The time was 4:04 PM when the ground in Tuve suddenly started moving. Five minutes later 65 houses had been destroyed and nine people had lost their lives in one of Sweden’s largest landslides.
‘Research has taught us more about the formation of quick clay and its preconditions since the Tuve landslide’, says geologist Mats Olvmo.
On 30 November it will be 40 years since an entire residential area in Tuve on the island of Hisingen was destroyed by a mudslide. This was by no means the first landslide along the Göta River Valley, but it is the one that had the greatest impact on many people in modern times. Besides the fact that nine people were killed, 400 were left homeless when their houses were destroyed.
What happened on that November afternoon? Like several other landslides around the Göta River, the Tuve slide was caused by quick clay. Quick clay is a kind of clay that is sensitive to mechanical disturbance and that can quickly lose its stability.
‘The structure of the clay collapses and it becomes completely fluid’, Mats says.
Quick clay slides develop in what are known as sensitive clays, which mainly occur along the coasts of Sweden, Norway, Canada and elsewhere. This type of marine blue clay had been deposited on the seabed adjacent to the ice sheet that melted away during the latest ice age. In connection with land elevation, the clay was then lifted above the water’s surface, where the salt was slowly leached out by rainwater or groundwater. In somewhat simplified terms, you could say that the salt acts as a kind of binder in the clay. When the salt disappears, the clay becomes sensitive and there is a risk that it will liquefy if it is subjected to the kind of shaking that occurs in connection with traffic or construction.
But it’s not just groundwater that affects the stability of the clay. A variety of other factors are involved. For example, quick clay is formed at different rates, depending on how soil layers are built up and how the landscape is configured. In the case of Tuve, many factors coincided, causing the landslide. It started in the area between two heights, Snarberget and Tångeklacken, where the surface of the bedrock tilts steeply downwards and the clay is not supported by rock down along the valley. There was good potential for leaching here, and after an extremely rainy autumn, what is called pore water pressure was sky-high. This meant that the clay was exposed to such high pressure from the groundwater that it was almost lifted off the substrate.
The Tuve landslide became the starting shot for more comprehensive research on quick clay. Because there are many factors that come into play when quick clay slides are formed, it is difficult to predict where they might occur.
‘To be able to locate quick clay, you need to find places where the preconditions for dehydration of the clay are favourable’, says Mats Olvmo.
Various methods have been developed to identify potential quick clay hills. In 2014 a new method was published in a thesis by Martin Persson from the Department of Earth Sciences. The method consists of a computer model that can be used to quickly predict the quick clay conditions for large areas.
‘My method can be used to utilise large amounts of archived geotechnical data, geological knowledge about soil layers and the established theory of how quick clay is formed. In this way, we can better predict where quick clay is found and utilise land properly to reduce the likelihood of large landslides’, says Martin Persson.
Today geotechnical investigations are routinely conducted before deciding to build in an area. But that does not mean that landslides do not occur. One example is the 2006 landslide at Småröd south of Munkedal, where the new motorway collapsed. The landslide occurred because a large amount of excavated material was piled up in connection with the motorway construction, which overloaded the hillside.
‘Even if you have good control of the geotechnical conditions, things can still go wrong. Lack of communication in connection with construction is one such example’, says Mats.
But the landslide area in Tuve will not be built upon. After the landslide a total of 70 houses that either were damaged or stood on the edge of the slide area were demolished, and the area has since been reinforced by piling. But you can still see traces of the landslide in the terrain, which moved the entire Kvillebäcken hillside.
‘The landscape in the slide area changed radically during the few minutes that the landslide took place. The ground was displaced as much as 200 metres down towards the stream Kvillebäcken along an area 800 metres long and 600 metres wide. The stream was moved tens of meters eastward, and even today you can see traces of the landslide.’
A method to forecast quick clay
The thesis Predicting Spatial and Stratigraphic Quick-clay Deployment in SW Sweden by Martin Persson presents a method for predicting quick clay preconditions over large areas.
The method, which can be used to quickly forecast quick clay preconditions over large areas, shows that the preconditions for quick clay are greatest in Bohuslän and the Göta River Valley. The Swedish Geotechnical Institute is currently evaluating its usefulness, along with other geotechnical and geophysical methods.