Assessment of shear strength characteristics of the unsaturated gypseous soil at various saturation degrees

Abstract The purpose of this study is to determine whether or not unsaturated gypseous soil can function well as a substrate for the foundations of carrying loads. A comprehensive program of testing was carried out with the objective of validating the geotechnical parameters and behavior of the unsaturated gypseous soils. The testing program included specific gravity, moisture content, classification tests, Proctor’s compaction, relative density, and the triaxial test. Additionally, chemical analysis was performed on the samples as well. This approach was employed in a granular soil suction process to eliminate gaps of air in the soil until the soil grains held together. The sample was prepared by using a pump of vacuum with a suction process (approximately −20.0 kPa), and this method was used in the granular soil suction process. As a consequence of this, the suction prevents a specimen from collapsing when it is removed from the apparatus. The next step consisted of conducting a consolidated-undrained triaxial test on the soil. Experiments were performed on materials with a relative density of 35% and several degrees of saturation, such as normal saturation (6%), unsaturated (30, 60, 80%), and 100% saturated. It was shown that there is a reduction in the internal friction angle for the effective and total stresses is caused by an increase in the water content of the soil at any saturation degree. This occurs in both the unsaturated and saturated states of the soil. The angle of friction decreased by 80% of the natural value for both stresses, effective and total. As gypseous soil moisture increases up to the saturation degree of 60%, the soil cohesion for the total and effective stresses rises, where it increased by (220% and 125%) of the natural value for both the effective stress and the total stress, respectively, leading to an increase in the soil’s shear strength (ϕ and c). After then, there was a steady weakening of the force when it reached saturation degrees of 80% and 100%, where it decreased by (44% and 47%) of the maximum value at 60% saturation degree for both the effective stress and the total stress, respectively.


