Assessing Shear Strength of Unsaturated Soil Using Non-Experiment Methods


Research Paper (postgraduate), 2022

68 Pages


Excerpt

Table of Contents

Abstract

1. Introduction

2. Literature Review
2.1 Unsaturated Soil Mechanics
2.1.1 differentiating between unsaturated saturated soils
2.1.2 Nature and genesis of unsaturated soils
2.2 Unsaturated soil characteristics
2.3 The Problem of using unsaturated soil mechanics
2.4 The Concept of Suction Stress and Suction Strength
2.4.1 Explaining Suction Stress
2.4.2 Suction Strength of Unsaturated Soils
2.5 Model for Shear Strength of Unsaturated Soils Based on the Hydraulic State
2.6 Model Verification
2.6.1 Strength Property under Single Drying Hydraulic Path

3. Methodology
3.1 Equation for Shear Strength
3.2 Verification Procedure Using Experimental Data

4. Results and Discussions
4.1 Results
4.2 Discussion

5. Conclusions and Discussions
5.1 Conclusions
5.2 Limitations
5.3 Recommendations for Further Research

List of Figures

References

Abstract

This paper seeks to explore the non-experimental methods for measuring shear strength of unsaturated soils. This study used non-experimental approaches to evaluate the shear strength of unsaturated soil. Numerous reasons exist for considering non-experimental approaches acceptable for determining the shear strength of unsaturated soils. One of the primary reasons why non-experimental techniques might be regarded a viable option is because performing tests to determine the shear soils is a complicated and time-consuming operation. Typically, the technique entails a number of operations that need specialized equipment and the execution of methodical stages. If the findings are required quickly, the time necessary to obtain them might be harmful to the entire operation. Due to the fact that engineering processes frequently need fast results, experimental approaches may not be effective in certain circumstances. The results showed that, the equations from previously computed calculations can be used to predict and measure the shear strength of unsaturated soils.

1. Introduction

In addition to soil scientists, geotechnical, geo-environmental, and agricultural engineers are all interested in understanding the shear strength behavior of soils. Soil shear strength is required for the construction of foundations, retaining walls, and pavements in civil engineering applications, as well as for the resistance to traction and tillage equipment in agricultural engineering applications, among other things. Saturated soil shearing behavior is related to one stress-state variable: the effective stress, denoted by the letter s. The effective stress is defined as (s - uw). Total stress and pore-water pressure are represented by the letters s and uw, respectively. Most geotechnical and agricultural labs have the capability of conducting shear strength tests on saturated soils. A detailed discussion of the test methods that are used to determine the shear strength of saturated soils is not included in this section.

As stated by equation (1), matrix suction may be defined as "the difference between pore-air pressure" (ua) and pore-water pressure" (uw). Unsaturated oils have negative pore-water pressures, whereas saturated oils have positive pore-water pressures (ua - uw). Different from the mechanical behavior of saturated soils, the mechanical behavior of unsaturated soils is affected by two distinct stress-state factors. A variable called the stress tensor (s + ua), sometimes known as net normal stress, and another called the matric suction (ua - uw) are in issue (Fredlund & Rahardjo, 1993). Soil behavior is independent of the individual valves of ua and uw, as well as the overall stress, s, as long as the stress-state variables (s - ua) and (ua - uw) remain invariant. It is well knowledge among soil physicists that the matric suction (ua - uw) in a pressure membrane apparatus is entirely responsible for the water content of unconfined soil specimens, independent of the individual values of ua and uw.

A number of research on the experimental determination of unsaturated soil shear strength have been carried out in the past (e.g., Fredlund et al., 1978; Escario and Saez, 1986; Rahardjo et al., 1995; Feuerharmel et al., 2005). The primary objective of these tests is to determine the shear strength characteristics of unsaturated soils in terms of net normal stress and matric suction in terms of net normal stress and matric suction in terms of matric suction. In unsaturated soils, tric suction is a major element that influences shear strength, and it is believed to constitute a component of cohesion in certain cases (Lu and Likos, 2004). The axis translation method has been shown to be successful in the regulation of suction in general applications. This technique has been used in a variety of tests, including the conventional direct shear test, the ring shear test, triaxial testing, the resonant column test, and the real triaxial test.

