Microstructural changes associated with thixotropic phenomena in clay soils

Abstract

Thixotropic phenomena in clay soils are accompanied by microstructural changes. The nature of these changes is unclear because of the short duration of the thixotropic processes and the difficulties involved in preparing a specimen at certain stages. However, these technological problems were overcome and SEM photographs were obtained of the thixotropic soil microstructure during deformation of samples in a rotary viscometer with and without vibration. The new data obtained clarify the mechanism of thixotropic phenomena. Soil microstructure was not ruptured by vibration during the shear process. On the contrary, it became more homogeneous over the entire volume and at the same time this induced a decrease in strengthin the system due to a reduction in cohesion at contacts. Disruption of some structural bonds is followed by their rapid restoration—the overall microstructure remaining intact. As a result, in all the samples studied the shear zone disappears. In kaolinite clay and coarser dispersed soils (silty clay, water saturated loess) the local areas of the structural framework (shear planes) become smoother and the orientation of the structural elements along the direction of shear disappears. After the cessation of vibration, the microstructure is rapidly restored to its initial state. Les phénomènes thixotropiques dans les sols argilacés sont accompagnts de changements microstructurels. La nature de ces changements n'est pas clair, à cause de la courte durée des phénombnes thixotropiques et des difficult de la préparation d'un échantillon a` de certaines étapes. Ces problémes technologiques ont été résolus et des images SEM ont été obtenues de la microstructure du sol thixotropique pendant la déformation des échantillons dans un viscométre rotatif avec et sans vibration. Les nouvelles données obtenues clarifient le mécanisme des phénoménes thixotropiques. La microstructure du sol n'a pas été rompue par la vibration pendant le cisaillement. Tout au contraire, elle est devenue plus homogéne sur le volume entier, tandis qu'en même temps ceci a causé une réduction de la résistance du systéme due à une diminution de la cohésion aux contacts. La rupture de quelques liens structurels est suivie de leur rétablissement rapide, tandis que la microstructure entiére reste intacte. Comme résultat, la zone de cisaillement disparaît dans tous les échantillons etudiés. Dans l'argile kaolinite et les sols dispersés plus grossiers (argile limoneuse, loess saturé d'eau) les zones locales du cadre structure1 (plans de cisaillement) deviennent plus lisses, et l'orientation des éléments structuraux le long de la direction de cisaillement disparait. Aprés que la vibration a cessé, la microstructure se rétablit rapidement dans son état initial.

Shear‐rate‐dependent strength control on the dynamics of rainfall‐triggered landslides, Tokushima Prefecture, Japan

“…This kind of friction and interlocking also depends on the grain orientation and mode of intergranular shearing (shear mode, Tika et al, 1996). Many studies have been made of the rate effect on residual shear strength of soils (Petley, 1966;Kenney, 1977;Lupini et al, 1981;Osipov et al, 1984;Lemos, 1986;Tika, 1989;Tika et al, 1996;Suzuki et al, 2004;Saito et al, 2007) and three types of variation of the residual strength with increase of shear displacement rate have been recognized: the residual shear strength (a) increases (positive rate effect), (b) decreases (negative rate effect), and (c) does not change (neutral rate effect). Two hypotheses have been proposed to explain the mechanism of shear-ratedependent residual shear strength of clayey soils, namely (a) excess pore water pressure is produced due to the changing shear rate, which changes effective normal stress and results in changing residual shear strength (The Japanese Geotechnical Society 1987), and (b) the shear mode varies with the shear rate and causes the internal friction angle of soils to change (Skempton, 1985;Tika et al, 1996;Saito et al,2007).…”

Fast shearing of pre-existing shear zones in soil

“…Previous hypotheses advanced to explain the low strengths observed in soils in ®eld and laboratory include mechanical¯uidization (Bagnold 1954; Howard, 1973;Hsu, 1975;Koerner, 1977;McSaveney 1978), frictional heating (Romero & Molina, 1974;Habib, 1975;Anderson 1980;Voight & Faust, 1982 and liquefaction (Terzaghi, 1956;Seed, 1968;Casagrande, 1975;Hutchinson, 1986), as well as those considering the rheological properties of the materials involved, such as thixotropy and the microstructural changes associated with it (Seed & Chan 1957;Mitchell, 1960;Kerr & Drew, 1968;Osipov et al, 1984), and the strength drop due to cyclic loading (Stark & Duncan, 1992). Consideration of all the observations on the behaviour of soils showing a negative rate effect suggests that the following mechanism may apply.…”

Undrained cyclic behavior of bentonite–sand mixtures and factors affecting it

“…5a), the clay particles tend to aggregate and accumulate on the surface of sand grains or at the contact points of sand particles, forming a bridge-like structure bonding the grains to each other. Such particle arrangements produce significant pore spaces, Osipov et al (1984) and Gratchev et al (2006b),, who reported that lowstrength ''clay bridges'' at the contact points of sand grains were easily destroyed during cyclic loading, destabilizing the whole structure. It is likely that for B7 the destruction of ''clay bridges'' during cyclic loading was the trigger for rapid liquefaction.…”

“…In that work, the greater loss of cyclic shear strength under a higher loading frequency was attributed to the thixotropic nature of the studied soil. Thixotropy, a process of bond restoration resulting in some strength recovery, has been well-studied (Mitchell 1960;Osipov et al 1984;Barnes 1997). It was found that thixotropic recovery was time-consuming and increased when the time interval between loading cycles also increased.…”