| |
Mikulůvka(map
village
The landslide at Mikulůvka lies to the north of the village. It is roughly 600m
long and 300-400m wide. At the time of activation, the landslide was moving at 1
to 2 metres per hour. This was considered fast movement but now, through some
extensive remediation, the landslide has calmed and only moves a gradual amout.The first movement started on the 24th
of July 1997, at 7am. At 7pm, the lanslide had taken full affect on the village.
5 houses were damaged as well as the main street, walls (see plates 1 and 2),
some of the forest, wells and other engineering facilities, under and overground.
The separate zones are
of irregular shape. The transport zone is partly covered by forest and partly by
meadows. In this zone there is cemetery. This has been damaged by the earth
movements, which have broken the perimeter wall, cracked the small building
present and offset some of the gravestones (see plates 1 and 3).
The accummulation zone covers a large
area roughly 400m wide (see plate 4). This zone has affected the village of
Mikulůvka and caused much damage. At least 2 houses were dismantled because of
too much damage to the structure impending them unsafe (see plate 5). An
eyewitness to the first movements of the accummulation zone’s movement quoted
the ground at the back of the house ‘rose up under-foot at least 3 or 4m
within a matter of minutes!’ Visual evidence of the effect the landslide had
on some houses is also present, (see plate 2).
Some of the reasons of
the origin of the landslide can be theorised and most likely come down to:
- Increased pore water pressure in the ground
-
Full saturation of the land strata above the slide
surface
- The effect of an upward pressure on parts of
the slope
-
Increased weight of land
- Degradation of shear strength of claystones
after saturation due to intense rainfall.
Geology of Mikulůvka
The slope is made form prequarternary sandstones and claystones of Paleogene age.
Overlying these rocks there are quarternary
sediments of deluvial origin.
These sediments are found at depths of 1.4 to 3.2m. They consist of clay to
sandy clay with 5cm fragments of claystone.
The prequarternary bedrock is of Paleogene age and is
mostly comprised
completely weathered claystones with alternating layers of disintergrated
sandstones. This information was gathered by borehole data, which also revealed
a possible skidding zone at 5.4 to 8.0m.
The reasons for this are due to the presnce of soft, pasty like sandy clay, with
frequent pebbles at 0.3cm in size.
The
boreholes J-2, J-3and J-5 (deep slide surface) have discovered similar
circumstances at depths of 5.0 to 5.4m, 4.0 to 4.7m and 5.5 to 6.5m respectively.
Hydrogeological
aspects of the Landslide
The problem with all landslides is that the slide occurs when the ground is over-saturated.
In areas like Vsetín where the rocks are of the flysch series, this becomes a
big problem when intense rainfall happens over a long time. Drainage of the
landslide is always the fisrt consideration when action is taken to remedy the
slide (see plate 6). Therefore, mapping of the water table, the occurrance of
springs and the recognition of dead water must be monitored.
This information was collected about the hydrogeological status at Mikulůvka:
-
Wet
areas have been located in the upper parts of the transport zone and in the east
end of the accumulation zone.
-
Springs were found on the northwest part of the transport zone and either side
of the accumulation zone.
-
Streams can be found in both zones.
-
Dead water is present. It accumulated 45 days after the heavy rains in the
middle and eastern parts of the landslide. An area of stagnant water also
collected in a ditch near the cemetery.
-
Four wells are located in the east part of the transport and accumulation zones.
The water level ranged from 1.2m to 4.0m between the four wells present.
The level of the underground water ranged from 4.0m to 6.8m. The permeability in
the prequaternary rocks is very low. Water travels via fissures and cracks
between the rocks. This causes the hydrostatic pressure to increase and
therefore activate the sliding plane at the weakest points.
Monitoring of the Landslide at Mikulůvko
During the period between March 1998 and December 1998, the following
observations were conducted:
- Variation
of water in boreholes and wells
- Inclinometric observations
- Measurements of the outflow of water from the horizontal boreholes
- Goedectic suveying, i.e. ground movements
Conclusions
The results obtained from the monitoring of the landslide
and from static
laboratory tests confirmed part success in the horizontal drainage systems. It
was concluded that the drainage was not enough to stop the landslide moving but
it was enough to slow down its progress.
The results of mapping and static analysis indicate the possible development of
the moving activities, i.e.:
-
High saturation and frequent cutt-offs in the upper part indicate further
activity of the landslide.
-
If the landslide s northeastern part moves, it is possible that the central
part of the landslilde will move as a reult of the higher movement.
-
Continuing movement of the upper part of the landslide will consequently
add to the lower accumulation zone and have a greater affect on the village.
|
|
