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Report May 19 05 01:43p Derek Brown & Hssoc. (971] 233 -DD7t p.2 "' • °' "" 11:41-, 1 � � rm. 5 U 1 570 9147 CARLSON /ESTING s `' Ca �S .___ _ _.. 00Ol on Geotechnjcal Akin Mkt S. A division or Cartoon Bating, Ina P.O. Box 23014 - "' °p Gectachnlcm Conlulfi7tA Tigr g , 4000 Hudson Ave., NE PO. Boa Ms p ct )n1c tneAedlort end Related 78Afe phony W03 S84-3490 Mono (b gR -T29a bond, OR 87708 FAX {50g) 6704447 FAX phone ( 330D•915S tBOaS 589 -1909 FAX .. 330 • Final Summary Letter 5 1720 -Oars/ 7 ,s.'6 5 SLc)(60i(7/72¢ Ro August 23, 2004 Mr. Mark Hanson Derek Brown $ Associates. Inc. 4949 SW Meadows Rd - Suite 400 Lake Oswego, Oregon 97035 Re: Final Letter of Geotechnical Compliance Bonita Town Home Subdivision Southwest Fanny Creek DT1Ye & Southwest Bonita Road Tigard, Oregon CGT Project G0302094A Catiscx, Geatecllnfcat (CGT) served as geotechnical engineer-of-record (EOR) for the Bonita Town Home Subdivision project in Tigard, O chnicaf lnve "fin• As g$otechnical engineer, we performed a stgation of the site, the results of which were reserl Geotachnical Engineering Services dated April 24, 2003. We also red in our Report of construction observation services during construction. Our observations oo f enio on-call. part -time basis during the months of April through July 2004. completed eted ®d o on an Foundation bearing surfaces were not prepared at the time of this report; therefore, we hav observed any of the foundation subgrade soli& for the proposed have not the foundation subgrades after they have p used r esldenGes. CGT Should observe eY rre been excavated to subgrade elevation, and prior to pouring concrete. Copies of our field reports have been forwarded to you, and are available in our office. W performed the following observations during construction: • Site stripping: Based on our field observations, the site was stri topsoil in genera/ accordance with our recommendations. ptJed of organic • Site grading: Based on our field observations as of our last visit dated 7/20/04, permanent slopes at . the site have been graded to a 2H:1V (2 horizontal 1 vertical) gradient or less in general eccondance with our recommendations_ • Lot structural 8ft: Based on our observations, the lot fills placed at the site that we observed were prepared in general accordance with our recommendations. tta� 19 OS 01:4210 Derek Brown & Assoc. (971) 233 -0071 P-3 anti 11: Z8 FAX 503 670 9147 CARLSON TESTING �` X1002 Final Summary Letter Bonita Town Home Subdivision Tigard, Oregon 00302094,A August 23, 2004 • Retaining wall subgrade and backfill: Based on our observations, the retaining well subgrade soils that we observed have been prepared in accordance with our recommendations. Based on our observations during backtilling, the two Lock and Load type walls located at the northeastern portion of the site had been backfilled with a combination of Imported crushed aggregate and native sift materials. (Note: The use of native materials as backfill was approved by David A. Hall, P.E. of David A. Hall Structural Engineering.) Imported crushed aggregate had been installed within 3 feet of the back of the wafts. Our density testing indicates that this material had been compacted to meet project specifications. Native silt materials had been placed behind the crushed aggregate section and within the zone of geo- grid reinforcement. We do have any records of CGT performing density testing of the silt material, with the exception of the 4 -foot high retaining wall located at the southwestern quadrant of SW Weaver Way and Cattail South. Based on our testing, this slit backlit/ section was compacted in general accordance with our recommendations_ We understand that the silt backfill within the other retaining walls at the site was installed using similar equipment and methods. Consolidation of the sift backfills may occur if the sections of backfill were not prepared In general accordance with our recommendations. • Roadway structural fill: Based on our observations, the roadway fills placed at the site that we observed were prepared in general accordance with our recommendations. • Roadway subgrade and base course: Based on our observations, proof roping and density testing (where applicable), the subgrade and base aggregate for the roadways were prepared in accordance with project plans, Specifications, and our recommendations. • Utility trench backfill compaction: Based on our observations and density testing, with the exceptions noted below, the utility trench backfill that we observed has been compacted in general accordance with project plans, specifications, and our recommendations. we understand that density testing was not required for power line trench backfill material within appeeximate 4.5 -foot deep trenches located parallel to existing curb lines, which will bisect private driveways to the lots. We understand that this material was compacted using similar equipment and methods as used in other • Carlson Ge Itechnlcai Page 2 or 3 t1ay 19 (971) 05 . t:4 3� . r De rek 8147 Brow & Assoc. 233-0071 p,4 L1 O sN av� ein ° CARLsON TESTING 4 0003 r• Final 3urizmelry Letter Bonita To Home SubdNlsion Tt7ard, ofecon 00302094.A August 23, 2004 utility trench backfilling operations. Consolidation of this 4.5 -foot deep bac trfill may occur if this section of backfifl was not prepared in general accordance with our fecorrmlendations. • Roadway asphalt compaction: Based on our observations and density testing, both lifts of asphaltic concrete on site roadways have been compacted in general accordance with project plans, specifications, and our recommendations. Based on our observations, as documented in o field report, and to the best of our knowledge, within our scope of work. the above geotechnicar observations were accordance with the recommendations completed in g eneral provided in our report. Sincerely, CARL-SON OEOTECHNJCAL Brad M. Wilcox. E.I.T. Geotechnicat Staff 11 1 0 -2- Jeanne M. Nierner, RE. Principal Geotechnicai Engineer Doc (f}_ P'10E0TECM PROJECT9ti2003 Pral(echeonita Torµntfomes • CO FInal Lntler.DOC Carlson Geottchrycal Pawn 2 of 3 :Can .son � G e ate clinic al Main Office Salem Office Bend Office � P.O. Box 23814 4060 Hudson Ave., NE P.O. Box 7918 A Division of Carlson Testing, Inc. Tigard, Oregon 97281 Salem, OR 97301 Bend, OR 97708 Geotechnical Consulting Phone (503) 684 -3460 Phone (503) 589 -1252 Phone (541) 330 -9155 Construction Inspection and Related Tests FAX (503) 670 -9147 FAX (503) 589 -1309 FAX (541) 330 -9163 s r Ta 7 5#" o v-r)-C9 78 - 6 , 5 &0.x, ?