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ABSTRACT
Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts shear strength in soil can be affected due to many reasons, we here in this paper drive our main concern towards the shear strength which is affected due to the variations in soil suction with respect to moisture content
Here we are concerned in determining the suction controlled shear strength of cohesive and cohesionless soil. For testing Montmorillonite clay and Indian standard sand are used as the respective cohesive and cohesionless soils
Further to improve the shear strength and reduce infiltration capacity granite residual soil is added in layers for the above tested samples the results are compared the effectiveness of the procedure is shown through the result
For this experiment various tests are conducted and mainly triaxial shear test is conducted to estimate the changes in shear strength in the following samples. By conducting triaxial shear test we can get to know about the changes in shear strength and reduction in infiltration capacity of soil this is achieved by using granite residual soil because of its interlocking and low water repelling character
To predict shear strength characteristics of cohesive, and cohesionless, soils and to improve it using residual soil by using drying and wetting process with respective to matric suction
The moisture content in the sample while doing initial tests are zero. Oven dried completely moisture less sample is used, and then the moisture content is increased gradually according to suction. Since the moisture increases from 0% to 100% the process is known to be as drying and wetting process
CHAPTER 1
INTRODUCTION
1. GENERAL
Soil mechanics and foundation engineering is the branch of civil engineering concerned with the engineering behaviour of earth materials. Geotechnical engineering is important in civil engineering, but also has applications in military, mining, petroleum and other engineering disciplines that are concerned with construction occurring on the surface or within the ground. Geotechnical engineering uses principles of soil mechanics and rock to investigate subsurface conditions and materials; determine the relevant physical mechanical and chemical properties of these materials; evaluate stability of natural slopes and man-made soil deposits; assess risks posed by site conditions; design earthworks and structure foundations; and monitor site conditions, earthwork and foundation construction.
A typical geotechnical engineering project begins with a review of project needs to define the required material properties. Then follows a site investigation of soil, rock, fault distribution and bedrock properties on and below an area of interest to determine their engineering properties including how they will interact with, on or in a proposed construction. Site investigations are needed to gain an understanding of the area in or on which the engineering will take place. Investigations can include the assessment of the risk to humans, property and the environment from natural hazards as earthquakes, landslides, sinkholes, soil liquefaction, debris flows and rock falls.
A geotechnical engineer then determines and designs the type of foundations, earthworks, and/or pavement subgrades required for the intended man-made structures to be built. Foundations are designed and constructed for structures of various sizes such as high-rise buildings, bridges, medium to large commercial buildings, and smaller structures where the soil conditions do not allow code-based design.
Foundations built for above-ground structures include shallow and deep foundations. Retaining structures include earth-filled dams and retaining walls. Earthworks include embankments, tunnels, dikes and levees, channels, reservoirs, deposition of hazardous waste and sanitary landfills.
Geotechnical engineering is also related to coastal and ocean engineering. Coastal engineering can involve the design and construction of wharves, marinas, and jetties. Ocean engineering can involve foundation and anchor systems for offshore structures such as oil platforms.
The fields of geotechnical engineering and engineering geology are closely related, and have large areas of overlap. However, the field of geotechnical engineering is a specialty of engineering, where the field of engineering geology is a specialty of geology.
Soil mechanics is a branch of engineering mechanics that describes the behaviour of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids usually air and water and particles usually clay, silt, sand, and gravel but soil may also contain organic solids, liquids, and gasses and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a sub discipline of civil engineering, andengineering geology, a sub discipline of geology. Soil mechanics is used to analyse the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as engineering geology, geophysical engineering, coastal engineering, engineering hydrology and soil physics.
This article describes the genesis and composition of soil, the distinction between pore water pressure and inter-granular effective stress, capillary action of fluids in the pore spaces, soil classification, seepage and permeability, time dependent change of volume due to squeezing water out of tiny pore spaces, also known as consolidation, shear strength and stiffness of soils. The shear strength of soils is primarily derived from friction between the particles and interlocking, which are very sensitive to the effective stress. The article concludes with some examples of applications of the principles of soil mechanics such as slope stability, lateral earth pressure on retaining walls, and bearing capacity of foundations.
Geotechnical engineers and engineering geologists perform geotechnical investigations to obtain information on the physical properties of soil and rock underlying and sometimes adjacent to a site to design earthworks and foundations for proposed structures, and for repair of distress to earthworks and structures caused by subsurface conditions. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves in-situ testing two common examples of in-situ tests are the standard penetration test and cone penetration test. In addition site investigation will often include subsurface sampling and laboratory testing of the soil samples retrieved. The digging of test pits and trenching particularly for locating faults and slide may also be used to learn about soil conditions at depth. Large diameter borings are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to be lowered into the borehole for direct visual and manual examination of the soil and rock stratigraphy.
A variety of soil samplers exist to meet the needs of different engineering projects. The standard penetration test (SPT), which uses a thick-walled split spoon sampler, is the most common way to collect disturbed samples. Piston samplers, employing a thin-walled tube, are most commonly used for the collection of less disturbed samples. More advanced methods, such as ground freezing and the Sherbrook block sampler, are superior, but even more expensive.
Atterberg limits tests, water content measurements, and grain size analysis, for example, may be performed on disturbed samples obtained from thick walled soil samplers. Properties such as shear strength, stiffness hydraulic conductivity, and coefficient of consolidation may be significantly altered by sample disturbance. To measure these properties in the laboratory, high quality sampling is required. Common tests to measure the strength and stiffness include the triaxial shear and unconfined compression test.
Surface exploration can include geologic mapping, geophysical methods, and photogrammetry or it can be as simple as an engineer walking around to observe the physical conditions at the site. Geologic mapping and interpretation of geomorphology is typically completed in consultation with a geologist or engineering geologist.
