Shear strength of soil Flashcards
True or False
In the short term, saturated clays experience undrained conditions where the shear strength of the soil is expressed solely by its undrained shear strength S_u.
True
What does the cohesion intercept c’ represent in the context of shear strength of soils under drained conditions?
A) The maximum stress the soil can withstand
B) The inherent soil strength without external support
C) The initial shear strength of the soil before any loading
B) The inherent soil strength without external support
The Coulomb failure criterion for soil failure under drained conditions is expressed as T_f = c’ + σ_n’tanɸ’ is the angle of _______.
internal shearing resistance
Why is the undrained shear strength (c_u or S_u) particularly important in the analysis of short-term stability in saturated clays?
In the short term, such as during construction or sudden loading, saturated clays do not allow for water to escape, resulting in undrained conditions.
S_u is crucial for assessing stability under these conditions as it represents the soil’s strength without any drainage of pore pressures.
True or False
During long-term stability analyses, the total stress equals the effective stress in drained conditions of saturated clay.
True (In drained conditions, pore water pressure dissipates fully, allowing total stress to equal effective stress)
In undrained conditions, a sample is sealed to ensure no water enters or leaves, meaning there is no overall volume change, a condition termed as _____.
hydraulically sealed
What is the primary difference between drained and undrained conditions in the context of shear strength of soils?
A) Drained conditions relate to long-term stability while undrained conditions are relevant in short-term scenarios.
B) Drained conditions test the soil’s compressibility, undrained do not.
C) Undrained conditions allow for water movement within the soil, drained do not.
A) Drained conditions relate to long-term stability while undrained conditions are relevant in short-term scenarios.
True or False
The shear strength parameters for clays (c’ and ϕ’) can vary significantly based on whether the condition is drained or undrained.
True
In the formula T_f = c_u = S_u for undrained shear strength,
T_f stands for _______.
failure shear stress
How does the angle of internal shearing resistance (ϕ’) affect the stability and strength of a soil under loaded conditions?
The angle of internal shearing resistance is a measure of the soil’s ability to resist sliding along internal surfaces when stressed. A higher
ϕ’ indicates greater resistance, contributing to enhanced soil stability and strength.
True or False
The direct shear test uses a cylindrical sample to measure the shear strength of soil.
False (The direct shear test uses a square shear box)
In foundations, the short term is critical because there is no time for water to flow, leading to a sudden increase in water pressure under the foundation, known as a jump in _______ pressure.
water
What does the factor of safety in soil mechanics typically represent?
A) The ratio of applied stress to shear strength
B) The ratio of shearing resistance to disturbing forces
C) The ratio of soil density to water content
B) The ratio of shearing resistance to disturbing forces
What is the main purpose of using a direct shear box in soil testing?
The direct shear box is used to determine the shear strength of soil by applying a normal load and measuring the force required to cause shear failure along a predefined plane within the soil.
True or False
In the context of excavations, the long term is critical due to swelling periods that increase pore water pressures, potentially leading to decreased slope stability.
True
During the direct shear test, vertical displacements are measured using a _______ gauge, which helps determine whether the soil sample contracts or dilates.
dial
Why are porous stones used in the direct shear test apparatus?
A) To absorb excess water from the soil
B) To allow free drainage and control pore water pressures
C) To increase the weight and stability of the test setup
B) To allow free drainage and control pore water pressures
True or False
The results from direct shear box tests on dense and loose sand provide insights similar to those from tests on normally and over-consolidated clays.
True
he shear stress in a direct shear test is calculated using the formula 𝜏 = F_H/L^2, where F_H is the _______ force.
horizontal
How does the consolidation process affect the factor of safety for foundations?
The consolidation process increases the strength of the soil due to the dissipation of excess pore pressures, which in turn increases the factor of safety. However, it typically does not return to its original state because the soil has been disturbed and subjected to a loading force.
What are dense and loose sand responses similar to?
Dense and loose sand responses are similar to normally and over-consolidated clays.
How do you calculate ∆L?
motor speed × time (or measure directly)
How do you calculate normal stress?
σ_n’=F_v/L^2
Porous stones allow ______.
Free-drainage
True or False
In the triaxial test, the soil sample is allowed to consolidate under the applied load before measuring shear strength to mimic in-situ conditions accurately.
True
The consolidation process during foundation loading is critical in the short term because it affects the _______ due to the applied load, influencing the foundation’s initial stability.
pore water pressure
In slope analysis for long-term stability, why is the unloading phase considered?
A) To assess the potential for erosion
B) To determine the effect of reduced total stress on pore water pressures
C) To calculate the optimal slope angle for new constructions
B) To determine the effect of reduced total stress on pore water pressures
What is the primary reason for using a motor in a direct shear test apparatus?
The motor in a direct shear test apparatus is used to apply a horizontal force at a controlled rate, driving the soil to shear along the split plane. This helps in determining the shear strength of the soil under varying normal stresses.
True or False
In the triaxial test, pore water pressures are controlled to simulate either drained or undrained conditions depending on the soil type and the purpose of the test.
True
Which of the following is a critical consideration when applying loads in an oedometer test?
A) The speed of load application
B) The duration each load is held
C) Both A and B
C) Both A and B
True or False
The triaxial test allows for the measurement of the angle of internal friction and cohesion in soils under controlled stress conditions.
True
During the consolidation phase under a foundation, the excess water pressure generated due to the load tends to dissipate over time, resulting in an increase in the soil’s _______.
effective stress
How does the swelling period affect the stability of slopes in the long term?
The swelling period increases pore water pressures as the soil unloads and expands, which can decrease the effective stress and shear strength, reducing the stability of slopes over time.
