ENGINEERING GEOLOGY/Problematic Soils 555 Table Effects of transportation on sediments Gravity Ice Water Air Size Various Unsorted Shape Angular Angular Surface texture Striated surfaces Striated surfaces Various sizes from boulder gravel to muds Sorting takes place both laterally and vertically Marine deposits often uniformly sorted River deposits may be well sorted From angular to well rounded Gravel: rugose surfaces Sand: smooth, polished surfaces Silt: little effect Sand size and less Sorting Varies from clay to boulders Generally unsorted Any system of soil classification involves grouping soils into categories that possess similar properties, so providing a systematic method of soil description by which soils can be identified quickly Although soils include materials of various origins, for purposes of engineering classification it is sufficient to consider their simple index properties, which can be assessed easily, such as their particle size distribution and consistency limits For instance, coarse-grained soils are distinguished from fine on a basis of particle size, gravels and sands being the two principal types of coarse-grained soils Plasticity also is used when classifying fine-grained soils, that is, silts and clays Quicksands As water flows through sands or silts and slows down, its energy is transferred to the particles past which it is moving that, in turn, creates a drag effect on the particles If the drag effect is in the same direction as the force of gravity, then the effective pressure is increased and the soil is stable Conversely, if water flows towards the surface, then the drag effect is counter to gravity, thereby reducing the effective pressure between particles If the upward flow velocity is sufficient, it can buoy up the particles so that the effective pressure is reduced to zero This represents a critical condition where the weight of the submerged soil is balanced by the upward acting seepage force If the upward velocity of flow increases beyond this critical hydraulic gradient, then a quick condition develops As the velocity of the upward seepage force increases further from the critical gradient, the soil begins to boil more and more violently (Figure 1A) At such a point structures fail by sinking into the quicksand (Figure 1B) Quicksands, if subjected to deformation or disturbance, can undergo a spontaneous loss of strength, causing them to flow like viscous liquids A number of Uniformly sorted Well rounded Impact produces frosted surfaces conditions must exist for quick conditions to develop Firstly, the sand or silt concerned must be saturated and loosely packed Secondly, on disturbance the constituent grains become more closely packed, which leads to an increase in pore-water pressure, reducing the forces acting between the grains This brings about a reduction in strength If the pore water can escape very rapidly the loss in strength is momentary Hence, the third condition requires that pore water cannot escape readily This is fulfilled if the sand or silt has a low permeability and/or the seepage path is long Quick conditions frequently are encountered in excavations made in fine sands that are below the watertable Liquefaction of potential quicksands also may be brought about by sudden shocks caused by the action of heavy machinery (notably pile driving), blasting, and earthquakes (Figure 1B) Such shocks increase the stress carried by the pore water, and give rise to a decrease in the effective stress and shear strength of the soil There is also a possibility of a quick condition developing in a layered soil sequence containing fine sands where the individual beds have different permeabilities Collapsible Soils Soils, such as loess and brickearth, commonly are regarded as being of aeolian origin and some believe that the material of which they are composed was formed initially by glacial action These soils often have a metastable fabric and so possess the potential to collapse In addition, some wind-blown silts, such as those found in subtropical arid areas like Arizona and the Kalahari Desert in southern Africa, have metastable fabrics and are potentially collapsible Certain residual soils also are prone to collapse For example, when granites undergo notable chemical weathering in subtropical regions that involves appreciable leaching, then the resulting saprolite and