Bulk Shear Strength Research Articles

Lessons Learned from NASA’s DART Impact about Disrupting Rubble-pile Asteroids

We present a series of numerical simulations using a shock physics smoothed particle hydrodynamics code, investigating energetic impacts on small celestial bodies characterized by diverse internal structures, ranging from weak and homogeneous compositions to rubble-pile structures with varying boulder volume packing. Our findings reveal that the internal structure of these rubble-pile bodies significantly influences the impact outcomes. Specifically, we observe that the same impact energy can either catastrophically disrupt a target with a low boulder packing (≲30 vol%), or result in the ejection of only a small fraction of material from a target with the same mass but high boulder packing (≳40 vol%). This finding highlights the pivotal role played by the rubble-pile structure, effectively acting as a bulk shear strength, which governs the size and behavior of the resulting impact. Consequently, understanding and characterizing the internal structure of asteroids will be of paramount importance for any future efforts to deflect or disrupt an asteroid on a collision course with Earth.

Investigating the effect of different adhesion materials on electrical resistance using a high pressure torsion rig

This paper presents an assessment of newly-developed conductive adhesion materials (Products A-E) in comparison to standard rail sand used in Britain. Current rail sand is an insulating material which can affect track circuits; newly-developed conductive materials could reduce the risk of this and allow for more material to be applied to further mitigate against low adhesion. The particles were characterised to determine their densities, and size and shape distributions. Bulk behaviour was assessed through three characteristics: angle of repose, bulk shear strength, and particle breakage index. Materials were then assessed using a high pressure torsion approach to measure their effects on adhesion and electrical resistance in dry, wet, and leaf contaminated conditions. It was found that all products produced better or equivalent conductivity compared to the currently used GB rail sand and that Product D and Product E should be considered for future field testing.

Insight on bulk shear strength parameters of clay using discrete element approach incorporating physics-based interparticle force model

This paper aims to provide insight on factors affecting the bulk shear strength parameters of clay, i.e., the cohesion and internal friction as well as the effects of memory of preconsolidation pressure. A unique Discrete Element Method (DEM) model is built on platy particles with customized particle interaction force model. The particle interaction force model considers non-contact forces (such as the long-range electrostatic repulsion, short-range van der Waals attraction) as well as the direct contact force. The long-range electrostatic forces are calibrated by measurement with Atomic Force Microscope (AFM). Parameters for direct particle contacts of the regular DEM contact model, such as the contact stiffness and the friction coefficient, are calibrated by use of experimental consolidation and direct shear testing data. Computational simulations are conducted on digital clay specimen subjected to virtual direct shear tests. The predicted experimental responses of the digital specimens are consistent with typically observed in experiments including the shear stress-shear strain relationship, volumetric contraction and dilation, shear band formation, etc. From the stress–strain curves, the soil strength parameters are obtained with common experimental criteria. The DEM simulation results show that higher preconsolidation pressure drives more particles to overcome non-contact force into direct contacts and consequently bonding by van der Waals force. Consequently, the bulk cohesion of clay increases with increasing preconsolidation pressure. The results show that bulk cohesion strength is mainly attributed to the attractive interparticle force as well as the interlocking of platy particle, while the bulk internal friction angle is affected by the particle friction coefficient and particle fabric. Overall, the simulation results indicate the experimentally observed macroscopic shear strength parameters c and φ are linked to the microscope characteristics of soil particles and their mutual interactions. The results offer insight on the microscopic properties of particles on the bulk macroscopic strength behaviors. Besides, this work demonstrates a new strategy for simulation-based prediction of bulk soil strength parameters, by incorporating microscale characterization of the interparticle interactions and particle fabric into the particle-based DEM model.

