Sandbox Experiments Research Articles

Non-pumping reactive wells filled with mixing nano and micro zero-valent iron for nitrate removal from groundwater: Vertical, horizontal, and slanted wells

Non-pumping reactive wells (NPRWs) filled by zero-valent iron (ZVI) can be utilized for the remediation of groundwater contamination of deep aquifers. The efficiency of NPRWs mainly depends on the hydraulic contact time (HCT) of the pollutant with the reactive materials, the extent of the well capture zone (Wcz), and the relative hydraulic conductivity of aquifer and reactive material (Kr). We investigated nitrate removal from groundwater using NPRWs filled by ZVI (in nano and micro scales) and examined the effect of NPRWs orientations (i.e. vertical, slanted, and horizontal) on HCT and Wcz. The dependence of HCT on Wcz for different Kr values was derived theoretically for a homogeneous and isotropic aquifer, and verified using particle tracking simulations performed using the semi-analytical particle tracking and pathlines model (PMPATH). Nine batch experiments were then performed to investigate the impact of mixed nano-ZVI, NZVI (0 to 2 g l−1) and micro-ZVI, MZVI (0 to 4 g l−1) on the nitrate removal rate (with initial NO3−=132 mg l−1). The NPRWs system was tested in a bench-scale sand medium (60 cm length × 40 cm width × 25 cm height) for three orientations of NPRWs (vertical, horizontal, and slanted with inclination angle of 45°). A mixture of nano/micro ZVI, was used, applying constant conditions of pore water velocity (0.024 mm s−1) and initial nitrate concentration (128 mg l−1) for five pore volumes. The results of the batch tests showed that mixing nano and micro Fe0 outperforms these individual materials in nitrate removal rates. The final products of nitrate degradation in both batch and bench-scale experiments were NO2−, NH4+, and N2(gas). The results of sand-box experiments indicated that the slanted NPRWs have a higher nitrate reduction rate (57%) in comparison with vertical (38%) and horizontal (41%) configurations. The results also demonstrated that three factors have pivotal roles in expected HCT and Wcz, namely the contrast between the hydraulic conductivity of aquifer and reactive materials within the wells, the mass of Fe0 in the NPRWs, and the orientation of NPRWs adopted. A trade-off between these factors should be considered to increase the efficiency of remediation using the NPRWs system.

Water flux characterization through hydraulic head and temperature data assimilation: Numerical modeling and sandbox experiments

Spatial distribution of groundwater recharge/discharge fluxes has an important impact on mass and energy exchanges in shallow streambeds. During the last two decades, extensive studies have been devoted to the quantification of one-dimensional (1-D) vertical exchange fluxes. Nevertheless, few studies were conducted to characterize two-dimensional (2-D) heterogeneous flux fields that commonly exist in real-world cases. In this study, we used an iterative ensemble smoother (IES) to quantify the spatial distribution of 2-D exchange fluxes by assimilating hydraulic head and temperature measurements. Four assimilation scenarios corresponding to different potential field applications were tested. In the first three scenarios, the heterogeneous hydraulic conductivity fields were first inferred from hydraulic head and/or temperature measurements, and then the flux fields were derived through Darcy’s law using the estimated conductivity fields. In the fourth scenario, the flux fields were estimated directly from the temperature measurements, which is more efficient and especially suitable for the situation that a complete knowledge of flow boundary conditions is unavailable. We concluded that, the best estimation could be achieved through jointly assimilating hydraulic head and temperature measurements, and temperature data were superior to the head data when they were used independently. Overall, the IES method provided more robust and accurate vertical flux estimations than those given by the widely used analytical solution-based methods. Furthermore, IES gave reasonable uncertainty estimations, which were unavailable in traditional methods. Since temperature can be accurately monitored with high spatial and temporal resolutions, the coupling of heat tracing techniques and IES provides promising potential in quantifying complex exchange fluxes under field conditions.

