Questions and Answers
Which factor is NOT mentioned as affecting the shear strength of soils?
What type of soil is characterized by only friction factors among particles?
Which type of friction is NOT a component of friction resistance in soils?
Which component of soil shear resistance is found only in clayey soils?
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According to the content, what effect does water content have on the shear strength of soils?
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What are the two primary components of shear strength in soils?
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Which type of loads are mentioned as affecting the shear strength of soils?
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Which term is used to describe soils like sandy soils in terms of shear strength?
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What does the Mohr-Coulomb law relate to?
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Friction resistance in soils includes resistance developed from all EXCEPT:
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What is the weight of the hammer used in the Standard Proctor Test?
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What is the drop height of the hammer in the Standard Proctor Test?
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What is the formula to calculate the compactive effort in the Standard Proctor Test?
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What is the volume of the mold used in the Standard Proctor Test?
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How many blows per layer are applied in the Standard Proctor Test?
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What is the mold size used in both Standard Proctor Test and Modified Proctor Test?
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How many times is the soil compacted in each lift?
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In the Modified Proctor Test, what is the weight of the hammer used?
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How many lifts are used in the Modified Proctor Test?
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What is the energy calculated in the Standard Proctor Test?
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How many layers are compacted in the Modified Proctor Test?
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What is the purpose of compacting the soil multiple times from dry to wet?
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What is the name of the test procedure described in the content?
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What is the drop height used in the Modified Proctor Test?
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When was the Modified Proctor Test developed?
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What is the equation used to calculate points from the ZAV curve?
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What is the name of the equipment used to determine the maximum dry density of a soil?
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What is the effect of entrapped air on the dry density of a soil?
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How is the compactive effort calculated?
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What is the purpose of the compaction curve?
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What is the primary objective of soil compaction?
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What is the term for the process of increasing soil unit weight by reducing the soil void ratio?
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What is the result of entrapped air in soil compaction?
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What is the purpose of laboratory work in soil compaction?
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Who is responsible for field compaction?
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What is the purpose of checking compaction execution in the field?
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What is the goal of achieving maximum dry density in soil compaction?
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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.
Soil Compaction
- Objective: increase density, strength, bearing capacity, and reduce permeability and compressibility
- Methods: laboratory work and field compaction using rollers and compactors
- Importance: check compaction execution in the field by several methods
The Standard Proctor Test
- Developed by Proctor in 1933
- Equipments:
- Hammer with 5.5 lb weight
- Mold with 1/30 ft³ volume, 4 in diameter, and 4.584 in height
- Procedure:
- Soil is compacted in three layers with 25 blows per layer
- Hammer drops from a height of 12 inches
- Compactive effort calculated as 12,375 ft-lb/ft³
- Standard Energy: 12,375 ft-lb/ft³
Modified Proctor Test
- Developed during World War II
- Modifications:
- Hammer weight: 10 lb
- Drop height: 18 inches
- Number of layers: 5
- Compactive effort: 56,250 ft-lb/ft³
- Modified Energy: 56,250 ft-lb/ft³
Comparison of Standard and Modified Proctor Tests
- Mold size: 1/30 ft³ (same for both)
- Height of drop: 12 inches (Standard), 18 inches (Modified)
- Hammer weight: 5.5 lb (Standard), 10 lb (Modified)
- Number of layers: 3 (Standard), 5 (Modified)
- Blows per layer: 25 (same for both)
- Energy: 12,375 ft-lb/ft³ (Standard), 56,250 ft-lb/ft³ (Modified)
Compaction Procedure in Lab
- Soil is divided into three lifts
- Each lift is compacted 25 times
- Procedure is repeated 4-6 times from dry to wet
- Objective: to obtain maximum dry density and optimum water content
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Description
Understand the importance of shear strength in determining the bearing capacity of soil for foundation design and its role in preventing structural failure.
