The phenomenon of Quick Clays

Introduction

Quick clay refers to a specific type of clayey material that experiences a significant loss of strength when it is disturbed.

Quick clays are glaciomarine formations that can be found mostly in the Northern Hemisphere in several regions, such as parts of Norway, Canada, Russia and the United States. Glaciomarine deposits are formed in a marine environment due to both marine and glacier geologic processes. As land uplift followed the withdrawal of the glaciers, marine clays deposits rose above the sea water level and have been detected at elevations up to more than 2,000 meters.

Soil Sensitivity

Quick clay behavior is primarily characterized by a material property known as sensitivity, a term utilized in soil mechanics to define the ratio between the undisturbed and the remolded shear strength of a soil material. In Figure 1, stress-strain response examples of clay materials (sensitive and non-sensitive) are presented. In sensitive clays, a loss of the ultimate undrained shear strength is observed when the material reaches its post-peak behavior. 

Given the schematic of Figure 1, the sensitivity of a clay is calculated as:

In its natural, pre-failure state, quick clays yield considerable strength, however, the sensitivity ratio that characterizes such materials is higher than 30-50, therefore their remolded strength is significantly lower.  Moreover, the remolded soil tends to behave more like a fluid than a solid, a phenomenon known as liquefaction. This radical behavior alteration can be explained by the loss of the clay particle structure. Clayey materials are characterized based on their sensitivity as presented in Table 1.

Detecting a quick clay deposit is challenging as the stratum is typically covered by topsoil and thus, soil sampling and testing needs to be conducted on the underlying layer. Geophysical techniques can be utilized to investigate the subsurface conditions and identify quick clay soils.

The undrained shear strength can be measured in-situ via Vane Shear or CPT tests. However, there is no specific test designed to measure the remolded strength of the material in the field so that the clay deposit can be classified. Therefore, the undisturbed strength has to be measured via laboratory testing. 

Table 1: Clayey soil classification based on sensitivity (data from Rosenquist, 1953)

Factors that affect quick clay sensitivity

The sensitivity of quick clays primarily depends on the following factors:

• Sea water salt leaching: Marine formations are deposited in sea water which contains a specific concentration of dissolved sodium chloride (NaCl). Leaching refers to the process of salt ions being removed from the soil. It is a procedure that strongly affects the capability of soil particles to regain their structure after being disturbed. Research has shown that leaching may also affect the undisturbed strength of a quick clay.
• The Ion composition of pore water: Higher concentrations of Na⁺ and K⁺ ions in the pore water of clays results in greater sensitivity as they create larger diffuse double layers around the clay particles which are responsible for increased repulsive forces between them.
• The level of PH: According to Mitchell (1976), when the PH in the pore water is high, hydrogen ions tend to dissolve leading to an expansion of the diffuse double later and resulting in higher sensitivity.
• Dispersing substances: Dispersing agents mainly consist of natural organic substances that can affect the ion composition of pore water leading again to an expansion of the diffuse double layer of clays and higher sensitivity.

Quick Clay mineral composition

Similar to other types of soil, quick clay is a three-phase material that consists of solid particles and voids filled with air and/or water.  Quick clays consist of non-swelling clay minerals such as kaolinite and illite. In general, clay minerals are known as secondary minerals that are formed as a result of silicate (e.g. feldspars, amphibolites, and mica) weathering.

Quick Clay Landslides

Quick clay may depart from its stable state due to multiple triggering factors (overloading, erosion, rainfalls, slope undercutting, earthquakes etc).  The phenomenon has been the main cause of numerous massive landslides that have occurred throughout the history (more than 250 recorded in Canada).

Quick Clay landslides tend to occur without a warning pattern and progress within a short time-period. Therefore, they resemble co-seismic landslides even if they may be triggered by other factors.

Progressive failure in quick clays that leads to extensive landsliding requires 3 major conditions:

1. The undisturbed shear strength of the material must locally be exceeded.
2. Quick clay must acquire sufficient differential strain to reach post-failure behavior.
3. The remolded shear stress of the clay must be low enough that stresses are redistributed and affect neighboring material that has not failed yet.

Quick clay landslides may have disastrous impact on human life and the built environment due to their enormity and the absence of a warning mechanism.

Two of the most well-known cases of quick clay landslides are presented below.

The Case of the Verdal Landslide

On a night of May 1893, a massive landslide struck the community of Verdal, in Norway, resulting in 116 casualties and major infrastructure damage.

The event consisted of 3 landslides that struck consecutively. It was reported that the third one was the most destructive. Landsliding was completed in just 30 minutes and 55 million m³ of clay flooded the municipality. It was estimated that the velocity of the debris reached approximately 60km/h. The failure was associated with the increased rainfalls that had struck the region before the incident.

The Verdal slide has been the deadliest in Norway’s history. An area of about 9 km² of land was covered by several meters of debris.

The Case of the Rissa Landslide

The Rissa Landslide occurred in 1978 in Norway and became widely known as it was filmed by two amateur photographers. Rissa is located close to a fiord, consisting of quick clay deposits, northwest of the city of Trondheim.

The landslide struck at the banks of Lake Botnen and affected a flat region of marine clay deposits between the lake and the fjords which had been used for agriculture.

In April 1978, an earth fill was placed at the shore of the lake after excavations works were completed at a nearby barn. Suddenly, an 80-meter part of the shoreline slid into the lake as a result of the load applied by the earth fill. However, the landslide development continued. Over the next 40 minutes, the slide progressed retrogressively in a series of small slides and affected a 450-meter width zone that covered an area of more than 25,000 m².

Nevertheless, the body of the main landslide had just began destabilizing. A large soil mass (about 150x200 meters) began moving downwards towards the lake. Its velocity was initially 10-20km/h and eventually reached almost 40km/h. The amateur photographer that filmed the development of the event had to run for his life. Most of the quick clay debris reached the lake while some stopped in a compression zone near the shoreline.

A series of smaller landslides followed the main event over the following period but the process halted when the slide area reached the mountain side. It was later estimated that the total landslide volume was 5–6 million m³.

To learn more details about the Rissa landslide, watch the short documentary prepared by the Norwegian Geotechnical Institute (NGI) below. It includes the original footage captured by the amateur photographers.

Credits: NGI - Norges Geotekniske Institutt

References

Baverfjord B. and Thakur, V. (2008). Landslides in Education: The Verdal and Rissa Landslides. 6th International Conference on Case Histories in Geotechnical Engineering. USA, September 2008.

Lundström, K., Andersson-Sköld, Y., Hultén, C., Larsson, R., Leroux, V. and Dahlin, T. (2004). Quick clay in Sweden. Swedish Geotechnical Institute. Linkoping. ISSN 0348-0755.

Mitchell, J.K. (1976). Fundamentals of soil behaviour. John Wiley & Sons, Inc., New York.

Rosenqvist, I. Th., (1953). Considerations on the Sensitivity of Norwegian Quick-Clays Geotechnique, Vol. 3, No. 5, pp 195-200.