Study On Failure Models And Strength Of Rockmass Containing Discontinuous Joints In Direct Shear

Rock mass is a complicated engineering medium widely founded in many kindsof construction engineering, such as water engineering, transportation engineering,mining engineering and petroleum engineering. Deformation and strength of the rockmass have great influence on engineering safety. The failure and instability of therock mass containing discontinuous joints is essential caused by the joints opening,closing, extending and connecting. In general, the failure includes joints failure androck bridges failure, so the failure plane is a composite one composed of originaljoints and new joints resulted from rock bridge sfailure at the end of the originaljoints. Many problems, such as how the joints and rock bridges influence thedisplacement and strength of the rock mass, what is the rule of the displacement andstrength of joints and rock bridge, and what is the joints extending rule, haven't beenresolved so far. After tension failure occurs at the end of the joints, the loads the rockmass can bear still exceeds its initial cracking strength. The initial cracking strengthand the peak strength are obviously different. It is widely accepted that the initialtension failure determines the initial cracking strength, though, how the tensionfailure influences the peak strength is actually still unknown. Direct shear test, whichcan simulate the stress state of rock mass well, is an effective test method. However,most theories and tests are concentrated on uniaxial or biaxial compression test ratherthan direct shear test.This paper studies the extension and failure rulers of rock mass containingdiscontinuous joints by testing after geometrical and mechanics analysis, andproposes some new approaches. The main research work of this paper can besummarized as following:The geometrical property of rock mass containing discontinuous joints has beenstudied. According to the relationship between joints and the plane of enforced shear,this paper classified rock mass containing discontinuous joints into two types: rockmass containing coplanar discontinuous joints and rock mass containing non-coplanar discontinuous joints. The former can be classified into three types. The latter can beclassified into two types.Electro-hydraulic servo-control shear system has been developed. The system iscomposed of four parts: shear apparatus, electro-hydraulic system, servo-controlsystem, and computer control and manage system. The devices can run under bothconstant stress and constant displacement expediently, exert loads automatically, andrecord all kind of data automatically. The scope of measure is large and the results areof high precision.The strength and deformation properties of rock mass containing discontinuousjoints, as well as the rules of stress and stain of observing points in the rock bridge,are studied under direct shear condition. The shear stress-displacement curve can bedivided into four phases: The phase in which the tension failure has not emerged, thephase in which the tension joints extends stably, the phase in which tension jointsextends unsteadily, and the phase after the rock mass failure. The relationshipbetween the characteristic strength in different phases and the peak strength is alsostudied in this paper. It is subsequently pointed out that there is correspondingrelationship between the different phases of curves and the states of stress and strainof rock bridges.In this paper, the failure mechanism of rock mass containing discontinuous jointsis studied by direct shear test, and Lajtai's rock bridges failure theory is improved.Then, this paper analyzed the stress state of rock bridges and drew the conclusion thatthe rock bridges failure has two possible patterns, say, tension failure and shearfailure. And an optional mechanical equation is suggested. The study focuses on theproperties of the failure path, and four failure models are proposed: TTTT model(failure in tension), TTTS model (combined action of shear and tension), TTSSmodel (combined action of shear and tension), SSSS model (failure in shear) .The mechanical behaviors reduction of rock mass containing discontinuousjoints is proposed in direct shear test, and the cause and reduction region are alsoanalyzed. It is proposed that the tension strength can be regard as the parameter ofglue cell, which is a mesoscopical cell. Also, the mesoscopical model of themechanical behaviors reduction is proposed. Based on the model, the simulating formula for shear stress-displacement curve is suggested, and the influence of theseparameters in the formula is respectively discussed.Subsequently, the shortcoming of the existing strength criterion is analyzed,with a new strength criterion for rock mass containing discontinuous joints beingsuggested. The new strength criterion can reflect the geometric characteristic andmechanical behaviors, such as the length and the shear strength parameter of jointsand rock bridge, and tension strength of rock bridge. It can also consider the influenceof different failure models. What is more, the reduction coefficient of mechanicalbehavior is introduced.A large number of direct shear tests for rock mass containing discontinuousjoints have been done. The new rock bridges tension failure theory can explain thephenomenon in the tests. The proposed failure shape models have been verified, andits mechanism can explain the observed failure shapes. The calculated peak strengthbased on the new strength criterion coincided well with the experimental results. Thesimulating curves based on suggested shear stress-displacement formula coincidewell with the experimental curves. And reduction of cohesion and angle of internalfriction of rock bridge shave been tested and verified.