Growing Faults in the Lab: Insights Into the Scale Dependence of the Fault Zone Evolution Process

Analog sandbox experiments are a widely used method to investigate tectonic processes that cannot be resolved from natural data alone, such as strain localization and the formation of fault zones. Despite this, it is still unclear to which extent the dynamics of strain localization and fault zone formation seen in sandbox experiments can be extrapolated to a natural prototype. Of paramount importance for dynamic similarity is the proper scaling of the work required to create the fault system, Wprop. Using analog sandbox experiments of strike‐slip deformation, we show Wprop to scale approximately with the square of the fault system length, l, which is consistent with the theory of fault growth in nature. Through quantitative measurements of both Wprop and strain distribution we are able to show that Wprop is mainly spent on diffuse deformation prior to localization, which we therefore regard as analogous to distributed deformation on small‐scale faults below seismic resolution in natural fault networks. Finally, we compare our data to estimates of the work consumed by natural fault zones to verify that analog sandbox experiments scale properly with respect to energy, that is, that they scale truly dynamically.

Estimation of borehole heat exchanger thermal properties

An important issue in the design of installations with borehole heat exchangers (BHE) is knowledge of the thermo-physical properties of the grouting material and soil. The objectives of this work is to estimate the thermal conductivity and specific heat of the grouting material and the soil. The estimated values were determined on the basis of two numerical heat transport models in the borehole and using the results of the small sandbox experiment. The design of experiment technique (DOE) and the response surface methodology (RSM) were used to achieve the aim of this work. Numerical calculations were carried out with the use of the new finite element with multiple degrees of freedom (MDF) and a quasi-3D model in ANSYS package. The estimation requires minimization of four output parameters (quality criteria), and therefore it is a multiple-objectives optimization problem. Based on the DOE, RSM and multiple-objective optimization the material properties of BHE and soil was determined. For both models the discrepancies Y1P1 are below 4 %, Y2P1 are below 11.5%, Y1P2 are below 6.5 % and Y2P2 are below 11.5 %. The average discrepancies below 5 % based on the verification measurements with different operating parameters was obtained. It is noteworthy that for the two different models the same values of the estimated parameters were obtained.

Influence of grout thermal properties on heat-transfer performance of ground source heat exchangers

Grout plays an important role in the heat-transfer performance of borehole heat exchangers. The current study developed a numerical model to investigate the effect of grout thermal conductivity and heat capacity on the heat-transfer performance of borehole heat exchangers. The “relative thermal conductivity” was defined to obtain the best matching relationship of thermal conductivity between grout and soil. The numerical model was validated through sandbox experiments. The results showed that, when the grout thermal conductivity and heat capacity were both varied by 47.00%, the effect of the grout thermal conductivity on the pipe wall temperature was 26.44% stronger than that of the grout heat capacity; whereas the effect of the grout heat capacity on the heat-transfer radius was 5.21% stronger than that of the thermal conductivity. As the same variation of the grout thermal conductivity, when the soil thermal conductivity change from 1.30 to 1.90 W/(m·K) (1.60 kcal/[m2 · h · °C]), the effect of the changing grout thermal conductivity on the heat-transfer performance decreased by 8.04%. The results also showed that the optimal relative thermal conductivity was 1.20–1.30. These results can serve as guidelines for selecting appropriate grouts for different geological conditions.

The effect of salt in dilatant faults on rates and magnitudes of induced seismicity – first results building on the geological setting of the Groningen Rotliegend reservoirs

