Loading...
cn-2011-00158.tif r Terracon GEOTECHNICAL ENGINEERING REPORT PROPOSED NEW GRAIN STORAGE BINS BLAIR, NEBRASKA Terracon Project No. 05995072 RECEIVED May 11, 1999 MAY 18 2011 CITY INTRODUCTION N NEBRASKA This report presents the results of the subsurface exploration for the proposed New Grain Storage Bins located at the existing Tyson Elevator in Blair, Nebraska. Two soil borings extending to depths ranging from about 30 to 50 feet below existing grade were drilled within the proposed new grain storage bins to obtain subsurface information. Individual boring logs and a location diagram are included with this report. 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 -regarding design and construction of the foundations for the proposed project. Also, general site - development and earthwork recommendations are provided. PROJECT DESCRIPTION We understand the proposed project will consist of the construction of two new grain storage _ bins. The grain bins will be 48 and 60 -foot diameter and 40 feet high. The bins will be separated by about 14 feet. Based on a grain unit weight of about 55 pcf, we anticipate that the bins will have an average bottom pressure of about 2,500 psf. Grade changes for construction of the bins are anticipated to involve up to about 4 feet of fill on the south side of the proposed bins. Based on conversations with George Tyson, the existing bins are constructed on 4 feet wide by 4 feet deep crushed rock footings with 2 to 2%2 feet of fill sand beneath the bottom of the tanks. SUBSURFACE EXPLORATION AND LABORATORY TESTING PROCEDURES The borings were drilled with a truck-mounted rotary drill rig equipped with a hydraulic head. The boreholes were advanced using continuous flight augers, and representative samples were obtained by thin - walled tube sampling procedures in general accordance with ASTM 1 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 Terracon May 11, 1999 Specification D -1587. In this method of sampling, a thinwalled, seamless steel tube with a sharp cutting edge is pushed hydraulically into the ground to obtain a relatively undisturbed sample. The samples were tagged for identification, sealed, and returned to the laboratory for testing and classification. Field vane shear tests were also performed in the borings to measure the in -situ undrained shear strength of the subsurface soils. The vane shear device consists of a four - bladed vane connected with steel rods to a calibrated torque measuring device. The vane is pushed into the undisturbed soils below the bottom of the borehole and torque is slowly applied to the rod and vane until the vane causes shear failure to occur in the soil. The ultimate torque measured during the test is then converted to a measurement of theundrained shear strength of the soils. The vane is especially useful in cohesive soils that are sensitive to sampling disturbance and for which conventional -sampling, sample handling and laboratory testing techniques commonly provide significantly lower than actual indication of shearing strength. The results of the vane shear tests are summarized on the boring logs. Field logs of the borings were prepared by the drill crew. These logs included visual classifications of the materials encountered during drilling as well as the driller's interpretation of the subsurface conditions between samples. The final boring logs included with this report represent an interpretation of the field logs and include modifications based on laboratory observation and tests of the samples. Representative samples of cohesive soils obtained by the thin-walled tube sampling procedure were tested -for water content and density. Unconfined compressive strength tests were performed on samples of . sufficient length and integrity. Hand penetrometer tests were performed on the thin - walled tube soil samples. The hand penetrometer is a device which has been correlated with laboratory unconfined compressive strength and provides an estimate of the strength and consistency of the soil sample. Results of the laboratory tests are provided on the attached boring logs. Descriptive classifications of the soils indicated on the boring logs are in accordance with the enclosed General Notes and the Unified Soil Classification System. Also shown are estimated Unified Soil Classification Symbols. A brief description of this classification system is attached to this report. All classification was by visual - manual procedures and-was performed by experienced personnel. .71 2 - Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 _ Terracon May 11, 1999 SITE AND SUBSURFACE CONDITIONS The proposed new grain storage bins are planned on the east side of the Tyson Elevator property. The bins are planned within an existing plowed bean field south of an existing soil berm. The site sloped to the south with a measured elevation difference of about 1 %Z -feet between the borings. The boring locations were laid out by representatives of Terracon in the approximate center of the tank area that was flagged prior to our field exploration. Distances from these locations to the reference