Geophysical exploration is also sometimes used. Geophysical techniques used for subsurface exploration include measurement of seismic waves pressure, shear, and Rayleigh waves, surface-wave methods and or down hole methods, and electromagnetic surveys magnetometer, resistivity, and ground-penetrating radar.
2. SHEAR STRENGTH
Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strains. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case, the peak strength would be followed by a reduction of shear stress. The stress-strain relationship levels off when the material stops expanding or contracting, and when inter particle bonds are broken. The theoretical state at which the shear stress and density remain constant while the shear strain increases may be called the critical state, steady state, or residual strength.
The volume change behaviour and inter particle friction depend on the density of the particles, the inter granular contact forces, and to a somewhat lesser extent, other factors such as the rate of shearing and the direction of the shear stress. The average normal inter granular contact force per unit area is called the effective stress.
If water is not allowed to flow in or out of the soil, the stress path is called an undrained stress path. During undrained shear, if the particles are surrounded by a nearly incompressible fluid such as water, then the density of the particles cannot change without drainage, but the water pressure and effective stress will change. On the other hand, if the fluids are allowed to freely drain out of the pores, then the pore pressures will remain constant and the test path is called a drained stress path. The soil is free to dilate or contract during shear if the soil is drained. In reality, soil is partially drained, somewhere between the perfectly undrained and drained idealized conditions.
The shear strength of soil depends on the effective stress, the drainage conditions, and the density of the particles, the rate of strain, and the direction of the strain.
For undrained, constant volume shearing, the Tresca theory may be used to predict the shear strength, but for drained conditions, the Mohr–Coulomb theory may be used.
Two important theories of soil shear are the critical state theory and the steady state theory. There are key differences between the critical state condition and the steady state condition and the resulting theory corresponding to each of these conditions
1. Undrained shear strength
This term describes a type of shear strength in soil mechanics as distinct from drained strength.
Conceptually, there is no such thing as the undrained strength of a soil. It depends on a number of factors, the main ones being:
1. Orientation of stresses
2. Stress path
3. Rate of shearing
4. Volume of material
Undrained strength is typically defined by Tresca theory, based on Mohr's circle as:
σ1 - σ3 = 2 Su
Where:
σ1 is the major principal stress
σ3 is the minor principal stress
is the shear strength (σ1 - σ3)/2
Hence, = Su , the undrained strength.
It is commonly adopted in limit equilibrium analyses where the rate of loading is very much greater than the rate at which pore water pressures that are generated due to the action of shearing the soil may dissipate. An example of this is rapid loading of sands during an earthquake, or the failure of a clay slope during heavy rain, and applies to most failures that occur during construction.
As an implication of undrained condition, no elastic volumetric strains occur, and thus Poisson's ratio is assumed to remain 0.5 throughout shearing. The Tresca soil model also assumes no plastic volumetric strains occur. This is of significance in more advanced analyses such as in finite element analysis. In these advanced analysis methods, soil models other than Tresca may be used to model the undrained condition including Mohr-Coulomb and critical state soil models such as the modified Cam-clay model, provided Poisson's ratio is maintained at 0.5.
One relationship used extensively by practicing engineers is the empirical observation that the ratio of the undrained shear strength c to the original consolidation stress p' is approximately a constant for a given Over Consolidation Ratio (OCR). This relationship was first formalized by (Henkel 1960) and (Henkel & Wade 1966) who also extended it to show that stress-strain characteristics of remoulded clays could also be normalized with respect to the original consolidation stress. The constant c/p relationship can also be derived from theory for both critical-stateand steady-state soil mechanics (Joseph 2012). This fundamental, normalization property of the stress-strain curves is found in many clays, and was refined into the empirical SHANSEP (stress history and normalized soil engineering properties) method.(Ladd & Foott 1974).
1.2.2 Drained shear strength
The drained shear strength is the shear strength of the soil when pore fluid pressures, generated during the course of shearing the soil, are able to dissipate during shearing. It also applies where no pore water exists in the soil (the soil is dry) and hence pore fluid pressures are negligible. It is commonly approximated using the Mohr-Coulomb equation. (It was called "Coulomb's equation" by Karl von Terzaghi in 1942.) (Terzaghi 1942) combined it with the principle of effective stress.
In terms of effective stresses, the shear strength is often approximated by:
= σ' tan(φ') + c'
Where σ' =(σ - u), is defined as the effective stress. σ is the total stress applied normal to the shear plane, and u is the pore water pressure acting on the same plane.
φ' = the effective stress friction angle, or the ‘angle of internal friction' after Coulomb friction. The coefficient of friction is equal to tan(φ'). Different values of friction angle can be defined, including the peak friction angle, φ'p, the critical state friction angle, φ'cv, or residual friction angle, φ'r.
c' = is called cohesion, however, it usually arises as a consequence of forcing a straight line to fit through measured values of (τ,σ')even though the data actually falls on a curve. The intercept of the straight line on the shear stress axis is called the cohesion. It is well known that the resulting intercept depends on the range of stresses considered: it is not a fundamental soil property. The curvatureof the failure envelope occurs because the dilatancy of closely packed soil particles depends on confining pressure.
1.3 FACTORS AFFECTING SHEAR STRENGTH
The stress-strain relationship of soils, and therefore the shearing strength, is affected by:
1. Soil composition: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
2. State (initial): Defined by the initial void ratio, effective normal stress and shear stress (stress history). State can be described by terms such as: loose, dense, over consolidated, normally consolidated, stiff, soft, contractive, dilative, etc.
