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Geotechnical Engineering 3 > Principles Second Term > Flashcards

Principles Second Term Flashcards

1
Q

What is the maximum depth of an unsupported trench?

A

1.2m

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2
Q

When analysing a vertical cut failure, where H is cut height, theta is base inclination and gamma is soil bulk weight, what is the equation for destabilising force?

A

(Gamma x H-squared)/(2 tan theta) x sin theta

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3
Q

What is the factor of safety defined as?

A

The stabilising force divided by the destabilising force

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4
Q

If a question gives you the value of gamma with a subscript k, what must be done?

A

The design value gamma d must be found using the table of factors

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5
Q

Why is the design approach 1-1 generally considered more critical for failure?

A

The material properties are unfactored

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6
Q

What happens to the Mohr’s circle as pore water pressure increases?

A

It moves towards the failure line

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7
Q

What is the most common type of dam in the world?

A

Embankment dam

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8
Q

Why can rapidly draining a reservoir cause a dam to fail?

A

Because it is designed to balance the force of the water

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9
Q

What is Darcy’s law for flow rate of fluid through a porous medium?

A

Cross sectional area x permeability x hydraulic gradient

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10
Q

How is the hydraulic gradient calculated?

A
  • change in hydraulic head / change in distance
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11
Q

For a first time failure, when does a slide occur?

A

After peak strength is exceeded

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12
Q

How can a reactivated slope failure be prevented?

A

Excavation to the failure surface

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13
Q

What is regrading?

A

Moving material from the crest of a slope to the toe

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14
Q

Which are (generally) the cheapest and most expensive methods of slope retention?

A

Cheap: surface drainage
Expensive: piles

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15
Q

What can happen in loose soils with high Void ratio, such as mine tailings?

A

Water inflow causes the soil to undergo liquefaction

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16
Q

What is the active portion of a retaining wall failure?

A

The slump behind a wall as it moves away

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17
Q

What is the passive portion of a retaining wall failure?

A

The compression and lifting of soil as the wall pushes into it

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18
Q

What is special about undrained retaining wall analysis?

A

Total stress is used as opposed to effective

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19
Q

What happens when negative Earth pressures are created?

A

Tension cracks form

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20
Q

What must be included when calculating the downward force of a wall on the soil below?

A

The downward friction between the retained soil and the wall

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21
Q

How can an Earth pressure diagram be used to find the destabilising force for sliding failure of a wall?

A

Area of the diagram

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22
Q

Where is the force of a triangular Earth pressure distribution considered to act?

A

At 2/3 of the total triangle depth

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23
Q

Where is the centre of rotation of an embedded wall?

A

If the wall is 1.2d deep, the centre is at d deep

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24
Q

How does adding a soil anchor affect retaining wall rotation?

A

The anchor is taken as the centre of rotation

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25
Q

What is the allowance for unplanned over-excavation?

A

0.1 x the depth of the excavation or 0.5m, whichever is smaller

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26
Q

What is the eccentricity of a load from the centre of a foundation that causes instability?

A

Greater than B/6, B is the width of the foundation

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27
Q

If the water table is shallower than the width of a foundation, what is assumed?

A

That the water table is at the base of the foundation

28
Q

What is the resistance to gravity wall sliding failure in undrained soil?

A

Stabilising force = wall cohesion (cw) x base width

29
Q

What is the resistance to wall slip in the drained case?

A

Stabilising force = (wall weight + vertical friction) x tan wall soil friction angle [delta]

30
Q

In the translational slip equations, what is m?

A

Relative water table height: 1-(water table depth/slip surface depth)

31
Q

In principle, what is the destabilising moment of an undrained slice

A

Weight x lever arm

32
Q

How do you calculate the weight of a slice?

A

Width x centre height x bulk density

33
Q

How to you calculate lever arm of a slice for destabilising moment?

A

Slip radius x sin (base inclination angle)

34
Q

In principle, how is the stabilising moment of an undrained slice calculated?

A

Undrained strength x length of slip surface x lever arm

35
Q

How is length of slip surface of a slice calculated?

A

Width/cos (base inclination angle)

36
Q

How is inclination angle measured?

A

From the vertical at the centreline of the slice

37
Q

What is the lever arm for stabilising moment?

A

Slip radius

38
Q

In undrained conditions, what is failure shear stress equal to?

A

Undrained strength

39
Q

In drained conditions, what is failure shear stress equal to?

A

cohesion + normal effective stress x tan (failure angle)

40
Q

How does drained/undrained affect the slope factor of safety calculations?

A

Only Mstb is affected

41
Q

What is the first step of deriving the expression for drained FoS of a slip slice?

A

Consider all the forces acting on the slice I.e. weight, lateral pressure, normal resistance, shear resistance, vertical friction

42
Q

What does bishop’s method for simplifying slice calculations assume?

A

Interslice shear is neglected

N acts through the base centre

43
Q

What is the shear resistance force at the base of a slice equal to?

A

Failure shear x base length

44
Q

What is normal resistance at the base of a slice equal to?

A

Total vertical stress x base length

45
Q

What is pore water pressure ratio Ru?

A

Pore water pressure / (bulk density x slice centre height)

46
Q

What is c’

A

Soil cohesion

47
Q

What is cu

A

Undrained strength

48
Q

What occurs in coarse grained soils at critical state and why

A

“Turbulent shear” angular particles rotate slightly while maintaining volume

49
Q

If a slope has not previously failed, what must occur for a landslide to trigger?

A

Peak strength must be overcome

Critical state must be reached

50
Q

What value changes based on the previous failure conditions of a slope

A

Failure angle theta’

51
Q

What failure angle must be used when designing slopes which have previously failed?

A

Theta’ residual

52
Q

What is the relationship between stabilising and destabilising shear for an infinite slope on a Mohr’s circle diagram?

A

Stabilising shear is at the tangent point of the circle and failure line
Destabilising is at any point directly below this on a smaller circle

53
Q

What is destabilising force for a cut slip block of weight W at inclination alpha

A

W sin alpha

54
Q

What is destabilising shear stress for a translational slip in a slice of an infinite slope?

A

Destabilising force / inclined width
Or
(W/b) sin alpha cos alpha

55
Q

What is the normal force in a slice of weight W at inclination alpha?

A

W cos alpha

56
Q

What is effective normal stress in a slice of an infinite slope?

A

Normal force / inclined width
Or
W cos squared alpha

57
Q

Which shear stress in the factor of safety for an infinite slope is changed by the presence of water and why

A

Stabilising shear as it is calculated using effective normal stress

58
Q

Knowing the stability condition of a dry slope, how can the failure angle of a saturated slope be found

A

Find the failure angle for the dry slope and apply this to the factor of safety equation for the saturated slope

59
Q

How does one start performing back analysis

A

Establishing that the factor of safety at failure is 1

60
Q

What is principally different about back analysis compared to slope design?

A

Eurocode 7 factors are not applied in back analysis

61
Q

What type of failure can occur in a vertical cut?

A

Translational slip (consider forces)

62
Q

What is the difference between serviceability and ultimate limit states?

A

Serviceability just requires that the structure is still able to perform it’s required function
Ultimate limit state requires that the structure does not collapse

63
Q

How are partial factors applied in design approach 1-1?

A

To increase the effect of individual actions

64
Q

How are partial factors applied in design approach 1-2?

A

To reduce the impacts of material properties

65
Q

How are design factors applied differently between actions and material properties?

A

Actions are multiplied by the factors, properties are divided

Geotechnical Engineering 3 (4 decks)

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