Compaction

Compaction of soils is a complex process in which several soil properties as well as compactor characteristics interact. General rules have been developed through years of experience in construction and through a need, in the recent past, to increase the sub-base and base strength of runways to accomodate higher aircraft wheel loads. General guidelines are adequate when there is no need for an accurate prediction of the number of compactor coverages required to effect a given level of soil compaction.During the conduct of a recent program, it was necessary that an estimate be made of the time required to compact soil to a certain strength. A review of the literature indicated that little recent work has been done on compaction and on the modelling of the compaction process. Similitude modelling has been used to predict the trafficability of soft soils. Although the soil compaction criteria are different from those of trafficability, it was felt that similitude modelling could also be applied to compaction. This paper describes the basis for CBR and density models and some indications of their form and prediction ability.     

Reduced testing and modelling of the bearing capacity of rooted soil for wheeled forestry machines

The next generation of forestry machines must be developed to be gentler to soil and to the root mat than present machines, especially in thinning operations. The bearing capacity of the soil is a key property for determining the terrain trafficability and machine mobility. This asks for better and more general terramechanics models that can be used to predict the interaction between different machine concepts and real and complex forest soil.This paper presents results from terramechanics experiments of rooted soil with a new and small-scale testing device. The force–deflection results are analyzed and compared with analytical root reinforcement models found in literature. The presented study indicates that rooted soil properties obtained with the new laboratory test device can be used to create an augmented soil model that can be used to predict the bearing capacity of rooted soil and also to be used in dynamic machine–soil interaction simulations.     

A proposed strength classification test for fine-grained soils

It is proposed that the linear semi-log relation of water content to soil strength be used as a means of identifying and classifying soils. The relation can be specified by two parameters proposed to be named trafficability limit and strength index. Together these two parameters will provide more useful information about a soil for the conduct and analysis of traction and mobility testing than any other classification system now in use. The parameters also have potential for additional correlations with measures of soil behavior in dynamic testing. Data are presented to demonstrate the feasibility of the proposal.     

Vorschlag für die Klassifizierung der Festigkeit Feinkörniger Böden

It is proposed that the linear semi-log relation of water content to soil strength be used as a means of identifying and classifying soils. The relation can be specified by two parameters proposed to be named trafficability limit and strength index. Together these two parameters will provide more useful information about a soil for the conduct and analysis of traction and mobility testing than any other classification system now in use. The parameters also have potential for additional correlations with measures of soil behavior in dynamic testing. Data are presented to demonstrate the feasibility of the proposal.     

Basic study of probabilistic approach to prediction of soil trafficability — Statistical characteristics of cone index

The principal purpose of this study is to present some of the fundamental data which might be used in the prediction of soil trafficability. These data are based on some sets of random cone index measurements which were taken from the areas of interest. In all cases, it was assumed that the individual measurements of the cone index being investigated would be independent. It is shown that the cone indices at critical layer depth can be regarded as normal random variables. The discussion in this study may be valid for the probabilistic approach of soil trafficability     

Design and development of a new transformable wheel used in amphibious all-terrain vehicles (A-ATV)

Conventional ground-wheeled vehicles usually have poor trafficability, low efficiency, a large amount of energy consumption and possible failure when driving on soft terrain. To solve this problem, this paper presents a new design of transformable wheels for use in an amphibious all-terrain vehicle. The wheel has two extreme working statuses: unfolded walking-wheel and folded rigid wheel. Furthermore, the kinematic characteristics of the transformable wheel were studied using a kinematic method. When the wheel is unfolded at walking-wheel status, the displacement, velocity and acceleration of the wheel with different slip rates were analyzed. The stress condition is studied by using a classic soil mechanics method when the transformable wheel is driven on soft terrain. The relationship among wheel traction, wheel parameters and soil deformation under the stress were obtained. The results show that both the wheel traction and trafficability can be improved by using the proposed transformable wheel. Finally, a finite element model is established based on the vehicle terramechanics, and the interaction result between the transformable wheel and elastic–plastic soil is simulated when the transformable wheel is driven at different unfold angles. The simulation results are consistent with the theoretical analysis, which verifies the applicability and effectiveness of the transformable wheel developed in this paper.     