In addition to directly measuring the unsaturated shear strength, methods for connecting the unsaturated shear strength to the saturated shear strength and the soil–water characteristic curve (SWCC) have been devised (e.g., Vanapalli et al., 1996; Vanapalli and Fredlund, 2000). It takes time to calculate the SWCC, on the other hand, and this is a job in and of itself. There are other empirical techniques for estimating unsaturated shear strength, which are discussed below.

Unsaturated soil shear strength is difficult to determine experimentally, regardless of the testing technique used. The most significant difficulties are: 1) the large number of tests required to establish the variation of shear strength with matric suction; and 2) the long testing times required to establish suction equilibrium in soil samples prior to shearing. Unsaturated shear testing methods are generally more complicated, time-consuming, and costly than standard shear testing procedures for saturated soils due to the fact that unsaturated soils are not saturated.

A growing number of people are becoming interested in evaluating the unsaturated soil characteristics; nevertheless, more research is needed before each method can be used more broadly. As a consequence, further research into practical testing methods to minimize the cost and time involved with shear strength testing of unsaturated soils is required. It is possible to conduct a multistage direct shear test in the laboratory to evaluate the shear strength of saturated soils. Time and money can be saved by using this method, since it has the ability to save both. It has not yet been thoroughly and critically studied whether and how to use the multistage direct shear test to unsaturated soils.

The primary goal of this article's research is to establish the validity of the multistage direct shear test for both saturated and unsaturated soils, which is presented in this paper. The investigation will be carried out in two phases. This is accomplished via the use of multistage testing, which involves carefully examining the effect of different test circumstances on the reported shear strength. Several soil samples were subjected to multistage testing, and the findings were compared to those obtained from conventional direct shear tests conducted on a larger number of soil samples. As a result of these results, suggestions are given for doing multistage direct shear testing in order to get accurate shear strength data for unsaturated soils. These recommendations are based on the findings of the study. Our previous study had shown that experimental techniques for measuring soil saturation were ineffective, therefore we utilized undisturbed soils obtained from riverside soil deposits to evaluate the shear strength of unsaturated soil.

Because of the suction created by an unsaturated soil, the shear strength of an unsaturated soil is represented mathematically as a nonlinear function, similar to that of a rubber band. Shear strength laboratory testing is both costly and time-consuming due to the fact that it necessitates the use of specialized laboratory equipment and testing methods. A technique that connects unsaturated soil shear strength to a more clearly measurable relationship, such as the soil–water characteristic curve, is needed rather than the use of multiple shear strength equations, as previously stated (SWCC). The development of an estimation method for the shear strength function, which is similar to the hydraulic conductivity function that is usually used in saturated–unsaturated seepage studies, is required by geotechnical engineers in order to estimate the shear strength function. For geotechnical engineering applications, it is essential to create a single estimate shear strength equation that is both accurate and cost-effective enough to be utilized in a variety of situations.

Geotechnical engineering practice for example, recognizes the use of nonlinear permeability and water storage functions based on the measurement of soil–water characteristic curves (SWCCs) as generally acceptable]. We suggest that shear strength studies use a method similar to this one in order to get more accurate results. There are many possible names for the quantity of water present in a soil, and the soil–water characteristic curve is a series of drying and wetting curves that may be expressed as a succession of drying and wetting curves for example, gravimetric water content, volumetric water content, and degree of saturation). A typical set of characteristic curves for soil–water interactions is shown in the diagram below. Soil suction may be estimated using any of the water content designations mentioned above if the total volume of the soil does not change while soil suction does. This is true even if the entire volume of the soil does not change while the soil suction changes. It has been shown that using the saturation SWCC (S-SWCC) designation when computing the permeability function is more preferred than using the degree of saturation designation. For about as long as there has been study on unsaturated soil shear strength the degree of saturation has also been employed as a descriptor of water content to characterize the amount of water present in a soil's water content. Additionally, the use of the S-SWCC main drying curve to estimate unsaturated soil property functions is suggested, as is taking into account the effect of moisture on the soil properties (hysteretic impact).

