Terramechanics, as an interdisciplinary field, focuses on the interaction between the ground and vehicles. Terramechanics concepts can be used to realize the design of underbodies and pass-planning of off-road vehicles, among other applications. However, conventional semi-empirical terramechanics models cannot accurately evaluate the terrain surface deformation associated with the interaction between the solid object and ground. Therefore, this article proposes an extended terramechanics model, which incorporates the terrain surface deformation mechanism based on cellular automata. First, to examine the effectiveness of the proposed model to capture variations in the terrain surface deformation and draft force, an analysis of a vertical plate drag test was conducted based on the proposed model. Next, a single-wheel traveling analysis was performed. The experimentally obtained drawbar-pull and sinkage could be reasonably simulated, which cannot be easily realized using conventional methods.

Finite element simulation of cone penetration in a "virtual" soil using the commercial finite element software MSC/DYTRAN was accomplished Parameters for DYTRAN'S crushable foam and soil constitutive model, DYMAT14, were estimated using the NSDL_AU soil compaction model developed by the USDA_ARS National Soils Dynamics Laboratory and Auburn University. Finite element simulations were conducted for two soils, a sandy load and a clay loam, and compared to existing cone penetration data.

Computer simulation of soil response and machine performance during soil-machine or soil-plant interaction requires a model of soil mechanical behavior. It is envisioned that future computer simulation will permit simulation and prediction of soil behavior and machine performance over a wide range of soil conditions and alleviate total reliance on experimental data from expensive testing equipment, facilities and personnel. This will aid our simulation of mobile machine dynamic performance and plant growth in a virtual proving ground environment. Thus, a computer model of the soil-machine and/or soil-plant system could serve as a "virtual" model of the system to enable "what-if" analysis and generation of simulated data for algorithm development and management decisions. A key element for computer simulation of the soil behavior is adequate mathematical models of significant soil behavioral responses to mechanical force systems applied by nature, machines, animals, or humans.The "critical state" concepts and theory applied to describe soil behavior appear to provide the most unified complete framework for mathematical description of soil behavior over a wide range of initial soil conditions. The NSDL-AU soil behavior model was developed based on many of the "critical state" concepts, to represent the behavior of compactible agricultural soil when unsaturated and subjected to both confining and shear stresses. The authors and others have utilized this model to estimate parameters in some constitutive material models available in both private and commercial finite element code to aid in the prediction of soil-machine behavior.

The tractive performance of a new tire, a worn tire, and a worn tire with forestry tire chains was measured in four soil types. Two of the soil types simulated forest-floor conditions with one soil type having a surface cover of pine straw and the other having a surface cover of sod. The two remaining soil types were bare. The worn tire with and without chains had higher net traction than the new tire. Tractive efficiency was highest for the worn tire without chains in all soil types.

Variable load test data were used to evaluate the applicability of an existing forestry tire traction model for a new forestry tire and a worn tire of the same size with and without tire chains in a range of soil conditions. The clay and sandy soils ranged in moisture content from 17 to 28%. Soil bulk density varied between 1.1 and 1.4g cm−3 with cone index values between 297 and 1418 kPa for a depth of 140 mm. Two of the clay soils had surface cover or vegetation, the other clay soil and the sandy soil had no surface cover. Tractive performance data were collected in soil bins using a single tire test vehicle with the tire running at 20% slip. A non-linear curve fitting technique was used to optimize the model by fitting it to collected input torque data by modifying the coefficients of the traction model equations. Generally, this procedure resulted in improved prediction of input torque, gross traction ratio and net traction ratio. The predicted tractive performance using the optimized coefficients showed that the model worked reasonably well on bare, uniform soils with the new tire. The model was flexible and could be modified to predict tractive performance of the worn tire with and without chains on the bare homogeneous soils. The model was not adequate for predicting tractive performance on less uniform soils with a surface cover for any of the tire treatments.

