Post on 11-Nov-2015
description
Mam Tor Fieldclass Sunday 12th March
depart from Department at 8.45am Pick up Greenbank at 9am 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 Bishops conventional method (can take into
account pore water pressure)
Rotational slip
total stress analysis or u = 0
strength parameters are those of undrained soil
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)
WeCrF
2
=
Method of slices
Swedish circle method
For use with cohesive and frictional soils
+= n
n
nn
T
NCrF
1
1tan
Effect of a tension crack
Reduces the angle of the sliding sector
Height of tension crack:
For frictionless soil
Chc
2=
C = cohesive strength (Pa)= unit weight of soil (N m-3) = friction angle
+=
245tan2
Chc
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
GWL
hP = h x f w
L
r
+= n
n
nnfn
T
LPNCrF
1
1tan)(
Other methods of analysis
Taylors stability analysis used for frictional and cohesive soils uses a dimensionless number to iterate
towards a solution Bishops 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 Yungayand 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 oversteepenedslopes
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 MamTor 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.