The determination of plough draught—Part II the measurement and prediction of plough draught for two mouldboard shapes in three soil series

The draught of a 3-furrow reversible plough fitted with two types of bodies was measured at five separate test sites. Each site was ploughed on four different days to provide a range of soil moisture contents. The plough was operated at three different speeds in sequence for each type of body. The horizontal and vertical forces transmitted to the tractor were measured on a three-point linkage dynamometer. Tachogenerators monitored tractor wheel speed and fifth wheel ground speed. Cone index and soil specific weight were recorded at 30 mm intervals throughout the top-soil profile. Cone index at median plough depth was found to be a satisfactory measure of soil strength for the prediction of plough draught. Characterising specific plough draught by soil cone index, specific weight, moisture content, plough mouldboard tail angle and ploughing speed provided predicted values in closer agreement with measured draught compared with earlier equations. The sensitivity of cone index to soil moisture content supports the use of the cone penetrometer as a practical monitor of soil conditions in the field and as a management tool for judging the opportune times for agricultural tillage operations.     

Cone penetration resistance equation as a function of the clay ratio, soil moisture content and specific weight

The cone penetrometer is a simple versatile device which is widely used to monitor the strength of a soil in terms of its resistance to the penetration of a standard cone. The soil penetration resistance is a function of soil moisture content, soil specific weight and soil type. The soil type is characterised by means of a clay ratio which is the ratio of the clay content of the soil to the content of silt and sand.Based on the classical bearing capacity theories for strip foundations, a general cone penetration resistance equation is developed to represent the variability of cohesion and friction angle by means of soil type and moisture content. The empirical relationship is shown to give an accurate prediction of the cone penetration resistance for a wide range of soils from a loamy sand to a heavy clay (clay ratios 0.10–1.60) and over a wide spectrum of soil moisture contents from 10 to 65% w/w.     

Evaluation of penetrometers for measuring soil strength

Cone index, as determined by a cone penetrometer, is frequently used as a measure of soil strength. The index is a compound parameter involving components of shear, compressive and tensile strength and soil metal friction. In order to assess the effect of soil type and condition on the relative contributions of these components to penetration resistance, the forces required to push blunt and sharp probes into two soils under a range of moisture contents and bulk densities were investigated. The maximum penetration force in homogeneous soil was not uniquely related to dry bulk density or cohesion, but varied with soil moisture content.At high and low moisture contents, the soil tended to interact with the shaft of the penetrometer thus increasing the resistance to penetration. At low moisture content, bodies of compressed soil formed in front of the probe, effectively changing the probe geometry.It was concluded that interpretation of cone index in typical layered field soils is difficult. Even in homogeneous soils, the proportion of shear, compressive and tensile components that the cone index reflects varies with soil condition.     

Determination of rating cone index using wheel sinkage and slip

It has been known from empirical equations that soil strength can be determined if wheel sinkage and slip of a vehicle can be measured on a soil surface. In this study, field data of wheel sinkage and slip were collected from two platform tractors of different sizes on gravely sandy and sandy loam soils. Using an empirical equation, the rating cone index was determined using the measured wheel sinkage and slip data. The data demonstrated that the same rating cone index can be obtained although the measuring platforms are different. It was also noted that the rating cone index can be estimated in real time by measuring the sinkage and slippage of a driving wheel.     

Effect of trash board on moldboard plough performance at low speed and under two straw conditions

Draught requirement is an important parameter for tillage tool performances. This study investigated the influence of trash board on the performance of moldboard plough and the system optimization. Draught and vertical forces acting on the plough were measured with and without a trash board under two straw conditions, i.e. with only stubbles and with dense straw cover. Field soil moisture content was kept at 30%. The study also used the finite element method (FEM) to simulate stress distribution on the plough. Results showed that draught significantly increased without trash board under dense straw cover as compared with only stubbles. The trash board attachment reduced draught significantly. Similar trend was also true for vertical force. FEM results were found to be compatible with the experiment. The simulated maximum equivalent stress acting on the mouldboard plough was 279.43 MPa, while the material’s yield stress was 250 MPa. Attaching trash board with the moldboard plough is important where straw cover was dense.     

