The phenomenon of Subsidence's and ground cracks
The importance of the study on Subsidence's and ground cracks is due to their spread in a very serious manner in many regions of the Kingdom of Saudi Arabia, especially in the central, eastern, northern, and southern parts of the Kingdom, due to their presence within formations characterized by the presence of limestone rocks and evaporites, including, for example, the Jubailah, Sulay, Hit, and Al Arab formations, which are spread in the region. Riyadh, the Jazan region, the Qassim region, Hail, Al-Jawf, and the Eastern region, and the phenomenon of landslides pose a great danger to citizens. Landslides are land subsidence that may occur slowly or suddenly in most cases due to the widening of naturally formed subsurface gaps and caverns or due to factors associated with human activity in limestone rocks and evaporites. Landslides represent one of the most important geological risks that can occur suddenly. In many agricultural, residential, and other areas (Figure 3). It has also been noted that the presence of evaporite deposits and rocks in any region of the world exposes it to the emergence of karst terrains of a special nature, which differ most of the time from the karst terrains of limestone rocks. One of the most important of these phenomena is what is called Karren, which may be visible on the surface or covered by other sediments, and it is widespread. Evaporite rocks are widespread in the region east of Riyadh due to their presence within the Hit and Arab formations, which directly affect the rocky and sedimentary formations above them. Sinkholes are considered one of the most common forms of karst landforms, in addition to the appearance of depressions associated with the processes of downward bending of layers. However, there are other landforms endemic to evaporite karst, especially those resulting from subsidence associated with the dissolution occurring in the interstratal and overlapping layers of other rocks, which is called interstratal dissolution, which occurs in covered karst environments (an example of this is what occurs below the Sulay Plateau). And the Darkal region in particular. Subsidence resulting from the dissolution of mutual and interstratal layers of evaporites located between other rocks causes the largest forms of karst terrain on the surface of the Earth, including the appearance of single folds on the edges of slopes, belts of depressions, and basins associated with dissolving fronts, which can reach hundreds of kilometers in length.
The special geomorphological features associated with evaporite rocks can be attributed to the following:
1- Evaporite minerals have a high degree of solubility, unlike carbonate minerals.
2- Halite and gypsum, in the presence of suitable hydrodynamic and hadrochemical conditions, can dissolve very quickly. Thus, karst features in evaporite rocks can develop within very short periods of time.
3- Evaporites can be eroded by mechanical processes more quickly than harder carbonate rocks.
4- Some of the important effects that give clear geomorphological shapes due to the processes of mechanical erosion and dissolution that occur on evaporite rocks can be listed as follows:
(a) The process of terrain formation and development takes place very quickly.
(b) The terrain can witness rapid changes as a result of its rapid response to changes. In the environment and ambient conditions,
(c) topographic features in evaporite rocks often have a limited persistence time (retained for short periods).
5- The mechanical properties of evaporites differ markedly from those of carbonate rocks. Gypsum has less strength and is more flexible. Furthermore, dissolution acting along the planes of joints and fractures in gypsum formations can lead to a rapid decline in rock strength.
Figure No. 3 shows the widespread karst features resulting from the dissolution processes of limestone rocks and evaporites.