Many formulations for unsaturated shear strength equations have been suggested by researchers since the 1970s, including the following: It was suggested by Fredlund (2019) that a novel method for estimating unsaturated shear strength be developed. The technique is based on two so-called anchor points that were discovered by the drying S-SWCC based on the results of many shear strength tests. It was suggested that a method be used; however, no adequate verification analysis of laboratory data was provided in support of the suggestion. Specifically, the aim of this research is to determine if the Fredlund (2019) approach is valid.

In order to accomplish this objective, a short examination of the parameters of previously suggested shear strength equations for an unsaturated soil is carried out in this section. In addition, the validity of the equation provided by Fredlund is tested for viability (2019). For the purpose of calculating the "slope" of the unsaturated shear strength function between two anchor points, namely the soil's air-entry value (AEV) and residual suction conditions, the Fredlund (2019) method employs the Bao et al. (1998) unsaturated shear strength equation, which was developed by Bao et al. (1998) and Fredlund (1998).

The results of a parametric research are used to illustrate the applicability of the Bao et al. (1998) function over a wide range of soil–water characteristic curve degrees of saturation. The conclusions of the study are based on the findings of the parametric study. During the course of the verification process, it is done out utilizing publicly accessible laboratory test data sets that have been made available. The premise for this research project is to estimate the unsaturated shear strength function using information from the drying branch of the soil–water characteristic curve (S-SWCC). The unsaturated shear strength function will then be utilized to predict the unsaturated shear strength function (together with the saturated shear strength parameters). The only item that was looked at in this study was the evaluation of laboratory measured data that had previously been published in academic journals, and that was all.

In addition to soil scientists, geotechnical, geo-environmental, and agricultural engineers are all interested in understanding the shear strength behavior of soils. Soil shear strength is required for the construction of foundations, retaining walls, and pavements in civil engineering applications, as well as for the resistance to traction and tillage equipment in agricultural engineering applications, among other things. Saturated soil shearing behavior is related to one stress-state variable: the effective stress, denoted by the letter s. The effective stress is defined as (s - uw). Total stress and pore-water pressure are represented by the letters s and uw, respectively. Most geotechnical and agricultural labs have the capability of conducting shear strength tests on saturated soils. A detailed discussion of the test methods that are used to determine the shear strength of saturated soils is not included in this section. According to the American Society for Testing and Materials (ASTM) methodology, the D2850-95e1, D3080-98, D2166-98a, and D4767-95 are standard testing methods for different shear strength tests for saturated soils that are based on D2850-95e1 and D3080-98 (ASTM, 1995a, b, 1998a, b). More information on the test methods may be found in Lambe (1951), Holtz and Kovacs (1981), Bishop and Henkel (1981), and others (1983).

As stated by equation (1), matrix suction may be defined as "the difference between pore-air pressure" (ua) and pore-water pressure" (uw). Unsaturated oils have negative pore-water pressures, whereas saturated oils have positive pore-water pressures (ua - uw). Different from the mechanical behavior of saturated soils, the mechanical behavior of unsaturated soils is affected by two distinct stress-state factors. A variable called the stress tensor (s + ua), sometimes known as net normal stress, and another called the matric suction (ua - uw) are in issue (Fredlund & Rahardjo, 1993). Soil behavior is independent of the individual valves of ua and uw, as well as the overall stress, s, as long as the stress-state variables (s - ua) and (ua - uw) remain invariant. It is well knowledge among soil physicists that the matric suction (ua - uw) in a pressure membrane apparatus is entirely responsible for the water content of unconfined soil specimens, independent of the individual values of ua and uw.

