MASS MOVEMENTS What are landslides Video clip 1

MASS MOVEMENTS What are landslides? Video clip 1 Video clip 2 Video clip 3 Video clip 4 Video clip 5 Video clip 6 Video clip 7 Video clip 8 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 6 000 m 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 1 a 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 1 b is a geological section through tip No. 7 and the underlying geology prior to the landslide.

(a) On the geological section (Figure 1 b), 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 1 b 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. 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