AbstractThe presence of salt in dilatant normal faults may have a strong influence on fault mechanics in the Groningen field and on the related induced seismicity. At present, little is known of the structure of these fault zones. This study starts with the geological evolution of the Groningen area, where, during tectonic faulting, rock salt may have migrated downwards into dilatant faults. These fault zones therefore may contain inclusions of rock salt. Because of its rate-dependent mechanical properties, the presence of salt in a fault may introduce a loading-rate dependency into fault movement and affect the distribution of magnitudes of seismic events. We present a first-look study showing how these processes can be investigated using a combination of analogue and numerical modelling. Full scaling of the models and quantification of implications for induced seismicity in Groningen require further, more detailed studies: an understanding of fault zone structure in the Groningen field is required for improved predictions of induced seismicity. The analogue experiments are based on a simplified stratigraphy of the Groningen area, where it is generally thought that most of the Rotliegend faulting has taken place in the Jurassic, after deposition of the Zechstein. This suggests that, at the time of faulting, the sulphates were already transformed into brittle anhydrite. If these layers were sufficiently brittle to fault in a dilatant fashion, rock salt was able to flow downwards into the dilatant fractures. To test this hypothesis, we use sandbox experiments where we combine cohesive powder as analogue for brittle anhydrites and carbonates with viscous salt analogues to explore the developing fault geometry and the resulting distribution of salt in the faults. Using the observations from analogue models as input, numerical models investigate the stick-slip behaviour of fault zones containing ductile material qualitatively with the discrete element method (DEM). Results show that the DEM approach is suitable for modelling the seismicity of faults containing salt. The stick-slip motion of the fault becomes dependent on shear loading rate with a modification of the frequency–magnitude distribution of the generated seismic events.

Sandbox rheometry: Co-evolution of stress and strain in Riedel– and Critical Wedge–experiments

Analogue sandbox experiments have been used for a long time to understand tectonic processes, because they facilitate detailed measurements of deformation at a spatio-temporal resolution unachievable from natural data. Despite this long history, force measurements to further characterise the mechanical evolution in analogue sandbox experiments have only emerged recently. Combined continuous measurements of forces and deformation in such experiments, an approach here referred to as “sandbox rheometry”, are a new tool that may help to better understand work budgets and force balances for tectonic systems and to derive constitutive laws for regional scale deformation.In this article we present an experimental device that facilitates precise measurements of boundary forces and surface deformation at high temporal and spatial resolution. We demonstrate its capabilities in two classical experiments: one of strike-slip deformation (the Riedel set-up) and one of compressional accretionary deformation (the Critical Wedge set-up). In these we are able to directly observe a correlation between strain weakening and strain localisation that had previously only been inferred, namely the coincidence of the maximum localisation rate with the onset of weakening. Additionally, we observe in the compressional experiment a hysteresis of localisation with respect to the mechanical evolution that reflects the internal structural complexity of an accretionary wedge.

Analogue modeling of through-going process and development pattern of strike-slip fault zone

Nine sets of sand-box experiments were designed according to actual geologic data to investigate the evolution pattern and development mechanism of simple shear strike-slip fault zone using the fault CT scanning technique. The experimental results show that R (Riedel) shears were developed early and more in number, and one set of these faults intersected with the principal displacement zone (PDZ) at relatively lower angle; the P shears (being in symmetrical distribution with the R shears) and Y shears (subparallel to PDZ) were developed later than the R shears, and the fault zone was through-going only after the formation of Y shears. The through-going process of strike-slip fault zone can be divided into four stages: embryonic stage, R (Riedel) shears development stage, P shears and Y shears development stage, and through-going stage of fault zone. In the process, the faults developed progressively from the basement to the top cover, the faults spread upward in the profile at embryonic stage, and then converged toward the PDZ on the plane at R shears development stage. The modeling supports that the en echelon structure developed in “helicoidally drag” pattern, however, the length of the en echelon fault grew linearly doubling the growth rate of its depth, and the fault intersection angle with the PDZ decreased in quadratic function with the increase of its depth. The analysis reveals that cover thickness and the maximum principal stress direction are the main factors causing the difference in “helicoidally drag” structure. The modeling provides guidance for the strike-slip fault interpretation and evolution study, and layering and segmenting faults in the marine carbonates of the Tarim Basin.

