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Geotechnics > Textbook - soil mechanics > Flashcards

Textbook - soil mechanics Flashcards

1
Q

What does the term soil mechanics refer to?

A

The mechanical properties of soil

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2
Q

What does the term geotechnical engineering refer to?

A

the application of those mechanical properties to design and construction

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3
Q

What is the principle of effective stress?

A

sigma_z’ = sigma_z - u

simga_z' = effective pressure 
sigma_z = total pressure
u = pore water pressure
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4
Q

What does the equation:
delta P = 2T/r
represent?

A

The pressure difference on either side of a surface
T= tensile force
r = radius of curved membrane

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5
Q

What makes up voids?

A

air and water

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6
Q

What is the equation for unit weight (gamma)?

A

rho*g

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7
Q

How do we calculate the specific gravity?

A

Gs = rho_solids/rho_water

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8
Q

What is the equation for density?

A

rho = mass/volume

so for water - it would be mass of water over volume of water

For solids it is mass of solid/ volume of solid
This can be rewritten as rho_s = Gs*rho_w

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9
Q

What does the symbol e stand for in geotechnics?

A

the void ratio

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10
Q

What is the equation for the void ration?

A

vol. voids/ vol. solids
Vv/ Vs

(Vw + Vg)/Vs

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11
Q

What does the specific volume represent and what is its symbol?

A

The volume occupied by unit volume of solid, v

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12
Q

What is the equation for specific volume?

A

v = total volume/volume of solid

= (Vv + Vs)/Vs = 1 + e

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13
Q

What is the equation and symbol for the total volume fraction?

A

f_vol = vol solid/total vol
= Vs/(Vs + Vv) = 1/v
= 1/(1+e)

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14
Q

What is the equation and symbol for porosity?

A

n = vol voids/ total vol
= Vv/(Vv+Vs)
=e/(1+e) = (v-1)/v

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15
Q

What is the equation for degree of saturation?

A

Sr = vol. liquid/ vol voids
=Vw/Vv
= Vw/(Vw+Vg)

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16
Q

What is the equation for the air void ration, a?

A

a = vol. gas/ total vol.

= (1-Sr)e/(1+e)

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17
Q

What is the equation for water content?

A

w = mass of water/mass of dry soil

= Mw/Ms

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18
Q

how can you express water content in volumetric ratios?

A

w = Mw/ Ms
= rho_wVw/(rho_sVs)
= 1/GsSre

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19
Q

Using volumetric ratios, how do you find the mass of water and the mass of solids?

A

Mw = Srerho_2Vs
Ms = rho_sV_s
= Gsrho_wVs

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20
Q

What is the equation for the bulk density, rho?

A

(Sr*e+Gs)/1+e *rho_w

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21
Q

What is the name of the process of increasing the density of soils?

A

compaction

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22
Q

How do you work out the density of an on site soil sample?

A

Dig a hole in the ground, and measure the mass of the sample
Put a thin membrane in hole and fill with water from a container of known volume or mass
Measure change in volume or mass

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23
Q

How can we make compaction easier?

A

By increasing the moisture content up to a point

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24
Q

What are the 4 methods of transport for rocks, to allow them to make up soils?

A

Gravity (landslide or debris flow)
Ice
Water
Air

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25
Q

What is the effect of glacier transport on rocks?

A

It smooths them, carries a range of particle sizes

26
Q

What happens as the speed of water transporting rocks drops?

A

The coarser rocks are deposited

27
Q

What are the defining features of clay minerals?

A

Appear as large packets of alumino-silicate molecules, characterised by large surface area and a larg ratio of lateral dimmensions to thickness

28
Q

What are three of the most common clay minerals?

A

Laolinite
illite
montmorillonite

All decompositition producs of feldspar and volcanic ash

29
Q

What is the symbol and equation or the coefficient of uniformity?

A

Cu = d60/d10

30
Q

What does a low value for the coefficient of uniformity mean?

A

poorly grade

ping pong balls = 1

31
Q

What shape has the lowest specific surface?

A

a sphere

32
Q

What are the properties of rain and shingl;e?

A

compact shape

low specific surface

33
Q

What causes clay soils to have high void ratios?

A

The electorstatic forces between adjacent particles

34
Q

What is the first order model in seiving?

