No preview available
HomeMy WebLinkAboutSHENANDOAH PUD - PRELIMINARY - 47-95 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTEarth Engineering Consultants, Inc. Shenandoah P.U.D. September 6, 1995 Page 2 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 and thin -walled tube sampling procedures in general accordance with ASTM Specifications D-1587 and D-1586, respectively. Ina 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 test were performed on each of the recovered samples. In addition, the unconfined compressive strength of appropriate samples was determined or estimated using a calibrated hand penetrometer. Atterberg limits and washed sieve analysis tests were performed on selected samples to evaluate the plasticity and quantity of fines in the subgrade materials. Swell/consolidation tests were also performed on selected samples to evaluate the potential for the subgrade materials to change volume with variation in moisture content. 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 laboratory by an engineer and classified in accordance with the attached General Notes and the Unified Soil Classification System, based on the materials' texture and plasticity. The estimate 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. -L I W z W y ryryI�� JF+i W ` / U W 4 W 1 "1 o W J � J O v a t m I � a r t I OD RI w o a I i Q (U I 3 = � I �D LJ ~ Q I I— I zG Gd❑J ,k1Nn❑3 NOT TO SCALE BORING LOCATION DIAGRAM SHENANDOAH P.U.D. FORT COLLINS, COLORADO AUGUST 1995 Il1. PROJECT No- 1952071 Earth EnLyineerim Consultants Earth Engineering Consultants, Inc. Shenandoah P.U.D. September 6, 1995 Page 7 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 Colorado Land Limited 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. Shenandoah P.U.D. September 6, 1995 Page 6 When selecting fill for the site, it should be noted that cutting expansive materials from one area and placing them as fill in another area may create the potential for a movement of footings and floor slabs in areas where these fill materials will be placed. Careful selection of borrow and placement areas will be required to reduce potential for creating an expansive soil problem in an area where one does not now exist. Zones of perched water are commonly encountered in overburdened soils immediately overlying highly weathered, less permeable bedrock. The location and amount of perched water can vary over time. Longer -term observations will be necessary to evaluate the potential for perched ground water occurring either during or after construction. Stabilization of the roadway subgrades may be necessary in some areas to reduce the potential for instability of the subgrades during construction. Stabilization can be used to reduce the overlying pavement section if the stabilization is appropriately designed. Although zones of well cemented sandstone were not encountered in the test borings on this site, we have observed well cemented zones in some areas of this bedrock formation. Those zones have required blasting in order to excavate for utility construction. 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 Earth Engineering Consultants, Inc. Shenandoah P.U.D. September 6, 1995 Page 5 In areas where drilled pier foundations are used to reduce potential for foundation movement, use of a structural floor slab should also be considered to reduce the potential for movement of floors supported directly on expansive site materials. Removal and replacement of a portion of those materials can be considered although a risk of future movement would remain. Foundations - Footings In areas where non -expansive or low expansion materials (lean clays or sandstone bedrock) are present, use of conventional footing foundations for support of lightly loaded structures could be considered. Footing bearing pressures ranging from 1500 to 3000 psf could be considered in the near surface cohesive materials. At locations where footing foundations would extend to the underlying highly weathered sandstone bedrock, higher bearing pressures could be used. To limit differential movement in the building, it would be desirable to support the entire structure on either the overburdened soils or underlying bedrock. In areas of the site with natural low volume change materials, the near surface materials could also be used for direct support of floor slabs. Care will be necessary to evaluate the moisture content of the subgrade materials in these areas immediately prior to placement of the floor slabs. General Considerations Several feet of fill may be required to develop the final site grades in lower lying areas of the site. The lower lying areas are typically streambed areas which have deposits of water borne silts and clays which have not been subjected to preconsolidation pressure. In these areas, the aerial fill will likely create general aerial settlement as the weight of the new fill causes consolidation of the underlying natural soils. Fill in these areas should be placed well in advanced of construction of site improvements to allow for the settlement to occur. Surcharging (placing additional fill in these areas) can be used to speed up the consolidation of the underlying materials. The softer materials could also be removed and replaced to speed construction. Earth Engineering Consultants, Inc. Shenandoah P.U.D. September 6, 1995 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 made while drilling and after completion of borings detect the presence and depth to the hydrostatic ground water table. Free water was observed only in boring B-8 at a depth of approximately 8 feet during and immediately after drilling. Longer term observations would be required in the fine grain cohesive materials to more accurately evaluate ground water levels and fluctuations. The ground water level can fluctuate over time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. ANALYSIS AND RECOMMENDATIONS Foundations - Drilled Piers The shale bedrock is moderately expansive and subject to volume change with variation in moisture content. In these areas, drilled pier foundations should be considered to reduce the potential for volume changes in the subgrade materials causing movement of building foundations. Drilled piers would develop load carrying capacity through skin friction and end bearing in the