Granular materials, ubiquitous in geophysics, chemical engineering, biophysics and soft matter physics, present unique challenges for optical imaging due to their opaque and heterogeneous nature. This perspective paper provides a comprehensive overview of opportunities in Light Sheet Microscopy (LSM) techniques for imaging granular materials, emphasizing refractive index matching as a critical tool for visualizing internal deformations and flow dynamics. By matching the refractive indices of solid particles and surrounding fluids, researchers can create “transparent soils” or analogous systems, enabling detailed examination of particle interactions, strain fields, and fluid flow. To comprehensively introduce opportunities for future research, the review explores the evolution of LSM, from early broadband light sources to modern laser and LED-based systems, highlighting advancements in wavefront shaping and contrast generation through scattering and fluorescence. The paper also surveys some innovative approaches for LSM and index matching, such as Sephadex spheres and cryolite. Emerging techniques, such as wavefront shaping and three dimensional imaging with event cameras, are presented as promising avenues for future research. Additionally, the review connects granular imaging to biological systems, demonstrating its relevance for studying microbial motility, biofilm growth, and tissue engineering. This perspective paper aims to guide and inspire researchers in selecting and refining LSM techniques for advancing the understanding of complex particulate systems.

This study presents the results from a series of centrifuge tests conducted in a novel permeameter capable of measuring both global and layer permeability. Six tests were undertaken at gravitational accelerations ranging from 10 to 40g. Using a parametric modeling approach, the research examined the influence of effective stress gradient on the onset and progression of suffusion in an underfilled, gap-graded soil. At a specimen scale, increased effective stress gradient appeared to increase suffusion susceptibility in identically prepared models. However, analysis of the permeability response of individual layers indicated that initial microscale variability and hydraulic loading history had more impact on the suffusion behavior than did effective stress. At the layer scale, suffusion and clogging events occurred from the onset of seepage flow, and there was no observed influence of effective stress level on suffusion onset. The study found that the definition of suffusion onset is highly dependent on experimental design, and critically evaluated the concept of threshold hydromechanical conditions for suffusion onset.

Waste foundry sands are a byproduct of cast-metal processes. There is minimal geotechnical characterisation of these artificial soils in the literature, and as such their potential applications in construction have not been extensively explored. Due to the presence of coal and bentonite, their unsaturated properties, hydraulic behaviour, and mechanical response can be complex, and need to be better understood before assessing the feasibility for geotechnical applications. This study provides hydromechanical characterisation of a waste foundry sand in terms of soil-water retention, swelling, compression, and shear behaviour.

Debris flows are extremely fast landslides whose complex dynamics are still not fully understood, primarily due to challenges in acquiring direct field measurements. In modern monitoring stations, cameras represent cost-effective data sources, providing essential information for characterising the documented events.Digital Particle Image Velocimetry (DPIV) algorithms have been extensively employed in the literature to reconstruct velocity fields in laboratory physical models under controlled conditions. However, the resolution of field camera footage is typically suboptimal due to weather and lighting conditions, as well as non-zenithal recording geometry, hindering a straightforward application of DPIV. Landslide flume experiments, conducted in collaboration with the Civil and Structural Engineering Department of the University of Sheffield and the British Geological Survey office in Keyworth, revealed that also suboptimal quality footage can be effectively utilised provided appropriate orthorectification algorithms are applied to eliminate the original image distortions.In this study, the methodology established through the laboratory flume experiments was applied to analyse a real debris flow event in an active catchment in the Camonica Valley (Lombardia, Italian Alps) between the municipalities of Ono San Pietro and Cerveno. The Blè Stream catchment, with a drainage area of approximately 3.5 km², a maximum elevation of 2,527 m a.s.l., and a main channel length of about 2.9 km, experienced a debris flow event on October 22, 2022. This was documented by several monitoring stations equipped with cameras and a flow-depth radar sensor along the main channel track.The frame-by-frame orthorectified surface velocity field of the recorded debris flow was obtained through a DPIV analysis, employing two open-source tools in Matlab sequentially: PIVlab (Thielicke & Stamhuis 2014) and RIVeR (Patalano et al. 2017). The discharge at a specific instant along a reference section was computed as the product of the reconstructed flow velocity distribution and the area of the section defined by its topography, known from pre- and post-event LiDAR and drone surveys, and the measured flow level. Throughout this phase, careful consideration was given to assessing the primary sources of uncertainty arising from the continuously changing section geometry and the measured surface velocity, which typically overestimates the actual depth-averaged velocity, with a divergence depending on flow rheology. Calculating the discharge for each frame along the reference section ultimately yielded the hydrograph of the documented debris flow event, along with an estimate of the involved volume of material. References:Patalano A, García C, Rodriguez A, 2017. Rectification of Image Velocity Results (RIVeR): A simple and user-friendly toolbox for large scale water surface Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). Computers and Geosciences. 109. 323-330. 10.1016/j.cageo.2017.07.009.Thielicke W, Stamhuis EJ, 2014. PIVlab – towards user-friendly, affordable and accurate digital Particle Image Velocimetry in MATLAB. J. Open Res. Softw. 2 http://dx.doi.org/10.5334/jors.bl.

