mod7

Question 1

three categories of failure

Answer 1

1. planar, “infinite” slopes
2. rotational failures
3. wedge-type failures

Question 2

F

Answer 2

Factor of Safety = available strength / actual stress

Question 3

If F = 1

Answer 3

tan (phi’mob) = tan (phi’) – slope just at point of failure

Question 4

If F > 1

Answer 4

tan ( phi’mob ) < tan (phi’) – slope is stable

Question 5

If F < 1

Answer 5

the slope should fail as tan (phi’mob) cannot be greater than tan (phi’)

Question 6

worst case scenario for infinite slope

Answer 6

fully saturated slope seepage parallel to slope

h = z cos^2 (beta)

Question 7

steps in calculating a limit equilibrium mechanism solution

Answer 7

a) draw an arbitrary collapse mechanism of slip surfaces
b) calculate the statical equilbrium of the components of the mechanism to determine the strength mobilised in the soil or the external forces
c) examine other mechanisms to find the critical one for which the loading is the limit equilibrium load

Question 8

Rotational failure of slopes assumes:

Answer 8

• that conditions of plane strain apply (conservative assumption)
• that failure occurs along a circular surface
  (strong assumption for a fairly homogenous deposit)
  (modification may be required for more inhomogenous slopes)

Question 9

Assumptions of Taylor’s chart methods

Answer 9

• no cracks
• no surface loading
• constant shear strength with depth
• homogenous soil

Question 10

Swedish Slip Circle Method

Answer 10

• used to examine more complex topography and situations where Cu is not constant throughout the slope
• can also include surface loads and tension cracks if need be

Question 11

Total strain analysis methods

Answer 11

• Taylor’s chart methods

- Swedish slip circle method

Question 12

Effective stress analysis methods

Answer 12

• Fellenius method

- Bishop’s simplified (or routine) method

Question 13

Fellenius method

Answer 13

• assumed that the resultant of both the horizontal and vertical interslice forces is zero
• underestimates F by 5-20%
• simple but not very accurate (leads to an expensive design)

Question 14

Bishop’s method

Answer 14

• assumed that vertical interslice forces have resultant of zero

Question 15

ru

Answer 15

pore pressure ratio = pore water pressure / total vertical stress
ru = u / sigma
- can be calculated at the base of each slice

Question 16

rapid drawdown failure

Answer 16

when the stabilising effect of exterior water pressure is removed

• high pore pressures in the slope can be maintained if the soil has low permeability
• factor of safety reduces as the level of the water is reduced
• over time, as drainage occurs, the factor of safety should increase

Question 17

residual friction angle of clays

Answer 17

in clay soils

• if the material in situ already contains slip planes, or shear surfaces, then the strength available on these surfaces will be less than the peak strength
• if sufficient displacement has taken place the strength may be as low as the residual strength (due to a low residual friction angle)

Question 18

residual friction angle can occur due to either:

Answer 18

• large movements in a particular direction; or
• repeated smaller movements on one plane
  causing the clay particles to align themselves in one direction

Question 19

principal processes to cause residual conditions:

Answer 19

• landsliding
• tectonic folding
• valley rebound
• glacier movements
• periglacial phenomena
• non-uniform swelling

Question 20

Identification of residual strength

Answer 20

• major difficulty is the fact that large displacements may be required to achieve the degree of orientation of the particles
• use ring shear aparatus

Question 21

ring shear apparatus

Answer 21

• provides large displacements required to define the complete shear stress displacement relationship
• consists of two metal rings which hold an annular sample
• sample is subject to a normal stress and then one pair of rings is subjected to rotation

Question 22

Desk study

Answer 22

1. satellite

2. aerial photographs

Question 23

walk over-study

Answer 23

• general signs of instability
• watercourses or related features
• disrupted artifical features

Question 24

general signs of instability

Answer 24

• slip scarps
• counterscarps or garbens
• soil waves; back-filled blocks
• toe bulges
• tensile cracks (easy to see)

Question 25

watercourses or related features

Answer 25

- landslip ponds - springs and sinks - marshy ground

Question 26

disrupted artifical features

Answer 26

- breaks in walls - distorted openings in buildings - damaged services - cracked/recently repaired roads - dislocated fencelines

Question 27

Why slopes can become unstable

Answer 27

i) if the slope is too steep ii) if the slope is too high iii) if the material is too weak iv) if the pore water pressure is too high v) if the slope is subjected to undesirable external forces

Question 28

Cut and Fill Solutions

Answer 28

1. Grade to a uniform, flatter slope 2. reduce overall height 3. add fill to toe of slope and/or use "berms" 4. use a lightweight fill, such as polystyrene or wood chips, to reduce surface loading at upper parts of failiure surface

Question 29

Drainage

Answer 29

- can be very effective method of stabilising slopes | - drains MUST be maintained to be effective at all times

Question 30

shallow drains

Answer 30

- designed to control movement of surface water and hence the hydraulic boundary conditions of the seepage regime of the slope

Question 31

deep drains

Answer 31

- modify the seepage pattern within the soil mass - more expensive than shallow drains - usually very effective because they remover or decrease the pore water pressure directly at the seat of the problem

Question 32

Structural Measures

Answer 32

- soil anchors - soil nails - retaining wall - piles - geogrids - grouting - line stabilisation - recompaction to destroy slip surface

Question 33

Considerations for type of remedial action

Answer 33

- immediary of effect/solution - cost - continued maintenance - land required - type/cause of instability

Question 34

First choice remedial action

Answer 34

usually cut and fill and/or regrading - effect is immediate, possibly improving with time - unlikely to be tampered with - often the cheapest option - may not be suitable in built-up areas

Question 35

considerations for structural stabilisation

Answer 35

- can be expensive | - does however allow emergency repairs to be done on a moving slope such as a railway embankment