The Quick Clay Landslide at Rissa - 1978 (English commentary)
Summary
TLDRThis documentary recounts a dramatic quick clay landslide in Rissa, Norway, in 1978. It explains how the unique conditions of marine clay deposits, shaped by glaciers and salt levels, can lead to landslides, posing risks to life and property. The video highlights the catastrophic event, with rare amateur footage capturing the landslide as it unfolded, destroying farmland and homes. The film also covers the geotechnical investigations, emergency response, and stabilization efforts that followed, aiming to prevent future disasters in areas with quick clay deposits.
Takeaways
- 🌍 The documentary discusses the significant risk posed by quick clay landslides in Norway, specifically highlighting their potential to cause loss of life and property.
- 🧊 During the last glaciation, large parts of the Northern Hemisphere were covered in glaciers, including a 3,000-meter-thick ice cap in Scandinavia.
- 🏞️ After the glaciers retreated around 11,000 years ago, marine clay deposits were formed in fjords and rose above water due to isostatic land uplift.
- 🌾 These marine clay areas, now valuable farming land, are heavily populated and prone to erosion and landslides.
- 💧 Quick clay is strong in its natural state, but if overloaded, it can collapse, leading to liquefaction and landslides, especially when salt concentration drops below 1g per liter.
- 📽️ The Rissa landslide of 1978 is the focus of the documentary, with amateur footage capturing the event for the first time in history.
- 🚜 The slide, caused by the weight of earthworks, spread over a 450-meter-wide area, destroying farms and homes as the liquefied clay rushed into the nearby lake.
- 🚁 Rescue efforts involved helicopters and evacuations, with 40 people in the area at the time of the landslide, though only one fatality occurred.
- 🔍 Geotechnical investigations were conducted after the slide to assess stability and remaining quick clay deposits, leading to stabilization work, including blasting and flattening of slopes.
- 🏡 Despite the devastation, within four years of the landslide, compensation was provided, new homes were built, and the affected area was rehabilitated for farming, with ongoing efforts to map and mitigate future quick clay landslides.
Q & A
What is the primary issue with marine clay deposits in Norway?
-The primary issue with marine clay deposits in Norway is the occurrence of quick clay slides, which pose serious risks, including potential loss of life and property damage.
What causes quick clay to become unstable and lead to landslides?
-Quick clay becomes unstable when the salt concentration in its pore water drops below 1 g per liter, causing the clay particle structure to collapse. This leads to remolding of the clay, drastically reducing its strength and triggering landslides.
How do glacial and marine processes contribute to the formation of quick clay in Norway?
-During deglaciation around 11,000 years ago, melting glaciers brought suspended material into fjords where silt and clay were deposited in marine environments. Isostatic uplift then raised these marine clay deposits above sea level, leading to the formation of quick clay.
What role does salt play in the stability of quick clay?
-Salt helps maintain the stability of quick clay by altering the interaction between the minerals and the pore water. When salt is removed through leaching by freshwater, the clay loses its strength, making it more susceptible to remolding and landslides.
What triggered the Rissa quick clay landslide in 1978?
-The Rissa quick clay landslide was triggered by the construction of a new wing on a farm. The excavation displaced about 700 cubic meters of soil, and the additional load caused a section of shoreline to slide into Lake Botn, initiating a retrogressive landslide.
How was the Rissa landslide captured on film?
-The Rissa landslide was captured on film by two amateur photographers who happened to be nearby. One of them recorded the initial slide and subsequent movements, providing the first footage ever of a quick clay landslide as it happened.
What were the immediate consequences of the Rissa landslide?
-The Rissa landslide covered an area of about 330,000 square meters and caused the destruction of seven farms and five homes. It also created large waves that damaged a village 5 km away. Only one person was killed, but many were displaced.
How was the area stabilized after the Rissa landslide?
-The area was stabilized by blasting the remaining quick clay terraces to cause controlled liquefaction and remove the unstable clay. Additionally, mechanical flattening of the surrounding slopes was performed to ensure the area's safety.
What steps were taken to prevent future quick clay disasters in Norway?
-Following the Rissa landslide, a national project was initiated to map quick clay deposits across Norway. This mapping aimed to identify high-risk areas and reduce the number and extent of future quick clay disasters.
How were the victims of the Rissa landslide compensated?
-The families affected by the Rissa landslide received compensation from the national disaster agency and their insurance companies. New homes and farms were built, and within four years, the area had largely returned to normal.