Shrub encroachment increases soil erosion risk in hillside alpine meadows of the Qinghai-Tibetan Plateau, NW China

Shrub encroachment is advancing in alpine meadow ecosystems under climate warming, which can alter meadow community and soil properties. Given the key role of alpine meadows in soil and water conservation, meadow changes that could lead to higher erosion are particularly worrisome. However, little is known about plant and soil characteristics related to erosion resistance change with increasing levels of shrub encroachment. To address this issue, we evaluated multiple plant and soil variables closely related to erosion resistance along a shrub encroachment gradient in the Qinghai-Tibetan Plateau. Plant variables were herbaceous density, richness, root biomass while soil variables included bulk density, moisture, temperature, compressive strength, and shear strength. These were measured at multiple plots at four treatments, including non-encroached hillside alpine meadow (control) and hillside alpine meadow with low (<30% shrub cover), moderate (30%-60% shrub cover), and high (>60% shrub cover) levels of shrub encroachment. Our results showed that shrub encroachment significantly decreased herbaceous plant density and herbaceous species richness of meadow communities and that this effect strengthened along the shrub encroachment gradient. Encroachment also reduced herbaceous root biomass by 40.6%, 59.4%, and 73.3% in low, moderate, and high levels of encroachment at the 0–10 cm soil depth. We also found that shrub encroachment decreased soil bulk density by 10.9%–21.8% and soil moisture by 3.7–13.5% at 0–10 cm soil. Most important, both compressive and shear strength of topsoil gradually decreased with increasing levels of encroachment, and were approximately twice as low on high encroached plots than non-encroached plots. Overall, our results indicate that shrub encroachment leads to sparse community density, fewer herbaceous roots, and topsoil looseness, ultimately decreasing the strength of hillside alpine meadows’ soils. These findings emphasize the role of shrub encroachment in driving the increase of soil erosion risk in hillside alpine meadows under climate changes.

What controls marsh edge erosion?

The survival of salt marshes depends on their ability to maintain vertical elevation and areal extent. In the lateral direction, marsh edges can expand laterally or undergo edge erosion through mass failure or continuous particle erosion through waves and tidal processes. In this study, we evaluate possible relationships between marsh shoreline type within the Great Marsh in Massachusetts and major geotechnical parameters along the marsh edge. We also explore if wave energy, using fetch as a proxy, affects the presence, type, and distribution of shoreline type. We mapped and classified the marsh into four categories: slumping, vertical and abrading, stable/accretionary, and bedrock or gravel, and sampled the marsh edge at 98 sites. Using over 450 measurements, we present typical ranges of values at these marsh edges for bulk density (0.10–1.43 g/cm3), organic content (0.99–55.07%), belowground biomass (0.11–36.76%), and shear strength (4.04–136.49 kPa at 20 cm into the marsh bank, and 4.04–131.03 kPa at 40 cm into the bank). We show that there are no significant differences in fetch or geotechnical properties for the different marsh edge classes. Thus, none of these parameters explain or correlate with edge erosion, even though the majority of previous edge erosion studies focus on these parameters as determinants of edge erosion. We further emphasize the heterogeneity of the marsh, as edge erosion can occur in highly exposed or sheltered areas alike with no trends in geotechnical properties, and that complex interactions between parameters not generally studied may be responsible for edge erosion.

A study on the effect of grain morphology on shear strength in granular materials

The Discrete Element Method (DEM) has been successfully used to further understand GM behaviour where experimental means are not possible or limited. However, the vast majority of DEM publications use simplified spheres with rolling friction to account for particle shape, with a few using clumped spheres and super quadratics to better capture grain geometric detail. In this study, we compare the shear strength of packed polyhedral assemblies to spheres with rolling resistance to account for shape. Spheres were found to have the highest shear resistance as the limited rolling friction model could not capture the geometric of rotation grains which caused reordering and dilation. This geometric arrangement causes polyhedra to align faces in the shear direction, reducing the resistance to motion. Conversely, geometric interlocking can cause jamming resulting in a dramatic increase in shear resistance. Particle aspect ratio (elongation and fatness) was found to significantly lower shear resistance, while more uniform aspect ratio’s increased shear resistance with shape non-convexity showing extremes of massive slip or jamming. Thus, while spheres with rolling friction may yield bulk shear strength similar to some polyhedra with a mild aspect ratio, the grain scale effect that leads to compaction and jamming from rotation and interlocking is missed. These results shed light on the complex impact that individual grain shape has on bulk behaviour and its importance.

