Distinct Shear Zones Research Articles

Enhancing Shear Strength of Weakly Consolidated Mudstone in Water-Diversion Tunnels Through Artificial Ground-Freezing Techniques: An Experimental and Theoretical Study

The utilization of artificial ground-freezing techniques is increasingly prevalent in the construction of water-diversion tunnels. The inadequate mechanical properties of weakly consolidated mudstone (WCM) pose significant challenges for tunneling construction. In this study, a series of triaxial shear tests were conducted on frozen specimens of WCM to elucidate its shear strength characteristics. The experiment involved four freezing temperatures (0, −5, −10, and −20 °C) and four confining pressures (1, 2, 3, and 4 MPa). The results indicate that the shear failure mode of the WCM exhibits distinct shear zone failure characteristics under artificial freezing conditions, particularly prominent in the lower temperature environments. As the freezing temperature gradually decreases, there is a substantial increase of over 200% in the shear strength of frozen specimens, accompanied by a corresponding decrease in yield strain. Furthermore, the cohesion and internal friction angle of frozen WCM increase as the freezing temperature decreases, following a complex exponential function relationship rather than a linear one. As the freezing temperature decreases from 0 °C to −20 °C, there is an increase in cohesion and internal friction angle from 1012 kPa to 1425 kPa, accompanied by a rise in the internal friction angle from 43.2° to 58.1°. Notably, the application of confining pressure exerts a pronounced influence on the shear strength of frozen WCM, with elevated levels of confining pressure resulting in a substantial augmentation of the shear strength. The failure mode of frozen WCM is significantly influenced by freezing temperatures. At low temperatures, the specimen of mudstone exhibits a shear failure behavior, while at high temperatures, it predominantly demonstrates expansion failure. This phenomenon can be attributed to the increased brittleness of specimens caused by ice crystals, rendering it more susceptible to brittle failure under shearing forces. These findings signify an enhancement in the mechanical behavior of WCM within the tunnel sidewall under artificial freezing conditions.

Material point method simulations of displacement pile and CPT penetration in sand considering the effects of grain breakage

Full displacement installation dramatically alters the stress and strain fields applying around piles and Cone Penetrometer (CPT) probes at sand sites. Calibration chamber tests have indicated that severe particle crushing occurs near to the pile or cone tips and a distinct shear zone may develop that leaves crushed sand adhering to the pile or cone shaft. It is therefore important to investigate how particle breakage affects the stress and deformation fields developed as this process occurs. In this study, installation by monotonic jacking of conically tipped model piles or CPT probes into silica sand is simulated by material point method (MPM) analyses. A thermodynamically consistent model which considers competing grain crushing and dilation is employed to model the sand behavior, while an MPM approach is adopted to cater for the large deformations involved in the penetration process. Particle crushing is predicted beneath the pile tip and around the pile shaft. The numerical results are compared to directly related instrumented pile calibration chamber tests. The numerical results show that the computed stress and displacement fields match key aspects of the experimental results well, especially in reproducing the volumetric compression observed experimentally. The results confirm that combining MPM with suitably sophisticated constitutive models creates new capabilities for analyzing large-deformation problems.

Quasi-static compressive mechanical properties of multilayer micro-lattice biomaterials for skull repair

Current skull repair biomaterials used in clinical surgery face problems such as unmatched mechanical properties and poor biocompatibility. To overcome these problems, the multilayer micro-lattice biomaterials (MB) combining both suitable mechanical properties and biocompatibility are proposed based on skull structural characteristics. The quasi-static compressive mechanical properties of MB are studied experimentally, numerically and theoretically, which are verified by experimental results. The typical deformations, such as distinct shear zone and stress concentration of nodes, are observed. The strength and modulus of the above MB specimens are in the range of 86.72 ± 0.84 to 197.73 ± 0.74 MPa and 2.99 ± 0.13 to 7.56 ± 0.54 GPa, respectively. Simultaneously, the properties of MB with gradient design, including positive, negative and hybrid gradient are investigated by finite element (FE) simulation. The MB with hybrid gradient can better match the structural characteristics of skull. Since the designed MB has out-of-plane compression characteristics comparable to that of skull and suitable biological space for cell growth, it can be implanted into the human body to matched surrounding skull tissue well. The insight of MB combining with design constraints of biomaterials provides a novel method for designing/tuning skull repair biomaterials that might result in the optimized clinical skull surgery effect.

Early Neoproterozoic Deformation Kinematics in the Chottanagpur Gneiss Complex (Eastern India): Evidence from the Curvilinear Hundru Falls Shear ZoneAnalysis

Abstract Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement-piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.

