"Imagine that a patch of land suddenly becomes as soft and runny as toothpaste and buildings literally float away," says Craig W. Christensen of EMerald Geomodelling. This is how quick clay can behave when the conditions facilitate it, typically due to man made disturbances of the ground. It is precisely for this reason that it is so important to uncover quick clay deposits in regions that are prone to such disturbance.
In Norway, national authorities (NVE) in collaboration with the Norwegian Geotechnical Institute and others started back in the 1980s to strategically map areas prone to a high risk of quick clay slides. A significant part of Norway’s urban and agricultural areas lies in the lowlands and are thus potentially located on or close to quick clay deposits. Over the decades, significant investments have been made in geotechnical research to understand, map and manage these challenging ground conditions. Today, engineers have access to a complex toolbox to manage and mitigate this risk. One of those tools is to map ground conditions early on with airborne geoscanning.
Quick clay was formed at the time when the glaciers covered the land during the last ice age. These clays were deposited in stagnant saltwater, and at that time, salt ions formed bonds between the particles in the clay and made it stable. But when the glaciers melted, the land and the seabed rose, and the marine clays ended up on dry land. Year after year, as fresh water ran through these sediments, the salt was slowly but surely washed out. Christensen relates that when the salt content falls below a certain level, it will take only a small disturbance or vibration to make the binding structure in these sediments collapse and fall apart, and in the blink of an eye, the stable ground under your feet turns into a soggy, unstable soup. A quick clay slide is then a certainty.
Dangerous settlements to live in
According to Christensen, Norwegians have settled themselves through history in the places that are the most dangerous from a purely geological point of view.
"The fact is that the best agricultural land in Norway, at the bottom of the deep valleys,can consist of marine, saline clay that can transform into a thin, fluid soup that sweeps away the foundations from right under your feet – quite literally," he says. We know many examples of this kind of landslide, such as the one at Rissa in 1978, where a farmer who had dug a foundation for a barn set off a chain reaction in the hill side. Between 5 and 6 million cubic metres of quick clay flowed out in the course of 45 minutes, leaving in its wake a 1.5 kilometre-long slide scar. Twenty houses and farms were destroyed, and one person lost their life.
Only a few weeks have passed since a major landslide near Alta, where a large area of clay suddenly broke loose and slid out into the sea, taking several cabins with it. Luckily no life was lost in this incident.
A northern problem
The problem with quick clay is by large limited to Norway, Finland, Sweden, Russia, Alaska and Canada, due to the aforementioned glaciers. In Norway, as soon as you get 200-300 metres above sea level, the dangers of quick clay largely disappear.
"The strange thing about quick clay is that its strength as a mass can appear perfectly fine, but when you disturb it, for example by stirring a sample in a bowl, you see at once how it loses its structure and turns into a thin soup," explains Christensen.
Difficult to find
It goes without saying that it is important to find deposits of quick clay before building work starts on any large housing developments and infrastructure projects.The traditional method of doing this is to carry out a site visit based on existing geological maps and local slope stability conditions. When this is combined with geotechnical drilling and sampling, it is then possible to produce a local map.
For most physical properties quick clay behaves very similar to stable marine clay as long as it is undisturbed in the ground. The aforementioned salt content is the key to scan for quick clay potential before drilling programs are designed.
"There aren’t always clear and distinct borders between stiff and stable clay and quick clay, and there can be strong local variations. We have, for example, geoscanned areas that had been characterised as quick clay zones, but we also found areas of stable clay within these zones. If the individual boreholes are several hundred metres from one another, you can miss the detailed information that may be crucial for the planning process," says Christensen.
Even though quick clay and stiff clay mixed with silt and sand have very different mechanica lproperties, they are still very similar from a geophysical point of view, and detecting quick clay is in fact fairly difficult. It is easier to confirm that a clay deposit is not quick than to confirm that it is quick, since a clay’s strength is so closely connected with its salt content. Marine clay with a high salt content – ergo, stable clay – has low resistivity and can be clearly distinguished from other uncompacted material in EMerald Geomodelling’s measurements and 3D models.
In order to obtain the highest possible precision in the measurements, they calibrate their methods for each area that is to be geoscanned.
"We have developed machine learning algorithms that take in borehole data and can classify them almost automatically. We can use this analysis to calibrate our algorithms for the actual geoscanning process, resulting in much more accurate interpretations," says Christensen. The immediate bonus and savings come from less expensive ground drilling campaigns and more detailed measurements.