soft to moderately hard highly weathered shale. End bearing ranging from 15 to 25 ksf could be expected from the shale bedrock with penetration of that bedrock on the order of 8 to 10 feet. It may be necessary to extend the drilled piers further into the bedrock to obtain additional download capacity or to resist uplift forces expected in the site materials. For the deep foundations, it would be necessary to re -enforce the drilled piers for their entire lengths to help transmit uplift forces throughout the entire pier shaft and help resist uplift of the foundations. In addition, use of void boxes beneath the grade beams would be appropriate. Earth Engineering Consultants, Inc. Shenandoah P.U.D. September 6, 1995 Page 3 SITE AND SUBSURFACE CONDITIONS The Shenandoah P.U.D. will be located west of South College Avenue between Larimer County Road 32 and Trilby Road in Fort Collins, Colorado. Most of the existing property is currently used as horse and cattle pasture with a few farm buildings located near the center of the site. Site drainage is towards several swales running east -west across the site. Maximum difference in ground surface elevations across the site is on the order of 50 feet. A field geologist from EEC was on site during drilling to evaluate the subsurface conditions encountered and to direct the drilling activities. Field logs were prepared by EEC personnel based on observation of disturbed samples and auger cuttings. The final boring logs included with this report may contain modifications to those field logs reflecting the results of laboratory testing and engineering evaluation. Based on the results of the field borings and laboratory testing, subsurface conditions can be generalized as follows. Approximately 3 to 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. The lean clay in borings B-1 and B-2 was moderately plastic indicating a tendency to change volume with variation in moisture content. Lower plasticity clays were encountered in the other borings. The overburden cohesive soils extended to depths ranging from approximate 2 to 14 feet at the boring locations. The overburden soils were underlain by highly weathered claystone/shale bedrock and/or by highly weathered sandstone bedrock. The ciaystone/shale bedrock is moderately to highly plastic and subject to volume change with variation in moisture content. The sandstone bedrock is not likely to show large volume changes with variation in moisture content although some clayeyer zones were encountered in the sandstone bedrock. The shale bedrock is soft to moderately hard it is colored generally gray and olive brown. The sandstone bedrock was poorly cemented; we have encountered zones of cemented sandstone in this formation. Borings were terminated at depths ranging from approximately 15 to 20 feet in the highly weathered bedrock. Preliminary Subsurface Exploration Report Shenandoah P.U.D. Fort Collins, Colorado EEC Project No. 1952071 INTRODUCTION September 6, 1995 The preliminary subsurface exploration for the proposed Shenandoah development to be located west of South College Avenue between Larimer County Road 32 and Trilby Road in Fort Collins, Colorado, has been completed. Ten (10) soil borings extending to depths ranging from approximately 15 to 20 feet below present site grades were advanced in the proposed 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 Shenandoah P.U.D. is planned as a mixed -used development including single and multi -family housing and retail/office park areas. It is anticipated the site buildings will include one - and two-story structures with many of those structures expected to include full basements. Foundation loads for the site buildings are expected to be light with continuous wall loads less than 3 kips per lineal foot and column loads less than 50 kips. Cuts and fills of up to 5 to 10 feet may be required to develop site grades for this property. Infrastructure improvements including roadways, water and sewer will be installed for the development. The purpose of this report is to describe the subsurface conditions encountered in the borings, analysis and evaluate the test data and provide preliminary geotechnical recommendations concerning the proposed construction. 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 estimating angles from the references indicated on the attached boring location diagram. The boring locations should be considered accurate only to the degree implied by the methods used to make the field measurements. 1 1 September 6, 1995 Colorado Land Limited 8101 Prentice Avenue, Suite 180 Englewood, Colorado 80141 Attn: Mr. Jim McCory Re: Preliminary Subsurface Exploration Shenandoah P.U.D. Fort Collins, Colorado EEC Project No. 1952071 Mr. McCory: I D) EARTH ENGINEERING CONSULTANTS, INC. Enclosed, herewith, the results of the preliminary subsurface exploration you requested for the referenced project. In summary, the subsurface materials encountered in the test borings consisted of variable blend of cohesive soils overlying highly weathered shale or sandstone bedrock. The overburdened soils are slightly to moderately plastic and would be subject to slight to moderate volume change with variation in moisture content. The shale bedrock shows higher plasticity and a greater potential for volume change with variation in moisture content. In our opinion, this ' property could be used for the proposed mixed -use development although special care and construction procedures will be required to mitigate the effects of the moderately expansive subgrade materials. Preliminary geotechnical recommendations concerning design and construction of 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 the enclosed report, or if we can be of fiuther service to you in any other way, please do not hesitate to contact us. Very truly yours, EarthEmagi;gggring Consultants, Inc. Reviewed by::.,, w ,E,,,a. •o°p' k 4y © " S Aeo _ � o" a •, L, J+j �e J o J �,�'� "a � 27315, ��peop ' Lester L. Litton, P.E. Principal Engineer Curtiss L. Palin, P.E. Principal Engineer ' cc: Northern Engineering - Bud Curtiss Centre For Advanced Technology 2301 Research Boulevard,$ulte 104 I Fort Collins, CO 80526 (970)224-1522 FAX 2244564 k I PRELIMINARY SUBSURFACE EXPLORATION REPORT SHENANDOAH P.U.D. FORT COLLINS, COLORADO EEC PROJECT NO. 1952071 EEf