Tracing the exhumation of the Eclogite Zone (Tauern Window, Eastern Alps) by 40Ar/39Ar dating of white mica in eclogites

New radiometric ages from the Subpenninic nappes (Eclogite Zone and Rote Wand — Modereck Nappe, Tauern Window) show that phengites formed under eclogite-facies metamorphic conditions retain their initial isotopic signature, even when associated lithologies were overprinted by greenschist- to amphibolite-facies metamorphism. Different stages of the eclogite-facies evolution can be dated provided 40Ar/39Ar dating is combined with micro-structural analyses. An age of 39 Ma from the Rote Wand — Modereck Nappe is in terpreted to be close to the burial age of this unit. Eclogite deformation within the Eclogite Zone started at the pressure peak along distinct shear zones, and prevailed along the exhumation path. An age of ca. 38 Ma is only observed for eclogites not affected by subsequent deformation and is interpreted as maximum age due to the possible influence of homogenously distributed excess argon. During exhumation deformation was localised along distinct mylonitic shear zones. This stage is mainly characterised by the formation of dynamically recrystallized omphacite 2 and phengite. Deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica. Flat argon release spectra showing ages of 32 Ma within mylonites record the timing of cooling along the exhumation path, and the emplacement onto the Venediger Nappe. Ar-release patterns and 36Ar/40Ar vs. 39Ar/40Ar isotope correlation analyses indicate no significant 40Ar-loss after initial closure, and only a negligible incorporation of excess argon. From the pressure peak onwards, eclogitic conditions prevailed for almost 8–10 Ma.

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

Comparison of structure development in injection molding of isotactic and syndiotactic polypropylenes

AbstractA comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X‐ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well‐defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α‐form and the hexagonal β‐form. Considerable amounts of β‐form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen‐in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold‐filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers.

Borehole deformation measurements and internal structure of some rock glaciers in Switzerland

AbstractIn order to understand the mechanical processes that influence the deformation patterns of active rock glaciers, information about local horizontal and vertical deformations as well as knowledge of the internal structure and the temperature distribution is necessary. Results from borehole deformation measurements of three sites in the Swiss Alps show that despite different internal structures, similar phenomena can be observed. In contrast to temperate glaciers, permafrost within rock glaciers has distinct shear zones where horizontal and vertical differential movements are concentrated. In addition, a reduction in volume can be caused by compressive flow due to the presence of air voids within the permafrost. The flow velocity depends on the temperature, the surface and bedrock slopes of the rock glacier, and the composition of the ice‐rich frozen ground. Within degrading permafrost, the ice content decreases, the creep velocity increases and the shear zone rises towards the surface, where seasonal temperature changes and the presence of liquid water might also influence deformation. Copyright © 2002 John Wiley & Sons, Ltd.

DEM Simulation of Particle Damage in Granular Media — Structure Interfaces

Using the discrete element method (DEM) with clustering, a novel means of numerically modeling damage of particles is presented. Damage, such as grain crushing, is treated by allowing clusters to break apart according to a failure criterion based upon sliding work. If the accumulated work done on an individual DEM particle of a cluster exceeds a threshold, that particle is allowed to break from the cluster. A value for the critical energy density is determined by comparing the degree of particle breakage from numerical simulations to data from laboratory tests. Numerical simulations were also conducted to determine the impact of particle damage on interface behavior. It was found that a very distinct shear zone was evident when particle damage was considered and that this occurred without significant reduction of the maximum shear strength of the medium. Also, the degree of damage was shown to be related to the angularity of the clusters.

Microstructural and textural development of calcite marbles during polyphase deformation of Penninic units within the Tauern Window (Eastern Alps)

The evolution of calcite microstructures and crystallographic preferred orientations (CPOs) is well understood due to well constrained experimental studies. However, the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths, a multiphase deformation history, and variable P– T conditions. The Penninic units within the Tauern Window (Eastern Alps) have been affected by several deformation events and metamorphic overprint. Generally, three major deformational events can be distinguished. D 1 is related to underthrusting of Penninic units beneath the Austroalpine nappe complex, and top-to-the-N nappe stacking within the Penninic continental units. Deformation stage D 2 is interpreted as reflecting the subsequent continent collision between the Penninic continental units and the European foreland. D 3 is related to the formation of the dome structure of the Tauern Window. This polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and CPOs. Three types of calcite-fabrics are distinguished within the Penninic units of the Tauern Window and the Lower Austroalpine unit. D 1-fabrics are characterized by equilibrated microstructures and LT-CPOs. The CPOs are generally strong and symmetric, with one well developed cluster near the Z-axis of the finite strain ellipsoid. These fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism. Generally, the occurrence of LT-fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central part of the Tauern Window, while HT-fabrics have been observed outside this area. Fabrics from the central part of the Tauern Window have likely been strengthened during subsequent thermal equilibration, while the fabrics that have been observed at the peripheral parts have been less affected by subsequent metamorphic overprint. Therefore, D 1-fabrics do not reflect D 1 conditions, but subsequent thermal equilibration. Similar observations have been made for the evolution of D 2-fabrics. LT-fabrics dominate inside the amphibolite facies isograde, HT-fabrics occur outside (greenschist facies metamorphic conditions). The fabrics are characterized by high finite strains near the margins of the Tauern Window, the intensity of which decreases towards the central parts, where microstructures are characterized by recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint. The strong CPOs document this influence. The HT-fabrics and microstructures within peripheral areas indicate that they have less been affected by this thermal event, and, therefore, are more indicative for the deformational conditions during D 2. D 3 is restricted to distinct shear zones along the tectonic boundaries of the Tauern Window. From the central parts to the shear zone boundaries, a clear evolution of microfabrics can be observed. In internal parts, microstructures are characteristic for intracrystalline plasticity with a dominating activity of r −-glide. Twinning was less important during the final phases of deformation. On approaching the shear zone boundaries, the grain size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed. Within these domains grain boundary sliding seems to have been dominant. In conclusion, calcite CPOs from polyphase areas do not only include information on the deformation conditions, but also bear information about the thermal overprint subsequent to the main deformational event.

