GEOTECHNICS

Quick Clay

Updated 26.04 2013

Introduction

Quick clay is mostly found in Norway and Canada. It is a clay which has been subject to a particular geological process and is stable in situ; however, if it is disturbed, it turns liquid. When it turns liquid, it can cause large landslides even in areas where the ground is relatively flat. Below you'll find a brief description of the origin of quick clay and a brief video of a simple quick clay experiment. In order to understand quick clay, I have chosen to start with describing a freshwater clay.

Freshwater clay

Clay minerals are tiny particles and can not be seen by the naked eye. In freshwater, clay minerals settles in a rather compact structure. The structure consists of the clay mineral which binds a small amount of water (bound water) and free water between the minerals. This is illustrated in the figure below.

Figure: freshwater clay

Marine clay

In a Marine environment where the salt content is higher than 3%, clay minerals will form a more chaotic structure than freshwater clay due to a polarisation of the clay particles. The structure of a Marine clay is illustrated in the figure below.

Figure: Marine clay

Due to the chaotic structure of the Marine clay, the free water content is much higher than for a freshwater clay. The clay may have the same strength properties however the water content is much higher than a freshwater clay.

Quick clay

A quick clay is a Marine clay which due to geological processes has ended up above sealevel. During thousands of years, the salt content has been washed out but the chaotic structure remains intact. That means that the clay is now stable; however the water content is higher than freshwater clay. The clay may therefore have a relative high strength; however if it is disturbed the chaotic structure will collapse and the clay behaves as a liquid (quick).

The quick clay can gain strength once again is salt is added since this will create a stable structure with the high water content. Quick clay is therefore particularly dangerous when disturbed. This typically occurs in Norway during construction work and courses very large landslides. The picture below is a video of a test performed on a quick clay by first disturbing the clay and then adding salt to regain strength. Unfortunately, the clay sample was a bit dry and didn't turn as quick as it was supposed to.