Outlines
🌊 The Threat of Quick Clay Slides in Norway
Marine clay deposits in Norway cover a vast area of 40,000 square kilometers, and quick clay slides present significant hazards to lives and property. This documentary explores the causes of such landslides, featuring unique footage captured by amateur photographers. During the last glaciation, glaciers covered much of the Northern Hemisphere. As glaciers retreated, marine clay deposits rose above sea level, forming valuable farmlands. Over time, erosion and the loss of salt in these sediments can lead to the collapse of clay particle structures, resulting in quick clay landslides.
📸 Capturing the Rissa Landslide in Real Time
In April 1978, an excavation near a farm in Rissa, Norway, triggered a quick clay landslide. The earth masses collapsed into a nearby lake, initiating a retrogressive slide. For the first time, the unfolding of such an event was captured on film by an amateur photographer. The slide extended over 200 meters, causing liquefaction of the clay, which flowed into the lake like water. The slide lasted for about 40 minutes, covering a large area, and culminated in a massive flake-type slide that caused significant destruction and prompted the photographer to flee for safety.
🚁 The Scale and Aftermath of the Landslide
The landslide spread rapidly, extending about 1 kilometer along the mountainside. A second photographer filmed the final stages of the event, capturing masses of sliding clay moving toward the lake. As the disaster unfolded, around 40 people were in the affected area, but most escaped. A helicopter arrived for rescue operations. Smaller slides continued to occur, and the landslide left a deep scar in the landscape. It caused damage to nearby villages, including the flooding of homes and destruction of a sawmill. The local authorities organized evacuations to ensure the safety of residents.
🔬 Geotechnical Investigation and Stabilization Efforts
After the slide, geotechnical investigations were conducted to assess the stability of the remaining areas. Field tests and soil sampling provided vital data about the quick clay deposits. It was discovered that a 15 to 20-meter high terrace of clay needed to be removed for safety reasons. Blasting was used to liquefy the remaining clay, which was then removed. The work was carried out during winter to ensure machines could operate on the frozen surface. Within ten months, the surrounding area was stabilized, and a new road was constructed to replace the one destroyed by the slide.
🌾 Recovery and Long-Term Impact of the Rissa Slide
Following the Rissa landslide, families affected by the disaster received compensation, and new homes were built. Within four years, nearly all signs of the catastrophe had disappeared, and the land was being farmed again. The event spurred a national initiative to map quick clay deposits across Norway, with the goal of preventing future disasters. The Rissa landslide serves as a reminder of the dangers posed by quick clay and the importance of ongoing geotechnical research and monitoring to mitigate risks.
Mindmap
Keywords
💡Quick Clay
💡Marine Clay
💡Deglaciation
💡Isostatic Land Uplift
💡Retrogressive Landslide
💡Remolding
💡Salt Leaching
💡Triaxial Test
💡Geotechnical Investigation
💡Liquefaction
Highlights
Marine clay deposits in Norway cover a total area of 40,000 square kilometers, posing a significant landslide risk.
Quick clay slides, such as the one described, represent a serious hazard, with potential loss of lives and property.
During deglaciation, melting glaciers deposited marine sediments, which later rose above sea level due to isostatic land uplift.
Marine clay areas now found at elevations up to 200 meters above sea level are among the most fertile and heavily populated regions.
Fresh groundwater flow through sand layers gradually leaches salt from marine sediments, weakening the clay's structure.
Quick clay exhibits strength in its natural state, but collapses when overloaded, causing landslides.
Remolding of quick clay occurs when the salt concentration in pore water drops below 1 gram per liter, leading to liquefaction.
The 1978 Rissa quick clay landslide in Norway was triggered by earthworks related to a new farm wing construction.
Amateur photographers captured the landslide on film, providing the first-ever footage of a quick clay slide in progress.
The slide expanded retrogressively, liquefying clay, and destroyed buildings as it moved rapidly towards the lake.
The slide area extended to 450 meters in width and covered 25 to 30,000 square meters within 40 minutes.
A massive flake-type slide occurred, displacing an area of 150 by 200 meters and dragging homes and farms into the lake.
Quick clay liquefaction and sliding continued, creating waves that damaged a nearby village and destroyed a sawmill.
Geotechnical investigations and stabilization efforts were implemented after the slide, including the removal of quick clay deposits.
A national mapping project of quick clay deposits was initiated following the Rissa disaster, aimed at reducing future landslides.
