slope stability ii 2
TRANSCRIPT
Mam Tor Fieldclass
• Thursday 17th March– depart from Department at 8.45am– return approx. 6pm (depending on traffic)
• You will need:– standard field gear
• mapping board• notebook• compass-clinometer• ruler• stationary• camera?• LUNCH
Slope stability II
EOSC316 Engineering Geoscience
Types of landslide
• Rock failure– failure plane pre-
determined
• Soil failure– failure plane along line
of max stress
Types of landslide
• Rock failure– failure along pre-determined planes of
weakness
• Soil failure– failure along lines of max. stress
• frictional, cohesive = rotational• frictional, incohesive = planar
Rotational landslip analysis
• For undrained frictionless failure– total stress analysis
• For cohesive and frictional failure– method of slices– Bishop’s conventional method (can take into
account pore water pressure)
Rotational slip
• total stress analysis oru = 0
• strength parameters are those of undrained soil
We
CrF
2
whereF = restraining moment disturbing moment
C = cohesive strength (Pa)r = slip circle radius (m)= slip sector in radiansW = weight of sliding sector (N)e = eccentricity of sliding sector (m)
Method of slices
• Swedish circle method
• For use with cohesive and frictional soils
n
n
n
n
T
NCrF
1
1tan
Effect of a tension crack
• Reduces the angle of the sliding sector
C
hc2
Height of tension crack:
C = cohesive strength (Pa)= unit weight of soil (N m-3)= friction angle
245tan
2 C
hc
For frictionless soil
Cohesive and frictional soil
Location of slip circle centre
• No simple way – trial and error
• F more sensitive to horizontal movements than vertical movements
Effective stress analysis
n
n
n
nfn
T
LPNCrF
1
1tan)(
GW L
hP = h x f w
L
r
Other methods of analysis
• Taylor’s stability analysis– used for frictional and cohesive soils– uses a dimensionless number to iterate
towards a solution
• Bishop’s method– effect of forces on each side of slice
considered– iterative method
Landslip monitoring
Flowslides
• Soil, clay, rock debris may behave like liquid; water content is > liquid limit– flowslide
• Flowslides are extremely mobile– e.g. Yungay, Peru, 1970
Mt. Huascaran, Peru, 1970
• earthquake triggered flowslide
• hit towns of Yungay and Ranrahirca, 18 km away, at around 150 km/hr
• Yungay completely buried, 66,000 dead
Flowslide, Slumgullion, Colorado
• National natural landslide laboratory
• Major slip ~3500 years ago, present slip ~1000 years ago
The Mam Tor head scar – looking west
Mam Tor landslide
• Occurred due to glacially oversteepened slopes
• Age ~3600 years, from radiocarbon dating of tree remains recovered from boreholes
• ~300 m wide and ~1000 m long• Upper part
– multiple rotation landslide
• Lower part– debris flow
Cross-section through the Mam Tor landslip
Geological map and movements at each station - 1996 to 2002
Correlation of movements with rainfall
An analogue for sedimentation in half-graben.
Derbyshire County Council is the transportation agent!
Mam Tor references
• Skempton, A.W. et al., 1989, The Mam Tor landslide, North Derbyshire, Phil. Trans. Royal Soc. Lond. 329, 503-547
• Rutter, E.H. et al., 2003, Strain displacements in the Mam Tor landslip, Derbyshire, England, J. Geol. Soc. Lond. 160, 735-744.