Evaluation of Model Concepts to Describe Water Transport in Shallow Subsurface Soil and Across the Soil–Air Interface

Soil water evaporation plays a critical role in mass and energy exchanges across the land–atmosphere interface. Although much is known about this process, there is no agreement on the best modeling approaches to determine soil water evaporation due to the complexity of the numerical modeling scenarios and lack of experimental data available to validate such models. Existing studies show numerical and experimental discrepancies in the evaporation behavior and soil water distribution in soils at various scales, driving us to revisit the key process representation in subsurface soil. Therefore, the goal of this work is to test different mathematical formulations used to estimate evaporation from bare soils to critically evaluate the model formulations, assumptions and surface boundary conditions. This comparison required the development of three numerical models at the REV scale that vary in their complexity in characterizing water flow and evaporation, using the same modeling platform. The performance of the models was evaluated by comparing with experimental data generated from a soil tank/boundary layer wind tunnel experimental apparatus equipped with a sensor network to continuously monitor water–temperature–humidity variables. A series of experiments were performed in which the soil tank was packed with different soil types. Results demonstrate that the approaches vary in their ability to capture different stages of evaporation and no one approach can be deemed most appropriate for every scenario. When a proper top boundary condition and space discretization are defined, the Richards equation-based models (Richards model and Richards vapor model) can generally capture the evaporation behaviors across the entire range of soil saturations, comparing well with the experimental data. The simulation results of the non-equilibrium two-component two-phase model which considers vapor transport as an independent process generally agree well with the observations in terms of evaporation behavior and soil water dynamics. Certain differences in simulation results can be observed between equilibrium and non-equilibrium approaches. Comparisons of the models and the boundary layer formulations highlight the need to revisit key assumptions that influence evaporation behavior, highlighting the need to further understand water and vapor transport processes in soil to improve model accuracy.

     

Validation of the GeoWATCH soil moisture model and proposed bias correction method

The US Army is required to be a good steward of the land per US Army regulation AR 200-1. Based on this regulation, Army installations need to manage lands, to reduce potential damage and impacts to water quality and habitat that may occur from training. Maneuver training does impact the vegetation and soil and this damage is directly related to soil moisture. Soil moisture is an important factor for understanding the potential for soil surface disturbance due to vehicle impacts and predicting soil resilience to vehicle traffic, however, producing accurate estimates of the spatial and temporal variation of soil moisture has historically been elusive. GeoWATCH, which stands for Geospatial Weather-Affected Terrain Conditions and Hazards (formerly DASSP), simulates soil moisture world-wide, at relatively small spatial and temporal scales. GeoWATCH uses a physics-based downscaling approach that uses weather-scale land surface model estimates of soil moisture and land surface water and energy fluxes, with high resolution geospatial data. GeoWATCH soil moisture outputs coupled with vehicle impact models, are anticipated to be useful for near-real-time estimation of ground disturbance, but will require ground validation. To validate GeoWATCH soil moisture estimates, we utilized Soil Climate Analysis Network (SCAN) gauge network soil moisture data from 127 sites across 34 states. Statistical analysis of the raw GeoWATCH output indicated the model performs statistically better in certain soil textures. Model bias is largest for sandy soils, whereas clayey soils were least biased. As a result, bias correction models were applied to the raw GeoWATCH simulated values using linear regression to predict correction factor (CF) values based on physical site characteristics. The bias correction models significantly improved the performance of the GeoWATCH soil moisture model in terms of average performance statistics and number of statistically cally unbiased sites. This process could easily be incorporated into GeoWATCH, allowing for a capability to rapidly estimate vehicle impacts and determine rehabilitation requirements by installation land managers.     

