No preview available
HomeMy WebLinkAboutHUNTINGTON HILLS WEST - Filed SER-SUBSURFACE EXPLORATION REPORT -SUBSURFACE EXPLORATION REPORT JENSEN DEVELOPMENT FORT COLLINS, COLORADO EEC PROJECT NO. 1952045 tl/ u N TI RJ (, TO nt t4 f L c- 5 w G 5 T 5145DI vI51O N EARTH ENGINEERINI CONSULTANTS, INC. SUBSURFACE EXPLORATION REPORT JENSEN DEVELOPMENT FORT COLLINS, COLORADO EEC PROJECT NO. 1952045 l' tl u"TI Aj C, TO nl t41 L L, 5 5uA5 U 15 10 N a a a 4 4 4 I IJuly 10, 1995 Ll 4 4 J.R. Engineering, Ltd. 4812 South College Avenue EARTH ENGINEERING CONSULTANTS, INC. Fort Collins, Colorado 80525 Attn: Mr. Norm Whitehead Re: Subsurface Exploration Report Jensen Development Fort Collins, Colorado EEC Project No. 1952045 Mr. Whitehead: Enclosed, herewith, are the results of the subsurface exploration you requested for the referenced project. In summary, the subsurface materials encountered in the test borings consisted predominately of moderately to highly plasticity claystone bedrock. That material would be subject to volume change with variation in soil moisture. We recommend the proposed lightly loaded structures on this parcel be supported on drilled pier foundations to reduce the potential for post -construction movement with volume changes in the subsurface materials. Use of structural floors would be required to eliminate the potential for movement of the floor slabs subsequent to construction. Overexcavation and backfill procedures could be considered to reduce the potential for movement of the floor slabs; however, this procedure would still involve risk of floor slab movement. Geotechnical recommendations concerning design and construction of the foundations and support of the 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 hesitate to contact us. Very truly yours, Earth Engineering Consultants, Inc. Principal Engineer cc: Mrs. Elaine Jensen Centre For Advanced Technology 2301 Research Boulevard, Suite 104 Fort Collins, CO 80526 970)224-1522 FAX 224-4564 Reviewed by: Principal Engineer SUBSURFACE EXPLORATION REPORT JENSEN DEVELOPMENT FORT COLLINS, COLORADO EEC PROJECT NO. 1952045 INTRODUCTION July 10, 1995 The subsurface exploration for the Jensen Parcel Development south of Skyline Drive in Fort Collins, Colorado, has been completed. Four (4) soil borings extending to depths ranging from approximately 10 to 15 feet below present ground surface were advanced in the Jensen Development area to develop information on subsurface conditions. Individual boring logs and a diagram indicating the approximate boring locations, are included with this report. We understand the Jensen property will be developed for single-family residences. Twelve single- family housing lots will be constructed as a part of this development. It is expected the residences will be one or two-story wood frame structures which will likely contain full basements. Foundation loads for those buildings are expected to 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. Infrastructure improvements, including installation of water and sewer service and a paved local access road, will be required for the development. Minor grade changes are expected to develop the final site grades for this parcel. 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 foundations and support of floor slabs and pavements. EXPLORATION AND TESTING PROCEDURES 4 The approximate 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 estimating angles from the references indicated on the attached boring location diagram. The 14 a aEarth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 2 boringlocations should be considered accurate only to they degree implied by the methods used to make the field measurements. The borings were completed using a truck mounted, rotary type 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 in the borings were obtained using split -barrel sampling procedures in general accordance with ASTM Specification 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 materials 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 each of the recovered samples. In addition, the unconfined compressive strength of appropriate samples was estimated using a calibrated hand penetrometer. Atterberg limits and washed sieve analysis tests were performed on selected samples to evaluate the plasticity of the site materials and to aid in soil classification. Swell/consolidation tests were also performed on selected samples to evaluate the subgrade material's tendency to change volume with variation in moisture content. Results of the outlined tests are indicated 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 on the boring logs and a brief description of that classification system is included with this report. 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. 