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Geotechnical Engineering > Section 2 > Flashcards

Section 2 Flashcards

1
Q

What are some practical applications of Compaction?

A
  • Earthen embankments ( e.g. highway embankments, earthen dams, levees)
  • engineered fills
  • pavement subgrades
  • landfill liner and cover systems
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2
Q

Why is compaction performed?

A
  • to get the soil to an adequate strength
  • to meet a minimal compressibility
  • minimizes shrink-swell potential (i.e. volume change upon wetting and drying
  • lowers permeability
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3
Q

What factors affect compaction?

A
  • the soil type
  • water content
  • compactive effort
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4
Q

What is compaction?

A

Compaction is the ‘densification’ of soils through the expulsion of air

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5
Q

What is the specific gravity of most soils and how can we interpret that?

A

The specific gravity of most soils is around 2.7 which means that the mass of soil solids is 2.7 times the mass of water for the same volume occupied.

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6
Q

What are the axes for the moisture density curve and what does the shape tell us?

A

x-axis: water content
y-axis: dry unit weight [ γdry = γtotal/(1+w) ]

The moisture-density curve is a bell shaped curve. The apex of the curve tells us the optimum water content to attain maximum dry unit weight.

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7
Q

What is the zero air voids line?

A

The zero air voids line is plotted with saturation at 100% (x-axis) against dry unit weight (y-unit) axis and will always be to the right of the moisture density curve

γdry = γwater / (water content + 1/Gs )

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8
Q

How can we compact things in the field?

A
  • static compaction
  • dynamic compaction
  • kneading compaction
  • vibration compaction
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9
Q

What is static compaction?

A

Static compaction is compaction via large volumes sitting for a time to compact the soil beneath

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10
Q

What is dynamic compaction?

A

Large weight hammered on the ground

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11
Q

What is kneading compaction?

A

Used for fine grain soils. Kneading compaction is a large machine with a special roller with numbs on it, rolled over soil to compact it.

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12
Q

What is vibration compaction?

A

A roller that vibrates or a rod drilled into the soil that vibrates to compact the soil. Used for course grain soils.

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13
Q

What types of compactive efforts are there?

A

There are different types from small to large compactive efforts which will affect water content and dry unit weight

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14
Q

What other engineering properties are affected by compaction?

A

Compaction affects permeability and strength

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15
Q

How does compaction affect permeability?

A

Compaction affects permeability because the more you compact soil the less permeability (waters ability to flow through a soil) there will be. So when soil is compacted in the field, it will be compacted wetter than the ‘optimum’ to reach the lowest permeability. “wet of optimum”

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16
Q

How does compaction affect strength?

A

Compacting closer to the dry unit weight and optimum water content will result in good strength level, but compacting at lower water content will improve strength. “dry of optimum”

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17
Q

What methods and equipment are used in a standard proctor test?

A
  • 5.5 lb Hammer w/ a 12 inch drop
  • 3 layers of soil compacted with 25 blows (drops) of the hammer per layer
  • this compactive effort comes out to 12,375 ft-lbs/ft^3
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17
Q

What machines are used for each soil type?

A

Sand: vibratory, smooth drum
Clay: sheeps-foot
Both/Mix: pneumatic roller, Tamping foot

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17
Q

What methods and equipment are used in a modified proctor test?

A
  • 10 lb hammer, 18 inch drop
  • 5 layers of soil compacted with 25 blows of the hammer per layer
  • this compactive effort comes out to 56,250 ft-lbs/ft^3
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17
Q

What is some of the equipment used in field compaction?

A
  • Pneumatic roller: big smooth rolling wheel(s)/drum
  • Vibratory compactor: big smooth rolling wheel(s)/drum that vibrates
  • tamping foot rollers: big rolling wheel with nubs/sheeps foot
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18
Q

What are the key variables involved with field compaction?

A
  • the mass or weight of the compaction equipment
  • type of compaction (static, dynamic, kneading, or vibration)
  • lift thickness: the layer of soil dumped on a site before it is compacted (6 to 12 in. is typical)
  • number of passes
  • towing speed
  • frequency of vibration for vibratory compaction (usually between 20 and 80 Hz)
19
Q

How is the end product of field compaction compared to levels found in the lab?

A

In the field, compaction levels should meet 90% to 100% of the compaction levels identified in the lab or +/-5% of the optimum water content

20
Q

What is the Teton Dam?

A
  • The Teton Dam is/was an earthen dam completed in 1975 by the Bureau of Reclamation on Snake River, Idaho.
  • The reservoir behind the dam held 80 billion gallons of water, which wasn’t full.
  • Water seepage was noticed down stream of the dam and ignored until June 5th when a leak began at the abutment and the dam ultimately failed. 1/3 of the dam was disintegrated by the water
  • 14 people were killed and 13000head of cattle drowned, caused millions of dollars of property damage
21
Q

What is hydraulic conductivity?

A

Hydraulic conductivity is the flow of water (or other liquids) through soils which is an important geotechnical application

22
Q

What are some practical applications where hydraulic conductivity is used?

A
  • in the construction of earthen dams and levees
  • landfill liners and leachates collection systems
  • flow of groundwater and contaminant transport through natural soils
  • dewatering for construction excavations
23
Q

What is a primary difference between course and fine grain soils with respect to hydraulic conductivity?

