INVESTIGATIONS OP AND INSTRUMENTATION FOR MEASURING PRESSURE DISTRIBUTIONS IN SOIL ByArthur Wiggins Cooper AN ABSTRACT Submitted to the School for Advanced Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Agricultural Engineering Year Approved by _____7^» 1956 ____________ ProQuest Number: 10008657 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10008657 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 -1 3 4 6 Arthur Wiggins Cooper 1 An Abstract The development of a strain gage cell for measuring pressures in soil caused by surface loadings is described in detail. The pressure cell is two inches in diameter and three-fourths of an inch in thickness. It consists of a stainless steel disk (0.025 of an inch in thickness) soldered to a brass box with a removable bottom. Two active SR-1| strain gages are cemented to the underside of the stainless steel disk. Two "dummy” gages are cemented to the inside of the brass box. Wheatstone bridge. The gages make up the four arms of a When pressure is applied to the stainless steel disk it causes a change in resistance in the active gages. The voltage signal is amplified and indicated or recorded. The cell gives accurate pressure measurements when suspended in a homogeneous soil. Comparative pressure measurements were made with the small pressure cell and a load cell (ij. l/8 inches high, 1 3/l| inch in diameter, with a six inch diameter base). The load cell was found to give pressure readings two or more times as high as the small cell. A small amount of work is reported using liquid-filled rubber pickups and a liquid pressure transducer. Rubber tubing pickups did not have a linear calibration while a rubber balloon did. All of the liquid-filled pickups indi­ cate a low reading of pressure when used in soil. An Abstract Arthur Wiggins Cooper 2 The measured variation of soil pressure with depth under the center of the rear tire of a tractor is reported. This change in pressure with depth followed the same general pattern as values calculated by a theoretical formula devel­ oped by Froehlich. A theoretical discussion is given of the effect of surface contact area on the pressure in the soil along the load axis of a uniformly loaded circular plate as calculated by Froelich's formula. The discussion points out that the pressure at various depths under the surface of the soil is a function of the total load and the surface contact area of the load. It is not a function of the unit pressure applied to the surface alone. For example, an l8-inch applying a load often psi would at 15 inches depth in soil. cause a diameterplate pressureof Ip.6 psi A 12-inch diameter plate apply­ ing ten psi would cause a pressure of 2.6 psi at 15 inches depth. If the same total load that was applied to the 12- inch plate was applied to a circular plate having twice the area of the 12-inch diameter plate (17 inches in diameter), the unit surface load would be five psi, but the resulting pressure along the load axis 15 inches below the surface would be 2.1 psi as compared to the 2.6 plate applying the ten psi at the surface. psi for the 12-inch INVESTIGATIONS OP AND INSTRUMENTATION FOR MEASURING PRESSURE DISTRIBUTIONS IN SOIL By Arthur Wiggins Cooper A THESIS Submitted to the School for Advanced Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OP PHILOSOPHY Department of Agricultural Engineering 1956 t*/&-Q/lT7 & /*7 7 ACKNOWLEDGMENTS The author wishes to acknowledge gratefully the in­ spiring guidance and many helpful suggestions contributed by Professor H, F. McColly, chairman of the author^ guidance committee, and by Doctor W. M* Carleton, Graduate and Research Advisor of the Agricultural Engi­ neering Department, He also wishes to thank the other members of his guidance committee, Doctors A, E, Erickson and D, J, Montgomery for their suggestions, help and guidance during the investigations. Grateful acknowledgment is extended to Mr, G, E, Vanden Berg who worked with the author throughout most of this in­ vestigation and made valuable contributions to the project. The author wishes to express his sincere thanks to Doctor E, G, McKibben, Chief of the Agricultural Engineering Research Branch, U* S* D, A,; to Doctor M. L. Nichols, Head of the Tillage Machinery Laboratory Section, U. S. D. A.; and to Doctor A. W, Farrall, Head of the Agricultural Engineering Department, Michigan State University, who made it possible for the author to work on the cooperative project between the United States Department of Agriculture and the Agricul­ tural Engineering Department of Michigan State University from which the data used in this thesis was accumulated. The author wishes to thank Mr. J. B, Cawood, Laboratory Foreman, and his staff for help in constructing equipment. The author is especially grateful to his wife, Dorothy, for help in editing the manuscript and continued encouragement during the investigation. VITA Arthur Wiggins Cooper candidate for the degree of Doctor of Philosophy Pinal examination, February 21)., 1956, 1:15 P.M., Agricultural Engineering Building Dissertation: Investigations of and Instrumentation for Measuring Pressure Distributions in Soil Outline of Studies Major Subject: Minor Subject: Agricultural Engineering Physics Biographical Items Born, March 3, 1918, Fairfield, Alabama Undergraduate Studies, Alabama Polytechnic Institute, 1935-39 Graduate Studies, Alabama Polytechnic Institute, 1939-1)1, Purdue University, 191)6-1)9, Michigan State University, 1951)-56 Experience: Instructor and Assistant Agricultural Engineer, 1939-1)1; Asst. Prof. and Assistant Agricultural Engineer, Alabama Polytechnic Institute, 191)1-1)5; United States Navy, 191)51)6; Associate Agricultural Engineer, Alabama Polytechnic Institute, 191)6; Assistant in Agricultural Engineering, Purdue University, 191)6-1)7; In Charge Farm Electrification Research, .Purdue University, 191)7-1)9; Project Supervisor, Soil Conservation Service Research, Auburn, Alabama, 191)9-53; Agricultural Engineer, Tillage Machinery Laboratory Section, Agricultural Engineering Research Branch, U. S. D. A., Auburn, Alabama and East Lansing, Michigan, 1953-56 Honorary Societies: Gamma Sigma Delta, Sigma Pi Sigma Professional Societies: American Society of Agricultural Engineers TABLE OF CONTENTS Page INTRODUCTION ....................................... Soil Compaction 1 . 3 Reasons for Revived Interest in Soil Compaction and S u b s o i l i n g ............................. ASAE-SSSA Committee on Soil Compaction • . • • Scope of This T h e s i s REVIEW OF LITERATURE Soil Pressure Cells 1) 6 . 7 ............................... .......... 9 9 Goldbeck Pressure Cell ........ • • . • 11 Carlson Stress Meter . .......... . . . . . 11 WES Soil Pressure C e l l ......................... 13 California State Highway Department Pressure C e l l ..........................................li) Carbon Pile Cells............................... 15 Acoustic Stress Meter • • • • • ......... . . 15 Methods of Measuring Soil D e f o r m a t i o n ............. 17 Plaster Cast and Glass Fronted Box • . • • • 17 Bead Displacement ............ 18 Plaster of Paris and Cement B a n d s ............. 19 Bulk Density ................................. 19 Penetrometer ............................. 20 APPARATUS 21 Type A Soil Pressure C e l l ................ 21 Equipment Used for Calibrating the Type A Pressure Cell ............................................ 27 Soil Science Load C e l l ............................. 30 Liquid-Filled Rubber Pickups and a Liquid Pressure Transducer .......... Amplification and Recording Equipment 30 . . . . . . 36 TABLE OP CONTENTS (Cont.) Page Switching Mechanisms Soil Box .................... ..........................................39 Hydraulic P r e s s .......... PROCEDURE 36 39 ............................................1)2 Testing Type A Cells in Soil Box ................. 1)2 Testing the Soil Science Load Cell in Soil Box . . 1)3 Testing Liquid-Filler Pressure Pickups in Soil Box 1)3 Measuring Change in Soil Pressure with Depth in the Soil Box 1)1) Testing Cells at Powerama • • • • .............. 1)1) RESULTS AND D I S C U S S I O N ................................. 1)9 Performance of the Type A Pressure C e l l ...........1)9 Comparative Pressure Measurements with the Bevelled Top and Plat Top Strain Gage Pressure .............................. 52 Cells Comparison of the Type A and Load C e l l s ...........52 Uses and Limitations of the Type A Pressure Cell • 57 Performance of Rubber Pickups and Pressure Transducer in Soil ......................... 60 Variation of Pressure with Depth in Hillsdale Sandy Loam in Soil Box .......... 62 Measurement of Pressure at Various Depths Under the 66 Rear Tire of a T r a c t o r .......... A Theoretical Discussion of the Effect of Load Area on Pressure in Soil at Various Depths Below the Surface 68 RECOMMENDATIONS FOR FUTURE RESEARCH BIBLIOGRAPHY APPENDIX '. . ............. 