features indicated on the attached diagram are approximate and were measured with a mechanical wheel. Right angles for the boring location measurements were estimated. Ground surface elevations indicated on the boring logs are approximate and were obtained by s the drill -crew using a surveyors level and rod . , These elevations were referenced to the top of the ringwall footing on the east side of the existing bin located just west of the proposed bins. The locations and elevations of the borings should be considered accurate only to the degree implied by the means and methods used to define them. Conditions encountered at each boring location are indicated on the individual boring logs. Stratification boundaries on the boring logs represent the approximate location of changes in soil types; in -situ, the transition between materials may be gradual. Based on the results of the borings, subsurface conditions on the project site can be generalized as follows. Boring 1 encountered silty clay possible fill soils at the surface. The possible fill extended to a _ depth of about 3 feet. A sample of the possible fill had a moisture content of about 27 percent and a dry density of about 91 pcf. Boring 2 encountered natural silty clay (weathered loess or loess) soils at the surface. Natural silty clay (loess) soils were also encountered beneath the possible fill in Boring 1. The natural silty clay soils extended to the approximate 50 and 30 foot termination depths of the borings. Samples of the silty clay were typically stiff to a depth of about 13 feet and medium to soft below this depth. The samples had measured moisture contents that ranged from about 23 to 36 percent and dry densities from about 82 to 98 pcf. WATER LEVEL OBSERVATIONS The borings were monitored while drilling and immediately after completion for the presence and level of groundwater. Water levels observed in the borings are noted on the boring logs. At these times, groundwater was observed in both borings at depths ranging from approximately 16 to 26 feet below existing grade. These water level observations provide an approximate indication of the groundwater conditions existing on the site at the time the 3 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 - Terracon May 11, 1999 borings were drilled. However, due to the low permeability of the cohesive soils encountered in the borings, longer term monitoring in cased holes or piezometers would be required for a more accurate evaluation of the groundwater conditions. Fluctuations of the groundwater level can occur due to seasonal variations in the amount of rainfall, runoff and other factors not evident at the time the borings were performed. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project. ANALYSES AND RECOMMENDATIONS Geotechnical Considerations The borings typically encountered variable depths of existing fill soils underlain by silty clay (Peorian Loess) soils.' The silty clay soils were typically stiff in consistency to a depth of about 13 feet, below this depth the soils were typically soft to medium in consistency. Both borings terminated in the soft to medium silty clay soils. In a conversation with Mr. George Tyson, we recommended performing additional soil borings and electronic cone soundings to aid in determining the thickness of these soft to medium soils and the relative compressibility of the these soils. However, Mr. Tyson indicated that the amount of settlement of the bins was not a major concern and the additional soils work was not desired. Based on the soils information from the two soil borings, we estimate settlements on the order of 14 to 16 inches at the center of the bins and about 6 to 9 inches at the bin edges for a grain bin designed for a bottom pressure of about 2500 psf. Additional settlements are possible depending on the depth of the soft to medium silty clay soils. However, this can not be determined without additional soil borings or other exploration to depths greater than 50 feet. In addition, larger settlements are anticipated if fill is placed on the tank foundation area. For example, if 4 feet of fill is placed within a tank foundation area, we estimate that the above settlement estimates would increase by about 3 to 4 inches. Therefore, any fill placed on the bin area should preferably be placed at least 8 weeks prior to construction of the bin foundations to allow settlement under this load. Settlement plates could be placed in areas of - = fill placement, if construction schedule does not allow for the estimated 8 weeks. The fill preload time could be reduced if settlement data indicates that the majority of the settlement due to the fill placement has occurred prior to construction. Terracon should be contacted to provide additional recommendations on monitoring and placement of the settlement plates prior to earthwork and site grading. 