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Snow is a complex porous material presenting various microstructural patterns. This microstructure controls the mechanical properties of snow, and this control still needs to be better understood. Recent numerical developments based on three-dimensional tomographic data have provided new insights into snow mechanical behaviour. In particular, the discrete element method combined with the snow microstructure captured by tomography and the mechanical properties of ice has been used to reproduce the brittle properties of snow. However, these developments lack experimental evaluation so far. In this study, we evaluate a numerical model based on the discrete element method with cone penetration tests on centimetric samples. This test is commonly used to characterise the snowpack stratigraphy but also brings into play complex mechanical processes and deformation patterns. We measured the snow microstructure on different samples before and after a cone penetration test with X-ray tomography. The cone test was conducted with the Snow MicroPenetrometer (5 mm cone diameter), which recorded the force profile at high resolution. The initial microstructure and the ice properties fed the model, which can reproduce the exact same test numerically. We evaluated the model on the measured force profile and the displacement field derived from the difference between the initial and final microstructures. The model reasonably reproduced the force profiles in terms of average force, force standard deviation, and the correlation length of the force fluctuations. When the contact law describing ice mechanics is adjusted in the range of reasonable values for ice, the agreement becomes good on all three parameters. The model also well reproduced the measured deformation around the cone tip, which is less sensitive to the contact law parameterization. Overall, the model is capable of distinguishing the different microstructural patterns tested. Therefore this confrontation of numerical results with experimental measurements for this configuration gives confidence in the reliability of the numerical modelling strategy. The model could be further applied with different boundary conditions and used to characterise the mechanical behaviour of the snow better.

The aim of the survey was to define the indications for preventive tracheostomy in transoral robotic surgery (TORS) for head and neck cancers.From October 2019 to January 2020, an online questionnaire was e-mailed to French surgical ENT teams with considerable experience of the TORS procedure (Gettec group). A descriptive analysis of the answers was performed.Eighteen French surgical teams answered the questionnaire. For 77.8% of the surgical teams, a past history of radiotherapy with residual edema was an indication for prophylactic tracheostomy, and for 88.9%, 75 mg of antiplatelet medication or anticoagulation treatment was an indication.Early preventive tracheostomy during TORS can protect airway from uncommon but potentially life-threatening complications, such as transoral hemorrhage or airway edema. We recommend it in high-risk situations, such as a past history of radiotherapy or antiplatelet therapy associated with large resections. Further studies are needed to establish evidence-based recommendations.

International audience; Background: The aim of this study was to assess the impact of the initial therapeutic strategy on oncologic outcomes in patients with HPV-positive OPSCC.Methods: All p16-positive OPSCCs treated from 2009 to 2014 in 7 centers were retrospectively included and classified according to the therapeutic strategy: surgical strategy (surgery ± adjuvant radiotherapy and chemotherapy) vs. non-surgical strategy (definitive radiotherapy ± chemotherapy). Univariate, multivariate propensity score matching analyses were performed to compare overall (OS), disease-specific (DSS) and recurrence-free survival (RFS).Results: 382 patients were included (surgical group: 144; non-surgical group: 238). Five-year OS, DSS and RFS were 89.2, 96.8 and 83.9% in the surgical group and 84.2, 87.1 and 70.4% in the non-surgical group, respectively. These differences were statistically significant for DSS and RFS after multivariate analysis, but only for RFS after propensity score matching analysis.Conclusion: In p16+ OPSCC patients, upfront surgery results in higher RFS than definitive radiotherapy ± chemotherapy but does not impact OS.

Introduction Although Human Papilloma Virus (HPV)-driven oropharyngeal cancer (OPC) prognosis is significantly better than that of other head and neck cancers, up to 25% of cases will recur within 5 years. Data on the pattern of disease recurrence and efficiency of salvage treatment are still sparse. Material and method Observational study of all recurrent OPCs diagnosed, following a curative intent treatment, in seven French centers from 2009 to 2014. p16 Immunohistochemistry was used to determine HPV status. Clinical characteristics, distribution of recurrence site, and treatment modalities were compared by HPV tumor status. Overall survival was examined using Kaplan–Meier and multivariate Cox regression modeling. Results 350 recurrent OPC patients (246 p16-negative and 104 p16-positive patients). The site of recurrence was more frequently locoregional for p16-negative patients (65.4% versus 52.9% in p16-positive patients) and metastatic for p16-positive patients (47.1% versus 34.6% in p16-patients, p = 0.03). Time from diagnosis to recurrence did not differ between p16-positive and p16-negative patients (12 and 9.6 months, respectively, p-value = 0.2), as the main site of distant metastasis (all p-values ≥0.10). Overall and relapse-free survival following the first recurrence did not differ according to p16 status (p-values from log-rank 0.30 and 0.40, respectively). In multivariate analysis, prognosis factors for overall survival in p16-negative patients were distant metastasis (HR 2.11, 95% CI 1.30–3.43) and concurrent local and regional recurrences (HR 2.20, 95% CI 1.24–3.88). Conclusion With the exception of the initial site of recurrence, the pattern of disease relapse and the efficiency of salvage treatment are not different between p16-positive and negative OPCs.

International audience; The paper presents a combined state and parameter estimation for a landslide model using a Kalman filter. The model under investigation is based on underlying mechanics that depicts a landslide behavior. This system is described by an Ordinary Differential Equation (ODE) with displacement as a state and landslide geometrical and material properties as parameters. The Kalman filter approach is utilized on a simplified model equation for state and parameter estimation. Finally, the presented approach is validated by two illustrative examples, the first one a synthetic case study and the second one on Super-Sauze landslide data taken from the literature.

