SLOPE FAILURE (LANDSLIDES) AND SURFACE DISPLACEMENT
GEOINDICATORS
Marek Graniczny1 & John Ridgway2
1
2
Polish Geological Institute
ST. Rakowiecka 4 - 00-975 Warszawa - Poland
[email protected]
British Geological Survey
Keyworth, NG12 5GG - Nottingham – UK
[email protected]
Every year, small landslides destroy homes and farmland. Occasionally, an enormous
landslide buries a town or big city, killing thousands of people. Landslides cause billions of
dollars in damage every year, about equal to the damage caused by earthquakes in 20 years. In
many instances, losses occur because people do not recognize the dangers of mass wasting
that are obvious to a geologist.
Mass wasting can occur slowly or rapidly. In some cases, rocks fall freely down the face
of a steep mountain, whilst in others, rock or soil creeps downslope so slowly that the
movement may be unnoticed by a casual observer.
Mass wasting falls into three categories: flow, slide and fall.
There is no very sharp boundary between rock falls and landslides, except that rock falls
only occur on bare rock walls and landslides also take place on less steep, soil covered slopes.
To be defined as a landslide, four criteria must be fulfilled:
1. the movement must be rapid, lasting seconds or at most a few minutes;
2. the sliding surface, the shear plane, must go through the bedrock composing the
largest part of the landslide;
3. the sliding mass must disintegrate during the movement;
4. the slope area affected by the movement and the volume of the rock mass in
motion must be large enough for it to be defined as a landslide by the people
living in the area. A relative definition of the size of the mass seems more
appropriate than one expressed by an arbitrary number of square metres, tonnes or
cubic metres.
Landslides commonly occur on the same slopes as earlier landslides because the
geologic conditions that cause mass wasting tend to be constant over a large area and remain
constant for long period of time.
Many towns were founded decades or centuries ago, before geologic disasters were
understood. Often the choice of a town site was not dictated by geologic considerations but by
factors related to agriculture, commerce or industry, such as proximity to rivers and ocean
harbors and the quality of farmland. Once a city is established, it is virtually impossible to
move it. Furthermore, geologists’ warnings that a disaster might occur are often ignored.
After all, predictions of of earthquakes and volcanic eruptions are sometime incorrect. Even in
areas known to be active, a quake or eruption may not occur for decades or even centuries.
Awareness and avoidance are the most effective defenses against mass wasting.
Geologists construct maps of slope and soil stability by combining data on soil and bedrock
stability, slope angle and history of slope failure in the area. They also include evaluations of
the probability of triggering an event.
Stereo pairs of aerial photos (B&W, normal colour and IR colour) have long been used
to recognise slides and slide-prone terrain. Zones of previous sliding activity are easily
identified on aerial photos by characteristic crescent scarps and the hummocky topography
exhibited by the debris flow. It is obviously more difficult to identify areas that have a
potential for sliding or slumping, but the detailed interpretation of aerial photos may help to
identify such zones. Because the key features in such cases are rather small, large-scale
photos (about 1:10 000 scale) have been found to be the most useful.
At the regional scale, satellite images could be also useful for landslide studies. On
these images recognition of unstable terrain where slides occur could be possible. Such
analysis is enriched when satellite images are applied together with DTM. Satellite images
could be also useful for monitoring land surface changes related to landslides activity.
Major landslides in Poland are most characteristic of the Carpathian Mountains in the
southeast part of the country.
The location of areas at risk from major landslides is controlled by two main factors: the
presence of slopes with a unfavourable geological structure, and high levels of precipitation.
Both of these conditions are fulfilled in the case of the flysch Carpathians, consisting of
interbedded shales and sandstones, deeply dissected by numerous valleys. The annual
precipitation here reaches 800 – 1100 mm, sometimes concentrated in rainstorms. According
to investigations carried out by the Polish Geological Institute, about 10% of the area of the
Carpathians within Poland, has been in the past, or is being at present, endangered by
landslides or other forms of mass movement.
