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HomeMy WebLinkAboutGREENSTONE PUD PHASE 3 - Filed SER-SUBSURFACE EXPLORATION REPORT -SUBSURFACE EXPLORATION REPORT GREENSTONE PUD, PHASE III FORT COLLINS, COLORADO Prepared for: Stoner and Company Real Estate Prepared by: Earth Engineering Consultants, Inc. I r Earth Engineering Consultants, Inc. 2600 Canton Court, Suite A Fort Collins, Colorado 80525 303)224-1522 Fax: 224-4564 r June 4, 1993 Stoner & Company Real Estate 605 South College Avenue Fort Collins, Colorado 80524 Attn: Mr. Jay Stoner Re: Subsurface Exploration Report Proposed Greenstone PUD, Phase III Fort Collins, Colorado EEC Project 1932009 Mr. Stoner: Enclosed, herewith, are the results of the subsurface exploration performed for the reference project. In summary, the soils i encountered in the exploration borings performed on this site consisted of lean clay with varying amounts of sand and silt. Those soils were underlain at varying depths by highly weathered bedrock consisting predominately of silty shale in the South portion of the site and sandstone/siltstone in the northern portion. It is our opinion the proposed single family residences could be supported on footing foundations bearing on the natural site soils or underlying bedrock. These materials could also be used for direct support of floor slabs and pavements for the development. With the near surface cohesive soils and highly weathered shale, some movement of lightly loaded elements should be anticipated. Geotechnical recommendations concerning design and construction of the foundations and support of floor slabs and pavements are presented in the attached report. We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or .if we can be of further service to you in any other way, please do not hesitate to contact us. 7 Very truly yours, Earth Engineering Consultants, Inc. Lester L Litton, P.E. Colorado Number 23957 Curtiss L. (alin, P.E. Colorado Number 27315 cc: 1) Parsons & Associates; Mr. Gary Odehnal 2) Empire Construction Management; Mr. DeWayne Walker 7 J SUBSURFACE EXPLORATION REPORT PROPOSED GREENSTONE PUD, PHASE III i FORT COLLINS, COLORADO EEC PROJECT 1932009 June 6, 1993 INTRODUCTION The subsurface exploration for the proposed Greenstone PUD, Phase III, has been completed. As a part of this exploration, sixteen 16) soil borings extending to depths ranging from approximately 10 to 20 feet below existing site grades were advanced to develop information on existing subsurface conditions. Individual boring logs and a diagram showing the approximate boring locations are included with this report. The 3rd Phase of the Greenstone Development will be constructed to the east of the first and second phases. The Greenstone Development is generally located north of Larimer County Road 32 a approximately 1/2 mile east of Lemay Avenue in Fort Collins. As a part of Phase III of the development 36 lots for single family residential construction will be developed. The residences are expected to be one or two story wood frame structures which will likely include full basements. Foundation loads for those structures are expected to be light with continuous wall loads less than three kips per lineal foot and column loads less than 30 kips. Except for the basement excavations, cuts and fills of less than 2 to 3 feet are expected to develop the site grades. As a part of this project, County Road 32 will be upgraded in the immediate vicinity of the development and interior roadways will be constructed. Projected traffic volumes for those roadways will be obtained from the City of Fort Collins Engineering Department although volumes have been estimated as a part of this report to develop preliminary pavement section recommendations. A detention pond will be constructed near the east -central portion of the site. An approximate configuration of the proposed site improvements is shown on the attached boring location diagram. i r JCaM Engincering Consultants, Inc. Greenstone PUD, Phase III June. 6, 1993 J Page 2 JThe purpose of this report is to describe the subsurface conditions encountered in the borings, analyze and evaluate the test data and provide recommendations concerning design and construction of foundations and support of floor slabs and pavements. Preliminary recommendations concerning design of the structural pavement Jsections for the interior roadways and upgrade of County Road 32 are also provided. EXPLORATION AND TESTING PROCEDURES The boring locations were determined by Earth Engineering Consultants, Inc. (EEC) personnel. Those locations were established in the field by EEC personnel by pacing and estimating angles from identifiable site references. The surface elevations at the boring locations (as indicated on the boring logs) were estimated by plotting the approximate boring locations on a topographic site plan and interpolating between the plan contours. The locations and elevations of the borings should be considered accurate only to the degree implied by the methods used to make the measurements. The borings were