Once fallen on the ground, snowflakes evolve quickly and form the snowpack. Under its own weight, the snowpack slowly deforms and settles. On a steep slope, some layers may rapidly deform and fail, which yield to the release of an avalanche. At the macroscopic scale, snow mechanical behavior is highly strain-rate dependent: ductile at low strain rates and brittle at high strain rates. The ductile-to-brittle transition has recently been shown to occur in two stages, with an intermediate regime of intermittent brittle failures, assumed to result from a competition between different time scales. At the micro-scale, this mechanical behavior is controlled by the microstructure and the visco-plastic and sintering properties of the ice skeleton. In this work, we investigate snow brittle-to-ductile transition by conducting displacement-controlled compression tests monitored with X-ray micro-computed tomogaphy.We specifically designed a loading apparatus to perform displacement-controlled compression tests in cold environment and the constrained space of the tomographic cabin, so that microstructure evolution could be followed by regular scans. Samples (14 mm in diameter, 14 mm in height) were prepared from natural snow, sieved directly into samples holders, in batch of 10 samples and sintered for 72h at -20°C then stored at -50°C to prevent further microstructure evolution. The sample were taken out and placed in the compression device 30min before the first scan. We explored strain rates from 10-6 s-1 to 10-2 s-1 by vertically compressing 30 samples, up to a peak stress of 250 kPa and at a constant temperature of -18.5 °C. At high strain rates, only the initial and final 3D microstructures were scanned and simple radiographs were acquired during loading at a rate of 5 frames per second. At low strain rates, the 3D microstructure was regularly scanned during the loading. The obtained time series comprises one of the most-resolved (8.5 µm, 1h) and complete dataset on snow microstructure evolution near the ductile-to-brittle transition to date.Results indicate a clear dependency of snow mechanical response on the strain rate. At strain rates larger than about 10-3 s-1, snow samples display heterogeneous deformations with the formation of compaction bands, while the stress-strain curve shows a serrated behavior. To relate this macroscopic behavior to micro-structural evolution, quantitative investigation of local density and specific surface area changes, as well as of bond network evolution, will be presented. These results should help identifying the micro-scale mechanisms at play during deformation of snow through both ductile and brittle range.

Once fallen, snow settles due to the combined effects of metamorphism and deformation of the ice matrix under gravity. To understand how these coupled processes affect snow evolution, we performed oedometric compression tests and continuously monitored the snow microstructure with X-ray tomography. Centimetric samples with an initial density between 200 and 300 kg m−3 were followed during an initial sintering phase and under two different loads of 2.1 and 4.7 kPa at $-8^\circ$ C for ~1 week. The microstructure captured at a voxel size of 8.5 μm was characterized by density, specific surface area (SSA) and two metrics related to bond network, namely the Euler characteristic and the minimum cut surface. Load-induced creep of the ice matrix was observed only for sufficiently low values of initial density (−3 in our tests), and was shown to be associated to a significant increase of the number of bonds. Application of the load, however, did not affect the individual bond size nor the SSA, which appeared to be mainly controlled by isothermal metamorphism. The uniaxial compression did not induce any creation of anisotropy on the microstructural characteristics. Overall, our results show that, for the considered conditions, the deformation of the ice matrix mainly leads to a reduction of the pore space and an increase of the coordination number, while metamorphism mainly affects the grain and bond sizes.

International audience; A two-velocity fluid model is presented to describe particle migration in mono-disperse suspensions of neutrally buoyant particles. In contrast to previous migration models, the proposed formulation allows us to impose explicit boundary conditions on particle velocity, and thereby to satisfy strict mass conservation for the particle phase. In addition, the upper bound on particle volume fraction (jamming limit) is strictly enforced through a non-smooth complementarity condition and the introduction of a particle jamming pressure. The model is applied to an axisymmetric Poiseuille flow and solved using a finite-element method. For that purpose, a specific, fully implicit algorithm based on non-smooth optimisation tools is developed and validated. Preliminary comparisons with experimental data from the literature show promising agreement. In particular, the model properly captures the formation of an inner plug region, in which the suspension is saturated and jammed.

International audience; Purpose Characterization of thyroid nodules is crucial to propose surgical intervention for histological verification. Cervical ultrasound potentially combined with fine needle aspiration is recommended, and fluorocholine positron emission tomography (FCH-PET), commonly used in prostatic cancers, has been evaluated in the diagnosis of thyroid cancers in recent publications. Methods We present two cases of patients with multinodular thyroid and primary hyperparathyroidism. The preoperative assessment consisted of an ultrasound, a MIBI scintigraphy and an FCH-PET in favor of a parathyroid adenoma. Results The imaging examinations pointed to a diagnosis of a parathyroid adenoma. In both cases, papillary thyroid carcinoma, missed by FCH-PET, was discovered incidentally at a distance from the parathyroid adenoma during the surgical procedure. Conclusions These are the first descriptions of thyroid papillary carcinoma without preoperative FCH-PET identification. These clinical cases are contrary to recent publications showing a benefit of this examination in the diagnosis of thyroid cancers.

