Groundwater Conditions
Description:
If you dig a hole into a soil mass that has all the voids fi lled with water (fully saturated), you will observe water in the hole up to a certain level. This water level is called groundwater level or groundwater table. The top of the groundwater level is under atmospheric pressure and is sometimes called the free surface. We will denote groundwater level by the symbol ..
Many construction failures, court battles, and construction cost overruns are due to the nonidentifi cation or nondisclosure of groundwater conditions at a site. The water table invariably fl uctuates depending on environmental conditions (e.g., rainfall patterns, winter rains, monsoons, drought), human activities (e.g., pumping groundwater from wells and drawdown during construction), and geological conditions.
Unconfi ned, confi ned,
and perched aquifers;
aquiclude; and aquitard
The water-bearing soils below the groundwater level are called an aquifer. Aquifers can be unconfined or confi ned or semiconfi ned . In an unconfi ned aquifer the groundwater level is free to fl uctuate up and down, depending on the availability of water. During winter and spring, the groundwater level usually rises. In drier months, the groundwater level drops. Aquifers are sometimes separated by geological formations that may restrict groundwater fl ow.
If the formations are impermeable, such as fi ne-grained soils (e.g., clay) and/or nonporous rock, they are called aquicludes. If the formations are semi-impermeable, they are called aquitards. A confi ned aquifer is a water-bearing stratum that is confi ned by aquicludes (impermeable geological formations) above and below it. The water held in an unconfi ned aquifer is under pressure because of the confi nement. If one of the impermeable formations, usually the top, is penetrated, water can rise above the ground surface.
In some unconfi ned aquifers, water has risen more than 50 m above the aquifer surface during well drilling. Confi ned aquifers (also called artesian aquifers) are not directly affected by seasonal climatic changes. There is really no true confi ned aquifer, as some infi ltration does occur from the overlying soil or rock. The geological formations are rarely continuous, especially in alluvial aquifers. Often, the aquifer consists of fi ngerings or zones or lenses of impermeable and semi-impermeable.
Soils Laboratory Tests:
Samples are normally taken from the fi eld for laboratory tests to characterize the physical and mechanical (strength and deformation) properties. These parameters are used to design foundations and to determine the use of soils as a construction material. Disturbed samples such as from a standard sampler are usually used for visual inspection and for tests to determine the physical properties such as plasticity and grain size and shape. Undisturbed samples such as from a thin-walled sampler are used for both physical and mechanical properties. Test results, especially those that relate to the mechanical properties, are strongly affected by sampling, handling, transportation, and sample preparation disturbances. Care must therefore be exercised to protect the intact condition of the soil samples. Wax is often used to coat the soil samples to prevent moisture losses.