Experimental Study on Shear Stress-Displacement, Creep and Shear Slip Instability Behavior of Rock Fractures

Rock mass inherently contains discontinuities, and shear sliding of rock masses along discontinuities is the most common failure mode of rock mass in engineering practice (such as slope, dam and tunnel). In this study, the mechanical and failure properties of rock fractures were examined through direct shear tests, pre-peak tiered cyclic shear tests and multistage loading shear creep tests. The results show that the deformable memory of rocks can be observed from shear stress-shear displacement curves, namely reloading curves continued to increase along the loading curve of the last cycle under the pre-peak tiered cyclic shear loading. The envelopes of shear stress-shear displacement curves were similar to the variation trend of shear stress-shear displacement curves obtained in the direct shear test. Besides, the variation trend of residual shear displacement (RSD) and relative residual shear displacement (RRSD) before slip instability were obtained by the data analyses of rock mass under pre-peak tiered cyclic shear tests. It is found that the change in the friction and sliding state of rock fracture is the main reason for the fluctuation of shear stress-shear displacement curves. The Chen's method was used to process the multistage loading shear creep curves and the three creep stages were analyzed. Moreover, the complex creep models in some previous studies were discussed and the data of shear creep tests were successfully fitted to the classical Burgers model. This study provides a guidance for the study on the shear instability of rock fractures under different shear loading paths.

Applicability of single sample method to test strength parameters of sandstone

The single sample method allows the mechanical parameters of rocks to be obtained with very few rock samples; however, the method has not been widely used. This is mainly because the yield point of the single sample method is more difficult to control than the conventional triaxial compressive test and the effect of the different control methods on the measured data is not well understood. The single sample method obtains the strength parameters of the rock by loading a single rock sample with multiple stages of confining pressure. Multistage loading tests are divided into peak strength control and long-term strength control according to yield point control. In this study, multistage loading tests of sandstone were carried out to obtain strength parameters using long-term strength control. The results show that sandstones undergo seriously brittle damage in conventional triaxial compressive tests. Although the sandstones have been rigorously selected, they still vary considerably, and long-term strength points are more difficult to control. The error of strength parameters of sandstone obtained using the single sample method may exceed 20% compared to those obtained by conventional triaxial compressive tests. So this method must be used with caution for sandstones.

Mohr-Coulomb Envelope Based on Geomechanical Studies: West Siberian Fields Case Studies. Applied Methods

The strength properties of the rock determine many physical processes occurring in the formation and at the bottom of the well. Deformation and destruction of rocks under the influence of external forces can be both a positive and a negative factor for working with formations at all stages of development. To create the optimal design of the well drilling trajectory, select the optimal development project, the fluids extraction rate from the formations, efficient planning and implementation of hydraulic fracturing procedures, prevention of emergencies during drilling and operation and reduction of oil recovery due to irreversible loss of reservoir properties and solving many other problems, it is necessary to consider possible destruction of the rock. The Mohr-Coulomb envelope (rock strength passport) can be used as a strength criterion for such tasks, it characterizes the boundary values of stresses in the rock, at which its destruction occurs according to the Mohr-Coulomb theory. At article discusses three methods for determinate strength passports based on the results of laboratory studies of rock samples: multistage loading of the sample, assessment of the sample fracture after triaxial compression strength test, the use of "twin" samples for testing. The features of each method, its advantages and limitations are disclosed, examples of construction of strength passports for rocks from fields in Western Siberia are shown. According to the research results, the most preferable is the use of "twin" samples. However, this method is associated with technical difficulties.

GROUND STRENGTHENING METHOD BY MULTISTAGE LOCAL CONSOLIDATION

Authors are investigating a ground strengthening method, which effectively consolidates the construction area of requisite minimum in a short time by local consolidation for the soft ground. In this paper, Authors research that the effect of increasing the strength of soft ground by using the three-dimensional elasto-plastic finite element analysis when the area of local loading is gradually changed. First, it was found that the strength of the soft ground just below the local loading surface increased with a short time by the local consolidation and ultimate bearing capacity increases to 2.6 times as much as the initial bearing capacity by the multistage loading. Next, analysis was carried out in the case of the multistage loading with cyclic loading. In the reloading process, loading area is reduced to 1/3 and 1/6 of the loading area of the previous stage. In this case, the ultimate bearing capacity increases to 8.25 times as much as the initial bearing capacity. In conclusion, it was found that the strength of the soft ground just under the loading surface is considerably increased by multistage local loading with reducing the loading area.

Consolidation Theory for a Stone Column Composite Foundation under Multistage Loading

The consolidation theories considering instant load cannot fully reveal the consolidation mechanism of a stone column composite foundation used in the expressway embankments due to the time effect of loading; that is, the expressway embankments are often constructed in several stages for a long time. Meanwhile, owing to the special property that the pile-soil stress ratio is larger than 1, the consolidation theory for sand drain well foundation cannot be used directly in the consolidation analysis of stone column composite foundation. Based on the principle that the vertical load applied on the composite foundation is shared by the stone column and the surrounding soil, the governing solutions for the stone column composite foundation under a multistage load are established. By virtue of the separation of variables, the corresponding solutions of degree of consolidation for loading stage and maintaining load stage are derived separately. According to the Carrillo theorem, the solution for the average total degree of consolidation of entire composite foundation is also obtained. Finally, the reasonableness of the present solution has been verified by comparing the consolidation curve calculated by the present solution with that measured by site test.

Fatigue Life Evaluation For Wind Turbine Blade Based on Multistage Loading Accumulative Damage Theory

The fatigue life of MW wind turbine blade was assessed by applying theoretical calculation and test verification. Firstly, the fatigue characteristic curve of FRP material was obtained based on Palmgren-Miner damage theory. Furthermore, The fatigue life of Aeroblade1.5-40.3 wind turbine blade using multistage loading accumulative damage theory could be evaluated over 20 years accordingly. Then the coordinate system of wind turbine blade and its Bladed simulation model were set. By calculating fatigue loading, the moment distribution of fatigue test was obtained. Finally, the blade’s fatigue loading system driven by an eccentric mass was built and the multi-level amplitude resonant mode was adopted to carry on the test. Almost three months’ test results showed that the blade vibrating amplitude was constant, which illustrate the little variation of stiffness of loading point. The stable of stiffness could testify the fatigue life of blade was over 20 years. The results of in-site experiment were basically consistent with the theoretical calculation.