Human Footprints: Fossilised Locomotion? by Matthew R. Bennett & Sarita A. Morse
Author:Matthew R. Bennett & Sarita A. Morse
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham
5.2 Models of Human Track Formation
As a foot makes contact with a sediment surface there is a transfer of force (pressure) to the substrate which is then deformed if the applied force exceeds some measure of the substrate’s strength and that there is no elastic response. There are three principal components to the creation of a track, essentially to the creation and retention of a surface void (Fig. 1.2): (1) the irreversible compression of sediment below the plantar surface of the foot, effectively providing thorough consolidation space for the foot; (2) sediment displacement (i.e., transport through failure and deformation) below and away from the foot from areas of high pressure, to adjacent areas of lower pressure; and (3) the physical excavation and removal of sediment through plantar shear beneath the sole of the foot or by adhesion to the foot. None of these mechanisms are mutually exclusive and some combination will hold in the formation of most tracks.
Figure 5.1a shows a track formed by simple compression; note the extensional tension fractures around the sides and the compressed plantar sediment layers. The degree to which the applied stress can be accommodated solely by compression will depend on such sedimentary properties as grain-size, sorting, grain shape, porosity, packing, consolidation and pore-water content all of which will control the degree of strain compression that can be accommodated (Allen 1997; Craig 2004). These properties are broadly captured by the sediment’s natural bulk density which is effectively the mass of an intact volume (Manning 2004). Sediment that is poorly consolidated has a low bulk density and will be able to accommodate much more compressive strain than a sediment with a higher bulk density and consequently less void or pore-space to accommodate compression as the individual grains move closer. In general terms a sediment’s compressive strength will increase with its bulk density (Craig 2004) and with its moisture content up to a critical hydraulic threshold (Manning 2004). Dry sand has little strength compared to one which is damp, since in damp sediment, water coats the grains and the surface tension of the water helps bind the grains together. As the water content increases however and the air is driven from the sediment’s pores this water begins to push the grains apart and lubricate their relative motion, which reduces the sediment’s strength. The water is effectively incompressible when loaded and unless it can drain to areas of lower pressure and it will cause the sediment to ultimately fail. The rate at which the stress is applied, the porosity and permeability of the sediment as well as its vertical variation are all critical at this point in determining whether and how the sediment will fail (Craig 2004). For example, rapid footfall does not always provide sufficient time for pore-waters in sediment with a low permeability to drain and the sediment may fail rather than compress, where a similar pattern of footfall may cause compression if the sediment had a higher permeability allowing pore-waters to drain more effectively.
Fig. 5.1A series of modern human tracks made on a beach at Bournemouth in the UK.
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