Slope Stability II

8/7/2019 Slope Stability II

1/22

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


2/22

Slope stability II

EOSC316 Engineering

Geoscience


3/22

Types of landslide

Rock failure failure plane pre-

determined

Soil failure failure plane along line

of max stress


4/22

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


5/22

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)


6/22

Rotational slip

total stress analysis

orJu = 0 strength parameters

are those of

undrainedsoil

We

CrF

U2!

where

F = restraining moment

disturbing moment

C = cohesive strength (Pa)

r = slip circle radius (m)

U= slip sector in radiansW = weight of sliding sector (N)

e = eccentricity of sliding sector (m)


7/22

Method of slices

Swedish circle

method

For use with cohesive

and frictional soils

! nn

n

n

T

NCr

1

1

tanJU


8/22

Effect of a tension crack

Reduces the angle of

the sliding sector

VC

hc

2!

Height of tension crack:

C = cohesive strength (Pa)

= unit weight of soil (N m-3)

J= friction angle

!2

45tan2 J

V

Chc

Forfrictionless soil

Cohesive and frictional soil


9/22

Location of slip circle centre

No simple way trial

and error

F more sensitive to

horizontal movements

than vertical

movements


10/22

Effective stress analysis

!n

n

n

nfn

T

LPNCr

1

1tan)( JU

G L

hP hf

L

U

r


11/22

Other methods of analysis

Taylors stability analysis

used for frictional and cohesive soils

uses a dimensionless number to iteratetowards a solution

Bishops method

effect of forces on each side of slice

considered

iterative method


12/22

Landslip monitoring


13/22

Flowslides

Soil, clay, rock debris may behave like

liquid; water content is > liquid limit

flowslide

Flowslides are extremely mobile

e.g. Yungay, Peru, 1970


14/22

Mt. Huascaran, Peru, 1970

earthquake triggered

flowslide

hit towns of Yungay

and Ranrahirca, 18km away, at around

150 km/hr

Yungay completely

buried, 66,000 dead


15/22

Flowslide, Slumgullion, Colorado

National natural

landslide laboratory

Major slip ~3500

years ago, presentslip ~1000 years ago


16/22

The Mam Tor head scar looking west


17/22

Mam Tor landslide

Occurred due to glacially oversteepenedslopes

Age ~3600 years, from radiocarbon datingof tree remains recovered from boreholes

~300 m wide and ~1000 m long

Upper part

multiple rotation landslide

Lower part

debris flow


18/22

Cross-section through the Mam Tor landslip


19/22

Geological map and movements at each station

- 1996 to 2002


20/22

Correlation

ofmovements

with rainfall


21/22

An analogue

for

sedimentationin half-graben.

Derbyshire

County Council

is the

transportation

agent!


22/22

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.

Documents

Slope Stability II