71) 76 82 LIST OF FIGURES FIGURE PAGE 1* First model of the type A strain gage pressure cell 22 2. Diagram of the six position multiple switch and the first model type A cell 23 3* 1). 5« 6. Calibration comparisons for three strain gage cell arrangements ......................... 21) Detailed sketch of the latest model type A strain ...................... ,gage pressure cell 26 Sample calibration of the latest model type A strain gage pressure cell . . . . . .............. 28 Sketch showing principles of construction of the .................... .soil science load cell 31 7. Sample calibration of the soil science load cell. 8. Statham transducer 9. Calibration of rubber tubing and balloon pressure pickups with Statham transducer • • • . . . * . 35 10. The Offner six-channel recording oscillograph . • 37 11. General view of equipment for making soil pressure measurements in the laboratory • • . . . . • • 38 12. 13. li). 15, 16. . . . . . ............ •• 32 33 Box for testing pressure cells and pickups, and for measuring pressure variations in soil with depth under various applied l o a d s ............... 1)0 Method of placing type A cells in the soil at the plowing demonstration )n Chicago . . • • • • • 1)7 Method of placing type A cells and load cells in soil at the plowing demonstration in Chicago • • 1)8 Calibration of type A strain gage pressure cells in Maumee sandy loam ........................ Comparison of pressure measurements in Maumee sandy loam made with the type A and load cells . 51 55 LIST OF FIGURES (Cont.) Pag© 17. Sketches showing the position of the type A and load cells before and after the pressure was applied ......................................... 59 18, Pressure measurements in Hillsdale sandy loam • . • 19# Effect of pressure on bulk density of Hillsdale sandy loam ♦ • . . . « .......... * .............. 65 20# Change in soil pressure with depth - Maumee sandy loam - very c o m p a c t .......................... 67 21. Effect of load area on pressure in soil at various depths as calculated by Froehlich's formula ... 69 Effect of doubling the surface area keeping the load constant as calculated by Froehlich’s formula 71 22. 63 23. Effect of moisture and bulk density on soil pressure at various depths under a surface load ........ 72 2i|, Strain and stress diagrams for circular flat plates ..................... 8L|. with fixed edges 25* Theoretical stress and strain in a uniformly loaded circular plate with clamped edges ............... 85 LIST OF TABLES TABLE I. II* III# IV# V# PAGE Water calibration of rubber tubing and balloon pressure pickups withStatham transducer . . . 34 Pressure measured with tops of type A cells flush with the top of a false bottom, cell suspended in soil, and cells resting on bottom of soil box 50 Comparative pressure measurements with the bevelled top and flat top strain gage pressure cells ................................... 53 Comparison of pressure measurements in Maumee sandy loam soil with type A and load cells . . . 54 Comparison of type A and load cells in very loose Hillsdale sandy loam . . . . . . ........ • • 58 VI# Performance of rubber pressure pickups used with the Statham transducer.......... .............. 61 VII# Effect of pressure on the bulk density of Hillsdale ........ ........................ 64 sandy loam VIII. Sample calibration data for type A strain gage .........................82 pressure cell IX# Change in soil pressure with depth — Maumee sandy loam ................................. 83 1 INTRODUCTION For many years agricultural workers have generally ac­ cepted the importance of the physical properties of soil to plant growth. A large portion of the statement, however, concerning this relationship has been vague, qualitative, and frequently unsupported by factual information. For this reason, a Joint Committee on Soil Tilth was established some years ago by the American Society of Agronomy and the American Society of Agricultural Engineers for the purpose of establishing methods and procedures for measuring and evaluating "soil tilth". The following extracts are from reports of the Joint committee on Soil Tilth. 19i;3: No amount of empirical experimentation will tell us whether sub-surface tillage is superior to plowing, whether plowing is superior to disking, or what changes are desirable in the design of tillage machinery. Before we can make real progress we must know what soil physical state is desired for a given crop under specified climatic conditions. We must be able to measure the changes produced in soil tilth by our different management practices. 191+1+-* The Committee has found that many re­ search people desire to measure soil tilth, but no one seems to know how to do it. Unfortunately, the Committee cannot provide an exact yardstick. There has been considerable discussion of soil tilth over recent years. This Committee has re­ ported annually that something ought to be done about 2 it. Among other things we have suggested the estab­ lishment of a national tilth laboratory. Despite all that has been said and all that has been recom­ mended, there has been very little done in the way of improving our situation with regard to measuring soil tilth and its effect upon plant growth. We believe the reason for this is that there is very little enthusiasm among research workers for the present methods of approach to the tilth problem that we are now making. It seems that it is going to be necessary for us to make some new approach. ip As a result of the deliberations of this joint committee a monograph (30)^ was prepared to meet a long-felt need among soil and plant scientists and agricultural engineers for a critical evaluation of the relation of soil physical conditions to plant growth. The monograph discusses: (a) soil as a physical system, (b) mechanical impedance and plant growth, (c) soil water and plant growth, (d) soil aeration and plant growth, and (e) soil temperature and plant growth. It was written by nine scientists and edited by B. T. Shaw, In the epilogue of the monograph, Shaw states: Having read this far, the thoughtful reader may well be amazed that a plant is able to grow in such a complex environment. He doubtless has arrived at the conclusion of the authors; namely, that although we have some understanding and limited techniques for control of single factors affecting plant growth, we understand very little of the complex interrelation­ ships among these factors, and hence are not In good position to modify them intelligently. Later he states: During a given season, first one and then another of the physical factors may limit plant growth. For ^Numbers in parentheses refer to References Cited. 3 exampla, a soil with a claypan may be slowly drained in the spring. Because the soil is nearly saturated with water, it warms up slowly. In addition to the unfavorable soil temperature, root growth may be limited also by lack of adequate quantities of soil air, because a large proportion of the pores are filled with water. As the season progresses, a shallow pattern of root growth is formed, whether root penetration is inhibited by mechanical impedance or by lack of aeration in the claypan. Still later in the season, lack of adequately distributed rain­ fall may result in soil moisture being limiting to the shallow rooted crop. Then, conceivably, satura­ tion of the soil by a heavy rain still later on brings soil aeration back into play as the factor mostser­ iously limiting plant growth atthat time. The writing of this monograph, published in 1952, was the last official act of the Joint Tilth Committee. A number of soil and plant scientists and agricultural engineers, however, are still working on these problems. They are trying to determine the best physical properties of the soil for plant growth, and the methods of tillage and soil management practices to obtain these soil physical factors. Soil Compaction One of the factors creating a soil physical condition which decreases plant growth is that of soil compaction, as described by Shaw in the case of the claypan soil. compaction often reduces crop yields (3» problem is not new. lAw 27). Soil This Interest in subsoiling to break up compacted layers has varied periodically in the past 50 k years. Recently there has been a revived interest among agricultural workers, including farmers as well as soil and plant scientists and agricultural engineers. Reasons for Revived Interest in Soil Compaction and Subsoiling 1. The traffic over agricultural fields with tractors, Implements, and trucks has increased rapidly in recent years. An example of increased traffic Is spraying to control in­ sects. In cotton, for instance, the farmer may spray five to ten times in a season to control insects. Also the total weight and in many cases the unit load of the traffic has increased. Spreading lime with trucks is a good example of this Increased total and unit load. In addition to the weight of the truck it may be carrying three to five tons of lime when it goes on the field. The total load on the rear wheels of one of the largest wheel type tractors while plowing was measured and found to be over 9,000 pounds. One company now reports measuring 16,000 pounds on the rear wheels of one of their experimental tractors. Even though the extent of damage due to this traffic has not been evaluated it is an effective selling point for subsoiling and the farmers have become quite Interested In this practice. Certainly the effect of this traffic on soil compaction should be studied and evaluated. 