Introduction
The bulk of Earth's surface and near-surface soils are gypseous, or unsaturated, soils.Its partially saturating characteristics produce pore air and pore water, which together create a contractile skin at the air-water interface.Unsaturated soils, as mentioned by Briaud (2023), are often described as a three-phase system consisting of air, water, and solids.However, new findings show that (airwater) interface is an essential component that must be taken into account when discussing the underlying physical processes.As a result, the contractile skin has an influence on the mechanical behavior of unsaturated soil when the aerobic phase is continuous (Ng & Menzies, 2007).
Predictions of global warming indicate a decrease in the groundwater table, expanding the allocation of an unsaturated soil around the world, Intergovernmental Panel on Climate Change, (IPPC, 2007).It making increasingly important to comprehend how these soils behave in the future.Soaking causes changes in the soil's volume, stiffness, and shear strength; a magnitude and character for these changes are determined by the structure of soil, stress level, and depth of the soaking (Ng & Menzies, 2007).
According to Al-Mufty (1997), the element of gypsum is Calcium hydrate (CaSO 4 .2H 2 O), a soluble salt with a solubility of around 2.2-2.6 g/litter.Gypseous soil is defined as soil that includes either hydrated or anhydrous calcium sulfate, often known as gypsum slats, gypsum deposits, or gypsum rocks (Obead et al., 2022).
(a) Primary gypsum is made up of two different minerals: gypsum anhydrite (CaSO 4 ) and alabaster (very small, white, crushed, amorphous gypsum grains).
(b) The chemical formula for secondary gypsum is CaSO 4 .2H 2 O, which describes aqueous gypsum.Stable in the environment, it is generated during reactions of chemical by two combining molecules of the water.Briaud (2023) stated that any two of the three possible combinations of the stress factors may reflect the stress status of unsaturated soils.In particular, a rise in the pore water pressure (u w ), pore air pressure (u a ) and standard stress (σ-u a ) may be combined with (u a -u w ), (σ-u w ) can be combined with (u a -u w ), or (σ-u a ) can be combined with (σ-u w ).These are the combinations that are conceivable.Razouki et al. (2007) conducted research on the impact of soaking on the shear strength behavior of gypseous soil, and their findings were published in a scientific journal.They found that extending the soaking duration led to a decrease in the internal friction angle (ϕ) and cohesiveness (c) of the soil.Infiltration of water into the soil results in a reduction in the soil's capacity to suction water, and it also may lead to unstable underground infrastructure.
According to Lu (2008), metric suction is a factor that may change the state of interest and should be considered a stress-state variable.A state mutable is any factor that can change state of an attention.As a consequence of this, every factor that has an effect on the way the soil behaves mechanically may be classified as a pressure state variable.
In a study by Rajeev et al. (2012), it was found that suction of soil is extremely susceptible to variables in a climate.There is no correlation between the climate of a place and the location of the groundwater table in that area.In climates that are semi-arid or dry, the water table gradually falls over time, but in climates that are mild or humid, the water table may remain relatively near the surface of the ground.There are two methods that water may be lost from the soils: one is via evaporation, and the other is through vegetation.These two processes are responsible for the water bending upwards into the form of vapor.On the others hand, the precipitation is major cause of water bending downwards and replenishing the soil.The variance between the downward and upward flow condition in profile of the soil is principally determined by state of the pore water pressure, which may be thought of as the soil's internal water pressure.
For the objective of determining the level of soil suction, the apparatus developed by Fattah et al. (2018) consisted of porous gypsum block sensors that were linked to a data logger.This apparatus is suitable for use in both large-scale models and a wide variety of situations encountered in the field.Additionally, the sensors inserted inside the porous gypsum blocks measured the "Soil Water Characteristic Curve" at a variety of depths.The size and form of the pores, the chemical characteristics of the water with soil particles, and the tension of surface all have an impact on the suction that the gypseous soil exerts.
Gypseous soils have been studied in the past within the classical framework of soil mechanics that is related to saturated condition.As such, they are characterised as collapsible, problematic soils that suffer large settlement and have significant loss of strength under long-term flooding.
A large number of academics have previously looked into the impact that gypsum concentration has on the numerous different properties of soil.Researchers (Al-Mufty, 1997;Fattah et al., 2014;Mahmood, 2017;Salman, 2011) investigated the effect that the amount of gypsum in the soil had on the shear strength of the soil using a number of different soil samples gathered from different locations around Iraq.While studies such as (Al-Gharbawi et al., 2022;Al-Obaidi et al., 2018;Fattah et al., 2017;Ibrahim & Khosravifar, 2017;Mahmood et al., 2018;Moula & Al-Saoudi, 2010;Obead et al., 2022) had investigated the distortion of the gypseous soil brought on by a number of different mechanisms, as the gypsum content grew, the shear strength declined, and increased of the stresses, according to the findings of all of these studies.
The main objective of this work is to increase our understanding of the mechanics of collapsible unsaturated particulate materials by modeling the behavior of the gypseous soil in the framework of unsaturated soil mechanics.To investigate the effect of suction, inundation vertical stress, initial dry density and initial degree of saturation on the collapsibility characteristics, this work is directed to predict the volume changes and collapse potential associated with the changes in soil suction, which is important for reliable prediction of settlements of buildings, failure mechanisms as base failure of square footing or strip footing, slope stability as well as the flow of fluid through dams in geotechnical engineering practice.Moreover, the importance of the soil-water characteristic curve is given, which is used as an important tool when dealing with unsaturated soils in combination with the shear strength for prediction of bearing capacity of unsaturated soils.
The aim of this study is to evaluate the viability of using unsaturated gypseous soil as subgrade layers for machines foundations.In addition, it is believed that such an objective is useful in determining the right techniques for researching the behavior and properties of unsaturated gypseous soils, such as shear strength characteristics with various varying saturation degrees of the water.

Soil properties
This study made use of gypseous collapsible soil for its soil experimentation.It was imported from the city of Tikrit in the Salah Al-Din province, which is located north of the Baghdad metropolitan area at coordinates N 34º 38'60'' E 43º 38'50''.In order to determine the physical properties of the soil, a typical set of tests was carried out.The samples were first crushed gently to get rid of lumps and then routine tests were conducted.Table 1 provides an illustration of the many facts about the physical qualities of the soil that were employed.Figure 2  In addition, the B.S. 1377 (British Standard Institution, 1990) specification was adhered to in order to carry out a standardized set of tests that were necessary in order to determine the chemical properties of the soil.Table 2 provides an illustration of the various chemical characteristics of the utilized soil in further detail.To measure the gypsum content in the soil (χ′).It depends on heating the gypsum salt between the soil grains, and it is done in two stages.In the first stage, the water is expelled from the soil particles by placing the gypsum soil specimen in the oven at a temperature of (45) °C for several days until the weight of the soil becomes constant (w45° C).Then in the second stage, the gypsum is heated by placing the soil specimen in the oven at a temperature of (105) °C for ( 24) hours (w105° C), then the gypsum content is calculated according to Equation (1).

