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Questions and Answers
What characterizes local shear failure in shallow foundations?
What characterizes local shear failure in shallow foundations?
Which theory of bearing capacity incorporates factors for shape, depth, and inclination?
Which theory of bearing capacity incorporates factors for shape, depth, and inclination?
In Terzaghi's Theory, what does the term $qN_q$ represent?
In Terzaghi's Theory, what does the term $qN_q$ represent?
Which failure type is associated with overloading conditions?
Which failure type is associated with overloading conditions?
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What is considered when using Skempton’s Method?
What is considered when using Skempton’s Method?
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What does the term $0.5 imes eta imes B imes N_eta$ represent in Terzaghi’s bearing capacity equation?
What does the term $0.5 imes eta imes B imes N_eta$ represent in Terzaghi’s bearing capacity equation?
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Which foundation design aspect is primarily influenced by the understanding of shear failure?
Which foundation design aspect is primarily influenced by the understanding of shear failure?
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What does the BIS Method emphasize when calculating bearing capacity?
What does the BIS Method emphasize when calculating bearing capacity?
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Which of the following is NOT a feature of general shear failure?
Which of the following is NOT a feature of general shear failure?
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What does the SPT N-value primarily indicate?
What does the SPT N-value primarily indicate?
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Which type of foundation is specifically designed to reduce pressure on weak soils?
Which type of foundation is specifically designed to reduce pressure on weak soils?
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What does the Plate Load Test measure?
What does the Plate Load Test measure?
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What is a primary advantage of using a Floating Foundation?
What is a primary advantage of using a Floating Foundation?
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Which test involves pushing a cone into the soil to measure resistance?
Which test involves pushing a cone into the soil to measure resistance?
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What does the Contact Pressure Distribution Diagram illustrate?
What does the Contact Pressure Distribution Diagram illustrate?
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Which of the following methods provides direct measurement of soil properties?
Which of the following methods provides direct measurement of soil properties?
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Which coefficient is essential in estimating bearing capacity based on SPT values?
Which coefficient is essential in estimating bearing capacity based on SPT values?
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What is the main purpose of the Pressure Meter Test?
What is the main purpose of the Pressure Meter Test?
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What is a critical factor for engineers when designing foundations?
What is a critical factor for engineers when designing foundations?
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What does Rankine's theory primarily address in the context of retaining walls?
What does Rankine's theory primarily address in the context of retaining walls?
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Which factor is considered to influence earth pressure in retaining wall design?
Which factor is considered to influence earth pressure in retaining wall design?
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What is a major purpose of a cofferdam in construction?
What is a major purpose of a cofferdam in construction?
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Culmann’s graphical method is primarily used for determining:
Culmann’s graphical method is primarily used for determining:
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Which class of structures utilizes sheet piles primarily?
Which class of structures utilizes sheet piles primarily?
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Study Notes
Bearing Capacity of Shallow Foundations
- The bearing capacity of a shallow foundation is its ability to support a structure's load without failure.
Shear Failure Concepts
- Local Shear Failure: Occurs in a limited area, characterized by soil sliding along a failure plane, leading to localized settlement or tilting.
- General Shear Failure: A wide failure zone develops, leading to significant soil displacement and usually associated with overloading.
Theories of Bearing Capacity
-
Terzaghi's Theory: Based on shear failure, the bearing capacity ( q_u ) is calculated with the following formula:
[
q_u = cN_c + qN_q + 0.5 \gamma BN_\gamma
]
where:
- ( c ) = soil cohesion
- ( q ) = effective overburden pressure
- ( \gamma ) = soil unit weight
- ( B ) = footing width
- ( N_c, N_q, N_\gamma ) = bearing capacity factors dependent on the angle of internal friction (( \phi )).
- Skempton's Method: Extends Terzaghi's work, incorporating factors for varying soil conditions, particularly useful for clay soils.
- Meyerhof's Method: Includes shape, depth, and inclination factors for more refined calculations based on footing shapes and embedment depths.
- BIS Method: Bureau of Indian Standards (BIS) provides guidelines (IS 6403), incorporating local conditions and empirical data.
Bearing Capacity of Granular Soils
- Estimated using Standard Penetration Test (SPT) values, where the N-value indicates soil density and strength.
