WORKSHOP – LIA D-FRACT -

COURMAYEUR – 2018

PROGRAM & Participants

Monday

Tuesday

Wednesday

09:00

E. Flekkoy

K. J. Maloy

A. Puisto

9:30

M. Ayaz

R. Planet

A. Skaugen

10:00

T. Makinen

T. Vincent Dospital

S. Ben Zev

10:30

Coffee Break

Coffee Break

Coffee break

11:00

V. Vidal

J. Mathiesen

J. Weiss

11:30

M. Moura

G. Linga

M. Korhonen

12:00

SKI / WORK…

WORK / SKI …

SKI / WORK…

16:30

Coffee Break

Coffee Break

17:00

J-C. Géminard

R. Toussaint

17:30

F. Eriksen

V. Levy

18:00

L. Viitanen

F. Dubourg

D. Dysthe

20:00

DINNER

List of Participants

1. S.Santucci  Organizer

ENS de Lyon

2. M. Alava    Organizer

Aalto

3. J.-C.Géminard

Mechanical stability of disordered media upon deformation

ENS de Lyon

4. V. Vidal

Formation of particle suspensions by gas injection

ENS de Lyon

5. V. Levy     

Evolution of the distance between plates in an experimental granular fault: Implications for earthquake forecast.

ILM Lyon

6. F. Dubourg

Granular Laboratory Quakes: Linking Local and Global Avalanches

ILM Lyon

7. KJ. Maloy

Pattern formation of frictional fingers in a gravitational potentia

Oslo, PoreLab

8. E. Flekkoy

Connecting transport and geometry in giant labyrinths

Oslo, PoreLab

9. M. Moura

Burst dynamics in porous media drainage flows

Oslo, PoreLab

10. F. Eriksen

Granular media on drop interfaces:

deformation, patterns & flow driven by electric fields

Oslo, PoreLab

11. M. Ayaz,

On transport and death of film-networks during slow drainage

PoreLab

/Unistra

12. A. Skaugen

Elasticity and plasticity in the phase field crystal model

PGP Oslo

13. R. Toussaint  

The art of sinking in saturated soils: the role of fluid on soil liquefaction during earthquakes, triggering without pressurization

Unistra,

Strasbourg

14. A. Cochard              

Unistra,

Strasbourg

15. T. Vincent Dospital

Thermal effects in the propagation of a crack front

in disordered papers and polymers

Unistra,

Strasbourg

16. S. Ben Zev

The coupling between compaction and pressurization in cyclically sheared drained granular layers: implications for soil liquefaction

Unistra,

Strasbourg

17. A. Puisto

Coarsening and mechanics in a mesoscale model of wet foams

Aalto

18. M. Korhonen

Interparticle friction controls submerged granular flows in simulations

Aalto

19. L. Viitanen,

Static versus Mobile: single intruders in two-dimensional foams

Aalto

20. T. Mäkinen

Fluctuations in plasticity :

deformation bands in the Portevin–LeChatelier effect

Aalto

21. J. Mathiesen           

Flow and stress in porous rock

NBI, Copenhagen

22. G. Linga

Multiphase electrohydrodynamics in complex geometries

NBI, Copenhagen

23. R. Planet

Drainage/imbibition displacements in a gap-modulated Hele-Shaw cell

UiB, Barcelona

24. J. Weiss

Fracture of disordered quasi-brittle materials and size-effects on strength from a statistical physics perspective - the case of concrete

UJF,

25. D. Dysthe

Confined crystal growth instabilities

PGP Oslo

Monday

1. E.G. Flekkøy (PoreLab)

Connecting transport and geometry in giant labyrinths

K. S. Olsen, J. Campbell, B. Sandnes, K.J. Måløy and E.G. Flekkøy

While the labyrinthine patterns that form during the slow drainage of a deformable porous medium have been known for a decade [1], little has been achieved in the way of characterizing these patterns. While the hydrodynamic forces that generate the labyrinths are known, the means of describing them has been limited to measuring their characteristic channel width. Here we start from the observation that the labyrinths are indeed folded trees and show how diffusion of random walkers on these structures may serve as a way of characterizing the geometry.

