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Civ Eng Y2 > Soil > Flashcards

Soil Flashcards

1
Q

How to characterise soils? *

A
  • particle die distribution curves
  • atterberg limits
  • moisture
  • colour
  • fabric
  • strength
  • compressibility
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2
Q

What is a particle size distribution curves?

A

Percentage passing through sives vs. Particle size (mm). Silt -> Sand -> Gravel

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3
Q

What is an Atterberg limits Casagrande chart?

A

Plasticity vs. Liquid limit. A line across with clays at the top left and silts bottom right.

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4
Q

What is the problem if silt and sand partings in rock?

A

When you dig a hole you relieve stress from the blocks which means that they will move along the silt and sand partings where water is.

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5
Q

Vv

A

Volume of liquid

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6
Q

Vs

A

Volume of solids

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7
Q

V

A

Volume

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8
Q

Fabric

A

Spatial and geometric configuration of all the elements that make up a rock

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9
Q

M

A

Overall mass of soil sample

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10
Q

Vs= Ms/ (Gs*ρw)

A

Solid volume = solid mass / specific gravity * water density

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11
Q

ν = V/ Vs

A

Specific volume = total volume/ solid volume

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12
Q

e= Vv/ Vs

A

Void ratio = liquid volume/ solid volume

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13
Q

n= Vv/ V

A

Porosity = liquid volume/ total volume

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14
Q

ν = 1 + e

A

Specific volume = 1 + void ratio

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15
Q

e= n/ (1-n)

A

void ratio = porosity/ (1-porosity)

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16
Q

Saturated soil?

A

All voids completely fill with water

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17
Q

Partly saturated soil (unsaturated soil)?

A

Some voids contain water

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18
Q

γ = ρg

A

Unit weight (kN/m^3) = density * gravitational acceleration

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19
Q

γw =

A

9.81 Kn/m^3

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20
Q

σa= F/A

A

Axial stress = force / area

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21
Q

E= σa / εa

A

Young’s modulus = stress/ strain

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22
Q

εa = δL/ L

A

Strain = change in length / original length

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23
Q

Principal stress?

A

Total force on a soil point

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24
Q

Effective principal stress?

A

Total force - pore pressure

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25
Q

Pore pressure?

A

(u) stress caused by the water acting in every direction with equal intensity

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26
Q

Suction?

A

Negative pore water pressure

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27
Q

S= ua - uw

A

Suction = pore air pressure - pore water pressure

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28
Q

What is pore water pressure at the water table or phreatic surface?

A

Zero

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29
Q

Perched water table?

A

Water lies on a stratum of low permeability above the level of the normal water table

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30
Q

Artesian conditions?

A

Water is confined by an overlying layer of low permeability and fed from a distant source where the water table is at a higher elevation

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31
Q

Seepage?

A

Flow through a soil

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32
Q

During steady state seepage what happens to pore pressure?

A

Pore pressures remain constant and no soil deformations occur

33
Q

What controls the flow in seepage?

A

The hydraulic driving head and in particular its gradient

34
Q

h= u/γw + z + v^2/2*g

A

h= total head (potential head)
u= pore water pressure
γ= bulk unit weight
z= elevation above a chosen datum
v= flow/seepage velocity
g= gravitational acceleration

35
Q

q= Aki or. v=q/A = ki (Darcy’s law)

A

q= volume of water flowing per unit time (m^3/s)
A= cross- sectional area of soil corresponding to flow q (m^2)
k= coeff of permeability (m/s)
i= hydraulic gradient
v= discharge velocity (m/s)

36
Q

What is the relationship between coefficient of permeability and particle size of soil.

A

The larger the particle size the larger the coefficient of permeability

37
Q

Hydraulic gradient (i)?

A

The rate of change of total head with distance in the direction of flow

38
Q

i= -dh/ds

A

i= hydraulic gradient
dh= change in total head
ds= the distance between the flow line between the two points being compared

39
Q

u=p γw (finding pore pressure using standpipe)

A

pore pressure = height of water in the standpipe * bulk unit weight of water

40
Q

Quick condition?

A

The upward water pressure gradient and water flow reduce the effective stress

41
Q

icrit - critical hydraulic gradient?

A

The hydraulic gradient when quick conditions occur

42
Q

icrit= (γs/ γw) - 1

A

icrit = critical hydraulic gradient
γs = bulk unit weight of soil
γw= water unit weight

43
Q

Assumptions for Darcy’s law (3)?

A
  • homogeneous, porous, saturated, constant volume medium
  • fluid homogeneous and incompressible
  • flow is steady and continuous (steady state)
44
Q

(dh^2dx^2) + (dh^2/dy^2) = 0

A

Laplace equation
dh= change in total head
dx= change in x direction
dy= change in y direction

45
Q

How can you show flow lines and total head lines?

A

You can draw flow lines as orthogonal trajectories against equal total head lines.

46
Q

EP lines?

A

Equipotential lines. These represent lines of equal total head

47
Q

FL lines?

A

Flow lines enclose flow ‘channels’ within which flow is constant

48
Q

H= Nd Δh (orthogonal trajectory flow lines)?

A

Δh= head drop between adjacent EP lines
H= total head drop across system
Nd= number of potential drops (6?)