Consolidated undrained triaxial test
The static triaxial test (CU-test) was carried out according to standard (ASTM D-4767).Several tests were performed to determine the behavior of gypseous soil.The characteristics of shear strength for gypseous soil (ϕ and c) at the 35% relative density were obtained through a consolidated undrained triaxial test, with the standard specimen mold of 76.0 mm in height and 38.0 mm in diameter.The sample was tested in its natural saturation degree of (6%), unsaturated saturation  degree of (30, 60 and 80%), and also fully saturated of (100%).Each specimen is subjected to three different cell pressures (50, 100 and 200 kPa).
In order to produce soil samples (at natural and unsaturated cases), a light fill of soil was used to progressively introduce the soil into the rubber membrane.The soil sample was immersed for 24 hours to achieve complete saturation.The specimen was sandwiched between two filter sheets, with 38.0 mm porous stones set on each side.The membrane was sealed off from the sample with the help of the two conventional O-rings.An O-ring tool, 2-filter papers, 2-porous stone discs, a membrane, a steel mold of sample, steel tamper, a collar, a funnel, a flexible tube, vacuum mold, and a suction pump.They are all needed to construct a granular soil triaxial specimen.The drainage lead, base pedestal, and top cover for the triaxial cell have all been installed.The cell has been decontaminated with airless water, and any leaks have been fixed.(c) Deactivating the vacuum mold in order to stretch the rubber membrane so that it is flush against the inner wall of the steel collar.

Establishment of the initial conditions and preparation pedestal base of triaxial cell
(a) Placing a porous disc, and then place filter paper on top of the porous disc.
(b) Wrapping the steel collar with a membrane (used membrane thickness was 0.50 mm), and then de-airing the rubber membrane in a careful manner.
(c) Wrapping a steel collar around the pedestal's base and covering it with a membrane.
(d) There is an excessively cut-away membrane surrounding the pedestal's base.

Attaching the top to the cap
(a) While holding the tamper, push down on the surface of the sample to determine whether or not it has a level surface.
(b) Covering the soil with filter paper and then, after the paper has been put on top of the soil, covering it with a porous stone disc.
(c) Placing the top pad over the specimen that is being examined in order to conduct the test.
(d) Winding the back pressure drainage line around the specimen in such a way that it does not interfere with the cell wall or the ram; putting an O-ring over the top cap; and, lastly, inserting the top cap into the porous disc in a careful manner.
(e) Peeling away the membrane that covers the top cap in a careful manner.
(f) Putting the O-ring tool over the O-ring in a way that is meticulous and accurate.
(g) Taking extra precautions to ensure that the O-ring is securely attached to the instrument's top cap.
(h) This step entails to the O-ring tool must be removed from the assembly.
Figure 4 depicts the proposed processes that should be followed in order to prepare the triaxial sample.
In order to expel air bubbles from the system, water that had been de-aired was pumped into the tubes.It was determined that a pressure difference of between 5.0 and 10 kPa is needed to be applied in order for the de-aired pure water to soak from upper of the sample down to bottom of the specimen.The pressure inside the cell was increased by adding water to it and then pressing out all of the air bubbles that were trapped within.
After that, back pressure was utilized in order to completely saturate the sample.According to Head (1998), the recommended value for the cell pressure (σ 3 ) is about (10.0) kPa higher than back pressure.Skempton's pore pressure parameter is used around 90% of the time and is denoted by B = u/3.The rise in pore pressure that accompanies a rise in cell pressure was used to generate this measure.

