MASS

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MASS MOVEMENTS
What are landslides? Video clip1 Video clip
2 Video clip 3 Video clip 4 Video clip 5 Video
clip 6 Video clip 7 Video clip 8 Preventing
Landslides Preventing Landslides 2 Preventing
Landslides 3
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Types of Mass Movement
FALL
SLIDE
SLUMP
FLOW
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Nevado del Ruiz Mudflow 1985
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Causes of Mass Movements
Shear stress
Gravity
slide component
Shear strength
stick component
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Causes of Mass Movements
In this example what has happened to the balance
between shear stress and the shear strength ?
Mass movements occur when the shear stress
increases or the shear strength decreases.
Shear stress
Shear strength
Shear stress has
Slope stability

Shear strength has
Shear strength
Slope failure
Shear stress

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Causes of Mass Movements
Think of factors that could either reduce the
shear strength or increase shear stress.
Shear Strength Shear Stress
Increase in water content of slope Increase in slope angle
Removal of overlying material Shocks vibrations
Weathering Loading the slope with additional weight
Alternating layers of varying rock types/lithology Undercutting the slope
Burrowing animals
Removal of vegetation
Explain how each of these either reduces shear
strength or increases shear stress.
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Water
Max angle angle of repose
Internal cohesion
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2. Water
Pore water pressure liquefaction
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Causes of Mass Movements
Shear Strength Shear Stress
Increase in water content of slope Increase in slope angle
Removal of overlying material Shocks vibrations
Weathering Loading the slope with additional weight
Alternating layers of varying rock types/lithology Undercutting the slope
Burrowing animals
Removal of vegetation
(Mt St Helens Elm)
(Aberfan, Vaiont Dam Nevado del Ruiz)
(Nevados de Huascaran Mt St Helens)
(Mam Tor, Avon Gorge)
(Vaiont Dam)
(Mam Tor, Vaiont Dam Holbeck Hall Hotel)
(Sarno)
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Vaiont Dam, North Italy, 1963
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Vaiont Dam, North Italy, 1963
Syncline structure
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Vaiont Dam, North Italy, 1963

• limestones inter-bedded with sands and clays. 

• bedding planes that parallel the syncline
  structure, dipping steeply into the valley from
  both sides.

• Some of the limestone beds had caverns, due to
  chemical weathering by groundwater

• During August September, 1963, heavy rains
  drenched the area adding weight to the rocks
  above the dam increasing pore water pressure

• Oct 9, 1963 at 1041 P.M. the south wall of the
  valley failed and slid into the reservoir behind
  the dam. 

• The landslide had moved along the clay layers
  that parallel the bedding planes in the northern
  wall of the valley

• Filling of the reservoir had also increased
  fluid pressure in the pore spaces of the rock. 

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Aberfan, South Wales 1966
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Nevados de Huascaran, Peru, 1970
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Nevados de Huascaran, Peru, 1970

• magnitude 7.7 earthquake

• shaking lasted for 45 seconds,

• large block fell from the 6 000m peak

• became a debris avalanche sliding across the
  snow covered glacier at velocities up to 335
  km/hr.

• hit a small hill and was launched into the air
  as an airborne debris avalanche. 

• blocks the size of large houses fell on real
  houses for another 4 km. 

• recombined and continued as a debris flow,
  burying the town of Yungay

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Mt St Helens, USA 1980

• Magma moved high into the cone of Mount St.
  Helens and inflated the volcano's north side
  outward by at least 150 m. This dramatic
  deformation was called the "bulge. This
  increased the shear stress.

• Within minutes of a magnitude 5.1 earthquake at
  832 a.m., a huge landslide completely removed
  the bulge, the summit, and inner core of Mount
  St. Helens, and triggered a series of massive
  explosions.

