In: very low moisture, Percent Fine - Grained C oarse- Percent Fine - Grained Coarse - dry dry to touch Soils Grained Soils Soils Grained Soils moist damp, without < 5 trace trace < 5 trace trace visible moisture 5 - 12 minor with 5 - 15 minor minor wet visible free water, > 12 some silty/clayey 1 -5 - 30 with with usually saturated Y;_ _, - . i ..., , . f ,. - s -b£F: - °<E�.'M :r.,:r...�_� � - 1.43 =. ,R,i > 30 sandy /gravelly sandy /gravelly GEODESIGN? SOIL CLASSIFICATION SYSTEM TABLE A -2 15575 SW Sequoia Parkway - Suite 100 Portland OR 97224 Off 503.968 8787 Fax 503.968.3068 z 0 0 H u w • BLOW COUNT DEPTH Q a Z a • • MOISTURE COMMENTS FEET a MATERIAL DESCRIPTION w w F 2 CONTENT% J W Q CC W I u HA -1 0.0 0 50 100 Medium stiff to stiff, light brown with orange and gray mottled SILT (ML), • • some clay, trace gravel, milled wood, red plastic fragments, organics (rootlets), and charcoal; moist (1-inch- ,hick root zone) - FILL. J 1.8 2.5 — Medium stiff, light brown SILT (ML), some clay, trace organics (rootlets); moist (alluvium). • becomes stiff at 3.5 feet stiff to very stiff, orange mottles at 4.0 ® Surface elevation was not \feet r 4.5 measured at the time of • 5.0— Exploration completed at a depth of 4.5 exploration. feet. • 7.5 — • HA -2 0 50 100 0.0 0 50 100 Loose, gray -brown SAND (SP), trace to IYI • Hand angered tour borings within minor gravel; wet - FILL. [ 0.s LV a 10 foot radius of HA -2 and o encountered concrete at 0.25 to Exploration terminated due to refusal 0.5 foot. on concrete at 0.5 foot. Surface elevation was not measured at the time of a exploration. 0 z 2.5 — • • • • O u z 0 5.0 — w u _ ai t 7.5 — l– z w U u a — • w 0 50 100 0 DRILLED BY: GeoDesign, Inc. staff LOGGED BY: JPW COMPLETED: 04/09/10 a 0 BORING METHOD: hand -auger (see report text) BORING BIT DIAMETER: 3-inch GEODESIGN CENTEX -16 -01 HAND AUGER 0 Q 15575 SW Sequoia Parkway • Suite 100 PROPOSED TIGARD RESIDENTIAL DEVELOPMENT = Portland OR 97224 APRIL 2010 FIGURE A -1 Off 503.968.8787 Fax 503.968.3068 TIGARD, OR z o O = u w • BLOW COUNT DEPTH u Q a z • MOISTURE FEET d MATERIAL DESCRIPTION w o F- g CONTENT % COMMENTS J I- Q W I— V HA -3 0 50 100 0.0 Stiff, brown SILT (ML), some clay, trace • organics (rootlets) and gravel; moist • • — - , (1- inch -thick root zone) - FILL. J- 1.0 Soft to medium stiff, brown SILT (ML), — - 1 trace clay and organics (roots and 1.8 \rootlets); moist (buried topsoil). 1 . 2.5— Medium stiff, light brown with orange - mottled SILT (ML), some clay, trace organics (rootlets); moist (alluvium). - becomes stiff at 3.5 feet stiff, light brown mottles at 4.0 feet ® Surface elevation was not measured at the time of Exploration completed at a depth of 4.5 4.5 exploration. 5.0— feet. 7.5 — HA -4 0 50 100 0 s0 100 0.0 Stiff, brown SILT (ML), trace clay, Y organics (rootlets), gravel, and asphalt El - fragments; moist (1- inch -thick root zone) - FILL. stiff to medium stiff, light brown at 1.5 El 1- - feet a 2.5 — I– z 0. Stiff, gray SILT (ML), trace fine sand, - 3.0 EI I- organics (rootlets), and gravel; moist, ° u - low plasticity - FILL. z u grades to gray and light brown at 4.5 o 5 feet — -, with orange mottles at 5.0 feet s.s u Stiff to medium stiff, light brown SILT • a (ML), some clay, trace organics