Gouge Development Research Articles

An experimental and numerical investigation on the hydromechanical behaviour of carbonate fault zones upon reactivation: the impact of carbonate mud sealing layers and overall research outcomes

Abstract To contribute to the understanding of the impacts of fault reactivation induced by reservoir exploitation, we describe the final series of laboratory experiments, numerical simulations and microstructural analysis conducted during the ‘Fault Reactivation in Carbonates’ research project. In the project, the structure and hydromechanical properties of carbonate-hosted fault zones were investigated. For the analyses here reported, faults were artificially generated by direct shearing composite blocks made of layers of reservoir analogue rocks (outcrop travertine or synthetic grainstone) intercalated with one layer of a sealing analogue rock (synthetic carbonate mudstone). Post-direct shearing, cylindrical plugs containing the fault zone and parts of intact rock were cored out from the blocks and tested in a triaxial test rig, simulating fault reactivation. Varied stress paths and pore-pressure conditions representative of fluid depletion and injection were considered. In parallel, two-dimensional mechanical models representative of the direct shear experiments were developed using smoothed particle hydrodynamics (SPH). We observed a continuous reduction in fault transmissibility during direct shearing, followed by a permeability reduction of 50–80% with increasing mean effective stress in the subsequent fault reactivation tests. Experimental fault zone geometries produced during direct shear were broadly reproduced by the two-dimensional modelling approach. We also detected that the inclusion of the carbonate mud sealing rock into the fault zone caused greater compaction of the fault materials when compared to experiments conducted without carbonate mud layers. We conclude that with fault displacement, increasing incorporation of carbonate mud sealing material into the fault zone and the concomitant development of gouge results in the continuous reduction of fault transmissibility/permeability. This occurs in the two very different limestone host-rock types and for all the stress configurations investigated. Discussions on these results and also on the outcomes of the research project as a whole are presented in the paper.

The Study of Hypervelocity Gouging Based on the Material Point Method

Gouging is a large nonlinear deformation phenomenon experienced in rail materials under severe loading conditions such as the tremendous heating caused by frictional effects at the slider/rail interface. The material point method (MPM) algorithm is designed specifically to solve such nonlinear, large deformation problems. The theories and steps of the MPM algorithm are described in this paper, and an investigation of hypervelocity gouging is conducted using the MPM algorithm. Because of frictional heating near the contact region, the effects of temperature on the gouge development are evaluated by several developed thermal environment scenarios. Moreover, the regular patterns of gouging at various impact velocities, horizontal velocities, and friction coefficients are analyzed. The results show that the MPM algorithm can successfully simulate the gouging phenomenon. With the gouge initiation and development, the temperature rapidly increases to the melting temperature, and the yield stress rapidly decreases at the gouge location. The regular patterns of gouging provide a new effective method for antigouging research.

Laboratory observations of fault transmissibility alteration in carbonate rock during direct shearing

AbstractWe present the results of hydromechanical changes across and along the evolving rupture surface of carbonate rocks during direct shear experiments. Direct shear experiments were performed on a laminated travertine of continental, microbial origin with calcite content of 99 wt%, chosen as a lithological analogue for Aptian presalt oil reservoir rocks found in South Atlantic presalt basins. Medical X‐ray CT images show that the porosity (~9–13%) is mainly composed of subplanar pores and vugs. Permeability is high along the laminations (~50–200 mD), controlled by interconnected pores and fractures, and extremely low across the laminations (≪1 mD). Six intact samples of travertine were sheared across the bedding direction to short displacement (20 mm), and a further three samples were sheared to a much longer displacement (120 mm). A constant effective vertical stresses of either 35, 40 or 45 MPa was applied throughout the tests. Fluid flow response across and along the fault zone was monitored continuously during both shear deformation (dynamic transmissibility) and hold periods (static transmissibility), while keeping a constant pore pressure throughout the measurement. While the samples show some microstructural variability, the set of samples sheared to 20‐mm displacement followed the same early kinematics and show very similar mechanical response to that seen in the same period of the samples where shear was continued on to 120‐mm displacement. After 20‐mm displacement, the microstructures are dominated by fractures in a zone 10–20 mm wide resembling a typical fault damage zone, with only thin and patchy development of gouge on the principal displacement plane. In all of the samples sheared to a high displacement of 120 mm, a continuous layer of gouge (6.2–13 mm thick) was formed, defining a distinct core of the shear zone. The dynamic transmissibility across the fault decreases progressively in all the sheared samples regardless of the applied effective stress. In general, the volume decreases as well, though at a tapering rate, throughout shear. A small vertical dilation occurred near the peak strength for travertine sheared under 35 MPa vertical stress, but this was not accompanied by an increase in transmissibility. From the results, we conclude that once the gouge material is developed in the core of the fault zone, the dynamic transmissibility across the fault is permanently decreased.

