Shear‐wave velocity as an indicator for rheological changes in clay materials: Lessons from laboratory experiments

Abstract

[1] Clay-rich geological formations are responsible for many landslides, the dynamics of which are still poorly understood and intensely debated. Analysis of landslide motion shows that slow clayey slope movements can suddenly accelerate and fluidize as a result of sudden loading or heavy rainfall. This solid-fluid transition, which involves disorganization of the particle network, is accompanied by a loss in rigidity that could potentially be monitored by S-wave velocity (Vs) variations. To investigate this hypothesis, two types of laboratory experiments were performed on clay samples originating from an area affected by numerous landslides (Trièves, French Alps). First, creep and oscillatory rheometric tests revealed the thixotropic behavior of the clay with a highly pronounced viscosity bifurcation at a critical stress t c . In relation with this reduction in apparent viscosity, a significant drop in Vs is also observed over t c . Second, at zero stress, acoustic surface wave propagation experiments showed a rapid linear Vs decrease with the gravimetric water content (w) in the plastic domain, and a much lower decay in the liquid domain. The geotechnically-defined liquid limit then appears as a break in the Vs-w curve. For water contents in the liquid domain in particular, both experiments gave consistent results. These laboratory results demonstrate that rheological changes in clay can be revealed through Vs variations, offering the possibility of monitoring solid-to-fluid transitions in the field.

Multiple Geophysical Techniques for Investigation and Monitoring of Sobradinho Landslide, Brazil

“…These changes (dV/V and natural period) are produced because of rainfall-induced changes in the landslide mass such as rheology/rigidity [5,6], natural period [22] and pore-pressure induced stresses [23]. An anomalous change in the shear wave velocity (related to landslide seasonal dynamics and detected by ANI) is reported before the occurrence of a landslide [5], and their correlation with changes in the rheology of material is verified under controlled laboratory scaled experiment [24]. The other applications of ANI are stress changes with magma migration in the case of active volcanoes [25][26][27], structural images i.e., in the case of oil and gas [28], stress changes due to active faults [29,30], stress level monitoring in gold and silver mining [31], deep earth investigation [32,33], monitoring of civil and geotechnical structures like buildings [34][35][36], temporal changes in earthen embankments [19] and tailing dams [37].…”

Dynamics of an Active Earthflow Inferred From Surface Wave Monitoring

“…A further point of discussion is the possible use of this technique for early warning of earthflow movement. Mainsant, Jongmans, et al () detected a decrease of the relative Rayleigh wave velocity well before the reactivation of their monitored landslide (a first 2% drop about 1 month before the movement, and a second 7% drop 4 days before). Mainsant et al () carried out some laboratory experiments on artificial clay slopes having different water content and confirmed a drop in V r values before the failure.…”

“…These standard techniques differ from the method used by Mainsant, Jongmans, et al () in a major aspect. Mainsant, Jongmans, et al () derived the velocity values in the subsoil from the cross correlation of the signal between two geophones at known distance. The two geophones are planted in the stable ground on both sides of the landslide and provide the average Rayleigh velocity across the investigated section.…”

Experiences from site-specific landslide early warning systems

“…Surprisingly, about half of the reported systems did not take into consideration technical sensor limitations before establishing threshold values, even if it would surely decrease the frequency of false alarms. In addition, recent research is focused on failure forecasting and/or flow initiations by looking for mass movement indicators (Baroň et al, 2012) such as precursory displacements (Abellán et al, 2010;Federico et al, 2012), changes in slope rheological settings (Mainsant et al, 2013), strain rates (Jaboyedoff et al, 2012), or hydrological conditions (Abellán et al, 2013;Mercogliano et al, 2013).…”