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HomeMy WebLinkAboutGREENSTONE PUD PHASE 2 - Filed SER-SUBSURFACE EXPLORATION REPORT -SUBSURFACE EXPLORATION REPORT GREENSTONE P.U.D., PRASE II LARIMER COUNTY, COLORADO JOB NO. 20935031 7 0 11 11 e 4 4 Empire Laboratories, Inc. A Division of The Terracon Companies, Inc. P.O. Box 503 • 301 No. Howes Fort Collins, Colorado 80522 303) 484-0359 FAX No. (303) 484-0454 February 23, 1993 Parsons & Associates 432 South Link Lane Fort Collins, CO 80524 ATTN: Mr. Gary Odehnal, P.E. RE: Subsurface Exploration Report Greenstone P.U.D., Phase II Larimer County, Colorado Job No. 20935031 Mr. Odehnal: Enclosed, herewith, are the results of the subsurface exploration performed for the referenced project. In summary, near surface soils encountered at the site consisted of low plasticity lean clays with varying amounts of sand and silt. These materials were underlain at depths ranging from approximately 2 to greater than 15 feet by highly weathered to weathered bedrock. A majority of the bedrock consisted of sandstone/siltstone materials although predominately claystone material was encountered in one of the borings. In the lower areas of the site, groundwater was encountered at shallow depth, on the order of 2 to 4 feet. In summary, we expect homes constructed in this area could be supported on conventional footing foundations. However, care will be necessary in low lying areas of the site to medicate an apparent high groundwater table. Geotechnical recommendations concerning design and construction of foundations and support of floor slabs and pavements are presented in the text of this 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. Very truly yours, EMPIRE LABORATORIES, INC. A DIVI I OF T T CON COMPANIES, Lester L. Litton, P.E. Principal Engineer INC. 46ink <J C:'1 a p n, ,(;i r" ? r Chester C. Smith, P.E. '! Division Manager Geotechnical, Environmental and Materials Engineers Offices of The Tertacon Arizona: Tucson Colorado: Colorado Springs, Denver, Ft. Collins, Greeley, Longmont Idaho: Boise Illinois: Bloomington, Chicago, Rock Island Iowa: Cedar Falls, Cedar Rapids, Davenport, Des Moines, Storm Lake Kansas: Lenexa, Topeka, Wichita Minnesota: St. Paul IIIMissouri: Kansas City Nebraska: Lincoln, Omaha Oklahoma: Oklahoma City, Tulsa Texas: Dallas Utah: Salt Lake City Wyoming: Cheyenne QUALITY ENGINEERING SINCE 1965 Terracon SUBSURFACE EXPLORATION REPORT PROPOSED GREENSTONE P.U.D., PHASE II LARIMER COUNTY, COLORADO Job No: 20935031 Date: February 23, 1993 INTRODUCTION The subsurface exploration for the proposed Greenstone P.U.D., Phase II near County Road 32 and Lemay Avenue in Larimer County, Colorado, has been completed. Ten soil borings extending to depths of approximately 15 feet below present site grades were advanced to develop information on subsurface conditions in the proposed Phase II development area. Individual boring logs and a diagram showing the approximate boring locations are included in this report. We understand the second phase of the Greenstone P.U.D. will involve the development of 42 single-family housing lots. The Phase II area will generally be to the west and north of the Greenstone Phase I area. The Greenstone development is presently outside the City of Fort Collins boundaries; however, we understand will be annexed into the city. We anticipate the proposed residences for the Greenstone development will be one or two-story wood frame houses which will likely contain full basements. Foundation loads for the residences will be light with continuous wall loads less than 3 kips per lineal foot and column loads less than 30 kips. Floor loads will be light, less than 100 psf. Except for basement excavations, grade changes less than 2 feet are expected to develop the site grades. The 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. Job No. 20935031 Terracon February 23, 1993 Page 2 EXPLORATION AND TESTING PROCEDURES The boring locations were selected by Empire Laboratories, Inc. personnel and located in the field by drilling personnel. Those locations were established by pacing and estimating angles from the references shown on the attached boring location diagram. Surface elevations at the boring locations were interpolated from 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 field measurements. The borings were performed using a truck -mounted, CME-55 drill rig equipped with a hydraulic head employed in drilling and sampling operations. The borings were advanced using continuous flight augers and samples of the subsurface materials encountered 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 obtain a relatively undisturbed sample of cohesive or moderately cohesive material. A CME automatic sampling hammer was used for driving the split -barrel sampler. 