A number of research on the experimental determination of unsaturated soil shear strength have been carried out in the past (e.g., Fredlund et al., 1978; Escario and Saez, 1986; Rahardjo et al., 1995; Feuerharmel et al., 2005). The primary objective of these tests is to determine the shear strength characteristics of unsaturated soils in terms of net normal stress and matric suction in terms of net normal stress and matric suction in terms of matric suction. In unsaturated soils, tric suction is a major element that influences shear strength, and it is believed to constitute a component of cohesion in certain cases (Lu and Likos, 2004). The axis translation method has been shown to be successful in the regulation of suction in general applications. This technique has been used in a variety of tests, including the conventional direct shear test, the ring shear test, triaxial testing, the resonant column test, and the real triaxial test.

In addition to directly measuring the unsaturated shear strength, methods for connecting the unsaturated shear strength to the saturated shear strength and the soil–water characteristic curve (SWCC) have been devised (e.g., Vanapalli et al., 1996; Vanapalli and Fredlund, 2000). It takes time to calculate the SWCC, on the other hand, and this is a job in and of itself. There are other empirical techniques for estimating unsaturated shear strength, which are discussed below.

Unsaturated soil shear strength is difficult to determine experimentally, regardless of the testing technique used. The most significant difficulties are: 1) the large number of tests required to establish the variation of shear strength with matric suction; and 2) the long testing times required to establish suction equilibrium in soil samples prior to shearing. Unsaturated shear testing methods are generally more complicated, time-consuming, and costly than standard shear testing procedures for saturated soils due to the fact that unsaturated soils are not saturated.

A growing number of people are becoming interested in evaluating the unsaturated soil characteristics; nevertheless, more research is needed before each method can be used more broadly. As a consequence, further research into practical testing methods to minimize the cost and time involved with shear strength testing of unsaturated soils is required. It is possible to conduct a multistage direct shear test in the laboratory to evaluate the shear strength of saturated soils. Time and money can be saved by using this method, since it has the ability to save both. It has not yet been thoroughly and critically studied whether and how to use the multistage direct shear test to unsaturated soils.

The primary goal of this article's research is to establish the validity of the multistage direct shear test for both saturated and unsaturated soils, which is presented in this paper. The investigation will be carried out in two phases. This is accomplished via the use of multistage testing, which involves carefully examining the effect of different test circumstances on the reported shear strength. Several soil samples were subjected to multistage testing, and the findings were compared to those obtained from conventional direct shear tests conducted on a larger number of soil samples. As a result of these results, suggestions are given for doing multistage direct shear testing in order to get accurate shear strength data for unsaturated soils. These recommendations are based on the findings of the study. Our previous study had shown that experimental techniques for measuring soil saturation were ineffective, therefore we utilized undisturbed soils obtained from riverside soil deposits to evaluate the shear strength of unsaturated soil.

Because of the suction created by an unsaturated soil, the shear strength of an unsaturated soil is represented mathematically as a nonlinear function, similar to that of a rubber band. Shear strength laboratory testing is both costly and time-consuming due to the fact that it necessitates the use of specialized laboratory equipment and testing methods. A technique that connects unsaturated soil shear strength to a more clearly measurable relationship, such as the soil–water characteristic curve, is needed rather than the use of multiple shear strength equations, as previously stated (SWCC). The development of an estimation method for the shear strength function, which is similar to the hydraulic conductivity function that is usually used in saturated–unsaturated seepage studies, is required by geotechnical engineers in order to estimate the shear strength function. For geotechnical engineering applications, it is essential to create a single estimate shear strength equation that is both accurate and cost-effective enough to be utilized in a variety of situations.