Landslides display heterogeneity in movement types and rates, ranging from creeping motion to catastrophic acceleration. In most of the catastrophic events, rocks, debris, or soil can travel at several tens of meters per year speed, causing significant cost in life losses, infrastructure, economy, and ecosystem of the region. In contrast, slow-moving landslides display typical velocities scaling from few centimeters to several meters per year. Although slow-moving landslides rarely claim life losses, they can still cause considerable damage to public and private infrastructure. Sometimes these slow, persistent landslides eventually lead to catastrophic acceleration, e.g., clayey landslides are prone to these transitions. Such events need to be detected by Early Warning Systems (EWS) in advance to take timely actions to reduce life and economic losses. Several approaches are proposed to forecast the time of failure; still, there is a need to improve prediction strategies and EWS’s. Here we present state and parameter estimation for a simplified viscoplastic sliding model of a landslide using a Kalman filter approach, which is termed as an observer problem in control theory. The model under investigation is based on underlying mechanics (physics-based model) that portray a landslide behavior. In this model, a slide block is assumed to be placed on an inclined surface, where landslide (slide block) motion is regulated by basal pore fluid pressure and opposed by sliding resistance governed by friction, cohesion, and viscosity. This model is described by an Ordinary Differential Equation (ODE) with displacement as a state and landslide material and geometrical properties as parameters. In this approach, known parameter values (landslide geometrical parameters and some material properties) and water table height time-series are provided as input. Finally, two illustrative examples validate the presented approach: i) a synthetic case study and ii) Hollin hill landslide (Uhlemann et al., 2016) field data. In both examples, displacement, friction angle, and viscosity are well estimated from known parameter values, water table height time-series, and displacement measurements. In the simulation results for the Hollin Hill field data, it is observed that friction angle almost remains constant while viscosity varies significantly through time. Uhlemann, S., Smith, A., Chambers, J., Dixon, N., Dijkstra, T., Haslam, E., Meldrum P., Merritt, A., Gunn, D., and Mackay, J., (2016). Assessment of ground-based monitoring techniques applied to landslide investigations. Geomorphology, 253, 438-451. doi:10.1016/j.geomorph.2015.10.027.

Wet snow avalanches present distinctive features such as unusual trajectories, peculiar deposit shapes, and a rheological behavior displaying a combination of granular and pasty features depending on the actual snow liquid water content. Complex transitions between dry (cold) and wet (hot) flow regimes can also occur during a single avalanche flow. In an attempt to account for this complexity, we report on numerical simulations of avalanches using a frictional-cohesive rheology implemented in a depth-averaged shallow-flow model. Through extensive sensitivity studies on synthetic and real topographies, we show that cohesion plays a key role to enrich the physics of the simulated flows, and to represent realistic avalanche behaviors. First, when coupled to a proper treatment of the yielding criterion, cohesion provides a way to define objective stopping criteria for the flow, independently of the issues incurred by artificial diffusion of the numerical scheme. Second, and more importantly, the interplay between cohesion and friction gives raise to a variety of nontrivial physical effects affecting the dynamics of the avalanches and the morphology of the deposits. The relative weights of frictional and cohesive contributions to the overall stress are investigated as a function of space and time during the propagation, and related to the formation of specific features such as lateral levées, hydraulic jumps, etc. This study represents a first step towards robust avalanches simulations, spanning the wide range of possible flow regimes, through shallow-flow approaches. Future improvements involving more refined cohesion parameterizations will be discussed.

Once on the ground, the microstructure of snow, i.e. the three-dimensional arrangement of ice and pores, quickly evolves with metamorphism and deforms under the overburden of the overlaying snow. Understanding these concurrent processes is important to predict the evolution of the physical and mechanical properties of snow which are crucial for many applications, such as avalanche forecasting. To this end, we monitored oedometric creep tests of snow under isothermal conditions at -8.6°C for about one week with X-ray tomography. We investigated the evolution of recent snow under a constant load of around 4 kPa, where both ice matrix creep and metamorphism are active. Our time-series comprises one of the most highly-resolved images of snow microstructure evolution, with a temporal resolution of 3 h and spatial resolution of 8.5 microns and thousands of images. Interestingly, we observed distinct effects of the overburden and of the vapor transport on the microstructure evolution. In particular, the quantification of the ice bond network through the Euler characteristic and the min-cut surface shows that metamorphism progressively increases the bond size almost independently of the applied overburden, while the application of an overburden yields a rapid increase of the bond coordination number. These distinct impacts exhibit the difficulty to accurately reproduce the time evolution of recent snow by snow cover models, whose snow microstructure representation with density and snow type remains too coarse.

International audience; The mass transfer mechanisms in landslides are complex to monitor because of their suddenness and spatial coverage. The active clayey Harmalière landslide, located 30 km south of Grenoble in the French Alps, exhibits two types of behavior: in its upper part, decameter-sized clay blocks slide along a listric slip surface, while a flow-like mechanism is observed in a clayey remolded material a few hundred meters below the headscarp. The landslide underwent a major retrogression affecting 45 ha in March 1981 and has experienced multiple reactivations since then. The last major event took place on the 26th of June 2016, and a large investigation survey was conducted to better understand the reactivation mechanism. A multi-method investigation was carried out at different temporal and spatial scales, including aerial photograph and light detection and ranging processing, correlation of optical satellite images, global navigation satellite system monitoring, continuous seismic monitoring, and passive seismic survey. The morphological evolution of the landslide was traced over the last 70 years, showing a headscarp retrogression of 700 m during multiple reactivations and a total mass transfer of more than 6 × 106 m3. The detailed study of the 2016 event allowed to track and understand the mechanism of a mass transfer of 1 × 106 m3 in 5 weeks, from a sliding mechanism at the headscarp to an earthflow at the toe.