The temporal incidence of mass movement is strongly correlated with climate. During
wet years, or soon after, an increasing number of fresh or revitalised landslides is reported.
Such a situation took place after heavy rainstorms and floods in the summer of 1997. Since
that time numerous landslides in the Polish Carpathians have been activated and serious
damages to houses and communications infrastructure reported.
The Polish Geological Institute is currently carrying out a large project including
registration of landslides in the Carpathians, monitoring their activity and making prognoses
for the future. These prognoses are connected with the necessity of changing local plans for
territorial development. Such plans are prepared in the smallest administration units (named in
Poland „gmina”). All available modern mapping technologies will be applied in the course of
the project, including remote sensing, GIS and GPS measurements.
The project, of 3 years duration, started in the spring of 2001 and is funded by the
Polish National Fund of Environmental Prtection and Water Resources.
The main aims of the project are as follow:
- full registration of landslides in the whole territory of the Polish Carpathians;
- detailed mapping of landslides at the scale of 1:10 000 in selected areas;
- establishment of a monitoring system in areas of active landslides – the 20 most active
landslides will be chosen for regular measurements;
- preparation of landslide hazard and prediction maps, at a scale of 1:10 000;
- development of a landslide data base.
-
The following working methods were applied:
analysis of archive materials, satellite images, DEM and aerial photos for selecting areas
for detailed studies;
application of GEORADAR techniques;
collecting samples for geotechnical analysis;
observation of groundwater levels in piezometers;
detailed measurements using GPS with an accuracy of 1 cm.
Several studies were carried out between Gorlice and Szymbark (Eastern Carpathians) .
Numerous landslides have occurred here, on the slopes of Maślana Mt. and Miejska Mt. The
landslides were mapped during geological mapping at a scale of 1: 50 000. Contours of the
landslides were superimposed on the land use map, prepared on the basis of Landsat TM
satellite image interpretation. It was found that about 50% of landslides are located in areas
covered by forests and thus constitute a low hazard. In the next stage of analysis mapped
landslides were compared with DTM. The analysis revealed that most of the mapped
landslide areas correspond to mountain slopes of 8 – 100 . This same analysis showed that the
landslide areas are located beneath slopes of 11 – 140 .Another aspect of analysis concerned
localities of landslides versus slope exposures. The method of digital analysis is very fast and
very useful for further terrain investigations.
During the field works GPS measurements are made using a Pathfinder ProXL
instruments providing measurements with an accuracy of half a metre, which is very suitable
for mapping at the scale of 1:10 000. For landslides monitoring purposes, another GPS
instrument – a LOCUS Ashtec - is used, with an accuracy of 1 cm.
Finally, on the basis of remote sensing data interpretation, field works and
measurements, a GIS database will be created. Maps of landslides prone areas will also be
prepared. In addition the landslides information system will be put on the INTERNET. A
special questionnaire on the PGI web site will facilitate the collection of information about
new landslides.This information will be processed and verified by PGI specialists and will be
taken into account during the preparation and verification of local territorial planning policy.
The last two years have seen considerable landslide activity throughout Britain, due
largely to sustained high rainfall. This has led to the first fatalities attributed to landslides for
at least 100 years. As part of its programme of landslide research, BGS has recently adopted
new monitoring methods for studying the role of landslides in coastal erosion of ‘soft’ cliffs.
These include the use of laser scanning instruments to directly produce 3-D models of cliff
and beach, enabling calculation of volume changes and movements with time. This
technology permits the most direct and detailed assessment of individual landslides to be
made. From the resulting models, slope stability analyses are carried out using specialist
geotechnical software, and factors of safety determined. Mapping and classification of
landslides is an important task, which is increasingly becoming part of the mainstream
geological mapping process in BGS. In addition, modern methods of measuring ground
displacement from satellites has potential for use in landslide monitoring.