Road Modeling

The soft soil road surface is represented as a rectangular structured grid of deformable springs in the vertical (z-axis) direction, as shown in the figure below. Using this approach, each spring represents a small road patch for which the necessary information is stored.

Typical parameters include the soil properties, the soil elevation (z coordinate) as well as information regarding the compaction of the soil, as functions of x and y coordinates. Moreover, the information stored for each road patch is set independently of the other springs. In addition, the grid spacing is set automatically based on the minimum tire width or track link dimension of the model.


Figure 1. Road surface modeled as a regular grid of deformable springs

Soil Library

A soil library containing ready-to-use road data files for a variety of soils are located in the installation <Installation>\hwdesktop\hw\mdl\autoentities\properties\Tires\ALTAIR_SOFTSOIL.

Road Data File

All the necessary (and optional) parameters for the soft-soil model are provided in the road data (.rdf) file. It can be used with tires, tracks, or both models. The soil properties are structured into Common Soil Properties, Tire Soil Properties, Track Soil Properties, and Advanced Track Soil Properties.

The Common Soil Properties contain information inherent to soil characteristics used in both tire and track soil equations.
Parameter Description
PHI Angle of internal shearing resistance [Units: angle]
C Soil apparent cohesion [Units: force/length**2]
KC Pressure-sinkage parameter [Units: force/length**(n+1)]
KPHI Pressure-sinkage parameter [Units: force/length**(n+2)]
SINKAGE_EXPONENT Sinkage exponent n [Units: -]
SOIL_STIFFNESS Soil elastic stiffness [Units: force/length**3]
SOIL_DAMPING Soil damping [Units: force*time/length]
SOIL_DENSITY Soil density [Units: mass/length**3]
KX1 Shear deformation module [Units: length]
KY1 Shear deformation module [Units: length]
The Tire Soil Properties contain information used only for the tire soft soil contact model.
Parameter Description
KX0 Shear deformation module [Units: length]
KY0 Shear deformation module [Units: length]
C1 Parameter for wheel angle of maximum normal stress [-]
C2 Parameter for wheel angle of maximum normal stress [-]
Besides the common soil properties, the additional requirements to simulate the track soft soil interaction are included in the Track Soil Properties.
Parameter Description
R_CASE The input report of the soft-soil model (0≤R_CASE≤2)
  • If R_CASE=0, the input is not reported
  • If R_CASE=1, the input is partially reported
  • If R_CASE=2, a full report is generated
WID_OR_MIN Definition of Bekker parameter b (1≤WID_OR_MIN≤2)
  • If WID_OR_MIN=1, b is the width of the track (recommended value)
  • If WID_OR_MIN=2, b is the minimum dimension of the track link
In addition, the optional parameters for the track model are available in the Advanced Track Soil Properties.
Parameter Description
DELTA The internal angle of soil-grouser friction.

If DELTA<0, then DELTA=PHI*|DELTA|

CA The cohesion between the soil and the grouser.

If CA<0, then CA=C*|CA|

R_JAN r of modified Janosi
JMAX jmax of modified Janosi

If JMAX<0, then JMAX=|JMAX|*(Link length)

JULT ju of modified Janosi

If JULT<0, then JULT=|JULT|*(Link length)

