Shear strength of soil

Question 1

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.

Answer 1

True

Question 2

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

Answer 2

B) The inherent soil strength without external support

Question 2

The Coulomb failure criterion for soil failure under drained conditions is expressed as T_f = c’ + σ_n’tanɸ’ is the angle of _______.

Answer 2

internal shearing resistance

Question 3

Why is the undrained shear strength (c_u or S_u) particularly important in the analysis of short-term stability in saturated clays?

Answer 3

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.

Question 4

True or False
During long-term stability analyses, the total stress equals the effective stress in drained conditions of saturated clay.

Answer 4

True (In drained conditions, pore water pressure dissipates fully, allowing total stress to equal effective stress)

Question 5

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 _____.

Answer 5

hydraulically sealed

Question 6

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.

Answer 6

A) Drained conditions relate to long-term stability while undrained conditions are relevant in short-term scenarios.

Question 7

True or False
The shear strength parameters for clays (c’ and ϕ’) can vary significantly based on whether the condition is drained or undrained.

Answer 7

True

Question 8

In the formula T_f = c_u = S_u for undrained shear strength,
T_f stands for _______.

Answer 8

failure shear stress

Question 9

How does the angle of internal shearing resistance (ϕ’) affect the stability and strength of a soil under loaded conditions?

Answer 9

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.

Question 10

True or False
The direct shear test uses a cylindrical sample to measure the shear strength of soil.

Answer 10

False (The direct shear test uses a square shear box)

Question 11

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.

Answer 11

water

Question 12

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

Answer 12

B) The ratio of shearing resistance to disturbing forces

Question 13

What is the main purpose of using a direct shear box in soil testing?

Answer 13

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.

Question 14

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.

Answer 14

False - intermediate

Question 15

During the direct shear test, vertical displacements are measured using a _______ gauge, which helps determine whether the soil sample contracts or dilates.

Answer 15

dial

Question 16

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

Answer 16

B) To allow free drainage and control pore water pressures

Question 17

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.

Answer 17

True

Question 18

he shear stress in a direct shear test is calculated using the formula 𝜏 = F_H/L^2, where F_H is the _______ force.

Answer 18

horizontal

Question 19

How does the consolidation process affect the factor of safety for foundations?

Answer 19

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.

Question 20

What are dense and loose sand responses similar to?

Answer 20

Dense and loose sand responses are similar to normally and over-consolidated clays.

Question 21

How do you calculate ∆L?

Answer 21

motor speed × time (or measure directly)

Question 22

How do you calculate normal stress?

Answer 22

σ_n’=F_v/L^2

Question 23

Porous stones allow ______.

Answer 23

Free-drainage

Question 24

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.

Answer 24

True

Question 25

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.

Answer 25

pore water pressure

Question 26

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

Answer 26

B) To determine the effect of reduced total stress on pore water pressures

Question 27

What is the primary reason for using a motor in a direct shear test apparatus?

Answer 27

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.

Question 28

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.

Answer 28

True

Question 29

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

Answer 29

C) Both A and B

Question 30

True or False The triaxial test allows for the measurement of the angle of internal friction and cohesion in soils under controlled stress conditions.

Answer 30

True

Question 31

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 _______.

Answer 31

effective stress

Question 32

How does the swelling period affect the stability of slopes in the long term?

Answer 32

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.

Question 33

When does failure occur within a soil mass?

Answer 33

When shear stress equals shear strength

Question 34

What is microfabric and macrofabric in soil samples?

Answer 34

Microfabric is small-scale particle arrangement; macrofabric is larger structural features.

Question 35

Define a Direct Shear Test.

Answer 35

A test where a soil sample is sheared along a plane to measure shear strength.

Question 36

Define a Triaxial Test.

Answer 36

A test using a cylindrical sample under controlled stress to measure shear strength.

Question 37

What happens to the volume of a saturated soil sample during undrained shearing?

Answer 37

No overall volume change.

Question 38

In a drained condition, if shearing is slow, pore pressure (u) becomes __________ and total stress equals __________ stress.

Answer 38

0; effective.

Question 39

In foundations, short-term stability is critical because of __________.

Answer 39

immediate loading creating excess pore water pressures.

