Loading...
HomeMy WebLinkAboutWEST PLUM STREET PUD - PRELIMINARY - 10-96 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTIEarth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 11 Stabilization of the subgrades could be considered to reduce the potential for instability during construction and could be used as part of the design pavement section. We would be pleased to provide additional information concerning stabilization of these subgrades, if desired. IGENERAL 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 observations during earthwork and foundation construction phases to help determine that the design requirements are fulfilled. This report has been prepared for the exclusive use of Messner Engineering Associates 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. Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 10 PAVEMENTS Subgrades for the pavements should be prepared as outlined for the floor slab subgrades. Positive drainage should be developed across the pavements and away from the pavement edges to avoid n wetting of the pavement .subgrades. Subgrades allowed to become wetted subsequent to ,I construction could result in premature failure of the pavements. We estimate a traffic loading of 5 equivalent daily load axles (EDLA) would be appropriate for n design of the interior streets for this development. A Hveem stabilometer R-value of 7 was 'I determined in testing of the site soils. That Hveem stabilometer R-value corresponds to a resilient modules value of 4885 using the American Association of State Highway Transportation Officials (AASHTO) correlation. The 1986 AASHTO publication "Design Guidelines of Pavement Structures" was used to evaluate alternative pavement sections for this project. Recommended pavement sections based on that evaluation are provided below in Table I. The assumed EDLA numbers will need to be verified with City of Fort Collins Engineering department prior to final design. 11 TABLE I - PRELIMINARY PAVEMENT DESIGN ACCOMMENDATIONS II a) Composite Section Surface Asphalt 3" Aggregate Base 6" c) Portland Cement Concrete 4 1/2" Asphaltic concrete used for the pavements should consist of high quality plant mix materials compatible with City of Fort Collins standard criteria for SCI or SCII blends. Base course materials should be compatible with Colorado Department of Transportation criteria for Class 5 or Class 6 base. Those materials should be placed and compacted in accordance with City of Fort Collins guidelines. Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 9 BELOW GRADE AREAS We recommend perimeter drains be constructed around all below grade areas to reduce the potential for development of hydrostatic loads on the below grade walls and to reduce the potential for seepage of infiltration water into the below grade areas. In general, a perimeter drain system would consist of perforated metal or plastic pipe placed around the exterior perimeter of the below grade area. The drain line should be placed at approximate foundation bearing level and should be sloped to provide positive gravity drainage to a sump area where it can be removed without reverse flow into the system. The drain line should be surrounded by a minimum of 6 inches of appropriately sized granular filter soil and the drain line or granular filter should be surrounded by an appropriately sized filter fabric to reduce the potential for infiltration of fines into the system. Backfill placed adjacent to the below grade walls should consist of approved, low -volume change materials free from organic matter and debris. We recommend those soils be placed in loose lifts not to exceed 9 inches thick, adjusted in moisture content as recommended for the subgrade fill materials and compacted to at least 90% of the material's standard Proctor maximum dry density. Backfill soils which will support driveways, patios, entryways, sidewalks or similar improvements should be compacted to at least 95 % of standard Proctor maximum dry density. Care should be taken when placing and compacting the backfill soils to avoid placing excessive lateral stresses on below grade walls. We recommend hand equipment or light mechanical equipment be used for the compaction of the backfill. Basement walls are commonly designed based on "active" lateral earth pressures. Active stress distribution assumes a slight rotation of the wall will occur. That rotation is approximately equal to 0.5 % of the wall height. We recommend an active lateral earth pressure, expressed as an equivalent fluid pressure, of 35 pounds per cubic foot be used for the below grade wall design. That pressure does not include an allowance for hydrostatic loading nor a factor of safety. Surcharge loads placed adjacent to below grade walls and/or point loads placed near the below grade walls could add to the lateral stresses on the below grade walls. Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 8 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 scarified soils should be adjusted to be within the range of ±2 % of standard Proctor optimum moisture. Scarification and recompaction of the subgrades in the below grade areas would not be required. Fill soils required to develop the floor slab subgrades should consist of approved, low -volume change materials which are 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. The near surface cohesive site soils could be used as fill in these areas although they may not strictly meet the requirements for low -volume change materials. The claystone bedrock should not be used for fill below slabs or pavements. Fill materials beneath floor slabs 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 standard Proctor maximum dry density. After preparation of the subgrades, care should be taken during to avoid disturbing the in -place materials. Materials which are loosened or disturbed by the construction activities or soils which are dry and desiccated or wet and softened should be removed and replaced or reworked in place prior to placement of the overlying floor slabs. Positive drainage should be developed away from the proposed structures with a minimum slope away from the structures of 1 inch per foot for the first 10 feet. Roof drains should be designed to discharge at least 5 feet away from the structures. Any lawn sprinking systems should be placed at least 5 feet beyond the perimeter of the structures and should be designed so as not to spray water on or immediately adjacent to the structure foundations. Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 7 replaced prior to placement of the foundation concrete. Care should be taken during construction to see that footing foundations are supported on suitable, non -expansive soils. Hand auger borings should be completed at several locations along the footing line to see that foundations are not placed on or immediately above the claystone bedrock. We estimate the long-term settlement of footing foundations designed and constructed as outlined above would be less than 3/4 inch. GENERAL The foundation type constructed throughout an individual structure should be consistent. Differential movement may occur for structures supported partially on drilled pier foundations and partly on footing foundations. FLOOR SLAB SUBGRADES For basement structures supported on drilled pier foundations, consideration should be given to supporting the basement floors as structural slabs. Placement of basement floor slabs on or directly above the weathered claystone bedrock may result in movement of the floor slabs and reflected distress to the rest of the structure. Non -basement structures or structures which are supported on footing foundations above the claystone bedrock, could use on -ground floor slabs. For the on -ground floor slabs, care should be taken to isolate the floor slabs from the structural elements of the buildings to reduce the potential for movement of the floor slabs causing damage to the remainder of the structure. Those details should include isolation joints around all structural elements and use of voided non -load bearing below grade walls. Door jambs should be provided with suitable voiding to allow movement and stairways should be suspended so they are not supported directly on the floor slabs. All topsoil and/or vegetation should be removed from beneath floor slabs and fill areas. After stripping and completing all cuts and prior to placement of any floor slabs or fill, we recommend Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 6 moisture fluctuations in the bearing materials. Care should also be taken during construction to avoid "mushrooming" the tops of the drilled piers. We estimate the long-term settlement of drilled pier foundations designed and constructed as recommended above would be small, less than 1/2 inch. FOOTING FOUNDATIONS If the design elevations of the structures are such that footings would not bear within 2 feet of the claystone bedrock, it is our opinion that footing foundations could be used for support of the proposed lightly loaded structures. We recommend those footing foundations extend through all existing vegetation and/or topsoil and bear in the natural, stiff to very stiff lean clay with varying amounts of sand and silt. For design. of footing foundations bearing in the natural, stiff to very stiff cohesive materials, we recommend using a net allowable total load soil bearing pressure not to exceed 2,500 psf. The net bearing pressure refers to the pressure at foundation bearing level ' in excess of the minimum surrounding overburden pressure. Total load should include full dead and live loads. We recommend a minimum dead load of 500 psf be used in design of the footing foundations to reduce the potential for wetting or drying of the cohesive soils causing movement of the footings. ' Exterior foundations and foundations in unheated areas should be located a minimum of 30 inches below adjacent exterior grades to provide frost protection. We recommend formed continuous footings have a minimum width of 16 inches and isolated column foundations have a minimum I width of 30 inches. Trenched foundations (grade beam foundations) could be used for support in the cohesive overburden soils. If used, we recommend the trenched foundations have a minimum width of 12 inches. No unusual problems are anticipated in completing the excavations required for the construction Iof footing foundations. Care should be taken during construction to avoid disturbing the foundation bearing materials. Materials which are loosened or disturbed by the construction ' activities or soils which become dry and desiccated or wet and softened should be removed and Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 5 likely be supported on stiff to very stiff lean clay with varying amounts of sand. For these materials, it is our opinion the lightly loaded structures could be supported on conventional footing foundations and the floor slabs could be supported as slab -on -ground. Recommendations are provided below for construction of