Shear sense

Investigation of microstructural relationships in major movement zones in metamorphic rocks, where the sense of displacement is known from regional geological relationships, indicates numerous problems with current concepts of shear-sense criteria and their application. The direction of apparent shearing commonly conflicts from one criterion to another (e.g. from the symmetry of quartz c -axis orientation diagrams to the asymmetry of extensional crenulation cleavages). This implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous.
A new approach to interpreting shear sense, involving the use of strain fields, resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear-sense histories extending back before the last shearing event for 'any foliated metamorphic rock'. It also provides a powerful tool for determining the structural/metamorphic path that a rock has followed within an orogen. For determination of the shear sense on the last foliation developed in a rock, this approach uses geometries developed around competent heterogeneities such as quartz pebbles, pegmatite pods, veins, porphyroclasts, porphyroblasts and breccia clasts. A shear-sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows.     

大连滨海粉质黏土剪切力学特性环剪试验

以大连滨海地区典型粉质黏土为研究对象,利用大型高速环剪仪,针对不同法向应力和剪切速率条件下该粉质黏土大剪切力学性质的变化情况进行了试验。环剪试验结果显示：1）该滨海粉质黏土在正常固结状态下出现明显的应变软化现象,分析表明其应变软化特性主要与土中黏土矿物质量分数及其在剪切过程中的定向排列有关。2）在相同剪切速率下,峰值强度和残余强度随法向应力的增加而增大,峰值强度与法向应力之间表现出良好的线性关系。3）由于峰值强度产生过程中土体内部黏聚力的变化,峰值强度随剪切速率的增加而增大;残余强度变化与剪切速率具有一定的相关性,这与不同剪切速率下剪切带（面）处黏土颗粒定向排列程度不同有关。     

用扭剪试验成果求剪切模量的计算公式

土的剪切模量是土工计算重要参数之一，通常是用弹性模量与们松比间接求得。本文结合扭转翼板仪的工作原理及其边界条件，推出用扭转力矩M和扭转剪切角θ直接求取剪切模量的计算公式。     

非饱和土的剪切强度

首先回顾了非饱和土剪切强度的研究现状,接着在控制吸力非饱和土三轴剪切试验的基础上提出了非饱和土剪切强度的双曲模型,它可预测不同吸力对强度的贡献———τｓ ,预测结果令人满意,具有很好的实用性。     

鲁西青邑韧性剪切带运动学涡度及剪切作用类型

青邑韧性剪切带是晚太古代末期发育在鲁西前寒武纪基底花岗岩中一条规模较大的韧性剪切带。剪切带NW走向,面理直立,线理水平,剪切标志反映右行剪切。石英光轴法求得运动学涡度在0．96～0．99之间变化,极摩尔圆法求得糜棱岩化岩石运动学涡度为0．91,初糜棱岩运动学涡度为0．87,糜棱岩运动学涡度为0．81,超糜棱岩运动学涡度为0．60。运动学涡度表明,剪切带剪切作用类型为一般剪切,变形初期以单剪为主,随应变的增大,运动学涡度值逐渐减小,变形的纯剪分量不断增加,最后以纯剪为主。剪切作用类型及三维参照变形分析表明,青邑韧性剪切带属加长一变宽类型的一般剪切带并且在Y轴方向上有所增长。韧性剪切在太古代末期克拉通化过程中具有加厚陆壳的作用。     

Undrained Shear Strength in Cohesive Soils Estimated by Directional Modes of In-Situ Shear Wave Velocity

The estimated undrained shear strength (s_u) is often not a unique value because it can be evaluated by various test types and/or procedures, such as different failure modes, shear strain rates, and boundary conditions. This study explores (1) the relationship between reference undrained shear strength and in situ shear wave velocity in terms of the effective overburden stress, and (2) the independent relationships to evaluate the undrained shear strength with special consideration of different directional and polarization modes (VH, HV, HH shear waves), which has not been reported. This evaluation is done via a worldwide database compiled from 43 well-documented geotechnical test sites associated with soft ground. Finally, new correlation models are proposed to estimate the undrained shear strength based on the in situ shear wave velocity as well as the plasticity index or the overconsolidation ratio. The application of the shear wave velocity–undrained shear strength relation is illustrated through two independent case studies. The proposed relationships are expected to contribute to reasonable estimates of undrained shear strength as well as offer practical guidance on even extrapolation beyond the data that is available to geotechnical engineers.     