Grey Cast Iron (GCI) water pipes are subjected to multiaxial, cyclic stresses caused by combinations of loads such as internal water pressure and road vehicle weight. However, the multiaxial fatigue performance of this material has not previously been characterised. To address this gap more than 45 fatigue tests, including some under nonproportional tension-torsion loading, were completed using a GCI material very similar to water pipe GCI. Of the four multiaxial fatigue criteria tested, the Smith-Watson-Topper (SWT) criterion provided the best predictions by a narrow margin, supporting the idea that a tensile cracking mode dominates the fatigue life of GCI.

Field observations of debris flows often show that a deep dry granular front is followed by a progressively thinner and increasingly watery tail. These features have been captured in recent laboratory flume experiments (Taylor-Noonan et al. , J. Geophys. Res.: Earth Surf. , vol. 127, 2022, e2022JF006622). In these experiments different initial release volumes were used to investigate the dynamics of an undersaturated monodisperse grain–water mixture as it flowed downslope onto a horizontal run-out pad. Corresponding dry granular flows, with the same particle release volumes, were also studied to show the effect of the interstitial fluid. The inclusion of water makes debris flows much more mobile than equivalent volumes of dry grains. In the wet flows, the formation of a dry front is crucially dependent on the heterogeneous vertical structure of the flow and the velocity shear. These effects are included in the depth-averaged theory of Meng et al. ( J. Fluid Mech. , vol. 943, 2022, A19), which is used in this paper to quantitatively simulate both the wet and dry experimental flows using a high-resolution shock-capturing scheme. The results show that velocity shear causes dry grains (located near the free surface) to migrate forwards to create a dry front. The front is more resistant to motion than the more watery material behind, which reduces the overall computed run-out distance compared with debris-flow models that assume plug flow and develop only small dry snouts. Velocity shear also implies that there is a net transport of water to the back of the flow. This creates a thin oversaturated tail that is unstable to roll waves in agreement with experimental observations.

Landslides comprised of a wide range of particle sizes (e.g. debris flows) exhibit flow structures arising from particle size segregation. Segregation influences the mobility of the flow, the development of debris fans, and the resulting impact forces to be expected when designing barriers and containment structures. In order to capture the flow dynamics of segregable materials in numerical simulations, experimental datasets quantifying segregation in the final deposit are required. However, the measurement of segregation is not a straightforward task as segregation observed at an external transparent boundary may not be indicative of segregation within the bulk of the landslide mass due to sidewall friction. In this paper, we explore the use of four different strategies to optically measure particle size segregation in large landslide flume tests, comparing measurements taken (i) at the external transparent flume boundary; (ii) using a thin transparent plane as a splitter plate along the centre of the flow; and using a (iii) vertically or (iv) horizontally inserted transparent plate into the static deposit after flow arrest. Relationships between concentrations measured by projected area (i.e. sidewall image) to concentrations by mass are derived and validated for a tridisperse mixture to assess which sampling method most closely represented the original source volume. Of the four strategies tested, the transparent splitter plane method was identified to cause the least amount of out-of-plane segregation of particles, provides a rich database of highly detailed observations of segregation of tridisperse granular flows that can be used to evaluate future numerical model outcomes, and is recommended for future laboratory flume investigations.

Fatigue cracking is thought to be a critical failure mode for Grey Cast Iron (GCI) water pipes; however, this failure mechanism is poorly understood. Using a novel approach to sourcing GCI pipe material for testing, the variation in fatigue strength between pipes from the same batch was experimentally quantified. These results were used to assess the impact of fatigue strength variation on a hypothetical but realistic GCI water pipe years-to-failure scenario. Full-pipe sections were used during fatigue testing and the observed failure mode gave physical meaning to the years-to-fatigue-failure predictions for the first time. The accumulation of fatigue damage was predicted to represent a terminal and very small part of a GCI pipe's life, so the loads applied to a GCI pipe early in its life are likely to have a limited impact on its remaining life and so do not need to be included in predictive modelling. For the scenario considered here, the predicted variation in lifetime was an 8.6-year range about an average of 59.4 years. GCI pipes in the UK are all > 50 years old, so this is a significant variation for asset and investment planners to account for as these pipes approach the end of their lives.

