Mass Movements Flashcards
What can increase shear stress in a slope?
Heavy rainfall, pore water pressure, erosion of cliff foot (reduction of lateral support), transient stresses
What can cause a reduction in shear strength?
Removal of vegetation, dry weather, pore water pressure, restructuring due to human activity, animals burrowing
How can shear strength be increased?
Afforestation, drainage, restrictions on drilling/quarrying, fences
How can shear stress be reduced?
Drainage, defences at foot of slope, restricted construction on conditionally stable slopes
When did the Venezuelan mudslides take place?
15th-16th December 1999
Why were many people near Caracas at risk?
Shanty towns had grown up steep valley slopes
What caused the 1999 Venezuelan mudslides?
Heavy rainfall; 40-50% above normal during the first two weeks of December
Why was there so much material in the Venezuelan mudslides?
High rainfall and temperatures leads to high rates of chemical weathering and therefore deep soils; 20-30m deep
Define mudslide.
A fast slide of mud, often after heavy rainfall, with high water content and a basal slide plane
The 1999 mudslides were the worst disaster in Venezuela in…
…200 years
Define shear strength.
The internal resistance of a body to movement
Define shear stress.
The force acting on a body that causes movement of the body downslope
What does pore water pressure do to a slope?
Reduces the cohesion of particles and decreases the frictional strength of the slope material
Where do mass movements often occur?
Where there are weak rocks, steep slopes and/or active undercutting
How m at people were made Ho else’s due to the Venezuela mudslides?
Over 150,000
How did most of the victims of the Venezuela mudslides die?
They were buried in mudslides 8-10m deep
What were some of the secondary impacts of the Venezuela mudslides?
Destroyed infrastructure, buried crops, impact on tourist industry
After the mudslides had run their course for a while, they turned into debris flows that included…
…water, sand, gravel, boulders, trees and other materials
Why are the Seward facing slopes in Vargas so steep?
The summit ridge (max 2700m) is just 10km from the sea
What was the geology of the mountains in Venezuela like?
The mountains were mantled in an easily eroded clay which made up the body of material that moved
What effect may widespread deforestation in Venezuela have had on the slopes?
Destabilised as roots increase shear strength
Why was it difficult to get some supplies to Vargas after the mudslides?
Caracas airport temporarily closed, costal highway destroyed, La Guaira seaport flooded
Why did people settle on alluvial fans and what are they?
Fan shaped deposits of debris where landslides have reached and extended into the sea; people settled here due to rapid population growth
How much of Venezuela’s population lives on the coast?
70%; people don’t want to live in the interior of the country, however governments tried to relocate refugees there
How many people were evacuated from Vargas?
100,000
How could Venezuela reduce the risk of another mass movement of the same scale?
Land-use planning in the mountains, constructing flood channels (particularly on alluvial fans), monitor and warn people of heavy rainfall, align new buildings and streets in the direction of debris flows
What was the immediate response to the Venezuela mudslides?
Search and rescue (fairly unsuccessful) and emergency aid (accommodation, water purification, food, medicine)
What are the governments development plans in Vargas?
Build new homes, expand airport, build canals that direct rivers away from communities, fixing infrastructure
Towns such as Carmen de Uria which were completely destroyed by the 1999 mudslides will be turned into…
…parks, bathing resorts, outdoor facilities
How much debris flowed from the mountains through Vargas?
2 million metres cubed
In some areas of Vargas the coastline was extended…
…250m
Why did undercutting near Holbeck Hall not affect the landslide?
There was a wave cut platform
How much material was involved in the Scarborough landslide?
60,000 m³
How far was the Scarborough coastline extended?
50 m
What kind of Mass movement occurred in Scarborough?
Rotational slump
What hard engineering strategies were implemented after the Scarborough landslide?
Geotextile bags and 30,000 tonnes of rock armour to protect the love
What soft engineering strategies could be used at Holbeck Hall?
Regrading/revegetating with halophytic plants
What happened to the hotel in the Scarborough landslide?
It partially collapsed and lost its gardens. There are also tension cracks in the car park
What year did the Scarborough landslide take place?
1993
How might the owners of the hotel have caused instability in the slope?
They built a conservatory in 1991
How many tilt metres where provided by geotechnic consultants?
Three; if movement is above a set figure, the council is warned
What date did the Scarborough landslide take place?
Fifth of June
Scarborough had two months of heavy rain after…
…A prolonged dry season
How much rain did Scarborough receive?
140 mm in two months
What is the geology around Scarborough?
Impermeable boulder clay overlying permeable mudstone and sandstone
How were pins along the cliff face in Scarborough used?
Surveyed by electronic distance measuring equipment
What was the response in Whitby after the landslide?
To extend a sea wall, costing £3.4 million
How much are the cliff top homes near Holbeck Hall worth?
£1.2 million
What evidence was there that the Holbeck Hall landslide was a slump?
Vegetation on landslide material after event
What differences could be seen along the cliff either side of Holbeck Hall?
The south side wasn’t defended and evidence of erosion could be seen due to the wave cut platform, exposed cliff face, areas that lack vegetation suggestion small mass movements; north bay is protected by a sea wall and ground, slope less steep, no evidence of past mass movements, may have been regraded
Strong cliffs’ strata dip in what direction?
Inward dipping; seaward dipping strata are more susceptible to movement
Where are the Spittles?
Near Lyme Regis on the Jurassic Coast
When did the Spittles rock falls take place?
6th May 2008
How far did the rocks fall at the Spittles?
300m
Why did the timing of the Spittles rock falls mean no one was in danger?
The falls occurred during a high tide when the area is inaccessible; during low tide it is popular with families and fossil collectors
Why does the geology at the Spittles encourage movement?
There are many strata in the rock; water percolates through sandstone, saturates clay and then slides along limestone terraces
What part of the cliff was affected at the Spittles in 2008?
The top part (shale)
When can rock anchoring be used to stabilise a slope?
When strata dip towards the protected area
How are roads and railway lines protected from avalanches in the Alps?
By building concrete shelters around them; these have deep foundations and are pinned into the ground and mountainside to stabilise them