drilled pier and footing foundations. DRILLED PIERS If conventional footing foundations would extend to be within 2 feet of the site bedrock, we recommend drilled piers be used to support the proposed lightly loaded structures. We recommend the drilled foundations extend to a total depth of at least 10 feet and penetrate the bedrock by a length of at least 8 feet. For design of the drilled pier foundations meeting those two criteria, we recommend using a total load end bearing pressure not to exceed 20,000 psf. A minimum dead load pressure of 4,000 psf should be used to reduce potential for uplift forces causing movement of the drilled pier foundations. ' Additional pier capacity or uplift resistance could be developed by extending the drilled shaft deeper into the bedrock and taking advantage of the development of frictional resistance between ' the drilled shaft concrete and surrounding bedrock. We recommend using an allowable friction resistance value of 2,000 psf for the surface area of the drilled shaft below 8 feet penetration into lthe bedrock for calculation of additional pier capacity. An uplift friction resistance of 1,400 psf could be used for that same area. IDrilled shafts should be reinforced for their full length to help transmit frictional uplift resistance I forces. Grade beams between the drilled piers should be constructed with a minimum 4-inch void between the base of the grade beam and underlying bedrock. Those void zones should be developed with cardboard void boxes or other approved means to prevent the accumulation of ' material within the voided zone and transmission of uplift forces to the grade beams. ' Based on soil and ground water conditions observed at the time of drilling, we do not anticipate that temporary casing would be necessary for the drilled shafts. Concrete should be placed in the ' drilled holes as soon as practical after completion of those borings to reduce the potential for I Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 4 The stratification boundaries shown on the boring logs represent the approximate location of changes in soil and rock types; in -situ, the transition of 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. WATER LEVEL OBSERVATIONS Observations were made while drilling and after completion of the borings to detect the location and depth to the hydrostatic groundwater table. At the time of drilling, no free water was observed in any of the test borings. Based on the lack of observable free water and on the moisture contents of the in -place materials, it is our opinion the borings terminated above the depth of any hydrostatic ground water present at the time the borings .were completed. Ground water levels may fluctuate over time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. Zones of perched and/or trapped water may be encountered in more permeable zones interbedded in the subgrade soils and perched and/or trapped water is often encountered in the overburden soils immediately overlying less permeable highly weathered bedrock. The location and amount of perched water may also vary over time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. ANALYSIS AND RECOMMENDATIONS FOUNDATIONS The materials observed below a depth of approximately 5 to 6 feet below present ground surface consisted of moderately to highly plastic claystone bedrock. If foundations would be supported on or within 2 feet of the claystone bedrock, we recommend considered be given to the use of a drilled pier foundation system. Use of structural floors should be included with the drilled piers. If garden level or non -basement structures are constructed, foundations for those structures would Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 3 SITE AND SUBSURFACE CONDITIONS The Plum Street P.U.D. will be constructed between West Elizabeth and Plum Streets, to the south and north, respectively, and between Rocky Road and Timber Lane to the west and east, respectively. The development parcel is presently undeveloped pasture with an apparent detention/retention pond excavated in the northeast corner of the site. Ground cover on the site is presently weeds and grasses. Site drainage is to the north with a maximum difference in ground surface elevations across the site estimated to be on the order of 5 to 8 feet. Evidence of prior building construction was not observed by EEC site personnel. An EEC representative was on site during drilling to direct the drilling activities and evaluate the recovered samples. EEC site personnel developed field boring logs of the materials encountered during drilling based on visual and tactual observation of disturbed samples and auger cuttings. The boring logs included with this report may contain modifications to those logs based on the results of laboratory testing and engineering evaluation. Based on results of the field borings and ' laboratory testing, subsurface conditions can be generalized as follows. Approximately 3 to 4 inches of vegetation and/or topsoil were encountered at the surface at the boring locations. The topsoil/vegetation was underlain by brown and light brown lean clay containing varying amounts of sand and/or silt. The lean clay extended to depths ranging from approximately 5 to 6 feet. In boring B-5, the cohesive overburden soils were underlain by fine to coarse sand and gravel. The granular materials were medium dense and extended to a depth of approximately 6 feet. The overburden soils at the boring locations were underlain by highly weathered and weathered