7.4 Factors affecting shear strength (drainage conditions, soil type, stress history)
4 min read•august 16, 2024
Shear strength is crucial in soil mechanics, determining how soil behaves under stress. This section explores key factors that influence shear strength: , , and . Understanding these factors is essential for accurate soil analysis and design.
Drainage conditions affect , impacting effective stress and shear strength. Soil type determines strength mechanisms, with cohesive and behaving differently. Stress history, particularly overconsolidation, shapes a soil's strength characteristics and response to loading.
Drainage Conditions and Shear Strength
Drainage and Pore Water Pressure
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- Drainage conditions determine water flow in soil pores during loading affects shear strength
- prevent water escape from soil pores during rapid loading leads to excess pore water pressure development
- allow excess pore water pressure dissipation results in effective stress changes and different shear strength behavior
- Loading rate relative to soil determines drained or undrained conditions prevail
- used for undrained conditions while effective stress analysis applies for drained conditions
- essential for understanding long-term soil behavior under different drainage conditions
- occur in intermediate loading rates require more complex analysis methods
Analysis Methods for Drainage Conditions
- conducted to simulate different drainage conditions (consolidated-drained, consolidated-undrained, unconsolidated-undrained)
- (Skempton's A and B) used to quantify pore pressure response in undrained conditions
- () applied to analyze drained behavior
- (Terzaghi's theory) used to estimate time for pore pressure dissipation
- employed to visualize stress changes during loading under different drainage conditions
- (finite element analysis) utilized for complex drainage scenarios in geotechnical problems
Cohesive vs. Cohesionless Soil Shear Strength
Shear Strength Mechanisms
- (clays) derive shear strength from friction and while cohesionless soils (sands) rely primarily on friction
- describes shear strength with cohesion (c) and (φ) as key parameters
- Cohesionless soils exhibit higher permeability allows rapid drainage and predominantly drained behavior
- Cohesive soils have lower permeability often results in undrained behavior under short-term loading conditions
- differs cohesive soils typically show more ductile behavior and cohesionless soils exhibit brittle failure
- for dense granular materials to expand during shear more pronounced in cohesionless soils
- Critical state concept particularly important for cohesive soils defines ultimate condition where shearing occurs at constant volume and effective stress
Soil Classification and Testing
- used to categorize soils based on grain size distribution and
- (liquid limit, plastic limit, plasticity index) determine cohesive soil behavior
- commonly used for cohesionless soils measures friction angle
- Triaxial tests performed on both cohesive and cohesionless soils provide comprehensive strength parameters
- conducted on cohesive soils estimates undrained shear strength
- utilized for in-situ measurement of cohesive soil strength
- employed for both cohesive and cohesionless soils correlates with shear strength parameters
Overconsolidation Ratio Impact on Shear Strength
OCR and Soil Behavior
- defined as ratio of maximum past effective stress to current effective stress
- (OCR = 1) have different shear strength characteristics compared to (OCR > 1)
- Overconsolidation affects cohesion intercept and friction angle in Mohr-Coulomb failure criterion
- in e-log p' space influenced by stress history affects ultimate shear strength of soil
- Overconsolidated clays typically exhibit higher undrained shear strength and tend to dilate during shearing
- Stress-strain behavior of overconsolidated soils generally stiffer and more brittle compared to normally consolidated soils
- Stress history affects pore pressure response during undrained loading overconsolidated soils potentially develop negative excess pore pressures
OCR Determination and Applications
- used to determine preconsolidation pressure and calculate OCR
- In-situ tests (CPT, pressuremeter) correlated with OCR for different soil types
- SHANSEP (Stress History and Normalized Soil Engineering Properties) method applied to estimate undrained shear strength based on OCR
- Recompression index (Cr) and influenced by OCR affect settlement calculations
- OCR consideration crucial in slope stability analysis affects choice of strength parameters
- Foundation design accounts for OCR in bearing capacity and settlement estimations
- vary with OCR impact retaining wall design
Factors Affecting Soil Shear Strength
Soil Structure and Composition
- including fabric and bonding between particles significantly influences shear strength especially in natural clay deposits
- both natural (calcium carbonate) and artificial (cement, lime) can increase cohesion and alter stress-strain behavior of soils
- resulting from depositional processes or stress history leads to directional variations in shear strength
- Presence of (peat, organic clay) can reduce shear strength and increase compressibility of soils
- (kaolinite, illite, montmorillonite) affects interparticle forces and shear strength characteristics
- influence friction angle and behavior in granular soils
- and control shear strength of cohesionless soils denser soils exhibit higher strength
Environmental and Time-Dependent Factors
- affect soil strength through thermal expansion changes in pore water pressure and alterations to clay mineral properties
- such as thixotropy in clays lead to time-dependent changes in soil strength
- including pH and pore fluid composition alter interparticle forces and affect shear strength particularly in clay soils
- (earthquakes, machine vibrations) can cause strength degradation and liquefaction in susceptible soils
- alter soil structure and strength properties in cold regions
- (plant roots, burrowing animals) modifies soil structure and affects shear strength
- (physical, chemical) gradually change soil composition and strength characteristics over time
Key Terms to Review (61)
Aging effects: Aging effects refer to the changes in physical and mechanical properties of soil over time due to various environmental and stress conditions. These changes can influence how soil behaves under load, affecting its strength and stability, which is crucial when assessing its shear strength related to drainage conditions, soil type, and stress history.