Characteristic loading of light mouldboard ploughs at slow speeds

A study on four mouldboard ploughs, that are commonly used with animal traction in Kenya, was conducted. Draught, suction and torsion loads were measured and specific draught evaluated in field tests on four sites with typical agricultural soil conditions. Draught and suction are the horizontal and vertical components of the reaction to soil force, respectively, while torsion is the resisting moment about the plough shank. The objective was to quantify these parameters and to study their characteristics under variable conditions at operation, at speeds up to 1.12 m/s and tillage depths between 0 and 150 mm in an attempt to optimize the design, selection and utilization of mouldboard ploughs for animal traction in Kenya. It was found that depth of tillage is the most critical factor, and draught and suction increased significantly with depth while specific draught increased or decreased depending on the soil type. Draught and specific draught increased significantly with speed. The increase in suction with depth probably implies an increased stability in the ploughing operation, while its reduction with speed indicates a potential instability of plough control with varying speeds. Consequently, aiming for steady motion in the utilization of animal traction may aid in the optimization. It was also found that ploughs with a high specific draught (kN/m) are expected to experience higher torsional loads on the shanks. The characteristic draught, specific draught and suction loads of the ploughs were described by quadratic functions in speed and depth of tillage with coefficients of determination (R₂) ranging from 0.55 to 0.99. A significant difference in the coefficient of variation of draught loads in the three soil types probably implies that optimal duration for use of animal traction in tillage should be dependent on soil type.     

Effect of wetting and drying on soil physical properties

Agricultural soils are subject to seasonal wetting and drying cycles. Effect of drying stress, as influenced by one cycle of wetting and drying, on physical properties of a clay–loam soil was investigated in the laboratory. The physical properties studied were soil bulk density, cone penetration resistance, shear strength, adhesion and aggregate size and stability. Three drying stress treatments were made by wetting air-dried soil of initial moisture content of 12% (on dry weight basis) to three different higher moisture contents, namely 27, 33 and 40%, and then drying each of them back to their original moisture content of 12%. Thus, the soil was subjected to three different degrees of drying stress. The results showed that the soil strength indicated by cone penetration resistance and cohesion, and soil aggregate size, increased with the degree of drying stress. However, the soil bulk density did not change significantly with the drying stress.     

Spatial distribution of soil forces on moldboard plough and draft requirement operated in silty-clay paddy field soil

Spatial distribution of soil forces on the surface of plough is an important aspect that can help engineers for improving efficiency of tillage implement. It was analyzed at eleven different points of the moldboard plough with the help of sensors accompanied with the virtual instrument developed in LabView software with the aid of other supporting instruments. It was observed that soil forces increased with an increase in speed and depth. Depth changed soil forces more at upper parts than lower parts whereas speed affected rear parts more than the front part of the plough. Draft forces followed almost similar trend and least value of 308.17 N experimental draft force was found at 1 m/s speed and 5 cm depth under 33% moisture content. Cumulative soil forces found too smaller than the draft as they represented the force spatial distribution of specific parts of plough. It was observed that sensor technology provided real time picture of force variation during tillage process that could save time and effort.     

Determination of the soil pressure distribution around a cone penetrometer

The objective of this paper was to investigate the pressure distribution around a cone penetrometer using a pressure sensing mat under laboratory conditions. The investigation was conducted under (1) constrained conditions using cylindrical split pipe molds and (2) unconstrained conditions using a soil box. These tests were conducted in Capay clay and Yolo loam soil containing two different moisture conditions and two compaction levels.In the constrained tests, a maximum radial pressure of 111 kPa was observed in the Capay clay soil with 3.4–4.3% d.b. moisture content and three blows of compaction (cone index value of 2040 kPa) when using the 41 mm diameter split pipe mold. These pressure levels decreased to 82 and 22 kPa, respectively, when 65 and 88 mm diameter molds were used. In both the Capay clay and Yolo loam tests, the average radial pressure and average cone index values showed similar trends.In the unconstrained tests, a maximum pressure of 9.0 kPa was observed in the Capay clay with 4.5% d.b. moisture content and three blows of compaction (cone index value of 550 kPa) at a horizontal distance of 25.4 mm from the vertical axis of the cone penetrometer and minimum pressure levels in the range of 0.2–0.3 kPa when the horizontal distance of the penetrometer was in the range of 56.8–66 mm. The pressure levels are much smaller than the ones obtained in the constrained tests and may suggest that the pressure distribution under field conditions is small at a distance of 25.4 mm or higher from the tip of the cone.The experimental data were statistically analyzed to identify significant factors. The results of the analysis for the constrained test indicated that the mold diameter and number of blows significantly increased the pressure readings within the soil mass. Increasing the mold diameter led to a decrease in the average radial pressure and increasing the number of blows contributed to an increase in the average radial pressure. In the unconstrained test, the average radial pressure distribution at a given point were significantly influenced by the horizontal distance of the point from the vertical axis passing through the center of the penetrometer shaft, soil type, and soil moisture content. Higher pressure values were obtained in the Capay clay tests compared to the Yolo loam tests. In all cases, the pressure levels were greater for the drier soil than for the moist soil.     