4 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 - Terracon May 11, 1999 In addition possible fill soils were encountered in Boring 1, these soils appeared to somewhat poorly to moderately well compacted. Therefore, we recommend that the fill.be removed and recompacted beneath the tank bottom. Based on the anticipated settlements and relatively soft natural soils, we recommend supporting the bins on a rigid ringwall foundation and at least a 24 -inch thick granular tank bottom subgrade. Site Preparation and Earthwork Site .stripping should include the removal of existing vegetation, topsoil, existing surfacing materials, and any other materials unsuitable for reuse as structural fill. Based on the boring information, a stripping depth of about 9 to 12 inches is generally expected to be adequate for - this purpose over most of the site. However deeper stripping could be required in localized areas. Following stripping, the fill should be removed beneath the bin foundations and to a distance of at least 5 feet beyond the perimeter. Groundwater was not observed near the expected depths of excavations. However, the silty clay soils at the bottom of the excavations will be sensitive to disturbance, particularly if wetted. Care should be taken to minimize disturbance of the soils in the bottom of the excavation. Grading should be provided around the excavation perimeters to divert surface runoff away from the excavations. Any water which collects in the excavations should be removed as soon -� as possible along with any softened subgrade soils. Following fill removal, the exposed soils should be scarified to a 6 -inch depth and moisture adjusted to within 3 percent of the material's optimum as determined in the standard Proctor procedure (ASTM D -698). The boring information indicates the existing soils may presently be somewhat above the recommended moisture range. Therefore, some soil moisture conditioning may be required to facilitate compaction. The scarified soils should be _ recompacted to at least 98 percent of the material's maximum standard Proctor density. -- Scarified soils which cannot be recompacted to the recommended degree should be undercut and replaced with stable fill. All fill and backfill used on this site should consist of approved granular or low plasticity, cohesive soil free of organic matter and debris. The low plasticity soil should have a liquid limit less than 45 percent and a plasticity index between 10 and 20 percent. The granularfill should consist of well - graded material ranging in size from gravel to fine sand. Angular, crushed 5 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 Terracon May 11, 1999 material would provide better support than rounded river sand. The on -site silty clay soils that - are free of organic matter and debris appear suitable for re -use as cohesive compacted fill. The suitability of off -site fill soils should be evaluated prior to construction. All fill placed beneath footings and tank bottoms in the bin areas and to a distance of at least 5 feet outside the bin perimeters should be compacted to at least 98 percent of the material's standard Proctor maximum dry density (ASTM D -698). The fill should "also be compacted at a: water content within 3 percent of the optimum determined in the standard Proctor test. The boring information indicates some moisture conditioning should be expected to achieve the recommended moisture range with the on -site soils. Ring Wall Foundations r. Ring wall footings for the proposed tanks are anticipated to be supported on approved and tested natural silty clay soils or new fill soils extending to the natural silty clay soils. A maximum net allowable soil bearing pressure of 2000 psf could be used for design of footings bearing in the tested and approved natural silty clay soils or newly compacted and tested fill soils underlain by the natural silty clay soils. This is the maximum pressure that should be transmitted to the bearing soil in excess of the minimum adjacent overburden pressure. The base of all foundation excavations should be free of water and loose soil prior to placing concrete. Concrete should be placed as soon as possible after excavating to minimize bearing soil disturbance. Should the soils at bearing level become disturbed or saturated, the affected soil should be removed prior to placing concrete. Foundations should extend to bear at least 3/2 feet below lowest adjacent finished grade for frost protection. Continuous formed wall footings should have a minimum width of 16 inches L and isolated column footings should have a minimum width of 30 inches. The allowable lateral load capacity of grade - supported shallow foundations and grade beams can be calculated using an allowable passive earth pressure based on an equivalent fluid weight of 180 pcf times the depth below the surface. It is recommended that the lateral resistance of the soils within the 3Y2-foot frost depth be neglected. Additional allowable lateral resistance developed by friction between the bottom of grade- supported foundations and the soil subgrades can be calculated using an allowable friction coefficient of 0.25 times the sustained bearing pressure. This is based on a factor of safety of 2. I 6 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 Terracon May 11, 1999 Settlement Considerations Maximum total settlements of the