Controlled laboratory-based physical evidence is presented, showing how the type of fatigue loading impacts the remaining life and the failure mode of corroded GCI water pipes. Leak-before-burst behavior is shown for pipes experiencing internal water pressure fatigue loading but not for four-point bending fatigue. Sharp pits are shown to reduce fatigue strength by up to 5.4 times, with the degree of reduction dependent on alignment. Condition assessments of corroded GCI water pipes must consider both the three-dimensional shape of the corrosion pitting and the loading experienced by the pipe to give a true assessment of the damage caused by a corrosion pit.

Pore fluid plays a crucial role in many granular flows, especially those in geophysical settings. However, the transition in behaviour between dry flows and fully saturated flows and the underlying physics that relate to this are poorly understood. In this paper, we report the results of small-scale flume experiments using monodisperse granular particles with varying water content and volume in which the basal pore pressure, total pressure, flow height and velocity profile were measured at a section. We compare the results with theoretical profiles for granular flow and with flow regimes based on dimensional analysis. The runout and the centre of mass were also calculated from the deposit surface profiles. As the initial water content by mass was increased from zero to around 10%, we first observed a drop in mobility by approximately 50%, as surface tension caused cohesive behaviour due to matric suction. As the water content was further increased up to 45%, the mobility also increased dramatically, with increased flow velocity up to 50%, increased runout distance up to 240% and reduced travel angle by up to 10° compared to the dry case. These effects can be directly related to the basal pore pressure, with both negative pressures and positive pore pressures being measured relative to atmospheric during the unsteady flow. We find that the initial flow volume plays a role in the development of relative pore pressure, such that, at a fixed relative water content, larger flows exhibit greater positive pore pressures, greater velocities and greater relative runout distances. This aligns with many other granular experiments and field observations. Our findings suggest that the fundamental role of the pore fluid is to reduce frictional contact forces between grains thus increasing flow velocity and bulk mobility. While this can occur by the development of excess pore pressure, it can also occur where the positive pore pressure is not in excess of hydrostatic, as shown here, since buoyancy and lubrication alone will reduce frictional forces. Graphical abstract

Ripon is a town affected by frequent collapse sinkholes that occur due to the rapid dissolution of the underlying gypsum. This gypsum is interbedded and mixed with low solubility but easily water weakened calcareous marl. Construction sites underlain by cavities can be remediated, but if even small flow paths remain, new cavities can appear in close proximity. Dissolution rates previously determined for gypsum have either been on high purity specimens or do not consider the insoluble impurities. It is therefore important to understand the role of interbedded calcareous marls in controlling cavity distribution and growth. A method is proposed to evaluate the effect of marl impurity on gypsum dissolution rates in this area.For the dissolution test, water is circulated through a hole drilled in a gypsum specimen from Ripon. As the gypsum dissolves, the marl could detach and settle, become suspended or also dissolve. If it remains attached, however, it could impede further gypsum dissolution. Conductivity, total dissolved solids (TDS) and pH of the water are monitored and the test continues until the conductivity has stabilised. This indicates that the water is saturated with gypsum and dissolution has ceased. The water is then evaporated to recover suspended solids, which are put through particle size distribution (PSD) sieves. The post-test specimen mass is added to the recovered solid mass and compared to the pre-test mass.After testing, mass loss is estimated from both conductivity and TDS curves, and these are compared to measured mass loss. Changes in pH are taken to indicate dissolution of calcareous components in the marl. The conductivity curve is used to find the dissolution rate constant of the specimen, and its cross-section is visually inspected to check the dissolution pattern. The PSD is used to study transport and deposition of insoluble material. Results are combined to assess the influence of marl on gypsum dissolution and sinkhole development, which can be applied both in Ripon and elsewhere.

Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Three-dimensional pore topology is extracted from a series of two-dimensional images and imported into computational fluid dynamics (CFD) simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV- and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterise the variance of flow velocity with the three-dimensional pore topology. Finally, the fluid–particle interactions obtained from CFD results show a larger variance in the angular particle packings.

