Soil Mechanics: Shear Strength
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Questions and Answers

What factor is significantly important for the shear strength of soils?

  • Grain size of soil particles
  • Temperature conditions
  • Type and conditions of test (correct)
  • Vegetation cover

    • Which type of soil relies on friction factors among particles for shear strength?

  • Clayey soil
  • Peaty soil
  • Loamy soil
  • Sandy soil (correct)

    • What is the term for the force developed in clayey soils due to particle bonding?

  • Cohesion force (correct)
  • Tension force
  • Capillary force
  • Adhesion force

    • Which of the following is NOT a component of friction resistance in soils?

    <p>Capillary action</p> Signup and view all the answers

    Which type of load can affect the shear strength of soils?

    <p>Static or dynamic loads</p> Signup and view all the answers

    What term describes the resistance created due to friction among particles under applied stress?

    <p>Friction resistance</p> Signup and view all the answers

    What are the shear strength components in soils commonly referred to as?

    <p>C &amp; φ</p> Signup and view all the answers

    Which of the following influences the shear strength of soils by changing the volume and sliding of soil particles?

    <p>Water content</p> Signup and view all the answers

    Which type of soil possesses a bonding among particles and provides cohesion?

    <p>Clayey soil</p> Signup and view all the answers

    What law describes the relationship between shear strength, friction, and cohesion in soils?

    <p>Mohr-Coulomb Law</p> Signup and view all the answers

    Which problem is directly related to the shear strength of soil?

    <p>Stability of slopes</p> Signup and view all the answers

    What is the primary concern in foundation design due to shear strength?

    <p>Ultimate bearing capacity</p> Signup and view all the answers

    Which of the following applications requires a thorough understanding of shear strength?

    <p>Earth dam construction</p> Signup and view all the answers

    What is the primary reason for analyzing shear strength in embankments?

    <p>To ensure slope stability</p> Signup and view all the answers

    Which of the following is NOT a factor affecting shear strength?

    <p>Thermal conductivity</p> Signup and view all the answers

    What is the term for the maximum shear stress that a soil can withstand?

    <p>Shear strength</p> Signup and view all the answers

    Which type of soil is more prone to landslides due to low shear strength?

    <p>Clayey soils</p> Signup and view all the answers

    What is the purpose of conducting triaxial tests on soils?

    <p>To measure shear strength</p> Signup and view all the answers

    Which of the following is a component of shear resistance in soils?

    <p>All of the above</p> Signup and view all the answers

    What is the primary advantage of using cohesionless soils in construction?

    <p>Easier to compact</p> Signup and view all the answers

    What type of stress acts on planes passing through a point in an element of mass that has been subjected to external loading?

    <p>Both normal and shear stress</p> Signup and view all the answers

    What is the direction of the normal stress σn?

    <p>Perpendicular to the plane</p> Signup and view all the answers

    What type of analysis is required to determine the stresses on different planes?

    <p>Mohr's circle analysis</p> Signup and view all the answers

    What is the primary concern in foundation design due to shear strength?

    <p>Bearing capacity of the soil</p> Signup and view all the answers

    What type of stress is responsible for the failure of soils?

    <p>Shear stress</p> Signup and view all the answers

    What is the term for the maximum shear stress that a soil can withstand?

    <p>Shear strength</p> Signup and view all the answers

    What type of test is used to determine the shear strength of soils?

    <p>Triaxial test</p> Signup and view all the answers

    What is the purpose of Mohr's envelope in soil mechanics?

    <p>To represent the relationship between shear stress and normal stress</p> Signup and view all the answers

    What type of soil is more prone to landslides due to low shear strength?

    <p>Cohesive soil</p> Signup and view all the answers

    What is the term for the resistance created due to friction among particles under applied stress?

    <p>Friction</p> Signup and view all the answers

    Study Notes

    Shear Strength of Soil

    • Shear strength of soil refers to its ability to resist deformation or failure due to external loading or imposed stress.
    • It is a critical characteristic that determines the soil's ability to support structures, foundations, and other loads.

    Importance of Shear Strength

    • Shear strength is crucial for determining the bearing capacity of soil for foundation design.
    • It is essential for calculating the ultimate bearing capacity of a soil for foundation design.
    • Shear strength also affects the lateral pressure against retaining walls, sheeting, or bracing.
    • It plays a vital role in the friction and stability of slopes, hills, slides, cuts, embankments, and earth dams.

    Shear Strength Development

    • Shear strength develops due to compression or tension forces, often due to compression.
    • When the stress applied on soil exceeds its shear strength, soil particles slide or move, causing a change in volume, leading to shear failure.

    Characteristics of Shear Strength

    • Shear strength is affected by soil density, with higher density leading to higher shear strength.
    • The line tangent to the Mohr's circle is called the Failure Envelope or Mohr envelope.