claystone bedrock. The claystone was soft to moderately hard and was moderately to highly expansive. The claystone bedrock extended to the bottom of the borings at depths of approximately 15 feet. Earth Engineering Consultants, Inc. EEC Project No. 1962013 February 28, 1996 Page 2 estimating angles from the references indicated on the attached boring location diagram. The locations 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 45 drill rig equipped with a hydraulic head employed in drilling and sampling operations. The boreholes were advanced using 4-inch nominal diameter continuous flight augers and samples of the subsurface materials encountered were obtained using split -barrel sampling techniques in general accordance with ASTM Specifications D-1586. In the split -barrel sampling procedure, a 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 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. Moisture content tests were performed on representative portions of each of the recovered samples. In addition, calibrated hand penetrometer tests were performed on appropriate samples. Washed sieve analysis and Atterberg Limits tests were performed on selected samples to establish the percentage and plasticity of fines in the subgrade materials. Swell/consolidation tests were performed on selected samples to determine the potential for the subgrade materials to change volume with variation in moisture content. One Hveem stabilometer R-value was completed on a representative sample of the subgrade materials to determine the remolded strength of those soils. The Hveem R-value is used in pavement design. 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. SUBSURFACE EXPLORATION REPORT PLUM STREET P.U.D. FORT COLLINS, COLORADO EEC PROJECT NO. 1962013 INTRODUCTION February 28,1996 The subsurface exploration for the proposed Plum Street P.U.D. in Fort Collins, Colorado, has been completed. Five (5) soil borings extending to depths of approximately 15 feet below present site grades were advanced in the development area to obtain information on existing subsurface conditions. Individual boring logs and a diagram indicating the approximate boring locations are included with this report. We understand the Plum Street P.U.D. will involve the development of approximately 3.4 acres ' of presently undeveloped pasture located north of West Elizabeth Street between Taft Hill Road and Overland Trail in Fort Collins, Colorado. The development will include 15 single-family housing lots with the proposed structures being one and two-story wood frame buildings. The proposed structures may be constructed as garden level or full basement. Foundation loads for the proposed wood frame buildings will be light with continuous wall loads less than 2.5 kips per lineal foot and column loads less than 30 kips. Floor loads will be less than 100 psf. Paved interior streets will be constructed for the development along with utility installations. Cuts and fills less than 2 to 3 feet will be required to develop the final site grades for this project. The purpose of this report is to describe the subsurface conditions encountered in the borings, analyze and evaluate the test data and provide geotechnical recommendations concerning design and construction of the foundations and support of floor slabs and pavements. EXPLORATION AND TESTING PROCEDURES The boring locations were selected and established in the field by representatives of Earth Engineering Consultants, Inc. (EEC). The field locations were established by pacing and February 28, 1996 Messner Engineering Associates 150 East 29th Street, Suite 225 Loveland, Colorado 80538 Attn: Mr. Dennis Messner, P.E. Re: Subsurface Exploration Report Plum Street P.U.D. Fort Collins, Colorado EEC Project No. 1962013 Mr. Messner: 10 i EARTH ENGINEERIN( CONSULTANTS, INC. IEnclosed, herewith, are the results of the subsurface exploration you requested for the referenced project. In summary, the subsurface materials encountered in the test borings performed at this site ' consisted of low to moderate plasticity cohesive soils overlying highly weathered and weathered claystone bedrock. A thin zone of granular soils was encountered at one of the test boring locations immediately overlying the bedrock. Based on results of the field borings and laboratory testing, it is our opinion that non -basement structures, including slab -on -grade or garden level structures, could be supported on conventional footing foundations bearing in the overburden soils. If the structures will include full basements and/or extend to the underlying weathered claystone bedrock, we ' recommend drilled piers be used for foundation support. In addition, those structures should be designed with structural slabs to eliminate the potential for floor slab movement causing distress to the structure. Slabs -on -grade and the development streets could be supported directly on the ' overburden soils. Geotechnical recommendations concerning design and construction of the foundations and support of floor slabs and pavements are presented in the text of 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 Ihesitate to contact us. Very truly yours, Consultants, Inc. au3957 Lester L. Litton, P.E. Principal Engineer i1 Principal Engineer Centre For Advanced Technology 2301 Research Boulevard, Suite 104 Fort Collins, CO 80526 (970)224-1522 FAX 224-4564 SUBSURFACE EXPLORATION REPORT PLUM STREET P.U.D. FORT COLLINS, COLORADO EEC PROJECT NO. 1962013 I P) EARTH ENGINEERIN( CONSULTANTS, INC.