Deep and very long trenches on the surface of some plains in Iran, along with abnormal and wide settlement of land in different regions of Iran, is a phenomenon that has been clearly visible in the recent years. The first time I saw this strange phenomenon was around 1364, around Shahid Sadougi Forestry Camp, near Ashkazar, Yazd. Although the large and giant trenches between Hojjatabad and Ashkzar (Yazd) were recently noticed, according to the local people, this phenomenon had existed for decades (even before the 1940s). According to the old and retired local caravanners, “In winter and in rain and snow a camel with its load could easily get lost in these trenches. In regional terms, the people of Ashkazar and Meybod call these deep trenches Shagh. Similar rifts are known in Kashan by the name of “Daqz” or Taqz.” Daqz in the Bakhtiari dialect means gap and seam.
Photo 1: Newly found deep cracks in the cay-silty soils of Shahid Sadouqi Ashkazar Forestry and Natural Resources Camp (1366).
These trenches are mostly found in lands made of clay (or a mixture of clay and silt with a higher percentage of clay) to a great depth, even if their upper part is covered with other materials such as alluvium or gravel. The nature of clay soil is such that it is impervious to water and retains the infiltrated water after absorption. Clay particles begin to swell when moisture reaches them, and in this way, all capillary seams are closed, and water loses its ability to penetrate. But the same clay soils that turn into mud after rain and moisture can remain on them for a while (according to the air temperature); after drying, they have many cracks, and their surface is full of seams and shallow cracks In contrast with these cracks, a deep, long and extensive trench forms in clay soils under special conditions, which is the subject of this article.
A bout very long and deep trenches
The very long and huge trenches are deep, vertical cracks, long, and with either narrow or wide widths forming on the surface of the earth (Kook & Yaran, 1993; Badu & Shelke, 2008). These trenches may be several meters deep, tens of meters to hundreds of meters long, and have various widths from a few millimeters to several meters. These trenches may appear from hairy seams with a depth of one or two meters to cracks with a width of several meters and more than ten meters deep; in this large group, there is even the possibility of surface water sinking. In the summer of 1366, when I was studying this phenomenon at the Shahid Sadouqi Forestry Camp (Quds Camp, Yazd), I came across young ruts whose width was about ten to twenty centimeters, and the new gravel road had cut through the campsite (photo one). This sand road was built about a year before the cracking event. On the other hand, these deep and fresh cracks continued into the administrative buildings or some of the warehouses of this camp, and the walls were also cracked. As far as could be seen, the width of some of these trenches, as mentioned, was ten to fifteen centimeters with a very vertical wall and the angle of contact with the surface or floor of the ground was precisely ninety degrees.
Due to the slightly wide and straight wall, such trenches are estimated to be very young (one year or less). On the other hand, another group of these furrows is more than one meter to several meters wide, more than twelve meters deep (Ashkzar region, 1383), and have a length of one hundred meters or more, and they join a group of other furrows that sometimes form They are seen as radial or divergent or convergent. In some places, such as Quds Camp (Shahid Sadouqi) and the Charkhab area of Yazd, the trend of deep cracks follow the state and direction of planting gaz and tag trees or irrigation canals and continue more or less parallel to them (1366).
Photo 2: Older trenches filled with sliding walls and continued landslides from both sides of the trench, Martyr Sadouqi Ashkzar Forestry and Natural Resources Camp (1366).
In general, the initial width of these trenches starts from a few millimeters to a few centimeters, but over time and with more land settlement, their width increases and reaches several meters in older cracks (several years old or several decades old). The side part and the edges of the walls of the cracks collapse into them. This collapsing or sliding occurs more often during heavy rains or after heavy snow melts. In this way, in older cracks, the depth is partially filled with collapsed mud or clay, and their depth is reduced because the fallen materials get caught in the middle of the crack and hide the greater depth from the eyes of the witnesses (Mehrshahi, 1369, p. 141).
How these deep trenches appear
More generally, deep cracks and trenches are primarily the results of tectonic processes such as earthquakes or tectonic movements in the earth’s crust. Such fissures, which may initially be more than hundreds of meters deep, occur over long distances of kilometers or hundreds of kilometers and have their characteristics and functions (see, for example, Cook and Yaran, 1993; Burbank & Anderson, 2004). On the other hand, some cracks are caused by other activities and processes, including deep cracks caused by uneven subsidence of the surface layers of the earth, due to the drop in the water level of the underground aquifers (Badu & Shelke, 2008). The emergence of such deep and long furrows caused by the drop of underground water can cause significant damage to industrial, transportation, and urban and rural infrastructures. Examples of these damages can be seen in various countries, including our land, Iran. For example, in Arizona, USA, land subsidence and the creation of long furrows negatively affected water supply networks, urban sewage channels, and gas supply networks (Glett, 1992, quoted by Bado and Schalke, 2008).
during my visits, from 1366 to 1368 (1997 – 1999), to Ashkzar, Hojjatabad and Meybod city, we saw deep and long cracks that had caused severe damage to the major urban water reservoir, disabling at, and it was not more possible to store water in it. On the other hand, several hundred houses were severely cracked by the new house-building facilities in the Shohdaye Meybod neighborhood due to the cracks, which required continuous and consecutive restoration.
Photo 3: A deep and long crack in an old waterway in the neighborhood of Bafrouyeh, Meybod, Ordibehesht 1390 (2011)
The progress and expansion of some of these gaps, especially those that are longer, is such that after decades, they become deep and narrow waterways and canals, which become a place for floods during sudden and heavy rains (example in photo 4).
Photo 4: Deep trenches that have become narrow waterways after several decades (from Dana Information Network, Bahman, 13967.
According to the statistical results of the study of the trend of rainfall between 1332 and 1380, as well as the changes in the subterranean water level of the Yazd-Ardakan plain from 1352 to 1380 and the large-scale increase of these furrows from the late 1960s onwards, researchers on the subject of deep cracks in These plains have come to the conclusion that the occurrence of these trenches and their progress was primarily caused by the drop in the underground water level (emptying of aquifers) and the subsidence of the lands (Mehrshahi, 1369; Jabari and Rezaian, 1383).
It is interesting to know that the number of authorized and unauthorized wells in the country has increased from about 47,000 (forty-seven thousand) rings in 1350 to about 750,000 (seven hundred and fifty thousand) rings in 1390. According to Mohammad Darvish, the number of reservoir dams in the country has increased to about sixty compared to fifty years ago. As a result of such activities without proper planning and according to the Minister of Energy of the current government, the amount of water taken from the country’s aquifers from 1357 to 2019 was at least one hundred and thirty billion cubic meters more than the water that entered these aquifers. Is! As a result of this drastic reduction in the water input or water supply of the country’s aquifers, with the lowering of the underground water, the pressure of the water in the upper layers has decreased. The empty spaces under the upper layers have led to their settlement in a heterogeneous and unbalanced way, which in turn causes tension in the higher lands. (Surface layers) become deep, and long trenches appear.
The remarkable thing here is that even though these terrible cracks are formed in a short time, the surface layers return to their previous state, if all the conditions, including the excessive extraction of water and the number of wells, are returned to the state of forty years ago (which It is impossible!) It will take much longer for the earth to return to its previous state, and it may never be possible in one to two hundred years. It is only possible to prevent the further development and increase of this phenomenon with compassion (commitment to duty), collective wisdom (getting help from experienced experts), and planning (seasonal and annual scheduling in the work process).