bins and their foundations designed and constructed as recommended in this report are estimated to be about 14 to 16 inches at the center and 6 to 9 inches at the edges for the bins constructed as recommended above. Settlements could be even greater, depending upon the depth of the compressible soils below the bottom of the borings. Based on conversations with George Tyson, other tanks at this site have experienced similar movements. Mr. Tyson also indicated that these ranges of settlements are not unusual CIA for the anticipated loading conditions and are generally tolerable for steel bins of this size if the differential is primary center to edge and uniform in distribution. Some differential settlements are possible along the bin perimeter due to varying soil conditions, foundation loads, and i adjacent bins, but are not expected to exceed about 2 to 5 inches. Steel Tank Bottom Recommendations We understand that the tank bottom will be supported directly on the recompacted structural fill L -:1 soils. We recommend that at least the upper 24- inches of fill below the tank bottom consist of well graded granular soils. The granular fill soils is anticipated to provide a stable working surface for tank construction and better tank bottom support. Initial Bin Loading Stage loading is recommended to help avoid abrupt movements of the bins upon initial filling Each stage should consist of about 20 to 25 percent of the bin's capacity, and a time period of several weeks is recommended between successive stages. Settlement monitoring of the bin perimeter should be performed during initial loading to help identify any unusually abrupt or differential movements and allow corrective action, if necessary. Site Grading Considerations ` If precipitation occurs immediately prior to or during construction, the clay soils in excavations would further soften and become more susceptible to disturbance. Construction staging should provide drainage of surface water and precipitation away from the bin area, and provide a for removal of water accumulation in excavations as soon as possible. Similarly, finished -grading slopes should promote drainage away from the storage bin. In addition, cohesive fill should also be used to form a cap around the outside perimeter of the r bin foundation to help promote surface water run -off away from the bin. This layer should be 7 Proposed New Grain Storage Bins Blair, Nebraska Project No. 05995072.001 Terracon May 11, 1999 12 to 18 inches thick and compacted to at least 95 percent of standard Proctor density. The — on -site soils can be used to form this cap layer. GENERAL COMMENTS - Terracon should be retained to review the final design plans and specifications so comments can be made regarding interpretation and implementation of our geotechnical recommendations in the design and specifications.' Terracon also should be retained to provide testing and observation during excavation, foundation installation, and construction phases of the project. _ The analyses and recommendations presented in this report are based upon the data obtained from the- 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 site.' The nature and extent of such variations may not become evident until construction. If variations appear evident, it will be necessary to reevaluate the recommendations of this report. The scope of services for this project does not include either specifically or by implication any - environmental assessment of the site or identification of contaminated or hazardous materials or conditions. If the client and /or owner is concerned about the potential for such contamination, other studies should be undertaken. 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 warranties, either express or implied, are intended or 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 Terracon reviews the changes and either verifies or modifies the conclusions of this report in writing. i 8 Ric • :• • 1161 t a" • 1lll /I 1 ililil . • ®� ® ®■ ®� I I 1 I/ Mal 1 /III/ .111 1 lIII/ - 1lIII/ 1 /II /I ® © ®�■ ®� 111 1III /I Ill/II 0 o 1I /I /I 1I /III 1I/ /II I / /III /// /// : • 1 • 1 0 goo / I ON ORR III //I 1 I/I /II OH 1IIII/ 0 e 1/� . . ... WATER LEVEL OBSERVATIONS, ft ' ® ♦ :•- • LOG OF BORING NO 'I page 2 of CLIENT ARCHITECT /ENGINEER TYSON'S INC. SITE 200 NORTH TENTH STREET PROJECT BLAIR, NEBRASKA PROPOSED GRAIN BINS SAMPLES TESTS �" O c o 0 CL J �. 2 W � z DESCRIPTION U=), U ct� > w — z c7 = w a _ W m > z� wL z U) W ? o�. r CO w 0 w w i. u, I— z >_ U n ` C7 0 z °' w a� Q0 W zH ¢w a W (0 m 0 0 a Drn SILTY CLAY, TRACE SAND (Loess) CL 9 ST 14 34.7 87 1000" Gray ML Medium to Soft 35 HS CL 10 ST 20 33.1• 86 500* ML 40 HS CL 11 ST 18 36.4 85 1280 ML 2000* 45 HS CL 12 ST 20 35.1 86 1000* ML 50 BOTTOM OF BORING 45'5 50 Z 0 U AP The stratification lines represent the approximate boundary lines 'Calibrated Hand Penetromei a. between soil and rock types: in -situ, the transition may be gradual. (V WATER LEVEL OBSERVATIONS, ft BORING STARTED 4 -23 -f WL � 26 WD T 17.2 1.5 hr AB BORING COMPLETED 4 -23 -E WL ir eirracon RIG FOREMAN P o WL APPROVED BAL JOB # 0599501