Self‐potential for monitoring soil remediation by smouldering: a proof of concept

ABSTRACTNear‐surface soils contaminated with non‐aqueous phase liquids, such as coal tar, crude oil, and chlorinated solvents, remain a serious problem. Smouldering remediation is a technique now being applied in the field for in situ destruction of non‐aqueous phase liquids. Based on a self‐sustaining exothermic reaction, smouldering remediation generates a hot region (>400 °C) that propagates through the subsurface. Self‐potential is here considered for the first time as a non‐destructive means for monitoring the smouldering remediation process. First, a series of sandbox experiments were conducted to investigate the magnitude of the thermoelectric coupling coefficient () for different sand sizes, water contents, and heat sources. Measured values ranged from ‐0.47 mV/°C for coarse, water‐saturated sand to ‐0.05 mV/°C for fine sand with a saturation of 30%. Next, self‐potential measurements were conducted during several laboratory smouldering remediation experiments, examining the response as a function of both space and time. A significant self‐potential anomaly was observed on the surface during the smouldering period. Moreover, the magnitude of the self‐potential anomaly was demonstrated to be highly correlated to the separation distance between the (moving) reaction front and the (stationary) self‐potential electrode positions. Overall, this research suggests that the self‐potential method has a significant promise as a non‐invasive monitoring tool for in situ smouldering remediation of contaminated sites.

RETRACTED: Experimental and numerical study for seawater intrusion remediation in heterogeneous coastal aquifer

The contamination of fresh groundwater by saltwater intrusion (SWI) becomes a worldwide alarming problem, which threatens all countries depending on groundwater abstraction from coastal areas. Various control and treatment strategies have been suggested to prevent SWI. The construction of subsurface physical barriers is one of the most practical implementation methods to prevent SWI. In this work, the use of subsurface dam as a remediation and protection tool was investigated in a heterogeneous aquifer via lab scale experiments and numerical simulation. The experiments depended on a novel automated imaged analysis method for SWI measurements. Glass beads of different grain sizes were used in sandbox experiments. The simulation works adopted the SEAWAT code for validation of the experimental results and making numerical sensitivity analyses for affecting parameters. Results proved the significant impact of using sub water dams with heterogonous aquifers. The remediation impacts of the dam was captured clearly in preventing and backwashing of the existed SWI. The results revealed also that the heterogeneous aquifers with high permeability in the bottom boundary behave closer to the homogenous aquifers in SWI than those having low hydraulic conductivity in the bottom. Sensitivity analyses results showed that the closer dam to seawater boundary led to the quicker and more effective backwashing process. Results exhibited also that the dam height with 50% of the aquifer dam has the ability to hold the seawater so long as the hydraulic gradient is high and dams with 67% of aquifer height prevent the saltwater intrusion completely.

Physical factors affecting the transport and deposition of particles in saturated porous media

AbstractSaturated sand box experiments were conducted to explore the effect of various physical factors on the transport and deposition of suspended particles in porous media. Red quartz powder and natural quartz sand were employed in the study and acted as suspended particles and porous media, respectively. Particles were injected into the sand box in two modes, i.e., pulse injection and continuous injection. Tests were performed at various particle concentrations, flow velocities, deposition rate coefficient and longitudinal dispersion coefficient by both injection modes. The breakthrough curves were described with the analytical solution of a convection–dispersion equation, in which first-order deposition kinetics were taken into account. Different behavior of suspended-particle transport and deposition in porous media was observed under different injection modes and experimental conditions. The results show that effluent concentration was approximately linear with the initial particle concentration. The deposition rate coefficient depends strongly on particle size and flow velocity, and the transport and deposition process was very sensitive to it. Furthermore, the longitudinal dispersion coefficient increases with increasing flow rate, and particles are easier to transport through pores as the longitudinal dispersion coefficient increases. This study shows the importance of particle concentration, flow velocity, deposition rate coefficient and longitudinal dispersion coefficient in the transport and deposition process of suspended particles.