Results of the tests and discussion
Table 3 provides an illustration of the specific outcomes of the consolidated undrained triaxial test.The relationship between stress and strain at various cell pressures with different saturation degrees is shown in curves (a, b, and c) in Figures ( 5-9).Also, the shear strength characteristics for the total and effective stresses are shown in (d) of Figures (5- f.The sample after compaction (bottom view).
g. Putting the sample into the desiccator.
h. Vacuum pump operation.
i.The mold with soil after suction.
j.The sample after suction.
k. Putting the lower porous stone and filter in the cell.
l. Putting the membrane about the collar, then deaired the membrane.show the influence of a change in the degree of saturation on the gypseous soil behavior when the properties of shear strength (angle of internal friction, and cohesion) are changed.
There is evidence of cohesion and an internal friction angle in every unsaturated and saturated sample, although the values were all different.The soil's cohesiveness will drop once it reaches a saturation level of 60 percent, but it will continue to rise up to the point where it reaches 100 percent.When there is a rise in the amount of moisture present in the gypseous soil, the angle of internal friction experiences a steady reduction in all of the samples.This is because the gypsum component when mixed with pure water produces a partial dissolution of itself.Furthermore, the solubility of the gypsum component rises when horizontal and vertical pressures are applied to the gypseous soil, it reasons the soil to collapse when it is at its extreme form.
The gypsum component dissolution is an essential property of the soil when it is subjected to soaking conditions, particularly when it is subjected to static and cyclic pressures.The characteristic is contingent on a wide variety of parameters, including gypsum content, hydraulic gradient, void ratio, and a host of other aspects.The following chemical process illustrates how the gypsum salts are able to dissolve when water is introduced to the soil (Al-Mufty, 1997): is knowing as the amount of a mineral that can be dissolved in one liter of a solution composed of calcium sulfate and four parts water (Al-Jumaily, 1994).The minerals started m.Putting the collar in the cell.
n. Gently putting the sample into the collar.
o. Placing the upper filter and porous stone.
p. Removing the collar from the cell (upward).
q. Installation of the O-rings about the sample.dissolving as soon as water made contact with them, and the process ceased once equilibrium was reached.Calcium sulfate is very minimally soluble in water compared to any water-soluble sulfates that may be present in a gypseous soil.The soil gypsum acts as a material of steady expansion and reduces dry density since it is a light mass substance with about 2.30 a specific gravity of the soils (Zhang & Solis, 2008).Since gypsum (CaSO 4 .2H 2 O) has two water molecules, it allows the soil to absorb water without altering the soil's chemical composition.However, owing to a change in its chemical composition, gypsum will turn into a solution when subjected to additional water (Lewry & Williamson, 1994).Increased water absorption owing to osmotic suction was another finding by Al-Daood et al. (2014) in soil samples with the soluble component gypsum.
The internal friction angle and cohesion of the shear strength characteristics were shown to decrease with increasing gypsum content, according to research by Fattah et al. (2008).Soils containing gypsum do not behave differently from soils without gypsum when subjected to loads, particularly when the gypsum is not exposed to water.The chemical bond between the gypsum and the soil particles is broken, however, when large quantities of clean water are rapidly supplied to the gypseous soil.Because of this, when the vertical settlement value or the longitudinal strain above the allowed range, the unsaturated gypseous soil collapses under relatively light loads because the gypsum element has decomposed and the shear resistance has decreased.
According to the findings in gypseous soil, the gypsum (CaSO 4 ) makes up the bulk of 44.7%.Zidan andHussein (2013), andAl-Gharbawi et al. (2022) both reported findings that were quite comparable.Soils rich in calcium carbonate (the most common type of carbonate in soils) have a lower mineral content of clays and a relatively high shear strength when they are natural or very slightly unsaturated.When water penetrates the soil, the high total sulfate (SO 33 ) and chloride (Cl −1 ) concentrations will cause interactions between the soil and the reinforced concrete foundation.Gypseous soil has a pH of about 7.23, which is within the normal range for soils.As may be noticed by its pale color and lack of odor, gypseous soil includes a negligible amount of organic matter (0.72%).High levels of total soluble salts (TSS) might indicate a deep water table during the winter or after extensive irrigation.
Figure 2 demonstrates that sand predominates over particles and gravel in the mixture.When sieved dry, retained of particles on a sieve No. 200 were 80% (1.0% gravel, and 79.0% sand), whereas particles passing the No. 200 sieve made up only 20% of the total.Classification tests indicate that the gypseous soil in Tikrit may be classified as "silty sand" or "SM", in accordance with the USCS.Salih (2003) performed preliminary studies on water's chemical characteristics.These tests demonstrated that the quantity and pace of dissolution in soil are affected by the water's initial chemical characteristics.Distilled water has been shown to be much more hostile than salt water.
The soils containing gypsum element do not behave differently from soils without gypsum when subjected to loads, particularly when the gypsum is not exposed to water.The chemical bond between the soil particles and the gypsum is broken, however, when large quantities of clean water are rapidly supplied to the gypseous soil.When the longitudinal strain or the vertical settlement value beyond the permissible range, the gypsum element decomposes, reducing the shear strength of the unsaturated gypseous soil and causing its collapse at low loads.
It was found that increasing how wet a gypseous soil is makes the internal friction angle go down for the effective and total stresses.This is true for both the unsaturated and saturated states.For effective stresses, the reduction value was between 27.5° and 5.5°, while for total stresses, it was between 25.5° and 5.0°.It was shown that when moisture of gypseous soil was raised up to the saturation degree of 60%, the cohesion strength of soil increased for the effective and total stresses.After reaching 80% and 100% saturation, the intensity progressively faded away.As a result, the soil's shear strength decreases by an amount that is 11.5 to 9.0 kPa lower for the effective stresses, and between 12.5 and 9.5 kPa lower for the total stresses.In addition, Lu and Likos (2004) demonstrated that when the gypseous soil was 100% saturated, very similar values were found for the effective and total shear strength properties.
The total suction is unaffected by the higher compaction effort required to maintain the same water content.Chemically speaking, more water molecules are produced per unit mass   of soil samples as gypsum content rises (CaSO 4 .2H 2 O).The fact that the gypsum percentage is increasing at the same time as the water activity may help to explain this pattern of behavior.Because the gypsum element in the gypseous soil is evenly distributed and made up of very small elements with a high specific surface zone, the total suction decreases as the relative humidity slightly rises.These results agree with the findings of Mahmood (2017), Obead et al. (2022) and Fattah et al. (2022).Abd et al. (2020) concluded that the relationship between the shear strength and the matric suction is divided into two parts, the first part is linear and the second is non-linear.The point between the two parts represents a point of reversal stress the two parts which is also considered as the peak stress point.The matric suction increases the soil elasticity, and all the stress variables also increase the angle of the internal friction between the soil particles.The maximum shear stress occurs at a larger matric suction with increase of confining pressure.