4 4 4 f 4 4 4 4 4 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 3 ITE AND SUBSURFACE CONDITIONS The Jensen property is located south of Skyway Drive approximately 1/4 mile east of College Avenue in Fort Collins, Colorado. Surface drainage at this parcel is to the south and east with maximum difference in ground surface elevations across the site on the order of 10 to 20 feet. Surface cover in the area is sparse vegetation. Evidence of prior building construction was not observed in the field by EEC personnel at the time of drilling. An EEC field geologist was on site during drilling to evaluate the subsurface conditions encountered and direct the drilling activities. Field logs prepared by the EEC geologist were based on visual and tactual observation of disturbed samples and auger cuttings. Final boring logs included with this report may contain modifications to those field logs based on 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 2 to 3 inches of vegetation and/or topsoil was encountered at the ground surface at the boring locations. The topsoil/vegetation was underlain by brown, highly weathered bedrock. The bedrock consisted predominately of claystone with occasional zones of sandstone. Those materials became less weathered with depth and ranged in consistency of soft to moderately hard. The bedrock is moderately to highly expansive. The borings were terminated at depths of 10 to 15 feet in the claystone bedrock. The stratification boundaries indicated on the boring logs represent the approximate locations of changes in soil and rock types; in -situ, the transition of materials may be gradual and indistinct. GROUND WATER CONDITIONS Observations were made while drilling and after completion of the borings to detect the location and depth to hydrostatic ground water. Free water was encountered at a depth of approximately 4 feet in boring B-2 as the borings were being advanced. Free water was not observed at the 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 4 other boringlocations. The surface of the site was relatively wet prior typ o drilling so that access for drilling was difficult. Based on the mineral deposits observed at the surface in some locations of the site, it appears as though perched water occasionally seeps down the slopes. Long-term monitoring of ground water conditions would be required to evaluate perched water conditions. Fluctuations in the level and amount of perched water and in the depth of the hydrostatic groundwater table can occur over time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. ANALYSIS AND RECOMMENDATIONS FOUNDATIONS The cla stone bedrock encountered in the test borings is moderately to highly lastic andYgygyp subject to volume change with variation in moisture content. To reduce the potential for the foundations of the structures moving subsequent to construction, we recommend the structures be supported on drilled pier foundations. Those drilled piers should extend at least 8 feet into the bedrock or to a depth of 12 feet below existing ground surface, whichever is greater. For design of drilled piers bearing in the natural, soft to moderately hard highly weathered claystone, we recommend using a total load end bearing pressure not to exceed 15,000 psf. A minimum dead load of 5000 psf should be developed on the piers or frictional uplift resistance should be included in the pier design to resist 5000 psf of uplift pressure. Additional pier capacity could be developed by extending the drilled piers deeper into the bedrock and taking advantage of friction between the drilled pier concrete and surrounding bedrock. We recommend a friction value of 1.500 psf be used for that portion of the drilled piers extending to depths greater than 8 feet into the bedrock and to a depth of greater than 12 feet below existing ground surface. We recommend drilled pier foundations be reinforced for their entire length to take advantage of frictional uplift resistance of the bedrock. IEarth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 5 In design of drilled pier foundations we recommend a minimum shaft diameter of 12 inches begP , used. Drilling equipment should be capable of thoroughly cleaning the bearing surface. If the bearing surfaces can not be thoroughly cleaned with the drilling equipment, a minimum shaft diameter of 30 inches should be utilized to allow for hand cleaning. During construction, care should be taken to avoid mushrooming the top of the drilled pier shafts. In addition, a void box should be used beneath the grade beams to allow for some expansion of the subgrade materials without developing uplift forces on the grade beams. We recommend the void boxes be at least 4 inches thick and be developed using cardboard void forms or similar methods to prevent adjacent materials from entering the voided area. Based on soil and groundwater conditions observed at the