3. Structure: Refers to the arrangement of particles within the soil mass; the manner the particles are packed or distributed. Features such as layers, joints, fissures, slickensides, voids, pockets, cementation, etc., are part of the structure. Structure of soils is described by terms such as: undisturbed, disturbed, remoulded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic
4. Loading conditions: Effective stress path, i.e., drained, and undrained; and type of loading, i.e., magnitude, rate (static, dynamic), and time history (monotonic, cyclic).
1.4 SOIL SUCTION
The water in soil voids below the water table is normally continuous. The soil may be saturated, with voids full of water or there may be occluded air bubbles present in the water. Pore pressures at depths below the water table are derived from a combination of the weight of the water lying above the given elevation and the drainage conditions below. The pore pressure normally has a positive value and can be measured using a saturated piezometer with a porous filter that is making intimate contact with the water in the soil.
If the water contained in the voids of a soil were subjected to no other force than that due to gravity, the soil lying above the water table would be completely dry. However, powerful molecular and physico-chemical forces acting at the boundary between the soil particles and the water cause the water to be either (a) drawn up into the otherwise empty void spaces or (b) held there without drainage following infiltration from the surface. The attraction that the soil exerts on the water is termed soil suction and manifests itself as a tensile hydraulic stress in a saturated piezometer with a porous filter placed in intimate contact with the water in the soil.
The magnitude of the attractive force that soil above the water table exerts on water is governed by the size of the voids in manner similar to the way that the diameter of a small bore glass tube governs the height to which water will rise inside the tube when it is immersed in water. The smaller the void, the harder it is to remove the water from the void.
The meniscus formed between adjacent particles of soil by the soil suction creates a normal force between the particles, which bonds them in a temporary way. Thus soil suction, if it can be relied upon, can enhance the stability of earth structures. However soil suction also provides an attractive force for free water, which can result in a loss of stability in loosely compacted soils or swelling in densely compacted soils.
1.4.1 Matric suction
Matric suction is the pressure dry soil exerts on the surrounding soils to equalise the moisture content in the overall block of soil. Matric suction conditions in the soil profile were obtained through steady state unsaturated seepage analyses. The initial matric suction profile is the same as suction varied from 409 kPa at tree root to 49 kPa at lower boundary of the soil domain. The contours of changes in matric suction are presented in Fig. 11. The closer to the tree, the more change in suction is observed. The results of stress-deformation analysis are shown in Fig 14 as contours of vertical displacement. A maximum foundation settlement of 80mm and minimum settlement of 25 mm was observed. A maximum settlement in the soil profile took place at tree location and decrease with horizontal distance and depth.
1.4.2 Osmotic suction
The osmotic pressure may be expressed, according to Van Laar, by the equation, where x is the molecular concentration of the dissolved substance, and a is an “ influencing ” coefficient, which expresses the consequences of the interaction of the molecules of the solvent with those of the dissolved substance. The logarithmic term is an essential feature of the thermodynamic equation, and it is urged that all kinetic theories which lead to expressions without
The thermodynamic equation, it is true, leads to an expression for dilute solutions which is identical with that of van’t Hoff. But in practice it is found that in more concentrated solutions deviations appear which are much smaller than those for non-ideal gases. We may therefore surmise that the so-called osmotic pressure has an entirely different ground from that suggested by van’t Hoff’s application of the gas-equation, and that there is here no close relation but merely an analogy.If the osmotic pressure were actually caused by the pressure of the dissolved substance, as Ehrenfest, reviving the old theory, suggests, the pressure of the sugar molecules against the semi-permeable membrane would, in van Laar’s opinion, cause the reverse effect to that which is actually observed. No water would pass from the pure solvent through the membrane into the solution, giving rise to a hydrostatic pressure in the osmometer; but, on the contrary, the inward flow of water would be checked, since the pressure in the solution would from the outset be greater than in pure water. In reality, osmotic pressure is caused by the water which penetrates through the semi-permeable membrane, giving rise to a hydrostatic pressure which prevents the further intrusion of the water. This excess of pressure is the so-called “osmotic pressure” of the solution.
Generally speaking, every theory which seeks to interpret osmotic pressure kinetically must be based on the diffusion of the water molecules on the two sides of the membrane. If this is done, the logarithmic member arises of its own accord, and finds a place in the equation, whether there is interaction between solvent or solute or not, i.e. the a-term appears quite independently of the logarithmic term. In van Laar’s opinion, the kinetic interpretation of osmotic pressure, which is always reappearing again in new forms, is moving, and has moved, in a wrong direction, 'and should again be founded on the simple diffusion phenomenon.
1.4.3 Total suction
Total soil suction is defined in terms of the free energy or the relative vapour pressure (relative humidity) of the soil moisture. The total suction consists of two components, matric suction (Ua - Uw) and osmotic suction (π). Total suction is the combination of the above two suction types which is matric and osmotic suction.
Both components are due to differences in relative humidity of the soil vapour.
1.5 MEASUREMENT OF MATRIC SUCTION
Soil suctions can be found in all ground that lies above the water table. It is one of the most importantparameters describing the moisture stress condition of unsaturated soils and laboratory measurements of suction can be very useful for assessing the quality of the samples, estimating the in situ effective stress and realistic applications of unsaturated soil mechanics. This paper reports on direct and indirect soil suction measurement methods. Direct suction measurement techniques mainly include axis transition technique, tensionometer and suction probe. Indirect suction measurement techniques are divided into three categories, namely, measurement techniques of matric suction, osmotic suction and total suction. Indirect matric suction measurement techniques include time domain reflectometry (TDR), electrical conductivity sensors , thermal conductivity sensor(TCS) and in-contact filter paper technique. Indirect osmotic suction measurement techniques chiefly include squeezing technique and saturation extract method.