Microstructural and mineral analysis on the fault gouge in the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake

The 2008 Mw 7.9 Wenchuan earthquake formed two coseismic surface rupture zones with the trend of N35°E, known as the Beichuan–Yingxiu rupture and the Pengguan rupture. The Beichuan–Yingxiu rupture is the principle one with abundant fault gouge development along its length. In the exploratory trench at the Saba village along the Beichuan–Yingxiu rupture, the new fault gouge zone is only ~3 mm wide, which suggests that fault slip was constrained in a very narrow zone. In this study, we thus carried out detailed microstructural and mineral component analysis on the oriented fault gouge samples from the Saba exploratory trench to understand their features and geological implication. The results show that different microstructures of localized brittle deformation can be observed in the fault gouges, including Y-shear, R1-shear, R2-shear, P-shear as well as tension fracture, bookshelf glided structure and so on. These microstructures are commonly recognized as the product of seismic fault slipping. Furthermore, within the area between two parallel Y-shears of the fault gouge, a few of microstructures of distributed ductile deformations were developed, such as P-foliation, elongation and asymmetrical trailing structure of detrital particles. The microstructure features of fault gouges implicate the thrust movement of the fault during the Wenchuan earthquake. In addition, the fault gouge has less quartz and feldspar and more clay than the surrounding rocks, which indicates that some quartz and feldspar in the surrounding rocks were transformed into clay, whereas the fault gouge has more illite and less illite/montmorillonite mixed layers than the surrounding rocks, which shows that the illite/montmorillonite mixed layer was partly converted into illite due to temperature increasing induced by coseismic fault slipping friction (also being affected partly by the chemical action of solutions). Such microstructures features and mineral component changes recorded the information of fault slip and provide criterions for discussing the genesis of fault gouge and recognition of the direction of fault movement.

Fault rocks from the SAFOD core samples: Implications for weakening at shallow depths along the San Andreas Fault, California

The drilling of a deep borehole across the actively creeping Parkfield segment of the San Andreas Fault Zone (SAFZ), California, and collection of core materials permit direct geological study of fault zone processes at 2–3 km depth. The three drill cores sample both host and fault rocks and pass through two currently active, narrow (1–2 m wide) shear zones enclosed within a broader (ca. 240 m wide) region of inactive foliated gouges. The host rocks preserve primary sedimentary features and are cut by numerous minor faults and small, mainly calcite-filled veins. The development of Fe-enriched smectitic phyllosilicate networks following cataclasis is prevalent in the presently inactive foliated gouges of the main fault zone and in minor faults cutting clay-rich host rocks. Calcite, anhydrite and minor smectitic phyllosilicate veins are interpreted to have formed due to local fluid overpressuring events prior to, synchronous with and after local gouge development. By contrast, the active shear zone gouges lack mineral veins (except as clasts) and contain numerous clasts of serpentinite. Markedly Mg-rich smectitic phyllosilicates are the dominant mineral phases here, suggesting that the fault zone fluids have interacted with the entrained serpentinites. We propose that weakening of the SAFZ down to depths of at least 3 km can be attributed to the pervasive development of interconnected networks of low friction smectitic phyllosilicates and to the operation of stress-induced solution-precipitation creep mechanisms.

