Shear Failure Zone Research Articles

Ultimate bearing capacity analysis of strip footings on reinforced soil foundation

Reinforced soil foundations (RSFs) have been employed in engineering practice to increase the soil bearing capacity and to reduce the potential footing settlement. The aim of this study is to develop analytical solutions for estimating the ultimate bearing capacity of strip footings on RSFs. A general failure mode for RSFs was first proposed based on previous studies conducted by the authors and test results from literature study. A limit equilibrium stability analysis of RSFs was performed based on the proposed failure mechanism. New bearing capacity formulas, which consider both the confinement and the membrane effects of reinforcements on the increase in ultimate bearing capacity, were then developed for strip footings on RSFs. Several special cases of RSFs were presented and discussed. The proposed model was verified by the experimental data reported in the published literature. The predicted ultimate bearing capacity was in good agreement with the results of model tests reported in the literature. The study showed that the depth of the punching shear failure zone (DP) depends on the relative strength of the reinforced soil layer and the underlying unreinforced soil layer, and is directly related to the reinforced ratio (Rr).

동축케이블과 시간영역반사법을 이용한 암반사면 전단변위 계측

본 연구에서는 동축케이블과 시간영역반사법(Time Domain Reflectometry, TDR)을 이용하여 실내실험을 수행하였으며, 전단변위에 따른 상대전압의 변화를 분석하여 전단변위와 상대전압차의 상관관계를 도출하였다. 전단변위에 따른 상대전압차를 이용하여 회귀분석을 실시한 결과, 전단변위에 대한 상대전압차가 2차 다항식의 관계를 가짐을 알 수 있었다. 또한 실제 사면에 TDR 기법을 적용하여 사면의 경사변화를 측정하였으며, 지반조사 자료를 통한 실제 사면의 전단파괴 영역과 현장에서 측정된 전단파괴 영역을 비교 분석하였다. 사면의 거동 변화를 측정하기 위하여 TDR 기법과 지중경사계를 설치하여 현장 계측을 실시한 결과, TDR 기법을 이용하여 지중경사계의 변위량과 비슷한 수준의 변위를 측정할 수 있었으며, 지중경사계의 경우 정확한 변위발생지점을 찾기 어려운 반면에 TDR기법을 이용하여 측정한 결과 정확한 변위발생 위치를 확인할 수 있었다. Through experiment using coaxial cable and Time Domain Reflectometry(TDR), the changes in relative voltage according to the shearing displacement were analyzed to derive correlation between the shearing displacement and relative voltage difference. As the result, it is found that relationship of shear displacements and relative voltage difference is polynomial. Also, TDR method was applied to real slope to measure the elapsed time, and shear failure zone of real slope through ground investigation data and those measured in the field was compared and analyzed. To measure the change of slope behavior, TDR method and inclinometer were installed. As a result, similar displacement to that of inclinometer could be measured using TDR method. In case of inclinometer, it was difficult to find accurate point of displacement while using TDR method, accurate displacement point could be found.

A Comparison of Mechanical Behavior and Frequency-Energy Relations for Two Kinds of Echelon Fault Structures Through Numerical Simulation

Heterogeneities of faults and rock blocks were included in the present numerical simulation to study differences between the compressive and extensional echelon fault structures. Released elastic strain energy due to shear and tensile failures of elements was calculated. An event is defined as a failed element releasing energy. For calculation of the frequency-energy relation, whose negative slope was labled b 0 value, a cut-off factor, used to delete a few larger events, and a classification factor, used to group the remaining events into several grades, were introduced. The evolution of b 0 value at different stages was obtained. Three kinds of changes in b 0 value were found. In the compressive echelon fault structure, no remarkable change in b 0 value is observed during the outward propagation of tensile failure zones, also during the jog failure in the extensional fault structure. This is due to the fact that tensile failure only increases the number of small events that are originally larger. In the compressive echelon fault structure, a rapid decrease of b 0 value is found during the jog failure since a shear failure zone links two fault tips, leading to the occurrence of larger events. For both kinds of echelon fault structures, b 0 value is found to decrease continuously during the fault reactivation. This is related to stress or strain enhancement over a larger area. In the extensional fault structure, a recovery in b 0 value in the vicinity of the stress peak is found after a global minimum, which is similar to phenomena caused by aftershocks. Moreover, evolution of b 0 value at different stages for the same echelon fault structure and that at the same stage for different echelon fault structures are compared and analyzed. It is found that the change in fault structure may not lead to the change in b 0 value. To obtain a more continuous and less fluctuant b 0 value curve, the suggested values of the cut-off and classification factors are 1/8 ~ 1/2 and 3 ~ 4, respectively. The proposed method can also be used to analyze the mechanical behavior and frequency-energy relations for other typical or complex fault structures. Our work can contribute to identify the regions where abnormalities or precursors can be monitored and to assess the positions of larger events.