Small-Strain Shear Modulus of Cement-Admixed Kaolinite

An experimental study is conducted to measure small-strain shear modulus of clay-cement mixture using bender element apparatus setup in a triaxial cell. Bender element tests were conducted on cement-treated soils and the results were analyzed to study the variation of shear modulus properties of soil specimens at different cement contents, confining pressures, curing times, and compaction moisture contents. Based on the obtained results increasing the cement ratio has a significant effect on the small-strain shear modulus of the treated soils, and this effect signifies with increasing the moisture content and curing time. Rates of shear modulus enhancements due to cement content, curing time, and compaction moisture content are quantified and presented. In this study, a clay–cement–water ratio formulation is proposed that enables one to calculate cement and water contents required to obtain specific small-strain shear modulus.     

颗粒材料的剪胀模型

本文研究了颗粒材料的剪切变形机制,建立了颗粒体在变形过程中应力与组构量的相互关系,论证了产生剪胀的微观组构条件,推导出各向异性组构颗粒体剪胀方程。做了相应的单剪试验,理论结果与试验结果相当吻合,从而验证了理论模型的正确性。     

Shear resistance of fissured Neogene clays

Two types of fissured Neogene clays (SM and WB) were tested in the laboratory using undisturbed and reconstituted specimens. Although of similar age, the clays differ in their geological history: SM clays were deposited in a marine environment, WB are lacustrine clays with a complex tectonic history.

The index properties of both clays are similar and, accordingly, their strength parameters are mutually close. Test results of the more compact and less wet clay (WB) are affected by the sampling and trimming procedures, especially for 38 mm diameter specimens.

Although the behaviour is similar, the origin of fissures is different. For the SM clays, weathering is the main cause and fissuring reaches a depth of about 20 m. WB clays are dissected by macro- and microfissures originating as the result of tectonic processes and volumetric changes (effect of the coal formation underneath the investigated massif).

Cementation of some samples was found and the transition from clays through cemented clays to claystones (soft rocks) is speculated.     

Shear behaviour of cemented pastefill-rock interfaces

The shear stress–strain behaviour and shear strength parameters of the interface between cemented paste backfill (CPB) and rock are of practical importance in the optimal and safe design of CPB structures. An understanding of the shear behaviour and properties at this interface is also required to develop comprehensive interface models for CPB-rock analyses, interface design methods for the static and dynamic stability analysis of CPB structures, and building high performance CPB structures. In this study, direct shear tests were conducted to investigate the interface shear strength behaviour between CPB and rock. All tests were carried out in a standard direct shear test apparatus for a range of curing ages of 1 to 28 days for the CPB. The procedures of the laboratory tests will be described. Results will be presented for interface shear behaviour, including stress–strain curves, vertical deformation and shear strength parameters. The test results show that the shear strength parameters and behaviour of the CPB-rock interface are time-dependent and significantly influenced by the normal load.     

A Laboratory Shear Cell Used for Simulation of Shear Strength and Asperity Degradation of Rough Rock Fractures

Different failure modes during fracture shearing have been introduced including dilation, sliding, asperity cut-off and degradation. Several laboratory studies have reported the complexity of these failure modes during shear tests performed under either constant normal load (CNL) or constant normal stiffness (CNS) conditions. This paper is concerned with the mechanical behaviour of synthetic fractures during direct shear tests using a modified shear cell and related numerical simulation studies. The modifications made to an existing true triaxial stress cell (TTSC) in order to use it for performing shear tests under CNL conditions are presented. The large loading capacity and the use of accurate hydraulic pumps capable of applying a constant shear velocity are the main elements of this cell. Synthetic mortar specimens with different fracture surface geometries are tested to study the failure modes, including fracture sliding, asperity degradation, and to understand failure during shearing. A bonded particle model of the direct shear test with the PFC2D particle flow code is used to mimic the tests performed. The results of a number of tests are presented and compared with PFC2D simulations. The satisfactory results obtained both qualitatively and quantitatively are discussed.     