Atterberg Limits: Atterberg limits are a set of tests used to determine the plasticity characteristics of fine-grained soils, specifically the liquid limit and plastic limit. These limits help in understanding how water affects soil behavior, providing essential insights into its consistency, workability, and classification, which are crucial for engineering applications.
Biological activity: Biological activity refers to the processes and interactions of living organisms, such as plants, microorganisms, and animals, within a soil environment that influence its characteristics and behavior. This activity plays a crucial role in shaping soil properties, including its shear strength, by affecting factors like drainage conditions, soil composition, and the history of stress applied to the soil.
Cementation: Cementation is the process where minerals precipitate from water and fill the spaces between sediment grains, binding them together and forming a solid rock. This process significantly influences the mechanical properties of soil and rock, particularly in how they respond to stress and shear forces, which can be affected by drainage conditions, soil types, and stress history.
Chemical factors: Chemical factors refer to the various chemical properties and reactions of soil constituents that influence soil behavior, particularly its shear strength. These factors play a significant role in determining how soil interacts with water, the presence of ions, and the overall chemical environment, which can all impact drainage conditions, soil type, and stress history.
Clay Mineralogy: Clay mineralogy refers to the study of the composition, structure, and properties of clay minerals, which are fine-grained natural soils primarily composed of phyllosilicates. These minerals play a critical role in determining the engineering properties of soils, especially in how they respond to different drainage conditions, soil types, and stress history.
Cohesion: Cohesion is the property of soil that describes the attraction between soil particles, which contributes to the soil's strength and stability. This internal binding force is essential in understanding how soil behaves under different conditions, including how it interacts with moisture, external loads, and other forces acting on it.
Cohesionless soils: Cohesionless soils are types of soils that do not exhibit any significant cohesion between their particles, primarily relying on friction to resist shear stress. These soils, such as sand and gravel, are typically granular in nature and behave differently under varying conditions like drainage, load stress, and historical stress paths. Understanding cohesionless soils is essential when analyzing factors affecting shear strength and evaluating the bearing capacity of foundations.
Cohesive Soils: Cohesive soils are types of soils that exhibit significant cohesion due to the presence of fine particles, such as clay, which have a strong tendency to stick together. This property makes cohesive soils critical in understanding engineering principles, as they affect the behavior of structures built on or within them.
Compression index (cc): The compression index (cc) is a measure of the compressibility of a soil, defined as the change in void ratio per unit change in effective stress during the consolidation process. It plays a significant role in understanding how soil behaves under loading conditions and is influenced by factors such as drainage conditions, soil type, and stress history. Understanding cc helps engineers predict settlement and stability of structures built on or within different soil types.
Cone penetration test (CPT): The cone penetration test (CPT) is a field test used to determine the geotechnical properties of soil by pushing a cone-shaped probe into the ground at a constant rate. This test provides continuous data on soil resistance, allowing for an understanding of various soil characteristics, which is essential for assessing shear strength and evaluating soil liquefaction potential in different drainage conditions, soil types, and stress histories.
