Shear-wave velocity as an indicator for rheological changes in clay materials : lessons from laboratory experiments 2in clay materials : lessons from laboratory experiments2

R´esum´e

Les formations g´eologiques riches en argiles sont responsables de nombreux glissements de terrain avec des dynamiques encore mal connues et vivement d´ebattue. L’analyse du mouvement des glissements de terrain montre que les mouvements lents argileux peuvent soudainement s’ac-c´el´erer et se fluidifier lors d’une augmentation de contrainte interne et/ou de variation de la quantit´e d’eau (par pr´ecipitations ou fonte du manteau neigeux). Cette transition solide-liquide, qui implique une d´esorganisation du r´eseau de particules, est accompagn´ee d’une perte de rigi-dit´e qui pourrait ´eventuellement ˆetre contrˆol´ee par la variation des vitesse d’ondes S Vs. Pour v´erifier cette hypoth`ese, deux types d’exp´eriences de laboratoire ont ´et´e effectu´ees sur des ´echan-tillons d’argile provenant d’une r´egion particuli`erement touch´ee par des glissements de terrain (la r´egion du Tri`eves, Alpes fran¸caises). Tout d’abord, des tests de fluage et d’oscillations dyna-miques ont r´ev´el´es le comportement thixotrope de l’argile avec une bifurcation de viscosit´e tr`es prononc´ee `a une contrainte critiqueτ_c. En relation avec cette r´eduction de la viscosit´e apparente, une baisse significative de Vs est ´egalement observ´ee lorsque la contrainte seuilτ_c est atteinte.

Deuxi`emement, `a contrainte nulle, des exp´eriences de propagation des ondes accoustiques ont montr´e une diminution rapide et lin´eaire de Vs en fonction de la teneur en eau gravim´etrique (w) dans le domaine plastique, et une d´ecroissance beaucoup plus faible dans le domaine liquide.

La limite de liquidit´e g´eotechnique (LL) apparaˆıt `a la rupture de pente de la courbeVs-w. Pour des teneurs en eau ´elev´ee (domaine liquide), les deux exp´eriences ont donn´e des r´esultats coh´e-rents. Ces r´esultats de laboratoire montrent que des changements rh´eologiques en argile peuvent ˆetre r´ev´el´es par les variations de vitesses d’ondes S, offrant la possibilit´e de suivre la transition solide-liquide de ces mat´eriaux.

Abstract

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 (Tri`eves, 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 τ_c. In relation with this reduction in apparent viscosity, a significant drop in Vs is also observed over τc. Second, at zero stress,

2. Auteurs : G. Mainsant (ISTerre), D. Jongmans (ISTerre), G. Chambon (IRSTEA), E. Larose (ISTerre), L.

Baillet (ISTerre). Publi´e dansGeophysical Research Lettersle 6 octobre 2012.

4.4. Shear-wave velocity as an indicator for rheological changes in clay materials : lessons from

laboratory experiments 117

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.

4.4.1 Introduction

Clay landslides are widespread throughout the world and pose serious problems for land management, because the slide can exhibit a dramatic increase in velocity (from a few cm/y to more than one m/s)(Van Asch and Malet, 2009). These acceleration phases, which generally occur during or after heavy and sustained rainfall, are regularly associated with a change in rheology, from a solid to a fluid-like behavior ([Angeli et al. (2000) ;Picarelli et al. (2005) ; Ei-lertsen et al.(2008)). The solid-fluid transition has aroused considerable interest in physics and soil mechanics and has been the object of significant research efforts (Coussot et al. (2005) ; Van Asch and Malet (2009)). In soil mechanics, fine-grained soils are usually classified using the so-called Atterberg limits (plastic limit PL and liquid limitLL), which are the water contents separating the three main states (solid, plastic and liquid) and measured experimentally using standard tests (for details, see Cornforth, 2005, chapter 8). Although the physical interpretation of these empirically-derived limits is still poorly understood, they are widely used in geotech-nical engineering and frequently correlated to other engineering properties (Cornforth (2005)).

However, if the Atterberg limits constitute good indicators of the macroscopic behavior of clay sediments, these parameters are not sufficient to fully understand the initiation and dynamics of landslides.

To gain a deeper insight into the constitutive law of such materials, i.e. the relationship bet-ween stress, strain, and strain rate, rheological tests are necessary. The rheological behavior of earthflows and debris flows has been the subject of intense research and debate in the scientific community, with the confrontation between elastoplastic and viscoplastic models (Iverson et al.