time of drilling, we do not anticipate it would be necessary to use temporary steel casing to prevent an influx of soils or groundwater into the excavations. Some seepage of water into the excavations may occur. It might be necessary to pump excess water from the excavations or place concrete by a tremie methods to develop the concrete shafts. Concrete should be placed in the drilled shafts as soon as practical after excavating to prevent drying of the bearing materials. We estimate the long-term settlement of drilled pier foundations design and constructed as outlined above would be small, less than 1/2 inch. If excessive wetting of the subgrade occurs, some movement of those foundations can occur due to expansion of the subgrade materials. Care should be taken to avoid wetting of the bearing and subgrade materials. FLOOR SLAB SUBGRADES The site materials are moderately to highly plastic and subject to volume change with variation in moisture content. If floor slabs are supported directly on these soils, some heaving of those floor slabs should be anticipated as the subgrade materials become wetted and expand. Use of structural floor slabs would be required to eliminate this potential for movement. Overexcavation 4 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 6 and backfill procedures could be considered to reduce the potential for movement; however, thatPP , potential would not be eliminated using this procedure. Concerning use of structural slabs, we recommend a minimum 6-inch void space be constructed beneath all structural floor systems. Care should be taken to have a positive method for removing water from beneath the floors to avoid excessive wetting of the subgrade materials. Concerning overexcavation and backfill procedures (if used), we recommend overexcavations extend to a depth of at least 3 feet below the top of the floor subgrade. Materials used as backfill in this zone should consist of approved, low volume change soils 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. We recommend those materials contain sufficient 4 fines to prevent ponding of water in the subgrade materials. Fill materials beneath the floor slabs should be placed in loose lifts not to exceed 9 inches thick. adjusted in moisture content and compacted to at least 95 % of the materials' maximum dry density as determined in accordance with ASTM Specification D-698, the standard Proctor procedure. The moisture content of the subgrade materials should be adjusted to be within the range of ±2% of standard Proctor optimum moisture at the time of placement. Direct support of floor slabs on properly placed and compacted fill as outlined above could be considered. This method of floor support would reduce but not eliminate potential for movement i of the floor slabs subsequent to construction due to moisture induced volume changes in the subgrade materials. Use of a structural floor system would be necessary to eliminate the potential for movement. BELOW GRADE AREAS We recommend a perimeter drain system be installed around all below grade areas to reduce the potential for buildup of hydrostatic loads on the below grade walls and/or seepage of infiltration F] IEarth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 7 water into the below grade areas. In general, a perimeter drains stem would consist ofggPY perforated metal or plastic pipe placed around the exterior perimeter of the structure at approximate foundation bearing level. The drain line should promote the free flow of water to a sump area where it can be removed without reverse flow into the system. We recommend the drain line be surrounded by a minimum of 6 inches of appropriately sized granular filter soil and that either the drain line or the surrounding filter soil be wrapped in a appropriate filter fabric to prevent infiltration of fines into the system. Backfill above the drainage system should consist of approved, low volume change materials which are free from organic and debris. If granular materials are to be used as fill, we recommend the top 2 feet of backfill material consist of an essentially cohesive soil to reduce surface water infiltration. The backfill materials should be placed in loose lifts not to exceed 9 inches thick, adjusted in moisture content and compacted to at least 90 % of -the materials' maximum dry density as determined in accordance with ASTM Specification D-698, the standard Proctor procedure. Compaction of the backfill materials should not proceed until the first floor floor -framing is in place or another system of bracing is in place to reduce potential for deflection of the basement walls. Compaction of the backfill materials should be completed with light mechanical equipment or hand operated equipment to reduce the potential for developing excessive lateral forces on the below grade walls. In