Consolidation: Consolidation refers to the process by which soil decreases in volume over time due to the expulsion of water from its pores under sustained load. This process is critical in understanding how soils behave under load and is closely linked to factors such as soil-water interaction, effective stress, and drainage conditions.
Consolidation Theory: Consolidation theory is a fundamental concept in geotechnical engineering that describes the process by which soil decreases in volume over time due to applied loads, primarily from overlying structures. This process involves the expulsion of water from the soil's voids and is crucial for understanding how foundations settle and how soil strength is affected under different conditions.
Critical State Concept: The critical state concept is a theoretical framework in soil mechanics that describes the behavior of soil at the point where it transitions from a consolidated, stable state to an unstable, flow-like state. This concept is crucial for understanding the shear strength of soil under various conditions, particularly focusing on how factors like drainage conditions, soil type, and stress history influence this transition and the overall stability of soil structures.
Critical State Line: The critical state line is a fundamental concept in soil mechanics that defines the boundary between different stress states of a soil, particularly in relation to its shear strength and volumetric behavior. It represents a condition where soil can undergo continuous deformation without a change in volume or effective stress, signifying the transition between stable and unstable states. Understanding this line helps to analyze how various factors such as drainage conditions, soil type, and stress history influence the shear strength of soils.
Cyclic loading: Cyclic loading refers to the repeated application of loads on a material or structure over time, which can cause fatigue and potential failure. This type of loading is critical in understanding how soils behave under varying stress conditions, especially when assessing their shear strength and susceptibility to phenomena like liquefaction. The effects of cyclic loading can vary based on factors such as drainage conditions, soil type, and the soil's stress history, making it essential to consider when designing structures in geotechnical engineering.
Dilatancy: Dilatancy is the property of certain materials, especially granular soils, that causes them to increase in volume when subjected to shear stress. This phenomenon occurs when the soil particles rearrange themselves during deformation, leading to an increase in void space. Dilatancy can significantly affect the behavior of soils under loading and influences their settlement characteristics and shear strength, particularly in different drainage conditions and with varying soil types.
Dilatancy tendency: Dilatancy tendency refers to the behavior of certain materials, particularly granular soils, where an increase in volume occurs when subjected to shear stress. This property is significant in understanding how soil responds under different conditions, especially in relation to factors like drainage conditions, soil type, and stress history. When materials exhibit dilatancy, they may initially resist deformation but then become more unstable under sustained loads, affecting their shear strength characteristics.
Direct Shear Test: The direct shear test is a laboratory procedure used to measure the shear strength of soil by applying a horizontal shear force along a predetermined plane. This test helps in understanding how different soil types behave under various loading conditions, and it plays a vital role in evaluating the shear strength parameters that influence stability in geotechnical engineering. The results from the direct shear test can be compared to other tests like triaxial and unconfined compression tests to provide a comprehensive view of soil behavior.
Drainage conditions: Drainage conditions refer to the status of water movement through soil and the surrounding environment, significantly influencing the behavior of soil under applied stress. Proper drainage conditions can affect the effective stress, shear strength, and overall stability of soil structures, as they determine the amount of pore water pressure and its contribution to soil mechanics. Understanding drainage conditions is essential in evaluating shear strength, as it interacts with factors such as soil type and stress history.
Drained conditions: Drained conditions refer to a state in which soil is able to freely allow water to flow through it, leading to effective stress being solely a function of the applied external loads. In this scenario, excess pore water pressure dissipates quickly, allowing soil particles to interact directly under the influence of these loads. This concept is crucial in understanding shear strength in soils and how different laboratory tests, like direct shear and triaxial tests, behave under varying moisture conditions.
Effective Stress Principle: The effective stress principle states that the strength and behavior of soil are determined by the effective stress, which is the total stress minus pore water pressure. This concept is crucial in understanding how soils respond to loads, as it impacts consolidation, shear strength, and overall stability in geotechnical engineering.
Freeze-thaw cycles: Freeze-thaw cycles refer to the repeated process of water freezing and expanding in soil during cold temperatures, followed by thawing when temperatures rise. This natural phenomenon significantly impacts soil behavior, particularly its shear strength, by altering soil structure and drainage conditions. The expansion and contraction can lead to increased pore pressure and changes in effective stress, affecting how different soil types respond to loading over time.