We present the combined efforts of a research network designed to address the many challenges in the experimental modelling of debris flow phenomena. The approach has been to use apparatuses of different functional arrangement and at different scale with identical and commonly sourced flow materials from the highly idealised (dry, coarse and uniform) to the highly complex (well graded, segregating, fluid saturated). Here we briefly present some key findings of the network and point to the research questions that are currently being addressed. This complementary view of experimental debris flows helps to constrain methodological artefacts/scale effects and to identify key processes responsible for the diverse appearance and often high mobility of debris flows.

<p>Particle size segregation is a phenomenon that generates preferential sorting of particles, based on size, in material flows of non-uniform size distribution. Landslide hazards, such as debris flows, involve materials of non-uniform particle sizes and therefore generate flow structures which arise from particle size segregation. The mobility, distal reach and impact forces associated with these natural hazards are influenced by these processes. Understanding the mechanisms of this phenomenon is essential for acquiring accurate input parameters that are needed to model these flows and properly design debris flow barriers and retaining structures. While the dynamics of particle size segregation in flow and deposition have been furthered through studying granular flows, studies to date have had several limitations. They primarily examine flows of bidispersed material, are small in scale, and rely on observations from flume sidewalls, precluding the study of dynamics along the centreline of flows. In this study, a large scale 6.8 m long and 2.1 m wide slope inclined at 30 degrees was used to generate dry tridispersed granular flows with 0.6 m<sup>3</sup> of material. The tridisperse mixture consisted of even proportions by mass of 3 mm, 6 mm and 12 mm diameter spherical particles. Replicate tests were conducted to observe flow dynamics and assess methods for sampling along the internal plane of the test deposit. Image analysis techniques were developed to quantify particle size distributions within the deposit. Flume sidewall and internal observations were found to differ significantly from each other, in that side wall observations contained significantly higher proportions of the largest particle size. Additional replicate tests were conducted with saturated material to further examine the impact of pore fluid on segregation. This work will allow for future calibration of both numerical and theoretical models of particle size segregation and ultimately enable better debris flow modelling and mitigation practices.</p>

Abstract Tsunamis are a major hazard along many of the world's coastlines. To understand the impact of these events, a sufficiently long record of previous events is needed, which can be provided by their sedimentary deposits. A number of past events have left extensive sedimentary deposits that can be used to understand the hydrodynamics of the tsunami. The ca 8.15 ka Storegga submarine slide was a large, tsunamigenic mass movement off the coast of Norway. The resulting tsunami had estimated run‐up heights of around 10 to 20 m on the Norwegian coast, over 30 m in Shetland and 3 to 6 m on the Scottish mainland coast. New cores were taken from the Ythan Valley in North‐East Scotland, where Storegga tsunami deposits have previously been found. High‐resolution sedimentary analyses of the cores, combined with statistical (changepoint) analysis, shows signatures of multiple waves. Moreover, detailed CT scans of the erosional basal surface reveal sole marks called skim marks. Taken in conjunction with the grain size and sedimentary fabric characteristics of the tsunami deposits, this indicates that the flow exhibited a high‐concentration basal component, with an initial semi‐cohesive phase and that deposition was dominantly capacity driven. A multiple wave hypothesis is tested by creating a high‐resolution numerical model (metre‐scale) of the wave inundation, coupled to a previously published regional model. The inundation model confirms that multiple waves passed over the site in agreement with the sedimentological analysis. The sensitivity of the model to the reconstructed palaeocoastal geomorphology is quantitatively explored. It is concluded that local palaeogeomorphological reconstruction is key to understanding the hydrodynamics of a tsunami wave group in relation to its sedimentary deposit. Combining sedimentological data with high‐resolution inundation modelling is a powerful tool to help interpret the sedimentary record of tsunami events and hence to improve knowledge of their risks.

The transmission of forces within high speed granular flows may be straightforwardly viewed in two-dimensional photoelastic experiments, but precise measurements have remained elusive due to difficulties in differentiating between particles and forces with sufficient accuracy at reasonable processing speeds. This paper presents a novel approach to detect the positions of disks embedded in this complex situation, which is a crucial step in applying the methodologies necessary for the analysis of the photoelastic response of individual disks. We have developed a Deep Learning based solution to perform the segmentation of experimental photoelastic images, disentangling with high fidelity the disk outlines from the rest of each image. The accuracy and the reliability of the proposed methodology are discussed in detail, demonstrating that this approach can be effectively adopted for the problem under investigation, improving the quality of the photoelastic analysis and dramatically accelerating the data processing procedure.