Shear zones in granular material

Summary Shear Zones in Granular Materials. An Experimental Study of Their Structure and Mechanical GenesisA special ring-shear apparatus was designed to study in granular materials under continued shearing the development of shear zones and the accompanying changes in texture and stress state.Various series of experiments were run with densely packed, cohesionless materials in both dry and fluid-saturated states. For comparison, in a few tests also moderately cohesive materials were used. Materials and vertical load were chosen to either allow or suppress shear dilatancy, the latter case being characterised by severe grain crushing (cataclasis) in the shear zone proper. While growth and texture of dilating and non-dilating shear zones differ quite systematically, the build-up of stresses is quite similar in both cases, in particular with respect to the peak in shear stress.In many experiments stress measurements were carried out inside the granular pack, mainly by inserting small photoelastic cylinders, to establish the orientation of the maximum compressive stress in the vicinity of the shear band. The results leave little doubt that the shear bands produced between rigid platens are bounded by planes of maximum shear stress rather than by Coulomb-type slip planes. Inside a shear band, however, Coulomb slips are operative in deforming the softened shear zone material. This may be concluded from the regularly observed sets of oblique minor slips (Riedels) and discrete gliding planes.

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Zusammenfassung Scherzonen in körnigen Stoffen. Eine experimentelle Studie ihrer Strukturen und mechanischen EntstehungEin Ringschergerät wurde eigens entwickelt, um bei hinreichend langem Schervorgang die Entwicklung von Scherzonen in körnigem Material sowie die zugehörigen Struktur- und Spannungsänderungen studieren zu können.Verschiedene Versuchsreihen wurden mit dicht gepackten, nicht-kohäsiven Stoffen durchgeführt, sowohl in trockenem Zustand als auch gesättigt mit Flüssigkeit. Zum Vergleich wurde in einigen Versuchen auch mäßig verfestigtes, körniges Material verwendet. Geeignete Wahl von Material und vertikaler Belastung erlaubte es, Scherdilatanz zuzulassen oder zu unterdrücken; im letzteren Falle mit starker Kornzertrümmerung innerhalb der eigentlichen Scherzone (Bruchfließen). Während sich dilatierende und nicht dilatierende Scherzonen hinsichtlich Wachstum und Struktur systematisch unterschieden, war der Spannungsaufbau in beiden Fällen ähnlich. Dies gilt im besonderen für die Schubspannungsspitze (peak stress).In vielen Experimenten wurden Spannungsmessungen innerhalb der Körnerpackung ausgeführt, und zwar hauptsächlich mittels eingelegter photoelastischer Zylinderchen, um die Richtung der größten Druckspannung in Nähe der Scherzone zu ermitteln. Die Ergebnisse lassen wenig Zweifel, daß die zwischen starren Platten erzeugten Scherbänder von Ebenen größter Schubspannung begrenzt sind und nicht von Coulombschen Gleitebenen. Im Inneren der Scherbänder sind allerdings Coulombsche Gleitflächen an der Verformung des entfesteten Materials maßgeblich beteiligt, wie die regelmäßig wahrgenommenen, gestaffelten Sekundärgleitflächen (Riedel-Gleitflächen) und Gleitebenen zeigen.