Terrain evaluation under water

A method to evaluate terrain properties of sediments under water is proposed for the design of new equipment, such as fully submersible bulldozers and cutter suction dredgers, as well as the evaluation of trafficability conditions.The method consists of a simple prediction of the soil profiles, such as water content, void ratio, undrained shear strength and effective pressure, using the liquid limit of surface sediments based on a compression law of soils.     

Trafficability analysis of lunar mare terrain by means of the discrete element method for wheeled rover locomotion

An irregularly shaped particulate system for simulation of lunar regolith is developed using discrete element modeling based on the fractal characteristics, particle shape, and size distribution of returned Apollo-14 samples. The model parameters are determined by dimensional analysis and biaxial test simulation with an improved boundary condition. Under terrestrial conditions, the trafficability of lunar mare terrain is estimated in terms of wheel-terrain interaction by experiment and simulation in order to validate the applicability of the wheel-terrain model employed here. The results show that the discrete element method combined with the wheel-terrain model is sufficiently accurate for mare terrain trafficability analysis without consideration of lunar environmental effects. To predict the trafficability of in situ lunar mare terrain, the non-contact forces attributed to the lunar surface environment are discussed and the initial mechanical model of discrete elements is modified by introduction of lunar gravitational force as well as electrostatic force. In the modified model, wheel-terrain interaction is analyzed under the same travel conditions as that of the experiment. The result shows the trafficability of the in situ lunar mare terrain is worse than that obtained by experiment and simulation with the initial model according to the value of horizontal force at any slip ratio. However, the wheel requires less drive torque on the moon than that on the earth. An explanation for these phenomena may be that lunar subsurface regolith particles are arranged in a looser manner under local environmental effects that effectively decrease the bearing and shearing strength of regolith.     

Integrated soil tillage force prediction models

This paper describes the integration of a series of models to predict the forces acting on a range of tillage tools from simple plane tines to mouldboard ploughs. The results show that the horizontal (or draught) and vertical forces can be predicted with average errors of −3% and +33%, with the majority of the predicted values within ±20% and ±50% of the measured values respectively. The models adequately reflect the changes in soil strength and implement geometry. All of the predicted values given have been estimated using a spreadsheet based model which is freely available.     

Assessment of low-altitude atmospheric turbulence models for aircraft aeroelasticity

We investigate the dynamic aeroelastic response of large but slow aircraft in low-altitude atmospheric turbulence. To this end, three turbulence models of increasing fidelity, namely, the one-dimensional von Kármán model, the two-dimensional Kaimal model and full three-dimensional wind fields extracted from large-eddy simulations (LES) are used to simulate ambient turbulence near the ground. Load calculations and flight trajectory predictions are conducted for a representative high-aspect-ratio wing aircraft, using a fully coupled nonlinear flight dynamics/aeroelastic model, when it operates in background atmospheric turbulence generated by the aforementioned models. Comparison of load envelopes and spectral content, on vehicles of varying flexibility, shows strong dependency between the selected turbulence model and aircraft aeroelastic response (e.g. 58% difference in the predicted magnitude of the wing root bending moment between LES and von Kármán models). This is mainly due to the presence of large flow structures at low altitudes that have comparable dimensions to the vehicle, and which despite the relatively small wind speeds within the Earth boundary layer, result in overall high load events for slow-moving vehicles. Results show that one-dimensional models that do not capture those effects provide fairly non-conservative load estimates and are unsuitable for very flexible airframe design.     

A CFD comparative study of bubble break-up models in a turbulent multiphase jet

In this paper several bubble break-up models are compared. They have been implemented in the CFX-4.4 fluid dynamic commercial code, which uses the population balance equations for describing liquid/gas multi-phase flows. The models have been assessed against published experimental data, obtained for air bubble break-up within a turbulent water jet. The model of Martínez-Bazán, based on purely kinematics arguments, has shown better agreement with the experimental data. The capabilities of using these models coupled to a CFD code for multiphase flow prediction in industrial applications have been demonstrated.     