Geotechnical engineering practice for example, recognizes the use of nonlinear permeability and water storage functions based on the measurement of soil–water characteristic curves (SWCCs) as generally acceptable]. We suggest that shear strength studies use a method similar to this one in order to get more accurate results. There are many possible names for the quantity of water present in a soil, and the soil–water characteristic curve is a series of drying and wetting curves that may be expressed as a succession of drying and wetting curves (for example, gravimetric water content, volumetric water content, and degree of saturation). A typical set of characteristic curves for soil–water interactions is shown in the diagram below. Soil suction may be estimated using any of the water content designations mentioned above if the total volume of the soil does not change while soil suction does. This is true even if the entire volume of the soil does not change while the soil suction changes. It has been shown that using the saturation SWCC (S-SWCC) designation when computing the permeability function is more preferred than using the degree of saturation designation. For about as long as there has been study on unsaturated soil shear strength the degree of saturation has also been employed as a descriptor of water content to characterize the amount of water present in a soil's water content. Additionally, the use of the S-SWCC main drying curve to estimate unsaturated soil property functions is suggested, as is taking into account the effect of moisture on the soil properties (hysteretic impact).

Many formulations for unsaturated shear strength equations have been suggested by researchers since the 1970s, including the following: It was suggested by Fredlund (2019) that a novel method for estimating unsaturated shear strength be developed. The technique is based on two so-called anchor points that were discovered by the drying S-SWCC based on the results of many shear strength tests. It was suggested that a method be used; however, no adequate verification analysis of laboratory data was provided in support of the suggestion. Specifically, the aim of this research is to determine if the Fredlund (2019) approach is valid.

In order to accomplish this objective, a short examination of the parameters of previously suggested shear strength equations for an unsaturated soil is carried out in this section. In addition, the validity of the equation provided by Fredlund is tested for viability (2012). For the purpose of calculating the "slope" of the unsaturated shear strength function between two anchor points, namely the soil's air-entry value (AEV) and residual suction conditions, the Fredlund (2019) method employs the Bao et al. (1998) unsaturated shear strength equation, which was developed by Bao et al. (1998) and Fredlund (1998).

The results of a parametric research are used to illustrate the applicability of the Bao et al. (1998) function over a wide range of soil–water characteristic curve degrees of saturation. The conclusions of the study are based on the findings of the parametric study. During the course of the verification process, it is done out utilizing publicly accessible laboratory test data sets that have been made available. The premise for this research project is to estimate the unsaturated shear strength function using information from the drying branch of the soil–water characteristic curve (S-SWCC). The unsaturated shear strength function will then be utilized to predict the unsaturated shear strength function (together with the saturated shear strength parameters). The only item that was looked at in this study was the evaluation of laboratory measured data that had previously been published in academic journals, and that was all.

The shear strength of an unsaturated soil is described in terms of two independent stress state variables, which are referred to as the stress state variables. In terms of the shear strength equation, it may be written as

Abbildung in dieser Leseprobe nicht enthalten

The transition from saturated to unsaturated soil is extremely noticeable. The shear strength equation may also be expressed in a second form.

Abbildung in dieser Leseprobe nicht enthalten

The contributions of changes in total stress and changes in pore-water pressure uw are clearly shown in this equation, which is easy to understand. There is a scarcity of data in the published scholarly literature. The results, on the other hand, show that shear strength may be represented by a flat surface of the shapes that are now recommended. It is also described how to estimate suitable shear strength values using laboratory test results.

Clay minerals are often found in high amounts in expansive soils, and this is not unusual. The initial water content of these clayey soils, as well as drainage and saturation conditions, are the factors that have the most influence on the shear strength characteristics of the soils. Expansive clays, depending on the size of the particles, show low friction, substantial cohesiveness (due to clay mineral adsorption), and a significant perceived cohesion (due to clay mineral adsorption) (due to water retention in a fine-grained soil matrix).