performed using a truck -mounted, Dietrich D-50 drill rig equipped with a hydraulic head employed in drilling and sampling operations. The boreholes were advanced using continuous flight augers and samples of the subsurface materials encountered J in the borings were obtained using thin -walled tube and split - barrel sampling procedures in general accordance with ASTM Specifications D-1587 and D-1586, respectively. In the thin -walled tube sampling procedure, a seamless steel tube with a sharpened cutting edge is pushed into the soil with hydraulic pressure to J obtain a relatively undisturbed sample of cohesive or moderately cohesive material. In the split -barrel sampling procedure, a J standard 2-inch O.D. split -barrel sampling spoon is driven into the ground by means of a 140 pound hammer falling a distance of 30 J 7 Greenstone PUD, Phase III June 6, 1993 Page 3 Earth Engineering Consultants, Inc. inches. The number of blows required to advance the split -barrel sampler is recorded and is used to estimate the in -situ relative density of cohesionless soils and, to a lesser degree of accuracy, the consistency of cohesive soils and hardness of weathered bedrock. All samples obtained in the field were sealed and returned to the laboratory for further examination, classification, and testing. At three of the boring locations, after completing the borings and obtaining the samples, field slotted PVC piezometers were placed in the open bore holes. Those piezometers were backfilled with auger cuttings and the piezometer pipes were covered with PVC slip caps. The piezometers were installed to allow for longer term observations of groundwater levels. That monitoring will be completed by others. Laboratory testing of the recovered samples included moisture content tests on all of the samples and dry density and unconfined strength tests on selected samples. Atterberg limits, minus 200 washed sieve analysis and swell/consolidation tests were also performed on selected samples. Results of the laboratory testing are shown on the attached summary sheets and boring logs. One JHveem R-Value test is being performed; however, it was not completed at the time of this report. As a part of the testing program, all samples were examined in the laboratory by an engineer and classified in accordance with the attached General Notes and the Unified Soil Classification System, based on the soil's texture and plasticity. The estimated group symbol for the Unified Soil Classification System is shown on the boring logs and a brief description of that classification system J is included with this report. 1 J Earth Engineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 4 SITE AND SUBSURFACE CONDITIONS The Greenstone PUD Development is located north of Larimer County Road 32, approximately 1/2 mile east of Lemay Avenue in Fort Collins, Colorado. Site drainage in the Phase III area is generally to the east with maximum difference in ground surface elevations across the site on the order of 60 feet. The development area is presently open agricultural land. Evidence of prior building construction was not observed by EEC personnel during the field exploration. An irrigation ditch appeared to be running on the south side of County Road 32 at the time of our field exploration. According to area geologic maps, the near surface soils at the project site are Eolium deposits (wind blown materials) of the upper Holocene geologic period. We would expect a portion of the materials in the drainage ways to be recent alluvial deposits. Those materials are reportedly underlain by the Pierre Shale formation (upper shale member) which consists of silty gray shale and friable sandstone in this vicinity. The shale bedrock would be expected to extend to depths of 2000 to 3000 feet below the project site. A An EEC engineer developed field logs of the materials encountered during drilling based on observation of recovered samples and auger cuttings. The boring logs included with this report are an interpretation of those field logs and may contain modifications based on results of laboratory observation and testing. Based on the results of the field borings and laboratory testing, subsurface conditions can be generalized as follows. Approximately 4 to 6 inches of vegetation and/or topsoil was encountered at the ground surface at the boring locations. The topsoil/vegetation was typically underlain by brown, lean clay r Lanh Fnginccring Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 5 containing varying amounts of silt and sand. The site cohesive materials contained occasional calcareous zones and extended to depths of approximately 4 to greater than 19 feet. The cohesive soils were absent in borings B-15 and B-16 at the north end of the site and extended to the bottom of borings B-3 and B-11. The cohesive materials in borings B-1, B-2, B-6, B-7, and B-8 were underlain by clayey fine sand. The predominately granular soils were loose to medium dense and extended to the bottom borings B-1 and B-2 and to depths of approximately 12 to 14 feet