Prediction of clay soil compaction

Field measurements were made of soil density and moisture patterns under different vehicle tire paths with varying external pressures and number of passes. In addition, laboratory index tests were performed to determine the compaction behaviour of the same soil. Using these results, a prediction equation of dry density in terms of applied pressure and moisture content was obtained for the clay soil. A previously developed equation for sandy soil was modified for the complete range of moisture contents encountered. Estimation of shear strength for the clay soil was made using plastic and liquid limits.     

An empirical model to predict soil bulk density profiles in field conditions using penetration resistance, moisture content and soil depth

Cylindrical soil probes measuring 300 mm in diameter by 300 mm in height were prepared in the laboratory using samples extracted from a well drained loamy soil (FAO classification: Vertic Luvisol). These probes were compacted at different moisture contents [3, 6, 9, 12, 15 and 18 (% w/w)] and using different compaction energies (9.81, 49.05, 98.1 and 981 J). The soil penetration resistance was determined by means of the ASAE 129 mm² base area cone and seven other different cones with base sizes of 175, 144, 124, 98, 74, 39 and 26 mm². The variability of the penetration resistance measurements increased as the size of the cone decreased. Nevertheless, the penetration resistance values proved to be independent of the cone used, as long as the size of the latter was equal to or greater than 98 mm². This confirms the possibility of using cones with areas smaller than the ASAE standard when measurements are to be carried out in dry soils with high levels of mechanical resistance. The experimental data were used to develop an empirical model, a linear additive model on a log–log plane, capable of estimating soil bulk density depending on soil penetration resistance, soil moisture content and depth. This model has provided good results under field conditions and has allowed soil bulk density profiles and accumulated water profiles to be accurately estimated.     

Measuring soil properties to predict tractive performance of an agricultural drive tire

The coefficient of traction for a 9.5–16 R-1 bias ply tire was measured and compared with predictions using equations developed by Janosi and Hanamoto [Proc. 1st Int. Conf. on Mechanics, p. 707–736 (1961)]; Wismer and Luth [J. Terramechanics10, 49–61 (1973)] Gee-Clough et al. [J. Terramechanics15, 81–84 (1978)] and Brixius [ASAE Paper No. 87–1622 (1987)]. For the soft soil condition, with a cone index of 120 kPa, Gee-Clough's equation predicted the coefficient of traction better, but predictions using the Brixius equation were better for soil with a cone index of 225 kPa. An experimental device was developed to simultaneously measure the horizontal and vertical stress-strain relationships of soil. The use of resultant stress from the experimental device data failed to show any improvement in coefficient of traction prediction over using the cone index. The resultant of the normal and shear stress from the experimental device data did not adequately represent the soil properties involved in terrain-vehicle mechanics.     

Modeling compaction in agricultural soils

Research was conducted to quantify the effect of tire variables (section width, diameter, inflation pressure); soil variables (soil moisture content, initial cone index, initial bulk density); and external variables (travel speed, axle load, number of tire passes) on soil compaction and to develop models to assess compaction in agricultural soils. Experiments were conducted in a laboratory soil bin at the Asian Institute of Technology in three soils, namely: clay soil (CS), silty clay loam soil (SCLS), and silty loam soil (SLS). A dimensional analysis technique was used to develop the compaction models. The axle load and the number of tire passes proved to be the most dominant factors which influenced compaction. Up to 13% increase in bulk density and cone index were observed when working at 3 kN axle load in a single pass using a 8.0–16 tire. Most of the compaction occurred during the first three passes of the tire. It was also found that the aspect ratio, tire inflation pressure and soil moisture content have significant effect on soil compaction. The initial cone index did not show significant effect. The compaction models provided good predictions even when tested with actual field data from previous studies. Thus, using the models, a decision support system could be developed which may be able to provide useful recommendations for appropriate soil management practices and solutions to site-specific compaction problems.     

How to calculate the effect of soil conditions on tractive performance

The paper presents an analysis and quantitative evaluation of the effect of soil conditions on tractive performance of off-road wheeled and tracked vehicles. The results of this study indicated that to accurately calculate the tractive performance of a vehicle in a given soil condition, soil properties and parameters and their changes as functions of soil moisture content and density should be taken into account. An effective Tractive Performance Analytical (TPA) model which takes into consideration the effect of soil conditions on tractive performance of the vehicles is developed. The TPA model uses invariant soil parameters that can be given or measured before the calculations by routine methods of classical soil mechanics. Soil parameters can also be obtained by recommended empirical equations using four physical soil parameters measured in the field with hand held instruments without time consuming and costly plate or vehicle tests. The model was validated in different soil conditions and compared with other models used in terramechanics for tractive performance predictions. The paper includes also an analysis of capabilities and limitations of the observed models.     