The Santiago Landslide and Associated Ridge-top Graben (Sackungen): Implications for Paleoseismic Landslide Studies

Some recent paleoseismic studies have focused on dating ridge-top graben deposits to evaluate the timing of paleoseismic events. By contrast, our study of the Santiago landslide demonstrates that ridge-top grabens also can be associated with aseismic, deep-seated landsliding. The Santiago landslide in Anaheim Hills, California, failed during the winter of 1992–1993 in response to elevated groundwater conditions associated with intense rainfall. The head of the active landslide included a zone of extensional deformation along the bounding ridgeline. Interpretation of historical, aerial photographs indicates that the active landslide is a re-activated, ancient, deep-seated, translational landslide and an associated ridge-top graben. Large-diameter borings within the ridge-top graben encountered thick colluvium, steeply dipping colluvium-filled fractures, and shears with normal offsets. In contrast to the rupture surface within the central part of the landslide, the basal rupture surfaces in the graben area had significantly less gouge. We interpret this contrast in gouge development as an indication that the ridge-top graben developed later than the original landslide by upslope progression of the deformation. Our limit-equilibrium, slope-stability analyses indicate that either high groundwater or seismic ground motion could have previously activated the ancient landslide and ridge-top graben. Because colluvial deposits preserved within the ridge-top graben and produced by these two different types of triggering events could be misinterpreted as representing the late Quaternary paleoseismic record, these features are not useful for paleoseismic studies unless aseismic activation can be clearly precluded.

Fault intersections as critical hydrocarbon leakage zones: integrated field study and numerical modelling of an example from the Timor Sea, Australia

Fault intersections are identified as important sites for hydrocarbon leakage from the Skua oil field in the Timor Sea, Australia. Integrated structural and fluid history data sets suggest that these fault intersections may be efficient and long-lived fluid conduits. Three-dimensional (3D) numerical modelling, based on fault patterns observed in the Skua Field, generated zones of high dilation in the vicinity of fault intersections during contraction, even at low bulk strain values. In nature, these dilational zones are likely to be sites of high structural permeability containing concentrated open fracture networks ideal for high fluid flux. The potential for fluid leakage from these zones may be further enhanced where low shear strain occurs due to mechanical locking at the fault intersection. Although not tested in the numerical experiments, fault gouge development is likely to be less extensive in these zones of low shear strain, reducing the probability of forming membrane seals. The modelling results support previously published charge and leakage history studies of the Skua Field and highlight the potential for large volumes of hydrocarbons to be lost where fault intersection zones breach the top seal. Fault intersections may therefore play a significant role in influencing trap integrity conditions in other areas.

Syn-deformational features of Carlin-type Au deposits

Syn-deformational ore deposition played an important role in some Carlin-type Au deposits according to field and laboratory evidence, which indicates that flow of Au-bearing fluids was synchronous with regional-scale deformation events. Gold-related deformation events linked to ore genesis were distinct from high-level, brittle deformation that is typical of many epithermal deposits. Carlin-type Au deposits, with brittle–ductile features, most likely formed during tectonic events that were accompanied by significant fluid flow. Interactive deformation-fluid processes involved brittle–ductile folding, faulting, shearing, and gouge development that were focused along illite–clay and dissolution zones caused by hydrothermal alteration. Alteration along these deformation zones resulted in increased porosity and enhancement of fluid flow, which resulted in decarbonated, significant dissolution, collapse, and volume and mass reduction. Carlin-type Au deposits commonly are hosted in Paleozoic and Mesozoic sedimentary rocks (limestone, siltstone, argillite, shale, and quartzite) on the margins of cratons. The sedimentary basins containing the host rocks underwent tectonic events that influenced the development of stratabound, structurally controlled orebodies.