Stress state transducers (SSTs) were used to determine the orientation of the major principal stress, σ1, in soil beneath the centeline of an 18.4R38 radial-ply R-1 drive tire operated at 10% slip. Two soils, a sandy loam and a clay loam, were each prepared twice to obtain two density profiles. One profile of each soil had a hardpan and the soil above the hardpan was loose. The soil in the second profile was loosely tilled. The stress state was determined at a depth of 358 mm in the sandy loam and 241 mm in the clay loam soil. The tire was operated at two dynamic loads (13.2 and 25.3 kN), each at two levels of inflation pressure (41 and 124 kPa). When the orientation of σ1 was determined directly beneath the axle, the mean angles of tilt in the direction of travel ranged from 6 to 23 degrees from vertical. Inflation pressure did not significantly affect the angle when the dynamic load was 13.2 kN in the sandy loam soil, and neither inflation pressure nor dynamic load significantly affected the angle in the clay loam soil. When the dynamic load was 25.3 kN in the sandy loam soil, the orientation of the major principal stress determined directly beneath the axle was tilted significantly more in the direction of travel when the tire was at 41 kPa inflation pressure than when at 124 kPa. These changes in stress orientation demonstrate the importance of measuring the complete stress state in soil, rather than stresses along only one line of action. The changing orientation of σ1 as the tire passes over the soil indicates the soil undergoes kneading and supports future investigation of the contribution of changes in stress orientation to soil compaction.

Soil stresses were determined just above a hardpan beneath the centerlines of tires representative of front and rear tires of a mechanical front wheel drive (MFWD) tractor with dual rear tires. Rut depths were measured at the centerline of each tire track. A 14.9R30 R-1 front tire and an 18.4R42 R-1 rear tire were operated on a sandy loam and a clay loam soil with loose soil above hardpans in soil bins, using three combinations of dynamic load and inflation pressure representative of a tractor hitched to a row-crop planter. When a tractor of this type is used with either an integral planter with liquid chemical carried on the tractor, or a towed planter without liquid chemical carried on the tractor, the front tires cause greater soil stresses and rut depths than the rear, so the front tires are likely to compact the soil more than the rear tractor tires. A tractor configured for use with a towed planter without liquid chemical carried on the tractor generates lesser soil stresses and rut depths, and therefore is less likely to compact soil beneath the tractor tires than a tractor configured for an integral planter with liquid chemical carried on the tractor.

Soil stresses were measured under a 18.4R38 R-1 radial-ply tractor tire, operated at two levels each of dynamic load and inflation pressure. Stress state transducers were placed at two depths beneath the centerline of the path of the tractor tire in two different compaction profiles in each of two soils. Peak soil stresses and soil bulk density increased with increases in both dynamic load and inflation pressure.

Several areas of study are contributing to the development of procedures to predict soil compaction from machinery traffic. Stresses at the soil-tyre interface are being measured. Analytical and finite element analyses are being used to predict stresses throughout the soil profile. Constitutive stress-strain relationships link soil stresses to compaction and strain. Measured stress states in the soil profile provide intermediate validation of the procedures. This paper summarizes research on measurement of soil-tyre interface stresses, measurement of stress states in the soil profile, the development of soil constitutive relationships and analytical and finite element methods to represent the soil compaction process.

A finite element model was modified to use a constitutive relationship of soil compaction that included the effects of both normal and shearing stresses, Predicted values of soil stress were compared against results from a laboratory experiment. All predicted values at final deformed depths less than 0.3 m were within the 95% confidence intervals of the measured values, bur at deeper depths most of the predictions fell outside the 95% confidence intervals of the measured values.

Soil stresses under a rigid wheel loading were predicted with a finite element model using each of three different soil compaction models as the constitutive relationship. An energy analysis compared the energy lost in traction to energy absorbed in compaction as predicted by the finite element model. One constitutive relationship resulted in underprediction of energy absorbed in compaction while the other two resulted in overprediction. The method of energy analysis is shown to be a useful tool for evaluating accuracy of soil stress and strain predictions by a finite element model.

A cubical triaxial unit (CTU) was designed and built that used pneumatically pressurized flexible cushions to apply a three-dimensional, independently controlled, compressive stress state to a cubical soil sample measuring 50.8 mm along each side. The apparatus was computer controlled and could operate safely up to pressure levels of 1 MPa. The performance of the CTU was evaluated by comparing it to data obtained from an existing cylindrical triaxial device. Also, data from the CTU were used to validate Bailey and Johnson's (1989) soil compaction model. The CTU will be used to determine how soils react to three independent principal stresses, and to aid in the development of finite element techniques to predict soil and soil-machine behavior.