AS The plasticity factor of the shearing (0≤AS≤1)
AM The plasticity factor of the grouser force (0≤AM≤1)
PHI_LONG The internal angle of soil friction. This value is used for the interaction between a link and the soil in the longitudinal direction. PHI_LONG=PHI*PHI_LONG
DELTA_LONG The internal angle of soil-link friction. This value is used for the interaction between a link and the soil in the longitudinal direction. DELTA_LONG=DELTA*DELTA_LONG
C_LONG The cohesion of the soil particles. This value is used for the interaction between a link and the soil in the longitudinal direction. C_LONG=C*C_LONG
CA_LONG The cohesion between the soil and the grouser. This value is used for the interaction between a link and the soil in the longitudinal direction. CA_LONG=CA*CA_LONG
PHI_LAT The internal angle of soil friction. This value is used for the interaction between a link and the soil in the lateral direction. PHI_LAT=PHI*PHI_LAT
DELTA_LAT The internal angle of soil-link friction. This value is used for the interaction between a link and the soil in the lateral direction. DELTA_LAT=DELTA*DELTA_LAT
C_LAT The cohesion of the soil particles. This value is used for the interaction between a link and the soil in lateral direction. C_LAT=C*C_LAT
CA_LAT The cohesion between the soil and the grouser. This value is used for the interaction between a link and the soil in the lateral direction. CA_LAT=CA*CA_LAT
CJ1 Cohesion, c of the Janosi approach, for the longitudinal direction. CJ1=C*CJ1
CJ2 Cohesion, c of the Janosi approach, for the lateral direction. CJ2=C*CJ2
PHIJ1 Angle of internal friction, φ of Janosi, for the longitudinal direction. PHIJ1=PHI*PHIJ1
PHIJ2 Angle of internal friction, φ of Janosi, for the lateral direction. PHIJ2=PHI*PHIJ2
RJ1 r of the modified Janosi, for the longitudinal direction. RJ1=R_JAN*RJ1
RJ2 r of the modified Janosi, for the lateral direction.
  • If RJ2≥0, then RJ2=R_JAN*RJ2
  • If RJ2<0, then RJ2= |RJ2|

Recommended value: RJ2=-1.

JMAX1 jmax of modified Janosi, for the longitudinal direction. JMAX1= JMAX*JMAX1
JMAX2 jmax of modified Janosi, for the lateral direction. JMAX2= JMAX*JMAX2
JULT1 ju of modified Janosi, for the longitudinal direction. JULT1=JULT*JULT1
JULT2 ju of modified Janosi, for the lateral direction. JULT2=JULT*JULT2
AS1 The plasticity factor of the shearing (0≤AS1≤1). The index 1 indicates the longitudinal direction. AS1=AS*AS1
AS2 The plasticity factor of the shearing (0≤AS2≤1). The index 2 indicates the lateral direction. AS2=AS*AS2
AM1 The plasticity factor of the grouser force (0≤AM1≤1). The index 1 indicates the longitudinal direction. AM1=AM*AM1
AM2 The plasticity factor of the grouser force (0≤AM2≤1). The index 2 indicates the lateral direction. AM2=AM*AM2

Property File Example

$----------------------------------------------------------------MDI_HEADER
[MDI_HEADER]
FILE_TYPE  =  'rdf'
FILE_VERSION  =  5.00
FILE_FORMAT  =  'ASCII'
$---------------------------------------------------------------------UNITS
[UNITS]
 LENGTH              	= 'm'
 FORCE               	= 'newton'
 ANGLE               	= 'radians'
 MASS                	= 'kg'
 TIME                 	= 'sec'
$---------------------------------------------------------------------MODEL
[MODEL]
 PROPERTY_FILE_FORMAT                = 'USER'
 FUNCTION_NAME                       = 'mbdtire::ROADSUB'
 METHOD                              = '3D'
 ROAD_TYPE                           = 'softsoil'
$---------------------------------------------------------------------PARAMETERS
[PARAMETERS]
 MU                        =  1.0              $ Friction coefficient
 LENGTH                    =  300.0            $ Range of soil region in x direction
 WIDTH                     =  5.0              $ Range of soil region in y direction
 NODES                     =  1                $ Number of integration points
 MULTIPASS                 =  'TRUE'
$----------------------------------------------------------------COMMON_SOIL_PROPERTIES
[PROPERTIES]
 PHI                                  = 0.65                $ Angle of internal shearing resistance [angle]  
 C                                    = 800.0               $ Soil apparent cohesion [force/length**2]  
 KC                                   = 1370.0              $ Pressure-sinkage parameter [force/length**(n+1)]  
 KPHI                                 = 8.14E5              $ Pressure-sinkage parameter [force/length**(n+2)]  
 SINKAGE_EXPONENT                     = 1.0                 $ Sinkage exponent n [-]  
 SOIL_STIFFNESS                       = 8.14E6              $ Soil elastic stiffness [force/length**3]  
 SOIL_DAMPING                         = 500.0               $ Soil damping [force*time/length]
 SOIL_DENSITY                         = 1600.0              $ Soil density [mass/length**3]  
 KX1                                  = 0.036               $ Shear deformation module [length]  
 KY1                                  = 0.013               $ Shear deformation module [length]                               
 