We study the microscopic origin of nonlocality in dense granular media. Discrete element simulations reveal that macroscopic shear results from a balance between microscopic elementary rearrangements occurring in opposite directions. The effective macroscopic fluidity of the material is controlled by these velocity fluctuations, which are responsible for nonlocal effects in quasistatic regions. We define a new micromechanically based unified constitutive law describing both quasistatic and inertial regimes, valid for different system configurations., Physical Review Letters, 125 (18), ISSN:0031-9007, ISSN:1079-7114

International audience; An intimate understanding of snow failure, particularly under mixed-mode shear and normal loading, is a key ingredient for numerical modelling of snow avalanche release. Due to its highly porous structure, the failure of snow can result in a volumetric collapse. Despite its importance, snow failure and collapse remain poorly understood, mainly due to the fragile nature of the material, which renders systematic experimental exploration difficult and complicates observation at the microscopic level. A microstructure-based discrete element model of snow has been developed and utilized to study snow failure under mixed-mode loading. Depending on applied normal stress, three distinct failure regimes are identified, characterized by different volumetric responses. Shear-induced macroscopic collapse is observed to develop above a critical level of normal stress. A deeper understanding of the mechanisms leading to this volumetric collapse is investigated on the microscopic scale. Force chain buckling within the snow sample leads to volumetric collapse, while stable force chains result in a localized failure and the absence of a volumetric collapse. The stability of the force chains is shown to be insured by the grain contacts between the force chains and the surrounding material. The ratio of contacts close to failure is identified to control the absence or presence of collapse after failure.

International audience; Patients with oropharyngeal squamous cell carcinoma (OPSCC) display a significant risk of synchronous primary neoplasia (SPN) which could impact their management. The aims of this study were to evaluate the risk and distribution of SPN in OPSCC patients according to their HPV (p16) status, the predictive factors of SPN and the impact of SPN on therapeutic strategy and oncologic outcomes.Material and methods: All OPSCC patients treated from 2009 to 2014 were included in this multicentric retrospective study. Univariate analyses were conducted using Chi-2 and Fisher exact tests. For multivariate analyses, all variables associated with a p ≤ 0.10 in univariate analysis were included in logistic regression models.Results: Among the 1291 patients included in this study, 75 (5.8%) displayed a SPN which was preferentially located in the upper aerodigestive tract, lung and esophagus. Comorbidity level (p = 0.03), alcohol (p = 0.005) and tobacco (p = 0.01) consumptions, and p16 tumor status (p < 0.0001) were significant predictors of SPN. In multivariate analysis, p16+ status was significantly associated with a lower risk of SPN (OR = 0.251, IC95% [0.133;0.474]). Patients with a SPN were more frequently referred for non-curative treatment (p = 0.02). In patients treated with curative intent, there was no impact of SPN on the therapeutic strategy (surgical vs. non-surgical treatment). We observed no overall survival differences between patients with or without SPN.Conclusion: P16 tumor status is the main predictive factor of SPN in OPSCC patients. This study provides crucial results which should help adapt the initial work-up and the global management of OPSCC patients.

International audience; This paper presents an observer-based state and parameter estimation for the extended sliding-consolidation model of a landslide. This system is described by a pair of coupled Ordinary Differential Equation (ODE) and Partial Differential Equation (PDE), with a mixed boundary condition for the PDE. The coupling appears both in the ODE and in the Neuman boundary condition of the PDE. The observer consists of a copy of the PDE part of the system and Kalman-like observer for the ODE. It is shown to ensure exponential convergence of the state and parameter estimates by means of Lyapunov tool. Finally, a simulation result of the extended sliding-consolidation model is presented to illustrate the effectiveness of the proposed observer.

Only few rheological models in the literature simultaneously capture the two main non-Newtonian trends of non-colloidal suspensions, namely finite normal stress differences and transient effects. We address this issue by extending a previously proposed minimal model accounting for microstructure anisotropy through a conformation tensor, which was shown to correctly predict transient effects (Ozenda, Saramito & Chambon, J. Rheol., vol. 62 (4), 2018, pp. 889–903). A systematic sensitivity study was performed to provide insights into the physical interpretation of the various model terms. This new model is compared to a large experimental dataset involving varying volume fractions, from dilute to concentrated cases. Both apparent viscosity and normal stress differences in steady state are quantitatively reproduced in the whole range of volume fraction, and qualitative agreement for transient evolution of apparent viscosity during shear reversal is obtained. Furthermore, the model is validated against particle pressure measurements that were not used for parameter identification. Even if the proposed constitutive equation for the Cauchy stress tensor is more difficult to interpret than in the minimal model, this study opens the way for the use of conformation tensor rheological models in applications where the effect of normal stress differences is prominent, like elongational flows or particle migration processes.

SUMMARYIn late June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-yr-long period of continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 2 × 10 6 m3. Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic metres. For 4 months, they continuously recorded seismic ambient vibrations and microearthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (fB), the relative variation in Rayleigh wave velocity (dV/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material.