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. A field engineer from Empire Laboratories, Inc. was on site during drilling to evaluate the materials encountered and adjust the sampling program to meet the field conditions. The field engineer also maintained a log of the materials encountered in the drilling program. All samples obtained in the field were sealed and Job No. 20935031 Terracon February 23, 1993 Page 3 returned to the laboratory for further examination, classification Nand testing. Moisture content, dry density and unconfined strength tests were performed on representative portions of the samples recovered in the thin -walled tubes. Moisture content tests were completed on representative portions of the samples recovered from the split - barrel sampler. In addition, Atterberg Limits, swell, consolidation, washed sieve and soluble sulfate tests were performed on selected samples. One Hveem stabilometer R-value test was also conducted on a remolded sample of the near surface highly weathered sandstone. Results of the outlined tests are shown on the attached boring logs and summary sheets. 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 in the appropriate column on the boring logs and a brief description of that classification system is included with this report. SITE AND SUBSURFACE CONDITIONS The Greenstone P.U.D. is located north of County Road 32 (the Windsor Road) and east of Lemay Avenue in Larimer County, Colorado. The Phase II area will be constructed to the west and north of the Phase I development. The site is currently open pasture and planted fields. Site drainage is generally to the north and west in the Phase II area, towards Stanton Creek. Approximately 30 feet of fall was noted across the building areas of the site. L 2 Job No. 20935031 Terracon February 23, 1993 Page 4 The field logs prepared by the Empire Laboratories field engineer were based on visual and tactual observation of recovered samples and auger cuttings. The boring logs included with this report may contain modifications to the field logs based on the results of laboratory testing and observation. Based on the results of our exploration and testing program, subsurface conditions in the Phase II area can generally be described as follows. Approximately 6 inches of vegetation and/or topsoil was encountered at the surface at the boring locations. The topsoil/vegetation was underlain by lean clay with varying amounts of sand and silt. In boring B-1, the cohesive soils extended to the bottom of the boring at a depth of approximately 15 feet. At the other boring locations, the cohesive materials extended to depths ranging from approximately lZ to 7? feet. The consistency of the overburden soils range from medium to very stiff with a majority of the materials exhibiting a medium to stiff consistency. The cohesive overburden soils were underlain by highly weathered to weathered bedrock. In boring B-2, the bedrock consisted of claystone/siltstone with the bedrock described as sandstone or sandstone/siltstone at the other boring locations. The bedrock was generally soft or poorly cemented near ground surface and became progressively less weathered and harder (more well cemented) with depth. The borings were terminated at depths of approximately 15 feet in weathered bedrock. The stratification boundaries shown on the boring logs represent the approximate location of changes in soil and rock types; in - situ, the transition of the materials may be gradual and indistinct. Classification of the bedrock was based on visual and tactual observation of disturbed samples and auger cuttings. Coring and/or petrographic analysis may reveal other rock types. Job No. 20935031 Terracon February 23, 1993 Page 5 WATER LEVEL OBSERVATIONS Observations were made while drilling and after completion of the borings to detect the presence and level of free water. Free water was observed only in borings B-1 and B-2 at these times. At those locations, the observed free water levels were 6 feet and 2 feet, respectively, at the time of drilling and 4 feet and 2 feet, respectively, approximately 24 hours after completion of the borings. The cohesive soils encountered have low permeabilities so that longer term observations, which should include the installation of piezometer and/or monitoring wells which are sealed from the influence of surface water, would be necessary to more accurately evaluate groundwater conditions. Fluctuations in groundwater levels can occur throughout time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. In addition, zones of perched and/or trapped water can occur in more permeable zones interbedded with low permeability cohesive materials and weathered bedrock. The location and amount of perched water may also vary throughout time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. 