The inception and evolution of earthmoving soil mechanics

Soil and test conditions important to earthmoving machinery have been found to be significantly different from all other fields of endeavour with the partial exception of tillage studies. This could be the subject of a long dissertation. Broadly, however, soil conditions which produce critical mobility problems are much too soft and/or wet to be of concern to the earthmoving contractor who has to meet rigid specifications on acceptable types and moisture contents of fill soils. Occasional soft spots are considered as nuisances instead of indicators of the need for major design compromises.

Civil engineers are concerned with the same types of soil, but in a vastly different context. They must design soil structures which will never reach initial soil failure. Earthmoving processes, on the other hand, must accept soil failures in many different forms and degrees and utilize post-failure soil strength to perform their tasks efficiently.

Tillage studies display many important similarities to earthmoving studies, particularly in regard to the types of soil failures of importance. They are, in reality, merely another form of earthmoving; by definition, if nothing more. Earthmoving processes can range into much stronger soils, but this alone is insufficient to set them apart.

The term Earthmoving Soil Mechanics was introduced in 1962⁽²⁰⁾. This paper more clearly defines the implications of the new terminology and illustrates the first successful application of soil mechanics and model analysis principles in the earthmoving industry.     

Traffic-soil-plant (maize) relations

Twenty-five treatments consisting of three vehicle contact pressures, 62, 41 and 31 kPa (0.63, 0.42, 0.32 kg/cm²), four numbers of tractor passes (1, 5, 10, 15,) before and after seeding groups, and a control of zero traffic were used to study the effect of soil compaction on corn plant root growth and distribution in a Ste. Rodalie clay soil. The average dry bulk density values for 0–20 cm depths measured during the season varied from a minimum of 0.89 g/cm³ to a maximum of 1.12 g/cm³ depending on the severity of the treatment. Root distribution maps were obtained for all the treatments by field measurements coupled with root washing methods. An average root density of 5.7 mg/g of soil in an uncompacted control plot was reduced to less than 2 mg/g in a plot with 15 passes of 0.63 kg/cm² contact pressure. Soil penetration resistance values in various plots were compared, and a statistical model was obtained in terms of the traffic treatments, soil moisture content and depth. Yield reductions and penetration resistance were compared to root distrubution density results.     

Dynamic load and inflation pressure effects on contact pressures of a forestry forwarder tire

A Trelleborg Twin 421 Mark II 600/55-26.5 steel-reinforced bias-ply forwarder drive tire at inflation pressures of 100 and 240 kPa and dynamic loads of 23.9 and 40 kN was used at 5% travel reduction on a firm clay soil. Effects of dynamic load and inflation pressure on soil–tire contact pressures were determined using six pressure transducers mounted on the tire tread. Three were mounted on the face of a lug and three at corresponding locations on the undertread. Contact angles increased with decreases in inflation pressure and increases in dynamic load. Contact pressures on a lug at the edge of the tire increased as dynamic load increased. Mean and peak pressures on the undertread generally were less than those on a lug. The peak pressures on a lug occurred forward of the axle in nearly all combinations of dynamic load, inflation pressure, and pressure sensor location, and peak pressures on the undertread occurred to the rear of the axle in most of the combinations. Ratios of the peak contact pressure to the inflation pressure ranged from 0 at the edge of the undertread for three combinations of dynamic load and inflation pressure to 8.39 for the pressure sensor on a lug, near the tire centerline, when the tire was underinflated. At constant dynamic load, net traction and tractive efficiency decreased as inflation pressure increased.     

Earthmoving in minature

This paper has described a similitude approach to earthmoving, using small-scale models based on a dimensional analysis with certain simplifying assumptions. An earthmoving mechanics laboratory has been developed, with facilities for testing two sizes of models.

Despite the variability of real soils, and the complexity of the soil action during earthmoving processes, the somewhat idealized model tests are useful and exceedingly realistic. Models yield design information that is not available by any other means. Full-scale testing is still needed for final evaluation of an actual machine.     

