mod8 Flashcards
three main types of wall
- gravity walls
- embedded or “insitu” retaining walls
- reinforced soil walls
what are retaining walls
- vertical or near-vertical structures that retain soil or rock
- used as permanent works, or temporary works such as excavation
externally stabilised systems
- gravity walls
- cantilever walls
- in-situ walls
internally stabilised systems
- reinforced soil
- in-situ reinforcement
Assumptions of Rankine’s theory
- 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
as the wall moves away from the soil
- horizontal effective stress decreases
- Mohr’s circle expands
- soil fails
Active case
wall moves away from soil
Passive case
wall pushes soil
effect of increasing phi’
- lower Pa
- higher Pp
- better wall stability
total stress analysis
undrained conditions - short term stability of a wall supporting clay
active failure - undrained conditions
- appearance of cracks (dry or flooded depending on GWL)
- adding surcharge, q, can prevent cracks
Effects of cracks on wall
- 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)
three modes of wall failure for gravity walls
- overturning
- sliding
- bearing capacity
Overturning
FOS = stabilising moment / destabilising moment
Sliding
F = horizontal resistance / horizontal drive Fslide = Vtan(delta) + Pp / Pa(horizontal)
General behaviour of embedded retaining walls
- 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
General characteristics of reinforced soil retaining walls
- 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
common reinforcing in NZ
more common to use geosynthetic reinforcement in NZ (GRS) than metal strip reinforcement which is popular in the US.
External stability of reinforced soil retaining walls
a) sliding
b) overturning
c) bearing capacity failure
d) global failure
Internal stability of reinforced soil retaining walls
a) reinforcement pull out (capacity)
b) reinforcement rupture
c) internal sliding of layers upon one another
two key design parameters for reinforced soil retaining walls
- the reinforcement length-to-wall-height ratio (L/H)
- inclination of the wall
Why is it common practice to reduce or ignore the soil’s passive resistance in th design calculations of gravity retaining walls?
because the soil needs to develop considerable strain to develop full passive resistance
- this may be unacceptable in terms of serviceability (deformation) requirements
Is the assumption of static pore pressure conditions a conservative assumption in the calculation of the stability of the wall
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