4 ANALYSIS AND RECOMMENDATIONS Foundations Based on the materials encountered in the test borings, we anticipate conventional footing foundations for the proposed structures would be supported on medium to stiff lean clay in the lower portions of the site and highly weathered to weathered siltstone/sandstone bedrock at the upper areas of the site. For design of footing foundations bearing on the natural, medium to 0 Job No. 20935031 Terracon February 23, 1993 Page 6 stiff cohesive materials, we recommend using a net allowable total load soil bearing pressure not to exceed 1,000 psf. For design of footing foundations bearing entirely on highly weathered to weathered sandstone bedrock, we recommend using a net allowable total load soil bearing pressure not to exceed 4,000 psf. The net allowable bearing pressure refers to the pressure at foundation bearing level in excess of the minimum surrounding overburden pressure and the total load pressure refers to the full dead and live load expected on the foundation. In the bedrock areas, we recommend a minimum dead load of 1,000 psf be designed into the foundations to reduce the potential for post -construction movement of the foundations caused by heaving of the bearing materials. Exterior foundations and foundations in unheated areas should be located a minimum of 2? feet below adjacent exterior grade to provide frost protection. In the lean clay areas, we recommend formed continuous footings have a minimum width of 16 inches and isolated column foundations a minimum width of 30 inches. We recommend trenched footing foundations not be used in these areas. In the bedrock areas, thinner footing widths could be used and we anticipate trenched foundations would also be acceptable. In lower lying areas of the site, it appears as though free water may be encountered at relatively shallow depth. In these areas, we recommend non -basement construction be considered or means be evaluated to lower the groundwater table in this area. Evaluation of those considerations are beyond the scope of this report. Bedrock was encountered at shallow depths throughout much of the site. It has been our experience that bedrock which can be augered with conventional soil augers (such as the augers used on this site) can be excavated without the use of blasting or jackhammers; however, heavy duty equipment may be required. Zones r Job No. 20935031 Terracon February 23, 1993 Page 7 of more well cemented sandstone can be interbedded within the bedrock which would make excavating of this material more difficult. Care should be taken during construction to avoid disturbing the bearing materials. The near surface lean clays near the water table would be relatively easily disturbed. Any loosened or disturbed material, or any extremely wet or dry material, should be removed from the foundation excavations prior to placement of Ireinforcing steel and foundation concrete. In areas of the expected lean clay foundation bearing soils, overexcavation and backfill procedures could be considered to increase the allowable bearing pressure. We would be pleased to provide additional recommendations concerning overexcavation and backfill procedures, if desired. We estimate the long-term settlement of footing foundations designed and constructed as recommended outlined above would be small, less than 3/-inch. Smaller settlements would be expected in the sandstone bearing areas. Floor Slab and Pavement Subgrade All existing vegetation and/or topsoil should be removed from floor slab and pavement areas. After stripping and completing all cuts and prior to placement of any fill, floor slabs or pavements, we recommend the in -place soils be scarified to a minimum depth of 9 inches, adjusted in moisture content and compacted to at least 95% 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 site materials should be adjusted within the range of ±2% of standard Proctor optimum moisture. 