� • rrT • Report of Geotechnical Engineering Services Bonita Townhome Subdivision SW Fanno Creek Dr & SW Bonita Rd Tigard, Oregon CGT Project: G0302094 Prepared for Mr. Mark Hansen Derek L. Brown & Associates, Inc. 9500 SW Barbur Boulevard #220 Portland, Oregon 97219 • April 24, 2003 • Carlson Georechnlcal Main Office Salem Office Bend Office P.O. Box 23814 4060 Hudson Ave., NE P.O. Box 7918 .—A Division of Carlson Testing, Inc. Tigard, Oregon 97281 Salem, OR 97301 Bend, OR 97708 Geotechnical Consulting Phone (503) 684 -3460 Phone (503) 589 -1252 Phone (541) 330 -9155 Construction Inspection and Related Tests FAX (503) 670 -9147 FAX (503) 589 -1309 FAX (541) 330 -9163 April 24, 2003 Mr. Mark Hansen Derek L. Brown & Associates, Inc. 9500 SW Barbur Boulevard #220 r Portland, Oregon 97219 Report of Geotechnical Engineering Services Bonita Townhome Subdivision SW Fanno Creek Drive & SW Bonita Road Tigard, Oregon c:. CGT Project G0302094 • INTRODUCTION Carlson Geotechnical (CGT) is pleased to present the results of our geotechnical investigation for the proposed Bonita Townhome subdivision to be located at the intersection of SW Fanno Creek Drive and SW Bonita Road, in Tigard, Oregon. The location of the project is shown on Figure 1. We have performed this investigation in general accordance with our Proposal Number P02727 dated April 8, 2003. The purpose of our work was to explore subsurface conditions at the site in order to provide geotechnical engineering recommendations for design and construction of the proposed development. Our scope of work was as follows: • Explore subsurface conditions at the site by excavating six test pits to depths of up to 10 feet below ground surface (bgs). • Classify the materials encountered in the explorations by American Society for Testing and Materials (ASTM) Visual - Manual Method. • Obtain soil samples at select depths in the explorations. • Collect representative soil samples for laboratory testing and to verify our field classifications. • Complete up to 10 moisture content determinations on representative samples obtained from the test pits. • Provide geotechnical- engineering recommendations for design and construction of shallow spread foundations, including allowable design bearing pressure and minimum footing depth and width. • Estimate settlement of footings and floor slabs for the design loading. • Provide geotechnical- engineering recommendations for the design and construction of concrete floor slabs, including an anticipated value for subgrade modulus and recommendations for a capillary break and vapor barrier. • Provide recommendations for subsurface drainage of foundations, floor slabs and pavements, if necessary. Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 • Provide recommendations for removal of tree stumps and set back distance from Fanno Creek. • Provide recommendations for cement amendment of on -site soils during construction. • Evaluate design pavement sections, including base course and asphalt concrete thicknesses for parking areas and access roads. • Provide geotechnical engineering recommendations for design and construction of retaining walls, including allowable design bearing pressure and minimum footing depth, width and setback. • Provide recommendations for the Uniform Building Code (UBC) site coefficient and seismic zone, as well as qualitatively evaluating liquefaction potential. • Provide a written report summarizing the results of our geotechnical investigation. r .- PROJECT INFORMATION AND SITE DESCRIPTION Project Information Development at the site will include construction of approximately 52 single - family townhomes and appurtenant pavements and utilities. No detailed structural information has been provided; however, we assume that building loads will be typical for these types of structures. Site Description Regional Geology • The site is located in the Tualatin Valley in Beaverton, Oregon. The Tualatin Valley is a northwest- southeast trending valley bordered by the Portland Hills to the north, the Tualatin Mountains to the east, the Chehalem Mountains to the south, and the Oregon Coast Range to the west. The Tualatin Valley is an extension of the Willamette Valley, which was formed when the volcanic rocks of the Oregon Coast Range, originally formed as submarine islands, were added onto the North American Continent. The addition of the volcanic rocks caused inland downwarping, forming a depression in which various types of marine sedimentary rocks accumulated. Approximately 15 million years ago, these marine sediments were, in turn, covered by Columbia River Basalts (Tcr) that flowed down the Columbia River Gorge and Willamette Valley, as far south as Salem, Oregon. Later, uplift and tilting of these Columbia River Basalts, the Oregon Coast Range, and the western Cascade Range formed the trough - like character of the Willamette and Tualatin Valleys that we observe today. The Tualatin Valley was subsequently filled with non - marine clay, silt, sand, and a few gravel units (Wilson, 1998') ' Wilson, 1998. Post - middle Miocene geologic evolution of the Tualatin basin, Oregon. Doyle C. Wilson. Oregon Geology Volume 60, Number 5, pp. 99, published by Oregon Department of Geology and Mineral Industries, • September /October 1998. Carlson Geotechnical Page 2 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 derived from weathering of the adjacent hills. During this same time period, local volcanic activity produced the Boring Lavas (Qtb) through several localized vents including Mt. Sylvania and also Mt. Scott and Mt. Tabor. Catastrophic floods later washed into the Willamette and Tualatin Valleys approximately 12,000 to 15,000 years ago and deposited fine- grained sedimentary assemblages (Pleistocene Flood Deposits) mapped throughout the area (Off). Site Geology The available mapping (Madin, 1990 indicates that the site is underlain by approximately 60 feet of fine- grained Pleistocene flood deposits (Off), which are in turn underlain by non - marine clay, silt, and gravel units. The mapping also suggests that the depth to the basement rocks (basalt of the Waverly Heights and associated undifferentiated sedimentary rocks of Eocene age) in the area is approximately 