Friction angle: The friction angle is a measure of the internal resistance of soil to shear stress, represented by the angle at which soil particles can slide past one another. This angle is crucial for understanding how soils respond to external loads, and it plays a vital role in determining the shear strength of soils in various conditions, such as drained and undrained states.
Lateral Earth Pressure Coefficients (k0, ka, kp): Lateral earth pressure coefficients are ratios used to quantify the horizontal pressures exerted by soil on retaining structures and other geotechnical elements. These coefficients include at-rest pressure (k0), active pressure (ka), and passive pressure (kp), and they are influenced by factors like drainage conditions, soil type, and stress history, which all play critical roles in determining the stability and behavior of soil in different scenarios.
Mohr-Coulomb Failure Criterion: The Mohr-Coulomb failure criterion is a mathematical model that describes the shear strength of soil and other materials based on their internal friction and cohesion. This criterion helps engineers predict when materials will fail under stress by relating shear strength to normal stress through a linear relationship defined by the cohesion intercept and the angle of internal friction.
Normally consolidated soils: Normally consolidated soils are soils that have been subjected to a stress level equal to their overburden pressure, meaning they have not been previously loaded or compacted beyond this level. This condition affects their mechanical properties, including shear strength, which is influenced by drainage conditions, soil type, and stress history.
Numerical modeling: Numerical modeling is a computational technique used to simulate physical phenomena through mathematical representations, allowing for the analysis of complex systems. This approach is particularly valuable in understanding the behavior of materials and structures under various conditions, such as different drainage scenarios, soil types, and stress histories. It enables engineers and scientists to predict outcomes and optimize designs without needing extensive physical experiments.
Oedometer Test: The oedometer test is a laboratory procedure used to assess the consolidation properties of soil by measuring its deformation under a controlled load over time. This test provides crucial insights into how soil behaves under stress, especially in relation to consolidation theory and its implications for settlement analysis and foundation design.
Organic matter: Organic matter refers to the decomposed remains of plants and animals, including their byproducts, that are found in soil. It plays a crucial role in enhancing soil structure, fertility, and overall health, influencing key properties such as moisture retention and nutrient availability, which are vital for the shear strength of soils under varying conditions.
Overconsolidated Soils: Overconsolidated soils are those that have been subjected to a higher maximum past vertical effective stress than the current vertical effective stress. This condition typically occurs due to historical loading events, such as glacial periods or previous construction activities, which compress the soil beyond its current load. Understanding overconsolidation is essential as it significantly influences shear strength, depending on drainage conditions and soil types, and is closely tied to the stress history of the soil.
Overconsolidation Ratio (OCR): The overconsolidation ratio (OCR) is a measure used in geotechnical engineering to express the extent to which a soil has been subjected to greater past vertical stress than its current effective stress. It is defined as the ratio of the maximum past effective stress to the current effective stress, indicating whether a soil is normally consolidated (OCR = 1) or overconsolidated (OCR > 1). This concept is critical in understanding how drainage conditions, soil type, and stress history impact shear strength.
Partially drained conditions: Partially drained conditions occur when the pore water pressure in soil changes during loading, but there is not enough time for complete drainage to happen. This situation affects the effective stress and shear strength of the soil, significantly influenced by factors like drainage conditions, soil type, and stress history. Understanding these conditions is crucial for predicting how soil behaves under load, particularly during construction and in geotechnical engineering.
Particle Shape and Size Distribution: Particle shape and size distribution refers to the way in which soil particles vary in size and the geometric form of those particles. This term is important because it influences the physical properties of the soil, affecting its density, porosity, and permeability. Understanding particle shape and size distribution helps in predicting how soil behaves under different conditions, especially concerning its shear strength, which is vital in geotechnical applications.
Particle Size Distribution: Particle size distribution refers to the variation in the size of particles within a given soil sample, typically expressed as a percentage of the total mass of soil within specific size ranges. Understanding this distribution is crucial, as it directly influences various soil properties, such as permeability, compaction, and shear strength, which are essential for evaluating soil behavior under different conditions.