Impact toughness and fracture analysis of linear friction welded Ti–6Al–4V alloy joints

Linear friction welding (LFW), as a relatively new solid-state joining technique, has great potentials in welding of non-axisymmetric components, especially for cost-effectively machining blade/disc (blisks) assemblies. In this study, Ti–6Al–4V alloy was jointed by the LFW process under the appropriate processing parameters developed before. The microstructure, impact toughness and fracture characteristics of LFW Ti–6Al–4V joint were investigated. The results showed that a sound weld was obtained consisting of a superfine α + β microstructure in the weld center (about 70 μm thickness). The weld presents a higher impact toughness (61.3 ± 5.8 J/cm 2) than the parent Ti–6Al–4V because of the superfine microstructure formed in the weld. The fracture surface exhibits three typical regions: the thin fibrous zone close to the notch, the radiation zone in the middle and the shear lip zone at the other three sides, corresponding to the crack initiation, propagation and shear failure zones, respectively. The crack develops a short distance along the weld center and thermomechanically affected zone after its initiation, and then extends into the parent metal due to the lowest impact toughness of the parent.

Earth Pressure due to Vibratory Compaction

This paper presents experimental data on the variation of lateral earth pressure against a nonyielding retaining wall due to soil filling and vibratory compaction. Air-dry Ottawa sand was placed in five lifts and each lift was compacted to achieve a relative density of 75%. Each compacted lift was 0.3 m thick. The instrumented nonyielding wall facility at National Chiao Tung University in Taiwan was used to investigate the effects of vibratory compaction on the change of stresses at the soil-wall interface. Based on the experimental data it has been found that, for a compacted backfill, the vertical overburden pressure can also be properly estimated with the traditional equation v=z. The effects of vibratory compaction on the vertical pressure in the backfill were insignificant. On the vertical nonyielding wall, extra horizontal earth pressure was induced by vibratory compaction. After compaction, the lateral earth pressure measured near the top of the wall was almost identical to the passive Rankine pressure. It is concluded that as the cyclic compacting stress applied on the surface of the backfill exceeded the ultimate bearing capacity of the foundation soil, a shear failure zone would develop in the uppermost layer of the backfill. For a soil element under lateral compression, the vertical overburden pressure remained unchanged, and the horizontal stress increased to the Rankine passive pressure. It was also found that the compaction-influenced zone rose with the rising compaction surface. The horizontal earth pressure measured below the compaction-influenced zone converged to the Jaky state of stress.

Cohesive strengthening of fault zones during the interseismic period: An experimental study

There is widespread evidence indicating that faults regain a portion of their strength during the interseismic period. Here, we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid‐rock reactions in fault zones. The triaxial experiments consisted of fracturing cores of Fontainebleau sandstone under dry conditions, forming a localized shear failure zone (stage 1). The specimens were then reacted hydrothermally under isostatic conditions, allowing the fault damage zone to compact, consolidate and strengthen (stage 2). Following reaction, the specimens were then reloaded to failure under nominally dry conditions, so that the increase in cohesive strength of the fault could be measured (stage 3). Experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction. After 6 hours of reaction at the highest temperatures (927°C) and pore pressures (200 MPa), cohesion increases by as much as 35 MPa. Microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together, exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed. A theoretical treatment of the data was conducted using these experiments in combination with results on time‐dependent changes in fault cohesion presented by Tenthorey et al. (2003). We find that the rate‐controlling process in our experiments has an activation energy (Q) of approximately 70 kJ mol−1. We use this information to develop a model for time‐dependent cohesive strengthening in fault zones within the continental seismogenic regime. We conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids.