5 2. With increased power available in recent years it has been easier for the farmer to stir the surface of the soil. In numerous cases he has done this several times in the spring or fall to prepare what he considers a better seedbed and to kill weeds. Much of this stirring operation has been done with a disk harrow which packs the soil below the depth of penetration of the harrow (9)* No matter what tool it is done with, in some soil types this stirring ac­ tion aids in the formation of filter pans. These pans are formed by fin© particles moving down to the bottom of the tilled area and filling the voids. 3. With increased power available it now costs the farmer less money and effort to accomplish subsoiling. He is much more willing to try It in hopes that it will in­ crease yield from the land. Furthermore in some states Agricultural Conservation Payments made for subsoiling have increased interest in this practice. Ij.. In many cases subsoiling has Increased the water intake of the soil, so that less moisture is lost due to runoff. On a few soil types substantial increases in yield have been obtained due to subsoiling. 5# The increased interest in deep fertilizer place­ ment has caused an increased interest in deep tillage. 6. Some very tough soils are easier to plow In the spring if they are ripped with a deep tillage tool in the fall. 6 ASAB-SSSA Committee on Soil Compaction Because of the recent interest in soil compaction and deep tillage a joint Soil Compaction Committee of the Ameri­ can Society of Agricultural Engineers, and of the Soil Science Society of America, was set up in 1955* The purpose of this committee is to study the soil compaction problem and to gather information to help guide the progress of understanding and solving the soil compaction problem. Three subcommittees from each group have been appointed, one to define terms involved in soil compaction, one to review the present knowledge of the subject, and one to study methods and instrumentation for making measurements involved in soil compaction studies. The Subcommittee on Terminology classified the types of compacted soils as Induced pans and genetic pans. In­ duced pans are those which are caused by applications of surface pressure to the soil (pressure pans), or caused by filtering of fine particles to form a dense layer (filter pans). Genetic pans are those dense layers of soil which occur naturally. The following classifications and defini­ tions of soil conditions have been proposed for reporting research on soil compaction: Type I. Induced Pans A. Pressure pans are horizontal layers having a higher bulk density and lower total porosity than the soil material directly above and below. The top 7 of the pressure pan usually coincides with the lower depth of normal cultivation and is never more than 12 inches from the surface of the soil* The mechanical analysis and chemical properties of the pressure pan layer is similar to that of the soil immediately above and below the pan. In cultivated fields the pressure pan may be more pronounced in traffic row middles than it is immediately under the crop row. Pressure pans are most common in medium-textured soils of low struc­ tural stability and in regions where the soil is not subject to frequent freezing and thawing when moist. B* Filter pans are closely related to pressure pans, possibly being caused by collection of fine particles from the surface cultivated layers washing down and collecting in a pressure pan. They have all the characteristics of pressure pans, plus having coatings of fine particles (silt and clay) on the surfaces of the structural aggregates near the top of the pan. For the purpose of this thesis these are the two defini­ tions of interest. The subcommittee also described the following genetic pans: (a) claypans, (b) fragipans or siltpans, (c) indurated hardpans, (d) alkali pans, and (e) dense C or D horizons. Scope of This Thesis This thesis deals with one small segment of the com­ paction problem, that of measuring the pressure distribution in soil caused by traffic over the land such as tractor, truck and implement tires, and tillage tools. If the pres­ sures in the soil could be measured it would help to evaluate the forces causing soil compaction. 8 The contents of the thesis include a description of the development of a small (2-inch diameter by 3/k inch thick) electrical resistance strain gage pressure eell (transducer); the characteristics of this cell and other soil pressure cells; a description of some auxiliary instrumentation used with cells; some experiences with rubber pressure pickups and a liquid pressure transducer; some results of the measurement of pressures at various depths under the rear tires of a tractor; and a theoretical discussion of pressure distribution in soil based on calculations by Proehlich's formula (5)* 9 REVIEW OF LITERATURE Soil Pressure Cells The U. S. Waterways Experiment Station report (36) gives a complete description of most types of soil pressure cells developed for soil mechanics studies, such as measurement of the pressures in the soil under walls, footings, and tunnels* In addition, the report describes four series of tests per­ formed at the Experiment Station with their newly developed pressure cell known as the WES cell* to evaluate: Their tests were planned (a) the effect of projection of the pressure cell from the surface of a rigid wall in terras of the indicated pressure of a sand mass bearing on the wall; (b) the effect of the compressibility of the cell mounted flush with the rigid wall in terms of the indicated pressure of the sand on the wall; (c) the effect of the relative dimensions of the cell on pressure indicated by a cell wholly within the sand mass; and (d) the effect of the cell compressibility on its ability to Indicate pressure within the sand mass* They found that if the ratio of cell diameter to its projection from a rigid surface exceeds 30, the pressure indicated is nearly the same as that indicated by the cell mounted flush with the surface. They determined that a pressure 10 cell with a diameter-thickness ratio greater than 5, when placed near the center of a sand mass in a pressure chamber, indicated nearly the same pressure relative to the pressure applied at the surface of the sand. Their data showed that for values of the ratio of cell diameter to deflection (com­ pression) exceeding 2000, there was very little change in indicated pressure. The exact relationship between the pres­ sures indicated by the cells and those existing in the ab­ sence of cells was not established. However, they considered it very probable that the criteria established for cells mounted in a rigid wall, diameter-projection ratio greater than 30 and diameter-deflection ratio greater than 1000, do limit the range within which the cells indicate approximately the pressures which act on the wall in their absence. Although the requirements for cells to measure pressures in soils under tillage implements and other agricultural traffic are somewhat different from those to measure soil pressures under walls, footings, and tunnels, there is enough similarity in the fundamental considerations to justify a brief description of some of the latter type of cells in this thesis. For a complete description of the Goldbeck cell, Carlson stress meter, WES soil pressure cell, California State Highway Department pressure cell, carbon pile cell, and acoustic stress meter, one should refer to the original publication or the Waterways Experiment Station report (3&)* 11 Goldbeck Pressure Cell As early as 1916 Goldbeck (6, 7, 8, 31) developed a cell which could be placed in soils to measure pressure through earth fills. one end open. It consisted of a cylindrical metal case with A movable metal piston was fitted loosely in the open end of the case and was held flush with the rim of the case by a thin metal diaphragm. Electrieal con­ tact was made between the movable piston and an insulated electrical contact button which was fastened inside the case. A small pipe, about l/8-inch inside diameter, was fas­ tened to the inside of the cell and extended to the ground surface. A single-conductor Insulated wire connected