Three-Dimensional Combined Finite-Discrete Element Modeling of Shear Fracture Process in Direct Shearing of Rough Concrete–Rock Joints

A three-dimensional combined finite-discrete element element method (FDEM), parallelized by a general-purpose graphic-processing-unit (GPGPU), was applied to identify the fracture process of rough concrete–rock joints under direct shearing. The development process of shear resistance under the complex interaction between the rough concrete–rock joint surfaces, i.e., asperity dilatation, sliding, and degradation, was numerically simulated in terms of various asperity roughness under constant normal confinement. It was found that joint roughness significantly affects the development of overall joint shear resistance. The main mechanism for the joint shear resistance was identified as asperity sliding in the case of smoother joint roughness and asperity degradation in the case of rougher joint asperity. Moreover, it was established that the bulk internal friction angle increased with asperity angle increments in the Mohr–Coulomb criterion, and these results follow Patton’s theoretical model. Finally, the friction coefficient in FDEM appears to be an important parameter for simulating the direct shear test because the friction coefficient affects the bulk shear strength as well as the bulk internal friction angle. In addition, the friction coefficient of the rock–concrete joints contributes to the variation of the internal friction angle at the smooth joint than the rough joint.

Short-term impacts of different tillage practices and plant residue retention on soil physical properties and greenhouse gas emissions

Reducing tillage intensity and plant residue retention have the potential to mitigate climate change by reducing soil greenhouse gas emissions. Few comparative studies have explored the effects of different tillage practices and plant residue retention on the net balance of GHG emissions in the short term (1–2 years). We hypothesised that reducing tillage and plant residue retention decreases GHG emissions compared to conventional ploughing and residue removal. We estimated the effects of different tillage practices; no-tillage, minimum-tillage and traditional ploughing as well as plant residue retention on; 1) bare-soil GHG emissions, 2) the net ecosystem exchange (NEE), and ultimately, 3) the potential for climate change mitigation, CO2 equivalent. GHG fluxes were measured using the static chamber method and soil physical properties were assessed using X-ray Computed Tomography (CT) and a range of traditional methods. No-tillage increased bulk density and shear strength compared to minimum-tillage and ploughed soil. Soil moisture content and organic matter content at 0−5 cm depth were higher in no-tillage and minimum-tillage compared to ploughed soil. No-tillage increased soil−CO2 emissions and CH4 uptake but reduced NEE−CO2 emissions and increased NEE−CH4 uptake. No and minimum-tillage resulted in 56 % and 40 % reduction in CO2 equivalent emissions respectively compared to ploughing for the combined crop-soil measurements. Changes in the CH4 and CO2 were interlinked to changes in soil pore architecture and physical properties. Taken together our data demonstrate that no-tillage and minimum-tillage have potential to contribute to climate change mitigation through reducing GHG emissions.

Fabrics-Shear Strength Links of Silicon-Based Granular Assemblies

ABSTRACTSilicon (Si)-based materials are sought in different engineering applications including Civil, Mechanical, Chemical, Materials, Energy and Minerals engineering. Silicon and Silicon dioxide are processed extensively in the industries in granular form, for example to develop durable concrete, shock and fracture resistant materials, biological, optical, mechanical and electronic devices which offer significant advantages over existing technologies. Here we focus on the constitutive behaviour of Si-based granular materials under mechanical shearing. In the recent times, it is widely recognised in the literature that the microscopic origin of shear strength in granular assemblies are associated with their ability to establish anisotropic networks (fabrics) comprising strong-force transmitting inter-particle contacts under shear loading. Strong contacts pertain to the relatively small number of contacts carrying greater than the average normal contact force. However, information on how such fabrics evolve in Si-based assemblies under mechanical loading, and their link to bulk shear strength of such assemblies are scarce in the literature. Using discrete element method (DEM), here we present results on how Si-based granular assemblies develop shear strength and their internal fabric structures under bi-axial quasi-static compression loading. Based on the analysis, a simple constitutive relation is presented for the bulk shear strength of the Si-based assemblies relating with their internal fabric anisotropy of the heavily loaded contacts. These findings could help to develop structure-processing property relations of Si-based materials in future, which originate at the microscale.