A
  • The ease of flow (of water or air)
  • In course grain soils water flows more easily because of the larger void spaces. Larger void spaces mean less interaction between water and the particle surface (small specific area.
  • In fine grained soils water flows less easily because of smaller void spaces and more interaction with particle surface resulting in a double layer formation of water particles on soil particles
24
Q

How is flow characterized?

A
  • Thermodynamics: where water flows - Bernoulli equation
  • Kinetics: How water flows - Darcy’s Law
25
Q

Summarize Bernoulli’s equation

A

total head equals velocity head (which we do not care about in geotech because its basically 0) plus pressure head plus elevation head

H(tot) = H(pressure) + H(z)
= P/γwater + z

26
Q

What is head loss?

A

Head loss is the change in total head (potential energy) that occurs when water passes through soils because it’s getting ‘held up’ by soil particles causing the water to lose energy

27
Q

What is pore water pressure (u)?

A

Pore water pressure (u) is γwater*z

28
Q

What does the Darcy equation convey to us?

A

The Darcy equation breaks down the kinetics of water flowing through soils. It helps us determine how fast water is flowing and how to describe that flow.

29
Q

When does water not flow?

A

When water is at the same pressure as other places around it, it won’t move anywhere. Water only moves from places of high pressure to lower pressure.

You can calculate total head of each location and if there is no difference (head loss) there is no flow.

30
Q

In Darcy’s law what term is used instead of flow?

A

Seepage

31
Q

What is Darcy’s law?

A

Q = kiA = v*A
v: is the velocity
A: cross sectional area
i: hydraulic gradient (i = change in head/ length of where the change occurs)
k: hydraulic conductivity

32
Q

What is the equation for seepage velocity?

A

v(seepage) = (k/n)*i
k: hydraulic conductivity
n: porosity
i: hydraulic gradient

33
Q

What is the difference between Darcy velocity and seepage velocity?

A

v(Darcy): 2:30am on a motorcycle to DC
v(seepage): 5pm on a Friday, on a motorcycle to DC

34
Q

What is laminar flow?

A

A slower form of Flow
- a babbling brook going pretty slow
- water is clear, you can see down to the bottom
- linear gradient, Darcy’s law applies

35
Q

What is turbulent flow?

A

White water rafting type of flow
- white rapids, bubbles, fast speeds
- turbulent speeds
- difficult to determine gradient, Darcy’s Law does NOT apply

36
Q

What hydraulic conductivity can you expect in different types of soil?

A
  • for gravel hydraulic conductivity can be on the order of (in cm/sec) from 10^2 - 10
  • for clays 10^-5 to 10^-9
37
Q

Horizontal flow between soil layers is analogous to what?

A

Parallel to the soil layers
- different velocity for each layers
Qtot = Q1+Q2+Q3 etc

k = Σ (kn*zn)/ (combined height of layers)

38
Q

Vertical flow between soil layers is analogous to what?

A

Perpendicular to soil layers
- speed of flow depends on the layer that has the slowest flow
Qtot = Q1 = Q2 = Q3

k = (combined height of layers) / (z1/k1 +z2/k2 + z3/k3 etc)

39
Q

Why does seepage matter?

A

Water is the great erode-er, can cause failures in the ground, can cause sinkholes, can cause failure of levees and dams.

40
Q

How is hydraulic conductivity measured in the laboratory?

A
  • constant head test
  • falling head test
41
Q

How is hydraulic conductivity measured in the field?

A
  • well pumping
  • boreholes
  • porous probe
  • infiltrometer
  • underdrain
42
Q

What is the constant head test?

A

The constant head test is used to measure permeability of coarse-grained soils. Head loss (change in head) remains constant throughout the test.

43
Q

What is the falling head test?

A

The falling head test is used to measure permeability of fine grain soils. Head loss varies throughout the test

44
Q

What are the assumption made for a well-pumping test?

A
  • the well penetrates water-bearing stratum
  • soils are homogenous, isotropic
  • Darcys law is valid
  • Radial Flow
  • Dupuit assumption
45
Q

What is a borehole test?

A

A drill rig drills a borehole
- without pumping: watch the water level go up and down or add a know volume of water and measure the resulting water levels over time
- with constant head pumping: (well pumping)
- as a step test - pump in a series of steps, each with a different discharge rate. determines short-term yield of borehole
- recovery test: after a test done with constant drawdown/discharge measure the water level as the borehole recovers without pumping.

46
Q

What is a porous probe?

A

a probe inserted into the ground hydraulically that can perform both falling and constant head tests by forcing pressurized water into or out of the chamber.

47
Q

What is an infiltrometer test?

A
  • does not require boreholes
  • looks like a big pot with out a bottom
  • hammer the infiltrometer into the ground and fill it with water
  • the double ring is better at facilitating one directional flow
48
Q

What is an underdrain test?

A
  • does not require boreholes
  • uses an infiltrometer (pot with no bottom that gets hammered into the ground and filled with water)
  • BUT before the infilitrometer is in place a layer of material with a large hydraulic conductivity is buried
  • water flows into the drainage layer which can then be collected and the Q acquired
  • can also do falling and constant head test

Geotechnical Engineering (3 decks)

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