Second Term Checklist Flashcards
When does a vertical cut in clay fail?
When shear stress equals undrained strength
What is the stabilising force of a vertical cut in clay?
Undrained strength x area (or length) of failure surface
What is the destabilising force of a vertical cut in clay?
Weight resolved in the slope direction
What is the depth of a vertical cut at which collapse occurs?
(2 x undrained strength) / (unit weight x sin theta x cos theta
In what three ways do tension cracks affect cut stability?
Reduced length of slip surface
Change in shape of falling block
Additional destabilising force from water pressure
What is the destabilising force of a water filled tension crack?
(Crack depth squared / 2) x unit weight of water
What is the lever arm of a rotational slip?
The horizontal distance between the radial centre of the slip and the line of action of the weight
Why is the lever arm for a slice “r sin alpha” where alpha is the base inclination?
The radius is perpendicular to the base so Alpha is also the angle of the radius to the vertical. Therefore r sin alpha is the horizontal distance
In principle, what is the factor of safety for a potential translational slip?
The ratio of stabilising to destabilising shear stress
In principle, what is the factor of safety for a vertical cut?
The ratio of stabilising to destabilising forces along the failure surface
List 5 methods of slope stabilisation
Regrading (moving material from crest to toe) Retaining walls Replace weak soil Improve drainage Nails, piles & anchors
How does the safety of an undrained slope vary with time?
In embankments, outflow of water increases effective stress and hence stability.
In cuttings, outflow has the opposite effect
How do you begin to derive the Rankine Earth pressure coefficients Ka & Kp?
Draw the Mohr’s circle at failure, with the principal stresses AND FAILURE ANGLE expressed in terms of the circle centre and the radius
What are the drawbacks of Rankine theory?
Does not account for wall friction or wall soil adhesion
Cannot deal with sloped ground and complex geometry walls
What are the limitations of analytical solutions for bearing capacity?
It requires the foundation to be assumed to be infinitely long, vertically loaded at the centre and with no groundwater
What are the three empirical correction factors for bearing capacity?
Foundation shape s
Base inclination b
Inclined loading i
If a load acts at eccentricity E from the shape centre, what area is it considered to actually act on?
B’ = B -2E
What changes when a load is eccentric?
Working load
Shape factor s gamma
Bearing capacity
How does effective unit weight gamma’ change based on the water table location for bearing capacity?
Above the water table use dry unit weight
Below, use saturated soil minus water unit weights
What is the virtual back of an L shaped gravity wall and what is it used for?
The vertical line up from the back edge of the wall foot
Earth pressure is calculated at this point
What are the 5 key steps in gravity wall design?
Apply EC7 design factors Calculate Earth pressures Draw Earth pressure diagrams Calculate horizontal forces Calculate vertical friction at virtual back (if applicable)
What is the vertical friction T caused by horizontal pressure P in soil where the soil wall friction angle is delta?
T = P tan delta
What is the difference between contiguous and secant pile walls?
Contiguous piles do not overlap, secant piles do
What is the minimum surcharge on the ground surface unless stated otherwise?
10kPa
How are checks for overturning stability in cantilever walls typically simplified?
By ignoring the active and passive pressures below the point of rotation
What must you always do before calculating the passive pressure from a cantilever wall?
Subtract the maximum unplanned over-excavation
What is the head h at a point equal to?
Elevation head z * (pore water pressure u/unit weight of water)
What is the active pressure coefficient Ka equal to?
Effective horizontal stress at failure/effective vertical stress at failure
What is the passive pressure coefficient Kp equal to?
Effective vertical stress at failure/effective horizontal stress at failure
What must be remembered about the EC7 surcharge?
Design factors must be applied
What design factor is applied to minimum surcharge?
Variable unfavourable
What is the equation for vertical friction due to horizontal force?
V = H tan delta
How does disturbing force from a water filled tension crack act on the falling block?
Horizontally towards the cut face, meaning it has to be resolved into the slip direction
What is the eccentricity of a moment load on a shallow foundation
Moment/Original vertical load
How does moment loading affect the bearing capacity of a shallow foundation?
Correction factors and effective area are changed, but the load Q is not
If the water table is considered level with the base of a foundation, how is this accounted for in the bearing capacity?
Nq term uses above wt gamma’
Ngamma term uses below wt gamma’