Permeability: Permeability is the ability of a material, such as soil, to transmit fluids through its pores or voids. This characteristic is essential in understanding how water interacts with soil and affects various engineering applications, from construction to environmental management.
Plasticity: Plasticity refers to the ability of a soil to deform permanently without breaking when subjected to stress or changes in moisture content. This characteristic is vital in understanding soil behavior, particularly in how soils respond to loading conditions, moisture fluctuations, and structural stability.
Pore Pressure Parameters: Pore pressure parameters refer to the pressures exerted by fluids within the void spaces of soil or rock, which significantly influence the behavior and strength of these materials under stress. Understanding pore pressure is crucial because it directly affects shear strength, stability, and drainage conditions in geotechnical engineering. The relationships between pore pressure, soil type, and stress history are essential for predicting how soil will perform under various loading scenarios.
Pore Water Pressure: Pore water pressure refers to the pressure exerted by water within the soil's pore spaces, influencing the behavior of soil under stress. It plays a critical role in various geotechnical processes, affecting how soil interacts with water, its effective stress, and ultimately its stability and strength under different loading conditions.
Recompression index (c_r): The recompression index (c_r) is a measure of the change in volume of saturated cohesive soils when they are subjected to unloading followed by reloading, reflecting their compressibility behavior. This index is crucial for understanding how soils respond to stress changes, particularly in terms of shear strength, which is influenced by factors like drainage conditions, soil type, and stress history. The recompression index helps predict how soil will behave under various loading conditions, impacting foundation design and stability assessments.
Relative Density: Relative density is a measure of the density of a soil relative to the maximum and minimum densities it can achieve, often expressed as a percentage. It helps in understanding the compaction state of granular soils and is vital for evaluating their engineering properties and behavior under loading conditions.
Saturation: Saturation refers to the condition in which all the void spaces within a soil are filled with water. This concept is critical as it influences the behavior of soil, especially its shear strength and stability. Understanding saturation is essential for evaluating how soil will react under different conditions, particularly in terms of slope stability and the factors affecting its strength.
Shansep Method: The Shansep Method is a technique used to evaluate and improve the shear strength of soil by analyzing factors like drainage conditions, soil type, and stress history. This method helps in understanding how these elements interact to affect the overall stability and behavior of soil under load. It combines empirical data with theoretical principles to predict how soils will perform in various conditions, making it crucial for effective geotechnical engineering.
Shear Strength Mechanisms: Shear strength mechanisms refer to the processes and factors that determine the ability of soil to resist sliding or shearing under applied loads. This concept is essential in understanding how various conditions such as drainage, soil types, and stress history impact a soil's stability and performance in engineering applications.
Soil Anisotropy: Soil anisotropy refers to the directional dependence of soil properties, meaning that soil strength, compressibility, and permeability can vary based on the direction of loading or flow. This characteristic is essential in understanding how different factors, such as drainage conditions, soil type, and stress history, influence the behavior of soil under various conditions. Anisotropy highlights that soils are not uniform, and their responses can significantly differ when subjected to forces or stresses from different angles.
Soil moisture content: Soil moisture content refers to the amount of water held in the soil, usually expressed as a percentage of the dry weight of the soil. This measurement is critical because it directly influences various soil properties, including shear strength, which is vital for understanding how soils behave under stress. The moisture content affects how different soil types interact with water, the drainage conditions they experience, and how their stress history impacts their current physical state.
Soil Structure: Soil structure refers to the arrangement of soil particles and the way they bind together to form aggregates. This arrangement impacts important soil properties, such as porosity, permeability, and overall soil stability, which are crucial for understanding how soil behaves under different conditions. The structure is influenced by factors like soil formation processes and environmental conditions, which in turn affect its physical behavior and engineering properties.
Soil Type: Soil type refers to the classification of soil based on its physical and chemical properties, including grain size, texture, structure, and mineral content. Understanding soil types is crucial because it directly influences construction practices, the behavior of soil under load, and how soils respond to environmental changes such as drainage and compaction.
Stress History: Stress history refers to the previous stress states that a soil has experienced over time, including the magnitude and duration of these stresses. This concept is crucial because it influences the soil's current behavior and strength, affecting how it responds under different loading conditions, which ties into aspects like drainage conditions and soil types.
