MASS
MOVEMENTS
What are landslides?
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Preventing Landslides
Preventing Landslides 2
Preventing Landslides 3
Types of Mass Movement
FALL
SLIDE
SLUMP
FLOW
Nevado del Ruiz Mudflow 1985
Causes of Mass Movements
Shear stress
Gravity
“slide component”
Shear strength
“stick component”
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 has ……
Shear
strength
Shear
stress
=
Slope
stability
=
Slope
failure
Shear strength has ……
Shear
strength
Shear
stress
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.
Water
Max angle = angle
of repose
Internal cohesion
2. Water
Pore water pressure =
liquefaction
Causes of Mass Movements
Shear Strength
Shear Stress
Increase in water content of
slope
Increase in slope angle
Removal of overlying material
Shocks & vibrations
(Aberfan, Vaiont Dam & Nevado del Ruiz)
(Mt St Helens & Elm)
(Nevados de Huascaran & Mt St Helens)
Weathering
(Mam Tor, & Avon Gorge)
Alternating layers of varying
rock types/lithology
(Mam Tor, Vaiont Dam & Holbeck Hall Hotel)
Burrowing animals
Removal of vegetation
(Sarno)
Loading the slope with additional
weight
(Vaiont Dam)
Undercutting the slope
Vaiont Dam, North Italy, 1963
Vaiont Dam, North Italy, 1963
Syncline structure
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 10:41 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.
Aberfan, South Wales 1966
Nevados de Huascaran, Peru, 1970
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
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 8:32
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
Holbeck Hall Hotel, Scarborough, 1993
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
Sarno, Italy, 1998
Sarno
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.
(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. [1]
(c) (i) State two geological factors that may have been responsible for
causing tip No.7 to fail. [2]
(ii) Give an explanation of the possible role played by one of the
geological factors you have identified in (c) (i). [2]
(d) Explain how appropriate action could have reduced the risk of mass
movement prior to the failure of tip No.7. [3]
(e) Explain one environmental problem (other than waste tipping)
associated with the extraction of rock or minerals from a mine you have
studied. [2]
Controlling Mass Movements
•
Stabilisation by retaining wall and anchoring
•
Terracing (benches) and drainage
•
Toe stabilisation and hazard-resistant design
•
Loading the toe and retaining walls
•
Drainage
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.
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.
Regrading the slope to produce
more stable angles to reduce shear
stress
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
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.
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.
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.
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)
Portway, Avon Gorge
Extensive network of
steel nets
Bolts to hold frost-shattered
rock together
Alpine canopy covered
with soil & vegetation
Mechanisms/Causes
Management/Control
1.
1. Slope Stabilisation
Shear strength
• benching
• pore water pressure
• rock anchors
• removal of overlying material
• mesh curtains
• weathering
• dental masonry
• lithology differences
• burrowing animals
• removal of vegetation
2. Shear stress
• shotcrete
Mass Movements of
Soil & Rock
2. Retaining Structures
• earth embankments
• gabions
• retaining walls
• slope angle
• vibrations & shocks
• loading slopes
• undercutting of slope
Prediction/Monitoring
3. Drainage Control
• hazard mapping
• underground drains
• surveying/site investigations
• gravel-filled trenching
• measurement of creep/strain
• shotcrete
• measurement of groundwater
pressures