Branching analysis of a medium sized labyrinth, showing 7 branching orders.

In order to make this connection a Horton-Strahler branching analysis is carried out, giving rise to a classification of the pattern in various branching orders. The figure shows these orders by different colors. If the labyrinth were realized as say, a maze of hedges, this figure would serve as a map that would show a lost person the way out. The random walker, which has no map, diffuses along in a way that is characterized by an anomalous exponent around 2/3 (normal diffusion being characterized by an exponent of 1). This transport exponent would also characterize the diffusion of heat if the labyrinth were realized as a structure with thermal conductivity, or the spreading of a tracer if it were realized as a substrate for some chemical substance. We predict this exponent from a simplified model that has the observed branching ratio and the fractal dimension of the branches. This dimension quantifies the folding of the branches. Simulations, which produce labyrinths more than 250 times the size of the experimental ones and therefore give results of good statistics, confirm the theoretical prediction.

Reference: [1] Sandnes, B., et al., Labyrinth patterns in confined granular-fluid systems. Phys. Rev. Lett., 2007. 99(3). 


2. Tero Mäkinen (Aalto)

Fluctuations in plasticity: deformation bands in the Portevin–LeChatelier effect

Some metal alloys exhibit deformation instabilities such as the Portevin-Le Chatelier effect. The associated strain localizations and their propagation have been studied experimentally with high spatiotemporal resolution which allows the study of not only the propagation velocities but also their fluctuations. This sheds light on the fluctuations of the underlying dislocation densities. The results are compared to a minimal one-dimensional model of plastic deformation in systems with dynamic strain ageing.

3. M. Ayaz (IPG-S / PoreLab)

On transport and death of film-networks during slow drainage.

M. Ayaz(1,2*) , R. Toussaint(1,2), K. J. Måløy(2) & G. Schäfer(1) 1 Institut de Physique du Globe de Strasbourg, CNRS / Université de Strasbourg 2 PoreLab, Physics Department, University of Oslo

* m.ayaz@unistra.fr

We study experimentally the residual saturation left behind as a fully saturated porous media is drained on a quasi two-dimensional porous model. The model is transparent, allowing the displacement process and structure to be monitored in space and time. Observations show the residual saturation to be interconnected by means of capillary bridges, allowing for seemingly entrapped fluid to be transported back to the bulk. This process shows dependence with the Bond number and a statistical decay with increasing distance from the invasion front. Furthermore, we have analyzed the spatial connectivity of the networks spanned by capillary bridges,  and examined the occurrence of rupturing of individual bridges.

4. V. Vidal (ENS de Lyon)

Formation of particle suspensions by gas injection

Particle suspensions are ubiquitous in various domains such as volcanology (crystal-rich magmas), marine geosciences (gas emission at the seafloor) or chemical engineering (catalytic reactors). In most examples, gas rises through a granular bed, then entrains particles in the above liquid layer. A suspension forms above the granular bed, resulting from the competition between particle entrainment by bubbles and sedimentation. We performed experiments in confined geometry (Hele-Shaw cell) to quantify the properties of the suspension formed by such process. In particular, we investigate the influence of the gas flow rate and effective gravity. The system either reaches a stationary state, or exhibits a puzzling oscillatory regime - more informations & nice movies in the talk!

5. Marcel Moura (PoreLab)

Burst dynamics in porous media drainage flows

M. Moura, K.J. Måløy, E.G. Flekkøy and R. Toussaint

We have given experimental grounding for the remarkable observation made 30 years ago by Furuberg et al. [1] of an unusual dynamic scaling for the pair correlation function N(r,t) during the slow drainage of a porous medium. Our experimental setup allows us to have full access to the spatiotemporal evolution of the invasion, which was used to directly verify this scaling [2]. We have connected two important theoretical contributions from the literature [3,4] to explain the functional dependency of N(r,t) and the scaling exponent for the short-time regime. A new theoretical argument was developed to explain the exponent for the long-time regime.