with the contact button was carried to the surface with the pipe. Pressure acting on the movable piston maintained it in electrical contact with the bottom. applied through the pipe. Air pressure was The opening of the electric circuit, as shown by a lamp or ammeter, indicated an air pressure equal to the applied soil pressure. The cell dimensions varied, but were usually 5 l/2 inches in diameter and 1 l/2 inch thickness between the parallel faces. Carlson Stress Meter The Carlson stress meter (2) is an adaptation of a strain meter to measure the stresses in concrete. This 12 stress meter has been used successfully for the measurement of soil pressures against rigid walls and in special mountings within earth structures. The cell operates by transmission of pressure, which acts on a flat, circular face plate, through a confined liquid to a metal diaphragm whose deflection ac­ tuates a strain meter. The face plate and a thick back plate are welded together at their perimeter so that a thin chamber is left between them. The edges of both plates are made sufficiently thin to be flexible. formed by boring the back plate. A central diaphragm is The strain meter is at­ tached to the rear of the base plate; the fixed arm and case being attached to the rigid portion of the plate, the movable arc being attached to the center of the diaphragm. The thin space between the face and back plate is filled with mercury. Pressure acting on the face plate is transmitted through the confined mercury to the diaphragm, which is de­ flected proportionately. Two electric resistance wires are coiled between insulators on the movable and fixed arms of the strain meter in such a manner that as strain is ap­ plied between the arms, tension is increased in one coil and diminished in the other. The electric resistance of the coils changes with the tension in the wire and these changes being opposite in the two coils, the effect is doubled. De­ flection of the diaphragm in the stress meter by the applied pressure produces a resistance change in the strain meter. 13 This change in resistance is a simple function of the applied pressure. Although the soil pressures are successfully measured when the cell is mounted flush in walls, there is some question of the effect of the projecting strain meter on stress distribution within the soil mass. The face of the cell is approximately 7 inehes in diameter and the strain meter, 1 inch in diameter, projects approximately ij. inches, “WES Soil Pressure Cell The Waterways Experiment Station pressure cell (36) consists of a circular face plate welded at its perimeter to a thicker base plate. The face plate has a peripheral slot which forms a flexible joint between the two plates, A diaphragm is formed in the base plate by boring. chamber thus formed is closed by a cover plate, The gage A connector eable enters the gage chamber through a packing gland in the side of the base plate. The thin disc chamber between the face and base plates is filled with oil (recently, modified cells are filled with mercury). Pressure applied to the face plate is averaged and transmitted by the oil to the diaphragm. The radial strain produced in the diaphragm by the pressure is measured by an SR-lj. electric resistance strain gage. An inactive or "dummy*1 strain gage mounted in the gage chamber on a piece of unstressed metal provides temperature compensation. Alteration in the active gage resistance produced by the diaphragm strain and indirectly by the applied pressure is amplified and indicated. A linear relation between applied pressure and resistance change can be attained. The WKS cells range in size from three to 12 inches in diameter and from 1/2 to 1 l/lf. inches in thickness. California State Highway Department Pressure Cell This cell was developed by the California State Highway Department (3&) principally for the purpose of measuring subgrade pressures produced by wheel loads on pavements. An outer diaphragm, attached rigidly at its circumference to the body of the cell, forms the outer end of a thin, cylin­ drical oil chamber. Pressure applied to the outer diaphragm is transmitted by the oil to a smaller weighing diaphragm. An iron disc is held against the weighing diaphragm by a flat spring, and is separated from the poles of the U-shaped iron core of an electromagnet by a small air space. De­ flection of the weighing diaphragm decreases the air gap between the disc and poles of the electromagnet. A rigid ring limits the travel of the disc and prevents damage by excessive pressures. Movement of the metal disc changes the magnetic flux in the gap and thus changes the reluctance of the circuit. A balancing unit consisting of a similar coil and gap is located separately from the cell in such a way as to be unaffected by the load on the cell. The cell 15 and balancing unit are connected in a bridge circuit. The unbalance due to pressure applied to the cell causes an Increase in current through the control arm of the bridge. The current change is a measure of the change in applied pressure. This cell is approximately seven inches in diameter and 1 5/8 inches in height. Carbon Pile Cells Carbon pile cells are perhaps the earliest type of soil pressure cell. In spite of many attempts to employ the carbon pile as a practical measuring unit in soil pres­ sure cells, there has been no success. The measuring ele­ ment of the cell consists of a stack of thin carbon discs (17) mounted between metal plates. When pressure is applied to the ends of the stack, its electrical resistance de­ creases. The change is of sufficient magnitude to be measured with a bridge. The principal difficulty with the stacks appears to be that they do not retain calibration. However, in the laboratory where it is possible to recali­ brate the stack frequently, good results have been obtained for dynamic tests and for short-duration static tests. Acoustic Stress Meter The basic principle of this instrument (15, 19) is the dependence of the natural frequency of a freely vibrating 16 string on the tension applied to it* Fundamentally, the cell consists of a face plate free to move relative to the cell housing, and acting through a hemispherical con­ tact at the midpoint of a steel beam supported by a knifeedge bearing near each end. A piece of steel wire is stretched between rigid arms which extend from the tension side of the beam. External pressure applied to the face plate bends the beam and increases the tension in the wire proportionately. The wire is set in vibration by means of a small coil through which an electric current is switched momentarily. The vibration of the wire is then picked up and transmitted by the same coil and connector wires to an indicating device (head phone or cathode ray oscillograph, for example). Since the wire is vibrating freely, its frequency will be the natural frequency which results from the altered tension in the wire. This frequency is matched, either by direct comparison or by superposition, with that of an adjustable standard wire. The standard wire consists of a similar taut wire with calibrated, adjustable tension. This wire is actuated and its vibrations detected by a coil similar to the one in the cell. The tension in the standard wire is adjusted so that its tone matches that of the cell, or so that the superposed signals from both vibrating wires do not interfere or beat. The tension in the standard wire then corresponds to a particular pressure onithe cell, as established by an initial calibration. 17 Soehne (33) evidently used some type of pressure cell in the soil for he made the following statement in his articles (as translated)* Measurement of pressure in the soil is not completely simple. A compression pressure cell, in order to make exact measurements, must be just as hard as the surrounding soil or infinitely thin. But it is either harder than the surround­ ing soil, which loads to a concentration of the force-lines toward the pressure cell or it is softer which results in an envelopment of the pressure cell by the lines of force. Moreover, on introduction of the pressure cell into the soil, the original stratification density is disturbed to some extent. For this reason it is not easy to ascertain by measurement what pressure distribution is present on the load surface. On the other hand, however, an error of 25 percent in measuring the pressure or in calculating results only introduces a maximum compaction error of one percent pore volume. An exactness of 25 percent ought, however, to have been obtained in the determination of the pressure stress. Methods of Measuring Soil Deformation Plaster Cast and Glass Fronted Box Nichols and Randolph in 1925 (21) developed the plaster cast method of studying soil stresses. The soil vjas stratified into layers by means of aluminum leaf or other delicate material and a definite pressure was applied. The soil was then removed one layer at a time and a cast made of the distorted surfaces of the aluminum leaf. A camera lucida was used for transferring the contours to 18 coordinate paper for study. In another visual method Nichols (20) placed layers of soil and thin aluminum foil in a glass fronted box. Known forces were then ap­ plied to the surface of the soil by implements of various shapes, and the distortion of the aluminum leaf was noted. The pressure at the bottom of the soil in the box was measured with a Goldbeck gage. Later Kummer (13) with the same glass front box, coated the glass with levigated alumina and noted the scratches caused by the movement of the soil as a force was applied to the surface. Reaves and Nichols (25), with this method, photographed these scratches in the alumina to study the soil stresses. Bead Displacement McKibben and G-reen (18) arranged small beads in the soil according to a predetermined color pattern. The beads were placed accurately at known distances vertically and horizontally. the beads. Wheels were then run over the soil containing The beads were dug up, and their displacement was taken to indicate the displacement of soil. Prom the displacements the deformation pattern was determined for steel wheels and for pneumatic tires. 