A2.3SP3 Practical Field Studies Bev Barras, Hugh Barras

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A2.3SP3Practical Field Studies Bev Barras,
Hugh Barras Mike Paul

• WELCOME TO THE MODULE

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Module Aims

• Integrated study of one specific problem -
  landsliding on the main postglacial cliff near
  Kinneil, Boness.
• Requires desk study, field survey, laboratory
  analyses and computer modelling.
• Based on a practical problem affecting the
  preserved steam railway.
• Intended to prepare you for working independently
  on your dissertation next year.

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Study Programme

• 1. Lecture and desk study
• 2. FIELDWORK FRIDAY 30th APRIL
• 3. Laboratory work
• 4. Laboratory work
• 5. Laboratory work
• 6. Lecture and data analysis
• 7. Report and poster preparation
• 8. Report and poster preparation
• 9. POSTER SESSION

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Assessment

• Literature Review (15) Thursday 6th May 2004
• Poster Presentation (25) Week 9 date TBA
• Final Report (60) Tuesday 15th June 2003

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Module Materials

• All paperwork and lecture notes can be downloaded
  from the module home page.
• Sets of reading materials will be made available
  in due course.

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SUMMARY

• Part 1 The Kinneil study area
• The geological setting
• The mid-Holocene cliff
• The local landslides
• Aims of the Desk Study
• Part 2 The analysis of landsliding
• Morphology of Landslides
• Stability analyses
• The planar slide

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INTRODUCTORY LECTURE

• PART 1
• INTRODUCTION TO
• THE KINNEIL STUDY AREA

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SUMMARY

• Part 1 The Kinneil study area
• The geological setting
• The mid-Holocene cliff
• The local landslides
• Aims of the Desk Study

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THE GEOLOGICAL SETTING
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THE MID-HOLOCENE CLIFF
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Barras 2000
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Mid-Holocene shoreline Nr Stirling Photo M.A.Paul
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Mid-Holocene shoreline Nr Stirling Photo M.A.Paul
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THE LOCAL LANDSLIDES
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THE AIMS OF THE DESK STUDY
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Aims of the Desk Study

• To establish the local geology and geomorphology
• To establish the history of movement
• To formulate a model of the landslide
• To prepare a plan for field work

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THE AIMS OF THE FIELDWORK
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Aims of the Filedwork

• To map the geomorphology of the site
• To locate the water table
• To seek evidence of movement and identify the
  style of landsliding
• To locate the outcrop of the main slip surface(s)
• To obtain a long profile of the slope
• To obtain a geological profile of the slide
• To collect samples for laboratory testing

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THE AIMS OF THE LABORATORY WORK
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Aims of the laboratory work

• To obtain parameter values for the stability
  analysis
• Natural water content
• Classification tests (liquid and plastic limits)
• In situ soil density
• Undrained shear strength (by shear vane)
• Drained shear strength (estimate from clay)

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THE AIMS OF THE MODELLING STUDY
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Aims of the modelling study

• To calculate the Factor of Safety (FoS) of the
  slope
• The FoS is required for
• The short term (rapid loading) condition
• The long term (equilibrium) condition
• Under each condition the FoS is modelled for
• Planar landsliding
• Circular rotational landsliding
• The results bracket the true FoS for a more
  complex condition.

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THE FINAL REPORT
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THE REPORT

• The report should identify the past and current
  status of the landslides
• History of movements
• Present factor of safety
• Sensitivity to future triggers
• The report should contain all the required
  evidence for the above
• Field notes, maps, survey profiles, summary calcs
• Discussion and conclusions
• Appendix of detailed field and laboratory data

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BREAK TIME!