The intermittent characteristic burst dynamics of the system was also investigated [5] and we have verified a theoretically predicted scaling law for the burst size distribution. We have shown that this system satisfies a set of conditions known to be true for critical systems, such as intermittent activity with bursts extending over several time and length scales, self-similar macroscopic fractal structure and a scaling behavior for the power spectrum associated with pressure fluctuations during the flow. The observation of a 1/f scaling region in the power spectra is new for porous media flows and, for specific boundary conditions, we notice the occurrence of a transition from 1/f to 1/f2 scaling. An analytically integrable mathematical framework was employed to explain this behavior.

Figure 1: Spatiotemporal map showing the evolution of the slow drainage process in which air displaces a viscous liquid from the porous network.

References:

[1] L. Furuberg, J. Feder, A. Aharony, and T. Jøssang, Dynamics of Invasion Percolation, Phys. Rev. Lett. 61, 2117 (1988).

[2] M. Moura, K.J. Måløy, E.G. Flekkøy and R. Toussaint, Verification of a Dynamic Scaling for the Pair Correlation Function during the Slow Drainage of a Porous Medium, Phys. Rev. Lett. 119, 154503 (2017).

[3] S. Roux and E. Guyon, Temporal Development of Invasion Percolation, J. Phys. A 22, 3693 (1989).

[4] S. Maslov, Time Directed Avalanches in Invasion Models, Phys. Rev. Lett. 74, 562 (1995).

[5] M. Moura, K. J. Måløy, and R. Toussaint, “Critical Behavior in Porous Media Flow,” EPL (Europhysics Letters) 118, 14004 (2017).

6. J.-C. Géminard (ENS de Lyon)

Mechanical stability of disordered media upon deformation

We first report on a cellular pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching. We present a simple model in which the mechanism responsible of the instability is the ''strain softening'' exhibited by humid granular materials above a typical strain. Our analysis indicates that such a type of instability should be observed in any system presenting a negative stress sensitivity to strain perturbations. We then extend the experimental study to the case of foam. We shall see that the mechanisms at play differ significantly.

7. Fredrik K. Eriksen (PoreLab)

Granular media on drop interfaces: Deformation, patterns & flow driven by electric fields

A. Mikkelsen, K. Khobaib, F. K. Eriksen, K. J. Måløy and Z. Rozynek

Drops covered by adsorbed particles display a wide range of research studies and applications, for instance in stabilizing emulsions or to encapsulate materials. To unlock the enormous potential of particle-laden drops, it is essential to understand and control particle organization at drop interfaces and how surface particles affect drop properties. We utilize electric fields to experimentally investigate both the mechanics of particle-covered silicone oil drops suspended in castor oil and the structuring of particles at drop interfaces. Interestingly, when subjected to DC electric fields, the deformation magnitude, shape and electrical properties of drops are altered by changing the electric field strength, the particle size, conductivity and particle coverage. Original particle image velocimetry experiments reveal that electrohydrodynamic (EHD) flows play an essential role in this regard. In competition with dipolar interactions, EHD flows also govern the organization of surface particles. This is especially demonstrated in the final part of this study where we present an unprecedented method for controlling the local particle coverage and packing of particles on drop surfaces by simply tuning the frequency of applied AC electric fields. The approach is expected to find uses in optical materials and applications.

8. Leevi Viitanen (Aalto)

Static versus Mobile: single intruders in two-dimensional foams

Foam moving around an obstacle exhibits complex flow behaviour. In this talk these are examined in two-dimensional channel flow with two distinct boundary conditions: static and dynamic. The foam velocity and shear rate fields are experimentally studied along with spatial distribution of topological rearrangements known as T1 events.

