J. Kenneth Torrance

Physical, chemical and mineralogical influences on the rheology of remoulded low-activity sensitive marine clay

Abstract The rheological response of remoulded Leda clay from the South Nation River landslide (1971) site has been investigated using a coaxial viscometer. The yield stress at constant water content was found to: decrease as the pore-water salinity decreased, with the decrease occurring at much lower salinity when Ca-saturated than when Na-saturated; decrease as the clay content of the soil was decreased; decrease upon the extraction of oxide minerals by dithionite–citrate–bicarbonate; decrease when the phyllosilicate content of the soil was depleted by a severe alternating acid/base extraction procedure; increase as the pH of the marine clay, whether carbonate-containing or carbonate-free, was lowered by acid addition. The effects are in agreement with expectations based on electrical double-layer theory. The relative effects of the various pretreatments are qualitatively discussed. Viscoplastic-shear-thinning and Bingham plastic response most commonly occurred, although at low salinities viscoplastic-shear-thickening response was exhibited for materials with high silt contents and after phyllosilicate-depletion. No relationship between flow type and the tendency to exhibiting quick clay behaviour is apparent.

Traditional farmers' knowledge of Sorghum (Sorghum bicolor [Poaceae]) landrace storability in Ethiopia

Traditional knowledge of grain storage was studied on small farms in Ethiopia. Sixteen of sixty landraces of sorghum collected from small farms in Shewa and Welo regions of Ethiopia were identified by the farmers as being stored sorghum landraces. Farmers were interviewed and asked to rate the storability of these stored-sorghum landraces with respect to the major insect pest of the area, the rice weevil. A farmers’ index of storability was then calculated for each landrace. The landraces were then assessed for rice weevil, Sitophilus oryzae (L.), susceptibility in standardized tests at 70% relative humidity and 27°C. The mean farmers’ index for the 16 landraces was found to be inversely related with the susceptibility parameters of F1 emergence (r2=0.80), oviposition (r2=0.76), weight loss (r2=0.88), and Dobie Index (r2=0.95). It was much less strongly related with the median development period (r2=0.20). The results show that farmer knowledge is an excellent guide to sorghum susceptibility to storage pests.RésuméSeize de soixante variétés de sorgho, récoltées sur des petites fermes dans les régions de Shewa et de Welo en Éthiopie, ont été identifiées comme des variétés destinées à l’entreposage. Les fermiers ont été interrogés et on leur a demandé d’évaluer la résistance des grains de céréales entreposés face au charançon du riz, le principal insecte nuisible de la région. Un indice des fermiers basé sur les évaluations de résistance a été calculé pour chacune des variétés. Les variétés ont été ensuite évaluées pour leur susceptibilité au charançon du riz dans des tests normalisées. L’indice des fermiers est inversement corrélé avec les paramètres de susceptibilité d’émergence de la génération F1 (r2=0.80), d’oviposition (r2=0.76), de perte de poids (r2=0.88) ainsi que l’indice de Dobie (r2=0.95). Il n’est pas corrélé avec la période médiane de développement. Les résultats démontrent que les connaissances des fermiers permettent de reconnaître la susceptibilité du sorgho face aux insectes nuisibles.

Chemical factors in soil freezing and frost heave

Chemical factors that are essential in frost heaving of soils are examined through consideration of the process of ice formation in soils and the role of temperature gradients in generating water potential gradients in freezing soils. Unfrozen films are maintained around soil particles in frozen soils. The osmotic potentials at the ice–water interface of the unfrozen films and in the frozen fringe, the thin zone between the frozen and unfrozen soil, generated by dissolved salts and exchangeable cations that satisfy soil particle surface charge, are controlled by the local temperature. The coldest location and the most negative osmotic potentials at the ice–water interface are located immediately below the base of the ice lens, in the unfrozen films that separate the underlying soil particles from the ice lens. An osmotic potential gradient is generated because the osmotic potential at the water–ice interface in the frozen fringe becomes less negative with increasing temperature and distance from the ice lens. As water freezes onto the ice lens, re-supply of water to the unfrozen film along the osmotic potential gradient is the temperature-gradient-induced mechanism that generates the force that lifts the overlying frozen soil. Models that recognize this driving mechanism should improve predictions of soil freezing and frost heave, analysis of contaminant transport in freezing and frozen soils, and other aspects of the soil-freezing and frost-heave processes.

Oxide minerals in the sensitive post-glacial marine clays

Abstract Oxide minerals increase the undisturbed and remoulded strengths of the sensitive marine clays. In Eastern Canada, the oxide mineral contents of sensitive post-glacial marine sediments are greatest in the near-surface, where weathering has occurred, and decrease to an approximately constant value (in the order of 1%) where unweathered. In these marine clays, crystalline hematite or magnetite dominate the iron oxide content, with little, if any, ferrihydrite. The total amorphous mineral content is very low. In Japanese quick clays, in which the minerals are of volcanic origin, ferrihydrite is the dominant iron oxide and total amorphous mineral content is not known. In order for the small quantity of oxide minerals present in the Canadian sensitive marine clays to have measurable effects on undisturbed behaviour, they must be concentrated at points of contact between silicate minerals. Their influence on remoulded behaviour must be the result of their decreasing the electrostatic repulsion between particles, thereby increasing the probability of flocculation. The imperfect mimicking of the natural processes complicates the interpretation of experiments in which oxide minerals are added and problems with the selectivity of chemical extractants complicate the interpretation of experiments in which oxide minerals are removed.

Chemistry, Sensitivity and Quick-Clay Landslide Amelioration

Quick clay’s combination of sufficient undisturbed shear strength to be stable in situ and liquefaction upon structural failure derives from chemical factors that act during and after sediment accumulation. Flocculation of silt and clay particles in marine water produces an interlinked, random structure with a water content that approximates its high-salinity liquid limit. Co-sedimentation of cementing agents may augment its strength. Upon uplift above sea level, fresh water displacement of the high-salinity pore water decreases the liquid limit while the structure and water content remain almost constant, yielding a liquidity index in the range of about 1.5–4. The central premise of this paper is that development of solutions begins with consideration of how problems have arisen. Specifically, chemistry’s dominating role in quick clay development provides insights into approaches that can be applied to amelioration of the quick clay landslide problem.

Chemistry: An Essential Key to Understanding High-Sensitivity and Quick Clays and to Addressing Landslide Risk

The property of silty-clay to clayey-silt quick clays, whereby apparently solid soil transforms to the liquid state when subjected to sufficient stress, derives from chemical factors: mineralogy (low activity); depositional environment (marine-to-brackish conditions causing flocculation and high water content); and post-depositional chemical changes (development of cementation and displacement of marine-to-brackish water by infiltrating rainwater). The stability of slopes developed by river incision is affected (negatively) by physical factors (drainage and fluctuating water tables) and chemical weathering reactions that have led to weak, fissured, blocky, nodular structures. Immediate causes of quick clay landslides are commonly the physical factors of: river erosion; high water contents in the fissured slopes; and human actions. Regardless, the characteristics of the resulting landslides are primarily determined by the chemical factors.