MASS MOVEMENTS What are landslides? Video clip1 Video clip 2 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 strength Shear stress Shear stress has …… Slope stability = Shear strength has …… Shear strength Slope failure 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 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)

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 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

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

Mass Movements of Soil & Rock Mechanisms/Causes Management/Control Shear strength 1. Slope Stabilisation 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 hazard mapping surveying/site investigations measurement of creep/strain measurement of groundwater pressures underground drains gravel-filled trenching shotcrete