Tuesday

9. Knut Jørgen Måløy (PoreLab)

Pattern formation of frictional fingers in a gravitational potentia

J. A. Eriksen, E. G. Flekkøy, R. Toussaint, B. Sandnes, O. Galland and K. J. Måløy

 

Aligned finger structures, with a characteristic width, emerge during the  slow drainage of a liquid/granular mixture in a tilted Hele-Shaw cell. A  transition from vertical to horizontal alignment of the finger structures is  observed as the tilting angle and the granular density are varied.  The dynamics is reproduced in simulations. We also show how the system may  explains patterns observed in nature, created during the early  stages of a dyke formation.

10. R. Planet (Barcelona)

Drainage/imbibition displacements in a gap-modulated Hele-Shaw cell

We consider drainage/imbibition displacements between an inviscid fluid (air) and a viscous fluid (oil) in a narrow channel with gap-thickness modulations. We derive the analytical solution of steady-state front morphologies in imbibition, and compare it to actual experimental realizations. We also predict the hysteretic behaviour of the front in the invasion of a single pore (Haines jumps), and verify it experimentally.

11. T. Vincent-Dospital (IPG-S)

Thermal effects in the propagation of a crack front in disordered papers and polymers

Tom Vincent-Dospital (1), Renaud Toussaint (1,2), Alain Cochard (1), Olivier Lengliné (1,2), Stéphane Santucci (3,2), Knut Jørgen Måløy (4,2)

(1) Institut de Physique du Globe de Strasbourg, CNRS / Université de Strasbourg, France, (2) Center for Advanced Study, Academy of Science, Norway, (3) LP ENS, ENS Lyon, France, (4) Department of Physics, University of Oslo, Norway

During the propagation of a crack in an elastic medium, some of the system’s energy brought by the external load is reversibly stored as elastic energy adapting to the crack morphology, while the rest gets irreversibly dissipated by three main processes: the creation of new fracture surfaces and defects/dislocations, the emission of mechanical waves transmitted to the far field and the Joule heating due to some friction in a damaged zone around the fracture front. The heat hence generated can in turn have a significant impact on the physics of the propagation. Notably, fracture propagation has been shown to be strongly affected by thermally activated rupture, even when the heterogeneity of the material properties determines strongly the fracture geometry and the intermittency of its propagation. This question is notably central in earth science, where a lot of attention has been recently set on thermal effects, with the possibility of thermo-pressurization of faults due the expansion of in situ fluids. Independently of this pressurization effect, the local rise of temperature of the zone enduring damage could significantly affect its creep and the global fracturing process, as understood by statistical physics and the Arrhenius law.

We are interested in quantifying these different effects with both experimental set-ups and numerical simulations. We present three sets of results:

- The first set is based on the infrared and optical imaging of a crack propagating in a sheet of paper. The temperature field in the sheet shows local increases of several degrees during the propagation. We present some numerical simulations that relate the increase of temperature to the speed at which the crack advances and the size of the zone around the crack tip in which the heat is generated.

- The second set is based on the imaging of a fracture in a heterogeneous interface inside an acrylic glass body. We show that modeling the crack kinetics based on the material disorder, the elastic interactions at the crack front, as well as on an Arrhenius law‚ hence depending on the room temperature ‚ shows good agreement with all the experimental observations‚ i.e. the scaling laws in the morphology of the crack front, and the distribution of the local rupture velocity.

- The third set shows numerical simulations combining both considerations: the crack propagation is modeled using an Arrhenius law with a temperature depending on the crack kinetics. We show that such a model leads to two possible propagation modes: a slow mode in which the temperature rise at the crack tip has little effect on the creep and a fast mode in which the crack is thermally weakened leading to high velocity avalanches. We propose that the mode at which the crack actually propagates is mainly dependent on the thermal properties and the toughness heterogeneities of the medium being fractured.

