Study On Strength And Creep Characteristics Of Composite Rock Mass With Inclined Soft-weak Interlayer

Layered rock mass are common object in rock engineering, and interlayer is its special form. If interlayer is weak and inclined, it will weaken mechanical properties of rock mass, and has negative impact on the stability of rock engineering with them.This negative effect will persist during the period of project building and operating.It is difficult to take samples due to the losses and damages in delivery and machining process. People always can not get enough natural layered composite rock mass samples, which lead to the absence of study on basic mechanical properties.There is not specific engineering project as aim, only as basic research this paper. The composite rock mass with inclined soft-weak interlayer is taken as research object, all experiments are done by artificial composite rock mass instead of natural rock mass to study their strength and creep characteristics.The main research content and research findings are as follows:(1) The stress component’s analytical solutions of composite rock mass samples with inclined soft-weak interlayer under uniaxial and triaxial compression are deduced,and the stress distributions of interlayer, main rock and interface are given respectively.After analysis, author think that the composite rock mass with different thickness ratio has different failure modes, rock mass with thick interlayer is easier to be broken than thin one; the strength under high confining pressure is higher than the strength under low confining pressure. The potential failure modes of composite rock mass under uniaxial and triaxial compression are given.Furthermore, the stress component’s analytical solutions of composite rock mass with inclined soft-weak interlayer are deduced. After analysis, author think that the topand bottom of the interlayer interface are easier to be destroyed than other areas; the strength of the material decrease firstly and then increase along with the dip angle increasing; rock mass under confining pressure is not easier to broken than that under zero confining pressure; the interlayer is easier to be broken than the main rock mass.The potential failure modes of composite rock mass under uniaxial compression are given.(2) A standard sample batch-pressure mold is specially developed. The layered material, dip angle, layer thickness, interlayer position could be controlled and one time forming quantity is adjustable; the uniformity of same batch is good; the whole mold can be disassembled, is suitable for small batch forming and fit to the general press machine, has obvious advantages in bedding control, forming efficiency and quality, using convenience and so on.Three kinds of common sedimentary rock combinations, shale, sandstone and limestone are chosen as simulation objects, cement, gypsum, sand are selected as simulation materials, and two kinds of artificial composite rock samples with different strength interlayer are made according to selected proportion.(3) Based on the uniaxial and triaxial compression testing result of two kinds of composite rock mass, the influence law of rock strength characteristics with soft-weak interlayer characteristics are obtained. The following conclusions are obtained:Accompanied with thickness increase of interlayer, rock compressive strength and elastic modulus decreased, yet. With the same total thickness of interlayer, the increasing number of interlayer, the strength increased, the damage possibility of each layer is same. Within a given dip angle range of layer, increased angle will weaken the strength of rock mass. However, different rock types have different characteristic angle,once exceeding this characteristic angle, the weakening effect will gradually weaken,the characteristic angle of the class Ⅰrock mass is larger than that of the class Ⅱ.Increased confining pressure can intensify the strength of composite rock mass, in the range of certain plastic deformation, even if interlayer is damaged, overall loading capacity of composite rock mass can still be maintained.(4) By numerical simulation, it is found that rock mass in excavation contour face nearly area stay in uniaxial stress state after excavation. The principal stress direction is always tangent with contour curve, regardless of shape of contour face, magnitude and direction of ground stress. In some rock mass project with soft-weak interlayer, it should be paid a special attention to the interlayer occurrence, the exposed position inexcavation face, and the direction of principal stress.(5) Based on the uniaxial compression creep test result of two kinds of composite rock mass, it is found the bearing stress of class II rock mass before failure is larger than class I, creep failure strength is smaller than uniaxial compressive strength for the same rock mass. It is found that current level creep value of two kinds of rock mass get bigger when stress increase, and diverge to high stress area; current level instant strain value decrease when stress increase, and convergence to high stress area; dip angle of interlayer has no certain relationship with current level creep value and instant strain.Absolute strain value, current level creep value and current level instant strain value of rock mass with interlayer lie between the corresponding values of interlayer material and main rock material. In practice, once getting the latter creep characteristic,technologists can deal with it properly, and then apply for rock mass with interlayer.At last, author chose mature creep models to fit test data and prove their applicability by numerical simulation.