Presentation on theme: "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."— Presentation transcript:
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
Types of Mass Movement FALLSLIDESLUMP FLOW
Nevado del Ruiz Mudflow 1985
Gravity Shear stress “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 ? Shear stress has …… Shear strength has …… Shear stress Shear strength = Slope stability Shear stress Shear strength = Slope failure Mass movements occur when the shear stress increases or the shear strength decreases.
Causes of Mass Movements Shear Strength Shear Stress Increase in water content of slope Increase in slope angle Removal of overlying material Shocks & vibrations WeatheringLoading 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. Think of factors that could either reduce the shear strength or increase 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 materialShocks & vibrations WeatheringLoading the slope with additional weight Alternating layers of varying rock types/lithology Undercutting the slope Burrowing animals Removal of vegetation (Aberfan, Vaiont Dam & Nevado del Ruiz) (Mam Tor, Vaiont Dam & Holbeck Hall Hotel) (Mam Tor, & Avon Gorge) (Sarno) (Mt St Helens & Elm) (Nevados de Huascaran & Mt St Helens) (Vaiont Dam)
Vaiont Dam, North Italy, 1963
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 The landslide had moved along the clay layers that parallel the bedding planes in the northern wall of the valley Oct 9, 1963 at 10:41 P.M. the south wall of the valley failed and slid into the reservoir behind the dam. 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
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
Boulder clay Dry & cracked due to 4 years of drought Above average rainfall in spring & early summer of 1993 Saturated clay is unstable Increase in weight Increase in pore water pressure Dissolves cement Cracked clay increased its permeability allowing water in
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.  (b) Draw a line on Figure 1b to show the probable surface of failure associated with the landslide. 
(c) (i) State two geological factors that may have been responsible for causing tip No.7 to fail. 
(ii) Give an explanation of the possible role played by one of the geological factors you have identified in (c) (i). 
(d) Explain how appropriate action could have reduced the risk of mass movement prior to the failure of tip No.7. 
(e) Explain one environmental problem (other than waste tipping) associated with the extraction of rock or minerals from a mine you have studied. 
Controlling Mass Movements
The toe is stabilised by gabions. The railway line is protected by hazard-resistant design structure. Toe stabilisation and hazard-resistant design Stabilisation by retaining wall and anchoring 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. 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. This increases the shear strength of the materials by reducing the pore-water pressure Regrading the slope to produce more stable angles to reduce shear stress Drainage Terracing (benches) and drainage
1.Drainage 2.Terracing (benches) and drainage This increases the shear strength of the materials by reducing the pore-water pressure 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.
Limestone interbedded with mudstones Portway, Avon Gorge Well jointed limestone Loose rock causes rockfall Frost shattering weathering Steep cliff Portway (main road at base of Avon Gorge)
Alpine canopy covered with soil & vegetation Extensive network of steel nets Bolts to hold frost-shattered rock together Portway, Avon Gorge
Mass Movements of Soil & Rock Mechanisms/Causes Management/Control benching rock anchors mesh curtains dental masonry shotcrete 1. Slope Stabilisation 2. Retaining Structures earth embankments gabions retaining walls 3. Drainage Control underground drains gravel-filled trenching shotcrete Prediction/Monitoring hazard mapping surveying/site investigations measurement of creep/strain measurement of groundwater pressures 1.Shear strength pore water pressure removal of overlying material weathering lithology differences burrowing animals removal of vegetation 2. Shear stress slope angle vibrations & shocks loading slopes undercutting of slope