- Empirical Relationship: ( q_u = (N \times C) + \text{other terms} ), where ( C ) is a coefficient determined based on SPT correlations.
Types of Foundations
- Raft Foundation: A large concrete slab supporting multiple columns and walls, spreading the load over a wider area for weak soil conditions.
- Floating Foundation: Counteracts settlement by balancing the structure's weight with displaced soil weight, useful for poor soil conditions.
In Situ Methods of Evaluation of Bearing Capacity
- Plate Load Test: A rigid plate is placed on the soil and loads are applied to measure settlement, providing an estimate of bearing capacity, particularly useful for site-specific conditions.
- Static Cone Penetrometer Test: Measures soil resistance to penetration by pushing a cone into the soil, correlating with soil strength and used to estimate bearing capacity.
- Pressure Meter Test: Measures the soil's in-situ pressure response to applied loads to determine deformation characteristics and bearing capacity.
- Contact Pressure Distribution Diagram: Illustrates the contact pressure distribution below the footing base, visualizing load transmission through the soil, aiding in design decisions.
Conclusion
Unit II provides a foundation for understanding shallow foundation bearing capacity, including shear failure concepts, various theories, and in situ testing methods. This knowledge is crucial for engineers to design stable and long-lasting structures.
Earth Pressure at Rest
- The pressure exerted by soil on a retaining wall when the wall is at rest, meaning no movement.
- Calculated using the coefficient of earth pressure at rest, K0, representing the ratio of horizontal to vertical stress.
- K0 generally lies between 0.4 and 0.5, depending on soil properties.
Plastic Equilibrium
- Soil reaching a state of plastic equilibrium when subjected to external forces, leading to permanent deformation.
- This state is crucial for analyzing active and passive earth pressures.
Rankine's Theory
- A theoretical approach to calculate active earth pressure, occurring when a retaining wall moves away from the soil mass.
- Formula: Pa = γh^2 * Ka, where Pa is active pressure, γ is soil unit weight, h is wall height, Ka is active earth pressure coefficient.
- Ka is affected by the soil's friction angle (φ) and is always less than 1.
Coulomb's Theory
- Similar to Rankine’s, but accounts for the wall's inclination and friction between soil and wall.
- Provides a more accurate estimate of active pressure, particularly for inclined walls.
Passive Earth Pressure
- The resistance offered by soil to the retaining wall when the wall moves towards it.
- Passive earth pressure is significantly higher than active earth pressure.
- Formula: Pp = γh^2 * Kp, where Pp is passive pressure, γ is soil unit weight, h is wall height, Kp is passive earth pressure coefficient.
- Kp is influenced by the soil's friction angle (φ) and is always greater than 1.
Influence of Surcharge
- Surcharge refers to additional loads applied on the soil surface above the wall.
- Significantly increases the active and passive earth pressures acting on the retaining wall.
Influence of Water Table
- The water table's presence affects both the weight of the soil and its internal stresses.
- Resulting in higher earth pressures compared to dry conditions.
Rebhann's and Culmann's Graphical Methods
- Simple methods for graphically determining active and passive earth pressures.
- Use a graphical approach, involving plotting the earth pressure envelope and determining its intersection with the wall.
Sheet Piles
- Interlocking steel sections used to construct retaining walls, especially in situations involving high ground water pressure.
- Available in various forms, including timber, steel, and precast concrete.
Bulkheads
- Similar to sheet piles, but typically used for more extensive applications, requiring higher strength and durability.
- Employed in waterfront projects, coastal structures, and other demanding environments.
Cofferdams
- Temporary structures used to create dry working areas within water bodies.
- Function by excluding water from a designated area, allowing construction or maintenance activities.
- Various types exist, including cellular cofferdams, diaphragm cofferdams, and sheet pile cofferdams.
- Choice of cofferdam depends on factors such as the depth of water, soil conditions, and project requirements.
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Description
This quiz covers the fundamental concepts of the bearing capacity of shallow foundations, including local and general shear failure. It also explores Terzaghi's theory and Skempton's method for calculating bearing capacity. Test your knowledge on how these theories apply to soil mechanics.
