Geotechnical Testing > Geotechnical Final > Flashcards

Geotechnical Final Flashcards

(76 cards)

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1
Q

True or False: The deviator stress gradually increases throughout the shearing stage of the test, until shear failure is generated.

A

True

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2
Q

What is the maximum deviator stress?

A

The shear strength of the soil

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3
Q

What are the two types of undrained triaxial tests?

A
  1. Unconsolidated Undrained (UU) Triaxial Test
  2. Unconfined Compression Test
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4
Q

True or false: An undrained state models long term soil strength

A

False, a drained state models the long-term soil strength.

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5
Q

What type of shear strength parameters does the unconfined compression & the unconsolidated undrained triaxial test have?

A

Reflects stress state at the end of construction (short term, total stress, undrained)

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6
Q

Which test is most commonly used for foundation design and why?

A

Unconfined Compression Test, inexpensive and quick to conduct

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7
Q

True or false: In the CUP and consolidated drained triaxial test, the sample is consolidated prior to shear

A

True

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8
Q

Which test is used in the analysis of long term stability of earth embankments and cut slopes?

A

The Consolidated Drained Triaxial Test

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9
Q

What type of stress state does the CUP and consolidated drained triaxial test reflect?

A

Reflects stress state after the dissipation of excess pore pressure (long-term, effective stress, drained)

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10
Q

Why is clay till an over consolidated material?

A

Preconsolidation from ice load

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11
Q

What does it mean when a sample is undrained?

A

It means that during the shear stage the load is applied relatively quickly and no drainage is permitted from the sample

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12
Q

What is Cell Pressure?

A

Pressure within the cell but outside of the membrane

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13
Q

What is pore (back) pressure?

A

Pressure within the sample membrane

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14
Q

What type of material is the CUP triaxial test generally used for?

A

Medium and high plastic clays

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15
Q

True or False: Clay soils have low permeability and a low rate of drainage of excess pore pressure

A

True

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16
Q

What is permeability?

A

The rate at which water under pressure can diffuse through the voids in a soil

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17
Q

What is hydraulic conductivity dependent on?

A

-Soil Properties
-Soil Structure
- Properties of the pore fluid

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18
Q

What soil properties affect soil permeability?

A

-Grain size distribution
-Particle shape and surface texture
-Mineral Composition

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19
Q

What parts of soil structure affect soil permeability?

A

-Void Ratio
-Degree of saturation
-Soil fabric

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20
Q

What pore fluid properties affect soil permeability?

A
  • Type of flow
  • Temperature or pore fluid
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21
Q

Why does temperature of pore fluid affect permeability?

A

As temperature increases, viscosity drops (water becomes more “fluid”)

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22
Q

True or false: Clean gravel has high permeability

A

True

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23
Q

Which materials have a very low permeability?

A

Silt and clay mixtures

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24
Q

What test is conducted for soils with high permeability?

A

Constant Head Test (consistently adding water)