450 feet. Four inferred faults are mapped within approximately 1 mile of the site. These faults have displaced the bedrock but the Pleistocene flood deposits do not appear to have been affected by this faulting episode, and are therefore considered to be non - active in the last 12,000 to 15,000 years. However, these faults may produce ground shaking at the site in the future. Site Surface Conditions The site is bordered by Fanno Creek to the north, SW Bonita Road to the south, and by residential development to the east and west. The site slopes down from the south to north at 3H:1V within approximately 50 feet of Fanno Creek. At the base of the slope, there is a 3 to 4 foot vertical drop off. At the time of our observations many large trees had been cut and removed from the site, but the stumps were still in place and many loose branches covered the site. Site Subsurface Conditions Field Exploration We excavated six test pits on April 20, 2003, to depths of 10 feet bgs. The approximate test pit locations are shown on Figure 2. A member of our geological staff logged the test pits, collected samples, and performed in -situ testing and sampling. Logs for the test pits are presented in the attached Figures 3 through 8. Our laboratory staff visually examined and classified all samples in general accordance with the Unified Soil Classification System. Subsurface Materials On the surface of the six test pits, we encountered a 3 to 12 inch layer of stiff to very stiff sandy silt with roots and organic matter. Below this organic layer, our subsurface explorations 2 Madin, 1990. Earthquake Hazard Geology Maps of the Portland Metropolitan Area, Oregon. Ian P. Madin. Oregon Department of Geology and Mineral Industries Open File Report 0 -90 -2. Carlson Geotechnical Page 3 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 encountered stiff to very stiff native silt underlain by stiff fine sandy silt that extended to the full depths of our excavations. We encountered ground water seepage during our explorations in test pit TP -5 at four feet bgs. We did not observe any ground water seepage in any of the other test pits. We anticipate that ground water levels will fluctuate due to seasonal variations in precipitation, changes in site utilization, or other factors. We did not encounter caving of the test pit walls during our explorations. In order to minimize disturbance to the slope and bordering vegetation near Fanno Creek, we evaluated the soils by excavating several small test pits with a shovel and probing with a %2 inch diameter foundation rod. Soils within 35 feet of Fanno Creek consisted of soft black organic silt to a minimum depth of 3 feet. CONCLUSIONS Based on the results of our explorations and analyses, it is our opinion that the proposed structures with the assumed building loads can be supported on shallow spread footings bearing on the stiff to very stiff native silt, or on new structural fill that is properly installed during construction. The following paragraphs present specific geotechnical recommendations for design and construction. RECOMMENDATIONS Site Preparation The surface soil with organic matter should be removed from proposed building and pavement locations and for a 5- foot - margin around such areas. - Based on our observations, stripping depths may be up to 12 inches. Stripped material should be transported off site for disposal or stockpiled for use in landscaped areas. The tree stumps covering the site should be removed from proposed building and pavement locations . and for a 5- foot - margin around such areas. After the stumps are removed, the resulting hole should be backfilled and compacted in accordance with the structural fill section of the report. A representative from CGT should observe the site subgrades prior to construction to identify areas of remaining organic soils or excessive yielding. If areas of organic soils and /or excessive yielding are identified, the material should be excavated and replaced with compacted soils as Carlson Geotechnical Page 4 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 recommended for structural fill. Areas that appear too soft and wet to support proof rolling equipment should be prepared in accordance with recommendations for wet weather construction given in the following section. Silt fences, hay bales, buffer zones of natural growth, sedimentation ponds, and granular haul roads should be used, as required, to reduce sediment transport during construction to acceptable levels. Measures to reduce erosion should be implemented in accordance with Oregon Administrative Rules 340 -41 -006 and 340 -41 -455 and Washington County regulations regarding erosion control. Fan no Creek Set Back Soils within 35 feet of Fanno Creek are not suitable for support of the proposed structures. Washington County's Department of Clean Water Services (CWS) recommends a 35 foot set - back between perennial stream ravines and proposed construction, unless a geotechnical engineer provides a stamped report stating that soil conditions will support the proposed construction. Based on our observations of soil conditions, it is our opinion that a 35 -foot set back should be retained between Fanno Creek and the proposed construction. Wet Weather Considerations The site soils are highly susceptible to disturbance during wet weather. Trafficability of the site soils will be difficult, and significant damage to subgrade soils will occur if earthwork is undertaken without proper precautions at times when the exposed soils are more than a few percentage points above optimum moisture content. If construction occurs during the wet season, or if site soils are wet at the surface, site preparation activities may need to be accomplished using track - mounted equipment, loading removed material into trucks supported on granular haul roads, or other methods to limit soil disturbance. A qualified geotechnical engineer should evaluate the subgrade by probing rather than proof rolling after the initial site preparation. Soils that have been disturbed during site preparation activities, or soft or loose areas identified during probing, should be removed and replaced with structural fill. _. Haul roads subjected to repeated heavy construction traffic will require a minimum of 18 inches of imported granular material. Twelve inches of imported granular material should be _sufficient for light staging