• Please return at 11.15

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INTRODUCTORY LECTURE

• PART 2
• INTRODUCTION TO LANDSLIDING

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Morphology of Landslides

• Landslides are the result of both environmental
  and geological factors
• oversteepening by erosion
• presence of elevated pore water pressures or
  seepage
• presence of weak or incompetent strata
• progressive failure
• renewal of loading, both natural and artificial

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Morphology of Landslides
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Morphology of Landslides

• Morphological classifications normally divide
  landslides into
• Rotational slides
• Circular
• Non-circular
• Planar (slab) slides
• Flows
• Complex slides

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Morphology of Landslides

• Circular slides
• occur in cohesive sediments
• low brittleness (little loss of strength on
  failure)
• homogeneous internal structure
• Circular slides can be single or multiple
• Seen on natural slopes in homogeneous clays or as
  shallow, late stage events in the weathered zone
• Distinguished by evidence of rotation such as
  back-tilting

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Morphology of Landslides

• Non-circular slides
• occur in layered cohesive sediments
• often associated with a competent base layer that
  prevents the formation of a circular slip
• alternatively with a weaker layer that guides the
  slip surface
• Common on many natural slopes
• Distinguished by evidence of both rotation and
  translation - back-tilted graben and counter-scarp

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Morphology of Landslides

• Planar (slab) slides
• these follow some shallow, planar structure in
  the ground
• this creates one or more shear surfaces in the
  upper few metres of the ground

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Stability Analysis of Landslides
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Stability Analysis of Landslides

• Landsliding occurs when the resisting force due
  to shear strength is insufficient to counteract
  disturbing force due to soil weight plus imposed
  load.
• The ratio of the resistance to the disturbance is
  termed the factor of safety (FoS).
• Thus at failure the FoS will be equal to unity.

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Stability Analysis of Landslides

• Slope failure are classified as being either
  short-term or long-term.
• The distinction is based on the ability of the
  soil to simultaneously consolidate during shear.
  This in turn is determined by the permeability of
  the soil and the rate of failure.
• Most natural slopes are in their long-term
  condition since landsliding continues over a
  period of years.
• The initial trigger event can be short-term.

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Stability Analysis of Landslides

• Short term stability analysis
• In short-term failure there is no change in the
  water content of the soil since drainage cannot
  occur in the time available (days)
• The slide is thus analysed using the undrained
  shear strength Su
• The pore pressure is transient and is usually not
  known. Using the undrained shear strength
  automatically takes account of this.

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Stability Analysis of Landslides

• Long-term stability analysis
• In the long term drainage will occur and so the
  pore pressure can achieve an equilibrium value.
• Slides are thus analysed using the drained shear
  strength parameters (c' and ?)
• The value of the pore water pressure u is
  calculated separately from the long-term flow
  condition

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THE PLANAR SLIDE

• Long term stability analysis

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Geotechnical Analysis of Landslides

• The Disturbing Force
• The disturbing force (F) is due to the weight of
  the soil
• It is equal to the downslope component of the
  weight (W)

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Geotechnical Analysis of Landslides

• The Resisting Force
• The resisting force (S) is due to shear strength
• Shear strength arises from the drained strength
  parameters

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Geotechnical Analysis of Landslides

• Thus to calculate the FoS of a planar slide we
  must know
• The soil density
• The shear strength
• The slope angle
• The depth to the slip surface
• The worst case pore water pressure

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Geotechnical Analysis of Landslides

• Under present climatic conditions the worst case
  pore pressure normally occurs when groundwater
  level is coincident with the ground surface and
  the flow parallel to the slope.

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Geotechnical Analysis of Landslides

• QUICK APPROXIMATION
• If we assume that
• the soil has a density twice that of water
• the pore pressure is the worst case value
• the strength is entirely friction (c0)
• This leads to a limiting angle of long-term
  stability of approximately one-half of the
  internal friction angle.

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Geotechnical Analysis of Landslides

• The internal friction angle is around 30? - 40?
  for the glacial tills of east-central Scotland.
• Thus natural slopes in these materials should be
  stable at around 15? - 20? under present day
  conditions.

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SUMMARY

• Part 2 Introduction to landsliding
• Evidence of movement
• Stability analyses
• The planar slide

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THE END