mod8

Question 1

three main types of wall

Answer 1

• gravity walls
• embedded or “insitu” retaining walls
• reinforced soil walls

Question 2

what are retaining walls

Answer 2

• vertical or near-vertical structures that retain soil or rock
• used as permanent works, or temporary works such as excavation

Question 3

externally stabilised systems

Answer 3

• gravity walls
• cantilever walls
• in-situ walls

Question 4

internally stabilised systems

Answer 4

• reinforced soil

- in-situ reinforcement

Question 5

Assumptions of Rankine’s theory

Answer 5

• soil is homogenous and isotropic
• critical shear surface at failure is a plane
• ground surface is a plane
• wall is infinitely long
• wall moves slightly to active/passive condition
• thrust is parallel to ground surface
• frictionless wall

Question 6

as the wall moves away from the soil

Answer 6

• horizontal effective stress decreases
• Mohr’s circle expands
• soil fails

Question 7

Active case

Answer 7

wall moves away from soil

Question 8

Passive case

Answer 8

wall pushes soil

Question 9

effect of increasing phi’

Answer 9

• lower Pa
• higher Pp
• better wall stability

Question 10

total stress analysis

Answer 10

undrained conditions - short term stability of a wall supporting clay

Question 11

active failure - undrained conditions

Answer 11

• appearance of cracks (dry or flooded depending on GWL)

- adding surcharge, q, can prevent cracks

Question 12

Effects of cracks on wall

Answer 12

• dry tension crack reduces the lateral stress on the active side of the wall (theoretically good for stability)
• however a flooded tension crack increases the lateral stress on the active side (bad for stability)
• important to know which case you have - if in doubt assume the worst (flooded crack)

Question 13

three modes of wall failure for gravity walls

Answer 13

1. overturning
2. sliding
3. bearing capacity

Question 14

Overturning

Answer 14

FOS = stabilising moment / destabilising moment

Question 15

Sliding

Answer 15

F = horizontal resistance / horizontal drive
Fslide = Vtan(delta) + Pp / Pa(horizontal)

Question 16

General behaviour of embedded retaining walls

Answer 16

• derive their stability from development of passive resistance (expected to deflect below excavation level)
• can be cantilevered (unpropped)
• often are propped and/or tied
• relatively light structures (self weight usually ignored)
• geotechnically they generally fail by overturning, sliding, bearing capacity
• structurally they may fail via bending and shear, or prop may fail

Question 17

General characteristics of reinforced soil retaining walls

Answer 17

• a reinforced soil block (reinforcement, cohesion-less granular fill)
• a concrete facing (can vary in design)
• a connection between the reinforcement and the facing
• a retained backfill behind the reinforced soil block

Question 18

common reinforcing in NZ

Answer 18

more common to use geosynthetic reinforcement in NZ (GRS) than metal strip reinforcement which is popular in the US.

Question 19

External stability of reinforced soil retaining walls

Answer 19

a) sliding
b) overturning
c) bearing capacity failure
d) global failure

Question 20

Internal stability of reinforced soil retaining walls

Answer 20

a) reinforcement pull out (capacity)
b) reinforcement rupture
c) internal sliding of layers upon one another

Question 21

two key design parameters for reinforced soil retaining walls

Answer 21

• the reinforcement length-to-wall-height ratio (L/H)

- inclination of the wall

Question 22

Why is it common practice to reduce or ignore the soil’s passive resistance in th design calculations of gravity retaining walls?

Answer 22

because the soil needs to develop considerable strain to develop full passive resistance
- this may be unacceptable in terms of serviceability (deformation) requirements

Question 23

Is the assumption of static pore pressure conditions a conservative assumption in the calculation of the stability of the wall

Answer 23

Yes

• if flow were taken into account, the pore pressure on the high side of the wall would be lower than hydrostatic
• this would lead to an increased effective stress, and less active pressure
• this would give a higher factor of safety against sliding/overturning