areas. The imported granular material should consist of crushed rock that is well - graded between coarse and fine, contains no unsuitable materials or particles larger than 4 inches, and has less than 5 percent by weight passing the U.S. Standard No. 200 Sieve. The imported granular material should be placed in one lift over the undisturbed subgrade and compacted using a smooth drum, nonvibratory roller until the material is well- keyed. Carlson Geotechnical Page 5 of 11 • Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 • We recommend that a geotextile be placed as a barrier between the subgrade and imported fill in areas of repeated construction traffic. The geotextile should have a minimum Mullen burst strength of 250 pounds per square inch (psi) for puncture resistance and an apparent opening size (AOS) between the U.S. No. 70 and No. 100 Sieves. Structural Fill On -site Soils Use of the on -site soils as structural fill may be problematic because silt is sensitive to small changes in moisture content and is difficult, if not impossible, to adequately compact during wet weather. Moisture conditioning should be expected in order to achieve adequate compaction. When used as structural fill, the on -site silts should be placed in lifts with a maximum thickness of 8 inches and compacted to not less than 92 percent of the maximum dry density, as determined by ASTM D -1557. If the on -site soils cannot be properly moisture conditioned or processed, we recommend using imported granular material for structural fill. Imported Granular Material Imported granular structural fill should consist of angular pit or quarry run rock, crushed rock, or crushed gravel and sand that is fairly well - graded between coarse and fine particle sizes. The • fill should contain no organic matter or other deleterious materials, have a maximum particle _. size of 3 inches, and have less than 5 percent passing the U.S. No. 200 Sieve. The percentage of fines can be increased to 12 percent of the material passing the U.S. No. 200 Sieve, if placed during dry weather and provided the fill material is moisture - conditioned, as necessary, for proper compaction. The material should be placed in lifts with a maximum uncompacted thickness of 12 inches and be compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D -1557. During the wet season or when wet subgrade conditions exist, the initial lift thickness should be increased to 24 inches and should be compacted by rolling with a smooth -drum, nonvibratory roller. Cement Amendment We recommend that subgrade soils over optimum moisture content be amended with approximately 5 percent Type I /II Portland cement. Amendment depths should be on the order of 2 feet bgs. Cement percentages are based on our experience with similar soils at similar moisture contents. The recommended percentage of cement is based on soil moisture contents at the time of placing the structural fill. The amount of cement added to the soil may need to be adjusted based on field observations and performance. • Carlson Geotechnical Page 6 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 After the application of the cement, the soils should be tilled and the cement thoroughly mixed into the soil. The tilled soil should be compacted with a sheep's foot roller. Upon completion of amendment and compaction, the compacted soils should be sealed with a smooth drum roller, and allowed to cure for three days. After the curing period, Carlson Geotechnical should observe a proof roll of the cement amended fill or subgrade to identify areas of excessive yielding. If areas of excessive yielding are identified, the soils should be re- amended and compacted. In locations where additional fill is required, filling should not continue until identified areas of excessive yielding heal, and proof rolling does not identify additional of excessive yielding. Because the permeability of cement - amended soil is extremely low, cement amendment should not be completed in landscape areas. Alternatively, the cement - amended material should be — removed from landscape areas prior to planting. Successful use of soil amendment depends on use of correct techniques and equipment, soil moisture content, and the amount of cement added to the soil. A contractor experienced in soil amendment techniques should be retained to complete this work. It is not possible to amend soils during heavy or continuous rainfall. Work should be completed during suitable conditions. Shallow Foundations We recommend that spread footings be founded on the stiff to very stiff native silt, or on new structural fill that is properly installed during construction. We recommend that all spread footings have a minimum width of 18 inches, and the base of the footings be founded at least 12 inches below the lowest adjacent grade. Continuous wall footings should have a minimum width of 12 inches and be founded a minimum of 18 inches below the lowest adjacent grade. Excavations near foundation footings should not extend within a 1 H:1 V plane projected from the bottom of the footings. Bearing Pressure and Settlement Footings founded as recommended should • be proportioned for a maximum allowable soil bearing pressure of 2,500 pounds per square foot (psf). This bearing pressure is a net bearing pressure and applies to the total of dead and long -term live loads, and may be increased by 1/3 when considering seismic or wind loads. For the recommended design bearing pressures, total settle ment of footings is anticipated to be less than 1 inch. Differential settlements should not exceed %2 -inch. Lateral Capacity We recommend using a passive pressure of 250 pounds per cubic foot (pcf) for design, for footings confined by stiff to very stiff native silt, or on new structural fill. In order to develop these Carlson Geotechnical Page 7 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 • capacities, concrete must be poured neat in excavations, the adjacent grade must be level, and the static ground water must remain below the base of the footing throughout the year. 