A physical and numerical modelling investigation of the roadway stability in longwall mining, with and without narrow pillar protection

The drivage of the supply gate of a development panel in the presumed low stress zone adjacent to the loader gate of an excavated panel with a very small width of intervening pillar between the two entries, known as skin-to-skin working, resulted in an extensive fall of the supply gate roof at a UK coal mine, Bilsthorpe colliery in 1993. This fall left a question over the application of skin-to-skin drivage of gate entries supported primarily by rock bolts. To investigate the possible reasons behind this fall, both physical and numerical modelling studies were carried out. Physical and numerical models were successful in demonstrating the potential danger of the working method with the rock bolt support system employed. Development of a shear failure plane from the rib edge into roof strata of the loader gate and development of the second shear failure plane at the abutment side of the supply gate exposed the supply gate to the fall of large rock blocks released by shear failure zones. Models demonstrated that the fall of the supply gate roof was not due to the inability of the rock bolts to prevent bed separation, but it was the method of working that made the bolts ineffective due to the height of the block delineated by the shear failure planes. Further physical and numerical models were undertaken to investigate the influence of 5, 7·5 and 10 m wide intervening pillars between the entries on the stability of the rock bolt supported supply gate roof. These studies showed that a 7·5 m or 10 m wide pillar would have prevented development of failure zones and fall of the roof while a 5 m pillar was found to be an undersized pillar width causing the development of extensive failure zones in the roof.

Finite element simulation of straight plunge grinding for advanced ceramics

The objective of this work is to model the grinding forces and the associated stress and deformation fields generated in a ceramic workpiece during plunge surface grinding. A two-dimensional finite element model is constructed with the grinding parameters and the mechanical properties of the workpiece as input variables. The size of the geometric model is several times larger than the size of the cutting zone, using approximately 5200 rectangular solid elements with a finer mesh in the cutting zone and with fixed remote boundaries. The loading in the cutting zone is imposed by displacement vectors proportional to the local undeformed chip thickness, which is a function of grinding parameters. For a given set of inputs, the model predicts the normal and tangential forces generated by the grinding wheel, as well as the deformation and the stress fields within the workpiece. As an example, the simulation is applied to a silicon nitride workpiece. Analysis of the stress fields developed in this material suggests that shear failure within the cutting zone is the dominant mode of subsurface failure, which could lead to the formation of shear micro- cracks at the grain interfaces. The depth of the subsurface shear failure zone increases with an increase in maximum undeformed chip thickness or the wheel depth of cut. The resulting local grinding force vectors, maximum stresses and damage zone sizes are predicted as a function of maximum undeformed chip thickness (or the wheel depth of cut).

Wave-Induced Seabed Instability: Difference Between Liquefaction and Shear Failure

The mechanism of wave-induced instability in a permeable seabed have been studied for more than two decades. The distinction between shear failure and liquefaction, however, has not been clearly defined. This paper presents a fundamental study on the differences in two failure modes for a fully saturated seabed of both finite and infinite thickness. The wave-induced effective stresses and pore pressure, obtained from an analytical solution of Biot’s pore-elastic consolidation theory, were employed to examine the failure modes under a two-dimensional plane strain condition. A case study is presented to examine the failure modes with respect to several parameters, such as excess pore pressure, seepage flow, seepage force, failure areas and stress path in the seabed. The conclusions obtained from this study were as follows; (1) the thickness of a permeable seabed affects the pore pressure and effective stress response to ocean waves and the failure mode of the seabed, (2) either a liquefaction or shear failure, or both, occur in the seabed, even in the saturated seabed, (3) the Mohr-Coulomb’s failure criterion, when combined with elastic stresses, can not be employed to estimate the liquefaction failure in the seabed, (4) the liquefaction can be evaluated by a criterion in terms of the excess pore pressure, (5) The liquefied zone in the seabed is significantly different from the shear failure zone. The shape beneath the seabed surface for the former is almost identical to the contour where the upward seepage flow is concentrated.