Stress path method: The stress path method is a graphical and analytical approach used to evaluate the behavior of soil under different loading conditions by plotting changes in stress and pore water pressure. This method is crucial for understanding how factors like drainage conditions, soil type, and stress history affect the shear strength of soils, making it essential for geotechnical engineering applications.
Stress-strain behavior: Stress-strain behavior describes how materials deform when subjected to external forces. In geotechnical engineering, this behavior is crucial as it illustrates the relationship between stress (force per unit area) and strain (deformation resulting from stress) in soils, which directly affects their shear strength. Understanding this relationship helps in evaluating how different soil types respond under varying conditions, particularly during laboratory tests that assess shear strength and how these factors interact with drainage conditions and stress history.
Temperature changes: Temperature changes refer to the variations in thermal conditions that soil experiences, which can significantly influence its physical and chemical properties. These changes can affect moisture content, soil structure, and ultimately impact the shear strength of the soil, which is crucial for understanding its behavior under load. As temperature fluctuates, it can alter drainage conditions, affect the type of soil through thermal expansion or contraction, and influence stress history as soils undergo different conditions over time.
Total Stress Analysis: Total stress analysis refers to the assessment of the overall stress acting on a soil mass, considering both the applied loads and the weight of the soil itself. This analysis is crucial in understanding how various factors, such as drainage conditions, soil type, and stress history, affect the shear strength of soil, which in turn influences its stability and behavior under different loading conditions.
Triaxial test: The triaxial test is a laboratory procedure used to determine the shear strength of soil by applying controlled stress conditions. It involves encasing a soil sample in a rubber membrane and subjecting it to various levels of confining pressure while applying axial stress until failure occurs. This method helps in understanding the behavior of soil under different drainage conditions and is crucial for evaluating soil stability and consolidation.
Triaxial tests: Triaxial tests are laboratory experiments designed to measure the mechanical properties of soil, particularly its shear strength, under controlled conditions of pressure and drainage. These tests help engineers understand how different factors, such as soil type, drainage conditions, and stress history, influence the behavior of soil when subjected to various loading scenarios, which is crucial for geotechnical engineering applications.
Unconfined Compression Test: The unconfined compression test is a laboratory procedure used to determine the compressive strength of cohesive soil without any lateral confinement. In this test, a cylindrical soil sample is subjected to axial loading until failure occurs, allowing for the assessment of its shear strength characteristics. This method is particularly significant when understanding how soil interacts with water and its phase relationships, as well as evaluating shear strength under varying conditions such as drainage, soil type, and stress history.
Undrained conditions: Undrained conditions refer to a situation in which the pore water within a soil does not have time to escape during loading or deformation, meaning the effective stress remains unchanged. This condition is significant in understanding how saturated soils behave under short-term loading situations, particularly when drainage cannot occur quickly enough to equalize pore pressures. Undrained behavior is critical for analyzing shear strength in saturated soils, particularly when considering how water content and drainage conditions affect soil stability.
Unified Soil Classification System (USCS): The Unified Soil Classification System (USCS) is a widely used method for categorizing soils based on their grain size, plasticity, and consistency. It helps engineers and geologists to classify soil types for various engineering applications, facilitating communication and understanding about soil behavior under different conditions. The USCS incorporates factors like particle size distribution, plasticity index, and moisture content, which are crucial in assessing soil properties relevant to construction and land use.
Vane Shear Test: The vane shear test is an in-situ test used to determine the undrained shear strength of saturated cohesive soils. This test involves inserting a vane into the soil and rotating it to measure the torque required to cause failure. Understanding the results of this test helps assess how drainage conditions, soil type, and stress history affect the shear strength of soils.
Void Ratio: The void ratio is a fundamental soil property defined as the ratio of the volume of voids (spaces between soil particles) to the volume of solid particles in a soil sample. This term is crucial for understanding soil behavior, including how water interacts with soil, its compaction characteristics, and its strength under different conditions.
Weathering Processes: Weathering processes refer to the physical, chemical, and biological breakdown of rocks and minerals at or near the Earth's surface. These processes play a crucial role in soil formation and the evolution of landscapes, significantly influencing factors like drainage conditions, soil type, and stress history which ultimately affect shear strength in geotechnical contexts.