49
Q

Stages in flow net construction?*

A
  • identify boundary condition relating to flow lines and equipotentials e.g. impermeable boundary usually coincides with a flow line; open water surface constitutes an equipotential
  • sketch trial flow lines remembering that flow ‘channels’ are like tubes within which flow occurs. Flow lines do not cross one another, nor do they converge completely
  • sketch trial equipotential lines such they are perpendicular to the flow lines and so that the enclosed areas between flow lines and equipotential lines form ‘curvilinear squares’
  • it is convenient if EPs drawn with equal head losses between adjacent EPs
50
Q

Δq = kH / Nd (flow through incremental channel)

A

Δq = change in vol of water flowing per unit time
K= coeff of permeability
H= total head across system
Nd= number of potential drops

51
Q

q = kH Nf / Nd (flow through system)

A

q= vol of water flowing per unit time
k= coeff of permeability
H= total head drop across system
Nf= number of flow channels
Nd= number of potential drops

52
Q

What is the impact on the change in total head over distance as EPs are closer of that given distance?

A

The closed the EPs are over a given distance, the greater the change in total head over that distance and hence the greater the hydraulic gradient

53
Q

(Gs + S*e) * γw = γ
—————
(1+e)

A

Gs= specific gravity
S= saturation
e= void ratio
γw = unit weight of water
γ= unit weight of soil

54
Q

e* S = w* Gs

A

e= void ratio
S= saturation
w= water content
Gs= specific gravity

55
Q

γdry = (Gs) * γw
——
(1+e)

A

γdry = unit weight of dry soil
Gs= specific gravity
e= void ratio
γw = unit weight of water

56
Q

w= Mw/ Ms

A

Water content = water mass / solid mass

57
Q

Sr = Vw/ Vv

A

Saturated = water volume / liquid volume

58
Q

Gs= ρs/ ρw

A

Specific gravity = density solid / water density

59
Q

When soil is not isotopic with respect to permeability, can FL and EP lines be orthogonal?

A

No

60
Q

Kx?

A

Coefficient of permeability in the x direction

61
Q

What happens when you change principal stress in granular soil?

A

Δ σ=Δσ´ drained condition

62
Q

What happens when you change principle stress for fine grained soil?

A

Δ σ = Δu undrained condition

63
Q

Consolidation?

A

When effective stress increases in fine-grained soil, the pore water pressure will increase. The increase in pore water pressure Δu is termed an excess pore pressure which induces a pressure gradient, resulting in a transient flow towards free- drowning boundaries of the soil layer. Flow continues until the excess pore water pressure have dissipated. In the long term when t=tinf and the excess pore water pressure have dissipated, the increment of total stress is carried entirely by the soil skeleton Δ σ = Δσ´ and is once again drained conditions. As drainage takes place the particles re orientated themselves and the soil compresses as inter-particle forces increase and there is a reduction in volume

64
Q

Oedometer test?

A

Measured compression and time on consolidation of settlement

65
Q

ΔH/ H0 = Δe/ (1+e)

A

Change in height/ original height = change in void ratio/ 1+ void ratio

66
Q

What do you plot from oedometer data to understand consolidation?

A

e vs σv’

67
Q

How does compression change in over consolidated and normally consolidated clays?

A

The settlement/ compression of over- consolidated clays is much less than normally consolidated clays. Therefore beware of exceeding pre consolidation pressure in over consolidated clays

68
Q

Over consolidated?

A

Clays that have been loaded up in the past to effective stresses greater than those acting at present

69
Q

3 causes of unloading?

A
  • melting of ice sheets
  • erosion of upper layers
  • rise in ground water table (affecting u rather than σv)
70
Q

mv = 1/(1+e0) (Δe/ Δ σ´)

A

Mv = coeff of volume compressibility
e0= original void ratio
e1= new void ratio
σ1´= new effective stress
σ2´= old effective stress

71
Q

mv= 1/H0 * (ΔH/Δσ´)

A

Μv= coeff of volume compressibility
H0= original height
H1= new height
σ´1= new effective stress
σ0´= original effective stress

72
Q

Assumptions of theory of one-dimensional consolidation?

A
  • soil is homogeneous
  • soil is fully saturated
  • solid particles & water are incompressible
  • compression and flow are one- dimensional
  • strains are small
  • Darcy’s law valid at all hydraulic gradients
  • the coefficient of permeability, k, and the coefficient of volume compressibility, mv, remain constant throughout the process
  • there is a unique relationship, independent of time, between void ratio and effective stress
73
Q

Cv= d2u/dx2 = du/dt

A

Cv= coeff of consolidation
du= change in pore water pressure
dz= change in distance along z
dt= change in time

74
Q

uz = Σ (2ui/M) * (sin(Mz/H))e^(-M^2Tv)
limits= m=inf and m=0

A

uz= value of the excess pore water pressure at depth z at time t
ui = initial excess pore water pressure, uniform over depth z
M= 0.5π(2m+1) where m= positive integer varying from 0 to infinity
Tv= dimensionless time factor
H= length of drainage path

75
Q

Tv= Cv * t / H^2

A

Tv= dimensionless time factor
Cv= coeff of consolidation
H= length of drainage path
t= time

76
Q

What is the length drainage path for 2 way drainage - permeable of both sides?

A

H/2

77
Q

What is the length drainage path of a 1 way drainage system - flow is one direction with one permeable side.

A

H

78
Q

Where in a shape of soil does consolidation immediately occur?

A

Top and bottom of shape

79
Q

U= (ui-ue)/ui = 1-ue/ui

A

U= average degree of consolidation
ui= initial excess pore water pressure, uniform over depth z
ue= value of the excess pore water pressure when σ´ is acting within the soil

Civ Eng Y2 (6 decks)

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