quick clays The term ‘quick clay’ is used to describe fine-grained, relatively low-density soils that exhibit a dramatic reduction in strength when subjected to either shearing or shaking. The geotechnical term
sensitivity is used to characterize these materials, and is defined as the ratio of the shear strength in the undisturbed state to the shear strength in the fully remoulded state. Most clays exhibit some drop in strength when sheared, but in the case of quick clays sensitivities well above 50 are quite common, and values above 100 have been reported. Under conditions of rapid shearing the material is transformed in a matter of seconds from a rigid solid to a fluid-like mass. Such unusual behaviour is made possible by the build-up of high pore-water pressure within the moving mass, which itself is caused by a collapse of the low-density material as it is sheared. Since water is virtually incompressible, any reduction in volume caused by structural collapse will cause a large increase in pore pressure. When the pore pressure at a given depth is equal to the total overburden pressure, the frictional forces between the particles are close to zero, and as a consequence the material can behave as a fluid. This is the origin of the ‘quick’ condition. Although this behaviour can be induced in most soils by adding water and stirring, the crucial difference with quick clays is that a fluid-like state can be induced without increasing the natural water content.
The extreme sensitivity of quick clays renders them very prone to landsliding, especially in Norway and eastern Canada where quick clay deposits are widespread. The term
flow slide has been coined to describe quick-clay landslides. Flow slides usually begin as a relatively small slump along a steep quick-clay embankment with failure initiated by river undercutting or marine erosion. As the initial slump moves away from the embankment, shear loads are placed on the material directly upslope, triggering a second cycle of failure. This headward progression of failure, known as
retrogression, may occur at a rate of several metres per minute. Eventually a large horseshoe-shaped crater of liquefied material is produced, and rapid evacuation of material from the crater may cause either river damming or significant displacement waves in lakes and fjords. The failure process is eventually arrested when the pore pressures within the moving mass start to dissipate by upward drainage of excess water. Decline of pore pressure then allows particles to interlock, thereby restoring frictional strength. Examination of aerial photographs in quick-clay areas of Canada and Norway reveals numerous flow-slide scars of greatly differing ages. As the St Lawrence Valley lowlands are subject to infrequent but large earthquakes, it is possible that many prehistoric flow slides were triggered by seismic shaking.
Given the unusual behaviour of quick clays, a considerable amount of research has been conducted into their origin and geotechnical properties. All major quick-clay deposits accumulated in marine or brackish-water embayments close to the margins of melting ice sheets at the end of the last glaciation, and were then raised above sea level as a result of isostatic uplift in the postglacial period. Given the source of the material, much quick clay does not consist of true clay minerals but rather of glacial rock flour: clay-sized particles of quartz and feldspar produced by glacial abrasion of bedrock. Such particles tend to be negatively charged and mutually repellant in fresh water, but in brackish or salt water the presence of dissolved salts provides swarms of positively charged salt ions which allow aggregation, or flocculation, of fine particles to occur. The small platelets of material tend to align themselves by forming bonds between particle edges and opposing particle faces in a three-dimensional card-house structure. This open, low-density structure favours the retention of large amounts of pore water, and over postglacial time, percolation of fresh groundwater through the deposits has leached out significant amounts of the original salt. As a consequence, the cohesive strength of the card-house structure is progressively reduced over time, although small amounts of cementing material such as iron oxide may be present. When the potentially unstable structure collapses because of shearing or shaking, the pore water is compressed and the ‘quick’ condition rapidly develops.
Michael J. Bovis
Bibliography
Evans, S. G. and and Brooks, G. R. (1994) An earthflow in sensitive Champlain Sea sediments at Lemieux, Ontario, June 20, 1993, and its impact on the South Nation River. Canadian Geotechnical Journal, 31, 384–94.
Mitchell, R. J. and and Markell, A. R. (1974) Flowsliding in sensitive soils. Canadian Geotechnical Journal, 11, 11–31.
Torrance, J. K. (1987) Quick clays. In Anderson, M. G. and Richards, K. S. (eds) Slope stability, pp. 447–73. John Wiley & Sons, Chichester.
