The Quick Clay Landslide at Rissa - 1978 (English commentary)

Summary

This 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

00:00

🌊 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.

05:03

📸 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.

10:03

🚁 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.

15:05

🔬 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.

20:06

🌾 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.

Keywords

💡Quick Clay

Quick clay refers to a type of clay that can change its consistency from solid to liquid when disturbed or remolded. In the video, it’s a central focus because the landslides in Norway are caused by the collapse of quick clay deposits after the salt leaches out of the clay. This drastic transformation leads to large-scale landslides, as seen in the Rissa event.

💡Marine Clay

Marine clay is clay that was originally deposited in a marine environment, often under the sea. As seen in the video, marine clay in Norway was deposited during deglaciation and later rose above sea level due to land uplift. These clay deposits are important because they constitute fertile farmland, but are also prone to quick clay landslides when disturbed.

💡Deglaciation

Deglaciation refers to the melting and retreat of glaciers after an ice age. In the video, this process occurred approximately 11,000 years ago, leading to the transgression of the sea and the deposition of marine clay in Norway. Deglaciation is important to the video’s narrative because it explains the origin of the quick clay deposits that cause landslides.

💡Isostatic Land Uplift

Isostatic land uplift is the rising of the Earth's crust after the removal of the heavy weight of glaciers, which had compressed the land during the last Ice Age. In the video, this process caused the marine clay deposits in Norway to rise above sea level, making them vulnerable to weathering and landslides.

💡Retrogressive Landslide

A retrogressive landslide is a type of landslide that moves backward into a slope as the soil progressively fails. In the video, the Rissa landslide developed in a retrogressive manner, where the failure started at the shoreline and spread inland, collapsing more and more ground into the lake.

💡Remolding

Remolding refers to the process by which the structure of quick clay breaks down, turning it into a liquid state. In the video, this process is key to understanding the behavior of quick clay during landslides. When disturbed, quick clay loses its strength, which allows it to flow like water and cause extensive damage.

💡Salt Leaching

Salt leaching is the process where groundwater removes salt from the clay, reducing the stability of the clay. In the video, salt leaching is presented as a critical factor in transforming marine clay into quick clay. When the salt concentration falls below a certain level, the clay becomes prone to liquefaction and landslides.

💡Triaxial Test

A triaxial test is a laboratory test used to measure the mechanical properties of soils, including their strength under various pressures. In the video, geotechnical tests like the triaxial test are conducted to determine the strength of the quick clay after the landslide and assess the safety of surrounding areas.

💡Geotechnical Investigation

Geotechnical investigation involves studying soil and rock properties to understand the stability of the ground. In the video, after the Rissa landslide, extensive geotechnical investigations were carried out to map the remaining quick clay and determine the safety of nearby areas.

💡Liquefaction

Liquefaction is a process where solid material, such as clay, transforms into a liquid-like state due to stress or disturbance. In the context of the video, liquefaction of quick clay is the primary cause of the landslide, where the soil becomes fluid and flows downhill, leading to large-scale destruction.

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

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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/

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hour this house you can see here

rotated around its vertical axxis as it

rushed towards the

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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]

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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

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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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