12. J. Mathiesen (NBI, Copenhagen)

Flow and stress in porous rock

13. G. Linga (NBI, Copenhagen)

Multiphase electrohydrodynamics in complex geometries

14. R. Toussaint (IPG-S / PoreLab)

The art of sinking in saturated soils: the role of fluid on soil liquefaction during earthquakes, triggering without pressurization

R. Toussaint1,6*, C. Clément1, E. Aharonov2, M. Stojanova1, S. Ben Zeev1,2, L. Goren2, G. Sanchez-Colina1,3, E. Altshuler3, A. Batista-Leyva4, L. Alonso-Llanes3,1 , M. Bousmaha 5 , S. Parez7

*Renaud.toussaint@unistra.fr

1Institut de Physique du Globe Strasbourg, Strasbourg Cedex, France,

2Hebrew University of Jerusalem, Jerusalem, Israel

3University of Havana, Havana, Cuba,

4INSTEC, Havana, Cuba

5 University Abdelhamid Ibn Badis of Mostaganem, Algeria

6 PoreLab, University of Oslo, Norway

7 Czech Academy of Science, Prague

Soil liquefaction is a significant natural hazard associated with earthquakes. Some of its devastating effects include tilting and sinking of buildings and bridges, and destruction of pipelines. Conventional geotechnical engineering assumes liquefaction occurs via elevated pore pressure. This assumption guides construction for seismically hazardous locations, yet evidence suggests that liquefaction strikes also under currently unpredicted conditions. We show, using theory, simulations and experiments, another mechanism for liquefaction in saturated soils, without high pore fluid pressure and without special soils, whereby liquefaction is controlled by buoyancy forces. This new mechanism enlarges the window of conditions under which liquefaction is predicted to occur, and may explain previously not understood cases such as liquefaction in well-compacted soils, under drained conditions, repeated liquefaction cases, far-field liquefaction and the basics of sinking in quicksand. We show how this mechanism allows explaining liquefaction triggering as function of Earthquake magnitude and epicentral distance. These results may greatly impact hazard assessment and mitigation in seismically active areas.

References:

· C. Clément, R. Toussaint, M. Stojanova, and E. Aharonov

Sinking during earthquakes: Critical acceleration criteria control drained soil liquefaction Phys. Rev. E 97, 022905 (2018)

· C Clément, R Toussaint, E Aharanov, Shake and sink: liquefaction without pressurization, arXiv preprint arXiv:1802.04391, 2018

· Zeev, S. B., Goren, L., Parez, S., Toussaint, R., Clement, C., & Aharonov, E. (2017). The Combined Effect of Buoyancy and Excess Pore Pressure in Facilitating Soil Liquefaction. In Poromechanics VI (pp. 107-116).

· Alonso-Llanes, L., Sánchez-Colina, G., Martínez, E., Batista-Leyva, A. J., Toussaint, R., & Altshuler, E. (2016). Intruder Penetration in Granular Matter Studied by Lock-In Accelerometry. Revista Cubana de Física, 33(2), 95-97.

· Bousmaha, M., Missoum, H., Toussaint, R., & Bendani, K. (2017, November). Saturated Sandy Soils Mechanical Instability Under Vibration Effect. In Euro-Mediterranean Conference for Environmental Integration (pp. 1857-1859). Springer, Cham.

· Sánchez-Colina, G., Alonso-Llanes, L., Martínez, E., Batista-Leyva, A. J., Clement, C., Fliedner, C., ... & Altshuler, E. (2014). Note:“Lock-in accelerometry” to follow sink dynamics in shaken granular matter. Review of scientific instruments, 85(12), 126101.

15. Florine Dubourg (ILM)

Granular Laboratory Quakes: Linking Local and Global Avalanches

F. Dubourg1, S. Lherminier1, R. Planet1,2,3, L. Vanel1, and O. Ramos1

1 Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, LYON, France.

2 Departament de FÍsica de la Matèria Condensada, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain.

3 Universitat de Barcelona, Institute of Complex Systems, Martí i Franquès 1, E-08028 Barcelona, Spain.

We present an experimental study aiming to understand the local origin of the acoustic bursts recorded globally during the shearing of a two-dimensional granular fault submitted to a constant pressure. The analysis focuses on their acoustic frequencies.