Evolution of structure and permeability of normal faults with clay smear: Insights from water‐saturated sandbox models and numerical simulations

AbstractClay smear is difficult to predict for subsurface flow applications and would benefit from an improved understanding of the processes controlling clay smear. We present water‐saturated sandbox experiments with large clay smear surfaces (~500 cm2) that couple cross‐fault fluid flow measurements with structural analysis of excavated clay smears. We compare measured flow data to numerical flow simulations to develop a tool to evaluate the evolving fault structure. Results show diagnostic relationships between fault structures and cross‐fault flow. In experiments with one or two clay layers and a cumulative thickness of 10 mm at 100 mm displacement, normally consolidated clay in a structural domain of graben faulting initially yields hybrid brittle/ductile failure with early breaching of the clay layer and increased cross‐fault flux. This is followed by fault backstepping, formation of clay smears, and reworking of clay fragments within the fault. Early formed holes remain open during the evolution of the faults. Fault zones are segmented by fault lenses, breached relays, and clay smears in which sand and clay mix by deformation. Experiments with two clay layers show that holes rarely form at the same position on the fault plane, producing a layered sand‐clay fault rock with greater flow path tortuosity and lower permeability than in one‐layer experiments. We compare our results with observations of faults in nature and discuss progress toward models with sufficient detail and understanding to allow prediction of flow across evolving faults, first in laboratory models and then in the subsurface.

Induced polarization response of porous media with metallic particles — Part 6: The case of metals and semimetals

We collected spectral induced polarization spectra with clean sand mixed with metallic particles (either silver, graphite, copper, steel, magnetite, or pyrite particles). The initial pore water conductivity was either 1500 or [Formula: see text] depending on the experiments (25°C, NaCl). For each of the 15 experiments, we used a narrow and unimodal grain size distribution for the metallic particles. The resulting polarization spectra display clear polarization peaks in the phase and can be fitted with a Cole-Cole complex conductivity model. In addition to this, the chargeability scales with the volume content of the metallic particles in a way that is consistent with the theory of disseminated metallic particles in a weakly polarizable background. Similarly, the phase scales with the content of the metallic particles in a predictable way. The Cole-Cole relaxation time shows a rough dependence with the mean particle size. The trend between these two parameters can be used to determine an apparent diffusion coefficient for the charge carriers responsible for the polarization. Finally, we conducted a laboratory sandbox experiment in which we put a copper plate in tap water-saturated sand. We use an approach based on self-potential tomography and compactness to invert the secondary source current density from the secondary voltages associated with time-domain induced polarization. With this approach, we localized the copper plate and determined a value for the relaxation time that is consistent with the laboratory core sample experiments.

Numerical Simulation of Forced External Circulation Standing Column Well Based on CFD

In view of the deficiency of numerical simulation research on the underground heat transfer system of the forced external circulation standing column well (FECSCW), based on the existing FECSCW sandbox experimental table, two multi-zone coupled 3D CFD models in different sizes were established. The models were validated by outlet water temperature (OWT) and accumulative heat absorption quantities (AHAQ) of thermal well. The experimental value of OWT agrees well with the simulation value. The relative error of AHAQ in 25 min is 3.28% and 5.56% respectively. The simulation results show that the size of the sandbox will affect the aquifer flow/temperature field. Sandbox size increases from 1.5×1.5×1.0 m3 to 2×2×1.58 m3, while the aquifer streamlines deform from "elliptical" into "butterfly", and the temperature field of top plate of sandbox is unaffected. The backfill gravel zone will induce the flow transfixion, while the width of the seal zone (WSZ) and the distances between pumping and injection screens (DPIS) will affect the rate of inlet water entering the pumping screen. Therefore, in order to avoid the influence of inlet water on boundary conditions. Actual engineering and sandbox experiment should increase the distance between injection screen and upper roof of aquifer, and the distance between thermal well and the walls. Moreover, the WSZ and the DPIS should be especially considered, in order to reduce the flow/thermal transfixion.

