kChapter 24 Guy LEFEBVRE SOFT SENSITIVE CLAYS 1. INTRODUCTION highly sensitive clays, therefore, it is not just the risk of a slope failure that is of concern, but ensitivity is defined as the ratio of the undis- also the area that can be affected by retrogressive S turbed to the remolded shear strength of a sliding. soil and thus expresses the loss of strength when a Their high sensitivity combined with the fluid- soil is remolded. A variety of clay deposits may ity of the remolded materials makes quick clays exhibit sensitivity, but some marine clay deposits very susceptible to retrogressive landsliding. Rapid exhibit very high sensitivity and very low re- and dramatic geotechnical failures are the result molded shear strength. Such deposits are termed (Figure 24-1). This type of retrogressive failure has quick clays, which is a direct translation of the been referred to as a flow slide or earth flow. The term kvikkleira used by Norwegian investigators to latter term is preferred, since it is in line with cur- describe these deposits, common in their country. rent international landslide terminology (see The Norwegians define quick clays as those soils Cruden and Varnes, Chap. 3 in this report). exhibiting a sensitivity greater than 30 and hav- Several cases have been documented in which ing a remolded shear strength less than 0.5 kPa such retrogressive failures affected areas larger (Norsk Geoteknisk Forening 1974). This defini- than 20 ha and extended more than 500 in be- tion requires the remolded soil to behave like a yondthe crest of the initial slopes (Eden and fluid. It should be noted that this definition of Jarrett 1971; Eden et al. 1971; Tavenas et al. 1971; quick clay considers soil behavior only, ignoring Brooks et al. 1994; Evans and Brooks 1994). geographic location, depositional environment, or At present, the risk and extent of retrogression any other factors related to origin or material. are difficult to evaluate. However, on the basis of In nonsensitive soils, landslide masses generally studies conducted in eastern Canada, retrogres- come to rest at the toe of the slope and act like a sion may be considered to be a consequence of an stabilizing mass in the new slope geometry. For initial slope failure. In terms of analysis, the first sensitive clays, however, the remolding involved step is to quantitatively examine the initial slope in the mass movement results in a drastic reduc- stability. The second step is to evaluate, in a semi- tion in shear resistance, causing the remolded clay quantitative manner, the risk of retrogression that to behave like a thick liquid so that the slide mass could develop following initial slope failure. In ad- moves away and leaves the new slope unsup- dition to numerical analysis, a certain compre- ported. A new instability may result and a series of hension of the formation of slopes in sensitive clay retrogressive failures will be triggered that can ex- deposits is helpful in order to put the processes tend far beyond the crest of the initial slope. In into a rational framework. 607 608 Landslides:Investigation and Mitigation FIGIJRE 24-1 Aerial view of I emieijx landslide. Ontario, Canada, late mnrnlng, Wednesday, June 23, 1993, involving failure of 2.5 to 3.5 million m3, much of which flowed into South Nation valley causing impoundment of river (foreground). Failure took place within 1 hour on June 20, temporarily damming river with debris. Approximate maximum height of river impoundment is shown during overtopping of dam. Ground surface within failure is about 12 m below 2. DESCRIPTION OF SENSITIVE CLAY 2.2 Geologic Origin original ground DEPOSITS surface and slopes Sensitive clay deposits are generally young marine gently toward river. The existence of sensitive clay deposits is related to clays deposited in preglacial and postglacial bod- Debris flowed about 1.6 km upstream ies of water that existed during the retreat of the Mineral sources and depositional conditions re- last Wisconsin ice sheet between 18,000 and and 1.7 km sponsible for an open soil fabric and a relatively downstream of 6,000 years before the present (BP). Many of the high water content and landslide topographic depressions left by the glaciers fonned (Brooks et al. 1994; Geochemical factors responsible for a reduction freshwater lakes, but some were connected to the Evans and Brooks of the liquid limit and the remolded shear oceans at some stage. 