19 Plaster of Paris and Cement Bands Soehne (33) placed bands of a mixture of plaster of paris and cement perpendicular to the direction of travel and at various depths to measure soil deformation under tractor and trailer tires. To place the mixture he drove a tube filled with plaster of paris and cement in the soil horizontally from a pit, and pushed the mixture out with a plunger as he pulled out the tube. After the tire had passed over the treated area he carefully excavated to de­ termine the soil deformation. Bulk Density By measuring the bulk density of a soil before and after loading, the amount of soil deformation caused by the load can be determined. A number (10, 11, 16, 3&# 39) of workers have used core samples for determining bulk density. The air pressure pycnometer (12, 22, 23# 29) can be used to determine the bulk density of the soil indirectly by determining the pore space volume of the soil. This instrument applies the principle of Boyle!s law to a sample in a closed system, and permits the direct determination of the volume of the gaseous constituents by measurement of pressure-volume relationships at a constant temperature. 20 Vomocil (37) recently reported a garnma-ray absorption technique for the measurement of soil bulk density. This instrument should aid greatly in the research of deter­ mining the extent of compaction due to various loads. Penetrometers Several penetrometers (l, 26, 28, 35) have been developed to locate compacted layers and indicate the degree of com­ paction. 21 APPARATUS Type A Soil Pressure Cell Three different models were designed, built and tested during the development of the type A strain gage pressure cell. The first model consisted of a small brass box (2 inches in diameter and 0.7 of an inch in height) with a thin metal disc diaphragm held in place by a threaded ring top (Figure 1). At the center of the lower side of the metal disc was cemented a l/2-inch SR-ij. electrical resis­ tance strain gage (type A-5, 120 fl, gage factor 2) which was the active gage of the circuit (Figure 2). Another SR-ij. strain gage was cemented to the bottom of the cell box for temperature compensation. This pressure cell operated satisfactorily, but had the disadvantage that its calibration was not linear (Figure 3, curve l). To overcome this difficulty of non-linear calibration, three new cells were constructed. The cell box construction was like those shown in Figure k> but the strain gage ar­ rangement was different. These cells had a 1/16-inch SR-lj. strain gage (Type A-19, 60 SI, gage factor l/68) cemented at the center of the metal disc, and a gage from the same lot cemented to the bottom of the brass box for temperature 22 gfk‘ M; ■ ilPW :il“ ^ g S £ w j Pig. 1. First model of the type A strain gage pressure cell. 0 z ui h 0: <1 D © H 2 < Ui hi 23 01 | o S CELL 2 ° x u p OF t in MULTIPLE SWITCH AND T^PE A •J'—> Ui -1 U Ui D/AGRAM < © FiG. 2. vD /I 2k ■M kb fiT-V-:-; F r:hi-'1 ttti m \ l}r ,i 4-t-U V !* i-"FT iffi ni ik I t - *■ if:1 tn IT ' dst ±t!T TT ±T FIG.3. CAL B R A T ON COMPARISONS ■BBS \5 ESSUR.E Ob/in'*') compensation. This arrangement gave a straight line calibra­ tion between 2 1/2 and 30 psi but curved slightly from 0 to 2 1/2 psi (Figure 3» curve 2). jections to this arrangement. There were two serious ob­ First, the cell was hard to assemble with the temperature compensating gage on the bottom plate. Second, this gage indicated a slight strain in the bottom cover plate when calibrated in water. This strain did not cause an error when the cell was suspended in soil, be­ cause pressure was applied to both top and bottom as in the water calibration. However, it caused a significant error when the cell was placed in the bottom of a soil box, be­ cause then the bottom plate of the cell was not strained as it was in the water calibration. read high. This caused the cell to Another minor difficulty was that the direet inking oscillograph used was equipped with a calibration re­ sistor for 120-ohm gages, the resistance of most of the gages used in the Agricultural Engineering Laboratories. This necessitated a different calibration setting from normal or changing of calibrating resistor. Due to these difficulties the third model of the type A strain gage pressure cell was developed. A detailed sketch of this model is shown in Figure 4* and a sample calibration (curve 3) is shown in Figure 3» This cell has twice the sensitivity of the two previous models because of its two active gages* Twelve of these latest model cells were built 26 [— D U M M Y GAGES ( S R - 4 TTPE A -/8) S T A IN LESS STEEL ^SOLDERED JO IN T BRASS r fcr- R U B B E R GASKET £ NO. 6 - 3 2 F, H. BRASS SCREW S NO. 2 0 - 4 W I R E R C S H IE L D E D 4 SLOTS CABLE ^ F b RASS N U T YBRASS W IT H F IT T IN G T A P E R E D THREADS S H IE L D SOLDERED T Y P E A-18 S R - 4 S T R A I N G A GE S C E M E N T E D TO S T A I N L E SS S T E E L C O V E R AT 90° TO EACH OTHER AS N E A R C E N T E R AS P O S S IB L E T U R N OFF THREADS TO PRE5S FIT AND SOUPER -H jh h DETAIL o f M O D IF IE D STANDARD B R A S S FIT T IN G FIG. 4 TYPE SCALE-ACTUAL A SOIL P R E S S U R E CELL SIZE AUGUST 1955 au o c 4 kiaU 27 for use at the Powerama Plowing Demonstration held in Chi­ cago, Illinois, in September 1955* Before the tests in Chicago the cells were calibrated from 0 to 30 psi. calibrations in this region were linear. The After these tests some of the chart readings indicated pressure measurements above 30 psi, so the cells were calibrated up to 60 psi. Above l|.0 psi the calibrations started to curve slightly so two of the cells were calibrated up to 90 psi to deter­ mine the characteristic of the cells in this region (Figure 5K *A'"!* For future construction of cells It is suggested that the two ’’dummy*1 gages shown in Figureij. be moved to opposite edges of the underside of the stainless steel disc. The length of the strain gages should be plaeed so that they indicate the radial stresses. all four gages active. This arrangement would make The two center gages should be in opposite arms of the Wheatstone bridge and the two outside gages should be in the other opposite arms. The reason for this arrangement can be seen by exam­ ining Figure 2ij.. The center gages would be in tension and the edge gages would be in compression. Equipment Used for Calibrating the Type A Pressure Cell The first equipment used to calibrate the type A pres­ sure cells consisted of a 5-quart pressure cooker for a 28 1 1 T TiV B FIG. 5 PLE CALIBRATIO M CURVE FOR LATEST MODEL T Y P E A RAIN GAGE PR ESSURE CE ;LQyjy ill Q \<6O A PRESSURE ( l b / i n ' 1') 29 pressure chamber, an air compressor to supply air under pressure, two control valves to regulate the air pressure accurately, and a mercury manometer to measure the air pressure in the chamber. The cells were placed in the chamber3which was filled approximately half full of water to reduce temperature fluctuations due to the compressed air. The top was then placed on the cooker and the leads were brought out the top through a rubber stopper to the amplifier and recorder. The air pressure above the water in the chamber was increased by 5 psi increments, and the resulting strain on the pressure cell measured and recorded. The second set of calibration equipment consisted of a heavy steel tank with a special top and a mercury manometer. In this arrangement city water pressure was used to supply increments of pressure. recorded. The resulting strain was read and The top of the tank had a four-inch pipe plug which could be removed to put in the cells* The leads of the cells could be brought out through four rubber stoppers in the top of the tank. By this arrangement four cells could be calibrated at the same time. Later a calibrated 0-100 psi Bourdon-tube pressure gage was used to indicate the pressure instead of the mercury manometer. 