Gouge development during hypervelocity sliding impact

The sliding interface of a hypersonic flyer/target system is subject to immense forces due to dynamic loads and the highly oblique impact of the flyer with the target. Under these severe loading conditions, the material in both the flyer and target will experience large non-linear deformation known as gouging. Using the hydrocode CTH, a numerical model of the gouging process has been created and studied to examine the underlying physics of the phenomenon. Results of simulated gouges are presented along with a discussion of the mechanics involved. Under the combination of a normal load sufficient to cause penetration of the flyer into the target, and hypervelocity tangential motion, stresses may exceed the plastic flow stress of the materials. In this case, material jets may develop at the material interface and impinge into the adjacent material. This jet formation, and the material deformation it causes, are the underlying causes of gouging.

Evolution of the Gouge Particle Size Distribution: A Markov Model

—The hydrologic and mechanical properties of faults are determined by their internal structures and zoning due to fracturing, grain breakage and diagenesis. The proposed model in this study is a first step in the development of a more complete model of fault internal structure. It describes the evolution of the gouge particle size distribution (PSD) with shear induced grain breakage within a small, assumed uniform volume element. A Markov formalism is applied for binary breakage within each time step over the many time steps constituting the evolution. The particle size, strain- and stress-dependent breaking probability is constructed based on physical arguments and data on natural and simulated gouge PSDs. The model is calibrated by using the results of on Ottawa sand PSD experiments. A perfect fit is obtained between experimental and numerically simulated PSDs for a range of normal effective stress (NES). Results of the numerical simulations capture the bimodal form of gouge PSD and also clearly define two different breakage mechanisms. The gouge development behavior makes a dramatic transition as the normal effective stress exceeds the grain crushing stress of the gouge mineral.¶For a more complete intrafault structure analysis, the interplay of diagenesis and the mechanics of grain breakage can be integrated into a single mechano-chemical model of the type presented here. The model proposed here has great potential for predicting the complex roles of faults as seals or fluid conduits.

Fault-related rocks: a photographic atlas

Preface Brittle Behavior Cataclasis and gouge development Pseudotachylyte Fluid-related features Semi-Brittle Behavior Fluid-related features Ductile Behavior Microstructures Foliation development Composite foliations in mylonites Porphyroclasts in mylonites Folds in mylonitic zones Evidence of coaxial deformational histories Metamorphic grade variations Fluid-related features Microstructural development of mylonitic rocks of specific composition References cited Index

Observation of fault gouge development in laboratory see-through experiments : Haggert, K; Cox, S J D; Jessell, M W TectonophysicsV204, N1/2, March 1992, P123–136

Observation of fault gouge development in laboratory see-through experiments : Haggert, K; Cox, S J D; Jessell, M W TectonophysicsV204, N1/2, March 1992, P123–136

Observation of fault gouge development in laboratory see-through experiments

We describe the results of a set of brittle sliding and wear experiments on thin sheets of polycrystalline sodium nitrate. We have built a new brittle deformation apparatus that allowed us to simultaneously observe the microstructural evolution under the microscope, and measure the mechanical response of the material. The microstructural history was recorded using a video microscopy technique and selected frames were then digitized and analysed. Fault gouge in these experiments was the direct product of frictional wear and was produced dominantly by intense intergranular fracturing and accompanying grain surface erosion. Although an increase in displacement was accompanied by an increase in gouge zone width, the development of a gouge zone is not a constant process either with respect to displacement, or with respect to the spatial distribution along the fault zone.

Friction of ice

The frictional strength of ice seems to be well below that for all other rocks. Triaxial testing of cylinders of pure water ice containing a 45° inclined sawcut, at temperatures of 77 ≤ T ≤ 115 K and confining pressures 0.1 ≤ P ≤ 250 MPa, reveals the frictional laws τ = 0.20 σn + 8.3 MPa for P ≥ 10 MPa and τ = 0.55 σn + 1.0 MPa for P ≤ 5 MPa, where τ and σn are the shear and normal stresses on the sawcut surface. Friction is independent of T and, over the one order of magnitude tested, of average sliding velocity. The sliding behavior is invariably stick slip. Examination of sliding surfaces shows only minor gouge development. A curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength. These results apply to models of near‐surface tectonic activity on Jupiter's three large icy moons, Europa, Callisto, and Ganymede. They indicate, for example, that a global expansion on Ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7±3 km.

