Abstract
Perculating and pumping of water through the earth material sometimes causes some critical events such as subsidence and fissures which might causes serious damages on industrial sites, urban areas and farms. Compression of aquifers in direct response to ground water head decline has been recognized for over 50 years. There is a simple relation between the increase in effective stress and decrease in pore water pressure. The worldwide scale of subsidence damage through groundwater withdrawal reached a peak between 1950 and 1970, at a time of inprecedented urban growth and industrialization (1)
Generally, sand is almost incompressible under the stress of shallow aquifers, except for a small amount due to grain rearrangement. As a results, the compaction of a sand aquifer in response to head decline is immediate and elastic, and is usually small.
Subsidence on clay soils as a direct consequence of the over-abstraction of groundwater from interbedded sand aquifers is a widespread phenomenon. Gentle subsidence bowls develop almost imperceptibly slowly but can extend over large areas. Their main effects are coastal inundation and deformation of surface drainage gradients, together with casing damages to the wells which initiated the subsidence and some cases of structural damage through ground strain.
Some of these events have been studied in Iran. Three places have been chosen as evidence and described in this paper.
Introduction
Subsidence at the surface can be regarded as ground movement which takes place due to the intensive abstraction of groundwater. It is attributed to the consolidation of sedimentary deposits in which the
Groundwater is present, consolidation occurring as a result of increasing effective stress. The total overburden pressure in partially saturated or saturated deposits is borne by their granular structure and the pore water(2).
In order to remove pore water from the ground, it is necessary to reduce the pressure in the water in the vicinity of the pump and son there will in general be an increase in the compressive effective stress state. When groundwater abstraction leads to a reduction in pore water pressure by draining water from the pores, this means that there is a gradual transfer of stress from the pore water to the granular structure. For instance, if the groundwater level is lowered by 1m, this gives rise to a corresponding increase in average effective stress will cause consolidation of the ground and may lead to large scale subsidence.
The decrease in pore pressure will not occur immediately . After pumping has commenced, the pore pressures will gradually decrease below their initial insitu values until a steady state distribution is established. Hence the resultant consolidation and surface subsidence will be time dependent. Scott(3) pointed out that surface subsidence does not occur simultaneously with the abstraction of fluid from an underground reservoir, occurring over a longer period of time than that taken for abstraction .
Generally, the amount of subsidence which occurs is governed by the increase in effective pressure, t he thickness and compressibility of the deposits involved, the length of time over which the increased loading is applied, and possibly the rate and type of stress applied (4). So many of the world's major cities have suffered subsidence self-induced by groundwater pumping that there is an extensive literature of case histories . subsidence at Tokyo, Osaka and some other Japanese cities has been controlled by reducing pumping (5). Probably the best known examples of this phenomenon occur in Bangkok, Venice and Mexico city where widespred subsidence has caused by withdrawal of water from aquifers for industrial and domestic purposes (6). Carillo (7) revealed that subsidence in parts of Mexico city occurred at a rate of 1 mm/day. This was due to the abstraction of water from several sand aquifers located in vary soft clay of volcanic origin. The aquifers extend under the city room an approximate depth of 50 m below ground surface to well below 500 m .water has been abstracted for over 100 years (8)
A problem that often occur in practice concerns the pumping of water on an aquifer a deep layer of homogeneous and isotropic soil. Since, this issue has been observed in different place of Iran, therefore as an evidence, subsidence at a few parts of Iran due to perculating and pumping water has been studied and described in following section.