The Effect of Shear Failure on the Cyclic Steam Process and New Pseudo Functions for Reservoir Simulation

Abstract The steam stimulation process is a proven, commercially viable recoveryprocess in oil sand reservoirs. However, an understanding of the recoverymechanism is far from complete. Recent studies of the geotechnical aspects oftar sands point to the importance of shear failure created by pore pressureincrease on fluid flow in the reservoir during steam injection. The changes inmass and heat transport properties in the heterogeneously deformed failure zonehave been shown to be important factors for the recovery mechanism. This paperdescribes the theoretical and practical aspects of representing the shearfailure zone in reservoir simulation, by the means of pseudo relativepermeability functions. Relative permeability hysteresis is commonly used in the simulation of thesteam stimulation process, but its mechanism is not clear. The paper reviewsthe existing methods for treating hysteresis and gives the theory for thepseudo relative permeabilities caused by heterogeneously failed media. The newfunctions are derived by considering the shear failure phenomenon. A new set ofstress-dependent pseudo relative permeabilities is proposed and a practicalmethod for the use of the pseudo functions (including hysteresis) in areservoir simulator is given. The method applies to injection as well asproduction cycles. Its effect on simulating the cyclic steam process is givenin examples and shows that the hysteresis should be stress-dependent ratherthan saturation-dependent. Introduction When the same set of water-oil relative permeability data is used tosimulate the injection and production periods of a cyclic steam stimulationprocess in tar sands or a heavy oil reservoir, the simulated water-oil ratiosare usually much greater than those observed in the field. To achieve a bettermatch of field data in the reservoir simulator, a higher relative permeabilityto water is used during the injection period than that used during theproduction period. The use of this technique in simulating a cyclic steamstimulation process is reported in a number of papers. It is based on theresults of laboratory studies on the hysteresis phenomenon of relativepermeability which will be briefly reviewed.

Continuous Monitoring of Metamorphic Changes of Internal Snow Structure as a Tool in Avalanche Studies

AbstractRelationships between snow structure, climatic factors, and the nature of avalanche events in the San Juan Mountains of Colorado, U.S.A., are analyzed. Physical and mechanical properties of the snow cover have been monitored on a continuous basis by conventional methods as well as through the application of a profiling snow-density gauge. Temperature-gradient metamorphism is the dominant mechanism which determines the character of the local snow structure. Properties unique to well-developed temperature-gradient snow (depth hoar) are discussed. Three specific metamorphic processes are described which contribute to a highly varied and mechanically weak snow structure. Analysis of fracture-line profile data allow general studies of the local snow climate to be compared with the stratigraphy of actual release zones. Fracture-line data support the conclusion that meteorological conditions prevailing at the time of new-snow deposition play a less significant role in the eventual release of slab avalanches than metamorphic processes which influence the snow cover between storm periods. Shear failure zones are consistently composed of weak layers metamorphosed weeks and even months prior to the precipitation event which triggers the release.

The Breakage and Stress Waves Caused by Detonative Loading

The dynamic stresses in the materials having various assumed properties caused by detonative loading are computed by using the numerical technique which involves the finite difference in approximation to the equation of motion in the Lagrangian coordinate system, and then, the patterns of the breakage around the explosion, and the behaviour of the stress waves projected into the materials are discussed by referring the results to the above mentioned stress analysis.Concerning the pattern of the breakage around the spherical explosion, it is concluded that the yield zone or shear failure zone is produced in close proximity to an explosion, and this zone is surrounded by the tensile fracture zone consisting of the radial cracks. Finally, the patterns of the breakage of the mild steel and the plexiglass (PMMA) subjected by detonative loading are presented, and it is pointed out that these are explained fairly well by the results of the above mentioned numerical analysis.