4 Job No. 20935031 Terracon February 23, 1993 Page 8 Scarification and recompaction of subgrades in the basement areas would not be necessary prior to placement of those floor slabs. Also, in the lower areas of the site near the existing water table, scarification of recompaction of the subgrades may be difficult and use of subgrade stabilization may be required. Fill required to develop the subgrade levels should consist of approved, low -volume change material, free from organic matter and debris. Normally, soils with a liquid limit of 40 or less and plasticity index of 18 or less could be used as low -volume change fill. We recommend the fill materials contain a minimum of 15% fines, material passing the No. 200 sieve, to reduce the potential for ponding of water in the fill. The site lean clays and highly weathered bedrock could be used for low -volume change fill. Fill materials in the pavement or floor slab areas should be placed in loose lifts not to exceed 9 inches thick, adjusted in moisture content as recommended for the scarified soils and compacted to at least 95% of the material's maximum dry density as determined in accordance with the standard Proctor procedure. The moisture content of the fill soils should be adjusted within the range of 2% of standard Proctor optimum moisture. In the pavement areas, we recommend the top 9 inches of fill be compacted to at least 98% of standard Proctor maximum dry density. After preparation of the subgrades, care should be taken to avoid disturbing the in -place materials. Materials which are loosened or disturbed by construction activity should be removed and replaced or, if possible, reworked in place prior to placement of the floor Islabs or pavements. 4 Job No. 20935031 Terracon February 23, 1993 Page 9 Positive drainage should be developed across the pavements and away from the pavement edges and site structures. Ponding of water on r or adjacent to these areas may allow wetting and softening of bearing soils or pavement subgrade and associated unacceptable performance. IBelow Grade Areas We recommend below grade walls be designed to resist hydrostatic loads or that a perimeter dewatering system be installed to prevent the buildup of hydrostatic pressures. A perimeter drain system would also reduce the potential for water seepage into the below grade areas. In general, a perimeter drain system should consist of perforated metal or plastic pipe placed at approximate foundation bearing levels around the exterior perimeter of the below grade areas. The drain line should be sloped to drain to a sump area where water can be removed without reverse flow into the system. The drain line should be surrounded by at least 6-inches of appropriately sized granular filter soil. The use of filter fabric around the drain should be considered to reduce the potential for fines entering the system. For design of the below grade walls with an appropriate drain system, we recommend using an equivalent fluid pressure of 60 pcf. The recommended design pressure is based on an "at rest" soil condition and does not include a factor of safety. Pavements Estimated 18-kip equivalent daily axle loads of 5 were provided by City of Fort Collins Engineering Department for the local cul-de- sacs and streets. Those roadways include Creekside Drive (aka Brookside Drive), Creekside Courts (aka Brookside Courts), Gold 4 9 11 It 11 It It It It It It it I It It Job No. 20935031 Terracon February 23, 1993 Page 10 Hill Court and Silver Moon Lane. These roads are defined as local streets and, as such, reliabilities of 70% were assigned to those traffic volumes. A Hveem stabilometer R-value of 12 was determined for a representative sample of the near surface siltstone/sandstone bedrock. The R-value corresponds to a resilient modulus value of 8,050 using the American Association of State Highway and Transportation Officials (AASHTO) correlation guidelines. An R- value of approximately 2.6, corresponding to a resilient modulus value of 2,400, was established for the site lean clays in previous testing completed for Phase I of the development. The 1986 AASHTO "Guide for Design of Pavement Structures" was used to help evaluate alternative pavements for the project. Based on this evaluation, we recommend the pavement section outlined below in Table 1. mauT.F i * PRrnmmRNnRD PAVEMENT SECTIONS Sandstone Subgrades Clay Subgrades Composite Pavement HBP 311* 311 Aggregate Base 411* 12" PCC Pavement 5" 5" Minimum recommended section per city o= ZUL U k.ullllln M, CL11ucll1 specifications. Overexcavation and backfill procedures in subgrade areas composed of clay soils could be used to reduce the required thickness of aggregate base and asphalt surfacing required to construct the roadways. Developing a subbase with