Principles Second Term

Question 1

What is the maximum depth of an unsupported trench?

Answer 1

1.2m

Question 2

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?

Answer 2

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

Question 3

What is the factor of safety defined as?

Answer 3

The stabilising force divided by the destabilising force

Question 4

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

Answer 4

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

Question 5

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

Answer 5

The material properties are unfactored

Question 6

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

Answer 6

It moves towards the failure line

Question 7

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

Answer 7

Embankment dam

Question 8

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

Answer 8

Because it is designed to balance the force of the water

Question 9

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

Answer 9

Cross sectional area x permeability x hydraulic gradient

Question 10

How is the hydraulic gradient calculated?

Answer 10

• change in hydraulic head / change in distance

Question 11

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

Answer 11

After peak strength is exceeded

Question 12

How can a reactivated slope failure be prevented?

Answer 12

Excavation to the failure surface

Question 13

What is regrading?

Answer 13

Moving material from the crest of a slope to the toe

Question 14

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

Answer 14

Cheap: surface drainage
Expensive: piles

Question 15

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

Answer 15

Water inflow causes the soil to undergo liquefaction

Question 16

What is the active portion of a retaining wall failure?

Answer 16

The slump behind a wall as it moves away

Question 17

What is the passive portion of a retaining wall failure?

Answer 17

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

Question 18

What is special about undrained retaining wall analysis?

Answer 18

Total stress is used as opposed to effective

Question 19

What happens when negative Earth pressures are created?

Answer 19

Tension cracks form

Question 20

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

Answer 20

The downward friction between the retained soil and the wall

Question 21

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

Answer 21

Area of the diagram

Question 22

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

Answer 22

At 2/3 of the total triangle depth

Question 23

Where is the centre of rotation of an embedded wall?

Answer 23

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

Question 24

How does adding a soil anchor affect retaining wall rotation?

Answer 24

The anchor is taken as the centre of rotation

Question 25

What is the allowance for unplanned over-excavation?

Answer 25

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

Question 26

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

Answer 26

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

Question 27

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

Answer 27

That the water table is at the base of the foundation

Question 28

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

Answer 28

Stabilising force = wall cohesion (cw) x base width

Question 29

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

Answer 29

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

Question 30

In the translational slip equations, what is m?

Answer 30

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

Question 31

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

Answer 31

Weight x lever arm

Question 32

How do you calculate the weight of a slice?

Answer 32

Width x centre height x bulk density

Question 33

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

Answer 33

Slip radius x sin (base inclination angle)

Question 34

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

Answer 34

Undrained strength x length of slip surface x lever arm

Question 35

How is length of slip surface of a slice calculated?

Answer 35

Width/cos (base inclination angle)

Question 36

How is inclination angle measured?

Answer 36

From the vertical at the centreline of the slice

Question 37

What is the lever arm for stabilising moment?

Answer 37

Slip radius

Question 38

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

Answer 38

Undrained strength

Question 39

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

Answer 39

cohesion + normal effective stress x tan (failure angle)

Question 40

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

Answer 40

Only Mstb is affected

Question 41

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

Answer 41

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

Question 42

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

Answer 42

Interslice shear is neglected | N acts through the base centre

Question 43

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

Answer 43

Failure shear x base length

Question 44

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

Answer 44

Total vertical stress x base length

Question 45

What is pore water pressure ratio Ru?

Answer 45

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

Question 46

What is c’

Answer 46

Soil cohesion

Question 47

What is cu

Answer 47

Undrained strength

Question 48

What occurs in coarse grained soils at critical state and why

Answer 48

“Turbulent shear” angular particles rotate slightly while maintaining volume

Question 49

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

Answer 49

Peak strength must be overcome | Critical state must be reached

Question 50

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

Answer 50

Failure angle theta’

Question 51

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

Answer 51

Theta’ residual

Question 52

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

Answer 52

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

Question 53

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

Answer 53

W sin alpha

Question 54

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

Answer 54

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

Question 55

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

Answer 55

W cos alpha

Question 56

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

Answer 56

Normal force / inclined width Or W cos squared alpha

Question 57

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

Answer 57

Stabilising shear as it is calculated using effective normal stress

Question 58

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

Answer 58

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

Question 59

How does one start performing back analysis

Answer 59

Establishing that the factor of safety at failure is 1

Question 60

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

Answer 60

Eurocode 7 factors are not applied in back analysis

Question 61

What type of failure can occur in a vertical cut?

Answer 61

Translational slip (consider forces)

Question 62

What is the difference between serviceability and ultimate limit states?

Answer 62

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

Question 63

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

Answer 63

To increase the effect of individual actions

Question 64

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

Answer 64

To reduce the impacts of material properties

Question 65

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

Answer 65

Actions are multiplied by the factors, properties are divided