Compaction increases the shear strength of soils in general, guaranteeing that they are appropriate for use in building construction projects. When it comes to growing soil, for example, high-quality borrow materials are in low supply, as is the case with high-quality borrow materials in the developing soil. Two examples of typical case studies that have been published in the literature are provided by Panesar et al. and Widger and Fredlund respectively. The use of chemical admixtures such as lime and calcium chloride to enhance soil stability has been tried in the past, but their use has been restricted mainly owing to the harsh local climatic conditions and the widespread use of de-icing salts in the area during the winter months. During the winter, it is important to examine the weather conditions outdoors as well as the use of deicing products.

Shear strength properties are frequently used in the following situations: I assessing the bearing capacity of strip, mat, and pile foundations for buildings; (ii) designing earth retaining structures for bridges and interchanges; and (3) evaluating the stability of raised or excavated slopes for embankments. Furthermore, shear strength characteristics are often utilized in the following applications: For freshly built buildings to be structurally stable, it is essential to understand the shear strength properties of the surrounding compacted expanding soil.

A study of the shear strength characteristics of compacted soils [friction angle (/0), cohesiveness (c0), and friction angle owing to suction (/b)] [friction angle due to suction (/b)] may be carried out using unsaturated soil mechanics theory [friction angle due to suction (/b)]. Saturated samples are used for the measurement of all parameters in a single test in the laboratory, whereas unsaturated samples are used for the measurement of /0 and C0 in a separate test using /b generated from the soil water characteristic curve (SWCC). There are many obstacles to using these techniques, including time limitations, equipment changes, and a lack of testing expertise.

A method that is quick (requiring no saturation time) and easy (requiring no equipment changes) for evaluating the shear strength characteristics of a local expanding soil across a wide suction range is desired by practitioners. This study describes a technique for calculating /b that takes use of compacted (unsaturated) samples to compute friction angle, cohesiveness, and soil suction (from the expanded failure envelope, as defined by Fredlund et al.). By using the Vanapalli et al. ] method to an expanding clay sample, it is possible to calculate /b.

One of the main objectives of this research is to investigate the unsaturated shear strength properties of compacted material. The investigation will be carried out in two phases.

This represents a significant quantity of dirt. When it came to first soil evaluation, geotechnical index characteristics were established. The compaction curve was created to help identify test samples that would be suitable for further study. To get a better understanding of the soil's water retention and volume change, as well as to prepare for future research, this work was carried out. The direct shear tests were carried out in order to get a better understanding of how shear strength characteristics vary with changes in water content and soil suction levels. The shear strength properties of compacted expanding clays are important in the construction of foundations because of their connection to slope stability and foundation carrying capacity.

2. Literature Review

2.1 Unsaturated Soil Mechanics

2.1.1 differentiating between unsaturated saturated soils

The solid phase of saturated soil is soil particles, while the liquid phase is water. Soil particles are the solid phase, water is the liquid phase, and air is the gaseous phase in unsaturated soil. The groundwater table, can be thought of as the border that divides the saturated and unsaturated zones of a soil slope. The saturated zone has a positive pore-water pressure (i.e., it is compressed), which reduces the effective stress and therefore the slope stability. The unsaturated zone has a negative pore-water pressure (i.e., tension), which raises the shearing levels of resistance between soil particles.

2.1.2 Nature and genesis of unsaturated soils

In geotechnical engineering, the word soil refers to a wide range of particle materials. All of the vacant spaces between the particles are filled with water in the saturated state, whereas some of the blank spaces are filled with air in the unsaturated state. The study of fine-grained soils containing clays and silts, as well as soils having coarser sand and gravel with a large percentage of fines, will be our primary focus. The finer clay soils have particle sizes less than 2 m that are barely visible to the naked eye, whilst the coarser gravels have particle sizes up to 60 mm. A rough guide to the range of particle sizes is as follows: if a clay platelet were stretched to the size of a saucer, a coarse gravel particle would have a proportionate diameter of about 10 km. Because of the vast variety of particle sizes and the intrinsic heterogeneity of soils, behavioral traits are difficult to analyze. Their multi-faceted behavior is further influenced by their stress history, particle shape, and time-dependent features, which necessitates simplifications and generalizations when proposing solutions to geotechnical problems.