International audience; This work proposes an optimal approach for parameter estimation in a landslide motion, based on the so-called adjoint method. The system is described by an extended sliding-consolidation model composed of an ordinary differential equation and 1D parabolic partial differential equation that represents landslide motion and pore pressure evolution respectively. The key feature of this model is pore pressure feedback, which regulates landslide motion and leads to coupling between both differential equations. Parameters to be estimated include the friction and dilatancy angle of the material. The objective functional for the optimal estimation is composed of: i) a cost function defined as the least square error between measurements and related simulated values, and ii) a product of Lagrange variables and system dynamics. A variational approach is applied to get the gradients of the cost functional with respect to parameters to be estimated and adjoint model. The cost functional is optimized, employing thesteepest descent method to estimate parameters. Finally, the presented optimal estimation method is validated on a test case.

Many regions of the world are exposed to landslides in clayey deposits, which pose major problems for land management and human safety. Clayey landslide activity is complex, showing a succession of periods of inactivity and reactivation phases that can evolve into sudden acceleration and catastrophic landslides and/or flows. Understanding the processes that control this activity therefore requires the continuous monitoring of specific parameters. At the end of June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble in the French Alps) was dramatically reactivated at the headscarp after 35 years of continuous but limited activity. The total volume involved, which moved in the form of tilted blocks of different sizes, was estimated at about 3,106 m3. Several sensors, including seismometers and GNSS stations, were installed immediately behind the main escarpment in early August 2016. They recorded a rupture involving a block of a few hundred cubic meters in November 2016. Additional data (seismology, meteorology, piezometer, etc.) were provided by a permanent observatory located a few hundred meters away in the nearby Avignonet landslide (RESIF2006). Two three-component seismic sensors were placed on the collapsed block and 10 meters aft on the stable part respectively.Thus, four seismic parameters were monitored for 4 months until the clay block rupture: the cumulative number of microseisms, the resonance frequency of the block, the relative variation in Rayleigh wave velocity (dV/V) and the correlation coefficient (CC) in the range 1-12 Hz. All these parameters showed a significant precursor signal before the rupture, but at very different times. During the monitoring period, they also showed different responses to environmental parameters and in particular to precipitation. The resonance frequency increased slightly but steadily from 8 to 9 Hz (+12%) during the pre-break period, then decreased from 9 Hz to 7 Hz (-22%) just one hour before the break. However, the other three parameters showed significant variations a few weeks before failure. The dV/V and CC parameters reacted 1.5 month before the failure, during a very heavy rain event. The CC showed a general decrease over time, first affecting the high frequencies, then gradually spreading to the low frequencies. Finally, seismic activity is almost constant during the first three months, with only slight temporary increases during precipitation. One month before the rupture, a significant increase in the number of events is observed.This study shows the potential of monitoring different seismic parameters over time in order to predict the slip of blocks in a clay material.

Recent winters saw a striking increase in wet snow avalanche activity. Compared to dry avalanches, wet snow avalanches present uniquely distinctive features such as slower velocities, larger depths, unusual trajectories and deposit shapes, and a paste-like rheology that can result in large shear and normal stresses. In addition, the behavior of wet avalanches may strongly vary depending on the actual snow liquid water content. Complex transitions between dry (cold) and wet (hot) behaviors have also been observed during the propagation of single avalanche events. Current numerical models of avalanche dynamics are challenged when it comes to capturing the full spectrum of these different regimes, and the transitions in between. In this contribution, we critically review the various rheological models that have been proposed in the literature to simulate dry and wet snow avalanches in the frame of depth-averaged shallow-flow approaches. On this basis, a simplified parametric rheological law is proposed, with the objective of representing both dry-like and wet-like behaviors and allowing for smooth transitions between them. The law is implemented in a robust 2D shallow-flow simulation code, and systematic sensitivity studies are performed on synthetic and real topographies. Simulation outcomes are analysed in terms of propagation dynamics and deposition patterns, and the ability of the model to capture both dry and wet regimes is discussed. Lastly, a specific calibration methodology is proposed to infer the relevant mechanical parameters from documented avalanche events.

International audience; The cone penetration test is widely used to determine the mechanical properties of snow and to delineate snow stratigraphy. Precise knowledge of the snow stratigraphy is essential for many applications such as avalanche forecasting or estimating the snowpack energy budget. With the development of sophisticated, high-resolution digital penetrometers such as the SnowMicroPenetrometer, the cone penetration test remains one of the only objective methods to measure snow stratigraphy. An accurate interpretation of the measured hardness profiles requires to understand the interaction between the cone tip and the snow material. In this study, we measured the displacement induced by the penetration of a conic tip with a radius of 2.5 mm in eight different snow samples using X-ray tomography. The experiments were conducted at a temperature of −10°C. To recover the full three-dimensional displacements between the tomographic images measured before and after the test, we specifically designed a tracking algorithm which exploits the unique shape of each snow grain. The tracking algorithm enables to recover most of the granular displacements and accurately captures the volumetric strain directly derived from density changes. The measured displacements are shown to be oriented downwards below the tip apex, upwards close to the snow surface, and nearly only radially in between. We observed and quantified the development of a compaction around and below the tip. Surprisingly, we also observed dilation of the snow material close to the snow and tip surfaces in very high-density samples. The radial extent of the compaction zone ranged between 1.6 and 2.3 times the tip radius. These results were compared to existing interpretative models. Although limited to relatively small samples and short penetration depths, these results provide new insights on snow deformation during a cone penetration test, and the validity of these models.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; The ability of viscoplastic fluids to self-select their flow geometry through the formation of unyielded dead zones has important consequences for flows in wavy channels, flows over obstacles, etc. Yet, the mechanisms controlling the formation and dimensions of the dead zones remain poorly understood. We present a detailed cross-comparison of experimental and numerical results concerning channel flows of a viscoplastic fluid over a rectangular cavity filled by the same material. In all the configurations studied, which correspond to moderate values of the Bingham number, a continuous dead zone forms inside the cavity. Both numerical and experimental data reveal that, unlike at high Bingham numbers, the shear-rate profiles above the dead zone display an asymmetric shape. Accordingly, two different flow zones can be distinguished: a Poiseuille-like zone, in which the velocity profile is similar to that over a rigid wall, and a boundary layer ensuring the transition with the dead zone below. It is shown that the effective boundary condition felt by the Poiseuille-like layer is essentially controlled by incoming flow characteristics, such that the thickness of this layer does not obey simple relations with cavity length. Interestingly, however, the thickness of the boundary layer appears to follow a generalized Oldroyd’s scaling with cavity length.

