6.2 Common terms used in simulation of water
Soils and rocks that transmit water with ease through their pores and fractures, respectively, are call aquifers. Typical aquifers are composed of gravel, sand, sandstone, limestone and fractured volcanic, igneous, or metamorphic rocks (figure 1 & 2). It is a wet underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which groundwater can be usefully extracted using water wells.
Aquicludes consist of strata or discontinuities that are sufficiently less pervious than the adjoining strata constituting barriers to groundwater. Silt or clay washing into a crack in the ground can produce an aquiclude that will block the flow in a sandy seam. Typical aquicludes are clay, shale and unfractured igneous and metamorphic rocks. Aquiclude is a formation which is although porous and capable of absorbing water, but does not permit its movement at rates sufficient to furnish an appreciable supply for a well or spring (figure 1 & 2). . Aquiclude is an impermeable body of rock or stratum of sediment that acts as a barrier to the flow of groundwater.
Perched Water Table
A perched water table (or perched aquifer) is an aquifer that occurs above the regional water table. A perched water table is sustained above an aquiclude or an impermeable stratum such as a clayey layer (figure 1 & 2). It may be transient and rapidly developing in response to heavy rainfall and dissipating quickly, or permanent in response to seasonal variations in rainfall levels.
Figure 1: Various in situ water conditions
Figure 2: Various in situ water conditions
An artesian aquifer is a confined aquifer containing groundwater under positive pressure. It is confined between impermeable rocks or clay which causes this positive pressure (figure 3). Artesian water is derived from an artesian aquifer, in which the piezometric head of the water pressure is higher than the upper surface of the aquifer, but the water is confined by an overlying aquiclude. In other words, if piezometers are installed in the artesian aquifer, the water in the piezometer tubes would rise to greater elevations from the deeper artesian strata than from the strata nearer to the ground surface. The presence of artesian water should not be overlooked and must be accounted for any slope stability analysis
Figure 3: Example of an aquifer system with artesian wells