slope stability ii 2

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.