The phases that make up a soil mass are solid particles, water, and air. Traditionally, academics have had some progress in obtaining a better knowledge of the behavior of saturated, fine-grained soils and dry, coarse materials like sand and gravel. Extending our knowledge to the behavior of unsaturated soils, particularly those with high fines content, has been difficult. This is mostly due to the presence of a fluid phase of air or other gases in the empty spaces, which further complicates the difficult task of determining the regulating stress regime. Differences in air and water pressures, phase compressibilities and their interactions, as well as chemical impacts, must all be considered when interpreting the behavior of unsaturated soils. The contractile skin between the fluid phases causes a surface tension effect, which is particularly important in the formation of fine-grained soils' distinctive aggregated structure. Later chapters look at the importance of the phases and their interconnections in affecting soil behavior.

Natural soils and artificial ground are both susceptible to becoming under-saturated. Natural soils that are unsaturated are prevalent in dry or semi-arid environments, where the groundwater table is frequently several meters deep. Arid or semi-arid areas cover around one-third of the earth's surface, when potential evaporation exceeds precipitation (Barbour, 1999). In a generally dry climate, however, any soil near the ground surface is likely to have a negative pore water pressure (water pressure relative to a datum of atmospheric air pressure) and may undergo de-saturation or air entry into the pore spaces.

Even though the soil is wet above the water table, air will enter the pore spaces if the pore water pressure decreases sufficiently. While the figure shows a decrease in negative pore water pressure at the ground surface, where precipitation would enhance the degree of saturation, in a warmer environment, more desiccation owing to evaporation is expected. Because complete compaction and closing of all air spaces is impracticable, unsaturated fillers such as those used in earth dams, road subgrades, and embankments are commonly used. Understanding unsaturated soil behavior and comprehending the importance to engineered structures is dependent on negative pore water pressure.

Climate plays a key part in the development of unsaturated soils (Lu and Likos, 2004), with evaporation drying up the ground in hot weather, causing fine-grained soil shrinkage and, eventually, shrinkage cracking. Following rain, further soaking causes swelling and fissure closure, but not always complete eradication of suction-induced soil structure. Following drying, fine particle aggregation has an impact on future behavior traits. Future climate change as a result of global warming might result in major changes in soil moisture regimes and, as a result, soil conditions throughout vast swaths of the globe. Water uptake by vegetation can lead to significant ground desaturation due to evapotranspiration, whereas plant removal can lead to subsequent re-saturation, potentially causing stability issues, such as slope instability due to deforestation. Large swelling and shrinkage phenomena can occur in unsaturated soils containing highly plastic clays due to water uptake or reduction in water content, resulting in ground movements capable of inflicting significant structural damage. The presence of expanding clay minerals like montmorillonite is the fundamental cause of expansive clays.

Seasonal volume fluctuation in clay is mainly limited to the upper 1.0–1.5 m in temperate zones such as the United Kingdom (Bell and Culshaw, 2001). Nonetheless, in the South and South Midlands of England, the presence of clay formations such as the London clay, Oxford clay, Kimmeridge clay, and Lias clay has resulted in a large number of insurance claims for cracking of domestic properties as a result of excessive ground shrinkage during dry summers, particularly in conjunction with tree root action.

Unstable soils are an essential phenomenon to be aware of. Wetted loess or weakly compacted fills in an unsaturated condition may suffer significant collapse settlements, resulting in damage to underlying buildings and other structures. Leftover soils from disintegrated rock show complicated behavior features in emerging regions of the globe, particularly in East Asia, which may be attributed in part to partial saturation of soils (for example. Lee and Coop, 1995; Ng and Chiu, 2003). The fast growth of structure and infrastructure in regions such as the tropics is likely to result in a greater understanding of the possible issues associated with unsaturated soils. In addition, Alonso and Olivella (2006) provide further reading on a broad variety of geotechnical issues in which a knowledge of unsaturated soil behavior is required before deciding technical solutions.