Application of Discrete Element Methods to Approximate Sea Ice Dynamics

AbstractLagrangian models of sea ice dynamics have several advantages over Eulerian continuum models. Spatial discretization on the ice floe scale is natural for Lagrangian models and offers exact solutions for mechanical nonlinearities with arbitrary sea ice concentrations. This allows for improved model performance in ice‐marginal zones, where sea ice is fragmented. Furthermore, Lagrangian models can explicitly simulate jamming processes that occur when sea ice moves through narrow confinements. While difficult to parameterize in continuum formulations, jamming emerges spontaneously in dense granular systems simulated in a Lagrangian framework. Here we present a flexible discrete element framework for approximating Lagrangian sea ice mechanics at the ice floe scale, forced by ocean and atmosphere velocity fields. Our goal is to evaluate the potential of models simpler than the traditional discrete element methods for granular dynamics. We demonstrate that frictionless contact models based on compressive stiffness alone are unlikely to produce jamming and describe two different approaches based on Coulomb friction and cohesion which both result in increased bulk shear strength of the granular assemblage. The frictionless but cohesive contact model displays jamming behavior which is similar to the more complex model with Coulomb friction and ice floe rotation at larger scales and has significantly lower computational cost.

Acoustics of biologically active marine sediments

Biological organisms are prevalent on and near the seabed; however, most predictive theoretical models do not explicitly address the acoustic behavior of biologically active marine sediments. Sediment-acoustics models, such as those based on Biot theory or grain-shearing theory, allow for parameterization of various physical properties including bulk density, porosity, shear strength, or pore fluid viscosity, all of which can be modified by the presence of biological organisms or bioturbation. Other models can include the effects gas bubble distributions, which can also be associated with the presence of benthic biology, on compressional and shear wave propagation. In this paper, we examine sediment-acoustics models in comparison with data from various field experiments focused on infauna- and seagrass-bearing sediments. The model parameters are partially constrained by physical data from core samples collected at the field experiment sites. Empirical relations between the core data parameters and acoustic data will also be examined. [Work supported by ONR and ARL:UT IR&D.]

Estimating machine impact on strip roads via close-range photogrammetry and soil parameters: a case study in central Italy

Several studies have been carried out to investigate soil compaction and rutting after logging vehicle traffic, based on time consuming and punctual field measurements. The objective of this study was to measure soil disturbances with two methods: (i) a new, image-based models derived by a structure-from-motion (SfM) photogrammetry approach; and (ii) a traditional soil sampling (bulk density and shear strength). Two trails were selected in a logging area (central Italy), one trafficked by a forwarder (FT) and one trafficked by a skidder (ST). Data collection was conducted before, during and after timber extraction. Image-based models derived by SfM photogrammetry was used to highlight the differences in the shape and distribution of the disturbances along ST and FT. Results showed that the physical parameters of soil significantly changed due to both FT and ST traffic. Machine passes increased bulk density (111% and 31% for FT and ST, respectively), penetration resistance (29% and 24% for FT and ST, respectively) and shear resistance (14% and 6% for FT and ST, respectively), whereas porosity decreased (46% and 9% for FT and ST, respectively). Significant differences between FT and ST were found when comparing ruts removal and bulges with SfM photogrammetry. After logging, FT clearly showed ruts and bulges, whereas in ST ruts and bulges were not visible, but soil displacement in the direction of extraction was evident and measurable. Nevertheless, although our result shows a larger soil disturbance caused by forwarders than skidders, it is not possible to draw any general conclusions about differences between the two machines. Data about the machine passes, or the wood volumes transported over each trial area were not available; therefore, any general conclusion is misleading. SfM photogrammetry give information not available via traditional methods, thus improving impact assessment.

Influence of weakening minerals on ensemble strength and slip stability of faults

AbstractWe explore the impact of phyllosilicate (weak but velocity strengthening) in a majority tectosilicate (strong but velocity weakening) matrix on bulk shear strength and slip stability of faults. Numerical simple shear experiments using a distinct element model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear elastic contact model with a slip‐weakening constitutive relation representing the essence of rate‐state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1 particle, ~50% shear strength reduction is observed with 3‐particles, and a dominant influence occurs at 5 particles. In terms of slip stability, a transition from velocity weakening to velocity strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3 particles to 5 particles in the textured mixtures. In addition, further analysis suggests that quartz has a high tendency toward dilation, potentially promoting permeability; while talc dilates with increased slip rate but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.