$--------------------------------------------------------------TIRE_SOIL_PROPERTIES
$-Properties used for the tire-soil interaction
$-
 KX0             = 0.043        	$ Shear deformation module [length/angle]  
 KY0             = 0.02         	$ Shear deformation module [length/angle]  
 C1              = 0.4          	$ Parameter for wheel angle of maximum normal stress [-]  
 C2              = 0.15         	$ Parameter for wheel angle of maximum normal stress [-]  

$--------------------------------------------------------------TRACK_SOIL_PROPERTIES
$-Properties used for the Track-soil interaction
$- 
 R_CASE            = 0     	 $ Report of softsoil input - 0: None, 1: Minimal, 2: Full
 WID_OR_MIN        = 1     	 $ Definition of Bekker parameter b - 1: Width, 2: Minimum dimension of link
 
$--------------------------------------------------------------ADVANCED_TRACK_SOIL_PROPERTIES
$-Advanced Properties used for the Track-soil interaction (OPTIONAL)
 DELTA              = -0.5               $ Internal angle of soil-grouser friction
 CA                 = -0.5               $ Cohesion between soil and grouser
 R_JAN              = 0.5                $ r of modified Janosi
 JMAX               = -0.8               $ jmax of modified Janosi
 JULT               = -1.5               $ ju of modified Janosi
 AS                 = 0.5                $ Plasticity factor of shearing
 AM                 = 0.5                $ Plasticity factor of grouser force
 PHI_LONG           = 1.0                $ The internal angle of soil friction. 
 DELTA_LONG         = 1.0                $ The internal angle of soil-link friction.
 C_LONG             = 1.0                $ The cohesion of the soil particles. 
 CA_LONG            = 1.0                $ The cohesion between the soil and the grouser. 
 PHI_LAT            = 1.0                $ The internal angle of soil friction. 
 DELTA_LAT          = 1.0                $ The internal angle of soil-link friction. 
 C_LAT              = 1.0                $ The cohesion of the soil particles. 
 CA_LAT             = 1.0                $ The cohesion between the soil and the grouser. 
 CJ1                = 1.0                $ Cohesion C of the Janosi approach, for the longitudinal direction
 CJ2                = 1.0                $ Cohesion C of the Janosi approach, for the lateral direction
 PHIJ1              = 1.0                $ Angle of internal friction, Φ, of Janosi, for the longitudinal direction
 PHIJ2              = 1.0                $ Angle of internal friction, Φ, of Janosi, for the lateral direction
 KJ1                = 1.0                $ k of Janosi, for the longitudinal direction
 KJ2                = 1.0                $ k of Janosi, for the lateral direction
 RJ1                = 1.0                $ r of the modified Janosi, for the longitudinal direction
 RJ2                = 1.0                $ r of the modified Janosi, for the lateral direction
 JMAX1              = 1.0                $ jmax of modified Janosi, for the longitudinal direction
 JMAX2              = 1.0                $ jmax of modified Janosi, for the lateral direction
 JULT1              = 1.0                $ ju of modified Janosi, for the longitudinal direction
 JULT2              = 1.0                $ ju of modified Janosi, for the lateral direction
 AS1                = 1.0                $ The plasticity factor of the shearing 0≤AS1≤1. 
 AS2                = 1.0                $ The plasticity factor of the shearing 0≤AS2≤1.
 AM1                = 1.0                $ The plasticity factor of the grouser force 0≤AM1≤1. 
 AM2                = 1.0                $ The plasticity factor of the grouser force 0≤AM2≤1.
$----------------------------------------------------------------------GRAPHICS
[GRAPHICS]
 ROAD_INCR   	= 0.050

CRG Road Data File

By default, a flat road model with the user-defined dimensions (length, width) is created using the soft-soil tire model, see the figure below.