Experiments with single grains are also performed; they allow separating the contributions related to collision between neighbours from shear movements between grains. The results show that both frictional sliding and collision mechanisms are involved in the origin of the acoustic bursts. Ultrafast imaging records the relaxation dynamics related to large acoustic events, identifying their nature as well-localized processes, instead of a “snow-ball” avalanche-like scenario. These large events take place along major force chains, indicating a relationship between the energy distribution of events and the structure of the force network, and highlighting the key role of the disorder of the network into a Gutenberg-Richter-like dynamics.

Typical acoustic signal

16. Victor Levy dit Vehel (ILM)

Evolution of the distance between plates in an experimental granular fault: Implications for earthquake forecast.

V. Levy dit Vehel1, F. Dubourg1, L. Vanel1, K. J. Måløy2 & O. Ramos1

1 Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, LYON, France.

2 PoreLab, Department of Physics, University of Oslo, P. O. Box 1048, 0316 Oslo, Norway.

We have recently developed an experimental system capable of --for the first time-- reaching a stationary regime following quantitatively the main laws of seismicity. The intermittent dynamics of our labquakes consists of frictional failures in the structure of a compressed granular medium submitted to a continuous shear. By quantitatively replicating the main laws of seismicity: Gutenberg-Richter law, Omori law, distribution of waiting times between events; as well as other qualitative (for the time being) similarities, our work strongly indicates that these two very different system: earthquakes and our experiment, are governed by a similar physics. Moreover, thanks to the possibility of a significant statistics and better quality measurements than the real phenomenon, our system corroborates the existence of magnitude correlation in the dynamics, a result that has been previously associated to catalogue incompleteness. Here we introduce new results focusing on the evolution of the packing fraction during the experiment, particularly around very large quakes and where we expect a dilatancy of the medium preceding mainshocks. To do that, we have directly monitored the distance between the plates that compress the granular fault. Preliminary results indicate a precursory behaviour that seems more reliable than the analysis delivered so far by our acoustic data.

Experimental setup

17. Dag Kristian Dysthe (UiO)

Confined crystal growth instabilities

We observe crystals “floating” on a fluid film of 25–50 nm in thickness due to the disjoining pressure. We find that in this fluid film there are three end-member nanoconfined growth behaviors: (1) smooth, (2) a Mullins-Sekerka-like instability (3) and rough intermittent growth, the latter being faster than the two former. The intermittent growth rims have regions of load- bearing contacts that move around the rim causing the crystal to “wobble” its way upwards. We present strong evidence that the transition from smooth to rough is a generic confinement-induced instability that is, until now unexplained.

Wednesday

18. Antti Puisto (Aalto)

Coarsening and mechanics in a mesoscale model of wet foams

Aqueous foams are an important model system that displays coarsening dynamics. Coarsening in dispersions and foams is well understood in the dilute and dry limits, where the gas fraction tends to zero and one, respectively. However, foams are known to undergo a jamming transition from a fluid-like to a solid-like state at an intermediate gas fraction, $\phi_c$. Much less is known about coarsening dynamics in wet foams near jamming, and the link to mechanical response, if any, remains poorly understood. In this talk, we discuss coarsening and mechanical response using numerical simulations of a mesoscale model for wet foams. As in other coarsening systems we find a steady state scaling regime with an associated particle size distribution. We relate the time-rate of evolution of the coarsening process to the wetness of the foam and identify a characteristic coarsening time that diverges approaching jamming. In addition, we probe the mechanical response of the system to strain while undergoing coarsening. We find two competing time scales, namely the coarsening time and the mechanical relaxation time. We relate these to the evolution of the elastic response and the mechanical structure.

19. Skaugen (PGP, Oslo)

Elasticity and plasticity in the phase field crystal model

The phase field crystal model is an efficient phenomenological model for describing crystal plasticity in the mesoscale without needing to resolve the fast timescales of lattice vibrations and phonons. However, this comes at the price of making all dynamics take place on the same diffusive timescale. In particular, elastic distortions relax on the same timescale as plastic deformation.