Influence of wood-derived biochar on the compactibility and strength of silt loam soil

Abstract Biochar is proven to enhance soil fertility and increase crop productivity. Given that the influence of biochar on soil compaction remains unclear, selected physico-mechanical properties of soil amended with wood-derived biochar were assessed. For unamended silt loam, the bulk density, maximum bulk density, optimum moisture content, plastic limit, liquid limit, and plasticity index were 1.05 Mg m-3, 1.69 Mg m-3, 16.55, 17.1, 29.3, and 12.2%, respectively. The penetration resistance and shear strength of the unamended silt loam compacted in the standard compaction Proctor mold and at its optimum moisture content were 1800 kPa and 850 kPa, respectively. Results from amending the silt loam with 10% particle size ranges (0.5-212 μm) led to relative decreases of 18.1, 17.75, 66.66, and 97.4% in bulk density, maximum bulk density, penetration resistance, and shear strength, respectively; a 26.8% relative increase in optimum moisture content; along with absolute increases in plastic limit, liquid limit, and plasticity index of 5.3, 13.7, and 8.4%, respectively. While the biochar-amended silt loam soil was more susceptible to compaction, however, soil mechanical impedance enhanced.

Probabilistic stability evaluation and seismic triggering scenarios of submerged slopes in Lake Zurich (Switzerland)

Subaqueous landslides and their consequences, such as tsunamis, can cause serious damage to offshore infrastructure and coastal communities. Stability analyses of submerged slopes are therefore crucial, yet complex steps for hazard assessment, as many geotechnical and morphological factors need to be considered. Typically, deterministic models with data from a few sampling locations are used for the evaluation of slope stabilities, as high efforts are required to ensure high spatial data coverage. This study presents a simple but flexible approach for the probabilistic stability assessment of subaqueous slopes that takes into account the spatial variability of geotechnical data. The study area (~2 km2) in Lake Zurich (northern Switzerland) shows three distinct subaquatic landslides with well-defined headscarps, translation areas (i.e. the zone where translational sliding occurred) and mass transport deposits. The ages of the landslides are known (~2,210 and ~640 cal. yr BP, and 1918 AD), and their triggers have been assigned to different mechanisms by previous studies. A combination of geophysical, geotechnical, and sedimentological methods served to analyse the subaquatic slope in great spatial detail: 3.5 kHz pinger seismic reflection data and a 300 kHz multibeam bathymetric dataset (1 m grid) were used for the detection of landslide features and for the layout of a coring and an in situ cone penetration testing campaign. The assignment of geotechnical data to lithological units enabled the construction of a sediment-mechanical stratigraphy that consists of four units, each with characteristic profiles of bulk density and shear strength. The thickness of each mechanical unit can be flexibly adapted to the local lithological unit thicknesses identified from sediment cores and seismic reflection profiles correlated to sediment cores. The sediment-mechanical stratigraphy was used as input for a Monte Carlo simulated limit-equilibrium model on an infinite slope for the assessment of the present slope stability and for a back analysis of past landslides in the study area, both for static and earthquake-triggered scenarios. The results show that the location of failure initiation in the model is consistent with stratigraphic analysis and failure-plane identification from sediment cores. Furthermore, today’s sediment-charged slopes are failure-prone, even for a static case. This approach of including an adaptable sediment-mechanical stratigraphy into a limit-equilibrium slope stability analysis may be applied as well to the marine realm.

Physical, Chemical, and Engineering Properties of Landfill Stabilized Waste

AbstractIn order to utilize landfill stabilized waste (LSW) as growing substrate for plants during restoration projects, LSW were sampled from five landfills; the properties of unconsolidated samples and those after 3 years of settlement were determined in comparison with farm soil. Results showed that (i) unconsolidated LSW was a soil‐like material with high porosity and infiltration rate, which also had low bulk density and shear strength. After 3 years of settlement, its physical properties were similar to those of farm soil. (ii) The contents of available K, available N, available P, and organic matter of LSW were 5·0–21·6 times higher than the values of surrounding farm soil. (iii) The waste samples from 3/5 landfills were suitable for cultivation based on their heavy metal contents, but two samples contained over‐limit Hg and/or Zn. (iv) LSW had higher water content, and generally, less surface runoff was generated on LSW slope under natural precipitation compared with the farm soil slope. (v) Water extract of LSW had no significant effect on seed germination, and the effect of LSW on seedling growth depended on plant species. On the basis of the results, LSW can be used in ecological restoration as plant‐growing substrate, but measures should be taken to prevent heavy metal contamination and soil erosion. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Epoxy‐based adhesives filled with flakes ag‐coated copper as conductive fillers

