class terrainMaterial

Class for describing terrain (soil/rock/snow) mechanical properties for improved contact simulation

Public Fields

General properties

Soil constants

Constants for Terzaghi's equation

Public Methods

void computeDerivedConstants(double gravity)

computes constants needed for later calculations, some of which depend on gravity

double computeSinkage(double &force, double width, double area, double zwMax, int assumeCohesionless = -1)

returns the sinkage of a wheel supporting a given force over a given area, which has a given width.

double computeSinkage(double &force, double width, double area, double dwDz, double zwMax, int assumeCohesionless = -1)

returns the sinkage of a wheel supporting a given force over a given area, which has a given width.

inline double computeRollingResistance(double force, double width, double area, double sinkage)

Computes the rolling resistance (in Newtons) for the given contact patch.

double computeMaximumTraction(double area, double force)

Computes the maximum traction that can be supported by the given contact area and force, based on soil cohesion and internal friction

int readParams(paramParser* p)

reads parameters from the current context of a parser.

Public Members

enum terrainMaterialType

Different classes of terrain materials.

Documentation

Class for describing terrain (soil/rock/snow) mechanical properties for improved contact simulation

enum terrainMaterialType

Different classes of terrain materials.

• ROCK - rocks or solid ice; effectively rigid
• SOIL - sand, clay, drift material, and other particulate materials
• SNOW - Snow or unconsolidated ice

General properties

int materialType

Type of material

double grainSize

size of particles in material (m)

double bulkDensity

density of material

double cohesion

Cohesion, in kPa

double internalFriction

internal friction (degrees).

Soil constants

Tergazhi's equation for bearing capacity requires several coefficients which depend on the angle of internal friction and whether a soil is cohesionless or not. These coefficients are usually obtained from a table, such as that on p. 375 of "Essentials of Soil Mechanics and Foundations" by McCarthy.

double Nc

Constant for cohesion in Terzaghi equation. Reasonable values are 3.5 to 18 for cohesionless soils and 5-70 for others. Worst case cohesionless Martian drift materials has Nc = 3.5 (though cohension is 0, so this term is not used)

double Nq

Constant for soil surcharge in Terzaghi equation. Reasonable values are 0-16 for cohesionless soils and 0-70 for others. Worst case cohesionless Martian drift materials has Nq = 3.5.

double Ng

Constant for gamma in Terzaghi equation. Reasonable values are 0-16 for cohesionless soilds and 0-70 for others. For phi <= 14 deg, Ng is 0. Worst case cohesionless Martian drift materials has Ng = 1.0.

double gamma

soil specific weight; equals density * gravity. Units = Newtons

double cNc

cohesion * Nc

double tanPhi

tangent of angle of internal friction

int cohesionlessSinkage

indicates whether or not cohesion should be considered when computing sinkage. cohesionlessSinkage = 1 is worst case (conservative)

void computeDerivedConstants(double gravity)

computes constants needed for later calculations, some of which depend on gravity

double computeSinkage(double &force, double width, double area, double zwMax, int assumeCohesionless = -1)

returns the sinkage of a wheel supporting a given force over a given area, which has a given width. Sinkage is computed using Terzaghi's equation. If the computed sinkage required to support the supplied force is greater than zwMax, then force will be clipped to the maximum force supportable at zwMax. Three different useful values for assumeCohesionless can be supplied:

• 1 - assume soil is cohesionless (worst case; cohesion = 0)
• 0 - use current value of 'cohesion'
• -1 - use the value of 'cohesionlessSinkage'

double computeSinkage(double &force, double width, double area, double dwDz, double zwMax, int assumeCohesionless = -1)

returns the sinkage of a wheel supporting a given force over a given area, which has a given width. Sinkage is computed using Terzaghi's equation, with a trapezoidal approximation to tire deflection. If the computed sinkage required to support the supplied force is greater than zwMax, then force will be clipped to the maximum force supportable at zwMax. Three different useful values for assumeCohesionless can be supplied:

• 1 - assume soil is cohesionless (worst case; cohesion = 0)
• 0 - use current value of 'cohesion'
• -1 - use the value of 'cohesionlessSinkage'

inline double computeRollingResistance(double force, double width, double area, double sinkage)

Computes the rolling resistance (in Newtons) for the given contact patch. The value for 'sinkage' can be obtained from terrainMaterial::computeSinkage().

double computeMaximumTraction(double area, double force)

Computes the maximum traction that can be supported by the given contact area and force, based on soil cohesion and internal friction

int readParams(paramParser* p)

reads parameters from the current context of a parser. The parameters are:

• enum { ROCK, SOIL, SNOW } materialType;
• double grainSize (meters)
• double bulkDensity (kg/m^3)
• double cohesion (kPa)
• double internalFriction (degrees)
• int cohesionlessSinkage (1 or 0)

For materials of type SOIL or SNOW, the following additional parameters are read:

• double Nc (dimensionless)
• double Ny (dimensionless)
• double Nq (dimensionless)

These last three coefficients (bearing capacity factors) depend on the angle of internal friction and can be found in texts on foundations and soil mechanics (e.g. "Essentials of Soil Mechanics and Foundations", McCarthy, p. 375).

This class has no child classes.

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