We present some analytical results on the elastic behavior of the PFC, giving a local expression for the stress tensor. We analyze kinetics of dislocations  as topological defects in the amplitude expansion, which allows us to relate the motion of dislocations to the stress. Thus we are able to derive the Peach-Koehler force under some simplifications.

20. S. BenZeev (IES / IPG-S)

The coupling between compaction and pressurization in cyclically sheared drained granular layers: implications for soil liquefaction

BenZeev Shahar 1,4, Goren Liran 2, Parez Stanislav 3, Toussaint Renaud 4 and

Aharonov Einat 1

shahar.benzeev@mail.huji.ac.il

1 Institute of Earth Science, Hebrew University of Jerusalem, Israel;

2 Geological & Environmental Science, Ben-Gurion University of the Negev, Israel;

3 Institute of Chemical Process Fundamentals of the CAS, Prague, Czech Republic;

4 Université de Strasbourg, CNRS, Institut de Physique du Globe de Strasbourg, UMR7516, F-67000 Strasbourg, France;

The dynamics of saturated granular layers during shaking is controlled by the coupling between grains and fluid. Understanding such systems is crucial for studies of soil liquefaction, seismically induced landslides and shear along faults. This study focuses on the compaction of a near surface well-drained saturated granular layer during seismic shaking. Compaction is known to promote soil liquefaction, but the exact feedback mechanism between compaction and pressurization remains poorly understood. We use Discrete Element numerical simulations composed of coupled solid grains and fluid phases under cyclic horizontal shear of the bottom undrained boundary and a free, completely drained, top layer. We compare the dynamics under two drainage conditions: First, simulations of “infinite” drainage, where the fluid pressure is maintained hydrostatic during the shaking. Second, simulations of “realistic” drainage in a high permeability layer, whereby fluid pressure dynamically deviates from hydrostatic values due to local granular compaction and dilatation. Simulation results show two end member behaviors, with a transition controlled by the magnitude of shaking acceleration: At low acceleration the system behaves rigidly, compaction is negligible and fluid pressure remains constant even during “realistic” drainage simulations, where it is allowed to evolve. At high acceleration, significant compaction occurs in both cases, but the compaction rate is higher in “realistic” drainage simulations. This rapid compaction trend is temporally correlated to a transient pore pressure increase that reaches lithostatic stress values before it drops back to a lower value. This is an evidence to a feedback mechanism in which compaction causes pressure increase that can persist under drained condition as long as the compaction rate is sufficiently high. On the other hand, this very pressure itself promotes the high compaction rate. From this we conclude that although well-drained soils are considered liquefaction-resistant, dynamic coupling between pore fluid pressure elevation and compaction during seismic shaking provides a previously unrecognized pathway to liquefaction.

21. J. Weiss (Univ. Genoble)

Fracture of disordered quasi-brittle materials and size-effects on strength from a statistical physics perspective - the case of concrete

22. Marko Korhonen (Aalto)

Interparticle friction controls submerged granular flows in simulations

Powders, sand and slurries exemplify a class of materials known as granular media, their constituent particles sharing the characteristic length scale of 1-1000 micrometers. When subjected to external stress, these materials can exhibit behavior typical of both solids and liquids. The latter behavior can be witnessed in the industrially relevant silo/hopper geometry, where granular particles flow from a hopper via an orifice under the influence of gravity, being embedded in either air (dry case) or in water (submerged case). Performing experiments and CFD-DEM simulations utilizing this flow geometry, we recovered the Beverloo equation as expected in the dry case, implying that the outflux of the granular particles remains constant in such a flow. However, in the submerged case, a time-dependent, non-monotonic outflux is instead observed in both experiments and simulations, which can be explained in the context of an effective shear-thickening rheology.

30 cm

3 cm

1

2

3

4

b a c

30 cm

3 cm

1

2

3

4

b

a

c