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Questions and Answers
A soil specimen is subjected to a direct shear test. Which of the following parameters can be directly determined from this test without needing further calculations or additional tests?
A soil specimen is subjected to a direct shear test. Which of the following parameters can be directly determined from this test without needing further calculations or additional tests?
- Pore water pressure
- Effective stress
- Shear stress at failure (correct)
- Angle of internal friction
Which of the following statements best describes the Mohr-Coulomb failure criterion?
Which of the following statements best describes the Mohr-Coulomb failure criterion?
- A method for determining the pore water pressure in saturated soils.
- A linear equation relating effective stress to shear strength. (correct)
- A non-linear equation relating total stress to shear strength.
- An equation defining the relationship between principal stresses at failure.
In which type of soil would you primarily use the Vane Shear Test to determine its shear strength?
In which type of soil would you primarily use the Vane Shear Test to determine its shear strength?
- Soft, saturated clay (correct)
- Overconsolidated clay
- Dense, well-graded gravel
- Loose, silty sand
Which of the following factors has the least influence on the shear strength of coarse-grained soils?
Which of the following factors has the least influence on the shear strength of coarse-grained soils?
What is the primary purpose of performing a Consolidated Drained (CD) triaxial test?
What is the primary purpose of performing a Consolidated Drained (CD) triaxial test?
Which of the following scenarios is most likely to lead to a quick sand condition?
Which of the following scenarios is most likely to lead to a quick sand condition?
How does increasing the effective stress in a soil mass typically affect its shear strength?
How does increasing the effective stress in a soil mass typically affect its shear strength?
A soil sample is tested in a Consolidated Undrained (CU) triaxial test. During the shearing phase, what parameter is typically measured that is not measured in a Consolidated Drained (CD) test?
A soil sample is tested in a Consolidated Undrained (CU) triaxial test. During the shearing phase, what parameter is typically measured that is not measured in a Consolidated Drained (CD) test?
Which of the following scenarios is LEAST likely to be analyzed using shear strength parameters of soil?
Which of the following scenarios is LEAST likely to be analyzed using shear strength parameters of soil?
The critical hydraulic gradient ($i_{cr}$) is a key parameter in assessing the potential for quick sand. Which of the following equations correctly defines the critical hydraulic gradient, where $G_s$ is the specific gravity of soil solids and $e$ is the void ratio?
The critical hydraulic gradient ($i_{cr}$) is a key parameter in assessing the potential for quick sand. Which of the following equations correctly defines the critical hydraulic gradient, where $G_s$ is the specific gravity of soil solids and $e$ is the void ratio?
Which of the following actions would be LEAST effective in preventing quick sand conditions in an excavation?
Which of the following actions would be LEAST effective in preventing quick sand conditions in an excavation?
Why are overconsolidated soils expected to have higher shear strength than normally consolidated soils at the same effective stress?
Why are overconsolidated soils expected to have higher shear strength than normally consolidated soils at the same effective stress?
In a triaxial test, if the confining pressure is increased while keeping the axial stress constant, how will it affect the effective stress and consequently the shear strength of a drained soil sample?
In a triaxial test, if the confining pressure is increased while keeping the axial stress constant, how will it affect the effective stress and consequently the shear strength of a drained soil sample?
Which of the following field conditions would be most conducive to the formation of quick sand?
Which of the following field conditions would be most conducive to the formation of quick sand?
A retaining wall is being designed to support a soil backfill. Which shear strength parameter is most critical for assessing the lateral earth pressure exerted on the wall under drained conditions?
A retaining wall is being designed to support a soil backfill. Which shear strength parameter is most critical for assessing the lateral earth pressure exerted on the wall under drained conditions?
What is the primary reason that loose sands are more susceptible to quick sand conditions than dense sands?
What is the primary reason that loose sands are more susceptible to quick sand conditions than dense sands?
In the context of shear strength, what does 'cohesion' represent in soil?
In the context of shear strength, what does 'cohesion' represent in soil?
During a CU test, the pore water pressure is positive. What does this indicate about the soil sample?
During a CU test, the pore water pressure is positive. What does this indicate about the soil sample?
Which soil type is unconfined compression test best used for?
Which soil type is unconfined compression test best used for?
Which factor is least likely to cause quick sand?
Which factor is least likely to cause quick sand?
Flashcards
Shear Strength
Shear Strength
Soil's resistance to shear stress, crucial for designing foundations, slopes, and retaining walls.
Cohesion (c)
Cohesion (c)
Inherent attraction between soil particles, independent of applied stress.
Angle of Internal Friction (φ)
Angle of Internal Friction (φ)
Measure of soil's resistance to sliding due to friction between particles.