A

assuming particles are spherical, as particles of all shapes can pass through square holes

35
Q

What are the units for viscosity?

A

kg m^-1 s^-1

can also be written in Pa s

As the viscosity of water is 1 MPa s

36
Q

What do you need to work out the properties of a clay or fine particle soil?

A

A dilute, well mixed suspension

This allows us to think of the soil particles as non interacting spheres

Therefore we add a deflocculating agent to ensure that electrostatic forces do not clump

37
Q

What is the process of measuring the properties of a clay or other fine particle soil?

A
  • Start with a shaken and stirred well mixed solution
  • stand the the tube and allow them to settle
  • take samples from the solution, at the same depth at regular time periods
  • Then dry the sample in an oven to detmine the quantity of the material that has been extracted
  • This allows us to compute a particle size distribution
  • see Stokes law
38
Q

How do we determine the minimum void ratio emin?

A

Through a standard vibratory procedure

39
Q

How do we determine the maximum void ration emax?

A

Put sample in a large tube and invert numerous times

40
Q

What does Bernoullis equation describe?

A

the one dimensional flow of an incompressible fluid along a streamline

41
Q

What kind of flow is usually in soil?

A

slow, massively subcritical, likely laminar

42
Q

What is bernoulis equation?

A

E = u + rho_wgz+1/2 * rho_w*v^2

where u is the fluid pressure

43
Q

What is the equation for energy head?

A

H = u/(rho_w*g) + z + 1/2 v^2/g

44
Q

What is Poiseuille’s equation?

A

tau = viscosity *dv/dz

Where tau is the viscous shear stress

45
Q

What does Posueuille’s equation define?

A

That the velocity varies smoothly from th edge of the tube

This means the water is being pulled forward by the water at the centre of a tube, and backwards by the water at the sides

46
Q

What does poiseuille’s equation mean for fluid mechanics?

A

That the ease of water flow through a void will be proportional to the square size of that opening

47
Q

What is the expression for permeability?

A

K = kviscosity/ (rho_wg)

this is the absolute or specific permeability

48
Q

What is the expression for the seepage velocity?

A

v_s = vbar*(1+e)/e

= vbar/n

49
Q

How is the Reynould’s number for flow through soil defined?

A

Re = rho_wvbard50/viscosity

50
Q

What direction does flow of waterneed to be to artificially increase the stresses in soil?

A

downwards

51
Q

What is an aquifer?

A

A water-bearing level that usually has some spatioal connectivity to an artificial or natural water source which controls the water pressure

52
Q

What is an annulus?

A

A ring shapes object, structure or region

53
Q

If you have two adjacent tubes, and you measure the head in the right hand one, with a soil sample in the middle, what happens when you increase, decrease and level the water levels in both tubes?

A

Increase: Water level in right hand tube will go down - +ve pressure gradient
Decrease: Water level in right hand tube will go up, -ve pressure gradient
Level: water level will be the same, no pressure gradient

54
Q

What is the definition of a pile?

A

A long, slender, stiff structural member which is used to transfer loads from some surface structure through more or less soft soils to a certain depth in the ground

55
Q

What is the effect of a pile foundation?

A

The surface load is shared with the soil over the length of the pile by the generation of shear stresses (tau) (shaft resistance) and by the end bearing stress (sigma_b)

56
Q

What are the disadvantages of shallow foundations?

A

They are subject to overturning

57
Q

What is a shear box?

A

A labatory test device which is used to force a failur surface to form withing a soil sample.

The soil sample is conatined within a split box

58
Q

How does a shear box test work?

A

The shear and split boxes are alligned and filled with soil.
The soil is loaded through a platen with a force P which generates a vertical stress (P/A A is the cross sectional area)
There may be a pore pressure, although it is difficult to retain any pore pressure in a moderatley permeable soil in the shear box
A load is then appliedforcing a relative displacement and the area of contact reduces

59
Q

How many degrees of freedom does the shear box have?

A

2 - normal and shear forces

60
Q

How does sliding ocur in many geotechnical systems?

A

Sliding on a clear slip surface, mobilising the shear strength of the surface

61
Q

What is cheaper, a pile or spread foundation?

A

spread

Geotechnics (12 decks)

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