Modelling in FEM the soil pressures distribution caused by a tyre on a Rhodic Ferralsol soil

Tyre traffic over soil causes non-uniform ground pressures across the tyre width and along the soil–tyre contact area. The objective of this paper was to obtain in the topsoil the shape, magnitudes, distribution and transmission in depth of the ground pressures from a finite element model of soil compaction. The influence of tyre inflation pressure, tyre load and soil water content over the pressures propagation in the soil was analysed. The model shows how to low inflation pressure the tyre carcass supports most of the total load and the biggest peak pressures are distributed in the tyre axes when it traffics over firm soil. For high inflation pressure the incremented stiff causes that pressure is distributed with parabolic shape. In wet soil the inflation pressure does not influence on the ground pressure distribution, this depends only on the tyre load. The inflation pressure and tyre load changed the shape of the vertical pressures distribution on the surface of a hard dry soil, but these variables did not affect the distribution of vertical stresses in a soft wet soil or below a depth of 0.15 m.     

Effect of variations in front wheels driving lead on performance of a farm tractor with mechanical front-wheel-drive

Most previous researches indicate that about 20–55% of available tractor power is lost in the process of interaction between tires and soil surface. Vertical wheel loads and tire performance are parameters that play a significant role in controlling slip and fuel consumption of a tractor. Tractor’s slip is adjusted by attaching additional weights and/or reducing tire pressures, and this may have an impact on driving lead of front wheels. Mechanical Front-Wheel-Drive (MFWD) tractors work efficiently when driving lead of front wheels is 3–4% in soft soil and 1–2% in hard soil. This research was aimed to experimentally determine such tire pressures that allow adjusting tractor’s slip without deviating from set value of driving lead of front wheels. The research was also aimed to determine the effect of driving lead of front wheels on MFWD tractor’s slip and fuel consumption. Experimental results showed that front/rear tire pressure combinations that generate a well-targeted driving lead of front wheels have no effect on slip on hard soil; however, it significantly affect fuel consumption. Results show that when air pressures in front/rear tires varied within 80–220 kPa, driving lead of front wheels varied in the range from +7.25% to −0.5%.     

Prediction of sinkage and rolling resistance for off-the-road vehicles considering penetration velocity

The use of plate pressure tests is common for the prediction of sinkage and rolling resistance for off-the-road vehicles. Most of these penetration tests are carried out with small penetration bodies in a quasi-static way. However, when vehicles are rolling on soft soil, the soil is charged dynamically. Consequently, this dynamic effect should be considered by the prediction of sinkage and rolling resistance. At the Institute of Automotive Engineering at Hamburg, tests were carried out with penetration bodies in sizes up to 900 cm², which is comparable to the contact area of wheels. The results confirm the influence of the penetration velocity. With a constant load, the body sinkage is smaller at a higher penetration velocity. Also shear tests with shear rings of different sizes have been carried out and an exponential equation for the shear strength, not depending on the shear ring size, was found. A dynamic pressure-sinkage relationship depending on the penetration velocity or driving speed was derived and transferred to the sinkage and rolling resistance of a rigid wheel.     

Agricultural tire deformation in the 2D case by finite element methods

The mechanical characteristics of the rubber tire and the interaction between a tire and a rigid surface were investigated by a two-dimensional (2D) finite element (FE) model. The FE model consists of a rigid rim and a rigid contact surface which interact with the elastic tire. Four distinct sets of elastic parameters are used to represent beads, sidewall, tread and lugs. Several sets of tire loads and inflation pressures were applied to the FE model as boundary conditions, together with various displacements and friction conditions. The deformation of the tire profile, the tire displacements in the vertical and lateral directions, the normal contact pressures, the frictional forces and the stress distribution of the tire components were investigated by the 2D FE model under the above boundary conditions. The calculated tire deflections were compared with the measured data. The results show a good fit between calculated and measured data, especially at high load and inflation pressure. The comparison shows that the FE analysis is suitable to predict aspects of the tire performance like its deflection and interactions with the contact surface. Compared with the experimental methods, the FE methods show many advantages in the prediction of tire deformation, contact pressure and stress distribution.     

