Terrain is the noun used to refer to an area of land which can be described, or defined, by its natural features, for example, if it is rough or flat, its surface vegetation and the properties of its constituent material. In terramechanics, terrain is the land on which vehicles and machines travel and operate, and its ability to support wheels, tracks, and objects and to provide resistance and reaction to normal loads and motion are key characteristics of interest. How terrain is transformed or modified during the passage of vehicles and machines is also of great interest, including the environmental impact generated and, in agriculture, the transformation of land (soil) to promote and facilitate the growth of crops.
The main types of terrain include (see Fig. 1):
• Soil – clay, loam, sand, predominately organic material such as peat
• Grass
• Rough vegetation
• Snow and ice
• Rough roads
• Boulder crawl
• Wading – water above hard/soft smooth/rough surface
Fig. 2 shows some variations on these main types of terrain.
In agriculture, and also in a great deal of off road vehicle studies, the main terrain material of interest is soil. A great deal of the science of soil mechanics is shared by off road vehicle, civil, mechanical and agricultural engineers but applications diverge and the focus on soil cutting, tillage, traction, vehicle mobility, vehicle dynamics and mechanics generally is different. The first part of this chapter concentrates on the main properties of soil of interest to agricultural engineering and how they are determined, the surface profile is often taken as flat and horizontal in many tests and calculations, but the effect of surface profile and slope is considered in a later chapter.
Soils are masses of mineral particles mixed with varying proportions of water, gases, salts, and often organic matter, which is important in the performance of growing crops. Soils can be formed physically and chemically in situ from parent rock in layers of up to 25 m. Particles can also be removed from rock material by physical and chemical weathering to subsequently be transported to a new location to form sediment.
Sand and gravel, and many silt materials (individual particles larger than 0.002 mm diameter) are generally of the same mineralogical composition as their parent rock (material) such as quartz, feldspars, calcite, dolomite, micas and others. They are usually removed from their parent material by wind, water, temperature change and glacial and other abrasion. Transportation of these granular materials, by wind, water, ice and gravity, can change the shape of individual particles by abrasion and impact and can also sort particles by size. This can be seen in the uniform size of sand particles in sand dunes and shorelines beaches. Alternatively, material moved by glaciers may have a wide range of particle sizes.
Clay minerals are usually formed by chemical changes to produce new minerals that break easily into small particles of less than 0.002 mm diameter that have more chemically active surfaces and large surface areas per unit of mass. See Mckyes (1989) and Craig (2012) for more detail of soil minerology.