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 0.32 may be used when calculating resistance to sliding. Floor Slabs Satisfactory subgrade support for building floor slabs supporting up to 250 psf areal loading can be obtained from the stiff to very stiff native silt, or on new structural fill, when prepared in accordance with the recommendations presented in the "Site Preparation" section of this report. A minimum 6- inch -thick layer of crushed rock should be placed over the prepared subgrade to assist as a capillary break. A subgrade modulus of 125 pounds per cubic inch can be used for ,.: the design of the floor slab. Floor slabs constructed as recommended will likely settle less than '/2 -inch. We recommend that slabs be jointed around columns and walls to permit slabs and foundations to settle differentially. Utility Trenches Trench cuts should stand near vertical to a depth of approximately 4 feet in the silt provided no ground water seepage is observed in the sidewalls. If seepage is encountered that undermines the stability of the trench, the sidewalls should be flattened or shored. Trench dewatering may be required to maintain dry working conditions if the invert elevations of the utilities are below the ground water level. Pumping from sumps located within the trench will likely be effective in removing water resulting from seepage. If ground water is present at the base of utility excavations, we recommend placing trench stabilization material at the base of the excavation consisting of 1 foot of well - graded gravel, crushed gravel, or crushed rock with a minimum particle size of 4 inches and less than 5 percent passing the U.S. Standard No. 4 Sieve. The material should be free of organic matter and other deleterious material and should be placed in one lift and compacted until well- keyed. While we have described certain approaches to the trench excavation, it is the contractor's • responsibility to select the excavation and dewatering methods, to monitor the trench excavations for safety, and to provide any shoring required to protect personnel and adjacent improvements. All trench excavations should be in accordance with applicable OSHA and state regulations. Permanent Slopes Permanent slopes should not , exceed Adjacent on -site and off -site structures and surfacing should be located at least 5 feet from the top of slopes. Footings constructed within Carlson Geotechnical Page 8 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 slopes should have a minimum of 5 feet between the face of the slope and the outer edge of the footing. Retaining Structures For walls not restrained from rotation, we recommend using an equivalent fluid pressure of 33 pcf for design. We recommend using an equivalent fluid pressure of 55 pcf for design of walls restrained from rotation. When computing resistance to lateral loads, we recommend using a base friction coefficient of 0.32. Footings for the retaining walls should be designed for a maximum bearing pressure of 2,500 psf. Wall drains should include perforated drainpipe wrapped in a non -woven geotextile filter installed behind the walls at the base. Walls should be backfilled with imported granular material, as described in the Structural Fill section of this report. The above design recommendations are based on the assumptions that: (1) the walls consist of conventional cantilevered retaining walls or embedded building walls, (2.) the walls are less than 10 feet in height, (3) the backfill is level and drained and consists of imported granular materials, and (4) no surcharges are imposed behind the wall. Reevaluation of our recommendations will be required if the retaining wall design criteria for the project vary from these assumptions. Pavements We recommend a pavement section of 3 inches of asphalt concrete over 8 inches of aggregate base be used in paved areas that will be exposed to less than five trucks and 200 passenger cars per day. The design of the recommended pavement section is based on an assumed California Bearing Radio (CBR) of 3 and on the assumption that construction will be completed during an extended period of dry weather. Increased base rock sections may be required in wet conditions to support construction traffic and protect the subgrade. Asphalt concrete should conform to Section 00745 of the Standard Specifications for Highway Construction, Oregon State Highway Division, 1996 Edition, for light -duty asphalt concrete. Aggregate base should • conform to Section 02630 of the same specifications. Place aggregate base in one lift and compact to not less than 95 percent of the maximum dry density, as determined by ASTM D -1557. Drainage Considerations We recommend that subsurface drains be connected to the nearest storm drain or other suitable discharge. We also recommend that paved surfaces and ground near or adjacent to the proposed buildings be sloped to drain away from the structures. Surface water from pavement surfaces and open spaces should be collected and routed to a suitable discharge point. Carlson Geotechnical Page 9 of 11 Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 Seismic Design The site is located in Seismic Zone 3 of the 1997 Uniform Building Code. Based on our understanding of the subsurface conditions, the UBC soil profile that best characterizes the site is "S We recommend using a seismic coefficient of C = 0.36 and C 0.54 for site conditions corresponding to the amplification of an SD soil profile. Based on their consistency, grain size distribution, and unsaturated condition, there is a relatively low potential for liquefaction of site soils. OBSERVATION OF CONSTRUCTION Satisfactory foundation, pavement, and earthwork performance depends to a large degree on the quality of construction. Sufficient observation of the contractor's activities is a key part of determining that the work is completed in accordance with the construction drawings and specifications. Subsurface conditions observed during construction should be compared with those encountered during subsurface explorations. Recognition of changed conditions often requires experience; therefore, we recommend that qualified personnel visit the site with sufficient frequency to detect whether subsurface conditions change significantly from those anticipated. We recommend that site stripping, rough grading, foundation and pavement subgrades, and placement of engineered fill be observed by the project geotechnical engineer or their representative. Because observation is typically performed on an on -call basis, we recommend that the earthwork contractor be held contractually responsible for scheduling observation. LIMITATIONS We have prepared this report for use by the owner /developer and other