Due to the collapsibility of gypsum, many structures built on gypsiferous soils have significant challenges, particularly in the drying-wetting loop, when compared to structures built on non-gypsum soils. When the water table is near the ground surface, evaporation of saline groundwater forms these soils. Gypsiferous soils are common in the Middle East, particularly in areas near the Red Sea and the Arabian Gulf. They span significant swaths of Iraq, possibly up to 20% of the country's entire territory [1]. Al-Najaf City is one of the governorates in Iraq that has various degrees of gypsum content. The city's soil is primarily made up of sand-sized particles bound together by varying concentrations of gypsum [2]. It has been reported that several structures built on these soils have cracks in various patterns and unleveled settlements when they are exposed to water [3].
A number of research has been looked into the impact of gypsum content on various soil qualities after soaking for various amounts of time. Razouki and Al-Azawi used CBR studies to show that soaking time has a significant impact on the deformation of gypsum-containing soils, and that this deformation increases as the soaking time increases [4]. Mahmood et al., investigated the effect of different soaking periods up to two weeks on low gypseous sand soils (5%) from Al-Najaf City using the Oedometer test and found that there was a noticeable increase in settlement, but no collapse potential in the soil samples [5].
The possibility for gypseous sand soils to collapse owing to settlement when wet is investigated by taking two differing gypsum contents that have been taken from Al-Najaf, Iraq (15 percent and 29 percent). Under six stress levels, the research is conducted utilizing a computerized normal Oedometer test cell. The impact of soil density as a percentage of field dry density is also explored (100-percent and 85-percent). To identify the influence of wetting on sample settlement-time for each stress level, the tests begin with unsaturated soil samples (water content of 4%), then the wetting process for a few minutes (short term) is conducted for each stress level. The findings show that as gypsum content was increased at the same conditions of stress level, and temperature rise, there is a considerable increase in the settlement [6].
Soil mechanics of gypsiferous soils has been researched in the past (saturated condition). Gypsiferous soils are found in arid and semi-arid (unsaturated) environments, and their features vary greatly [7]. Water penetration reduces soil suction, potentially affecting subsurface services such as water pipelines [8]. The main causes of collapse are soil deformation and shear strength loss due to presence of water [9].
Abdul-Hussain et al. published a study in which they studied volumetric strains of Oedometer device tests with varied wetting intervals and associated with the unsaturated triaxial testing. In Iraq, the soil of Al-Najaf City is thought to be rich in gypsum materials. The gypsum content of the district's soil was found to be 29-percent. The tests were carried out in an Oedometer device with different soaking intervals: first in normal gypsum content, then again, half an hour, and lastly two weeks. The triaxial hydration stimulus allowed for the quantification of volumetric stresses at two different levels of stress (2.5 kg/m2 and 5 kg/m2) and four levels of matric suction: zero ψ, 0.3 ψo, 0.6 ψo, and initial matric suction (ψo). The findings of unsaturated tests in the triaxial device show that as matric suction decreases, volumetric strains increase, and the stress-strain curve becomes steeper. The volumetric strains are not drastically changed when the sample is wetted in natural test for a half hour and are near to the high matric suction, according to Oedometer tests There is a significant rise in volumetric stresses when the soaking duration is prolonged to one week. However, subsequently the two weeks of wetting process in the Oedometer device, the vertical volumetric strains become more visible as the matric suction increases, and they are strikingly similar to the volumetric strains observed in unsaturated tests with low matric suction [10].
Sh. Mahmood et al., investigated into how the soaking method affected gypsum sand soil. Recent research looks into a time-based soaking approach for soil samples with high gypsum content of 29%. The soaking process softens gypsum components, breaking the connections between soil particles and stabilizing the structures. Samples were obtained from a specific location in Al-Najaf, Iraq, and then reconstituted to a density of 85% of the maximum dry density of the Proctor test and a moisture content of 4%. The specimens were tested under varied pressure levels (1.11, 2.23, and 4.47 kg/cm2) using a computerized Oedometer device. The findings revealed that as soaking periods and stress levels increase, the likelihood of these soils collapsing increases. After soaking for half an hour, the chance of breakdown climbed to around 8% after two weeks [11].
Under varied loading circumstances, the inspiration of matric suction term on the shape deformation of gypsum sand soil in unsaturated conditions was described. The soil specimens came from Al-Najaf, Iraq, and contained 14-percent, 22-percent, and 29-percent gypsum, respectively. On these soils, in a modified triaxial cell, wetting-process studies were performed. This procedure is employed after any building has been constructed and the degree of saturation of the foundation soil has been raised (reducing in matric suction). Under two distinct mean net stresses of 100 and 200 kPa, four levels of matric suctions were used: 100, 60, 30 and zero percent initial matric suction. Rainfall, rising water tables, and/or sewage and water pipe leaks could all contribute to variations in saturation levels. The volumetric strains grow as the matric suction decreases and the gypsum content rises, according to the results of this approach [12].
In this research, the influence of matric suction on the amount of precipitation in the soil was tested using samples taken from the soil of the Al-Najaf city, which includes 14% and 29% gypsum content and 90% of the maximum dry density of the Proctor test.