International audience; Introduction: Patients with oropharyngeal squamous cell carcinoma (OPSCC) display a significant risk to develop a metachronous second primary neoplasia (MSPN). HPV and non-HPV-related OPSCC are 2 distinct entities with biological, clinical and prognostic differences. The aims of our study were to analyze the impact of tumor HPV status and other relevant clinical factors, such as tobacco and/or alcohol (T/A) consumption, on the risk and distribution of MSPN in OPSCC patients and to assess the impact of MSPN on patient survival.Material and methods: All OPSCC patients treated from 2009 to 2014 were included in this multicentric retrospective study. P16 immunohistochemical expression was used as a surrogate maker of tumor HPV status. The impact of tumor p16 status on the risk of MSPN was assessed in uni- and multivariate analyses. Overall survival (OS) was determined by Kaplan-Meier analysis.Results: Among the 1291 patients included in this study, 138 (10.7%) displayed a MSPN which was preferentially located in the head and neck area (H&N), lung and esophagus. Multivariate analyses showed that p16- tumor status (p = 0.003), T/A consumption (p = 0.005) and soft palate tumor site (p = 0.009) were significantly associated with a higher risk of MSPN. We found no impact of p16 tumor status on the median time between index OPSCC diagnosis and MSPN development, but a higher proportion of MSPN arising outside the H&N, lung and esophagus was found in p16 + than in p16- patients. MSPN development had an unfavorable impact (p = 0.04) on OS only in the p16 + patient group.Conclusion: P16 tumor status and T/A consumption were the main predictive factors of MSPN in OPSCC patients. This study provides crucial results with a view to tailoring global management and follow-up of OPSCC patients.

The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting., The Cryosphere, 11 (1), ISSN:1994-0416, ISSN:1994-0424

International audience; Snow sintering is investigated at microscopic and macroscopic scales with high-resolution cone penetration tests. In a cold room at −10 °C, vertical profiles of penetration force were measured periodically during 24 h using the SnowMicroPenetrometer in four snow samples, which differed only by their grain sizes. We estimated the evolution of snow micromechanical properties, namely the bond rupture force, the deflection at rupture and the number of ruptures per penetration increment, by applying a statistical analysis to penetration profiles. The upper part of the profiles is transient due to the progressive formation of a compaction zone in front of the cone tip. In order to explicitly account for this process in the statistical analysis, we used a non-homogeneous Poisson shot noise model which considers a depth dependency of the rupture occurrence rate. On simulated transient profiles, this analysis is shown to provide accurate estimates of the micromechanical properties. On our experimental data, the method effectively revealed that the vertical heterogeneity of penetration force was essentially due to variations of the rupture rate. Conversely, the time evolution of the macroscopic force was mainly due to microstructural bond strengthening. Both macroscopic force and bond rupture force followed a power law with an average exponent of 0.27 and 0.29, respectively. On our samples, a higher exponent for larger grains was observed on the microscopic bond force, while no trend with grain size was visible in the exponent characterizing the macroscopic force evolution.

[Departement_IRSTEA]Eaux [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; The mechanical response of snow to mixed‐mode shear and normal loading is the key ingredient for snow avalanche modeling and strongly depends on microstructural characteristics. A discrete element numerical model was developed, which enables the simulation of large‐strain response of snow samples directly described by their full microstructure obtained through X‐ray microtomography. The model offers new insights into the failure mechanism as well as postfailure response of snow in mixed‐mode loading. Three distinct failure modes are identified, depending on the value of applied normal stress. Above a certain threshold normal stress, the failure is characterized by a structural collapse that decomposes the snow sample into a set of cohesionless grains. It is shown that the collapse is a dynamic process, which, once initiated, develops independently of shearing. This behavior was consistently observed for different snow types, including faceted crystals typically composing weak layers.

Flow-like landslides in clayey soils represent serious threats for populations and infrastructures and have been the subject of numerous studies in the past decade. However, despite the rising need for landslide mitigation with growing urbanization, the transient mechanisms involved in the solid-fluid transition are still poorly understood. One way of characterizing the solid-fluid transition is to carry out rheometrical tests on clayey soil samples to assess the evolution of viscosity with the shear stress. In this study, we carried out geotechnical and rheometrical tests on clayey samples collected from six flow-like landslides in order to assess if these clayey soils exhibit similar characteristics when they fluidize (solid-fluid transition). The results show that (1) all tested soils except one exhibit a yield-stress fluid behavior that can be associated with a bifurcation in viscosity (described by the critical shear rate $$ \dot{\gamma_c} $$ ) and in shear modulus G; (2) the larger the amplitude of the viscosity bifurcation, the larger the associated drop in G; and (3) the water content (w) deviation from the Atterberg liquid limit (LL) seem a key parameter controlling a common mechanical behavior of these soils at the solid-fluid transition. We propose exponential laws describing the evolution of the critical shear stress τc, the critical shear rate $$ \dot{\gamma_c} $$ , and the shear modulus G as a function of the deviation w-LL.