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25
What test is conducted for soils with intermediate to low permeability?
Falling head test (letting water flow naturally)
26
What type of material is a falling head test conducted on?
Silts & Clays
27
What type of material is a constant head test conducted on?
Sands & Gravels
28
True or false: Soil permeability can be estimated from a grain size analysis
True
29
What is the purpose of dewatering?
To temporarily lower groundwater levels to conduct construction, such as excavations, and the placement of underground facilities or foundations
30
True or False: Open pumping from excavations is an appropriate method for soils of high permeability
False, open pumping is for soils of low to medium permeability
31
What are the five methods of lowering groundwater levels in advance of excavation
1. Drainage Ditches 2. Wells 3. Wellpoints 4. Geosynthetic Products 5. Installation of cutoffs
32
What type of soils are used in drainage ditches
open graded sands, gravels
33
True or false: Drainage ditches are ineffective for soils of low or medium permeability
True
34
How is a drainage ditch created?
Dig perimeter ditches around the the site to a sufficient depth to permit outflow of groundwater
35
What situation is a Deep Well System suitable for?
Suitable for pumping large water volumes from sand and gravel deposits of high permeability
36
If a Deep Well is needed for long term, what needs to be maintained?
The pump needs to be maintained consistently
37
Name of advantage and one disadvantage of a deep well system
Advantages: - Lower operating costs than wellpoints - Higher lift capacity Disadvantages: - Higher installation and maintenance cost - Not feasible in soils of low or moderate permeability
38
True or false: A deep well system can be used for clays
False, a deep well system will not work for low permeable material
39
How does a wellpoint work?
Vacuum pressure applied to the narrow gauge wells to remove groundwater
40
To what depth is a wellpoint applicable?
Wellpoints are suitable for lowering the groundwater table by depths up to 6m
41
True or false: Rigging is required for wellpoint installation
True
42
In what situation do you achieve soil stabilization by Electro Osmosis?
Used in soft clays and silts where vacuum wellpoints are ineffective
43
How does a geosynthetic wick drain work?
Promotes consolidation of clay soils, installed vertically through clay allowing for drainage paths for groundwater
43
List 4 methods of cutoff
1. Sheet pipe 2. Slurry Trenching 3. Grout injection 4. Ground freezing
44
What's one reason for field tests for coefficient of permeability
- Lab samples are subject to disturbance - Samples may be conducted on remolded materials - Field tests reflect actual in situ conditions
45
In cases where flow is irregular or if water is entering and leaving a permeable soil using Darcy's law can be difficult. What other method can be used to evaluate flow?
Flow Nets
46
Why is uplift pressure important to consider?
If the upward force exceeds the downward force the structure will be unstable
47
What is a dispersive clay?
Clays that disaggregate easily and rapidly in water of low salt concentration
48
True or false: Dispersive clays are saturated with sodium cations
True
49
Why is dispersion potential important?
important to identify clay soils that are susceptible to erosion or dispersion
50
What are the 3 methods for assesing dispersion potential in soil?
1. Pinhole test 2. Crumb Test 3. Double Hydrometer
51
During a pinhole test, how can you tell the clay is dispersive?
the 1mm hole will increase in size and there will be a cloudy effluent
52
For a pinhole test method what three items are typically assessed?
1. Final flow rate 2. Final size of hole 3. Visual assessment of the collected water
53
For a crumb test, explain the difference between a 1 & 4 rating
1. There will be no reaction and no sign of cloudiness, therefore nondispersive 4. There will be a strong reaction and cloud covers nearly the entire bottom of the beaker, therefore highly dispersive
54
True or false: Before a crumb test the soil is dried in an oven
False, oven drying can cause irreversible changes to soil properties
55
True or false: During a crumb test its possible for a soil to give a "false negative" but not a "false positive"
True
56
For a double hydrometer in the formula Percent Dispersion = A/B x100 state which sample (A or B) contains the dispersing agent
Sample B is the standard hydro with the dispersing agent
57
What's the typical units for Esecant?
MPa
57
True or false: For very dispersive clays both values A & B will be relatively close for a double hydrometer
True
58
In a UU triaxial test, the confining pressure is equal to?
σ3
59
What's the major principle stress of a triaxial test given: confining pressure = 100 kPA, deviator stress=140 kPa
= 240 kPa
60
What does the Mohr Coulomb diagram plot?
Shear Stress vs Normal Stress
61
On a Mohr Coulomb diagram for a UU triaxial test, the deviator stress is?
The diameter of the circle
62
True or False: In the UU triaxial test, drainage is permitted from the specimen
False
63
What is the direct shear test most commonly used for?
Slope stability analysis
64
What is the main advantage of a CUP triaxial test?
Pore pressure is measured
65
Which test can measure residual shear strength?
Direct Shear Test
66
What is the difference in head over the difference in length referred to as?
Hydraulic Gradient
67
True or false: Density of soils solids is a direct factor of soil permeability
False
68
Which of the laboratory permeability tests requires initial and final head readings
Falling head test
69
An aquifer bound in both vertical directions by an impervious stratum is referred to as a?
Confined Aquifer
70
Fill in the blank: If the uplift pressure exceeds the downward force on a structure the structure will be _______
Unstable
71
Dispersive clays are saturated with what cations?
Sodium
72
True or false: A dispersive clay is ideal for liquid containment structures
False
73
In the pinhole test, what parameter is examined in method A but not in method B?
Final Flow Rate
74
In the crumb test, "cloudiness around most particles and undefinable boundaries describe what grade of soil?
3-4 Moderate-Strong Reaction

Geotechnical Testing (3 decks)

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