Figure 2. Road Model with user-defined dimensions (length, width)

In order to represent complex terrain geometries, the soft soil road model enables the use of curved regular grid (CGR) files. For this, the attribute CRG_FILE=’path to .crg file’ needs to be set in the PARAMETERS block of the road (.rdf) file.

$---------------------------------------------------------------------PARAMETERS
[PARAMETERS]
 MU              =  1.0                 $ Friction coefficient
 LENGTH          =  300.0               $ Range of soil region in x direction
 WIDTH           =  6.0                 $ Range of soil region in y direction
 NODES           =  1                   $ Number of integration points
 MULTIPASS       =  'TRUE'
CRG_FILE         =  '<road_path>.crg'
Note: The obstacles defined in the soft soil road are still superimposed on the existing road geometry. In a road containing a .crg file, the parameters LENGTH and WIDTH are still used to define the limit of the road. If the .crg surface is smaller than the specified dimensions, a flat surface is created extending the .crg surface to the road limits, instead if the .crg surface is bigger, the road limits crops the .crg surface. If the specified .crg file cannot be found, the simulation proceeds using a flat road model.

Deformable Road

MotionView provides the ability to include soft-soil road deformation in simulations using the soft-soil tire model. The deformable road graphic is created in MotionView using the Road Tools and sent to MotionSolve to simulate the tire-soft-soil dynamics. The soft-soil road deformation can be visualized in HyperView.



Figure 3. a) Soft-soil road graphics in MotionView; b) Soft-soil road graphics deformed in HyperView

The Road Tool supports the creation of flat and Curved Regular Grid (.crg) roads using the road property file (.rdf). It is also possible to include the different obstacle types available in the soft-soil tire model that deforms along with the road depending on the obstacle material. Refer to Obstacles for more information.

Note: The soft-soil deformation can be plastic or elastic depending on the soil material and load applied on the soil by the tire. In case of an elastic deformation where no plastic sinkage is present, the road graphics will not display any deformation.

Road Positioning

The soft-soil road is positioned according to the road reference marker. In case of flat surfaces and an obstacle, the road reference marker is positioned at the center of the road in both the X and Y direction. For .crg surfaces, the road marker is positioned at the beginning of the road. The figures below, highlight the different cases of road positioning in the deformable road.


Figure 4. Flat surfaces and an obstacle positioned with the road reference marker


Figure 5. CRG surfaces positioned with the road reference marker on the deformable road
Note: The .crg files have by definition the length direction along positive X. The vehicle models built using MotionView vehicle libraries point along negative Global X direction. When using the .crg surfaces with the vehicle library models, it is recommended to rotate the road reference maker to align the vehicle direction with the road.

Add a Soft-Soil Deformable Road and Visualize the Results

  1. Load the soft-soil road property file (.rdf) in the Road Tools (see the Visualization - Road Tools topic for additional information).
    When loading the road property file, an .h3d file and .mdl file are created at the selected path. The road graphics are displayed in MotionView's modeling windows and a graphic system containing the road graphics is defined in the model browser.


    Figure 6.
  2. Add the soft-soil tire and road property files in the Tires entity or use the track builder to create a track system with soft soil.
  3. Run the analysis in MotionSolve.

    MotionSolve reads the road .h3d created by the Road Tools and generates an additional SSR (soft-soil road) file which has the name <MotionSolve_result_xml_name>_softsoil_road.ssr during run time. The softsoil_road.ssr contains the information about the road graphic nodes deformed during the simulation.

  4. Post-process the analysis in HyperView.

    Once the simulation in completed, open the model animation results in HyperView.