Indirect total suction measurement techniques include psychrometertechnique, relative humidity sensor, chilled-mirror hygrometer technique and non-contact filter paper method. These techniques have been widely used in research laboratories and in engineering practice. However, each of these methods has its own limitations and disadvantages, and active research to improve these techniques need to be done in research laboratories and universities. This paper demonstrates working principles, measurement, and application of these methods based one recent literature and geotechnical engineering practice.
1.5.1 Direct methods
Matrix suction can be obtained through direct measurement of the negative pore-water pressure. The pore-air pressure usually equal to on-site atmospheric pressure, and matric suction is the difference between air pressure and pore-water pressure. The direct measurement of matric suction requires aseparation between water and air phase by means of a ceramic disk or a ceramic cup. The maximum value of matric suction that can be measured is limited by the air entry value of the ceramic disk or the ceramic cup used.
Axis-translation Technique
The working principle of this technique is artificially raising the atmospheric pressure experienced by a soil sample while maintaining the pore-water pressure to a positive reference pressure to avoid measuring negative pore-water pressure. Therefore, pressure difference Ua– Uw , otherwise known as the matric suction, also does not change. The measurement of matric suction using this technique is only limited by the air entry value of the ceramic disk used since cavitation can be avoided due to elevated pore-water pressure. Ceramic disks with a maximum air-entry value of 1500 kPa are available in the market (Soil moisture Equipment Corp, 2002). Since water pressure in the water compartment is maintained as close as possible at a zero value, the technique is called null-type axis-translation technique (Fredlund, 1989).The technique was adopted to determine unconfined wetting and drying curves in the low suction range (i.e., less than 1500 kPa). The range of axis translation technique to measure or control matric suction is limited by two factors, namely, the maximum air pressure which can be imposed on the experiment system and the air entry value of the ceramic disk. One challenge of the axis translation technique is it does not yield instantaneous results when used to impose matric suction, another drawback is the long equilibrium times associated with the axis translation technique make these experiments particularly susceptible to the process of air diffusion.
Tensiometer
Tensiometer is normally used for directly measuring the negative pore-water pressure of soil. The basic principle is that the pressure of water contained in a high air entry material will come to equilibrium with the soil water pressure making it possible to measure negative soil water pressures. Since a true semi-permeable membrane for soluble salts does not exist in tensiometer, the effect of osmotic component of suction is not measured. Thus, the measurement only provides the value of matric suction component in the soil. A small ceramic cup is attached to a tube filled with deaired water which is connected to a pressure measuring device. Saturate the ceramic cup and tube by filling with water and
applying a vacuum to the tubing. Allow the ceramic tip to dry to reduce the water pressure in the sensor and remove any air bubbles that appear. Due to the cavitation problem, the use of a ceramic cup with a higher air entry value will not increase the measurement range of the tensiometer. However, improvements have been made to the tensiometer technique to enable measurements of matric suction greater than 100 kPa to be performed. The limitation is that air in the sensor will result in bad or less negative measurements of the pore water pressure for the following reasons: a) water vaporizes as the soil water pressure approaches the vapor pressure of water at the ambient temperature. b) air in soil can diffuse through the ceramic material; c) air comes out of solution as the water pressures decrease.
Suction Probe
The direct measurement of matric suction is preferred in unsaturated soil tests since measured porewater pressures are more rapidly reflected. Ridley and Burland (1993) developed a suction probe for measuring matric suction of soil. The principle of making suction measurements using a suction probe is based on the equilibrium between the pore-water pressure in the soil and the pore-water pressure in the water compartment. Before equilibrium is attained, water flows from the water compartment into the soil, or vice versa. The suction probe measures the pore-water pressure(uw). The matric suction can be computed since the applied air pressure (ua) is known, and the matric suction is the difference between the pore-air pressure and the pore-water pressure (ua– uw).Basically, a suction probe consists of a pressure transducer with a high-air entry ceramic disk mounted at the tip of the transducer. The diaphragm of the pressure transducer responds to the pressure applied. In the suction probe, the volume of water reservoir beneath the ceramic disk or ceramic cup is minimized. Water in the water reservoir is pre-pressurized such that benefit of the high tensile strength of water can be utilized (Marinho and Chandler, 1995).Recently, Meilani et al. (2002) developed a mini suction probe for measuring matric suction along The specimen’s height during triaxial test on an unsaturated soil.
It is unique in its ability to make direct measurements over a wide range of soil suctions (i.e. up to 1500 kPa) and has been used extensively in both laboratory and field applications for a variety of clients and on a range of soil types. Measurements can be made in a borehole at depths between 0 and 5m or on samples after they have been recovered from the ground. Similar to the null-type axis-translation technique, the upper limit of matric suction that can be measured using this technique is governed by the air-entry value of the ceramic disk or ceramic cup used. The main problem is that there may be cavitation and air diffusion through the ceramic head during the suction measurement.
1.5.2 Indirect methods
The indirect measurement of matric suction is commonly performed using a standard porous sensormade of a special material (e.g., filter paper, fiberglass, gypsum, nylon, sintered glass, clay ceramics, and metal). The measurement is performed by equilibrating the porous sensor with the matric suction in the soil. Therefore, the water content of the porous sensor represents the magnitude of matric suction of the soil.