at the other locations. The essentially granular soils in borings B-6, B-7, and B-8 and cohesive soils at the other locations, were underlain by highly weathered bedrock. The bedrock materials consisted of silty shale in the south portion of the site and sandstone/siltstone in the north portion. The shale bedrock was gray and soft to moderately hard. The sandstone was colored tan to olive brown and was poorly cemented. The bedrock materials extended to the bottom of the borings at depths of approximately 15 feet below ground surface. The bedrock was generally less weathered with depth. The stratification boundaries shown on the boring logs represent J the approximate location of changes in soil and rock types; in- 1 situ, the transition of materials may be gradual and indistinct. WATER LEVEL OBSERVATIONS Observations were made while drilling and after completion of the borings to detect the presence and level of apparent ground water. In addition, field slotted piezometers were installed at three of the boring locations to allow AN for a longer term water level observations. During drilling, water levels were observed in borings B-1, B-2, B-3, B-4, and B-6 at depths of approximately 6 to AN i 8 feet below present ground surface. Approximately 24 hours after drilling, water was observed in the piezometers at depths of i Earth fsgineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 6 y approximately 6 to 11 feet with no water observed at the location of B-14. Based on the field observations performed at the time of this report, we estimate groundwater levels at a depth of approximately 6 to 10 feet underlying the south portion of the site. We suspect the higher water levels in this area may be caused by irrigation of the property to the south and development J of perched water in the more permeable silty sand zone. Zones of perched and/or trapped water may be encountered in more permeable zones within subgrade soils and perched water is commonly encountered in soils overlying less permeable highly weathered J bedrock. J The location of the groundwater table may vary with time depending on variations and hydrologic conditions and other conditions not apparent at the time of this report. Zones of perched water may also vary in location and depth with hydrologic conditions and other conditions not apparent at the time of this report. ANALYSIS AND RECOMMENDATIONS FOUNDATIONS Based on the materials observed in the test borings locations, it is our opinion the proposed lightly loaded foundations could be supported directly on the natural site soils or highly weathered bedrock. For design of footing foundations bearing in the natural, stiff lean clay with varying amounts of sand and silt, we recommend s using a net allowable total load soil bearing pressure not to exceed 2000 psf. If foundations for the residences will bear within the near surface highly weathered bedrock stratum, we recommend using a net allowable total load bearing pressure not to exceed 4000 psf. The net bearing pressure refers to the pressure A. at foundation bearing level in excess of the minimum surrounding overburden pressure; the total load pressure refers to dead and s 7 PAM Engineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 7 full live loads. We recommend a minimum dead load of 1000 psf be used for the foundations bearing on the bedrock stratum to reduce post -construction heaving. Exterior foundations and foundations in unheated areas should be located a minimum of 30 inches below adjacent exterior grade to provide frost protection. Grade beam foundations (trenched foundations) could be used in the near surface cohesive soils and highly weathered bedrock. If used, we recommend those foundations extend a minimum of 42 inches below adjacent exterior grade. If a portion of the footings for a residence (excluding the garage foundations) would extend to bear in the highly weathered bedrock, we recommend extending the rest of the foundations to the bedrock stratum to reduce the potential for differential movement. Differential movement may occur between portions of the structures supported on significantly different materials. No unusual problems are anticipated in completing the excavations required for construction of the footing foundations. Care should be taken during construction to minimize disturbance to the bearing soils particularly in the area where wetter silty sands are encountered. Soils which are loosened or disturbed by the construction activities or materials which are wet and softened or dry and desiccated, should be removed from the foundation J excavations prior to placement of reinforcing steel and foundation concrete. In the areas of saturated silty soils, removal and replacement of a portion of the heaving soils may be required if foundations extend to near the water levels. Drilled piers could also be considered for foundation support in those areas and we would be pleased to provide additional recommendations at your request. We estimate the long term settlement of footing foundations designed and