1994). strength. The weight of the continental ice sheets caused i'I-IOTOGRAPII 193.254F COURTESY OE GEOLOGICAl. depression of the land surface up to several hun- SURVEY OF CANADA These factors naturally restrict the geographic dis- tribution and geologic characteristics (including dred meters. Isostatic rebounding of the land sur- composition, stratigraphy, groundwater geochem- faces occurred after the ice retreated, but this is a istry, and groundwater volumes) of soft sensitive relatively siow process that is still continuing in clay deposits. some regions. The enormous amounts of water in- corporated into the continental ice sheets caused the ocean volumes to be somewhat less than at 2.1 Geographic Distribution present, and the ocean levels dropped worldwide Although soft clay deposits are found in many by at least 120 m (Kenney 1964). However, the parts of the worki, the most extensive areas of rate of ice-sheet melting and the consequent rise in highly sensitive clays occur in Scandinavia and sea level were much more rapid than the rebound- eastern Canada. They have also been reported in ing of the land surface following ice-sheet removal. Alaska, Japan, the former Soviet Union, and New Isostatic rebound and eustatic sea-level changes Zealand (Torrance 1987). Small areas of similar after the last deglaciation have been well docu- deposits may still be discovered, but it appears un- mented (Kenney 1964; Andrews 1972; llillaire- likely that any large areas of such deposits remain Marcel and Fairbridge 1978; Quigley 1980). The undiscovered. isostatic rebound was much larger than the in- Soft Sensitive Clays crease in sea level. The net result of the interac- mineralogy. The silt fraction is most often high, tion between changing land elevations and sea typically in the range of 30 to 70 percent. The levels was a period of inundation and subsequent fraction with grain sizes smaller than 2 ,um gener- reemergence of certain portions of the continental ally contains more rock flour than clay minerals, land masses in the time between the melting of and the clay minerals are predominantly illite the continental ice sheets and the present day. (Quigley 1980; Locat et al. 1984). Sensitive clays are considered to have been de- posited in marine, or at least brackish, bodies of 2.5 Stratigraphy water. In North America the last glaciation covered most of Canada, parts of the conterminous north- In large depositional basins, marine clay deposits ern United States, and part of Alaska. The largest are generally massive and notably uniform. In deposits of postglacial marine clays were formed in small basins, or in the proximal areas of large the Champlain Sea, which occupied the St. Law- basins, marine clay deposits can be interlayered rence lowlands from the Gulf of St. Lawrence to the with sand and gravel and even with glacial till region around the city of Ottawa during the period (Gadd 1975). For most slope stability analyses, the approximately 12,500 to 10,000 years BP (Elson stratigraphy of typical postglacial soft clay deposits 1969; Gadd 1975; Hillaire-Marcel 1979). can generally be simplified (Figure 24-2). The massive clay formation is confined between two 2.3 Groundwater Geochemistry relatively pervious boundary layers. The upper boundary consists of either a weathered fissured FIGURE 24-2 The emergence of these postglacial marine clay crust or sand and silt derived from the final depo- Simplified formations above sea level altered their ground- sitional stages. The lower boundary is formed ei- down-valley cross water regimes, and because of hydraulic or ther by till deposited on the bedrock by glaciers or section of typical soft marine clay concentration gradients, their pore-water salt con- by fissured bedrock. The overall coefficient of per- centration, which was initially as high as 32 gIL, deposits of eastern meability in the boundary layers is generally at was reduced. Present-day pore-water salinity in Canada least two orders of magnitude higher than that in (modified from many sensitive clay deposits has been reduced to the massive clay deposit. Lefebvre 1986). values below 1 g/L. Reduction of pore-water cations (salinity) is known to reduce the liquid limits (Rosenqvist 1966) and the remolded shear strength of clays. Because of an open fabric and high water con- tent retained since deposition, some lacustrine SURFICIAL LAYER clays and unleached marine clays can also be fairly a, fussured sensitive. The soft Barlow-Oj ibway lacustrine clay "' ,clay crust in northern Quebec Province, for example, has a water content of as much as 100 percent, a plas- ticity index of 40, a liquidity index of 1.5, and a INTACT CLAY sensitivity of about 20. However, geochemical fac- tors, such as salinity reduction, are considered necessary for the development of extrasensitive or quick clays, which have a liquidity index greater than 2 and flow when remolded. However, it should be noted that geochemical factors other than salinity reduction, such as the introduction TILL of dispersants, can also lead to an increased li- quidity index and sensitivity. BEDROCK 2.4 Composition and Mineralogy Because of their geologic origin, sensitive clays fre- quently have a characteristic composition and .M Landslides: Investigation and Mitigation 3. VALLEY FORMATION AND reflects either cementation (Locat and Lefebvre GROUNDWATER REGIME 1986; Lefebvre et al.