members of the design and construction team for the proposed development. The opinions and recommendations contained within this report may be used for bidding or estimating purposes. They are not intended to be, nor should they be construed as a warranty of subsurface conditions, but are forwarded to assist in the planning and design process. We have made observations based on our explorations that indicate the soil conditions at only those specific locations and only to the depths penetrated. These observations do not necessarily reflect soil types, strata thickness, or water level variations that may exist between explorations. If subsurface conditions vary from those encountered in our site exploration, CGT should be alerted to the change in conditions so that we may provide additional geotechnical recommendations, if necessary. Observation by experienced geotechnical personnel should be considered an integral part of the construction process. Carlson Geotechnical Page 10 of 11 • • Bonita Townhome Subdivision Tigard, Oregon April 24, 2003 The owner /developer is responsible for insuring that the project designers and contractors implement our recommendations. When the design has been finalized, we recommend that the design and specifications be reviewed by our firm to see that our recommendations have been interpreted and implemented as intended. If design changes are made, we request that we be retained to review our conclusions and recommendations and to provide a written modification or verification. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Within the limitations of scope, schedule, and budget, our services have been executed in accordance with the generally accepted practices in this area at the time this report was prepared. No warranty or other conditions express or implied, should be understood. We appreciate the opportunity to serve as your geotechnical consultant on this project. Please contact us if you have any questions. Sincerely, CARLSON GEOTECHNICAL , 1 Ti/ Jared Mechetti Geotechnical Staff ( �9 . 0 " ' GON s Jeanne M. N emer, " h' Principal Geltechn' al Engineer EXPIRES:t2.3i - . '4- ttachments: _Figures 1 through 8 • Doc ID: P: \GEOTECH \PROJECTS\2003 Projects \Bonita Town Home Subdivision \Geo Report.doc Carlson Geotechnical Page 11 of 11 BONITA TOWN HOME SUBDIVISION SITE LOCATION . 1 - �� • i, _ -gyp . r. / � 1�. ` � • ► Witt,./t1. � � ; S; Q� S €J rIr.. i i.p �' i ► \\ � i7 S 7 • ' ; V. , � „= ,.. i '• : l �ir` . : :< o P "". ' '•.7 i ik,' "'' `, N. • t ; e ,1 r + 9 t tii i , � f ,t IN ��+- -+ r4: fir' nt -r y � A �.., .� �� • ; mrr�. ,.: y �1 , �,�7 r t e fz, L.:. 01 1...--;046,,-7.4....---.- ,/..,,,....,v, s u � .,< k'�'. j7 ' r .._ i ° .; fi . ` � �•>, • a - 39 *�.'" .► ,. ,) ,e . 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' -- Kq �./ . 1 � \ I J . . a„ '-',.'•' ( 43=';-. _A �; j L - .. • , te t r k •se'.� • — - .- --4. * ` r� a • `= • �'ra i r ® ! = `' ' ` ' � /e 1 '1 ► 1 , i � - h5c cv Map created with TOPO! Widower Productions (www.topo.com) Scale 1 Inch = 2,000 feet USGS 7 :5 Minute Topographic Map Series, Beaverton, OR Quadrangle. Map version 1981. Current as of 1984. T.1 W, R.2S, Section 12 Willamette Meridian, Thomas Guide 655 - F6 0 2000 4000 • - G � O Carlson Geotechnical ® P.O. Box 23814. CGT Job No. G0302094 FIGURE 1 503 Tigard, Oregon 97281 BONITA TOWN HOME SUBDIVISION SITE PLAN NORTH Fanno Creek _ _ - - ~ - TP -2 a Existing House TP -1 Weaver Way T -3 Existing Barn Existing To Be House Removed TP -6 TP -5 • TP-4 Existing Existing House House To Be To Be Removed Removed S. W. Bonita Road NOTES: LEGEND * All locations are approximate. TP -1 CGT Job No. G0302094 ® Location of test pit explorations Drawing based on sketch by CGT staff. Date: 03/03/03 Gp► •,l, Carlson Geotechnical � P.O. Box 23814 FIGURE 2 503.6 Tigard, Oregon 97281 Drawing Not To Scale BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 Location: See Figure 2 Surface Elevation: Unavailable a c m or, T 2° m o m E� o ~ c 3 �. a o= y m a m � Material Description o c c p Q . co Z � U c m 0 U _ 2.5 S-1 31 ML Very stiff black fine sandy SILT with roots and organics; moist. ML Very stiff brown SILT with occasional mottles; moist. 1- 2— 3- 2.0 S -2 a 29 4 5 6 light brown, fine sandy silt below 6 feet. 7 — 8 S -3 24 9 — 10 •— Test pit terminated at 10 feet. 11- 12- 13 14- 15 16— NOTE: No ground water seepage or caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 1 Figure: 3 P L ® ,� Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503. 664.3460 BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 • Location: See Figure 2 Surface Elevation: Unavailable m o m c 3� ; ° s a o m ; a c 0 Material Description 0 a cc coz � U o !c m ML Stiff black fine sandy SILT with roots and organics; moist. — 2.0 S -1 61 48 1 ML Stiff brown SILT with occasional mottles; moist. 2 - 3— 1.75 S -2 a 29 5 - light brown, fine sandy silt below 6 feet. 6 - 7— S -3 23 8 9 10 • — Test pit terminated at 10 feet. 11- 12- 13 14- 15 16— NOTE: No ground water seepage or caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 2 Figure: 4 • te ® Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503.684.3460 BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 Location: See Figure 2 Surface Elevation: Unavailable a m E a � =5 � 3 g 5 »� E E @ •M m ° Material Description Q a c c E ° o g " p a° co - v °` ° `° 0 �U ML Stiff black fine sandy SILT with roots and organics; moist. — 2.0 ML Stiff brown SILT with occasional mottles; moist. 1 — 2— S -1 26 3— 1.75 4- 5 b light brown, fine sandy silt below 6 feet. 7- 8 S -27 18 9 — 10 - - Test pit terminated at 10 feet. 11 12- 13 14- 15 16— NOTE: No ground water seepage or caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 3 Figure: 5 Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503. 684.3460 BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 Location: See Figure 2 Surface Elevation: Unavailable w c _ 8 m .i o a 0-.- E * E Material Description o a co z E g ° o " � P Cl) v o �v ML ,Stiff black fine sandy SILT with roots and organics; moist. , — 2.0 S-1 a 18 ML Stiff brown SILT with occasional mottles; moist. • 1- 2— 3— 1.75 S-2 Pr 4 5 light brown, fine sandy silt below 6 feet. 6- 7— S -3 8— g— 10 • Test pit terminated at 10 feet. 11- 12- 13 14- 15 16— NOTE: No ground water seepage or caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 4 Figure: 6 • P�.� Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503.664.3460 • BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 • Location: See Figure 2 Surface Elevation: Unavailable m a o e EE m 6 m ' .r Material Description Q dm E oc m ar d V)Z � U 2 I'= W _ 0 U — 1.75 S -1 28 ML Stiff black fine sandy SILT with roots and organics; moist. 1 ML Stiff brown SILT with occasional mottles; moist. • 2— 3— 1.25 S -2 P r 30 • 4441) ground water seepage 0 • 8— light brown, fine sandy silt below 6 feet. 9 — 10 — Test pit terminated at 10 feet. 11- 12- 13 14- 15 16— NOTE: Ground water seepage observed at 4 feet. No caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 5 Figure: 7 23 Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503.684.3460 BONITA TOWN HOME SUBDIVISION Logged by: Jared Mechetti Date Excavated: 04/20/03 Location: See Figure 2 Surface Elevation: Unavailable r a o g m a m' W 7 a • m E Material Description cn m c .) 2 `� W 0- 0 C9 U ML Very stiff black fine sandy SILT with roots and organics; moist. ML Very stiff brown SILT with occasional mottles; moist. 1- 2— 3 — 4 light brown, fine sandy silt below 6 feet. 5 — 6— S -1 mr' 29 7- 8 9- 10 Test pit terminated at 10 feet. 11- 12- 13 14- 15 16— NOTE: No ground water seepage or caving observed during excavation. 17 Job No. G0302094 Log of Test Pit 6 Figure: 8 p L Carlson Geotechnical - P.O. Box 23814 - Tigard, Oregon 97281 - 684 -3460 - Fax 670 -9147 503.684.3460 11 /V,7 /VJ 1YLU LJ:J/ ran UVO VII/ 0191 VAALOVIV 1LJ1117V W_IVV1 Sa Ca rlson - Geotechnical Main ► ®m On Bend Office P.O. Box 23814 4060 Hudson Ave., NE P O. Box 7918 A Division of Carlson Testing, Inc. Tigard, Oregon 97281 Salem, OR 97301 Berl, OR 97708 Geotechnical Consulting Phone (503) 684 -3460 Phone (503) 589.1252 Phone (541) 330.9155 ConstruCtlOn Inspection and Related Tests FAX (503) 670.9147 FAX (503) 589 -1309 FAX (541) 330.9163 RECEIVED November 4, 2003 MAR 4 2004 CITY OF TIGARD Mr. Mark Hansen BUILDING DIVISION Derek Brown & Associates 9500 SW Barber Boulevard Portland, OR 97219 Report of Geotechnical Engineering Services Reinforced Retaining Wall Design Bonita Town Homes SW Fanno Creek Drive and SW Bonita Road Tigard, Oregon • CGT Project G0302180 INTRODUCTION Carlson Geotechnical (CGT) is pleased to submit our Report of Geotechnical Engineering Services for the proposed retaining walls at the proposed Bonita Town Homes Subdivision located on the corner of SW Fanno Creek Drive and SW Bonita Road in Tigard, Oregon. We performed our work in general accordance with our proposal P02873 dated October 23, 2003. The purpose of our work was to provide recommendations for design and construction of geo- grid reinforced retaining walls. Project Information Carlson Geotechnical completed a geotechnical investigation for development of the site, the results of which were presented in our Report of Geotechnical Services dated April 24, 2003. We used subsurface information gathered during that investigation for our current work. Development at the site will include construction of approximately 53 town homes and associated pavements and utilities. In conjunction with site development, several geo -grid reinforced retaining walls of varying heights will be constructed on the site. These walls will be up to six feet high with an approximately three -foot high 2H:1V sloping backfill. The walls will be surcharged by a roadway located roughly two feet from the back of the wall. Retaining Wall Analyses We completed analyses for wall heights of two, four and six feet using software provided by the Allan Block Corporation. The wall analyses require geometric properties and soil strength parameters. We assumed unit weight, cohesion, and internal angle of friction parameters based on our observations of the site soils during our previous geotechnical investigation. For the • native silt, we assumed a unit weight of 110 pounds per cubic foot (pcf), a cohesion value of • 1 DLBC70 1 • 11 /Va /VJ nnu LJ:J/ CAA JVJ vlv 0191 VAt \LJVN auJa a •ci Bonita Town Homes Reinforced Retaining Well Design Portland, Oregon November 5, 2003 200 pounds per square foot (psf), and an internal angle of friction of 30 degrees. For the aggregate backfill, we assumed a unit weight of 130 pcf and an internal angle of friction of 40 degrees. Since the walls will be located adjacent to the proposed roadways, we assumed a live load surcharge of 200 psf. We analyzed the wall heights with a three -foot high 2H:1V sloping backfill. Geo -grid Retaining Wall Recommendations Based on the results of our analyses, we determined the minimum geo -grid lengths shown below for static loading conditions: Geo -grid Locations _Wall Height I Geo -grid Location' Minimum Length of (feet) (feet) Geo -grid (feet) 2 • 1.29 3.0 4 1.29 3.0 3.23 3.0 129 3.0 6 5.17 4.5 • Measured from bottom of wall The following recommendations should be incorporated into the plans and construction of the walls: • The walls should be founded on undisturbed stiff to very stiff native silt subgrade or on new structural fill that is properly installed during construction, • The walls should be constructed on level surfaces (i.e. benches). • The foundation for the walls should consist of a minimum of six inches of 1 -inch -minus crushed rock or approved equivalent compacted to a minimum of 95 percent of the material's maximum dry density, as determined in accordance with ASTM D1557. The crushed rock layer should extend a minimum of 6 inches beyond the front and back faces of the wall and should be founded on undisturbed, native subgrade or on new structural fill that is properly installed during construction. • The foundation for the walls should be set back a minimum of 10 feet from the nearest slope as measured from the face of the wall to the crest of the nearest slope. - - Carlson Geotechnical Page 2 of 5 • Bonita Town Homes Reinforced Retaining Wall Design Portland, Oregon November 5, 2003 • The wall should be .embedded a minimum of six inches, or one regular block height, whichever is greater, below surrounding site grade. • A 4- inch - diameter, perforated, plastic drainpipe, with a minimum slope of one percent, should be located at the base of the walls and should be connected to a positive outlet. • The wall should be constructed with standard sized blocks that measure a minimum of 0.65 feet high by 1.47 feet long by 0.97 feet deep. • The reinforcement layers should consist of Miragrid 3XT or approved equivalent, and