constructed as recommended above would be small, less J J J Latli Engineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 8 J than 3/4-inch. For footing foundations supported on the bedrock stratum, the long term settlement should be less than 1/2-inch. FLOOR SLAB AND PAVEMENT SUBGRADES JAll existing vegetation and/or topsoil should be removed from beneath floor slabs and pavements. After stripping and completing all cuts and prior to placement of any fill, floor slabs, or pavements, we recommend the exposed materials be scarified to a minimum depth of 9 inches, adjusted in moisture content and compacted to at least 95 percent of the material's maximum dry density as determined in accordance with ASTM Specification D-698, the standard Proctor procedure. The moisture content of the scarified soils should be adjusted to within the range of minus 1 to plus 3 percent of standard Proctor optimum moisture. Fill required to develop the floor slab and pavement subgrades should consist of approved, low volume change material, free from J organic matter and debris. Normally, soils with a liquid limit of 40 or less and plasticity index of 18 or less could be used for low volume change fill. Based on the materials observed at the test i boring locations, it is our opinion the site cohesive soils could J be used for fill in the floor slab and pavement areas although they may be slightly above the low -volume change material criteria. Those fill soils should be placed in loose lifts not to exceed 9 J inches thick, adjust in moisture content as recommended for the scarified soils and compacted to at least 95 percent of the J material's standard Proctor maximum dry density. The near surface site materials are slightly to moderately plastic and some volume change may occur in these materials with fluctuations in moisture contents. Placing the materials with moisture contents near optimum and maintaining those moisture contents prior to construction of the floor slabs or pavements will i J Greenstone PUD, Phase III June 6, 1993 Page 9 Earth P.ngincering Consultants, Inc. j be helpful in reducing the potential for post -construction movement JJ of these materials. If the prepared subgrades become dry and desiccated or wet and softened, those materials should be removed and replaced or reworked in place prior to construction of the overlying floor slabs or pavements. The above recommendations for moisture control of fill placed below the floor slabs will help reduce but not eliminate post construction movement of those lightly loaded elements. Construction of a zone of low volume change material beneath the floor slabs and pavements could be considered to further reduce the potential for movement. Use of structural floor systems j JJ constructed above void areas would be required to eliminate t e d potential for movement. JBELOW GRADE AREAS ' All below grade walls for the new residences should be designed to resist hydrostatic loads or a perimeter drainage system designed and constructed to effectively eliminate the build-up of hydrostatic pressures. In general, a perimeter drain system would include perforated metal or plastic drainage line placed at approximate foundation bearing level and sloped to a sump area where water may be removed without reverse flow into the system. JThe drain line should be surrounded by a minimum of 6 inches of appropriately sized permeable granular filter soil. The drain line or filter soil should be surrounded by an appropriate filter fabric Jto reduce the potential for the influx of fines and eventual clogging of the system. J Lateral earth pressures on the below grade walls would vary with the materials used as wall backfill. For walls backfilled with the J near surface site cohesive materials, we recommend using an equivalent fluid pressure of 55 pounds per cubic foot. Those Earth Engineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 10 materials should be compacted using appropriate hand operated equipment to densities of at least 90 percent of standard Proctor maximum dry density. Areas under sidewalks or driveways should be compacted to the 95 percent minimum of standard Proctor maximum dry density as recommended for pavement and floor slab areas. The allowable lift thickness would vary with the type of compaction equipment used for the soils; however, lift thickness generally should not exceed 9 inches in loose thickness. OTHER CONSIDERATIONS Positive drainage should be developed away from the proposed structures with a minimum 10 percent slope established for the 10 feet perimeter around the structures. Roof drains should be designed to discharge water beyond the backf ill areas and sprinkler systems should be designed and constructed to avoid spraying water on the foundation elements. Sprinkler systems should also be designed with water lines placed outside of the wall backfill. Plantings should not be placed adjacent to the foundation walls. Floor slabs for the residences should be effectively isolated from structural elements and non -load bearing partitions. We recommend