Quantitative assessment of electrical resistivity tomography for monitoring DNAPLs migration – Comparison with high-resolution light transmission visualization in laboratory sandbox

Real-time monitoring of dense non-aqueous phase liquids (DNAPLs) migration and distribution is essential for the decision of an effective remediation strategy. Light transmission visualization (LTV) has shown its accuracy and efficiency for measuring DNAPLs saturation and water content in the laboratory, but it cannot be implemented in three dimensional sandbox or field-scale sites. Recently developed electrical resistivity tomography (ERT) has been applied in monitoring the migration and distribution of DNAPLs in bench- and field-scale studies. However, the evaluation of the ability of ERT for monitoring DNAPLs migration by a direct comparison of ERT with high-resolution techniques such as LTV within an experimental system is still lacking. Two sandbox experiments with different permeability conditions are conducted to quantitatively assess the capability of ERT for monitoring the DNAPLs migration. During the injections, LTV method is used to visualize the DNAPLs migration and provide high-resolution saturation data while ERT method is applied to capture the change of resistivity. The results from the comparison between LTV and ERT methods show that ERT is successful in detecting the accumulation and flow bypassing phenomenon around the low-permeability lenses, as well as the penetration through the high-permeability lenses. There is a fair correlation between the resistivity and saturation with overall correlation coefficients above 0.6, except at last stage. However, using classical regularization techniques (based on smoothness), the area of DNAPLs plume determined by ERT is commonly overestimated. Compared to the plume around the low-permeability lenses, the plume around the high-permeability lenses estimated by ERT is more extensive due to larger resistivity contrasts. In addition, ERT measurements indicate that the resistivity increase caused by the low-saturation DNAPLs is not apparent enough, which is likely to be covered up under the changing hydrogeochemical environments in field investigations.

Hamiltonian Monte Carlo algorithm for the characterization of hydraulic conductivity from the heat tracing data

Estimating spatial distributions of the hydraulic conductivity in heterogeneous aquifers has always been an important and challenging task in hydrology. Generally, the hydraulic conductivity field is determined from hydraulic head or pressure measurements. In the present study, we propose to use temperature data as source of information for characterizing the spatial distributions of the hydraulic conductivity field. In this way, we performed a laboratory sandbox experiment with the aim of imaging the heterogeneities of the hydraulic conductivity field from thermal monitoring. During the laboratory experiment, we injected a hot water pulse, which induces a heat plume motion into the sandbox. The induced plume was followed by a set of thermocouples placed in the sandbox. After the temperature data acquisition, we performed a hydraulic tomography using the stochastic Hybrid Monte Carlo approach, also called the Hamiltonian Monte Carlo (HMC) algorithm to invert the temperature data. This algorithm is based on a combination of the Metropolis Monte Carlo method and the Hamiltonian dynamics approach. The parameterization of the inverse problem was done with the Karhunen-Loève (KL) expansion to reduce the dimensionality of the unknown parameters. Our approach has provided successful reconstruction of the hydraulic conductivity field with low computational effort.

2D sandbox experiments of surfactant foams for mobility control and enhanced LNAPL recovery in layered soils

The ability of surfactant foam to enhance mobility control and LNAPL recovery in a heterogeneous porous media was investigated through sandbox experiments with p-xylene used as LNAPL. A layered heterogeneous porous media was represented in a 2D sandbox filled with two layers of coarse and medium silica sand. Based on previous tests, the surfactant solution used was Ammonyx Lo at a concentration of 0.1% (w/w). The sandbox experimental program included tracer tests done under both uncontaminated and contaminated conditions, foam injection under uncontaminated conditions and surfactant solution and foam injection under contaminated conditions. Tracer tests indicated that the permeability contrast between sand layers was increased by LNAPL contamination. Foam injection under uncontaminated conditions presented a S-shaped front that indicated a better mobility control than the piston-shaped front obtained during tracer tests. During foam injection, complete sweep of the sandbox was achieved with 1.8pore volume (PV) compared to 2.8PV during tracer injection, thus indicating better mobility control with foam. Pre-flush of the contaminated sandbox with surfactant solution initiated p-xylene mobilization but no free phase was recovered at the effluent. A negligible p-xylene residual saturation was reached following foam injection in the contaminated sandbox. However, mass balance indicated a total recovery of only 48% of the initial p-xylene, thus indicating an underestimation of the recovery by volatilization. Recovery by volatilization was corrected, which gave the following proportions of LNAPL recovery mechanisms: 19% by mobilization, 16% by dissolution and 65% by volatilization. Results show the potential efficiency of foam remediation of LNAPL source zones in heterogeneous porous media. Still, further developments are needed prior to field scale application, which could benefit from in situ foam production that would simplify on-site operations.

