How it is formed and influencing factors: -
Usually, sabkha consists of a layer or several layers of heterogeneous, non-compressed sediments with high porosity and permeability, which allows subsurface water to move to the top of the sabkha surface through capillary action under the influence of evaporation processes, which leads to an increase in the concentration of salinity, which allows the crystallization of many evaporative minerals that It loses its consistency and strength when exposed to water.
There are many factors, each of which affects the physical and chemical characteristics of sabkha.:
1- Weather factors: It includes rain - temperatures - humidity - prevailing winds in the region. The small amount of rain and high temperatures in dry areas affect the level of groundwater that rises to the surface of the sabkha by capillary action, which leads to an increase in the concentration of salts in the sabkha soil. As for humidity, it controls the processes of mineral precipitation in highly saline sabkha water, while the main precipitate is sulphate salts at a high humidity of 76% - 95%, table salt is the precipitate at a humidity of 67% - 76%, and potassium is at a humidity of 67% or less. As for the winds, sand-laden winds play an essential role in the formation of sand sabkhas and in the accumulation of sand dunes over the sabkha. 2-Chemical composition: The chemical composition of the sabkha determines its features and characteristics. It is either coal clay deposited by the tidal process and marine wind activity, or calcareous sand by desert winds.
3-Earth geomorphology: The extent of the slope of the surface, the accumulation of water in it, the re-wetting of the sabkha with water, its proximity to the beach, and the level of water below the surface of the soil. All of these factors help in the formation of the sabkha.
Degree of salts and comparison with sea water: -
The sabkha waters in the sabkhas extending along the coast are characterized by a high concentration of salts, which ranges between 37% - 60%, while the percentage of salts in the waters of the Gulf ranges between 3% - 4.2%.
Soil Types: -
Soils are divided into two types according to their formation: -
1- A sabkha containing minerals that emerge upon evaporation, such as argonite, gypsum, celestite, polyhalite, sylvite, hentite, and apsomite.
2- A sabkha dominated by minerals resulting from the interaction between sabkha fluids and sediments such as dolomite and magnesite.
Engineering properties of sabkha soil:-
The marshy soil extends on the shores of the Arabian Gulf for a distance of more than 1,700 km, and in some areas its width reaches 20 km. Its thickness varies from one place to another, although it does not exceed a few metres. It is heterogeneous, non-compact, and saturated with highly salinity water. The properties of both soil and sabkha water differ in both the horizontal and vertical directions in coastal sabkha, such that the horizontal properties depend on the distance of the sabkha from the coast, while the vertical differences depend on the successive temporal and weather stages that the sabkha has passed through since the beginning of its creation. The sabkha soil is considered to have variable characteristics in terms of its particle sizes, shapes, distribution, texture, strength of bonding, and compactness. As a result, the sabkha is in the form of cohesive and non-cohesive layers alternating with each other, as well as in the form of blocks. As for the materials that bind its granules, they are gypsum, hydrated calcium sulphate, anhydrate, calcite, aragonite, and halite, all or some of them. The difference in the materials that make up the sabkha soil leads to the difference in the mechanical and physical characteristics of the sabkha.
Expected problems: -
1- Variation in the compressibility of sabkha layers, especially in the non-cohesive layers.
2- The decrease in the strength of the upper layer of the sabkha as a result of rainfall, tides, or humidity leads to the surface of the sabkha being unable to bear any weight.
3- The gypsum in the sabkha goes through stages of transformation into anhydrite, which leads to a clear change in the size of the sabkha as a result of variations in temperature and humidity.
4- When the sabkha is exposed to fresh water, it leads to the dissolution of the salts and the disintegration and compaction of the sabkha grains.
5- The change in the amount of salts present in the sabkha soil leads to a change in the size, density, hardness and strength of the soil, which results in swellings and contractions in the soil layers that lead to serious damage to the foundations of the facilities built on them.
6- The classification of the sabkha changes from sand to clay in the same location, which leads to varying subsidence of the soil and thus the collapse of the facility.
7- The presence of salts and high-salinity water that have a high corrosive effect on both concrete and reinforcing steel.
8- The groundwater level of the sabkha is close to the surface. Therefore, every increase or decrease in the groundwater level may result in severe subsidence of the soil as a result of dry soil getting wet or re-wetting.