Effective Stress (σ')
Effective Stress (σ')
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Mohr-Coulomb Failure Criterion
Mohr-Coulomb Failure Criterion
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Soil Density Effect
Soil Density Effect
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Water Content Effect
Water Content Effect
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Stress History Effect
Stress History Effect
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Drainage Conditions Effect
Drainage Conditions Effect
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Direct Shear Test
Direct Shear Test
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Triaxial Test
Triaxial Test
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Unconfined Compression Test (UC)
Unconfined Compression Test (UC)
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Vane Shear Test
Vane Shear Test
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Consolidated Drained (CD) Test
Consolidated Drained (CD) Test
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Consolidated Undrained (CU) Test
Consolidated Undrained (CU) Test
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Quick Sand Condition
Quick Sand Condition
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Upward Seepage
Upward Seepage
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Critical Hydraulic Gradient (icr)
Critical Hydraulic Gradient (icr)
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Prevention of Quick Sand
Prevention of Quick Sand
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Examples of Quick Sand Situations
Examples of Quick Sand Situations
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Study Notes
- Soil mechanics is a branch of civil engineering that studies the engineering properties of soil and its behavior under stress and strain
- It applies principles of mechanics, hydraulics, and engineering geology to analyze and predict soil behavior
Shear Strength
- Shear strength is the soil's ability to resist shear stress
- Critical in geotechnical engineering for designing foundations, slopes, and retaining walls
- Shear failure occurs when shear stress on a plane exceeds the shear strength
- Shear strength: shear stress at failure
Components of Shear Strength
- Cohesion (c): Inherent attraction between soil particles, independent of applied stress
- Angle of internal friction (φ): Soil's resistance to sliding due to friction between particles
- Effective stress (σ'): Stress carried by soil solids
- Mohr-Coulomb failure criterion describes shear strength (τ) as a function of effective stress: τ = c + σ' * tan(φ)
Factors Affecting Shear Strength
- Soil composition: Mineralogy, particle size distribution, and shape affect shear strength
- Soil density: Denser soils generally have higher shear strength
- Water content: Affects effective stress and pore water pressure
- Stress history: Overconsolidated soils typically exhibit higher shear strength
- Drainage conditions: Drained conditions allow pore water pressure to dissipate, increasing effective stress
- Soil structure: Particle arrangement and cementation influence shear strength
- Temperature: Variations can affect pore water viscosity and soil particle behavior
Measurement of Shear Strength
- Direct Shear Test: Soil sample is sheared along a horizontal plane
- Measures shear stress at failure under constant normal stress
- Provides values for cohesion (c) and angle of internal friction (φ)
- Quick test, but the failure plane is predetermined
- Triaxial Test: Cylindrical soil sample is subjected to confining pressure and axial stress
- Allows control of drainage conditions
- Different types include Consolidated Drained (CD), Consolidated Undrained (CU), and Unconsolidated Undrained (UU) tests
- Provides more detailed information about the stress-strain behavior of soil
- Unconfined Compression Test (UC): Triaxial test with zero confining pressure
- Used for cohesive soils
- Measures unconfined compressive strength (qu); undrained shear strength (cu) is cu = qu/2
- Vane Shear Test: Field test to determine undrained shear strength of cohesive soils
- A vane is inserted into the soil, and the torque required to rotate the vane is measured
- Suitable for soft, saturated clays
- Consolidated Drained (CD) Test: Soil sample consolidated under pressure then sheared slowly under drained conditions
- Allows full dissipation of pore water pressure
- Provides effective stress parameters (c' and φ')
- Consolidated Undrained (CU) Test: Soil sample consolidated then sheared under undrained conditions
- Pore water pressure is measured during shearing
- Provides both total stress parameters (c and φ) and effective stress parameters (c' and φ')
Applications of Shear Strength
- Slope Stability Analysis: Stability of natural slopes and engineered embankments
- Foundation Design: Determining soil bearing capacity to support structures
- Retaining Wall Design: Calculating lateral earth pressure on retaining walls
- Pavement Design: Assessing stability and durability of pavement layers
- Tunnel Design: Analyzing tunnel excavation stability
Quick Sand Condition
- Quick sand: Saturated, loose sand loses strength and behaves like a fluid due to upward seepage
- Not a special type of sand, but a condition in granular soils under specific hydraulic conditions
Conditions for Quick Sand
- Upward Seepage: Upward water flow increases pore water pressure
- Critical Hydraulic Gradient: When hydraulic gradient (i) reaches a critical value (icr), effective stress becomes zero
- Critical hydraulic gradient: icr = (Gs - 1) / (1 + e). Gs is specific gravity of soil solids, e is the void ratio
- Loose Sand: Loose sands are more susceptible due to higher void ratio and lower effective stress
- Saturation: The soil must be fully saturated
Effects of Quick Sand
- Loss of Bearing Capacity: Structures can settle or collapse
- Instability of Slopes: Slopes can fail
- Difficulty in Excavation: Excavations can become unstable and collapse
Prevention of Quick Sand
- Lowering the Water Table: Decreases upward seepage force
- Providing Drainage: Controls water flow and reduces pore water pressure
- Increasing the Effective Stress: Applying a surcharge load
- Soil Stabilization: Grouting or soil compaction can improve strength and stability
Examples of Quick Sand Situations
- Excavations below the water table
- Riverbeds with upward hydraulic gradients
- Coastal areas with tidal fluctuations
- Areas with high rainfall and poor drainage
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