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Geotechnics > Soil Mechanics Lecture 3 > Flashcards

Soil Mechanics Lecture 3 Flashcards

1
Q

What do earth dams consist of?

A

A core, a filter and a shoulder material

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2
Q

What are the properties of an earth dam?

A

Strong
Low permeability
Minimise settlement

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3
Q

What is the definition of the nature of a soil?

A

The essential characteristics or basic qualities of the soil

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4
Q

How is the nature of a soil assessed?

A

Through intrinsic parameters

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5
Q

What is the definition of the state variables of the soil?

A

The physical condition in which the soil exists

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6
Q

How is the state behaviour of a soil quantified?

A

State variables

i.e. dense or loose soil

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7
Q

What are examples of essential characteristics of soil?

A 
Particle sizes (PSD)
Mineralogy (plasticity)
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8
Q

What are the main methods of finding the particle size distribution?

A

Seiving methods and sedimentation methods

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9
Q

What needs to be measured for dry/ wet seiving?

A

Apeture size
mass retaines
percentage passing

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10
Q

What needs to be measured for sedimentation methods?

A

Sedimentation, settling velocity, dispresing agent

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11
Q

What are 2 examples of sedimentation methods?

A

Pipette/ hydrometer

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12
Q

What are the particle size boundaries

A 
Clay <2 microns
Silt <60 microns
Sand <2mm
Gravel <60mm 
Cobbles <20 cm 
Boulders >20cm
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13
Q

What can you use dry sieving for?

A

Soils containing insignificant quantities of silt and clay

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14
Q

What can wet sieving be used for?

A

Mainly coarse-grained soil

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15
Q

How does sieving work?

A

Soil sample is loaded into the top sieve with the larges apeture and then the stack of sieves is vibrated or shaken for a time period, then you measure the mass change in each sieve and calculate the percentage passing

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16
Q

How do you plot particle size distribution?

A

With percentage passing on a linear scale on the y axis, and the particle size on a log10 scale on the x

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17
Q

what do d50, d90 and d10 mean?

A

d50 = the particle size such that 50% of the particles are smaller than d50
d90 = the particle size that 90% of the particles are smaller than d90
d10 follows the same ruleas the other two

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18
Q

What words can be used to describe particle size distribution?

A
  • Poorly graded/ well sorted/ uniform (ping pong balls)
  • well graded/ poorly sorted
  • gap - graded
  • glacial till
  • clay
  • silty clayey sand
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19
Q

How do you calculate the coefficient of uniformity C_u?

A

C_u = d60/d10

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20
Q

What does a C_u <4 mean?

A

poorly graded

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21
Q

How do you calculate to coefficient of curvature C_z?

A

Cz = (d30)^2/(d60*d10)

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22
Q

What is the range of coefficient of curvature for a well graded sample?

A

1

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23
Q

What kind of soil do you use sedimentation methods on?

A

<425 um

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24
Q

What are the three main methods of sedimentation analysis?

A

Pippette, hydrometer, laser diffraction

25
Q

What is the definition of settling velocity?

A

The speed with which particles of a certain diammeter settle out from a fluid

26
Q

What is a dispersing agent?

A

it is added to soil/ water to stop particles sticking together

27
Q

What does a hydrometer consist of?

A

A graduated stem and a glass bulb

28
Q

How do you use a hydrometer?

A

Place it in the fluid and read the water level off the scale on the stem

Uses stokes law

29
Q

What 3 phases do soils have?

A

voids: air + water

solids (soils)

30
Q

How do you calculate density?

A

rho = mass/ volume (kgm^-3)

31
Q

How do you calculate specific gravity?

A

Gs = rho_s/rho_water

32
Q

How do you caclulate the water content?

A

w = Mw/Ms

33
Q

How do you calculate the void ratio?

A

e = Vv/Vs

34
Q

How do you calculate the bulk density?

A

rho = M/V (kgm^-3)

35
Q

What is the min void ratio?

A

the densest soil packing that can be determined with a given testing method

36
Q

What is the max void ratio?

A

the loosest soil packing that can be determined with a given testing method

37
Q

What are the constituents of deposition?

A

Genesis

Formation of fabric

38
Q

What are the constituents of Burial?

A

Overburden
Development of bonds
consolidation

39
Q

What are the constituents of erosion?

A

unloading
swelling
over-consolidation

40
Q

What is the definition of fabric?

A

The physical arrangement and organisation of particles

E.g. sizes, shapes, orientation, layering, packing

41
Q

What is the definition of bonding?

A

Soil particles connections and their stability

e.g. Cementation, bonding nature and druability

42
Q

What is the definition of structure?

A

fabric + bonding

43
Q

What is the definition of a undisturbed sample?

A

Soil sample in its natural state

44
Q

What is the definition of a remoulded sample?

A

Soil sample that has been distrubed form its natural state with some of its natural structure removed due to mechanical means

45
Q

What is the definition of a reconstituted sample?

A

Soil sample that has all of its natural structure removed due to mechanical mixing at ~1.5 to 2 times its liquid limit

46
Q

What can be used to parameterise the state of clays?

A

The liquidity index

I_L = (w-wp)/ (wl - wp)
w = water contnet 
wp = water content at plastic limit 
wll = water content at liquid limit
47
Q

What can be used to parameterise the state of sands?

A

The relative density

Dr = (emax - e)/(emax - emin)

e: void ration
emax: maximum void ration
emin : min void ration

48
Q

What is the definition of plasticity?

A

Soil ability to undergo irrecoverable deformation without cracking/ crumbling

49
Q

What is the definition of Plastic limit?

A

The lowest water content at which the soil remains deformable without cracking/ crumbling

50
Q

What is the definition of liquid limit?

A

The lowest water content at which the soil cannot maintain its shape

51
Q

How is the plasticity index calculated?

A

Ip = Wl - Wp

52
Q

How is the liquidity index calculated?

A

Il = (w-wp)/(wl-wp)

53
Q

What is the test for plastic limit?

A

Mix with spatula, adding water little by little

Weigh the sample

Roll soil into a sausage shape of a diammeter of roughly 3mm
Gradually increase the water content, when the soil breaks do the same for each of the broken pieces until the rolling thread crumbles

Reweigh the total sample when dry

54
Q

What are the tests to find the liquid limit?

A
  • Cone penetrometer

- casagrandge cup

55
Q

What are the benefits of the cone penetrometer over the Casagrande cup?

A
  • Less operator dependent
  • Less subjective

Hence more repeatable/reproducible results

56
Q

What are the main components of a cone penetrometer?

A
  • Dial guage
  • Stem locking mechanism
  • Cone
  • Metal cup
57
Q

What is the method for using the cone pentrometer?

A
  • Mix soil using metal spatulas and distilled water
  • fill metal cup with the mixed soil, take care not to entrap air
  • strike off extra soil to give a smooth level surface
  • Lower the tip of the cone to the surface of the soil
  • Lower the stem of the dial gauge to contact the cone shaft and record the initial reading
  • release the cone for 5 seconds then lock
  • lower the stem of the dial guage to contact the cone shaft and record the final reading
  • extract a small sample to measure the water content
58
Q

What is the equation for the activity of a soil?

A 
A = Ip/c
Ip = plasticity index
C = clay fraction

Geotechnics (12 decks)

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