Modeling agricultural soil compaction is important as one input to a system of effective management of soil physical condition to improve crop production. The desired degree of compaction depends on the intended purpose; for example, the requirements for traction and mobility are quite different from those for infiltration and root propagation. Our goal is to develop a compaction model and related soil and soil-machine behavior models which can be used to design systems for effective management of soil physical condition. In this article we discuss our rationale in modeling soil compaction and related soil-machine systems. The status of the various modeling efforts is discussed, as are plans and needs for the future.

Tractive thrust components across the width of an 18.4R38 radial-ply tire were examined to determine how the tire generated thrust in loose and compacted soil conditions at each of four levels of dynamic load. The tire developed thrust more uniformly across the lug face in the loose soil conditions than in the compacted soil conditions. Increased dynamic load resulted in significant increases in the thrust at the tire centerline and lug edge positions. The thrust developed at the center of the lug was not significantly affected by increasing dynamic load on the compacted soil conditions.

The effect of disk spacing and gang angle on gang forces was investigated. Two conventional disk shapes were used at two gang angles. One angle provided soil clearance on the back side of the disk; the other was a commonly used angle which resulted in soil pressure on the back side of the disk. All tests were run at a speed of 1.3 m/s and a depth of 150 mm. The forces on a disk gang were found to vary linearly with the spacing as disk spacing was varied from 100 to 500 mm. A model was developed which described the soil forces as a function of the number of disks and the spacing of the disks in a gang. Results showed that changing disk spacing significantly changed disk gang forces. The disk gang force model was used to explore possibilities for design of disk harrows for maximum or minimum forces. The model was also used to determine the disk spacing at which no ballast was needed to force the harrow to operating depth.

The effect of disk spacing and gang angle on gang forces was investigated. Two conventional disk shapes were used at two gang angles. One angle provided soil clearance on the back side of the disk; the other was a commonly used angle which resulted in soil pressure on the back side of the disk. All tests were run at a speed of 1.3 m/s and a depth of 150 mm. The forces on a disk gang were found to vary linearly with the spacing as disk spacing was varied from 100 to 500 mm. A model was developed which described the soil forces as a function of the number of disks and the spacing of the disks in a gang. Results showed that changing disk spacing significantly changed disk gang forces. The disk gang force model was used to explore possibilities for design of disk harrows for maximum or minimum forces. The model was also used to determine the disk spacing at which no ballast was needed to force the harrow to operating depth.

Any mechanical manipulation that changes the soil condition may be considered as tillage. Most often machines are used to apply forces to the soil to effect this change. Soil dynamics includes a description of the behavioral response of soil to applied forces and a description of soil-machine behavior. The state of development of soil dynamics (quantitative descriptions of soil behavior, soil-machine behavior, and resultant soil condition) is explored. Research needs and directions in soil dynamics related to tillage and to prediction of the resultant soil condition are discussed. The wisdom and vision of Professor Henk Kuipers are embodied in this discussion.

ABSTRACT A method was developed to predict the complete stress state at a point in the soil due to an imposed load from a tire on the soil surface. The method may be used to investigate the influence of stress distribution at the soil-tire interface on the stress state in the soil. The mean normal, octahedral shear, maximum principal, and vertical normal stress and their directions may be predicted. The complete stress state is predicted based on the Froelich equation and the modified Cerruti equation.

ABSTRACT Asoil compaction model that includes soil behavior under compressive normal and shearing stresses great enough to attain maximum compaction was developed. The model was developed and verified with triaxial tests on several agricultural soils. This model predicts soil compaction under vehicle loads better than earlier models because the input stress state can more realistically represent field conditions.

The effect of number of discs in a gang on soil reaction forces was evaluated for gangs containing one to five discs. The performance of two different disc shapes was investigated, each at two operating angles. The angles and shapes were chosen to represent a wide range of operating parameters. Interaction was found to occur between discs; however, the interaction was not a function of the number of discs. The magnitude of the interaction was such that large errors would occur if gang draught force was calculated from single disc data only.