30 Soil Science Load Cell* Figure 6 shows the construction of the load cells borrowed from the Soil Science Department of Michigan State University* A sample calibration curve for one of the cells is given in Figure 7. Liquid-Filled Rubber Pickups and a Liquid-Pressure Transducer A small amount of work was done with liquid-filled rub­ ber tubing and balloons connected to a pressure transducer. A Statham model No, P6-306-120 unbonded strain gage pressure transducer was used (Figure 8)• It had a maximum input voltage of 7* resistance of 125*5 ohms and a calibration factor of 8]4,.11j.. The manufacturer *s calibration was checked with a dead weight tester and found to be accurate. Red rubber and latex tubing and a balloon were used for pressure-pickup bulbs. These were calibrated with water pressure in the same manner as were the type A cells. As seen from the calibration data (Table I, and Figure 9), the balloon gave a linear calibration but the rubber tubing calibrations varied from a straight line due to the rigidity of the tubing walls, *The load cells were loaned to this project by A. E. Erickson, Soil Science Department, Michigan State University, They were designed and built by P, J, DeKoning, Applied Mechanics Department, Michigan State University, to be used by N. A. Willits for his research which will be reported in detail in his Ph. D. thesis, Michigan State University, 1956, 31 WIRING DIAGRAM TC " 100 ^ / - A C T I V E STRAIN GAGES < 90° APART a r o u n d ' C Y L IN D E R TEM PERATURE STRAIN GAGE lA\\\X\\\\\ v \ \ R \ \ ^ x ^ \ \ \ \\\X\1 FIG.6 SO IL SCIENCE LOAD CELL COM PEK1SATINJG 32 SAMPLE CALIBRA ION OF 5 0 1L 5C MCE L O A D CE L- I Load C lb ) a £ 60 12.0 24-0 33 THE S T A T H A M TRANSDUCER ELEM ENT BELLOWS FIG. 8 UMBOMDED S T R A IN STATMAM TRANSDUCER 3k co -d" X vO -d- C\J rH CM fA o vO -d" 1A O' 1A P o vO fA rH CO o IA X o p vO 0- p o p o p -d- d. XI P 60 H d (D (D P P P 03 CO m K PH & o o X d P s 0CM CM X CO P fA O' XI X P W X W) 5 © « •d P H TABLE I <*s p Q rH o rH TJ • S « H op d IA CM with made a o d. XJ O d P rH © 60 x d X X © P at 3! d G O 03 © o . -d CM co • O' CM f• P X • O' p ri 1A . C^ d p* W ) W . c H ■H indicated CM CO o are d •rl N. d •H oil. Readings Indicator. o P P at P instrument with Young « o O P o O J O E PRE5SURE CELL5 V1EE S A M D Y -CIO A M ii PLUNGER PLUNGER PLUNGER Soi L S O IL ,C .E L I_ TOP OF C E LL5 FLUSH W I T H FALSE B O TTO M G P R E S S U R E A P P L I E D □' □ 1=1 ' SOI1- Ct-Ll- C E L IP CELLS SUSPENDED M 50 1u b v O b / i n 7' ) CELLS ON BOTTOM OF BO X' 52 soil is compacted. The soil to the side of the cell compacts causing a concentration of load on top of the cells resulting in a higher pressure in the soil than would be present if the cells were not there. This error constitutes a maximum in that the bulk density of the soil was a minimum. If the bulk density of the soil is increased there is less compac­ tion to the side of the cell and therefore less concentration of load on the cell. Comparative Pressure Measurements with the Bevelled Top and Plat Top Strain Gage Pressure Cells There was some question as to whether the slight bevel on the top of the first model pressure cells (Figure 1) would affect the pressure measurements. the data in Table III was obtained. In order to determine this The first model cells were compared with the latest model cells (Figure ij.) by placing them in the bottom of the soil box with their tops flush with the false bottom. As can be seen from the data there was no significant differences in the pressure measure­ ments made with the two cells. Comparison of the Type A and Load Cells In Figure 16 pressure measurements made with the type A cell are plotted against measurements made with the load cell 53 TABLE III COMPARATIVE PRESSURE MEASUREMENTS WITH THE BEVELLED TOP AND PLAT TOP STRAIN GAGE PRESSURE CELLS Test No. Pressure Applied (lb/in2) 2 lb/in2 6 4 lb/in2 lb/in2 8 lb/in2 10 12 lb/ in2 lb/in2 Bevelled Top Cell1 1 1.7 3.0 5.2 7.0 9.1 11.0 3 2.5 4-5 6.4 8.5 10.5 12.0 5 1.7 4.3 6.2 8.2 9.8 11.8 Average 2.0 3.9 5.9 7.9 9.8 11.6 Flat Top Cell 9 2 1.8 3.5 5.7 7.5 9.2 11.4 4 2.0 3.7 5.7 7.5 9.7 11.7 6 2.0 4.0 5.7 7.7 9.5 11.4 Average 1.9 3.7 5.7 7.6 9.5 11.5 TABLE IV COMPARISON OP PRESSURE MEASUREMENTS IN MAUMEE SANDY LOAM SOIL* MADE WITH TYPE A AND LOAD CELLS Distance Below Soil Surface Pressure Measured With Load Cell Type A Cell in. lb/in2 lb/ in2 10 3 10 10 10 10 10 10 10 10 10 10 1 10 10 10 10 10 10 9.1+ 37.5 5.2.0 37.5 5-2.0 42.0 18.7 9.1* 13.9 5-6.7 23.59.556.1 k.7 16.3 18.7 28.0 6.9 28.1 18.1* 18.5. 19.6 21.1* 16.0 l*.l* 5.0 16.5 6.3 2.1 29.7 2.5 11.6 13.5 17.2 6.6 13.2 28.0 Load Applied With Plow Tire R. Trac. Tire R. Trac. Tire Sprayer Tire R. Trac. Tire Sprayer Tire R. Trac. Tire S. P. Tamper P. Trac. Tire R, Trac. Tire R. Trac. Tire Plow Tire R. Trac. Tire P. Trac. Tire R. Trac. Tire F. Trac. Tire R. Trac. Tire P. Trac. Tire R. Trac. Tire (Plowing) .(Spraying) (Spraying) (Tamping) (Tamping) (Tamping) (Rolling) (Plowing) (Plowing) (Plowing) (Spraying) (Spraying) (Rolling) (Rolling) **The mechanical analysis of the Maumee sandy loam was 66*5-$ sand, 28.2$ silt, and 5*5-$ clay. 55 FIG. 16 brltr _ iMPARISOk) OF PRESSURE MEASUREMF. MAUMEE SANDY LOAM 5 0 1 L MADE. WITH T Y P E A AS D L O A D C E L L S I © PRESSURE 20 MEASURED WITH A CELL ( I b / i r ? ) 56 under various surface loadings. The tire or implement apply­ ing the load can be found in Table IV. It Is not surprising that the points of the curve are scattered because of the nature and position of the load. Even though the same tire or Implement applied the load to both cells, the lugs were probably not in the same position above both cells especially in case of the rear tractor tire therefore one would expect the readings to vary. On close examination it can be seen that the pressures measured under the front tire of the tractor, and the plow and sprayer tires fall closer to the "average comparison" line than do the pressures measured under the rear tractor tires. If both cells had indicated the true pressure in the soil where measurements were made they should have deviated from points along the i+5-degree line in Figure 16. Because they deviated from points along a line above the lj.5-degree slope it must be concluded that either the load cell indi­ cated a higher pressure than the true value or the type A cell indicated a lower pressure than the true value. From the analysis shown in Figure 15 the indicated pressure measurements made with type A cells should not be signifi­ cantly low. Therefore from the data available it is assumed that the load cell indicated a pressure approximately twice as high as the true pressure in this Maumee sandy loam soil with a very high bulk density. 57 Figure 17 is a sketch of the position of the type A and load cells before and after pressure was applied to a loose Hillsdale sandy loam soil. A comparison of pressures measured in very loose Hillsdale sandy loam is given in Table V. This shows that in a very loose soil the indicated pressure of the load cell would be quite high. This test should be repeated several times before accepting these readings as the exact ratios of the indicated pressure to the true pressure for this bulk density* It is an indication, however, of the maximum error that might occur in using the load cells for measuring pressures in very loose soil. There are two basic reasons for the load cell to indi­ cate a pressure higher than the true pressure. First, the base has 26.3 square inches as compared to an area of the top of the cell of 2.Ip square inches. This allows less settling of the cell with the soil which causes a concentra­ tion of load on the top of the cell. Second, the soil around the cell compacts and the cell does not compress appreciably causing a concentrated pressure on top of the cell which results in a high indicated pressure. Uses and Limitations of the Type A Pressure Cells The type A pressure cell should give relatively accurate pressure measurements when it is placed within a homogeneous soil mass. When it is being used near a hardpan it should be placed so the top of the cell is flush with the top of the 58 TABLE VCOMPARISON OP TYPE A AND LOAD CELLS IN VERY LOOSE HILLSDALE SANDY LOAM Pressure Applied Pressure Measured (lb/in2) Type A lb/in2 Cell 1 Cell 2 Average Cell 1 and 2 Cell 2 1.8 2.3 2.0 2.2 k 3.8 3.8 3.8 ’ 13.0 6 5.8 5.7 5.8 1-1-3*3 8 7.7 7.3 7.5 95.5 10 10.0 9.5 9.8 186.6 12 11.6 10.7 11.2 21^3.0 Height of load cell -.i|..l in. Diameter of top of load cell - 1 3/ij. in. Diameter of bottom of load cell - 6 in. Height of type A cells - 0.7 in. Diameter of type A cells - 2 in. Initial depth of soils Above all cells - 2 in. Below typeA cells 1 and 2 Below load cell - 6.i| in. Pinal depth of Above type Above load Below type Below load - 9.8 in. soil: A cell 1 and 2 - 1.3 in. cell - 0,6 in. A cell 1 and 2 - 5*3 in. cell - 2.6 in. 59 “ r~i 1-- 1 S o |L a. BEFORE PRESSURE WAS A PP LIE D , i 1~ 1 1 1 ' Sol u b. A F T E R PRESSURE W A S a p p l ie d FIG. 17 POSITION OF TYPE A AND LOAD CELLS BEFORE AND AFTER PRESSURE WAS APPLIED. 