Luigi Mongiovi

Experimental procedures and cavitation mechanisms in tensiometer measurements

In recent years tensiometers for direct measurement of matric suction have been developed at Imperial College and later on at the University of Saskatchewan. The major drawback of these instruments is water cavitation which may occur before pressure equalisation. A better understanding of the mechanisms that control cavitation inside the tensiometer may therefore help optimise their design and define adequate experimental procedures. This paper presents some of the experiences gained over the past four years using three tensiometers manufactured by Imperial College. Some of the anomalous responses recorded by tensiometers are first discussed. These were probably due to inadequate saturation of the porous ceramic rather than malfunctioning of the instruments. The results from a series of tests conducted with two or three tensiometers positioned on the same sample are then presented. These were helpful in defining suitable experimental procedures. Finally, the history of tension breakdown of each tensiometer is examined to identify some of the factors that affect the maximum sustainable tension as well as measurement duration. On the basis of data recorded before and after cavitation, a possible mechanism of cavitation in a tensiometer is described.

Suction induced by static compaction

The paper aims at investigating suction induced by static compaction in clay specimens. To this end, kaolin powders prepared at target water contents were statically compacted in a shear box apparatus. Trento (TNT) high-suction tensiometers (0-1800 kPa) were in stalled through the loading pad to monitor suction changes during the loading-unloading paths. Specimens were compacted at water contents ranging from 0.22 to 0.30 and at vertical stresses of 300,600, and 1200 kPa. This made it possible to explore abroad spectrum of compaction conditions. Experimental results are presented and discussed in the paper. The most striking aspect of the results is that suction of unloaded specimens increased as degree of saturation increased. This can be explained by the dependency of the main wetting curve upon void ratio.

Development of an Apparatus to investigate the Stress Variables Governing Unsaturated Soil Behavior

The paper presents the development of an apparatus to investigate the stress variables governing unsaturated soil behavior. This apparatus was devised to explore the range of negative pore-water pressures. Tests were performed by preventing deformation and water content change of an unsaturated sample and measuring changes in normal net stress and matric suction in response to a change in pore-air pressure. The condition of null deformation and null water content change was achieved by specially designing the cell and the air supply system. Vented flush diaphragm transducers and vented Imperial College tensiometers were used to measure directly normal net stress and matric suction, respectively. A preliminary test demonstrated that an interaction occurred between the soil and the flush measurement diaphragms. A specific procedure was then devised to account for the error due to this interaction. The results from the experimental tests support the assumption that the stress variables governing unsaturated soil behavior can be reduced to two stress state variables, e.g., net stress and matric suction.

Effects of unsaturation on the stability of a moraine slope

The papers presents a numerical analysis of the stability of a moraine slope that reactivated after two heavy rainfall events in November 2000. To analyse the pore water pressure distribution within the slope, rainfall and evapotranspiration were applied to the slope over a period of four years that preceded the main landslide reactivation. Because of the uncertainty about the saturated hydraulic conductivities of the landslide soils, different combinations of these coefficients were analysed. The factor of safety of the slope was then calculated at the time where the slope moved, as recorded by the inclinometer measurements. This made it possible to verify which combinations of saturated hydraulic conductivities were compatible with the slope movements.