On the importance of geological data for hydraulic tomography analysis: Laboratory sandbox study

This paper investigates the importance of geological data in Hydraulic Tomography (HT) through sandbox experiments. In particular, four groundwater models with homogeneous geological units constructed with borehole data of varying accuracy are jointly calibrated with multiple pumping test data of two different pumping and observation densities. The results are compared to those from a geostatistical inverse model. Model calibration and validation performances are quantitatively assessed using drawdown scatterplots. We find that accurate and inaccurate geological models can be well calibrated, despite the estimated K values for the poor geological models being quite different from the actual values. Model validation results reveal that inaccurate geological models yield poor drawdown predictions, but using more calibration data improves its predictive capability. Moreover, model comparisons among a highly parameterized geostatistical and layer-based geological models show that, (1) as the number of pumping tests and monitoring locations are reduced, the performance gap between the approaches decreases, and (2) a simplified geological model with a fewer number of layers is more reliable than the one based on the wrong description of stratigraphy. Finally, using a geological model as prior information in geostatistical inverse models results in the preservation of geological features, especially in areas where drawdown data are not available. Overall, our sandbox results emphasize the importance of incorporating geological data in HT surveys when data from pumping tests is sparse. These findings have important implications for field applications of HT where well distances are large.

Scaling the sandbox—Mechanical (dis) similarities of granular materials and brittle rock

AbstractAnalog sandbox experiments are an important tool to understand brittle tectonic deformation. To date, most experiments are interpreted kinematically only. With the advent of reliable, small‐scale force sensors, however, their dynamic evolution becomes available for analysis, offering new insights into the transient evolution of tectonic systems. Both rock and granular materials show an evolution of strain hardening and weakening during loading in the brittle‐plastic regime, but so far, this similarity has only been appreciated qualitatively. As strain weakening is a vital parameter controlling fault reactivation and lifetime, it requires proper scaling. We therefore measured and analyzed two common granular analog model materials (quartz sand and glass microbeads) using ring‐shear tests at a range of normal loads typical for analog experiments. We find two different modes of strain weakening as a function of normal load: Strain weakening at normal loads <1 kPa is due to partial loss of extrapolated cohesion, while at normal loads >1 kPa it is controlled by reduction of internal friction, which is consistent with previous measurements in this range. We show that this introduces a scale dependence into the scaling and restricts the possible use of the tested materials to crustal‐scale models with a length scaling factor of . For these we quantitatively compare the model materials' transient strength evolution to that known from natural rock and the Earth's crust.

Kinematic History of a Salient-recess Junction Explored through a Combined Approach of Field Data and Analog Sandbox Modeling.

Within fold-thrust belts, the junctions between salients and recesses may hold critical clues to the overall kinematic history. The deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess. We examine one such junction within the Sevier fold-thrust belt (western United States) along the Leamington transverse zone, northern Utah. Deformation within this junction took place by faulting and cataclastic flow. Here, we describe a protocol that examines these fault patterns to better understand the kinematic history of the field area. Fault data is supplemented by analog sandbox experiments. This study suggests that, in detail, deformation within the overlying thrust sheet may not directly reflect the underlying basement structure. We demonstrate that this combined field-experimental approach is easy, accessible, and may provide more details to the deformation preserved in the crust than other more expensive methods, such as computer modeling. In addition, the sandbox model may help to explain why and how these details formed. This method can be applied throughout fold-thrust belts, where upper-crustal rocks are well preserved. In addition, it can be modified to study any part of the upper crust that has been deformed via elastico-frictional mechanisms. Finally, this combined approach may provide more details as to how fold-thrust belts maintain critical-taper and serve as potential targets for natural resource exploration.