(1997) ; Ancey (2007)). In elastoplastic models, fluidization is related to the observed increase in pore water pressure, resulting from water infiltration or internal stress variations (Iverson (2005)). In contrast, numerous laboratory experiments performed on water-clay dispersions de-monstrated the potential of viscoplastic models for homogenized fine-grained materials, whereas, for poorly sorted materials, the bulk rheology can obey either elastoplastic or viscoplastic models (Ancey2007). In clays, rheology is thus usually described by a viscoplastic law (e.g. the Herschel-Bulkley model) in which the solid-to-fluid transition is characterized by a critical, yield stress τ_c (Huang and Garcia (1998)). However, in some clays like bentonite, Coussot et al. (2002a) showed that this transition can be more complex, and occurs with a viscosity bifurcation. Below the critical stress, the viscosity increases with time and the initial creep eventually stops, while for a slightly higher stress the fluid abruptly starts to flow with finite viscosity. This thixotropic behavior, which results from the competition between aging and shear rejunevation, was also observed in quick clays byKhaldoun et al. (2009), who found that the viscosity decreased dra-matically during flow, which may explain the unexpected large runout distances of clayey mass

slides.

Unfortunately, viscoplastic parameters for fine-grained soils can only be determined in the la-boratory (Ancey (2007)) and no in-situ monitoring of these properties is possible at the present time. An alternative is to continuously measure changes in the geophysical parameters cha-racterizing the sliding material behavior. In this respect, recent field and laboratory studies (Jongmans et al. (2009) ;Renalier et al. (2010)) have shown that the shear wave velocity (Vs) is very sensitive to clay deconsolidation (and the corresponding increase in porosity). In case of clay liquefaction,Vsshould tend to zero. Recently, a significant drop inVs (from 360 m/s to 200 m/s) was measured at the base of a small active clayey landslide (Pont Bourquin, Switzerland), several days before the triggering of an earthslide-earthflow of a few hundred m³(Mainsant et al.

(2012b)). This result opens up promising perspectives in relating geophysical and rheological pa-rameters for landslide prediction.

This paper aims to investigate through laboratory tests the potentiality of Vs as an indica-tor for rheological changes and the solid-fluid transition in clay. The tested clay was sampled in the Tri`eves plateau (Western Alps, France), at the southern limit of the large Avignonet land-slide (Bi`evre et al.(2011a)). Atterberg limits were measured on 3 samples, yielding a mean value of LL = 0,44±0.03, with a plasticity index of about 0.2. First, parallel-plate rheometric tests were performed at different water contents w in order to determine the viscoplastic behavior of the Avignonet clay. In parallel, the dynamic shear modulus under loading was measured by applying harmonic shear stress variations, in order to be able to relate Vs values to rheological parameters. In a second step,Vs values at zero stress were measured in saturated clay, with wa-ter content ranging from the plasticity limit to over the liquidity limit. Vs values were obtained by studying the propagation of surface waves in a clay-filled box. Combining the two series of tests, the Vs-w curve was drawn, showing the pertinence of Vs for monitoring the clay state over a wide range of porosity values.

4.4.2 Rheometric tests

Rheological measurements on Tri`eves clay were performed using a 60 mm diameter Bohlin-CVOR rotational rheometer, with a parallel-plate geometry. Before each test, the samples were pre-sheared at a strain rate of 50 s⁻¹for 10 s, and then left at rest for 5 s, to ensure a reproducible initial state. The temperature in the rheometer was fixed at 18^◦C. As several phenomena might hinder the repeatability of the rheometric results (Coussot (2005)), tests were systematically duplicated to quantify uncertainty. Creep tests, which consist in subjecting the sample to a steady shear stress and monitoring the strain rate induced versus time, were conducted for 8 different water contents (fromw= 0,54 tow= 0,72) over the liquid limitLL= 0,44. Tests for w <0,54 were not possible due to the development of heterogeneous deformation and cracking in the samples. Figure 1 shows the creep curves measured at constant stress for four values of w. For w = 0,54 (Figure 4.14 a), a clear transition in the mechanical response is observed at a critical shear stress τc ≈ 617 Pa. Below τc, the strain rate systematically decreases (i.e., the apparent viscosity increases) with time and progressively tends to zero (solid behavior).

4.4. Shear-wave velocity as an indicator for rheological changes in clay materials : lessons from