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Ce 11 > Lecture 9: Geologic Investigations > Flashcards

Lecture 9: Geologic Investigations Flashcards

1
Q

Aims to get more detailed information about the subsurface materials.

A

Site Exploration

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2
Q

How much soil sample is enough information to model the plausible site conditions

A

Fraction of the soil in the site

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3
Q

The tests and activities in sampling depends on the site itself. True or False?

A

True. Some projects require standard sample testing and underground drilling while more complex ones may require more advanced tests and deeper drilling to get more samples

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4
Q

DIRECT METHODS OF SITE EXPLORATION

A 
TEST PITS or trenches
DRILLHOLES/BOREHOLES 
ROCK CORING
SAMPLING OF SOIL
SITE TESTS
LAB TESTS
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5
Q
  • Excavations where samples are taken.
  • Allows direct examination of layering.
  • Depth varies but quite limited.
A

TEST PITS or trenches

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6
Q
  • Holes in the ground are drilled and samples are taken at regular intervals.
  • The samples are then tested to characterize the soil/rock and a model of the soil/rock layering can be made.
  • Commonly reaches several tens of meters in depth.
A

Drill holes

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7
Q

Drilling devices/drilling bits to excavate soil from a
hole. Can be hand operated or powered. For both cases, they may have “continuous flights” or helix around them that brings materials upwards.

A

Augers

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8
Q

depth reach of Hand Augers

A

3-5 cm

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9
Q
  • Powered by an external source.
  • Can reach greater depths.
  • Can be solid stem or hollow stem.
  • The difference lies in the stem of the auger being solid or hollow. A hollow stem allows removal of the center part and insertion of a sampling rod.
A

Powered Auger

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10
Q

To core rocks, a special coring bit is attached at the end and it cuts through rock.
The coring bit may have ________ leaving a rock core sample in the tube

A

diamonds, tungsten, or other materials that are hard

enough to cut the rock

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11
Q

The degree of disturbance of soil sample is usually related to

A

𝑨𝒓𝒆𝒂 𝑹𝒂𝒕𝒊𝒐 =𝑫𝒐𝟐−𝑫𝒊𝟐/𝑫𝒊𝟐(𝟏𝟎𝟎)

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12
Q

• If area ratio is less than 10%

A

Sample is undisturbed

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13
Q

soil sample must be REPRESENTATIVE of

A

the layer where it was taken

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14
Q

Ways to Sample a Soil

A
  • Split spoon sampler
  • Shelby Tubes
  • Scraper bucket
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15
Q

Split spoon sampler

A
  • Hollow tube with driving shoe at the bottom. The tube can be split longitudinally.
  • Used in determining STANDARD PENETRATION NUMBER, N or SPT N value, in the STANDARD PENETRATION TEST (SPT).
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16
Q

Shelby Tubes

A
  • Also known as thin-walled tubes (and gets undisturbed soil samples)
  • Usually used in obtaining clay samples
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17
Q

Scraper Bucket

A
  • Used for soils with pebbles which may be hard for split spoon sampler.
  • The tube has an opening on its side (rather than on one end) and it is twisted/rotated so it scrapes the material into the opening and into the tube itself.
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18
Q

Types of Site Tests

A
  • STANDARD PENETRATION TEST (SPT)
  • CONE PENETROMETER TEST (CPT)
  • VANE SHEAR TEST
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19
Q

Most commonly used site test as there are some literature correlating N value to other soil properties.

A

Standard Penetration Test

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20
Q

SPT is done by

A

• A hammer (W = 622.72 N or 140 lbs) is dropped (drop height = 0.762 m or 30 in.) onto the sampler to drive it.
• The number of drops/blows required
to drive the sampler 152.4 mm or 6 in. interval is recorded. This is done 3 times for a total of 18 in..
• The combined number of blows for
the 2nd and 3rd interval (last 12 in.) is the N value.

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21
Q

Test done to determine soil’s characteristics

A

Lab test

22
Q

Soil characteristics determined in the lab

A

• Soil density and specific gravity
• Moisture content (how much water is in the soil)
• Atterberg limits (how much water is needed to change the
behavior of the soil)
• Compaction test (how much water is needed to compact the soil
in its most dense form possible)
• Permeability (how fast water can pass through soil)
• Soil strength

23
Q

• These are methods where soil is not directly sampled, although, may still be disturbed in some manner.

A

Indirect Site Exploration Method

24
Q

Uses rocks’ physical properties or forces on them to determine the geological structure.

