R. Ajalloeian (B.Sc, M.Sc, Ph.D)
Assistant professor, Department of Geology,
university of Isfahan, Iran
B. Bahadoran (B.Sc, M.Sc)
Lecturer, Department of Geology,
university of Isfahan, Iran
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.
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.
Case studies in Iran
There are some evidences in Iran show subsidence has been caused by withdrawal of water from aquifers for industrial and other purposes. Three places have been considered to describe in this paper (a,b and c in figure 1).
A -iranshahr province
Iranshahr is in southeast of Iran. Nearly 12 years ago 2 residential areas were established next to the Iranshahr. A few cracks has been observed all over the buildings at the time of construction. One of the buildings was made of bricks and cement blocks. In this type of building a few cracks have been appeared all over them in all directions with different sizes. Other building were made of armed cement. In these sort of buildings the cracks have appeared right on the connections. All factors of causing crack such as faulting, earth movement, folding, subduciton and subsidence have been studied and explained (9). Since construction, some instruments such as seismographs have been installed and geological studies have been done. None of the geological factories are in the consideration size to cause cracking. For a long time there have been a few small quakes around the area, but there have not been of high shocks. The historical evidence around the area also shows no high quakes and the people also do not remember any considerable quake, so the effect of earthquakes on the formation of the crack is not suggested.
There are three major faults around the area. The study of these faults shows no considerable movement during the past few years. The phenomena of subsidence also have been studied. There are two reasons to reject the formation of these phenomena due to subsidence. First of all the movement is slowly with the scale of centimeters per year. Secondly, the distance of the area from subsidence margine (Aman margin) is about almost 300 km. With in this far distance this phenomena never happens at all.
Finally, water level fluctuation due to gain and recharge of waster and also sensatory of the clays, which form the most continents of soil profile will cause the expansion and shirinking and absolutely is the most important cause of the fractures.
This area is located at 25 km. South if isfahan. There are almost 200 square km. Of agricultural planes and several residential areas. Almost 150-200 meters of alluvium consisting of sand, silt and clay are also located under this plane. The bed rock is containing 50 meters clay. During past 40 years almost 250 water wells are pumping the ground water, 24 hour a day. The drowdown of the water table due to high pumping of water is about 120 meters per year.
During last 10 years a group of fissures have been appeared in the area. The strike of these fissures in N50-70W. the length of them is almost 10 km. The width of them is almost 5 to 50 cm. The fissures are opening almost 5 cm. Per year. These fissures pass through a highway, a village and an agricultural area which till now have caused some damages. Usually there are several factors which can cause cracks on the earth. Earthquake, landslides, underground solution, water , oil and gas evacuation and drawdown of water table are the most usual phenomena. All of these phenomena have been studied and traced within this area. Expect water extraction none of them is present.
Gaining high amount of water from the aquifer without enough recharge and rapid draw down of water table have caused subsidence of the aquifer. Because of the presence of few meters of clay on the surface of the plane the reaction of subsidence is cracking. There are several places in the world such as Huston in texas, Goos Creek in North Dakota, Santa Clara in California and kerman in Iran, that drawdown of the water in them have caused subsidence and cracking of the earth. Stopping the drawdown or injection of water in the aquifers have ceased or stopped subsidence and cracking .
This area is located 50 km. South west of Isfahan. There are two industrial complexes in this area, each of them is using huge amount of water for several purposes, such as cooling systems, watering the green areas and even agricultural activities. They get the needed water straightly from the zayandehrood river which is passing through north side of the areas. The excess water goes right to the aquifer.
Hydrogeological studies have been done extensively through the area almost for 3 years. The hydrodynamic coefficients have been calculated precisely. The prosity coefficient (K) towards the drainage area become lower and lower. In this condition the ground water level comes up considerably .
The annual piezometric map also shows the change of hydraulic gradient. There are several residential areas along the drain aged area. Therefore the ground water have been appeared in the houses, foundations and basement of buildings, and have caused considerable amount of damages, such as cracking the walls, roofs and running the farms.
The piezometric maps of the last decades show much lower water table and upthrouwn of ground water during the drainage area have been started right after installation of the two industrial complexes.
As a result the natural fluctuating of water table, pumping and perculating high amount of water from or to an aquifer might cause subsidence and fracturing of the earth material and cracking the buildings which will cause considerable amount of damages. Reducing or stopping each of these phenomena will cease the fracturing and slowdown the subsidence.
- Waltham, A.C.(1989) Ground subsidence, Blackie & son Limites. First published, 1989
- Bell, F.G. (1987) subsidence, Ground Engineer’s reference book. Edited by F.G.Bell, Butterworths and co. publishers.
- Scott, R.F. (1979) Subsidence-a review in evaluation and prediction of subsidence, Ed. By Saxema, S.K., Proc. Conf. ASCE, Gainsville, pp 1-25
- Lofgren, B.N. (1968 ) Analysis of stress causing land subsidence, U,S. Goel. Surv., Proc. Paper 600-B, pp 219-225
- Yananiti, S.(1984) case history no. 9.4. Tokyo, japan. In Poland, J.F. (1984). Op. cit., pp 175-184.
- Booker, J.R, Small, J.C. and Carter, J.P. (1985) Prediction of subsidence caused by pumping of groundwater. 21 the IAHR congress, Melbourne, Australia, pp 130-134.
- Carillo, N. (1984) Influence of artesian wells on the sinking of Mexico city, Proc. 2nd int. conf. soil Mech. Foundation Engg, Rotterdam, 3 pp 156-159.
- Bell, F.G. (1983) Fundamentals of engineering geology. Butterworth & Co. publishers ltd. 1983.
- Iran khak consultant report.