We propose a new, minimal tensorial model attempting to clearly represent the role of microstructure on the viscosity of noncolloidal suspensions of rigid particles. Qualitatively, this model proves capable of reproducing several of the main rheological trends exhibited by concentrated suspensions: Anisotropic and fore-aft asymmetric microstructure in simple shear and transient relaxation of the microstructure toward its stationary state. The model includes only few constitutive parameters, with clear physical meaning, that can be identified from comparisons with experimental data. Hence, quantitative predictions of the complex transient evolution of apparent viscosity observed after shear reversals are reproduced for a large range of volume fractions. Comparisons with microstructural data show that not only the depletion angle, but the pair distribution function, are well predicted. To our knowledge, it is the first time that a microstructure-based rheological model is successfully compared to such a wide experimental dataset.We propose a new, minimal tensorial model attempting to clearly represent the role of microstructure on the viscosity of noncolloidal suspensions of rigid particles. Qualitatively, this model proves capable of reproducing several of the main rheological trends exhibited by concentrated suspensions: Anisotropic and fore-aft asymmetric microstructure in simple shear and transient relaxation of the microstructure toward its stationary state. The model includes only few constitutive parameters, with clear physical meaning, that can be identified from comparisons with experimental data. Hence, quantitative predictions of the complex transient evolution of apparent viscosity observed after shear reversals are reproduced for a large range of volume fractions. Comparisons with microstructural data show that not only the depletion angle, but the pair distribution function, are well predicted. To our knowledge, it is the first time that a microstructure-based rheological model is successfully compared to such a wid...

International audience; Fluidisation occurring in clay-rich landslides poses serious threats to populations and infrastructures and has been the subject of numerous studies to apprehend its rheological origin. In parallel, noninvasive geophysical techniques on landslides have known considerable development as a means to approach in-situ geotechnical parameters. This study investigates the influence of fluidisation on two geophysical parameters: the shear wave velocity, Vs, and the electrical resistivity, ρ. Both parameters are widely used in landslide monitoring as they are sensitive, respectively, to soil stiffness and water content, two key parameters for material fluidisation. Laboratory tests were carried out on soil samples collected in five flow-like landslides occurring in very different geological conditions. A plate–plate rheometer was used to provoke fluidisation, and Vs was measured during oscillatory tests. The rheometer was redesigned for resistivity measurements, incorporating circular electrodes in polyvinyl chloride plates. Results show that (i) all soils exhibit a dramatic drop in Vs at the fluidisation, and (ii) the resistivity does not significantly vary at the solid–fluid transition. These last results are analysed in terms of clay particles arrangement using the electrical laws of Archie and Waxman–Smits, and the impact on landslide geophysical monitoring is discussed.

Rapid and large deformations of snow are mainly controlled by grain rearrangements, which occur through the failure of cohesive bonds and the creation of new contacts. We exploit a granular description of snow to develop a discrete element model based on the full three-dimensional microstructure captured by microtomography. The model assumes that snow is composed of rigid grains interacting through localized contacts accounting for cohesion and friction. The geometry of the grains and of the intergranular bonding system are explicitly defined from microtomographic data using geometrical criteria based on curvature and contiguity. Single grains are represented as rigid clumps of spheres. The model is applied to different snow samples subjected to confined compression tests. A detailed sensitivity analysis shows that artifacts introduced by the modeling approach and the influence of numerical parameters are limited compared to variations due to the geometry of the microstructure. The model shows that the compression behavior of snow is mainly controlled by the density of the samples, but that deviations from a pure density parameterization are not insignificant during the first phase of deformation. In particular, the model correctly predicts that, for a given density, faceted crystals are less resistant to compression than rounded grains or decomposed snow. For larger compression strains, no clear differences between snow types are observed.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; This paper presents an experimental characterization of the flow of an elasto-viscoplastic fluid (Carbopol) over a dead zone constituted of the same material. The studied configuration consists of a closed rectangular channel with a cavity in its base. A curved solid-fluid interface forms into the cavity, separating a yielded flowing layer above from an unyielded dead zone below. The hydrodynamics of the flow is investigated by means of high-resolution optical velocimetry (PIV). We focus in particular on the velocity profiles and shear-rate evolution in a quasi-longitudinal flow domain located around cavity mid-length. Our measurements show a non-monotonous evolution of the shear rate, which increases from zero at the solid-liquid interface, passes through a peak (sometimes leveling off at its maximum value), and returns to zero in a plug zone sufficiently far above the cavity. Two main flow zones can be distinguished: a Poiseuille zone, in which velocity profiles are the same as in the flow over a rigid wall, and a boundary layer ensuring the transition with the dead zone. Hence, consistently with our previous work (Luu et al., Phys. Rev. E, 2015), the flow self-organizes to partially smooth out the bottom perturbation. The characteristic thicknesses of the flow zones are shown to evolve with cavity length and hydrodynamic properties of the incoming flow. The study also points out the influence of elastic effects, notably on the shape (asymmetry) of the solid-fluid interface.