The flake core–shell Cu‐Ag powders are prepared and characterized. Isotropic conductive adhesives (ICAs) filled with flake silver‐coated copper was prepared by using epoxide resin as matrix and tetraethylenepentamine as curing agent. The flake silver‐coated copper was characterized by scanning electron microscopy, X‐ray diffraction, energy dispersive X‐ray spectroscopy, and TG‐DTA. The results show Ag content in the surface of coated copper is up to 96.32%. Oxidation resistance of flake Cu‐Ag powders with high surface Ag content is improved greatly investigated by TG‐DTA. And bulk resistivity and shear strength of ICAs are measured. It was found that the percolation threshold of ICAs filled with flake Cu‐Ag powders is as low as 40 wt%. The ICAs have good overall performances. And the main influence factor on electric resistivity was analyzed. POLYM. COMPOS., 38:846–851, 2017. © 2015 Society of Plastics Engineers

Quality assessment of mushroom slices dried by hot air combined with an electrohydrodynamic (EHD) drying system

In this study, hot air combined with an electrohydrodynamic (EHD) drying system was designed and examined for drying button mushroom (Agaricus bisporus) slices. The effects of three levels of voltage (17, 19 and 21kV) and electrode gap (5, 6 and 7cm) on solid and bulk density, porosity, shear strength, water absorption capacity (WAC) and total color difference (ΔE) of dried mushroom slices in comparison to oven dried mushroom slices were investigated. ANOVA showed that the hot air combined with EHD drying method had significant effects on bulk density and shear strength (p≤0.01) reduction as well as on porosity (p≤0.001) and WAC increase (p≤0.01), but this system had no significant effects (p≥0.05) on the solid density and ΔE parameters in comparison to the oven drying system. In addition, results indicated that increasing the voltage or decreasing the electrode gap resulted in some advantages such as bulk density and shear strength reduction or porosity and WAC increase that introduced this combined drying method as an improved method for drying mushroom slices.

수중문화재 탐사기법 개발을 위한 퇴적물 물성분석

Abstract: Since distribution and preservation of cultural artifacts in the submarine sediments are directly affected by not only ocean currents and tides, but also their composition, it is very important to investigate geological characteristics of sediments and ocean-sediment interactions for maritime archaeological survey. Physical properties of submarine sediments, which are collected by grab sampler and vibro-corer, are analyzed in order to investigate effects of submarine environment on development of maritime archaeological survey techniques. Result of physical property analysis showed that bulk density, shear strength, and magnetic susceptibility increase with depth, while water contents and porosity decrease with depth. Since the magnetic susceptibility of bedrock is about 40 times that of submarine sediments, it might impact greatly on the response of magnetic survey. Physical properties of sediments with depth and sediment classification by Folk’s ternary diagram indicate that submarine sediment mainly consists of silt, and cultural artifacts can not penetrate no deeper than 1.5 m in sediments.

A microphysical model for fault gouge friction applied to subduction megathrusts

A microphysical model is developed for the steady state frictional behavior of illite-quartz fault gouge and applied to subduction megathrust P-T conditions. The model assumes a foliated, phyllosilicate-supported microstructure which shears by rate-independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts. At low slip rates or high temperatures, the deformation of the clasts is easy, accommodating slip on the foliation without dilatation. With increasing velocity or decreasing temperature, the shear of the clasts becomes more difficult, increasing bulk shear strength, until slip is activated on inclined portions of the phyllosilicate foliation, where it anastomoses around the clasts. Slip at these sites leads to dilation involving clast/matrix debonding, balanced, at steady state, by compaction through thermally activated clast deformation. Model predictions, taking pressure solution as the thermally activated mechanism, show three regimes of velocity-dependent frictional behavior at temperatures in the range of 200–500°C, with velocity weakening occurring at 300–400°C, in broad agreement with previous experiments on illite-quartz gouge. Effects of slip rate, normal stress, and quartz fraction predicted by the model also resemble those seen experimentally. Extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity-weakening behavior at the updip seismogenic limit on subduction megathrusts. The model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz, which counteracts dilatation due to slip on the fault rock foliation.