Duplex geometry: an example from the Moine Thrust Belt

Abstract The geometry and microstructure of a small duplex formed in one bed from the Moine Thrust Belt of northwest Scotland is reported. The structure is seen in oblique section, within the Cambrian Pipe-rock, in an area of low strain. A range of movement direction indicators are present in the structure. An early grain shape fabric developed close to the roof thrust is taken as the best estimate of the overall movement direction towards 287°. Slickensides in the gouge developed on movement planes within the duplex show varied orientations on a given plane, and are not considered useful indicators of thrust transport direction. Branch lines exposed converge and diverge, suggesting little lateral continuity of the exposed structure. The microstructures present within the structure indicate an increase in localised deformation, and in cataclastic behavior as the duplex evolved. Early layer parallel shear is ubiquitous, giving rise to an elongate grain shape fabric close to bedding surfaces. In early formed horses, a layer-parallel, oblate grain shape fabric, which shows localised slip zones, is overprinted by gouge formation. Later formed horses show only fracturing and gouge development. This sequence is attributed to stick-slip behavior in the propagation or displacement of the original fault, now the floor thrust.

Incredible Tale of Texasgulf Well 7 and Fracture Permeability, Paradox Basin, Utah

ABSTRACTNormal faults and associated extended joints form an extremely permeable zone in Pennsylvanian and Permian limestones and arkoscs along the crest of the Cane Creek salt growth anticline of southeastern Utah. These same rocks in unfaulted states comprise an extensive regional confining layer sequence.Fluids in the fault zone are sodium chloride/hydrogen sulfide brines which have bleached all permeable flow paths within the units. Bleaching patterns in outcrops reveal that circulation in the fault zone is largely restricted to the fault planes and nearby extended joints. Orthogonal joints within the fault zone are also bleached but to a lesser extent. In contrast, bleaching of all joints a short distance from the faults is rare. The highly anisotropic permeability distribution observed is characterized by maximum principal permeability tensors oriented parallel to the strike of the faults and minimum principal tensors perpendicular to the fault planes.The productive zone has a transmissivity considerably in excess of 10s gallons/day‐foot parallel to the strike of the faults. Fracture permeability is attributed to minute separations developed along faults and joints as the rocks containing them underwent extension perpendicular to the strike of the anticline. The causative extensional stress regime was imposed on the rocks as they were bent and stretched over the growing core of the anticline.The general conclusion of this article is that extensional tectonic regimes are ideally suited for imprinting fracture permeability on brittle rock sequences. Such fractures include extensional joints and high angle normal faults. Small offsets appear to favor development of porosity because surface irregularities in the fracture planes are not ground off thereby forcing the facing surfaces to separate, and gouge development is minimized.

Petrogenesis of cataclastic rocks within the San Andreas fault zone of Southern California U.S.A.

This paper petrologically characterizes cataclastic rocks derived from four sites within the San Andreas fault zone of southern California. In this area, the fault traverses an extensive plutonic and metamorphic terrane and the principal cataclastic rock formed at these upper crustal levels is unindurated gouge derived from a range of crystalline rocks including diorite, tonalite, granite, aplite, and pegmatite. The mineralogical nature of this gouge is decidedly different from the “clay gouge” reported by Wu (1975) for central California and is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote and oxide mineralogy representing the milled-down equivalent of the original rock. Clay development is minor (less than 4 wt. %) to nonexistent and is exclusively kaolinite. Alterations involve hematitic oxidation, chlorite alteration on biotite and amphibole, and local introduction of calcite. Electron microprobe analysis showed that in general the major minerals were not reequilibrated with the pressure—temperature regime imposed during cataclasis. Petrochemically, the form of cataclasis that we have investigated is largely an isochemical process. Some hydration occurs but the maximum amount is less than 2.2% added H 2O. Study of a 375 m deep core from a tonalite pluton adjacent to the fault showed that for Si, Al, Ti, Fe, Mg, Mn, K, Na, Li, Rb, and Ba, no leaching and/or enrichment occurred. Several samples experienced a depletion in Sr during cataclasis while lesser number had an enrichment of Ca (result of calcite veining). Texturally, the fault gouge is not dominated by clay-size material but consists largely of silt and fine sand-sized particles. An intriguing aspect of our work on the drill core is a general decrease in particulate size with depth (and confining pressure) with the predominate shifting sequentially from fine sand to silt-size material. The original fabric of these rocks is commonly not disrupted during the cataclasis. It is evident that the gouge development in these primarily igneous crystalline terranes is largely an in situ process with minimal mixing of rock types. Fabric analyses reveal that brecciation (shattering), not shearing, is the major deformational mechanism at these upper crustal levels.

