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

ah ah Oh ah deposits of marine clay in Norway covered a total area of 40,000 square kilometers 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 glaciers in Scandinavia the weight of an approximately 3,000 meter thick ice cap loaded the bedrock during the glaciation 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 was 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 two hundred meters above present sea level in Norway in Sweden such areas constitute the best farming land and are among the most heavily populated areas the bedrock surface it's 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 assured 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 pore 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 remoulding of the clay the most characteristic feature of a quick clay is the complete and drastic change of consistency by remoulding such extreme quick clay behavior is encountered only when the salt concentration in the pore water drops below 1 gram 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 for water the salt ions alter the interaction between the minerals and the pore water the remoted strength increases dramatically and the clay is no longer a liquid now let's look at the actual ERISA quick clay landslide ERISA is situated close to the Trondheim fjord northwest of the city of Trondheim this our photo was taken a few years before the slide occurred in the bottom right hand corner we see the marine water of the Trondheim field on the left-hand side we see a part of the freshwater lake bottom between the fjord and the lake lies a fairly flat area of marine clay deposits which for centuries has been farmland this model larissa shows the locations of the small farms and of the connecting roads before the slide let's focus on this farm here where the slide was initiated in April 1978 a new wing was to be added to the existing bomb the excavation for the 700 cubic metre basement took two days and the soil masses were placed in a two meter high earth fill down by the shoreline of Lake bahtman just after the earth work had been completed a 70 to 90 meter length of the shoreline suddenly slid into the lake and disappeared the load from the earth fill caused this initial failure the slide developed retrogressive lee into the clay slope towards the right an amateur photographer living in the area took up a position at the shoreline of Lake bottom with his new 8 millimeter 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 meter width successive minor sliding then took place continuously the remoulded soil masses move down into the make creating small waves each new slide resulted in a complete liquefaction of the quick clay you are witnessing on the screen the remoulded 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 lake in the background we can see the bomb where the excavation was made the retrogressive sliding process continued for about 40 minutes the slide now extended over a 450 meter width and covered an area of 25 to 30,000 square meters the slide area had now the shape of a long and narrow pit with a narrow gateway towards the lake however at this time the real disaster started almost instantaneously a large flake type slide started and our amateur photographer had to run for his life in his own words a wave of Earth came rolling behind me an eyewitness recounts suddenly an area of about 150 by 200 meters including the old school building sank down and moved monolithically towards the lake the sliding mass didn't move through the existing slide scar but in the direction of the terrain slowed most of the slide debris flowed into the make 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 large flakes of dry crust in some cases with intact buildings on top floating on the remote lead quick clay stream this house moved with a velocity of the order of 30 kilometres per hour this house you can see here rotated around its vertical axis as it rushed towards the 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 five minutes the slide had propagated one kilometer along the mountainside a second amateur photographer who had heard about the landslide had rushed to 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 escaped a rescue helicopter had arrived at the site small failures from the steep slide edges occurred continuously in the inner part of the slide scope some sliding still took place here a large clay block slid out becoming more and more remoulded suddenly the quick clay completely liquefied this helicopter panoramic view of the final slide scar was taken in the evening about four hours after the initial slide occurred the first slide movement occurred in the direction of the terrain slopes from the hills and 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 books brought water into the slide earlier 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 five kilometers at the other end the waves flooded the small village of Leiter causing a great deal of damage this amateur film taken from a car window in later that same evening shows damaged houses and cars and flooded basements the sawmill and lumber yard 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 slide area covered 330 thousand square meters and the slide volume was of the order of five to six million cubic meters seven farms and five single-family homes were taken by the slide or had to be abandoned for safety reasons of the forty people who were within the area when the sliding started only one was killed several farms and homes were situated close to the slide scope 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 geo nose 54 millimeter piston sampler everybody's had before on it 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 full cone tests as shown tri-axial 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 percent it should be noted that in the zone of quick clay the liquid limit is 5 to 10 percent lower than the in-situ water content the green curve shows the vane shear strength and the red curve the remoulded shear strength the results of geotechnical analysis showed that stabilization works had to be carried out the 15 to 20 meter high remaining terrorists along the foot of the hillside contained quick clay and had to be taken down for safety reasons it was decided to do this by blasting holes 5 to 10 meters deep were drilled both from the top of the clay terrace and along the foot of the steep slope one kilogram of dynamite was used a hundred cubic meters of clay this blasting caused the remaining quick clay and the terraces to liquify and run out into the slide area these masses then eroded into the newly formed thin dry crust of the original slide bottom after several rounds of blasting the clay Terrace was removed over its entire 1 kilometer length the 5 to 10 meter 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 ten months after the slide the entire zone surrounding the slide had been stabilized the following year work started on a new road substituting the one taken by the slide for a distance of about one kilometer 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 barns and outhouses were erected and four years later nearly all traces of the slide disaster had been wiped out once again the slide area is being cultivated and crops harvested Jerusalem 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 disasters

Channel: NGI - Norges Geotekniske Institutt

Views: 1,162,613

Rating: 4.8666005 out of 5

Keywords: Quick, clay, landslide, Rissa, 1978, NGI, Norwegian, Geotechnical, Institute

Id: 3q-qfNlEP4A

Length: 21min 17sec (1277 seconds)

Published: Tue Mar 22 2011

Reddit Comments

[u/wonsnot]

That was awesome.

[u/trashypandapoppi]

This was a welcomed repost.