Solute transport through layered soil profiles: Zero and perfect travel time correlation models

In this paper the problem of solute transport through layered soil is modeled with a transfer function, representing the travel time to any depth in the soil as the sum of the travel times through the individual layers. The probability density function (pdf) of this travel time depends on the joint pdfs of the travel times through the individual layers, and hence on the correlation structure as well as the layer properties. Analytic expressions are derived for the flux and resident concentrations in layered soil for two limiting cases of correlation structure, for both convective-dispersive and stochastic-convective model representations of transport within the individual layers. These models are contrasted to the layered convection-dispersion model, which asumes continuous resident fluid concentration at the interface. Calculated concentrations are used to illustrate features of the different models, as well as to suggest experiments that might deduce the appropriate boundary condition to use at the interface between layers of different properties. The physical significance of various boundary condition assumptions is also discussed.     

Optimization of tandem wheel spacing for trafficability in sand

Heavy vehicles use multi-axle layouts to meet axle weight regulations and for better off-the-road mobility. Wheels in tandem are often used on these layouts. A study to optimize the wheel spacing for trafficability in sand was taken up. It was shown by field experimentation that soil pressure zones under wheels in sand do not exceed the wheel diameter and thus the criteria for selection of tandem wheel spacing shall be decided by the type of suspension on which the wheels are accommodated.     

Experimental evaluation of cutting dynamic models in soil bin facility

Computer Aided Engineering methods in earthmoving machines design and their automation require the development of soil-cutting models. These models both in two or three dimensions, static or dynamic, fitted for frictional or cohesive soils, must be mutually compatible and must function with soil transportation models and with machine locomotion characteristic models. In this work two different methods of soil cutting have been evaluated, both of them based on the classical wedge method, in order to verify their applicability to test conditions in the new soil bin facility of CEMOTER. From experimental results the possibility of using dynamic models of soil cutting in the frequency domain is discussed, to improve earthmoving machinery performance by automation and implementation of open and closed-loop control. After a preliminary analysis of a plane blade under different test conditions in sandy soil, soil cutting theoretical models of a simple implement are compared with respective scale models by tests performed in a soil bin facility at various operating speeds and depths, in order to investigate their applicability and the dynamic behaviour of the soil cutting force.     

Implementation of rotational resistance models: A critical appraisal

Contact models that simulate rotational resistance at particle contacts have been proposed as a means to capture the effect of shape in DEM simulations. This contribution critically explores some key issues related to the implementation of rotational resistance models; these include the need for physically meaningful model parameters, the impact of the model on the overall numerical stability/critical time increment for the DEM model, model validation, and the assessment of model performance relative to real physical materials. The discussion is centred around a rotational resistance model that captures the resistance provided by interlocking asperities on the particle surface. An expression for the maximum permissible integration time step to ensure numerical stability is derived for DEM simulations when rotational resistance is incorporated. Analytical solutions for some single-contact scenarios are derived for model validation. The ability of this type of model to provide additional fundamental insight into granular material behaviour is demonstrated using particle-scale analysis of triaxial compression simulations to examine the roles that contact rolling and sliding have on the stability of strong force chains.     

The present state of force prediction models for rotary powered tillage tools

In the past, investigators of rotary tillage tools have concentrated their efforts in developing relationships between the power requirement and operational parameters of tillers. Many researchers have developed empirical force and torque prediction models without giving due consideration to the strength properties of soil. In the absence of proper soil-tool behaviour equations, the designers of rotary tools have relied on these empirical approaches. In recent years, the authors have proposed the first theoretical model for two-dimensional soil failure by a rotary powered blade. The presently available state-of-the-art ideas on rotary tilling force prediction models has been presented in this paper.