Heat transfer in bubble layers at elevated pressures

Heat transfer with steam condensation under moderate pressure on the surface of a horizontal tube immersed in a bubbling layer was experimentally investigated. A copper test section 16 mm in outer diameter and 400 mm in length was placed in a bubbling column 455 mm in diameter. Experiments were made under pressures of 0.14–0.8 MPa, with void fraction 0.04–0.23, vapor superficial velocities 0.05–0.42 m/s, liquid-wall temperature differences 47–105 K, and heat flux densities 0.12–0.8 MW/m². The heat transfer process in the bubbling layer is shown to be of a high intensity: with moderate values of steam content, heat transfer coefficients reach 12–14 kW/(m² · K). Data obtained showed that the known correlations do not consider the influence of pressure on heat transfer. For the first time, data on radial steam content distribution under pressures higher than atmospheric were obtained by an electroprobe method. A table of experimental data is presented.     

Effects of reduced inflation pressure and vehicle loading on off-road traction and soil stress and deformation state

Field experiments on off-road vehicle traction and wheel–soil interactions were carried out on sandy and loess soil surfaces. A 14 T, 6 × 6 military truck was used as a test vehicle, equipped with 14.00-20 10 PR tyres, nominally inflated to 390 kPa. Tests were performed at nominal and reduced (down to 200 kPa) inflation pressures and at three vehicle loading levels: empty weight, loaded with 3.6 and 6.0 T mass (8000, 11,600 and 14,000 kg, respectively). Traction was measured with a load cell, attached to the rear of the test vehicle as well as to another, braking vehicle. Soil stress state was determined with the use of an SST (stress state transducer), which consists of six pressure sensors. Soil surface deformation was measured in vertical and horizontal directions, with a videogrammetric system. Effects of reduced inflation pressure as well as wheel loading on traction and wheel–soil interactions were analyzed. It was noticed that reduced inflation pressure had positive effects on traction and increased stress under wheels. Increasing wheel load resulted in increasing drawbar pull. These effects and trends are different for the two soil surfaces investigated. The soil surface deformed in two directions: vertical and longitudinal. Vertical deformations were affected by loading, while longitudinal were affected by inflation pressure.     

PIV measurement of the flow past a generic car body with wheels at LES applicable Reynolds number

Experiments by using 2D–2C Particle Image Velocimetry (PIV) were carried out and reported concerning the flow field past a generic car body (modified Ahmed body) which is equipped with wheels and wheel-arches. The Reynolds number was chosen to not exceed 2E+5 based on the height of the Ahmed body which makes it possible to investigate the same configuration by means of Large Eddy Simulation (LES). The wheels were rotating but the ground was stationary. The wheel-ground contact was realized by means of small rectangular openings below the wheels in the ground plane in which the wheels were immersed. The transition contour of the immersed wheels and the ground, as well as the rectangular openings below the wheels were properly sealed to prevent parasite flow and to provide well defined boundary conditions for an upcoming LES investigation.The flow field was measured in several planes with normal vectors pointing towards the directions normal to the free stream. Statistical characteristics of the flow are provided and discussed.     

The ability of agricultural tyres to distribute the wheel load at the soil-tyre interface

Bigger tyres with lower inflation pressure at equivalent wheel loads are expected to reduce the stresses transmitted to the soil. We measured the contact area and the vertical stress distribution near the soil-tyre interface for five agricultural implement tyres at 30 and 60 kN wheel load and rated inflation pressures. Seventeen stress transducers were installed at 0.1 m depth in a sandy soil at a water content slightly lower than field capacity and covered with loose soil. The recently developed model FRIDA was successfully fitted to the experimental stress data across the footprint. The contact area reflected the size of the tyres. The small tyres had identical contact area at the two loads, while it increased with load for the two biggest tyres. The small tyres presented uneven stress distributions with high peak stresses. Across the tests, the tyre inflation pressure described well the measured peak stress as well as the modelled maximum stress. The latter seems to be appropriate in evaluating vehicle trafficability. We found significant differences among tyres for the slope of a linear regression between the mean ground pressure and the inflation pressure, while the tyres displayed the same interception on the mean ground pressure axis. Our results therefore suggest that the slope of this relation is the most sensitive expression of tyres’ ability to deflect and transfer stresses to the soil. The two small tyres performed poorer in this respect than the larger tyres. Tests were limited to one soil strength, with future research directed toward a broader spectrum of soil strengths.     