Transcripts
deposits of marine clay in Norway
covered a total area of 40,000 square
kilm within this area quick clay slides
represent a serious problem involving
potential loss of lives and
property this documentary film presents
the story of such a
landslide it contains unique pictures
taken by two amateur photographers who
happen to be at the
scene during the last glaciation the
Northern Hemisphere was more or less
entirely covered by glassiers in
Scandinavia the weight of an
approximately 3,000 M thick ice cap
loaded the
Bedrock during deglaciation the sea
transgressed in front of the retreating
ice about 11,000 years ago the coasts of
Norway were submitted to conditions
similar to those in spitsbergen and
Greenland
today melting water from the glaciers
brought large amounts of suspended
material into the fjords where silt and
Clay were sedimented in the Marine
environment due to isostatic land uplift
following the withdrawal of the glaciers
Marine clay deposits Rose above the
water level such Marine areas are shown
with light green color on the map and
can today be found up to surface
elevations of 200 m above present sea
level in Norway and Sweden such areas
constitute the best farming land and are
among the most heavily populated areas
the Bedrock surface is very undulated
without cropping Hills and Valleys
filled to some extent with Marine
sediments rain and surface water subject
the Marine clay surface to
erosion with time the top five meters
have altered to a weathered crust of
stiff fissured
clay at the same time an upward flow of
fresh groundwater from fissures in
Bedrock and through sand layers
gradually leeches the salt out of the
poor water of the Marine
sediments
in its natural undisturbed state quick
clay exhibits considerable
strength however if the load becomes too
heavy as shown in this example a failure
takes place as the clay particle
structure collapses followed by
remolding of the clay the most
characteristic feature of a quick Clay
is the complete and drastic change of
consistency by
remolding such extreme quick clay
behavior is encountered only when the
salt concentration in the poor water
drops below 1 G per
liter to illustrate the important role
of salt on the material properties we
can add a little ordinary table salt to
the clay Sample When the salt goes back
into the poor water the salt ions alter
the interaction between the minerals and
the poor
water the remolded strength increases
dramatically and the clay is no longer a
liquid
now let's look at the actual Rissa quick
clay
Landslide Rissa is situated close to the
Trondheim F Northwest of the city of
Trondheim this air photo was taken a few
years before the slide occurred in the
bottom right hand corner we see the
Marine water of the Trondheim fu
on the left hand side we see a part of
the freshwater lake botan between the F
and the lake lies a fairly flat area of
marine clay deposits which for centuries
has been
Farmland this model of Rissa shows the
locations of the small farms and of the
connecting roads before the
[Music]
slide let's focus on this Farm here
where the slide was initiated in April
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1978 a new Wing was to be added to the
existing
B the excavation for the 700 cubic met
basement took 2 days and the soil masses
were placed in a 2 m High Earth fill
down by the shoreline of Lake
Boton just after the earthwork had been
completed a 70 to 90 M length of the sh
line suddenly slid into the lake and
disappeared the load from the earthfill
caused this initial failure the slide
developed retrogressively into the clay
slope towards the
right an amator photographer living in
the area took up a position at the
shoreline of lake botman with his new 8
mm movie camera for the first time in
history we can actually see film of a
quick clay Landslide as it's happening
at this moment the slide extended over
an approximate 200 M width successive
minor sliding then took place
continuously the remolded soil masses
moved down into the lake creating small
[Music]
waves
each new slide resulted in a complete
liquefaction of the quick clay you are
witnessing on the screen the remolded
quick clay at its natural water
content the debris literally splashed
against the lower slide Edge and move
like streaming water down the scarp and
into the
[Music]
lake
in the background we can see the barn
where the excavation was
made the retrogressive sliding process
continued for about 40
minutes the slide now extended over a
450 M width and covered an area of 25 to
30,000 sare
M the slide area had now the shape of a
long and narrow pit with a narrow
Gateway towards the lake
[Music]
however at this time the real disaster
started almost instantaneously a large
flake type slide started and our Amor
photographer had to run for his life in
his own words a wave of Earth came
rolling behind me and I witness
recounts suddenly an area of about 150
by 200 m including the old school
building sank down and moved
monolithically towards the
lake the sliding Mass didn't move
through the existing slide scarp but in
the direction of the terrain slope most
of the slide debris flowed into the lake
while some came to a stop in a
compression Zone close to the
Shoreline a new large Gateway had now
been opened into the
lake the persons living on this Farm
behind the new steep slide scarp escaped
with private cars just before the farm
slid
out the persons living on this second
Farm were warned by their escaping
neighbors and managed to get away just
before their houses were taken by the
slide back on safe ground our amateur
photographer again started his camera
just in time to film a new large flake
type
slide
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[Applause]
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large flakes of dry crust in some cases
with intact buildings on top floated on
the remolded quick clay stream this
house moved with a velocity of the order
of 30 km/
[Music]
hour this house you can see here