    Stress at a Point

    • A point in soil is subjected to stress in all directions, represented as a cube with three axes.
    • At any point in an element of mass, normal and shear stresses act on planes passing through that point.

    General State of Stress

    • There are nine components of stresses that define the state of stress at a point.
    • Stresses can be normal or parallel to a given plane, with normal stresses acting on planes at right angles.
    • The three principal planes have normal stresses, and the planes act on are called principal planes, where shear stresses equal zero.

    Normal and Shear Stress

    • On any plane passing through a soil mass, there are two stresses: σn (normal stress) and τ (shear stress).
    • The inclined angle of a plane with respect to the major principal plane is θ, and the inclined angle of the failure plane is θf.

    Mohr's Circle

    • Stresses (σ and τ) at a failure plane can be represented using Mohr's circle.
    • The characteristic obtained from Mohr's circle includes:
    • Maximum shear stress occurs at an angle of 45° with the major principal plane.
    • The failure envelop is tangent to Mohr's circle.
    • The relation between σ1 and σ3 at failure when C=0 is σ1/σ3 = (1+sin φ)/(1-sin φ).
    • The angle of failure plane θf is 45+φ/2.

    Mohr-Coulomb Theory

    • The relation between shear strength and friction and cohesion is described by the Mohr-Coulomb law: S = c + σ.tan φ.
    • For pure cohesive soil, φ = 0.
    • For pure cohesionless soil, C = 0, and τ = σn*tan φ.
    • For natural soil, C ≠ 0, φ ≠ 0, and τ = C + σn*tan φ.

    Factors Affecting Shear Strength of Soils

    • Factors affecting shear strength include water content, type and amount of loads, soil structure and disturbance effects, and the type and conditions of the test.

    Types of Soils with Respect to Shear Strength

    • Cohesive soils, like clayey soil, possess bonding among particles.
    • Cohesionless soils, like sandy soils, have only friction factors among particles.

    Components of Shear Resistance of Soils

    • Friction resistance, developed due to friction among particles, consists of friction due to sliding, rolling, and interlocking of particles.
    • Cohesion force, developed in clayey soils, is due to bonding among particles.

    Principal Stresses and Planes

    • σ1, σ2, and σ3 are principal stresses that act on major, intermediate, and minor principal planes, respectively, where shear stresses are zero.

    Biaxial Stresses in Soil

    • In soil study, biaxial stresses can be considered, as shown in the two-dimensional stress diagram.

    Mohr's Envelope

    • Mohr's Envelope represents the boundary between failure and stability states in soil.
    • All points below the failure envelope represent stress states less than the soil's bearing capacity.
    • Points on the line represent the maximum stress that the soil can withstand before failure.
    • No points exist above the failure envelope, as the soil would have already failed due to excessive stress.

    Total Stress and Pore Water Pressure

    • Omitting pore water pressure in design leads to over-estimation of bearing capacity, resulting in failure.
    • When the applied stress exceeds the soil's shear strength, soil particles slide, causing volume changes and possible displacement.

    Stress Components

    • σ represents normal stresses (stresses perpendicular to a plane).
    • τ represents shear stresses (stresses parallel to a given plane).
    • Nine components of stress define the state of stress at a point.

    Principal Planes

    • There are three planes at right angles where only normal stresses act.

    Applications of Shear Strength

    • Shear strength is crucial in solving problems related to:
      • Stability of slopes (e.g., hillsides, cuts, embankments, earth dams).
      • Ultimate bearing capacity of soil for foundation design.

    Stress Components

    • σ represents stresses normal to a plane (normal stress)
    • τ represents stresses parallel to a given plane (shear stress)

    State of Stress at a Point

    • Nine components of stresses define the state of stress at a point
    • Three planes at right angles where only normal stresses act
    • Principal stresses: σ major, σ intermediate, σ minor, acting on major, intermediate, and minor principal planes, where shear stresses = 0

    Biaxial Stresses in Soil Study

    • Can be considered as biaxial stresses acting on a soil element in two-dimensional space
    • σ1 and σ3 are the principal stresses acting on the soil element

    Mohr's Envelope

    • Represents the boundary between the failure and stability states of soil
    • Points below the failure envelope represent states of stress less than the soil's bearing capacity
    • Points on the failure envelope represent the maximum stress that the soil can withstand before failure
    • Points above the failure envelope do not exist, as the soil would have failed due to increased stress beyond its bearing capacity

    Principal Stresses and Planes

    • The stresses (σ1, σ2, σ3) are called principal stresses
    • The planes on which they act are called principal planes

    Stresses on a Soil Element

    • Both normal and shear stresses act on planes passing through a point in an element of mass subjected to external loading
    • Normal stress: σn

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