areas where the backfill materials will support sidewalks, steps, driveways or similar structures, we recommend that backfill be compacted to at least 95 % of standard Proctor maximum dry density. For design of below grade walls where appropriate steps have been taken to eliminate the buildup of hydrostatic loads, we recommend using an equivalent fluid pressure of 60 pounds per cubic foot. That equivalent fluid pressure does not contain a factor of safety nor an allowance for hydrostatic loading. Surcharge loads placed adjacent to the exterior walls could also add to the lateral stresses on the walls. I 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 8 PAVEMENT SUBGRADES All existing vegetation and/or topsoil should be removed from beneath the pavements. After stripping and completing all cuts and prior to placement of any fill or pavements, we recommend the exposed subgrades be scarified to a minimum depth of 9 inches, adjusted in moisture content and compacted to be within the range of 93 to 97 % of standard Proctor maximum dry density. The moisture content of the scarified materials should be adjusted to be within the range of -1 to 3 % of standard Proctor optimum moisture. Fill materials required to develop the pavement subgrade should consist of approved soils which are free from organic matter and debris. The near surface highly weathered bedrock could be used as fill beneath the pavements although those materials exhibit expansion characteristics and some movement of the pavement subsequent to construction should be expected: Fill materials in the pavement areas should be placed in loose lifts not to exceed 9 inches thick, adjusted in moisture content as recommended for the scarified materials and compacted to be within the range of 93 to 97 % of standard Proctor maximum dry density. A Hveem R-value of 6 was determined in laboratory testing of the site subgrade materials. We anticipate an equivalent daily load axle rating of 5 be appropriate for this local roadway. The 1986 American Association of State Highway and Transportation Officials (AASHTO) publication Guidelines for Design of Pavement Structures" was used to help evaluate alternative pavement sections. Based on that evaluation, we recommend the pavement section for the entry road consist of 3 inches of hot bituminous pavement overlying 6 inches of aggregate base coarse. Hot bituminous pavement should be compatible with City of Fort Collins standard criteria for SC -I or SC -II materials. The aggregate for the base should consist of materials compatible with Colorado Department of Transportation (CDOT) requirements for Class 5 or Class 6 Base. Positive drainage should be developed across the pavements and away from the pavement edges to avoid wetting of the subgrades. Pavement subgrades allowed to become wetted subsequent to r L 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 9 construction could result in softening of the subgrades and premature failure of the pavement section. OTHER CONSIDERATIONS Care should be taken to avoid wetting the subgrade and bearing materials around the residences. Lawn watering systems should be installed at least 5 feet away from the structures and the spray heads should be designed to avoid spraying on or adjacent to the foundations. Down spouts for the structures should be designed to transmit water at least 5 feet away from the buildings. Positive drainage should be developed away from the structure with a minimum slope of 1 inch per foot for the first 10 feet away from the perimeter. In addition, care should be taken in selecting landscaping adjacent to the structures to avoid plantings which would create significant moisture fluctuations in the subgrade materials or cause ponding of water adjacent to the subgrades. After construction, care should be taken to quickly repair any underground utility leaks in the area. Particular care should be taken with any lawn watering systems. Because of lawn watering and reduced evapo-transpiration, some expansion of subgrade materials should be expected and subsequent movement of overlying pavements and other structures anticipated. 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. 4 Earth Engineering Consultants, Inc. Jensen Development July 10, 1995 Page 10 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 Mrs. Elaine Jensen 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. 4 4 4 4 4 maxi [•..•-a,1 u . •s. N I I , R13-4 I 2 BLOCK 1. 141 14 PROJECT No: 1952045 o0R0 t Y _Ix(fw[IT N Ix.N[f I anon i [ a tlrtln n\\ I-rrra v-Y'v. ¢. • a moaO J+ n rm J/ I ' II I I I I I I I I , I , I , I , I ; II II I I1 II I I I 1 I IOx turf s2[ D<<xxul;:) 1 I II I I I I I i IIII O I I I I I ; III III1 I t I I I I iI I TEST BORING LOCATION DIAGRAM JENSEN DEVELOPMENT FORT COLLINS, COLORADO JUNE 1995 NOT TO SCALE Earth Engineering Consultants