• As the landslide moved down the volcano at a
  velocity of nearly 300 km/hr, the explosions grew
  in size and speed and a low eruption cloud began
  to form above the summit area

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Holbeck Hall Hotel, Scarborough, 1993
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Holbeck Hall Hotel, Scarborough, 1993

• Boulder clay

• Dry cracked due to 4 years of drought

• Above average rainfall in spring early summer
  of 1993

• Cracked clay increased its permeability allowing
  water in

• Saturated clay is unstable

• Increase in weight

• Increase in pore water pressure

• Dissolves cement

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Sarno, Italy, 1998
Sarno
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Figure 1a shows the site of the former Aberfan
coal-waste tips (South Wales), one of which (tip
No.7) suffered a major landslide and associated
debris flow in 1966.
Figure 1b is a geological section through tip
No.7 and the underlying geology prior to
the landslide.
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(a) On the geological section (Figure 1b), mark
with a labelled arrow ( S) the location of the
spring beneath tip No.7. Account for the presence
of a spring at this location. 2
(b) Draw a line on Figure 1b to show the probable
surface of failure associated with the landslide.
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(c) (i) State two geological factors that may
have been responsible for causing tip No.7 to
fail. 2
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(ii) Give an explanation of the possible role
played by one of the geological factors you have
identified in (c) (i). 2
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(d) Explain how appropriate action could have
reduced the risk of mass movement prior to the
failure of tip No.7. 3
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(e) Explain one environmental problem (other than
waste tipping) associated with the extraction of
rock or minerals from a mine you have studied. 2
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Controlling Mass Movements
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• Stabilisation by retaining wall and anchoring

• Terracing (benches) and drainage

• Toe stabilisation and hazard-resistant design

• Loading the toe and retaining walls

• Drainage

This increases the shear strength of the
materials by reducing the pore-water pressure
The toe is stabilised by retaining wall which
reduces the shear stress. The upper slope has
rock anchors and mesh curtains. Drains improve
water movement and shotcrete is used to reduce
infiltration into the hillside.
Material deposited at the slope foot (toe)
reduces the shear stress. Retaining walls are
used to stabilise the upper slope. In this case
a steel-mesh curtain is used.
The toe is stabilised by gabions. The railway
line is protected by hazard-resistant design
structure.
Regrading the slope to produce more stable angles
to reduce shear stress
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Mass Movement Stabilisation
1.Drainage
This increases the shear strength of the
materials by reducing the pore-water pressure
2.Terracing (benches) and drainage
Re-grading the slope to produce more stable angles
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Mass Movement Stabilisation
3.Loading the toe and retaining walls
Material deposited at the slope foot (toe)
reduces the shear stress. Retaining walls are
used to stabilise the upper slope. In this case
a steel-mesh curtain is used.
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Mass Movement Stabilisation
4.Stabilisation by retaining wall and anchoring
The toe is stabilised by retaining wall. The
upper slope has rock anchors and mesh curtains.
Drains improve water movement and shotcrete is
used to reduce infiltration into the hillside.
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Mass Movement Stabilisation
5.Toe stabilisation and hazard-resistant design
The toe is stabilised by gabions. The railway
line is protected by hazard-resistant design
structure.
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Portway, Avon Gorge
Limestone interbedded with mudstones
Well jointed limestone
Loose rock causes rockfall
Frost shattering weathering
Steep cliff
Portway (main road at base of Avon Gorge)
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Portway, Avon Gorge
Extensive network of steel nets
Bolts to hold frost-shattered rock together
Alpine canopy covered with soil vegetation
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Mechanisms/Causes
Management/Control
1. Slope Stabilisation

1. Shear strength

• benching
• rock anchors
• mesh curtains
• dental masonry
• shotcrete

• pore water pressure

• removal of overlying material

• weathering

• lithology differences

• burrowing animals

Mass Movements of Soil Rock
2. Retaining Structures

• removal of vegetation

• earth embankments
• gabions
• retaining walls

2. Shear stress

• slope angle

• vibrations shocks

• loading slopes

Prediction/Monitoring
3. Drainage Control

• undercutting of slope

• underground drains
• gravel-filled trenching
• shotcrete

• hazard mapping
• surveying/site investigations
• measurement of creep/strain
• measurement of groundwater pressures