GL 3 MASS MOVEMENT MONITORING ESSAY
Describe how sites of potentially hazardous mass movement can be monitored. In your answer examine
why monitoring may not be effective. Illustrate your answer with examples where possible.
(25)
Mass movements on a hillside or a cliff can be monitored directly and indirectly. Some of the methods
used are shown in the diagram below.
Mass movements can be measured directly using laser surveying. Fixed reference points, such as studs
cemented into stable rocks, can be used to see if the distance between them is changing because the slope in
between is moving. GPS (Global Positioning System) can also be used as satellites can carry devices that
measure altitude and position very accurately.
Cracks in the surface of a slide can be measured using strain meters, which consist of wires fastened to
fixed points. The change in width of a crack can be constantly monitored so that warning can be given if
unexpected changes occur. Strain meters can also be put down boreholes to see if the deeper parts of a slope
are moving. These can be automatically monitored and computer analysed to see if the movement is
dangerous. Strain gauges are used on the slopes of volcanoes, such as Hekla in Iceland, or across known
faults, such as the San Andreas fault. Volcanic eruptions and earthquakes can trigger landslides so the
information can be used in several ways.
Geophones, which measure vibrations, are used in some places. These assess slow movements in
rocks and subsoil to see if any movement is taking place. In Sweden and Japan they are linked to an automatic
warning system which tells train drivers taking routes across the unstable areas if there is any danger. That
means that the trains can slow down and stop if a landslide is detected on the line ahead.
Monitoring needs to take place over a long period of time for warning signs to be observed and
analysed. There needs to be a full understanding of the geology and water movements in the slope if
monitoring is going to be valuable.
Indirect monitoring can take place on the factors which could cause a mass movement.
Many mass movements are triggered by a rise in the water content of the slope, which can increase
pore water pressure and weight. The Water Authorities can measure the level of groundwater in a slope.
Boreholes are monitored so that the authorities are aware of the level of the water table. If the slope is getting
saturated with water, then drains can be cleared and water pumped out
The Meteorological Office has rain gauges in many sites so that an increase in local rainfall is known
and warnings can be given.
Planning departments of local authorities demarcate land use areas for housing and industry and need
to be aware of any potential landslips. The British Geological Survey makes maps which include surface
deposits as well as solid rock and their geologists can identify old mass movement sites from the landscape
and the deposits that they contain. The Mam Tor landslide near Castleton in Derbyshire has very hummocky
ground with many drainage pipes to remove the surplus water. The toe of the landslide has irregular ground
which can be photographed regularly to see if there are any changes in shape.
Hazard maps are made by engineering geologists in areas which are vulnerable to mass movements. In
the Alps maps showing known rock falls are published so that building does not take place in the path of a
landslide. Avalanche monitoring allows similar hazard maps to be made.
The Mam Tor hillside and other steep slopes in the Derbyshire area are well known as having the
potential to slide because of their characteristic geology and high rainfall. The main road over Mam Tor was
closed in 1977 because its safety could not be guaranteed after it moved seriously following a wet winter. It is
monitored by surveying and the network of fixed markers are mapped every year. The arrows on the map
below show how far (length of arrow) and in what direction that part of the slope moved during the year.
After about 15 years of monitoring it is clear that the Mam Tor landslip moves most after wet winters. The
researchers are beginning to be able to work out the maximum amount of water that the slope can hold without
moving significantly and that allows them to predict when a movement will occur.
Monitoring may not be effective because interpretation of results is difficult. Some landslides occur
suddenly, although most start moving gradually so that if any one is aware of the signs then warning can be
given. The Aberfan coal spoil tip moved gradually as more waste was added and the men working there were
aware of how much settling took place each day. However the danger of the build-up of water in the tip was
not recognised so the tip failed massively in 1966 and 130 people died. It was not systematically monitored at
all.
In some landslides the critical factors may not be recognised. The Vaiont Dam landslide in Italy was
monitored as the reservoir was filled with water once the dam had been completed. However the geological
weakness in the hillsides above the reservoir was not fully understood so no one realised how rapidly the slope
would fail once it was saturated by water at its foot.
Sometimes a factor acts as a trigger to start a landslide, even though the slope is unstable.
Exceptionally heavy rain soaks the ground suddenly and may cause many landslides. Earthquakes may shake
a slope so that it begins to slip.