60 pan or placed within the pan with the soil above the cell compacted to its original bulk density. This cell should give relatively reliable measurements in uncemented soil. It is probable that it would not give accurate results in cemented soils unless the cell is placed in the soil and then the soil given enough time to wet and dry sufficiently to cement around the cell. Performance of Rubber Pickups and Pressure Transducer in Soil There were not enough measurements made with the rubber tubing pickups or the balloons to establish any definite conclusions. The indicationsjas shown in Table VI,are that the rubber tubing and balloon pickups read low at the higher pressures. Since the indications were that it would take a con­ siderable amount of development to devise a satisfactory liquid-filled pickup it was decided to concentrate on the strain gage pickups and postpone the work on the former. One definite disadvantage of the liquid-filled pickups was that it would be difficult to devise a piping system so that more than one bulb could be used with one pressure transducer. 61 TABLE VI PERFORMANCE OF RUBBER PRESSURE PICKUPS USED WITH THE STATHAM TRANSDUCER Pressure Applied lb/in 2 Pressure Measure (lb/in^) With Red Rubber Tubing Soil Depth (inches) 2 k 6 Balloon* Test. No. 2 3 Ij- 1 Ave. 0 0 0 0 0 0 0 0 0 2 1.8 1.8 2.0 1.7 1.6 1.6 1.9 1.7 k 3.8 3.8 I4..I 3.2 2.4 2.9 3.5 3.0 6 5.5 5.5 5.8 4.2 4.1 5.0 4.4 8 7.0 6.8 7.2 5.8 5.6 5.3 6.6 5.8 10 8.2 8.0 8.8 7.2 7.0 6.6 8.1 7.2 12 9.8 8.5 8.3 7.9 9.4 8.5 ^2M loose soil above balloon 611 compacted soil below balloon 62 Variation of Pressure with Depth in Hillsdale Sandy Loam in the Soil Box The pressure measurements given in Figure 18 were made with the first model type A cells and not enough data were taken to establish reliable curves. The data are presented here to indicate how change in pressure with depth of confined soils might be determined with relatively few readings. The curves in Figure 18 were fitted to the data by linear regression and in all cases except the twoinch depth the curves crossed the zero line of pressure at the bottom of the box at an initial depth in the vicinity of 36 inches. Using 36 inches as the point of convergence and the pressure applied at the surface as a starting point, the lines were drawn as shown. The curves indicate that with this type soil at the given moisture content all of the load applied to the surface would be carried by the sides of the box when the depth reached approximately 36 inches with a reasonable applied load. If this assumption is true the convergence point for any soil at a given moisture could be determined by several duplications of a few points. Any of the needed curves could then be drawn. This assumption needs further investigation to draw definite conclusions. Table VII and Figure 19 show the effect of applied pressure on the bulk density of Hillsdale sandy loam. It FS L 63 HILLSDALE o j 10 CO ®o 1 oZ n // H B/ / O f / / BI [ / Q / (tUII/qi) ^ 3 A V 1 HIOS d o «o ITIAL FIG. 16. PRESSURE *i i . •C N o f-c?/-. 2 -J . <1 0 I 3 <1 p> CM ' i •c/ > o OF I # < DEPTH /// / ! i Vi 3/ // 3 to' SOIL f / / H i ■" / , 11 t(VIo r // / (INCHES) m **< MEASUREMENTS IN ■ # < o , 'r II c o , f" T e 3 o to t> / 0 /o WOXXOQ b xv 3ans93ad 64 TABLE VII EFFECT OF PRESSURE ON THE BULK DENSITY OF HILLSDALE SANDY LOAM** Applied Pressure Average Initial Depth of Soil Cinches) t' 2 k 6 8 gm/cc gm/ cc gm/cc gm/cc gm/ cc gm/ cc gm/ cc 0 0.90 0.93 0.92 0.93 0.93 0.93 0.92 2 1.06 1.09 1.05 1.07 1.05 1.08 1.07 4 1.12 1.16 1.14 1.15 1.12 1.14 1.14 6 1.20 1.20 1.18 1.20 1.18 1.18 1.19 8 1.2k 1.26 1.22 1.21). 1.20 1.23 1.23 10 1.28 1.28 1.2i| 1.27 1.24 1.30 1.27 12 1.28 1.30 1.26 1.30 1.27 1.34 1.29 lb/in^ 10 12 The various depths of soil were placed in. the soil box and the pressures indicated were applied to each batch of soil. After each load was applied, the depth of the soil was recorded. The bulk density was calculated after determining the weight of the soil and the moisture con­ tent. The mechanical analysis of the Hillsdale sandy loam was 58.2$ sand, 3&«4$ silt, and 5*4$ clay. 65 loSURE ON THE BULK D OF.HILLSDALE 5ANDY" LOAM 12,9 % M O IS T U R E * 1.30 -6 l-ZO ^[.10 r A P P L ED PRE55URE & ( l b / i n 2' ) 66 is surprising to note that the bulk density did not decrease when the depth of the soil was increased up to 12 inches. Measurement of Pressure at Various Depths Under the Rear Tire of a Tractor The pressure measurements plotted in Figure 20 (except the 5b psi at zero distance below soil) were measured with type A pressure cells in Maumee sandy loam under the center of the rear tire of the Oliver 99 tractor at the Plowing Demonstration in Chicago in September 1955- The points, although scattered, give an indication of how the pressure in the soil decreases with depth under a surface load. The soil was very dense and entire weight of the tractor was carried on the lugs of the rear tires. The lugs penetrated the soil less than a quarter of an inch. Each rear tire carried about 1}.505 pounds and the area of the lugs in con­ tact with the soil was 8b square inches, giving an average surface pressure of 51+- psi. was 16 psi. The tire inflation pressure Each point represents a different pass of the tractor over the cells, so it is quite likely that the lugs were in a slightly different position, with respect to the cells each time which would cause a slight scatter of the point. Also since the average pressure applied to the surface was approximately 5b P si» surface pressure just over the cell might have been slightly higher or lower than 67 tD Ui kD t Vj LU OQ£ < (\IQ_ o L O d _J LU 68 the average. In any event the data line up in reasonable magnitude so that it can be concluded that the type A cell measures the pressure in the soil with a reasonable degree of accuracy. A Theoretical Discussion of the Effect of Load Area on Pressure in Soil at Various Depths below the Surface Figure 21 shows the pressures at various depths under the centers of three different diameter circular plates, uni­ formly loaded, as calculated by Froehl3chfs formula on*. = P (1- cos^OC) * m where<7^ = pressure at some distance under the load along the load axis, Pm = surface unit pressure, lb/in , oC = one-half the aperture angle between the point in question and the edge of the plate. From the three curves using 18-inch, 12-inch, and 6-inch diameter plates, it can be seen that the pressure in the soil under the surface is not a function of the unit pressure alone but also depends on the total load applied to the sur­ face of the soil. It has been the common belief of many people that if one wants to reduce compaction in the soil all he has to do is to reduce the unit load applied to the soil and the pressure in the soil would be reduced proportionally. This is true for 69 £=SSi!!iSS!E! I 70 the surface pressure applied but is not true for the pressures in the soil (Figure 22). Both curves are for circular plates carrying the same total load* The 12-inch plate has exactly one-half the area of the 17-inch plate, thus it applies twice the unit load to the surface of the soil. As can be seen}with ten psi applied to the surface by the 12-inch plate, the pressure at 15 inches would be 2.6 psi, while with five psi applied to the surface by the 17-inch plate, the pressure in the soil at 15 inches would be 2.1 psi. While it helps some to double the area of contact surface of the tires of the tractor it does not reduce to one-half the compacting pressure in the soil below plowing depth. It is Interesting to note the shape of the curve of the measured pressure under the rear tractor tire in Figure 21. This is the same data as in Figure 20 with all the values divided by 5*^1- to bring the data to the same unit surface pressure as the data calculated by Froehlich’s formula. The shape of the contact surface of the tire was not a circle so the shape of the curves would not be expected to be iden­ tical. Figure 23 represents the effect of bulk density and moisture content on the distribution of pressure in soils as calculated by Froehlich's formula. As explained by Soehne (33) a - v a l u e of four represents a dense dry soil, aV -value of five represents a fairly moist relatively 71 UJ U vD a 72 73 dense soil (about the proper condition for plowing), and a V-value of six represents a wet soil or relatively loose soil. Soehne (33) gives a very good discussion of the effect of the size of tires and soil conditions on deforma­ tions of soil. 7k RECOMMENDATIONS FOR FUTURE RESEARCH 1* Make further tests with the type A cells to deter­ mine their usefulness and limitations for measuring pres­ sures in various textures of soil with various moisture contents and bulk densities. 