Polyphase wrench tectonics in the southern french Massif Central: kinematic inferences from pre- and syntectonic granitoids

In the Variscan French Massif Central, the South Limousin area consists of low- to medium-grade metamorphic rocks intruded by two granitic bodies. The structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area. This study shows that the South Limousin area experienced successivelly two strike-slip events along two geographically distinct shear zones, from north to south the left-lateral Estivaux and the right-lateral South Limousin strike-slip faults, respectively. These ductile faults subdivide the South Limousin into three structural units, from north to south they are the Upper Gneiss unit, Thiviers-Payzac unit .and Genis unit. The two granitic bodies intrude the Thiviers-Payzac unit only. The younger Estivaux granite is a syntectonic pluton which emplaced during left-lateral wrenching. 40Ar/39Ar dates from biotites indicate an Early Carboniferous age (346 ± 3 Ma). The older granite is a pretectonic body. It is the Ordovician “Saut du Saumon” augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid-state deformation. Our microtectonic data indicate that the right-lateral motions overprint the left-lateral ones and produce apparently symmetrical fabrics.

Observed performance of a deep excavation in clay : J Geotech Engng Div ASCE V115, N8, Aug 1989, P1045–1064

A test section was established next to a 40-ft-deep, braced excavation made through soft to medium-stiff, saturated clays in Chicago. Surface and subsurface three-dimensional ground movements, pore water pressures, sheet pile deformations and strut loads were measured. Results of these observations were correlated with construction activities at the test section. Larger than expected ground-surface settlements adjacent to the excavation were observed. The large movements occurred as a result of overexcavation during construction. The largest incremental ground movements occurred when the excavation was approximately half completed. Base stability computations indicated that the factor of safety against basal heave at that point in construction was 1.1. Two distinct shear zones developed in the soil mass; their initiation corresponded to the times when the largest incremental movements occurred. Soil displacements were always directed towards the excavation with magnitudes, at later stages of construction, larger than those measured on the sheet pile. Pore water response was markedly influenced by sheet pile installation and strut preloading; as a result, little net change in pore pressures was observed at the end of construction. Magnitudes of measured strut loads were within the levels expected based on standard design procedures.

Observed Performance of a Deep Excavation in Clay

A test section was established next to a 40-ft-deep, braced excavation made through soft to medium-stiff, saturated clays in Chicago. Surface and subsurface three-dimensional ground movements, pore water pressures, sheet pile deformations and strut loads were measured. Results of these observations were correlated with construction activities at the test section. Larger than expected ground-surface settlements adjacent to the excavation were observed. The large movements occurred as a result of overexcavation during construction. The largest incremental ground movements occurred when the excavation was approximately half completed. Base stability computations indicated that the factor of safety against basal heave at that point in construction was 1.1. Two distinct shear zones developed in the soil mass; their initiation corresponded to the times when the largest incremental movements occurred. Soil displacements were always directed towards the excavation with magnitudes, at later stages of construction, larger than those measured on the sheet pile. Pore water response was markedly influenced by sheet pile installation and strut preloading; as a result, little net change in pore pressures was observed at the end of construction. Magnitudes of measured strut loads were within the levels expected based on standard design procedures.

Analysis of underground openings with a distinct shear zone

Analysis of underground openings with a distinct shear zone

Analysis of underground openings with a distinct shear zone

for the analysis of deformations and stresses in the surrounding of an underground opening is demonstrated. For duplication of rock discontinuities in the finite element analysis the joint element of Goodman, Taylor and Brekke is used. A special technique was developed in order to consider the limited tensile strength, the reduction of shearing strength after reaching the peak strength and the limited closure of the joint. A Coulomb failure criterion is used for determining shear fracture. Results of two examples of tunnels with one distinct joint are shown. Berechnung yon Felshohlrdumen mit einer ausgeprdgten Scherzone. Ein Verfahren zur Berechnung der Verformungen und Spannungen in dem einen Hohlraum umschliet~enden Gebirge wird erl~iutert. Zur Nachbildung yon Diskontinuit~iten wird das yon Goodman, Taylor und Brekke entwickelte Kluftelement verwendet. Ffir dieses linienf6rmige Element wurde eine spezielle Reehentechnik entwickelt, die die begrenzte Zugfestigkeit der Kluft, den Abfall der Scherfestigkeit nach Erreichen der Spitzenfestigkeit auf die Restscherfestigkeit und die begrenzte Zusammendrfickbarkeit erfaf~t. Zur Bestimmung des Schubbruches wird das Coulombsche Bruchkriterium benutzt. Berechnungsergebnisse yon zwei Beispielen aus dem Tunnelbau mit einer diskret nachgebildeten Grof~kluft werden gezeigt.