Time domain reflectometry
Time domain reflectometry (TDR) was first suggested for measuring volumetric water content of soils by Topp et al. (1980). In the TDR technique, apparent dielectric constant of the soil (i.e., the bulk soil water) is measured, which is related to volumetric water content of the soil (Topp et al., 1980).Since then, the method has been used by a number of researchers involving various disciplines (e.g., Dalton et al., 1984; Kalinski and Kelly, 1993; Benson and Bosscher, 1999; Amente, et al., 2000; and Yu andDrnevich, 2004). For a quite large range of water content and suction encountered in clay soils, water is held in the pores that are located within the clay clusters (i.e., intra-cluster pores). The pore-water held in the pores between the clay clusters is the bulk pore-water, which gives rise to the capillary phenomenon (i.e., matric suction) in the absence of a true semi permeable membrane. Therefore, time domain reflectometry practically measures matric suction instead of total suction. Time domain reflectometry requires soil-water characteristic curve of the soil tested to relate the measured volumetric water content to matric suction. Yu and Drnevich (2004) improved the technique such that gravimetric water content of the soil specimen can also be measured without separately testing the soil for determining its specific gravity and dry density. The advantage of the technique is mainly that reliable measurements ofvolumetric water content can be conducted within a short time duration (Benson and Bosscher, 1999).
However, the limitation is that the technique requires a very sophisticated electronic device and the accuracy of TDR for measuring matric suction depends on the precise determination of SWCC of the soil tested.
Electrical conductivity sensors
Electrical conductivity sensors are commercially available (e.g. Soilmoisture Inc., Irrometer Company Inc., Measurement Engineering Australia and Environmental Sensors Inc.). The electrical conductivity sensor consists of a porous block and two concentric electrodes embedded inside the block. The electrical conductivity sensor measures the electrical conductivity of the porous block. As the moisture content of the porous block increases, the electrical resistance of the block decreases. The electrical resistance of the porous block can be related to the matric suction of the block. Usually the electrical conductivity sensor is read manually from a meter, limiting the number of readings when used in the field (Skinner et al., 1997). Gypsum was found to be the most suitable porous block material as gypsum took the shortest time to saturate and responded fastest in matric suction measurements (Buoyoucos and Mick, 1940). This however has the unintended effect of degrading the electrical
Conductivity sensor as the gypsum eventually dissolves completely into the soil solution. Similar to the thermal conductivity sensor, the gypsum block of the electrical conductivity sensor also suffers from hysteresis. The electrical conductivity sensor has a long equilibration time (2–3 weeks) in a rapidly changing moisture environment (Aitchison and Richards, 1965). The equilibration times of the gypsum electrical conductivity sensors were found to vary with matric suction ranging from 6 h for a matric suction of 50 kPa to 50 h for a matric suction of 1,500 kPa. The sensitivity of the electrical conductivity sensor becomes very low when the matric suction exceeds 300 kPa. Besides, the electrical resistance of the porous block is also dependent on the salt concentration of the soil solution and may not be a direct indication of the moisture content of the porous block. These shortcomings had led to a diminished use of electrical conductivity sensor for matric suction measurement even in the agricultural industry (Skinneretal., 1997). Currently, research on the electrical conductivity sensor is still ongoing to overcome its limitations.
Thermal conductivity sensor
Thermal conductivity sensor (TCS) is an equipment proposed by Shawand Baver(1939) toMeasurematricsuction.Since then,manyresearchershaveusedthetechniqueand examined other materials thatcan be used to enclose the TCS (Phene et al., 1971; Lee and Fredlund, 1984; Fredlund and Wong, 1989).
A thermal conductivity sensor employs a porous block, typically ceramic, as a medium to measure matricsuction indirectly. The basic principle is if a matric suction gradient exists between the soil and porous block, water flux will occur until their suctions are equal. The thermal conductivity of the block consists of the thermal conductivity of the solid and the fluid (air or/and water) that fills the voids in the porous block. As the moisture content of the porous block increases, the thermal conductivity of the block increases. The moisture content of the block is measured by heating the porous block with a heater embedded in the centre of the porous block and measuring the temperature rise during heating. The temperature rise which is related to the thermal conductivity of the porous medium and the moisture content can then be used as an index of matric suction in the soil. The time to equilibrate depends on the temperature gradient and the hydraulic conductivity of the porous medium and surrounding soil. Thermal conductivity sensors have been used in the laboratory as well as in the field (Fredlund and Wong 1989; Oloo and Fredlund 1995; O’Kane et al. 1998; Marjerison et al. 2001; Nichol et al. 2003). Currently, thermal conductivity sensors are available commercially (e.g. Campell Scientific, Inc. and GCTS). The attractiveness of the thermal conductivity soil suction sensor lies primarily in its ability to produce a reasonably reliable measurement of soil suction over a Relatively wide range of suctions and the measurements are essentially unaffected by the salt content of the soil (Lee and Fredlund, 1984 and Fredlund and Wong, 1989). Another advantage of thermal conductivity sensors is their versatility and ability to be connected to a data acquisition system for continuous and remote monitoring. There have been numerous shortcomings and difficulties experienced with previously developed versions of thermal conductivity suction sensors. These difficulties and shortcomings can be identified as: 1.) low strength and poor durability of the ceramic tip, 2.) insensitivity and inaccuracy particularly in the higher range of suctions, and 3.) poor stability of the electronic signal. Nowadays, the thermal conductivity sensor shows hysteretic behaviour on drying and wetting. In addition, the main problem with the thermal conductivity sensor is the variable uniformity of the porous block from sensor to sensor. This means that a separate calibration curve is required for each thermal conductivity sensor.