a minimum 1 1/2-inch void be developed between the partitions and the floor system to prevent transmission of uplift forces to the upper floor and/or roof systems. All site excavations should be cut with stable side slopes. Local codes and/or appropriate OSHA regulations should be complied with for the site excavations. If the sloped excavations are not feasible in some areas of the site, shoring and bracing should be used to develop stable below grade excavations. Eardi Engineering Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 11 DETENTION POND AREA To reduce seepage from the detention pond areas, we recommend scarification and compaction of the pond sides and bottoms be completed after excavating to design grades. In general, the exposed soils should be scarified to a minimum depth of 9 inches, adjusted moisture content as recommended for the scarified soils beneath floor slabs and pavements and compacted to at least 95 percent of standard Proctor maximum dry density. Further reduction in seepage loss from the pond areas could be achieved by blending site materials with bentonite or by developing a clay liner for the entire area. We would be pleased to provide additional information regarding these alternatives, if desired. PAVEMENTS / Equivalent daily load axles (EDLA's) of 5, 6, and 120kare estimated r for Bluewater Courts and Streamside Drive, Greenstone-Tral-1-and County Road 32, respectively, based on previous designs completed for Phases I and II of the Greenstone PUD. Those estimated traffic projections should be verifi'd with the City of Fort Collins Engineering staff. Previous laboratory testing indicated an R- value of 2.5 for near surface cohesive material at this site. That R-value would correspond to a resilient modulas value (Mk) of approximately 2400 using the American Association of State Highway and Transportation Officials (AASHTO) correlation. Additional R- value testing is being performed to verify the value for the near surface lean clay in the Phase III area. The 1986 AASHTO "Guide For Design of Pavement Structures" was used to help evaluate alternative pavement sections for this project. A reliability factor of 70 percent was used for the local streets and 90 percent used for the County Road 32 arterial. Alternative pavement design recommendations are provided below in Table I based I Panh P.nginccring Consultants, Inc. Greenstone PUD, Phase III June 6, 1993 Page 12 on that analysis. Other alternative pavement sections could be considered and we would be pleased to review additional pavement alternatives to be considered for use on this project. TABLE I - Recommend Pavement Sections Bluewater Cts Greenstone County Road 32 Streamside Dr Trail Composite Pavement HBP Surface 3" 3" 3" Bituminous Base 0 0 7" Aggregate Base 12" 13" 12" Clay Subgrade 9" Reworked 9" Reworked 9" Reworked Aggregate base in the asphalt pavement areas should consist of materials consistent with Colorado Department of Transportation CDOT) standard specifications for Class 5 or Class 6 base. The asphalt surfacing should be compatible with City of Fort Collins designs specifications for SCl or SC2 Hot Bituminous pavement. The aggregate base course material should be placed and compacted as I recommended for the fill soils beneath the roadways. The asphalt surfacing should be placed and compacted in accordance with City of i Fort Collins standards specifications. CO GENERAL COMMENTS The analysis and recommendations presented in this report are based upon the data obtained from the soil borings performed at the indicated locations and from any other information discussed in this report. This report does not reflect any variations which may occur between borings or across the site. The nature and extent of such variations may not become evident until construction. If variations appear evident, it will be necessary to re-evaluate the recommendations of this report. 1 ea Lurch FAgincering ConsulMnts, Inc. Greenstone PUD, Phase III June 6, 1993 Page 13 b It is recommended that the geotechnical engineer be retained to review the plans and specifications so that comments can be made regarding the interpretation and implementation of our geotechnical recommendations in the design and specifications. It is further recommended that the geotechnical engineer be retained for testing and observations during earthwork and foundation construction phases to help determine that the design requirements are fulfilled. a This report has been prepared for the exclusive use of Stoner and Company for specific application to the project discussed and has been prepared in accordance with generally accepted geotechnical engineering practices. No warranty, express or implied, is made. a In the event that any changes in the nature, design or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless the changes are reviewed and the conclusions of this report modified or verified in writing by the engineer. ineer. 9 9 43 v Sz 9 9 9L \ Boring Location Diagram Greenstone PUD, Phase III Fort Collins, Colorado Drawn: LLL Scale: None