2.2 Unsaturated soil characteristics

The amount of water in an unsaturated soil is determined by the amount of suction in the soil (Leong and Rahardjo, 1997a). The soil-water characteristic curve (SWCC), defines the connection between water content and suction. When the suction is below the air-entry value (AEV), the earth is essentially saturated. Water may be readily evacuated by raising the suction between the AEV and the residual suction. Water can scarcely be extracted above the remaining suction with additional increases in suction.

Water can only flow along continuous water pathways, thus the coefficient of permeability in relation to the water phase is proportional to the soil's water content (Leong and Rahardjo,1997b). When the suction is below the AEV, the soil permeability (kw) equals the saturated permeability. When the AEV is surpassed, air enters the vacuum, reducing the water permeability significantly. At high suction, water permeability should be very low.

Shear tension is transferred through the contact region between water and soil particles (Vanapalli et al., 1996). As a result the shear strength of unsaturated soil is connected to the water content and then to the suction level. Because of the complete saturation, the soil particles and water are totally touched below AEV. The effective angle of internal friction' is equal to the rate of increase in shear strength due to matric suction b. The soil particles and water are no longer entirely touched above the AEV. As a result, matric suction's contribution to shear strength is less effective than net normal stress.

2.3 The Problem of using unsaturated soil mechanics

Matrix suction (or negative pore-water pressure) is widely recognized for increasing soil shear strength and improving the stability of geotechnical constructions (Fredlund and Rahardjo,1993). Rainwater infiltration, on the other hand, may influence matric suction and its contribution to shear strength and stability. The matric suction that can be reliably depended upon must be determined in order to assess the contribution of matric suction in a geotechnical design (Zhang et al., 2004).

The soil-water characteristic curve determines the water storage capacity of an unsaturated soil. The permeability function determines how quickly water can percolate through an unsaturated soil. To conduct a transient seepage study, the soil-water characteristic curve and permeability function must be determined. The rate of increase in shear strength relative to matric suction (b) must be determined in order to undertake a stability analysis integrating matric suction.

The difficulty in measuring the soil-water characteristic curve, permeability function, and shear strength parameter ϕb is the main challenge of using unsaturated soil mechanics, which can be attributed to the following reasons (Fredlund and Rahardjo, 1993; Vanapalli et al., 1996; Nam et al., 2011; Zhang and Fredlund, 2015):

1. The soil characteristics at various suction levels must be tested in order to get the water content, permeability, and shear strength over a wide range of suction levels. Due to the poor permeability of unsaturated soil, establishing suction equilibrium takes a long time. Because permeability decreases with increasing suction, the equilibrium period in the high suction range may be longer than in the low suction range.
2. To accommodate the suction-controlled or suction measurement need, the traditional permeability permeameter, triaxial, and direct shear apparatuses for shear strength testing must be changed. As a result, evaluating unsaturated soils is more expensive and technically difficult than testing saturated soils.

If all unsaturated soil characteristics are anticipated using empirical techniques, the findings of a rigorous analysis will likewise be "approximate" and will no longer be deemed "rigorous." As a result, the rigorous technique would lose its “rigorous” benefit while maintaining its drawback of requiring sophisticated calculations. It is preferable to have a simpler technique that just requires easily obtained parameters and includes simple computation in situations when unsaturated soil characteristics cannot be directly assessed.

[...]

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Details

Title
Assessing Shear Strength of Unsaturated Soil Using Non-Experiment Methods
Author
Year
2022
Pages
68
Catalog Number
V1172310
ISBN (Book)
9783346598127
Language
English
Keywords
assessing, shear, strength, unsaturated, soil, using, non-experiment, methods
Quote paper
Joseph Kariuki (Author), 2022, Assessing Shear Strength of Unsaturated Soil Using Non-Experiment Methods, Munich, GRIN Verlag, https://www.grin.com/document/1172310

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