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21 Soil Most soils form from physical processes as material is eroded from one location, transported by wind, water or gravity, and then deposited in a new location. These soil deposits are named based on their depositional environment. Some examples of these materials include alluvium, colluvium, eolian, and glacial drift. There are other depositional environments that have unique types of deposits such as material deposited by landslides or other mass wasting events. Soils formed by deposition will consist of unconsolidated particles of clay, silt, sand, gravel, or boulders depending on the parent material and the depositional environment (i.e., high energy river deposits that have gravels and boulders or low energy marine deposits that have clays and silts). Alluvium describes materials deposited by moving water. These deposits are stratified layers of clay, silt, sand, gravel, cobble and boulders depending on the energy of the flow. If the flow of water is very fast, the material that is deposited is coarse-grained (gravel, cobble and boulder). If the flow of water is very slow the material deposited is fine-grained (clay and silt). Figure 13 shows an example of an alluviual fan deposit at the mouth of a canyon. In this example, coarse- grained material is deposited near the mouth, where the flow of water is fast and fine-grained material is deposited near the perimeter of the fan, where the flow of water slows and loses energy. Alluvial deposits are heterogeneous and can have interbedded layers of clay and silt within thick beds of gravel and cobble. Due to the action of water and movement of the particles, alluvial deposits are characterized by an assemblage of sub-rounded to rounded particles. Some typical engineering properties of alluvium are listed below: Coarse-grained alluvium  Highly permeable  Good source of aggregate  Good bearing capacity in gravel and cobble  Low shrink and swell potential  Sand deposits can be liquefiable Fine-grained alluvium  Low permeability  Low bearing capacity  High shrink and swell potential  Low shear strength Colluvium describes material deposited by gravity. Weathered rock and soil can creep slowly downslope by gravity or material can be deposited relatively quickly as blocks of rock and other lithic fragments fall to the bases of slopes and are incorporated into a matrix of material. The matrix of material at the base of a slope can be coarse-grained or fine-grained. Coarse-grained deposits are sometimes called tallus. Colluvium is a heterogeneous deposit that usually consists of a random mixture of large fragments of rock in a fine-grained matrix that can be composed of material ranging from sand to clay. Since colluvial deposits have been transported relatively short distances, the larger particles, or clasts, are characteristically angular to sub-angular. A few typical engineering properties of colluvium are listed below.  Extremely heterogeneous  Usually contains large rock fragments  Usually at natural angle of repose; cut slopes can be unstable  Can have tendency to move  Coarse deposits are usually difficult to excavate Eolian soils are deposited by wind. These soils are usually composed of sand and silt sized particles. The coarser sand sized particles can form dunes such as the dunes found in the Mojave Desert in California. The fine-grained silt (loess) stays suspended in the air longer than the sand, and as a result, loess deposits Figure 13 - Typical Alluvial Fan Deposit. Rock is Eroded from Above and Deposited Below as Soil.