mod7 Flashcards
three categories of failure
- planar, “infinite” slopes
- rotational failures
- wedge-type failures
F
Factor of Safety = available strength / actual stress
If F = 1
tan (phi’mob) = tan (phi’) – slope just at point of failure
If F > 1
tan ( phi’mob ) < tan (phi’) – slope is stable
If F < 1
the slope should fail as tan (phi’mob) cannot be greater than tan (phi’)
worst case scenario for infinite slope
fully saturated slope seepage parallel to slope
h = z cos^2 (beta)
steps in calculating a limit equilibrium mechanism solution
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
Rotational failure of slopes assumes:
- 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)
Assumptions of Taylor’s chart methods
- no cracks
- no surface loading
- constant shear strength with depth
- homogenous soil
Swedish Slip Circle Method
- 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
Total strain analysis methods
- Taylor’s chart methods
- Swedish slip circle method
Effective stress analysis methods
- Fellenius method
- Bishop’s simplified (or routine) method
Fellenius method
- 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)
Bishop’s method
- assumed that vertical interslice forces have resultant of zero
ru
pore pressure ratio = pore water pressure / total vertical stress
ru = u / sigma
- can be calculated at the base of each slice
rapid drawdown failure
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
residual friction angle of clays
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)
residual friction angle can occur due to either:
- large movements in a particular direction; or
- repeated smaller movements on one plane
causing the clay particles to align themselves in one direction
principal processes to cause residual conditions:
- landsliding
- tectonic folding
- valley rebound
- glacier movements
- periglacial phenomena
- non-uniform swelling
Identification of residual strength
- major difficulty is the fact that large displacements may be required to achieve the degree of orientation of the particles
- use ring shear aparatus
ring shear apparatus
- 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
Desk study
- satellite
2. aerial photographs
walk over-study
- general signs of instability
- watercourses or related features
- disrupted artifical features
general signs of instability
- slip scarps
- counterscarps or garbens
- soil waves; back-filled blocks
- toe bulges
- tensile cracks (easy to see)
watercourses or related features
- landslip ponds
- springs and sinks
- marshy ground
disrupted artifical features
- breaks in walls
- distorted openings in buildings
- damaged services
- cracked/recently repaired roads
- dislocated fencelines
Why slopes can become unstable
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
Cut and Fill Solutions
- Grade to a uniform, flatter slope
- reduce overall height
- add fill to toe of slope and/or use “berms”
- use a lightweight fill, such as polystyrene or wood chips, to reduce surface loading at upper parts of failiure surface
Drainage
- can be very effective method of stabilising slopes
- drains MUST be maintained to be effective at all times
shallow drains
- designed to control movement of surface water and hence the hydraulic boundary conditions of the seepage regime of the slope
deep drains
- 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
Structural Measures
- soil anchors
- soil nails
- retaining wall
- piles
- geogrids
- grouting
- line stabilisation
- recompaction to destroy slip surface
Considerations for type of remedial action
- immediary of effect/solution
- cost
- continued maintenance
- land required
- type/cause of instability
First choice remedial action
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
considerations for structural stabilisation
- can be expensive
- does however allow emergency repairs to be done on a moving slope such as a railway embankment