Question 40

Hydrostatic pore water pressure (u) formula

Answer 40

u = γw * z.

Question 41

After construction loading, pore water pressure increases and reaches a __________ during short-term conditions.

Answer 41

peak.

Question 42

Factor of safety (F) formula

Answer 42

Sum of shearing resistances / Sum of disturbing forces.

Question 43

As pore pressure dissipates over time, does the original factor of safety return?

Answer 43

No.

Question 44

In undrained conditions, __________ strength should be used for design.

Answer 44

cu.

Question 45

Excavation causes a __________ in pore water pressure.

Answer 45

drop.

Question 46

Unloading during excavation leads to __________ pore water pressures.

Answer 46

negative.

Question 47

Do negative pore water pressures make slopes more stable in the short term?

Answer 47

Yes.

Question 48

Despite a drop in pore water pressure, excavation causes a decrease in __________.

Answer 48

factor of safety.

Question 49

Direct shear box samples are split at the __________.

Answer 49

mid-height.

Question 50

Porous stones in the shear box allow __________.

Answer 50

drainage.

Question 51

Rollers in the direct shear box minimize __________.

Answer 51

friction.

Question 52

In a direct shear box, the dial gauge measures __________.

Answer 52

vertical displacement.

Question 53

The proving ring measures __________ in a direct shear box.

Answer 53

force.

Question 54

Why apply area correction during a direct shear test?

Answer 54

Because the shearing area reduces as displacement increases.

Question 55

Relationship between peak shear stress (τmax) and normal stress (σn')

Answer 55

τmax = c' + σ'n tan(φ').

Question 56

Strength of sand increases with __________.

Answer 56

density.

Question 57

Dense sand __________ during shearing, while loose sand __________.

Answer 57

dilates; contracts.

Question 58

What is dilation in soil mechanics?

Answer 58

Expansion during shearing due to particle interlocking.

Question 59

Critical state is reached when __________.

Answer 59

shear stress and volume change become constant.

Question 60

Over-consolidated soils behave more __________ than normally consolidated soils.

Answer 60

brittle.

Question 61

Define strain softening.

Answer 61

Reduction in shear strength after peak strength is reached.

Question 62

Residual strength compared to critical state strength is __________.

Answer 62

lower.

Question 63

What causes residual strength in clays?

Answer 63

Large strains realign clay particles parallel to the shearing plane.

Question 64

Most widely used shear strength test suitable for all soil types

Answer 64

Triaxial test.

Question 65

What does CP stand for in triaxial testing?

Answer 65

Cell Pressure.

Question 66

In triaxial testing, σa = ________.

Answer 66

σr + Fa/A

Question 67

What are the two main stages of the triaxial test?

Answer 67

(1) Consolidation, (2) Shearing.

Question 68

What is the function of porous stones in a triaxial test?

Answer 68

To allow drainage and control pore pressure.

Question 69

In a triaxial test, what seals the sample and prevents water loss?

Answer 69

Rubber membrane and o-rings.

Question 70

In the U-U test, is drainage allowed at any stage?

Answer 70

No.

Question 71

What is the formula for principal stress difference (deviator stress)?

Answer 71

σa - σr

Question 72

What happens to sample area during axial compression in a triaxial test?

Answer 72

It increases (area correction needed).

Question 73

What is the volumetric strain formula in drained conditions?

Answer 73

εv = ΔV / V0

Question 74

What consistency corresponds to an Su < 20 kPa?

Answer 74

Very soft.

Question 75

What is the relationship between Su and (σa - σr)?

Answer 75

Su = (σa - σr) / 2

Question 76

Is Su constant for all soils in reality?

Answer 76

No, it varies.

Question 77

What are some factors that affect Su?

Answer 77

Swelling, drying, sample size, rate of shearing, anisotropy, apparatus.

Question 78

In a C-U test, when is drainage closed?

Answer 78

Before axial loading.

Question 79

In a CU test, what advantage does applying back pressure give?

Answer 79

Easier to maintain full saturation.

Question 80

Can CP be lower than BP?

Answer 80

No.

Question 81

In U-U tests, what parameters are obtained?

Answer 81

Su or cu.

Question 82

In C-U tests, what parameters are obtained?

Answer 82

φ' and c'.