o. Peat. Peat displays the following engineering characteristics:
Compressibility/expansion: Very high
Permeability when compacted: Impervious
Shear strength when compacted and saturated: Poor
Potential frost action: Slight
Value as a construction material: Poor
2. Trafficability. The term trafficability, if used in the general sense, refers to the overall suitability of terrain for
the cross-country movement of military forces. However, the emphasis in this course is placed on soil
trafficability, which is a measure of the capacity of a soil to support vehicular movement.
a. Qualitative Estimations of Soil Trafficability. Qualitative estimations concerning soil trafficability
may be made based on the gradational characteristics of the soil as well as the soil type.
(1) Gradational Characteristics. The trafficability of a soil is significantly affected by the soil's
gradational characteristics. Poorly graded soils tend to exhibit loose soil structures because of the large amount of
void space that exists between individual particles within the soil mass (see figure 3-8).
In contrast, because well-graded materials are composed of a relatively uniform distribution of a wide range of
particle sizes, the spaces surrounding large grains may become filled with smaller grains, creating what is called
an interlocking soil structure (see figure 3-9, page 3-26).
Compared to the particles within a poorly graded soil, those within a well-graded soil are more closely surrounded
by other particles; therefore, well-graded soils possess a higher degree of grain-to-grain contact. This increased
amount of contact between grains serves to lock the soil particles into place, thereby reducing the likelihood of
individual grain displacement with the application of an external load. For this reason, the trafficability of well-
graded soils tends to be better than that of poorly graded ones.
(2) Soil Type. Because trafficability characteristics vary from one soil type to another, qualitative
estimations of trafficability may be made based on the soil types present within an area.
(a) Gravel. In general, the trafficability of gravels is excellent for tracked vehicles; however, where
gravels are pure, loose particles may roll under pressure, hampering the movement of wheeled vehicles. Weather
has very little effect on the trafficability of gravel.
(b) Sand. Dry sand normally impedes the movement of both tracked and wheeled vehicles,
especially in areas of sloping terrain. However, where sands are mixed with clay or where enough moisture is
present to allow for compaction, the sand exhibits improved trafficability.
(c) Silt. When dry, silts usually offer excellent trafficability; however, because dry individual silt
particles are very small and have little or no cohesion, conditions may become