Tire footprint characteristics as a function of soil properties and tire operations

The main objective of the following presentation is to examine the possibility of predicting agricultural tire footprint parameters under different operational conditions. The experimental part of the research involved the operation of two agricultural transport tires on two soils, under variations of tire load, inflation pressures and soil moisture contents. Results obtained show that tire footprint parameters, such as contact area, length, width and sinkage, can be reliably predicted using multifactorial linear and total regressions, within the range of recommended tire loads, inflation pressures and soil moisture contents around the plastic limit.     

三向受压混凝土的动态特性研究

为探究混凝土在三向受压状态下的动态特性,利用自行研制的大型多功能三轴材料试验机,进行不同应变速率(10-5/s、10-4/s、10-3/s、10-2/s)下混凝土不同定侧压比(1∶1、2∶1、3∶1、4∶1)的动态真三轴抗压试验,研究了混凝土在动态抗压下的强度和变形特性.结果表明:混凝土在三向受压状态下表现出明显的应变...     

Soil compaction and tires for harvesting and transporting sugarcane

Four tire types (A, block-shape tread; B, rib-shape tread; C, low-lug tread; D, high-lug tread) used to harvest and transport sugarcane were compared regarding the compaction induced to the soil. Tires were tested at three inflation pressures (207, 276, 345 kPa) and six loads ranging from 20 to 60 kN/tire. Track impressions were traced, and 576 areas were measured to find equations relating inflation pressure, load, contact surface and pressure. Contact surface increased with increasing load and decreasing inflation pressure; however, the contact pressure presented no defined pattern of variation, with tire types A and B generating lower contact pressure. The vertical stresses under the tires were measured and simulated with sensors and software developed at the Colombian Sugarcane Research Center (Cenicaña). Sensors were placed at 10, 30, 50 and 70 cm depth. Tire types A and B registered vertical stresses below 250 kPa at the surface. These two tires were better options to reduce soil compaction. The equations characterizing the tires were introduced into a program to simulate the vertical stress. Simulated and measured stresses were adjusted in an 87–92% range. Results indicate a good correlation between the tire equations, the vertical stress simulation and the vertical stress measurement.     

Tire load rating to reduce soil compaction

Both experience and research warn that heavily loaded wheels of agricultural transport vehicles and heavy machinery may cause severe compaction damage to the farmland. A remedy consists of reducing both the wheel load and the contact pressure.Early in the 1990s, the author suggested an experimental examination of the problem of soil compaction under fully controlled conditions. The ensuing research program, which was sponsored by the Grant Agency of the Czech Republic, included a series of experiments with loaded wheels carried out in the experimental grounds of the Czech University of Agriculture and, subsequently, their physical modelling in the laboratory of the Department of Motor Vehicles, Technical faculty. This program has corroborated the idea that physical modelling under controlled conditions, complemented by an adequate evaluation procedure, has a promising potential to predict full-scale ground compaction and become a sound basis for practical measures. This paper describes the laboratory equipment, testing technique, and the way of evaluating the compaction potential of tires in terms of soil dry bulk density, leading to a Compaction Number (CN) rating of individual tires. Practically, the CN rating is supposed to be included in agricultural tire catalogues to complement the load capacity/inflation pressure values for hard ground (e.g., ETRTO specifications based on tire strength and wear).     