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Résumé Zones de cisaillement en milieu granulaire. Etude expérimentale de leur structure et du mécanisme de genèseOn a conçu un dispositif annulaire spécial pour étudier le développement des zones de cisaillement en milieu granulaire, sous l'effet d'un effort de cisaillement continu ainsi que des modifications qui en résultent aussi bien dans la texture du milieu étudié que dans l'état de contrainte.Plusieurs séries d'expériences ont été effectuées sur un matériau fortement comprimé et denué de cohésion, tour à tour sec ou saturé de fluide. Dans un but de comparaison, on a aussi utilisé dans quelques cas, un matériau légèrement consolidé. Matériau et charge verticale furent choisis en sorte que l'effet de dilatation dû au cisaillement existe ou non: en l'absence de dilatation on a un important broyage des grains (cataclase) au sein de la zone de cisaillement. Alors que le développement et la texture des zones de cisaillement en dilatation ou sans dilatation présentent des différences systématiques, le développement des contraintes est très semblable dans les deux cas, en particulier en ce qui concerne la résistance au pic.Dans de nombreuses expériences, des mesures de contraintes furent faites à l'intérieur du milieu granulaire, par insertion de cylindres photoélastiques afin d'établir l'orientation de l'effort principal de compression, au voisinage de la zone de cisaillement. Les résultats laissent peu de doute sur le fait que la bande de cisaillement, entre les plaques rigides du dispositif est limitée par des plans de contrainte de cisaillement maximum et non par des plans de glissement du type des plans de Coulomb. Cependant, à l'intérieur d'une bande de cisaillement, des plans de glissement du type de Coulomb contribuent essentiellement à la deformation du matériau affaibli de la bande. On est amené à cette conclusion par l'observation systématique de familles de glissements obliques secondaires (Riedels) et de plans de glissements individualisés parallèles au cisaillement principal.

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With 33 Figures     

Fluid-Rock Interaction of Shear Zones in Continental Crust:as Evidenced by Xincheng-Xishui and Hetai Shear Zones

There is a coupling of thermal, mechanical, chemical and fluidal processes in a continental shear zone. Both Xincheng-Xishui and Hetai shear zones are typical continental crust shear zones of greenschist facies environment. The representative mylonite zones of the shear zones are studied with whole rock major and trace element analyses. The chemical compositional variation tendencies in both shear zones are very similar and the gain-loss ratios of various components in the mylonitic rocks are reflected in the mass balance equations. The enrichment of those immobile high-field-strengh elements is considered to he related to the volume loss of the myionitic rocks in a shear zone. Based on the volume loss expression C_s/C_o=1/(1-V),the fractional volume losses (V)are 37.5％ and 36.5％-42.3％ respectively for mylonites and ultramylonites in the Xincheng-Xishui shear zone and 11％ and 28％ respectively for mylonites and phyllonites in the Hetai shear zone. The high volume loss and large removal of SiO_2 from     

Comparison between Empirical Estimation by JRC-JCS Model and Direct Shear Test for Joint Shear Strength

In order to study the reliability of the empirical estimation of joint shear strength by the JRC(joint roughness coefficient)-JCS(joint compressive strength) model,natural rock joints of dif-ferent lithologic characteristics and different sizes were selected as samples,and their shear strengths under dry and saturated conditions were measured by direct shear test and compared to those esti-mated by the JRC-JCS model.Comparison results show that for natural rock joints with joint surfaces closely matched,the...     

Shear Behaviors of the Binary Structural Plane

Binary-medium contact interfaces widely exist in rock engineering. They have significant impacts on the safety of rock engineering due to their poor shear behavior. A material of different strength is produced by pouring mortar of a different sand-to-cement ratio on the top of a rock-like mortar material (with the ratio of 1:1), thereby forming a binary-medium structural plane. Then, direct shear test is performed on the structural plane by applying different normal stresses. The shear strength parameters of the structural plane (cohesion \(c\) and friction angle \(\varphi\)) are obtained from the Mohr–Coulomb criterion. Moreover, the mechanical behaviors of the structural plane are compared with the unitary-medium specimen in the shearing process. A similar shear stress–shear displacement rule is observed in the shearing process. However, the peak and residual shear strengths of the binary structural plane are far lower than those of the unitary ones. The difference between the unitary and binary planes at cohesion \(c\) decreases with the increasing sand–cement ratio, whereas a up-down trend is observed in the friction angle \(\varphi\) with the increasing sand-to-cement ratio. When the upper and lower parts of the structural plane are different in sand-to-cement ratio, the cohesion \(c\) of the structural plane slightly increases with the increasing ratio of the upper specimen. However, when the two parts are identical in ratio, the cohesion of the structural plane reaches the peak, and its friction angle \(\varphi\) substantially increases with the increasing ratio of the upper part.