EI (rootlets); moist (alluvium). Surface elevation was not • Exploration completed at a depth of 7.0 7.0 explo mea at the time of ration. � 7.5 — feet. F- Z - a, . L.) _ u s a N r 0 50 100 z 0 0 u DRILLED BY: GeoDesign, Inc. staff LOGGED BY: JPW COMPLETED: 04/09/10 3 0 -, BORING METHOD: hand -auger (see report text) BORING BIT DIAMETER: 3 -inch 3 z Lu ° G EODESIGN? C ENTEX -16 -01 HAN Z 15575 5575 SW Sequoia Parkway -Suite lo PROPOSED TIGARD RESIDENTIAL DEVELOPMENT = Portland OR 97224 APRIL 2010 FIGURE A -2 OH 503.968.8787 Fax 503.968.3068 TIGARD, OR z 9, u W • BLOW COUNT DEPTH Q a — a - ' • MOISTURE COMMENTS FEET a MATERIAL DESCRIPTION w o H g CONTENT % u Q W F VI V HA -5 0.0 0 50 100 Medium stiff to stiff, brown SILT (ML), • minor clay, trace organics (rootlets); moist (27inch -thick root zone). 2.5 — • Surface elevation was not Exploration completed at a depth of 4.0 4.0 measured at the time of P p P exploration. feet. 5.0 — • • 7.5 — • • • o So 100 HA -6 0 50 100 0.0 z c-,.1 Loose GRAVEL with silt and fine sand 0.2 Y \(GP) (2 inches) - FILL. Stiff, light brown SILT (ML), some clay, trace organics (rootlets); moist. • with orange and gray mottles at 2.0 2.5 — feet a ® • measured elevation at the time of not Exploration completed at a depth of 3.0 10 • exploration. 0 feet. u - z O 5.0 — V Q - 2 Q - x7 • 7.5 — I- Z - U V a - 0 50 100 z O DRILLED BY: GeoDesign, Inc. staff LOGGED BY: JPW COMPLETED: 04/09/10 0 BORING METHOD: hand -auger (see report text) BORING BIT DIAMETER: 3-inch CENTEX -16 -01 HAND AUGER 0 G EO D ESIGN? (continued) z • 15575 SW Sequoia Parkway - Suite 100 Portland OR 97224 APRIL 2010 PROPOSED TIGARD RESIDENTIAL DEVELOPMENT FIGURE A -3 Off 503.968.8787 Fax 503.968.3068 TIGARD, OR . z p a_ = v u„ • BLOW COUNT DEPTH u Q d — Q- •MOISTUR COMMENTS FEET a MATERIAL DESCRIPTION w w I g CONTENT % w ICJ Q W I- 0 HA-7 0 50 100 0.0 Medium stiff to stiff, brown SILT (ML), • some clay, trace organics (rootlets), gravel, and concrete fragments; moist (1- inch -thick root zone) - FILL. Very stiff, light brown -gray with 2.0 . 2.5 — orange mottled SILT (ML), some clay, trace gravel; moist - FILL. - Stiff, light brown with gray mottled SILT 3.3 (ML), some clay, trace organics (roots); moist (alluvium). Surface elevation was not 5.0 sand at 5.0 feet l' 5.0 measured at the time of - Exploration completed at a depth of 5.0 exploration. feet. . 7.5 — HA-8 o 50 100 0.0 o so ioo Stiff, brown SILT (ML), minor clay, trace Y organics (rootlets) and gravel; moist (1- inch -thick root zone) - FILL. - so sr cLi o Medium stiff to stiff, light brown with 2.0 Z 2 5— orange and gray mottled SILT (ML), - some clay, trace organics (rootlets); moist (alluvium). I 0 Q. . z u w Surface elevation was not 5.0 Exploration completed at a depth of 5.0 5.0 ® measured at the time of exploration. - feet. • L co x • o - - 7.5 — I- z • - w u • Q a N ul I- 0 50 100 z 0 0 U DRILLED BY: GeoDesign, Inc. staff LOGGED BY: JPW COMPLETED: 04 /09/10 3 0 J BORING METHOD hand -auger (see report text) BORING BIT DIAMETER: 3 -inch 3 z G EODESIG N? CENTER 16 Ol HAND AUGER 0 (continued) Z 15575 SW Sequoia Parkway -Suite 100 PROPOSED TIGARD RESIDENTIAL DEVELOPMENT = Portland OR 97224 APRIL 2010 Off 503.968.8787 Fax 503.968.3068 TIGARD, OR FIGURE A -4