Experimental folding of rocks under confining pressure, Part VIII—Forced folding of unconsolidated sand and of lubricated layers of limestone and sandstone

Research Article| May 01, 1980 Experimental folding of rocks under confining pressure, Part VIII—Forced folding of unconsolidated sand and of lubricated layers of limestone and sandstone M. FRIEDMAN; M. FRIEDMAN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar R.H.H. HUGMAN, III; R.H.H. HUGMAN, III 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar J. HANDIN J. HANDIN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar Author and Article Information M. FRIEDMAN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 R.H.H. HUGMAN, III 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 J. HANDIN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1980) 91 (5): 307–312. https://doi.org/10.1130/0016-7606(1980)91<307:EFORUC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. FRIEDMAN, R.H.H. HUGMAN, J. HANDIN; Experimental folding of rocks under confining pressure, Part VIII—Forced folding of unconsolidated sand and of lubricated layers of limestone and sandstone. GSA Bulletin 1980;; 91 (5): 307–312. doi: https://doi.org/10.1130/0016-7606(1980)91<307:EFORUC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Field and laboratory data suggest that variations in structural style are associated with differences in lithologic composition, stratigraphic sequence, and the mechanical behavior of the layers that are drape (forced) folded by differential vertical movements of underlying essentially rigid blocks. This hypothesis is tested by study of experimental, faulted, drape folds in veneers of loose, dry, unconsolidated sand and in multilithologic layered sequences with lubricated interfaces produced under confining pressures to 200 MPa (2 kb) at 25 °C. Deformation of the sand veneer provides a classic example of cataclastic flow. Forced folds develop as a result of microfracturing, rigid-body rotation of grains and fragments, and faulting and gouge development. The sand veneer is thinned drastically in the zone of faulting. The multilithologic, layered veneers with lubricated interfaces exhibit the same magnitudes of “bedding-plane” slip and somewhat more variability in the senses of slip than do specimens not lubricated. With lubrication, however, there is less deformation of the leading edge of the forcing block, less extensile faulting in the upthrown block, and more folding without faulting in the veneer. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effect of anisotropy on the coefficient of sliding friction in Schistose rocks

Foliated schist samples were sawcut at 45° to th cylinder axis and tested at 500 bars confining pressure, 25°C, and 10 −4sec −1 shortening rate, to examine the effect of anisotropy on the coefficient of sliding friction (μ). Cylinders were cored from sample blocks in a variety of orientations to produce sawcut-to-foliation (STF) angles ranging from 0° to 180° in 15° increments. The variation of μ with foliation orientation is sinusoidal. Maximum values of μ occur when the foliation is parallel to principal stress planes ( STF = 30°−45° and 105°−120°; τ = 0), and minimum values of μ correspond to planes of maximum resolved shear stress ( STF = 75° and 165°, τ = τ). Sliding-surface damage and gouge development vary inversely with μ. Surface damage results from the interaction of asperities with the sawcut surface, and failure and cataclasis of asperities produces gouge. Precursor events visible on force-time records, and partial loading tests terminated before megascopic sliding, indicate that surface damage and gouge are generated before megascopic sliding on the sawcut. Apparently, high resolved shear stress produces slip along the foliation prior to sliding on the sawcut. Foliation slip generates gouge at the sawcut interface; and the effect of the gouge is to lower μ.