EXPERIMENTS ON EVOLUTION CHARACTERISTICS FOR THE MUSHROOM-LIKE VORTEX STRUCTURE GENERATED BY A SUBMERGED LAMINAR ROUND JET

利用溢流恒压装置产生具有稳定出流速度的层流圆管潜射流,结合染色液流态显示方法,在多种射流时间和雷诺数组合下,实验研究了该射流动量在密度均匀黏性流体中的演化机理及其表现特征,定量分析了蘑菇型涡结构的无量纲射流长度L^*、螺旋型涡环半径R^*及其包络外形长度d^*等几何特征参数随无量纲时间t^*的变化规律。系列实验结果表明,蘑菇型涡结构的形成与演化过程可分为3个不同的阶段,分别为启动阶段、发展阶段和衰退阶段。在启动阶段,L^*和d^*随t^*线性变化,而R^*则近似为一个常数。在发展阶段,蘑菇型涡结构的演化具有自相似性,在各种射流时间和雷诺数组合下,L^*,R^*和d^*与t^*1/2均为同一正比关系,而且实验结果与基于斯托克斯（Stokes）近似的理论解结果一致。在衰退阶段,蘑菇型涡结构会发生两类形式的演化,第1类衰退出现在射流结束之后,其间L^*和R^*与t^*1/5相关,而d^*近似为一个常数;第2类衰退出现在射流结束之前,当射流动量达到某个临界值后,蘑菇型涡结构就会发生破碎等现象。     

Mobility algorithm evaluation using a consolidated database developed for wheeled vehicles operating on dry sands

A substantial number of laboratory and field tests have been conducted to assess performance of various wheel designs in loose soils. However, there is no consolidated database which includes data from several sources. In this study, a consolidated database was created on tests conducted with wheeled vehicles operating in loose dry sand to evaluate existing soil mobility algorithms. The database included wheels of different diameters, widths, heights, and inflation pressures, operating under varying loading conditions. Nine technical reports were identified containing 5253 records, based on existing archives of laboratory and field tests of wheels operating in loose soils. The database structure was assembled to include traction performance parameters such as drawbar pull, torque, traction, motion resistance, sinkage, and wheel slip. Once developed, the database was used to evaluate and support validation of closed form solutions for these variables in the Vehicle Terrain Interface (VTI) model. The correlation between predicted and measured traction performance parameters was evaluated. Comparison of the predicted versus measured performance parameters suggests that the closed form solutions within the VTI model are functional but can be further improved to provide more accurate predictions for off-road vehicle performance.     

A NEW HIGH-ORDER ACCURACY EXPLICIT DIFFERENCE SCHEME FOR SOLVING THREE-DIMENSIONAL PARABOLIC EQUATIONS

1.IntroductionInalotoffields,suchas'diffusion,seepageandheatconductionetc.,weoftenmeetproblemsofsolvingparabolicequations.Inthecaseofthree-dimension,themathematicalmodelissuchaninitialandboundaryvalueproblemasbelow:Thedifferenceschemesforsolvingtheproblemabove,whichhavehigh-orderaccuracyandcanbefiguredoutwithexplicitschemearesuchschemesasthoseinII]and[21.Theexplicitdifferenceschemeproposedinthispaperkeepsthehigh-orderaccuracy(it'hasthesameaccuracyasthosein[ifand[21),butitsexpressionismuchsimp…     

不同含氧量环境下紫铜/黄铜电接触微动磨损性能研究

电连接器常因接触界面磨损发生严重失效,因此有必要研究电接触模式下的微动磨损行为。本文中基于电接触模式和不同环境含氧量(即氧气体积分数分别为10%、20%和30%,后文统一称作10%O₂、20%O₂和30%O₂环境),着重研究含氧量对紫铜/黄铜微动磨损行为与磨损机制的影响。研究发现：10%O₂、20%O₂和30%O₂环境时所对应的摩擦系数稳定值分别为0.77、0.71和0.80,摩擦耗散能和磨损体积的结果变化趋势一致,即10%O₂环境下最高,20%O₂最低,30%O₂介于两者之间,可以推测,含氧量最低条件时的损伤比含氧量高时更严重。通过电接触寿命可以看出, 20%O₂环境中电接触性能最优,10%O₂次之,30%O₂最差,不同含氧量时的微动磨损区均发生不同程度的氧化,理论上含氧量越高氧化现象越严重,但10%O₂