In-contact filter paper technique
Filter paper technique was established for measuring soil suction by soil scientists and agronomists (e.g., Gardner, 1937; Fawcett and Collis-George, 1967; Al-Khafaf and Hanks, 1974; Hamblin, 1981; Greacen et al., 1987; and Deka et al., 1995). In geotechnical engineering fields, many researchers have also used the technique as a routine method for suction measurement
The in-contact filter paper technique is used for measuring matric suction of soils direct contact between the filter paper and the soil allows water in the liquid phase and solutes to exchange freely. In the in-contact filter paper technique, water content of an initially dry filter paper increases due to a flow of water in liquid form from the soil to the filter paper until both come into equilibrium. Therefore, a good contact between the filter paper and the soil has to be established. After equilibrium is established between the filter paper and soil, the water content of the filter paper is measured. Then, by using the appropriate filter paper calibration curve, the suction of the soil is estimated. The in-contact filter paper method becomes inaccurate in high matric suction range since water transport is dominated by vapour transport (Fredlund et al., 1995). The water content of filter paper is converted to matric suction using an in-contact filter paper calibration curve. The calibration curve for the filter paper matric suction measurement is commonly established using a pressure plate apparatus (e.g., Al-Khafaf and Hanks, 1974; Hamblin, 1981; Greacen et al., 1987; Deka et al. 1995; and Leong et al., 2002) or against known suction pressures of solutions. Leong et al. (2002) found that the filter papertechnique exhibits hysteresis. It is also important to note that only ash-less filter papers should be used in the filter paper technique and only Whatman 42 and Sleicher and Schuell 59 (or SS 59) filter papers are commonly used (Leong et al, 2002).The filter paper method is a simple technique and can be reliable if the basic principles of the method are understood and a strictly practiced laboratory protocol is carefully followed. Filter papers should be allowed to equilibrate for a sufficient time and 1 week of equilibration
Period is usually considered satisfactory for most soil suction measurements. Therefore, the limitations are response times (7-14 days) may be too long and filter may not make good contact with soil.
1.6 SOIL WATER CHARACTERISTIC CURVE
The soil-water characteristic curve for a soil is defined as the relationship between water content and suction for the soil (Williams 1982). The water content defines the amount of water contained within the pores of the soil. In soil science, volumetric water content,, is most commonly used. In geotechnical engineering practice, gravimetric water content, w, which is the ratio of the mass of water to the mass of solids, is most commonly used. The degree of saturation, S, is another term commonly used to indicate the percent- age of the voids that are filled with water.
The above variables have also been used in a normalized form where the water contents are referenced to a residual water content (or to zero water content). The suction may be either the matric suction (also known as capillary pressure) of the soil (i.e., ua- uw, where ua is the pore-air pressure and uw is the pore-water pressure) or total suction (i.e., matric plus osmotic suction).
PREVIOUS LITRATURE WORK
This procedure includes to get to know about evaluation of suction controlled shear strength, and to improve it. For the above reasons many literatures, and journals were collected and studied. From these literatures we can know about different techniques and methods for conducting our project
The following literatures were used to get important techniques and various methods which involves in successive completion of the project
1.PREDICTION OF SHEAR STRENGTH OF UNSATURATED SOILS USING DRYING AND WETTING PROCESS
Anuchit Uchaipichat , vongchavalikul university , Thailand 2010
An experimental research program of shear tests is conducted on a compacted kaolin using a conventional triaxial equipment modified for testing unsaturated soils. The modified triaxial cell is capable of independent measurement and control of pore air and pore water pressures at the top and the bottom boundaries of the specimen. The suction values varied from 0 to 300 kPa. The experimental results show the difference in relationships between shear strength and suction for drying and wetting processes. The shear strength predictions using simple failure equations for unsaturated soil in the literature are compared with the experimental results. The predictive capability of the proposed failure equations is discussed
This paper reviews only simple failure criterions (e.g., Alonso et al. 1990; Sun et al. 2000; Khalili et al. 2004; Sheng et al. 2008) since a few additional parameters are required and measurable in a typical geotechnical laboratory. Alonso et al. (1990) proposed constitutive models for unsaturated soils incorporated simple shear strength criterion.
An experimental program of shear tests was conducted on a compacted kaolin using aconventional triaxial equipment modified for testing unsaturated soils. The modified triaxial cell was capable of independent measurement and control of pore air pressure and pore water pressure at the top and the bottom boundaries of the specimen. The experimental results show the difference in relationships between shear strength and suction for drying and wetting processes.
The shear strength predictions using the simple failure equations for unsaturated soil in the literature were compared with the experimental results. Favourable agreements are achieved between the predictions using equation proposed by Khalili et al. (2004) and the experimental test data over the range of testing suction value for both wetting and drying processes. The comparisons also show that the equation proposed by Sheng et al. (2008) fails to predict the shear strength for drying process. Moreover, an underestimate from the predictions using equation proposed by Sun et al. (2000) occurs over the ranges of suction below the air entry and the air expulsion values for drying and wetting processes respectively.
2. REVIEW ON GRANITIC RESIDUAL SOILSGEOTECHNICAL PROPERTIES
Asmaa Gheyath Salih, University Technology Malaysia, Malaysia, 2012
Knowledge on characteristics of granite residual soil offers important information on soilstrength and its behavior for safe and economic geotechnical structure design. In Malaysia and Singapore, residual soils were investigated extensively because they are widespread in tropical areas due to their many applications. Residual soils’ properties depend mainly on weathering degree of the natural rock. Therefore, this would make the soil geotechnical characteristics vary according to the degree of weathering. This paper attempts to summarize the basic geotechnical properties of granite residual soils as obtained by different researchers that conducted in several sites and conditions. The important geotechnical properties of the soil such as specific gravity, particles size, clay contents percentage and shear strength of the soil mass which determine the suitability and ability of the soil for construction. The significant of collecting such data will develop a clear vision for new researchers and civil engineering activities by providing the major characteristics and the nature of the composition
of this type of tropical soils.