International audience; Objectives:Parathyroid hormone (PTH) is a risk marker for hypoparathyroidism (hypoPTH). This study aimed to determine the predictive values of early PTH assays carried out at the moment of skin closure (PTH SC), to establish a treatment algorithm, identifying two threshold values. We assessed the reproducibility of this approach with two different immunoassay kits (hypoPTH) after total thyroidectomy, but its practical application is not consensual.Study Design:We conducted a prospective descriptive study, including all patients who underwent a total thyroidectomy between March 2012 and November 2013. Postoperative PTH SC levels, corrected calcium on postoperative days, and occurrence of hypoPTH symptoms were collected.Results:Of 257 patients, the rate of hypoPTH was 20%. Threshold values to obtain a 100% positive predictive value to identify patients for whom hypoPTH was absolutely certain were: PTH SC

International audience; This paper reports on high-resolution measurements of the velocity field within the front of free-surface viscoplastic surges in an inclined channel. Free-surface shape and position of the unyielded plug layer inside the surges are also monitored. The experimental results are compared to the predictions of a thin-layer model based on lubrication approximation. Despite its simplicity, the model appears to provide an accurate description of the flow in most of the surge. As predicted, a 2-layer flow structure is clearly observed, with an upper plug layer progressively thinning towards the front. It is only in the tip of the flow, close to the contact line, that discrepancies with the model are observed. The plug layer completely disappears, and velocity profiles become sheared across the whole flow depth. The experimental free-surface shape also deviates from its theoretical counterpart. The distance from the contact line at which lubrication model fails, has been quantified for each experiment, and varies between 1 and 2 times the maximum height of the flow. In the future, this experimental dataset shall provide a unique benchmark to assess the predictive capabilities of more sophisticated flow models.

International audience; Weak layers of snow are thin layers of low density and cohesion that consist of a complex network of sintered ice grains. These layers are very fragile and their collapse is considered to be the primary cause of dry slab snow avalanche release. This study reports on an attempt at modelling the mechanical response of these weak snow layers with discrete element (DEM) simulations, using x-ray tomographical images of real snow samples as input data on the microstructure of the material. An original method for representing irregular grain shapes in the DEM is first introduced. The method is based on utilizing the medial axis concept to represent an arbitrary grain shape with an optimal set of overlapping spheres. A thorough study of the effect of the grain approximating technique parameters on the key geometrical features of the grains is then carried out. Finally, the functionality of the model is demonstrated by performing an oedometric test on an image of a real snow sample.

Natural sintering in ice is a fundamental process determining mechanical properties of various ice forms. According to the literature, limited data are available about the complex subjects of snow sintering and bond formation. Here, through cold laboratory mechanical tests with a new shear apparatus we demonstrate time-dependent effects of isothermal sintering on interface strengthening at various normal pressures. Measurements showed that interfacial strength evolved rapidly, conforming to a power law (mean exponent &approx;0.21); higher pressure corresponded to higher initial strength and sintering rates. Our findings are consistent with observations on homogeneous snow, provide unique records essential for slope stability models and indicate the significant importance of normal load on data interpretation.

International audience; Rapid deformations in snow are mainly con- trolled by particle rearrangements and contact interactions. To study this deformation regime, the description of the snow microstructure in terms of grains, which could eventually be handled by discrete element models, is relevant. In practice, microtomography has become a standard method to image the three-dimensional distribution of ice and pores, as a set of binary voxels. Here, we propose a new method to directly identify individual snow grains defined as zones separated by regions of potential mechanical weakness, in the microtomo- graphic images. In general, these grains are not well sepa- rated but rather sintered together. Our new method, based on local geometrical criteria, is shown to detect contacts directly inferred from an explicit numerical mechanical experiment. The developed algorithm is tested on snow but is generic and applicable to various geomaterials with a granular-like microstructure.

Objectives/Hypothesis To determine the feasibility and the preliminary oncological results of transoral robotic surgery (TORS) for the treatment of early stage laryngeal tumors. Study Design Retrospective single center study. Methods We reviewed the medical records of 23 patients who underwent TORS for the treatment of T1 or T2 laryngeal squamous cell carcinoma between August 2009 and March 2012. Results Laryngeal tumors were involving the glottis (13 cases) and the supraglottis (10 cases). They were classified T1 N0 in 16 cases, T2 N0 in four cases, and T2 N+ in three cases. The median TORS operative time was 60 minutes (ranging from 30 to 118 minutes). The median hospitalization time was 7.5 days. Histopathological examination of the resected tumors confirmed 14 cases in which the margins were clear, four cases in which the surgical margins were close (less than 1 mm), one case in which the margins were microscopically positive, and in four cases margins status were not able to be recorded. A tracheostomy was carried out for three patients, and 11 patients got a nasogastric feeding tube postoperatively. The local recurrence rate was 8.7% (2/ 23 cases), and in both cases the recurrence occurred in the anterior commissure area only. The overall larynx preservation rate was 95,7% (22/ 23 cases). Conclusion In this preliminary study, we demonstrated that laryngeal TORS is feasible and may result in oncological results comparable with other treatment strategies, including laser CO2 surgery. This innovative approach needs to be evaluated through randomized multi-institutional trial. Level of Evidence 4. Laryngoscope, 123:2485–2490, 2013