Second Term Checklist

Question 1

When does a vertical cut in clay fail?

Answer 1

When shear stress equals undrained strength

Question 2

What is the stabilising force of a vertical cut in clay?

Answer 2

Undrained strength x area (or length) of failure surface

Question 3

What is the destabilising force of a vertical cut in clay?

Answer 3

Weight resolved in the slope direction

Question 4

What is the depth of a vertical cut at which collapse occurs?

Answer 4

(2 x undrained strength) / (unit weight x sin theta x cos theta

Question 5

In what three ways do tension cracks affect cut stability?

Answer 5

Reduced length of slip surface
Change in shape of falling block
Additional destabilising force from water pressure

Question 6

What is the destabilising force of a water filled tension crack?

Answer 6

(Crack depth squared / 2) x unit weight of water

Question 7

What is the lever arm of a rotational slip?

Answer 7

The horizontal distance between the radial centre of the slip and the line of action of the weight

Question 8

Why is the lever arm for a slice “r sin alpha” where alpha is the base inclination?

Answer 8

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

Question 9

In principle, what is the factor of safety for a potential translational slip?

Answer 9

The ratio of stabilising to destabilising shear stress

Question 10

In principle, what is the factor of safety for a vertical cut?

Answer 10

The ratio of stabilising to destabilising forces along the failure surface

Question 11

List 5 methods of slope stabilisation

Answer 11

Regrading (moving material from crest to toe)
Retaining walls
Replace weak soil
Improve drainage
Nails, piles & anchors

Question 12

How does the safety of an undrained slope vary with time?

Answer 12

In embankments, outflow of water increases effective stress and hence stability.
In cuttings, outflow has the opposite effect

Question 13

How do you begin to derive the Rankine Earth pressure coefficients Ka & Kp?

Answer 13

Draw the Mohr’s circle at failure, with the principal stresses AND FAILURE ANGLE expressed in terms of the circle centre and the radius

Question 14

What are the drawbacks of Rankine theory?

Answer 14

Does not account for wall friction or wall soil adhesion

Cannot deal with sloped ground and complex geometry walls

Question 15

What are the limitations of analytical solutions for bearing capacity?

Answer 15

It requires the foundation to be assumed to be infinitely long, vertically loaded at the centre and with no groundwater

Question 16

What are the three empirical correction factors for bearing capacity?

Answer 16

Foundation shape s
Base inclination b
Inclined loading i

Question 17

If a load acts at eccentricity E from the shape centre, what area is it considered to actually act on?

Answer 17

B’ = B -2E

Question 18

What changes when a load is eccentric?

Answer 18

Working load
Shape factor s gamma
Bearing capacity

Question 19

How does effective unit weight gamma’ change based on the water table location for bearing capacity?

Answer 19

Above the water table use dry unit weight

Below, use saturated soil minus water unit weights

Question 20

What is the virtual back of an L shaped gravity wall and what is it used for?

Answer 20

The vertical line up from the back edge of the wall foot

Earth pressure is calculated at this point

Question 21

What are the 5 key steps in gravity wall design?

Answer 21

Apply EC7 design factors
Calculate Earth pressures 
Draw Earth pressure diagrams
Calculate horizontal forces
Calculate vertical friction at virtual back (if applicable)

Question 22

What is the vertical friction T caused by horizontal pressure P in soil where the soil wall friction angle is delta?

Answer 22

T = P tan delta

Question 23

What is the difference between contiguous and secant pile walls?

Answer 23

Contiguous piles do not overlap, secant piles do

Question 24

What is the minimum surcharge on the ground surface unless stated otherwise?

Answer 24

10kPa

Question 25

How are checks for overturning stability in cantilever walls typically simplified?

Answer 25

By ignoring the active and passive pressures below the point of rotation

Question 26

What must you always do before calculating the passive pressure from a cantilever wall?

Answer 26

Subtract the maximum unplanned over-excavation

Question 27

What is the head h at a point equal to?

Answer 27

Elevation head z * (pore water pressure u/unit weight of water)

Question 28

What is the active pressure coefficient Ka equal to?

Answer 28

Effective horizontal stress at failure/effective vertical stress at failure

Question 29

What is the passive pressure coefficient Kp equal to?

Answer 29

Effective vertical stress at failure/effective horizontal stress at failure

Question 30

What must be remembered about the EC7 surcharge?

Answer 30

Design factors must be applied

Question 31

What design factor is applied to minimum surcharge?

Answer 31

Variable unfavourable

Question 32

What is the equation for vertical friction due to horizontal force?

Answer 32

V = H tan delta

Question 33

How does disturbing force from a water filled tension crack act on the falling block?

Answer 33

Horizontally towards the cut face, meaning it has to be resolved into the slip direction

Question 34

What is the eccentricity of a moment load on a shallow foundation

Answer 34

Moment/Original vertical load

Question 35

How does moment loading affect the bearing capacity of a shallow foundation?

Answer 35

Correction factors and effective area are changed, but the load Q is not

Question 36

If the water table is considered level with the base of a foundation, how is this accounted for in the bearing capacity?

Answer 36

Nq term uses above wt gamma’

Ngamma term uses below wt gamma’