Knowing of granite residual soil characteristics will assist in construction of strong bases such as roads highways, airports, dams, foundations, embankments, slopes, etc. Intensive rainfall weather cause massive slope failures each year (Taha et al., 1998). Thus, strong slope development with a large cuttings construction are requires for optimum shear strength of residual soils in order to obtain safe and economic design. The residual soil properties are varying in the single original rock although it has sameWeather condition. Townsend (1985) stated that residual soil is the result of chemical weathering and thus the characteristics of engineering residual soil depend on climatic factors, raw materials, topography, flow and age. These factors will tremendously influence the engineering characteristics of residual soil. The in situ behaviour of soils is complex because it is heavilydependent on many factors (Ahmed et al., 2006).
For that, it is necessary to analyse the factors through geotechnical engineering and other associated disciplines like geology, geomorphology, hydrogeology, climatology and other earth and atmosphere related sciences
The intensive investigations through many researches show that, the degree of weathering process and clay content has a significant influence on the engineering properties of the granite residual soils. The integration of the engineering properties information obtained that, the granite soil has similar properties to the same ground depth. However, these properties vary gradually at different depths depends on the pore-size distributions, which is vary in contrast with weathering process degree. The conducted studies confirmed that, a higher degree of weathering process would cause higher pore volume with larger range of pore-size distribution. These studies also showed that, the clay content has a large influence in determining granite residual soil properties. Thus, it can be concluded that residual soils properties are deeply affected by clay content percentage and they are a function of depth for various degree of weathering
3. EQUATIONS FOR SOIL WATER CHARACTERISTIC CURVE
D.G Fredlund and A.Xing, University of Saskatchewan, Canada,1994
A theoretical framework for unsaturated soil mechanics has been established over the past two decades. The constitutive equations for volume change, shear strength. and flow through unsaturated soil have become generally accepted in geotechnical engineering (Fredlund and Rahardjo 1993a). The measurement of soil parameters for the unsaturated soil constitutive models, however, remains a demanding laboratory process. For most practical problems, it has been found that approximate soil properties are adequate for analysis (Papagiannakis and Fredlund 1984). Hence, empirical procedures to estimate unsaturated soil-parameters would be valuable
Laboratory studies have shown that there is a relationship between the soil-water characteristic curve for a particular soil and the properties of the unsaturated soil (Fredlund and Rahardjo 1993b). For example, it has become an accept- able procedure to predict empirically the permeability func- tion for an unsaturated soil by using the saturated coeffi- cient of permeability and the soil-water characteristic curve (Marshall 1958; Mualem 1986; University of Saskatchewan 1984).Similar procedures have been suggested for the shear strength properties of an unsaturated soil (Fredlund and Rahardjo 1993b). Since the soil-water characteristic curve is used as the basis for the prediction of other unsaturated soil parameters, such as the permeability and shear-strength functions, it is important to have a reasonably accurate characterization of the soil-water characteristic curve.
General empirical equations have been proposed to describe the soil-water characteristic curve. Each equation has its own limitations. A general form of the relationship between water content and suction was developed based on the pore-size distribution of the soil.
If the pore-size distribution of a soil can be obtained or predicted, then the soil-water characteristic curve is uniquely determined from the proposed general equation
The analysis in this paper provides not only a theoretical basis for most of the empirical equations but also proposes a new, more general equation to describe the soil-water characteristic curve. Based on the proposed equation, a curve-fitting utility, was coded. It was found that the equation fits experimental data reasonably well over the entire suction range from 0 to 106 kpa
4.VOLUME CHANGE BEHAVIOR OF UNSATURATED RESIDUAL SOIL
Bujang B.K Huat, university putra Malaysia,2004
Residual soils occur in most countries of the world but the greater areas and depths are normally found in tropical humid areas. Most residual soils exhibit high soil suctions for most of the year. The absence of positive pore water pressure except immediately after rain makes conventional soil mechanics for saturated soil irrelevant. This research examines the volume change behavior of unsaturated residual soil under various levels of matric suction (ua - uw), and net mean stress (- Ua) in a predetermined stress path. The volume change of the soil is found to be sensitive to both the matric suction and net mean stress
Generally,the void ratio of the soil with matric suction is greater than the soil without matric suction due to the additional rigidity provided by the matric suction to the soil structure. On the other hand, the water content and degree of saturation of soil with matric suction was lower than the soil without the matric suction. It is also observed that there is an apparent unique relationship between the void ratio, matric suction and net mean stress of unsaturated residual soil when there is collapse (decrease in void ratio) due to the reduction in applied matric suction at constant net mean stress
From the results of this study, the following conclusions can be made with regard to the volume change behaviour of unsaturated residual soil subjected to various levels of net mean stress and matric suction. When the applied suction is first increased at constant net mean stress condition, the void ratio of the soil sample generally decreased. When the net mean stress is increased to higher levels at constant matric suction, the soil sample experience a further decrease in void ratio. There is an apparent unique relationship between the void ratio, matric suction and net mean stress.
The uniqueness in void ratio is observed when there is collapse (decrease in void ratio) due to the reduction in applied matric suction at constant net mean stress. This decrease (collapse) in void ratio is believed to be due to loss of additional rigidity provided by the suction to the soil structure as the suction is suddenly reduced.
At a similar net mean stress level, the void ratio, water content and degree of saturation of the soil sample subjected to matric suction of even as low as 25 kPa appear to have marked difference from soil sample without the matric suction. Generally, when the soil is at the similar level of net mean stress, the void ratio of the soil sample subjected to applied suction is found to be greater than the soil sample without the matric suction due to the additional rigidity provided by the matric suction to the soil structure.
