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

GravityShear stress

“slide component”

Shear strength“stick component”

Causes of Mass Movements

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

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.

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 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

(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

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

•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

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

• 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 thelandslide.

(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

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

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