rotated around its vertical axxis as it
rushed towards the
[Music]
[Music]
lake the major sliding process was now
completed however debris was still
flowing into the
lake
in a very short period of time only some
5 minutes the slide had propagated 1
kilometer along the
Mountainside a second Amer photographer
who had heard about the landslide had
rushed for his camera and found a safe
position on the hillside from which he
started filming the last portions of the
slide activity below
him sliding masses moved in the
direction of the slope hit the lower rim
of the slide scarp and were forced to
flow to the right towards the
lake just before the main flake slide
started about 40 persons were known to
be within the affected area at this
moment it was not clear how many had
escaped a rescue helicopter had arrived
at the
site
small failures from the Steep slide
edges occurred
[Music]
continuously in the inner part of the
slide scarp some sliding still took
place here a large clay block slid out
becoming more and more
remolded some suddenly the quick clay
completely
[Music]
liquefied this helicopter panoramic view
of the final slide scar was taken in the
evening about 4 hours after the initial
slide
occurred the first slide movement
occurred in the direction of the terrain
slopes from the hills in the back the
debris was then forced to move towards
the lake to the
left this White House was taken by one
of the many small slides which occurred
during the first few days after the main
Slide the neighboring Barn also ended up
in the
debris small Brooks brought water into
the slide
area the two main roads through the area
were taken away by the main slide from
this Farm situated on an outcrop many
people watch the Dreadful sliding
activity we now see the farm where the
initial slide took
place when the masses from the major
flake type slide rushed into the lake
two or three large water waves were
created these waves propagated across
the lake a distance of 5 km at the other
end the waves flooded the small village
of laer causing a great deal of
damage this amateur film taken from a
car window in lra that same evening
shows damaged houses and cars and
flooded
[Music]
basements The Sawmill and Lumberyard
located close to the Shoreline were
destroyed this view shows the same
Sawmill the following
morning
the first night after the major slide
200 persons living close to the slide
area were asked to
evacuate the local police were
responsible for dealing with the
situation they were assisted by military
personnel and civilians in the
evacuation of cattle and clearing of the
area the SL area covered 330,000 sare M
and the slide volume was of the order of
5 to 6 million cubic
M seven farms and five single family
homes were taken by the slide or had to
be abandoned for safety
reasons of the 40 people who were within
the area when the sliding started only
one was
[Music]
killed several farms and homes were
situated close to the slide scarp an
important task was therefore to
establish the margin of safety of these
areas as soon as
possible geotechnical investigations
were carried out to map the extent of
remaining quick
clay the site investigations included
rotation soundings vein tests and
undisturbed soil
sampling the undisturbed soil samples
were taken by geonor 54 mm piston
[Music]
[Music]
sampler after field classification the
sample tubes were
registered and put into boxes for
transport to the
laboratory
after transport of the sample tubes to
the ngi laboratory in Oslo the clay was
extruded and prepared for various
geotechnical
tests routine investigations were first
carried out including fall cone tests as
shown
triaxial and simple Shear tests provided
more reliable strength parameters for
the stability
calculations this is a typical
geotechnical profile from Rissa the Blue
Line shows an average natural water
content of about
30% it should be noted that in the zone
of quick clay the liquid limit is 5 to
10% lower than the insitu water
content the green curve shows the vein
Shear strength and the red curve the
remolded shear
strength the results of geotechnical
analysis showed that stabilization Works
had to be carried
out the 15 to 20 M High remaining
Terrace along the foot of the hillside
contained quick clay and had to be taken
down for safe reasons it was decided to
do this by
blasting holes 5 to 10 m deep were
drilled both from the top of the Clay
Terrace and along the foot of the steep
slope 1 kgr of dynamite was used 100
cubic M of
clay
this blasting caused the remaining quick
clay in The Terraces to liquefy and run
out into the slide
area these masses then eroded into the
newly formed thin dry crust of the
original slide
bottom
[Music]
[Music]
going
after several rounds of blasting the
Clay Terrace was removed over its entire
1 km
length the 5 to 10 m High slopes at the
opposite side could be stabilized by
mechanical
flattening the clay was very soft and
sensitive the work was therefore carried
out during the winter so that the
necessary machines could be moved across
the Frozen top
crust within 10 months after the slide
the entire Zone surrounding the slide
had been stabilized
IED the following year work started on a
new road substituting the one taken by
the slide for a distance of about 1 kilm
the hillside was blasted out for the new
road the families who suffered material
losses recovered compensation partly
from the national disaster agency and
partly from their insurance
companies new dwellings B and ouses were
erected and 4 years later nearly all
traces of the slide disaster had been
wiped
out once again the slide areas being
cultivated and crops
harvested the Rissa land slide gave the
impulse to start a project of Nationwide
mapping of quick clay
deposits hopefully in the future the
results of this work will lead to a
reduction in both the number and extent
of quick clay
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disasters
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oh
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