On the other hand, the water content and degree of saturation of soil sample subjected to applied matric suction was lower than soil sample without the matric suction. When the net mean stress was increased, the water content of the soil sample generally decreased. The decrease in the water content of the soil sample without the matric suction appeared to be greater than the soil sample with the matric suction. The degree of saturation of the soil sample without the matric suction appears to remain unchanged with increase in the net mean stress level. But the degree of saturation of the soil sample with the matric suction increased with the increased in the applied net mean stress. There is an apparent approximate linear relation (normality) between the [e/eL]√Sr and log (σ – ua) in the unsaturated residual soil particularly at high net mean stress level
5. EFFECT OF COMPACTION ON THE UNSATURATED SHEAR STRENGTH
An experimental program was undertaken on compacted clay till at three different initial water contents to study the unsaturated shear strength under drained and undrained loading conditions. The three different initial moulding water contents (dry of optimum, optimum, and wet of optimum) induced differing soil structures to the compacted soil. The research study presented in this paper shows that the variation in suction in a soil that arises due to the influence of soil structure is a function of the initial compaction water content. The influence of soil structure on the drained and undrained shear strength parameters in unsaturated conditions is reflected through the shear strength contribution due to suction
Compacted fine-grained soils are commonly used in the construction of geotechnical and geo-environmental structures. Both drained and undrained shear strength analyses of compacted soils are of significance in dealing with these structures.
The shear strength behavior of compacted soils in an unsaturated condition is significantly influenced by properties such as the initial moulding water content, stress state and soil structure. For example, a fine-grained soil compacted at various initial water contents and to various densities produce “different” soils from a soil mechanics behavioural standpoint even though their mineralogy, plasticity, and texture are the same. The engineering behaviour will vary from one specimen to other due to differences in soil structure or aggregation which are related to the initial moulding water content. Limited information is available in the literature with respect to the influence of soil structure on the shear strength behaviour of unsaturated soils in drained and undrained loading.
An experimental program was undertaken on a compacted clay till to study the shear strength in drained and undrained loading. The initial water contents were chosen to represent dry of optimum, optimum, and wet of optimum conditions. The influence of soil structure on the drained and undrained shear strength parameters in unsaturated conditions and the contribution of shear strength due to suction is presented in this paper.
The influence of soil structure on the drained and undrained shear strength characteristics of statically compacted clay till with three different initial water contents representing dry of optimum, optimum, and optimum water contents have been presented and discussed in this paper. The drained and undrained shear strengths and the soil-water characteristics are different due to the varying soil structures in the specimens. The results of this study support the contention that the resulting soil structure in a compacted soil is a function of initial compaction water content. The influence of soil structure on the drained and undrained shear strength parameters in unsaturated conditions is reflected in shear strength contribution due to suction (i.e., φb values). In spite of varying soil structures resulting due to different initial water contents, there appears to be a unique relationship between tan φb/ tan φ' versus {(S-Sr)/(100-Sr)} or tan φb/ tan φ' versus Sκ for the clay till tested. More studies are required on different compacted soils to verify these relationships.
6. DETERMINATION OF THE SHEAR STRENGTH PARAMETERS OF AN UNSATURATED SOIL
D. G. Fredlund and H. Rahardj, University of Saskatchewan, Canada ,1998
Multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till. Aconventional direct shear apparatus was modified in order to use the axis-translation technique for direct shear tests on unsaturated soils. The soil can be subjected to a wide range of matric suctions. The testing procedure and some typical results are presented. Nonlinearity in the failure envelope with respect to matric suction was observed. Suggestions are made as to how best to handle the nonlinearity from a practical engineering standpoint.
Most man-made earth structures involve the use of compacted soils. The compaction process produces a soil with a degree of saturation usually in the range of 75 -90%. Earth fill dams, embankments, and highways are typical examples of earth structures made of compacted, unsaturated soils. The theory and measurement of shear strength of unsaturated soils have gained increasing attention during the past three decades. A brief review on the development of our understanding of the shear strength behaviour of unsaturated soils is presented in this paper. This paper presents the results of a series of direct shear tests on an unsaturated soil. Direct shear testing of unsaturated soils is desirable since less time is required to fail the soil specimen than when using the triaxial test. The time to failure in the direct shear test is greatly reduced because the specimen is relatively thin. The lengthy testing period for unsaturated soils is due to the low coefficient of permeability of the soil and the high air entry disc in contact with the specimen
The shear strength parameters of an unsaturated soil can be obtained using a direct shear apparatus, modified in order to apply matric suctions greater than 101 kPa (1 atm) to the soil specimen. The direct shear test uses a relatively thin specimenin comparison with the triaxial test, and can significantly reduce the time required for testing unsaturated soils of low permeability. The multistage direct shear test on Indian Head glacial till yields consistent results. It appears that the failure envelope for an unsaturated soil may be somewhat nonlinear with respect to the matric suction axis. When the soil remains saturated, the angle is approximately equal to the angle. As the soil desaturates at higher matric suctions, the angle appears to decrease to a relatively constant value. The nonlinearity in the failure envelope becomes noticeable when the soil is tested over a wide range of matric suctions
2.2 OBSERVATIONS FROM LITERATURE REVIEW
The above literatures have been reviewed and studied, from theabove literatures the method to calculate the suction controlled shear strength of cohesive and cohesionless soils. The above literatures shows us various methods to find the suction values, moisture content, shear strength and various other soil parameters.
The methods for predicting soil-water characteristic curve for cohesive and cohesionless soils are listed in the above literatures. These soil water characteristic curves are drawn relating moisture content and matric suction in soils by using those graphs the suction controlled shear strength of soils can be calculated