The document provides details about an upcoming fieldclass to Mam Tor on March 17th. Students are instructed to meet at the Department at 8:45am and return around 6pm, bringing standard field gear including a mapping board, notebook, compass, ruler, stationary and camera. The fieldclass will involve studying the Mam Tor landslide, a large rotational landslide that occurred approximately 3,600 years ago on glacially oversteepened slopes. Students will examine the geological map and movements monitored between 1996-2002, and discuss how the landslide provides an analogue for sedimentation in half-graben structures.

1. 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

2. Slope stability II
EOSC316 Engineering
Geoscience

3. Types of landslide
• Rock failure • Soil failure
– failure plane pre- – failure plane along line
determined of max stress

4. 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

5. 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)

6. Rotational slip
• total stress analysis
or φu = 0
• strength parameters
are those of
undrained soil
where
F = restraining moment
disturbing moment
Cr θ 2
F= C = cohesive strength (Pa)
r = slip circle radius (m)
We θ= slip sector in radians
W = weight of sliding sector (N)
e = eccentricity of sliding sector (m)

7. Method of slices
• Swedish circle
method
• For use with cohesive
and frictional soils
Crθ + ∑1 N n tan φ
n
F=
∑T
n
1 n

8. Effect of a tension crack
• Reduces the angle of
the sliding sector
Height of tension crack:
For frictionless soil
2C
hc =
ρ
Cohesive and frictional soil
C = cohesive strength (Pa)
ρ= unit weight of soil (N m-3)
2C  φ φ = friction angle
hc = tan  45 + 
ρ  2

9. Location of slip circle centre
• No simple way – trial
and error
• F more sensitive to
horizontal movements
than vertical
movements

10. Effective stress analysis
θ
G W L r
P f = h x ρw
h
L
Crθ + ∑1 ( N n − Pf Ln ) tan φ
n
F=
∑T
n
1 n

11. 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

12. Landslip monitoring

13. Flowslides
• Soil, clay, rock debris may behave like
liquid; water content is > liquid limit
– flowslide
• Flowslides are extremely mobile
– e.g. Yungay, Peru, 1970

14. 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

15. Flowslide, Slumgullion, Colorado
• National natural
landslide laboratory
• Major slip ~3500
years ago, present
slip ~1000 years ago

16. The Mam Tor head scar – looking west

17. 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

18. Cross-section through the Mam Tor landslip

19. Geological map and movements at each station
- 1996 to 2002

20. Correlation
of
movements
with rainfall

21. An analogue
for
sedimentation
in half-graben.
Derbyshire
County Council
is the
transportation
agent!

22. 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.