should be installed in the minimum lengths shown above, with the strong axis perpendicular to the wall. Adjacent and adjoining rolls shall be overlapped a minimum of one foot, and tied in accordance with the manufacturer's recommendations and details. • A wall batter of six degrees should be maintained. • Backfill for the walls should slope at a maximum gradient of 2H:1V with a maximum slope height of three vertical feet. • Backfill in the reinforced area behind the walls should consist of %- inch -minus crushed rock or an approved equivalent. Native material may not be used as backlit' in the reinforced area behind the walls. Backfill material should be placed in lifts with a maximum uncompacted thickness of 12 inches and compacted to not less than 90 percent of the material's maximum dry density, as determined by ASTM D1557. Backfill placed within three feet of the back of the wall should be compacted using hand - operated compaction equipment to ensure that undue lateral loads are not placed on the wail. Because the stability of a geo -grid reinforced wall is highly dependent on the quality of workmanship, we recommend that CGT provide observations during construction of the retaining walls. Drainage Considerations A minimum 4 -inch diameter wrapped perforated drain pipe should be installed along the base behind the wall to relieve hydrostatic pressure and seepage. Subgrade surfaces should be sloped such that the water collected by the wall drain is collected and routed to suitable discharge points. Permanent Slopes Permanent slopes should not exceed 2H:1V. Adjacent on -site and off -site structures and surfacing should be located at least 5 feet from the top of slopes. Footings should have a minimum set back of 10 feet between the face of the slope and the outer edge of the footing. Carlson Geotechnical Page 3 015 11 /vU( vv ..u✓ av vv .•a•. vvv ... .. ..� -. _.- .._. -_•. - __ -_.. _. .. ._ Bonita • wn Homes Reinforc -d Retaining Wall Design Portland Oregon Novemb r 5, 2003 Excava ions should not extend within a 1 H:1 V line projected from the bo om outside edge of the too Zings. OBSE • VATION OF CONSTRUCTION Satisfa • tory earthwork and wall performance depends to a large degr: e on the quality of constru tion. Sufficient observation of the contractor's activities is a key pa of determining that the wo k is completed in accordance with the construction drawings and specifications. Subsu : ce conditions observed during construction should be co pared with those encoun iered during subsurface explorations, and recognition of chang- • conditions often require- experience. We recommend that qualified personnel visit th= site with sufficient frequen to detect whether subsurface conditions change significantly fro those observed to date an anticipated in this report. We recommend that site stripping, rough grading, foundation subgrade -, and placement of enginee ed fill. and wall backfill are observed by the project geotechnical engineer or their represe tative. Because observation is typically performed on an on -call basis, we recommend that the - arthwork contractor be held contractually responsible for schedulin ! observation. LIMITA IONS We hay- prepared this report for use by the owner /developer and other me bers of the design and con - truction team for the proposed development. The opinions ano recommendations containe • within this report are not intended to be, nor should they be cons rued as a warranty of subsu ace conditions, but are forwarded to assist in the planning and des gn process. We hay= made observations based on our explorations that indicate the s • I conditions at only those s•ecific locations and only to the depths penetrated. These observations do not necessa ily reflect soil types, strata thickness, or water level variations that may exist between explorations. If subsurface conditions vary from those encountered In our si e exploration, CGT should b- alerted to the change in conditions so that we may provide add tional geotechnical recomm= ndations, if necessary. Observation by experienced geotechnical • - rsonnel should be consider- d an integral part of the construction process. The own - r /developer is responsible for insuring that the project designe s and contractors impteme t our recommendations. When the design has been finalized, we r-commend that the design a d specifications be reviewed by our firm to see that our recomme dations have been interprets• and implemented as intended. If design changes are made, we equest that we be retained • review our conclusions and recommendations and to provide a Mitten modification or verifi ion. Carlson Ge. technical Page 4 of 5 li/ VV/ VV 11 La., LV vv •{aaa vvv v. v vrar v...uv v.. �uv��..0 - Bonita Town Homes - Reinforced Retaining Wall Design Portland, Oregon 'November 5, 2003 • Within the limitations of scope, schedule, and budget, our services have been executed in accordance with the generally accepted practices in this area at the time this report was prepared. No warranty or other conditions express or implied, should be understood. We appreciate the opportunity to serve as your geotechnical consultant on this project. Please contact us if you have any questions. Sincerely, CARLSON GEOTECHNICAL • 2 C.Ra nney 7T 1 , ... .N.. .. .. Geotechnical Staff V,' , r • EGO' . T,7 V -G . r ; nne M. emer P.E. Prin ipal G otec ical Engineer Ex71F E7s 13.31.0.+ chmen . Figure 1 — Wall Detail Doc ID: P:IGEOTECH\PROJECTS12003 ProjectslBonita Town Home Subdivision Geo -Grid Retaining Wall\ Wall design report.doc Carlson Geotechnical Page 5 of 5 it _°-aiv BONITA TOWN HOMES SUBDIVISION - GEO-GRID WALL CONSTRUCTION DETAIL • 3 foot max. Sloping Backfill at 2H :1 max. 1 Walt Batton 6 degrees • 9 41/ Geogrid 314 inch minus crushed rock backfill compacted to at least 90% Minimum embedment of ASTM 0.1557 6 inches or one block height —�•.• L Infill Soil 4 Inch Dia. backfill drain 6 inch min. i 4 inch dia. Subdraln, subject to 1 ft Soil Engineer's review 1 inch minus crushed rock compacted to at least 95% , 2 ft minimum ASTM D -1557 Note: *Keyway, Subdrains, Foundation and Wail Embedment should be observed by CGT. 'Foundation for walls requires minimum 6 -inch base (non - reinforced) 'Geogrid lengths and locations as noted In text 'Drawing Not to Scale owes '4, Carlson Geotechnical .EOTE P.O. Box 23814 CGT Job No. G0302180 FIGURE 1 s03•584.3460 Tigard, Oregon 97281