2. Develop and test sensing bulbs for use with the liquid pressure transducer. 3. Develop a probe type measuring unit for studying the pressure distribution in field soils under various applied pressures. k* Could use information from 1 and 2. Determine the effect of pressure on change in bulk density of soils of various types and at various moisture contents. These should be confined and unconfined soils. 5. Measure the total load and area of contact of various load applying units that operate in agricultural fields. 6. Measure the pressure distribution in various type soils at various moisture contents caused by agricultural traffic. 7. Create pressure pans and filter pans in various types of soil to study how they are formed. 8. Study methods of preventing induced pans. 75 9* Develop methods for loosening induced and genetic pans. 10. Study the effect of various shapes of tillage tools on the physical structure of soils. 11. Develop methods to determine the physical proper­ ties of soil before and after tillage tools are passed through them. 12. Determine the effect of plant root systems on force distributions in soil. 13. In cooperation with plant and soil scientists, determine methods for handling soils for optimum plant production. 76 BIBLIOGRAPHY References Cited 1* Carleton, W. M. Principles affecting the performance of mechanical sugar beet planters. Unpublished Ph. D. thesis, Michigan State College, 1949, 54-55. 2. Carlson, R. W. Five years’ improvement of the elasticwire strain meter. Engr. News-Record, vol. Ilk, no. 20, p. 697. 1935. 3* Cook, R. L. Structure and sandy soils. Water Conserv. 6 :31. 1951. 4* Free, G. R. Compaction as a factor in soil conservation. Soil Sci. Soc. Amer. Proc. 17:66-70. 1953* 5. Froehlich, 0. K. Druckverteilung in Baugrunde (Pressure distribution in the soil) Wien 1934* 6. Goldbeok, A* T, and E. B. Smith. An apparatus for determining soil pressures. Proc. Am. Soc. for Test. Mat. vol. 16, no. 2, pp. 310-319. 1916. 7. Goldbeck, A. T. The distribution of pressure through earth fills. Proc. ASTM, 1917. 8. Goldbeck, A. T. Measurement of earth pressures on retaining walls. Proc. Highway Research Board, Part II, vol. 18, 193y. 9. Gordon, E. D. Physical reactions of soil on plow disks. Agri. Engr. vol. 26, no. 6 , 1941* Jour. Soil and 10* Jamison, V. C. , H. A. Weaver and I. F. Reed. A hammer driven soil core sampler. Soil Sci. 69:4^7-496, 1950. 11. Jamison, V. G., H. A. Weaver, and I. F. Reed. 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McKibben, E. G. and R* L. Green. Relative Effect of steel wheels and pneumatic tires on agricultural soils. Agr. Engr. vol. 21, no. 5, PP. 183-185, 191+0. 19* Morrison and Cornish. Description of a pressure cell for the measurement of earth pressures. Canadian Jour, of Rex., vol. 17, no. 1, 1939* 20. Nichols, M. applied to 1929. L. Methods of research in soil dynamics as implement design. Alabama Sta* Bui. 229, 21. Nichols, M* L. and J. W. Randolph. A method of studying soil stresses. Agri. Engr. Trans. 19:168, 1925. 22. Nitzsch, W. Der porengehalt des Ackerbodens. Messvefahren und ihre Brauch Barkeit. Bodenk. u. Pflanzenemahr• 1: 110-115, 1935. 23. Page, J. B. Advantages of the pressure pycnometer for measuring the pore space in soil. Soil Sci. Soc. Am. Proc. 12:81-81+, 19U-7. 2I4.. Randolph, J. W . , I. F. Reed, E. G. Gordon. Cotton-tillage studies on Red Bay Sandy Loam. U. S. D. A. Cir. No. 51+0, 191+0. 78 25. Reaves, C. A. and. M. L. Nichols. Surface soil reaction to pressure. 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Heft. 1, 1951. 33# Soehne, Walter. Druckverteilung in Boden und Bodenverformung unter Schlepperreifen (Distribution of pressure in the soil and soil deformation under tractor tires). Grdlgn. d. Landtechn. Heft 5 -i-t-9-83, 1953* 31*.• Taylor, D. W. Soil Mechanics. Chap. 11. theory for estimating stresses in soils. and Sons, Inc., New York, 19ip8. 35. Terry, C, W. and H. M. Wilson. The soil penetrometer in soil compaction studies, Agri. Engr. 3^:831, 1953* 36. U. S. Waterways Experiment Station Technical Memorandum No. 210-1. Soil pressure cell investigation (Interior report) • 37. Vomocil, J. A. In situ measurement of soil bulk density by gamma ray absorption technique. Agr• Engr. 35*851* 1951*-. Use of elastic John Wiley 79 38* Weaver, H. A. Tractor use effect on volume weight of Davidson loam* Agri. Engr, 31:182-183, 1950. 39* Weaver, H. A, and V. C. Jamison. Effect of moisture on tractor tire compaction of soil. Soil Sci. 71:15-23, 1951. Other References 1. Chilcott, E. C. and J. S. Cole. Subsoilingj deep tilling, and dynamiting in the Great Plains. Jour. Agr. Res. ll+:l+8l-521, 1918. 2. Chase, L. W. A study of subsurface tiller blades. Engr. 23:1+3, 1914-2. 3. Cook, R. L., L. M. Turk and H. P. McColly. Tillage methods influence crop yields. Soil Sci. Soc. Amer. Proc. 17:1+10, 1953. 1+. Cook, R. L. 1950. 5. Doner, R. D, Dynamics of soil on plow moldboard sur­ faces related.to scouring. Agri. Engr. l5J9, 1931+. 6. Fletcher, L. J. The development of deep tillage in California. Agri. Engr., Trans. 17:202, 1923. 7. Hume, A. N. Crop yields as related to depth of plowing. S. Dak. Agri. Expr. Sta. Bui. 3&9. 191+3* 8. Jamison, V. C. and H. A. Weaver. Soil hardness measure­ ments in relation to soil moisture content and porosity. Soil Sci. Soc. Amer. Proc. 16:13-15, 1951. 9. Jones, G. D. Method and effect of deep tillage. Engr. 20:61-63, 1939. Agri* Proc. Amer. Soc. Sugar Beet Tech. 6:286, Agri. 10. Learner, R. W. and J. T. Lutz. Determination of poresize distribution in soils. Soil Sci. l+9:3l+7-3°0, 191+0. 11. Lee, G. H. Analysis* An Introduction to Experimental Stress John Wiley and Sons, Inc., New York, 1950. 80 12, Lill, J. G. Plowing depths and fertilizers affect sugar-beet crop. Mich. Agr. Exp. Sta. Quart. Bui. 13: 122-127 (1931). 13* Love, A. E. H. A Treatise on the Mathematical Theory of Elasticity. Cambridge (England) University Press, 193)+* ll*. Nichols, M. L «, A. W. Cooper and C. A. Reaves. Design and use of machinery to loosen compact soil. Soil Sci. Soc. of Amer. Proc., vol. 19, no. 2, pp. 128-130, 15* Proctor, R. R. Fundamental principles of soil compaction. II. Description of field and laboratory methods. Engr. News-Record 3 0 8 6 -389, 1933* 16. Seaton, L. F. Compaction of soil due to tractors. Engr. Trans. 10:68, 1916. 17* Sjogren, 0. W. 12:21*9, 1931. 18. Smith, F. W. and R. L. Cook. 191*6; 11:1*02, 191*7. Principles of deep tillage. 19. Smith, R* S. Experiments and subsoil dynamiting. (1925). 20. Agri. Agri. Engr. Soil Sci. Soc. Amer. Proc. with subsoilingy deep tilling, 111. Agr. Expt. Sta. Bui. 25$, Soehne, Walter. Druckverteilung in Ackerboden und Verformbarkeit des Ackerbodens (Pressure distribution in the soil and deformation of the soil surface) Sonderdruck aus der Kolloid-Zeitschrift. Heft. 2, Seite 89, 1953* 21. Timoshenko, S. Theory of elasticity. 1931*, PP. 328-339. McGraw-Hill, 22. Thatcher, L. E. How deep should we plow. Ohio State Monthly Bui. (Wooster) 10, no. 1-2, pp. 3-6, 1925* 23. Torstensson, G. and S. Ericsson. A new method for determining the porosity of soil. Soil Sci. 1*2:1*051*17, 1936. 21*. Trullinger, R. W. The fundamental approach to tillage and traction research problems. Agri. Engr. 18:17, 1937. 81 25. Veihmeyer, F. L. and A. H. Hendrickson* Soil density and root penetration. Soil Sci. 652ip87—Ip93» 194®* 26. Visser, W. C, Pore space determination as a field method. Soil Sci. 44:478-479, 1937* 27* Williams, Ira L. Measurement of soil hardness. Engr. 20:25, 1939. 28. Woodruff, C. M. and D. D. Smith. Subsoil shattering and subsoil liming for crop production on claypan soils. Soil Sci. Soc. Amer. Proc. 1948; 11:539-542, 1947* Agri. 82 APPENDIX TABLE VIII SAMPLE CALIBRATION DATA FOR TYPE A STRAIN GAGE PRESSURE CELL Pressure (lb/in2 ) 0 Indicated Strain Reading (Two active gages) u in/in x 10 0 10 38.6 20 77.6 30 116.1* 1+0 151.2 5o 181*. 2 60 216.8 70 21+1*.*8 80 270.8 90 296,0 83 TABLE IX CHANGE IN SOIL PRESSURE WITH DEPTH MAUMEE SANDY LOAM - VERY COMPACT1 Distance Below Soil Surface (in.) Measure Pressure Under Center of Tire (lb/in2) 0# 5 ^ .0 1 .5 1+1.8 2 .5 1+2.9 2 .8 3 0 .0 3 .0 3 3 .0 5 .0 21+.3 6 .0 2 2 .8 1 0 .0 1 2 .8 1 0 .0 13.2 OJ . 0 H 1 1 .8 1 2 .2 9 .2 1 3 .0 1 0 .3 1 3 .6 8 .1 3 4 .0 1 2 .1 Pressure applied to surface of soil by rear tire of Oliver 99 GM diesel tractor. *Total load applied to soil by rear tractor tire - 1+505 lb. Lug area of tire in contact with soil - 81+ in • % a. -x ^ 1/1^ JO J p)Ml^a^UCfO a o _ L o s at Ul .J o o Oo O — o O iwfityi-fctoio ^ajsuiejG HoUl'3aiMi >° ssaujjoi.m a x UJ <0 $ s hi of I vO 0 1 o k UJ I- o 2 THEORETICAL Q. CLAMPED 11 WITH