A

Geophysical Methods

25
Q

Artificial Field of force

A

• Artificial includes seismic and resistivity as these can be “controlled” to a certain degree.

26
Q

Geophysical methods relies on 2 fields of force

A

Artificial, Natural

27
Q

Natural Field of Force

A

Natural includes gravity and magnetic methods where the forces are only observed and measured but can’t be controlled.

28
Q

Geophysical Methods

A
  • Seismic methods
  • Seismic Reflection
  • Seismic Refraction
  • Resistivity methods
  • Electromagnetic Methods
  • Magnetic Methods
  • Gravity Methods
  • Drillhole Logging methods
  • Crosshole methods
29
Q

Seismic MEthod

A

Uses the energy from an explosion or pounding/hammering in/on
the ground. The energy creates seismic waves and the waves usually go faster
as you go deeper into the ground

30
Q

In seismic methods, when the waves hit a different material, some if gets REFLECTED and some get REFRACTED giving birth to 2 methods:

A
  • SEISMIC REFLECTION

* SEISMIC REFRACTION

31
Q

During seismic refraction the wave that is travelling along the surface is called the

A

Direct Wave

32
Q

What is the Blue wave in seismic refraction?

A

Wave that will be “critically refracted”

33
Q

in seismic refraction, due to the oscillating stress just below the boundary, new disturbances are created that travel up and are received by the geophones. What are these disturbances called?

A

RED, usually referred to simply as REFRACTED WAVES

34
Q

The point where refracted and direct waves come at the same time is called the

A

critical distance.

35
Q

Seismic Refraction

A
  • This only works if the lower layer has higher velocity than the layer above.
  • It can also be used for multiple layers as long as the thickness is considerable and the velocities continue to go higher as you go deeper.
  • Aside from getting the depth to the layer with higher velocity, seismic refraction can also be used to estimate the position of faults.
36
Q
  • Used mostly in oil industry.
  • Requires much deeper area compared to the shot-to-detector-array distance – this makes sure that refracted waves are not going to be a problem in the measurement.
A

Seismic Reflection

37
Q

RESISTIVITY METHODS

A
  • The concept behind this is that subsurface variations means there will be variation in the conductivity and pattern of current of the subsurface as well. This is also reflected in the potential on the surface.
  • Most parts of rocks are insulators, however, electric current can be conducted in some of its conducting mineral parts and water it has.
38
Q

used to run an electric current on the

ground and the potential difference and apparent resistance is measured which is then converted to apparent resistivity.

A

2 current electrodes

39
Q

Involves measurement of one or more magnetic fields.

A

Electromagnetic method

40
Q

How do you detect the presence of an anomalous body using the electromagnetic method?

A

When the received field is drifferent from the released primary field

41
Q

It is a device with transmitter and receiver.

A

Terrain conductivity meter

42
Q

It uses radio transmitters and is

wider in range (thousands of kilometers)

A

Very Low Frequency (VLF) method

43
Q

– uses pulsed electromagnetic

waves and their time needed to be reflected (from surface to subsurface interface back to surface) and high resolution.

A

Ground probing radar

44
Q

Magnetic method

A

• The local geomagnetic field will have anomalies related to the
magnetization of rocks.
• Rocks have their own magnetic susceptibility which will interact and
change the magnetic field in a locality (compared to what is expected based on Earth’s magnetic field), giving insight as to what
rocks may exist.

45
Q

Magnetic method done on an airplane

A

aeromagnetic surveying

46
Q

relies on the variations of gravity field.

A

Gravity Method

47
Q

This affects the measurement of gravity during gravity metofd

A

densities of rock

48
Q

• Holes are drilled and probes are sent down the hole to measure
certain properties of along the wall of the hole.
• This method in essence may use other concepts as the “method” itself represents only the way the data may be read.

A

Drillhole Logging Technique

49
Q

include devices that record resistivity (or variations of
resistivity), spontaneous potential, velocities of refracted waves (waves are induced by pulses), radioactivity, and even drillhole diameters (measures diameter of hole and may provide insight as
to what kind of rock there is, as some materials tend to cave,
making the hole larger than the drill), among many others.

A

Probes

50
Q

2 holes are used with one hole having the disturbance source (at a certain depth) and a receiver is at the other hole (at same depth as disturbance source) and receives seismic waves from the first hole.
• This provides insight as to what material is in between the 2 holes at certain depths.

A

Crosshole Metbod

Ce 11 (11 decks)

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