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.
b-Mahyar area:
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 .
c-mobarekeh province:
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.
Conclusion
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.
References
- 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.
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