Principles Second Term Flashcards
What is the maximum depth of an unsupported trench?
1.2m
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?
(Gamma x H-squared)/(2 tan theta) x sin theta
What is the factor of safety defined as?
The stabilising force divided by the destabilising force
If a question gives you the value of gamma with a subscript k, what must be done?
The design value gamma d must be found using the table of factors
Why is the design approach 1-1 generally considered more critical for failure?
The material properties are unfactored
What happens to the Mohr’s circle as pore water pressure increases?
It moves towards the failure line
What is the most common type of dam in the world?
Embankment dam
Why can rapidly draining a reservoir cause a dam to fail?
Because it is designed to balance the force of the water
What is Darcy’s law for flow rate of fluid through a porous medium?
Cross sectional area x permeability x hydraulic gradient
How is the hydraulic gradient calculated?
- change in hydraulic head / change in distance
For a first time failure, when does a slide occur?
After peak strength is exceeded
How can a reactivated slope failure be prevented?
Excavation to the failure surface
What is regrading?
Moving material from the crest of a slope to the toe
Which are (generally) the cheapest and most expensive methods of slope retention?
Cheap: surface drainage
Expensive: piles
What can happen in loose soils with high Void ratio, such as mine tailings?
Water inflow causes the soil to undergo liquefaction
What is the active portion of a retaining wall failure?
The slump behind a wall as it moves away
What is the passive portion of a retaining wall failure?
The compression and lifting of soil as the wall pushes into it
What is special about undrained retaining wall analysis?
Total stress is used as opposed to effective
What happens when negative Earth pressures are created?
Tension cracks form
What must be included when calculating the downward force of a wall on the soil below?
The downward friction between the retained soil and the wall
How can an Earth pressure diagram be used to find the destabilising force for sliding failure of a wall?
Area of the diagram
Where is the force of a triangular Earth pressure distribution considered to act?
At 2/3 of the total triangle depth
Where is the centre of rotation of an embedded wall?
If the wall is 1.2d deep, the centre is at d deep
How does adding a soil anchor affect retaining wall rotation?
The anchor is taken as the centre of rotation
What is the allowance for unplanned over-excavation?
0.1 x the depth of the excavation or 0.5m, whichever is smaller
What is the eccentricity of a load from the centre of a foundation that causes instability?
Greater than B/6, B is the width of the foundation
If the water table is shallower than the width of a foundation, what is assumed?
That the water table is at the base of the foundation
What is the resistance to gravity wall sliding failure in undrained soil?
Stabilising force = wall cohesion (cw) x base width
What is the resistance to wall slip in the drained case?
Stabilising force = (wall weight + vertical friction) x tan wall soil friction angle [delta]
In the translational slip equations, what is m?
Relative water table height: 1-(water table depth/slip surface depth)
In principle, what is the destabilising moment of an undrained slice
Weight x lever arm
How do you calculate the weight of a slice?
Width x centre height x bulk density
How to you calculate lever arm of a slice for destabilising moment?
Slip radius x sin (base inclination angle)
In principle, how is the stabilising moment of an undrained slice calculated?
Undrained strength x length of slip surface x lever arm
How is length of slip surface of a slice calculated?
Width/cos (base inclination angle)
How is inclination angle measured?
From the vertical at the centreline of the slice
What is the lever arm for stabilising moment?
Slip radius
In undrained conditions, what is failure shear stress equal to?
Undrained strength
In drained conditions, what is failure shear stress equal to?
cohesion + normal effective stress x tan (failure angle)
How does drained/undrained affect the slope factor of safety calculations?
Only Mstb is affected
What is the first step of deriving the expression for drained FoS of a slip slice?
Consider all the forces acting on the slice I.e. weight, lateral pressure, normal resistance, shear resistance, vertical friction
What does bishop’s method for simplifying slice calculations assume?
Interslice shear is neglected
N acts through the base centre
What is the shear resistance force at the base of a slice equal to?
Failure shear x base length
What is normal resistance at the base of a slice equal to?
Total vertical stress x base length
What is pore water pressure ratio Ru?
Pore water pressure / (bulk density x slice centre height)
What is c’
Soil cohesion
What is cu
Undrained strength
What occurs in coarse grained soils at critical state and why
“Turbulent shear” angular particles rotate slightly while maintaining volume
If a slope has not previously failed, what must occur for a landslide to trigger?
Peak strength must be overcome
Critical state must be reached
What value changes based on the previous failure conditions of a slope
Failure angle theta’
What failure angle must be used when designing slopes which have previously failed?
Theta’ residual
What is the relationship between stabilising and destabilising shear for an infinite slope on a Mohr’s circle diagram?
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
What is destabilising force for a cut slip block of weight W at inclination alpha
W sin alpha
What is destabilising shear stress for a translational slip in a slice of an infinite slope?
Destabilising force / inclined width
Or
(W/b) sin alpha cos alpha
What is the normal force in a slice of weight W at inclination alpha?
W cos alpha
What is effective normal stress in a slice of an infinite slope?
Normal force / inclined width
Or
W cos squared alpha
Which shear stress in the factor of safety for an infinite slope is changed by the presence of water and why
Stabilising shear as it is calculated using effective normal stress
Knowing the stability condition of a dry slope, how can the failure angle of a saturated slope be found
Find the failure angle for the dry slope and apply this to the factor of safety equation for the saturated slope
How does one start performing back analysis
Establishing that the factor of safety at failure is 1
What is principally different about back analysis compared to slope design?
Eurocode 7 factors are not applied in back analysis
What type of failure can occur in a vertical cut?
Translational slip (consider forces)
What is the difference between serviceability and ultimate limit states?
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
How are partial factors applied in design approach 1-1?
To increase the effect of individual actions
How are partial factors applied in design approach 1-2?
To reduce the impacts of material properties
How are design factors applied differently between actions and material properties?
Actions are multiplied by the factors, properties are divided