Conclusions
On the basis of the findings presented in this study, one might reach the following conclusions:   (1) A method has been proposed for preparing test samples for testing gypseous soil with different levels of saturation using the CU-triaxial test.This method has been described briefly.
(2) The cohesion and the internal friction angle are present in all unsaturated and saturated samples, although with varying values in each case.The effective and total cohesion increase with increasing saturation degrees of soil up to 60%, then drop after this degree until the soil is entirely saturated.This pattern repeats itself until the soil is completely  saturated.When there is an increase in the amount of moisture present in the gypseous soil, the angles of internal friction experience a steady reduction in all of the samples.
(3) There is a reduction in the internal friction angle for the effective and total stresses is caused by an increase in the water content of the soil at any saturation degree.This occurs in both the unsaturated and saturated states of the soil.The angle of friction decreased by 80% of the natural value for both stresses, effective and total.As gypseous soil moisture increases up to the saturation degree of 60%, the soil cohesion for the total and effective stresses rises, where it increased by (220% and 125%) of the natural value for both the effective stress and the total stress, respectively, leading to an increase in the soil's shear strength (ϕ and c).After then, there was a steady weakening of the force when it reached saturation degrees of 80% and 100%, where it decreased by (44% and 47%) of the maximum value at 60% saturation degree for both the effective stress and the total stress, respectively.
(4) When the gypseous soil is fully saturated, the values of the effective and total shear strength parameters were quite close to one another.

Figure 1
Figure 1 provides an illustration of the various aspects of the testing program.
depicts distribution of the grain size of Tikrit gypseous soil, and Figure 3 displays the maximum dry density (ρ d-max ) with the content of optimum moisture (OMC) by standard Proctor compaction.

Figure 2 .
Figure 2. The grain size distribution of the gypseous soil.
(a) Attaching the plastic valve and rubber tubing to the suction connection point on the steel collar.(b)The second step involves deflating the vacuum cleaner's attachment and attaching the rubber tubing to it.
(e) The meticulous insertion of the sample into the collar.(f) Attach the right-sized O-ring to the O-ring tool.(g) Step seven involves positioning the tool over the membrane and precisely positioning the O-ring on the pedestal's base.(h) Carefully enclosing the O-ring inside the supplementary membrane.(i) Stacking two O-rings, one on top of the other, and setting them on top of the O-ring tool and the pedestal base.
9) for Mohr's circles at 35% relative density and different saturation degrees.In addition, (Figures 10,11) a. Preparing the mold with coating surface by oil.b.Gently mix the soil to the desired water content.

Figure
Figure 4. Unsaturated gypseous soil triaxial preparation of the sample.

r.
Closing the cell perfectly.

d.
Mohr's circles properties of the effective and total shear strength.

d.
Mohr's circles properties of the effective and total shear strength.

d.
Mohr's circles properties of the effective and total shear strength.

d.
Mohr's circles properties of the effective and total shear strength.

Table 1 . The physical characteristics of the gypseous soil Property Value Reference
d-min , g/cm 3 1.20 USCS = Unified Soil Classification System.