4 Causes of Mass Movements Shear stressGravity“slide component”Shear strength“stick component”
5 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 strengthShear stressShear stress has ……Slope stability=Shear strength has ……Shear strengthSlope failureShear stress=
6 Causes of Mass Movements Think of factors that could either reduce the shear strength or increase shear stress.Shear StrengthShear StressIncrease in water content of slopeIncrease in slope angleRemoval of overlying materialShocks & vibrationsWeatheringLoading the slope with additional weightAlternating layers of varying rock types/lithologyUndercutting the slopeBurrowing animalsRemoval of vegetationExplain how each of these either reduces shear strength or increases shear stress.
7 WaterMax angle = angle of reposeInternal cohesion
9 Causes of Mass Movements Shear StrengthShear StressIncrease in water content of slopeIncrease in slope angleRemoval of overlying materialShocks & vibrationsWeatheringLoading the slope with additional weightAlternating layers of varying rock types/lithologyUndercutting the slopeBurrowing animalsRemoval 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)
14 Vaiont Dam, North Italy, 1963limestones 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 groundwaterDuring August & September, 1963, heavy rains drenched the area adding weight to the rocks above the dam & increasing pore water pressureOct 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 valleyFilling of the reservoir had also increased fluid pressure in the pore spaces of the rock.
17 Nevados de Huascaran, Peru, 1970 magnitude 7.7 earthquakeshaking lasted for 45 seconds,large block fell from the 6 000m peakbecame 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
18 Mt St Helens, USA 1980Magma 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
20 Holbeck Hall Hotel, Scarborough, 1993 Boulder clayDry & cracked due to 4 years of droughtAbove average rainfall in spring & early summer of 1993Cracked clay increased its permeability allowing water inSaturated clay is unstableIncrease in weightIncrease in pore water pressureDissolves cement
22 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 thelandslide.
23 (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. 
24 (c) (i) State two geological factors that may have been responsible for causing tip No.7 to fail. 
25 (ii) Give an explanation of the possible role played by one of the geological factors you have identified in (c) (i). 
26 (d) Explain how appropriate action could have reduced the risk of mass movement prior to the failure of tip No.7. 
27 (e) Explain one environmental problem (other than waste tipping) associated with the extraction of rock or minerals from a mine you have studied. 
29 Stabilisation by retaining wall and anchoring Terracing (benches) and drainageToe stabilisation and hazard-resistant designLoading the toe and retaining wallsDrainageThis increases the shear strength of the materials by reducing the pore-water pressureThe 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
30 Mass Movement Stabilisation 1.DrainageThis increases the shear strength of the materials by reducing the pore-water pressure2.Terracing (benches)and drainageRe-grading the slope to produce more stable angles
31 Mass Movement Stabilisation 3.Loading the toe and retaining wallsMaterial 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.
32 Mass Movement Stabilisation 4.Stabilisation by retaining wall and anchoringThe 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.
33 Mass Movement Stabilisation 5.Toe stabilisation and hazard-resistant designThe toe is stabilised by gabions. The railway line is protected by hazard-resistant design structure.
34 Portway, Avon Gorge Limestone interbedded with mudstones Well jointed limestoneLoose rock causes rockfallFrost shattering weatheringSteep cliffPortway (main road at base of Avon Gorge)
35 Portway, Avon Gorge Extensive network of steel nets Bolts to hold frost-shattered rock togetherAlpine canopy covered with soil & vegetation
36 Mass Movements of Soil & Rock Mechanisms/CausesManagement/ControlShear strength1. Slope Stabilisationbenchingrock anchorsmesh curtainsdental masonryshotcretepore water pressureremoval of overlying materialweatheringlithology differencesburrowing animalsMass Movements of Soil & Rock2. Retaining Structuresremoval of vegetationearth embankmentsgabionsretaining walls2. Shear stressslope anglevibrations & shocksloading slopesPrediction/Monitoring3. Drainage Controlundercutting of slopehazard mappingsurveying/site investigationsmeasurement of creep/strainmeasurement of groundwater pressuresunderground drainsgravel-filled trenchingshotcrete