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.