reworked site sandstone could be considered to develop the subbase. At least 18 inches of this material should be placed with the overlying minimum pavement section designated by the City of Fort Collins. Materials in the I Job No. 20935031 Terracon February 23, 1993 Page 11 subbase should be placed and compacted as previously recommended for fill beneath the pavements. Aggregate base course materials should be compatible with Colorado Department of Transportation (CDOT) standard specifications for Class 5 or Class 6 aggregate base. Those materials should be placed in maximum 9-inch thick loose lifts and compacted as recommended for fill beneath the pavements. Asphalt for use in the pavement areas should consist of high - quality, plant mix, hot bituminous pavement compatible with City of Fort Collins standard specifications for SC-1 or SC-2 material. These materials should be placed and compacted as outlined by City of Fort Collins standard specifications. 4 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. 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 observation during earthwork and foundation construction phases to help determine that the design requirements are fulfilled. 4 Job No. 20935031 Terracon February 23, 1993 Page 12 This report has been prepared for the exclusive use of our client 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. 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 geotechnical engineer. GENERAL NOTES I I I I I I I F1 DRILLING & SAMPLING SYMBOLS: SS Split Spoon - 1%" I.D., 2" O.D., unless otherwise noted PS Piston Sample ST Thin -Walled Tube - 2" O.D., Unless otherwise noted WS Wash Sample PA Power Auger FT Fish Tail Bit HA Hand Auger RB Rock Bit DB Diamond Bit - 4", N, B BS Bulk Sample AS Auger Sample PM Pressuremeter HS : Hollow Stem Auger DC Dutch Cone WB Wash Bore Standard "N" Penetration: Blows per foot of a 140 pound hammer falling 30 inches on a 2 inch OD split spoon, except where noted. WATER LEVEL MEASUREMENT SYMBOLS: WL Water Level WS While Sampling WCI Wet Cave In WD While Drilling DCI Dry Cave In BCR Before Casing Removal AB After Boring ACR After Casing Removal Water levels indicated on the boring logs are the levels measured in the borings at the times indicated. In pervious soils, the indicated levels may reflect the location of groundwater. In low permeability soils, the accurate deter- mination of ground water levels is not possible with only short term observations. DESCRIPTIVE SOIL CLASSIFICATION: Soil Classification is based on the Unified Soil Classification System and ASTM Designations D-2487 and D-2488. Coarse Grained Soils have more than 50% of their dry weight retained on a #200 sieve; they are described as: boulders, cobbles, gravel or sand. Fine Grained Soils have less than 50% of their dry weight retained on a #200 sieve; they are described as: clays, if they are plastic, and silts if they are slightly plastic or non -plastic. Major con- stituents may be added as modifiers and minor constituents may be added according to the relative proportions based on grain size. In addition to gradation, coarse grained soils are defined on the basis of their relative in -place density and fine grained soils on the basis of their consistency. Example: Lean clay with sand, trace gravel, stiff CL); silty sand, trace gravel, medium dense (SM). CONSISTENCY OF FINE-GRAINED SOILS: Unconfined Compressive Strength, Qu, psf Consistency 500 Very Soft 500 - 1,000 Soft 1,001 2,000 Medium 2,001 4,000 Stiff 4,001 - 8,000 Very Stiff 8,001-16,000 Hard 16,000 Very Hard RELATIVE PROPORTIONS OF SAND AND GRAVEL Descriptive Term(s) of Components Also Percent of Present in Sample) Dry Weight Trace 15 With 15 - 29 Modifier 30 RELATIVE PROPORTIONS OF FINES Descriptive Term(s) of Components Also Percent of Present in Sample) Dry Weight Trace < 5 With 5 - 12 Modifier > 12 RELATIVE DENSITY OF COARSE -GRAINED SOILS: N-Blowslft. Relative Density 0-3 Very Loose 4-9 Loose 10-29 Medium Dense 30-49 Dense 50-80 Very Dense 80+ Extremely Dense GRAIN SIZE TERMINOLOGY Major Component Of Sample Size Range Boulders Over 12 in. (300mm) Cobbles Gravel Sand Silt or Clay 12 in. to 3 in. 300mm to 75mm) 3 in. to #4 sieve 75mm to 4.75mm) 4 to #200 sieve 4.75mm to 0.075mm) Passing #200 sieve 0.075m m) Form 108-6-85 Nerracon TEST BORING LOCATION PLAN LZI L 18 L23 B Ida L 17 s J0.8 w LZq LS LIle L25 L71 L14 7 ST 1ToJ Liz' f1' L 5, }- L5 too. L55 rrr L`i L 5(0 r 1,7 L57 Sri i L too L4 Id L Co 2 r, I u u SE J Z f r I.t,3 L4p4 f f Lto5rr L7o o Z Lto7 L I L 9 L oB I I 4- 1 3a oil U 32Goti! TY b EMPIRE LABORATORIES, INC.