Reducing and preventing leakage is a priority for water distribution network managers in many countries, including the UK. Understanding the mechanisms that cause leaks to form, and developing the ability to model these processes, will enable proactive replacement of water pipes before they start to leak. Smaller diameter Grey Cast Iron (GCI) water pipes are understood to experience biaxial, repeating loads, so fatigue cracking may be a cause of leakage for these pipes. To investigate this fatigue cracking mechanism a small-scale biaxial fatigue experiment is under development at The University of Sheffield. A large number of small diameter, un-corroded GCI pipes are needed to serve as specimens for this experiment. Therefore, in this work off-the-shelf BS416-2 DN 50 mm soil pipes are explored as an alternative to using exhumed pipes, which are often corroded and ≥76.2 mm diameter. The graphite microstructure and tensile stress-strain behaviour of a BS416-2 pipe were characterised and compared with literature data for exhumed spun-cast GCI water pipes, and a good agreement was found. This work concluded that BS 416-2 soil pipes can be used to represent spun-cast GCI water pipes in small-scale destructive experiments.

The presence of a pore fluid is recognized to significantly increase the mobility of saturated over dry granular flows. However, the mechanisms through which pore fluid increases mobility may not be captured in experimental flows of small volume typical of laboratory conditions. Here we present the results of dry and initially fluid saturated or “wet” experimental flows of near‐monodisperse coarse‐grained ceramic particles in a large laboratory flume for five source volumes of 0.2–1.0 m 3 . Measurements include flow height, velocity profile, pore pressure, and evolving solid volume fraction, as well as the final deposit shape. The dry experiments constrain the frictional properties of the common granular material and comparison with wet flows permits an independent evaluation of the interstitial fluid effects. These results demonstrate that flow dilation and strong variation in the velocity profile are directly linked to a greatly increased mobility for wet granular flows compared to dry, and a significant influence of scale as controlled by source volume on flow behavior. Excess pore pressure need not be present for these effects to occur.

Small landslides make up the vast majority of landslide incidents that affect communities and individuals globally. However, this reality is not well reflected in the landslide literature, which tends to focus on larger, high-individual-impact events. Globally, the large number of small to medium landslides that occur each year in hill slope and mountainous areas results in large economic, social, and environmental costs. These costs are mostly associated with human habitation and transport linkages. The decision whether to actively manage small landslides—either by event reduction using slope stabilization methods or by mitigation of their impacts—is a function of cost-benefit analyses that are undertaken either explicitly or implicitly. Although the direct costs and benefits are generally assessed to inform a decision about whether to mitigate the effects of a potential or actual landslide, relatively few analyses attempt to quantitatively determine the wider (economic, social, and political) implications of slope failure on communities, although evidence suggests that doing so would probably result in a greater number of interventions. This chapter considers a range of case studies from different countries that include larger and smaller landslide scenarios in order to examine and illustrate current management practice and likely future trends.

The granular temperature is an index of the level of collisional activity in a granular flow, and increasingly important in the verification of extended kinetic theories. The granular temperature is related to the square of the difference between a particle’s velocity and that of the group mean. Image analysis of high-speed video is the most common method to measure granular temperature in experimental flows and depends on correlation of a search mask or a portion of the original image to the next image frame to determine the particle’s movement. This invariably involves some level of estimation of the location at a resolution finer than the pixels that make up the image. However, errors in determining particle movement at the subpixel level can be shown to have a significant impact on granular temperature identification. We show that taking particle movement to be a chain of displacement vectors provides context to the apparent impulses on the particle. Here we propose two novel methods for determining the granular temperature of experimental flows, namely a novel method of initializing Particle Image Velocimetry (PIV) for granular systems where each search subset is centred on a previously determined particle location to reduce bias, and a method of filtering the apparent impulses on a particle on a frequency basis. We term these methods Guided-PIV and Impulse Frequency Filtering (IFF), respectively. In a verification exercise using synthetically generated images, we show Guided-PIV to produce substantially more accurate results than ordinary applications of PIV. The IFF method is shown to greatly reduce the influence of analyzed framerate on granular temperature results. Our results demonstrate practical improvements for granular temperature identification from image analysis, throughout a range of experimental image quality levels, and we anticipate that these improvements will enable experimental assessment towards verification of theorized models of collisional-frictional granular flows.