Non-linear finite element analysis of cone penetration in layered sandy loam soil – Considering precompression stress state

Axisymmetric finite element (FE) method was developed to simulate cone penetration process in layered granular soil. The FE was modeled using ABAQUS/Explicit, a commercially available package. Soil was considered as a non-linear elastic plastic material which was modeled using variable elastic parameters of Young’s Modulus and Poisson’s ratio and Drucker–Prager criterion with yield stress dependent material hardening property. The material hardening parameters of the model were estimated from the USDA-ARS National Soil Dynamics Laboratory – Auburn University (NSDL-AU) soil compaction model. The stress–strain relationship in the NSDLAU compaction model was modified to account for the different soil moisture conditions and the influence of precompression stress states of the soil layers. A surface contact pair (‘slave-master’) algorithm in ABAQUS/Explicit was used to simulate the insertion of a rigid cone (RAX2 ABAQUS element) into deformable and layered soil medium (CAX4R ABAQUS element). The FE formulation was verified using cone penetration data collected on a soil chamber of Norfolk sandy loam soil which was prepared in two compaction treatments that varied in bulk density in the hardpan layer of (1) 1.64 Mg m⁻³ and (2) 1.71 Mg m⁻³. The FE model successfully simulated the trend of cone penetration in layered soils indicating the location of the sub-soil compacted (hardpan) layer and peak cone penetration resistance. Modification of the NSDL-AU model to account for the actual soil moisture content and inclusion of the influence of precompression stress into the strain behavior of the NSDL-AU model improved the performance of FE in predicting the peak cone penetration resistance. Modification of the NSDL-AU model resulted in an improvement of about 42% in the finite element-predicted soil cone penetration forces compared with the FE results that used the NSDL-AU ‘virgin’ model.     

On the applicability of soil water finite difference models to operational trafficability models

Certain current operational trafficability models (e.g. SMSP) use soil moisture models which are empirically based and have a relatively coarse time increment. It is proposed that the inclusion of a soil water finite difference model may more realistically represent the detailed soil water flux changes that occur near the surface, and thus provide an improved forecasting capability. Results are present which show the important role evaporation can play in its effect upon rating cone index values, emphasizing the need need to have a dynamic soil water model in trafficability modelling.     

Influence of soil and vehicle parameters on soil rut formation

Soil and vehicle parameters have significant effects on soil rut formation. A randomized design was used to investigate the effects of five treatments: soil texture, soil moisture, vehicle type, turning radius and velocity, on rut depth, rut width and rut index, which measure the degree of soil disturbance. This vehicle rutting study was conducted on four off-road military vehicles under two soil moisture conditions and two soil texture conditions at Fort Riley, Kansas. A GPS-based vehicle tracking system was used to track the vehicle dynamics, and rut measurements were taken manually. SAS 9.1 was used to investigate the effects of soil and vehicle parameters on rut formation. Results show that all the vehicle parameters (vehicle type, weight, velocity and turning radius) and soil parameters (soil texture and moisture) are statistically significant to affect rut formation.     

Comparison of soil strength measurements of agricultural soils in Nebraska

In 2014 the University of Nebraska, Lincoln (UNL) was engaged in field testing program to investigate a soil moisture mapping system as a crop management tool. In conjunction with this work, the US Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory (ERDC-CRREL) deployed a team to perform soil characterization and strength measurements at three agricultural test sites. The primary objective was an investigation of the Lightweight Deflectometer (LWD) as a soil surface strength tool for the purposes of assessing bearing capacity of soft soils. The LWD measurements were performed with those from more “standard” tests, i.e. the Dynamic Cone Penetrometer, Cone Penetrometer, and Clegg Impact Hammer to determine if the LWD produced results that compared with these methods. The strength test data were also used to calculate California Bearing Ratio (CBR) values using existing equations in order to see if the different test methods produced similar CBR values that could in turn be used to predict the bearing capacity of the sites. The secondary objective was to compare the strength data with the corresponding soil water content data taken by UNL to determine if soil moisture was an indicator of soil strength.     

Trafficability tests with the marsh screw amphibian

A program of field tests was conducted with the Marsh Screw Amphibian to evaluate its performance on a range of soil conditions. Performance was expressed in term of ability to travel 50 passes in the same path, manoeuvrability, speed, drawbar pull, ans slope-climbing ability. Soil conditions were described in terms of soil type, moisture content, density and cone index. Relations between vehicle performance and soil condition are presented.