Numerical simulations of failure behavior around a circular opening in a non-persistently jointed rock mass under biaxial compression

Pre-existing discontinuities change the mechanical properties of rock masses,and further influence failure behavior around an underground opening.In present study,the failure behavior in both Inner and Outer zones around a circular opening in a non-persistently jointed rock mass under biaxial compression was investigated through numerical simulations.First,the micro parameters of the PFC^(3D) model were carefully calibrated using the macro mechanical properties determined in physical experiments implemented on jointed rock models.Then,a parametrical study was undertaken of the effect of stress condition,joint dip angle and joint persistency.Under low initial stress,the confining stress improves the mechanical behavior of the surrounding rock masses;while under high initial stress,the surrounding rock mass failed immediately following excavation.At small dip angles the cracks around the circular opening developed generally outwards in a step-path failure pattern;whereas,at high dip angles the surrounding rock mass failed in an instantaneous intact rock failure pattern.Moreover,the stability of the rock mass around the circular opening deteriorated significantly with increasing joint persistency.

Relationship Between Serum Leptin Concentration and Lipids & Body Mass Index in Patients With Chronic Congestive Heart Failure

Objectives To investigate the relationship of serum leptin concentration with the lipids and body mass index （BMI） in patients with chronic congestive heart failure（CHF）. Methods The serum concentration of leptin in 39 patients with CHF （14 in cardiac function class Ⅱ , 21 in class Ⅲ, 4 in classⅣ , NYHA） and in 46 patients with cardiac function class Ⅰ （NYHA） were assessed by radioimmunoassay. Results The serum concentration of leptin were 9.018±4.519 μg/l in CHF group （cardiac function class Ⅱ 11.492±5.649 μg/l, class Ⅲ 7.763±3.321 μg/l, class Ⅳ 6.100±2.657 μg/l）; 11.674± 6.911 μg/l in class Ⅰ group. The serum concentrations of leptin were significantly lower in CHF group, as compared with class Ⅰ group （P〈 0.05）. Moreover, the decrease of serum leptin concentration was significantly correlated with the decreased serum concentrations of total cholesterol, triglyceride, body mass index and left ventricular ejection fraction in CHF group, respectively （P 〈 0.05）. Conclusions The significance of the decrease in serum leptin in CHF patients needs further study.

Reply to Discussion on “A generalized three-dimensional failure criterion for rock masses”

General:Jaiswal and Shrivastva(2012)proposed the mathematical formulations,i.e.the J–S criterion for converting generalized H-B failure criterion into 3D smooth convex failure criterion at deviatoric plane.The J–S strength criterion is in two versions:uniform and variable extension ratio.It has been observed from the analysis that at uniform extension ratio,the required strength parameters are only UCS and m(other parameters such as Ls,a,b and c are related with m).In the case of variable extension ratio,extra parameter f is required along with UCS and m.Thus,it has minimal strength parameters compared to You strength criterion.Furthermore,You strength criterion does not obey the smooth convex condition at deviatoric plane.

Real-time arrival picking of rock microfracture signals based on convolutional-recurrent neural network and its engineering application

Accurately picking P-and S-wave arrivals of microseismic(MS)signals in real-time directly influences the early warning of rock mass failure.A common contradiction between accuracy and computation exists in the current arrival picking methods.Thus,a real-time arrival picking method of MS signals is constructed based on a convolutional-recurrent neural network(CRNN).This method fully utilizes the advantages of convolutional layers and gated recurrent units(GRU)in extracting short-and long-term features,in order to create a precise and lightweight arrival picking structure.Then,the synthetic signals with field noises are used to evaluate the hyperparameters of the CRNN model and obtain an optimal CRNN model.The actual operation on various devices indicates that compared with the U-Net method,the CRNN method achieves faster arrival picking with less performance consumption.An application of large underground caverns in the Yebatan hydropower station(YBT)project shows that compared with the short-term average/long-term average(STA/LTA),Akaike information criterion(AIC)and U-Net methods,the CRNN method has the highest accuracy within four sampling points,which is 87.44%for P-wave and 91.29%for S-wave,respectively.The sum of mean absolute errors(MAESUM)of the CRNN method is 4.22 sampling points,which is lower than that of the other methods.Among the four methods,the MS sources location calculated based on the CRNN method shows the best consistency with the actual failure,which occurs at the junction of the shaft and the second gallery.Thus,the proposed method can pick up P-and S-arrival accurately and rapidly,providing a reference for rock failure analysis and evaluation in engineering applications.

Zonal character of failure near the wells and openings in high depth conditions

Rock mass failure on the high depth near the underground openings often has zonal character.The mechanism of this phenomenon consists in the periodical character of stresses in surrounding rock mass and developing of tensile macrocracks at the places(zones)of maximum tangentional stresses.Mathematical model of the high stressed rock mass is developed on the base of the defect media mechanics and nonequilibrium thermodynamics principals.The correspondence between the experimental research of faulted zonal structures near the high depths openings and mathematical model calculation is achieved.Relationships between the width of cracking zones and rock mass strength property have been determined.

Failure behavior of highly stressed rocks under quasi-static and intensive unloading conditions

Unloading failure of rocks,especially highly stressed rocks,is one of the key issues in construction of underground structures.Based on this,analytical models for rocks under quasi-static and intensive unloading conditions are established to study the failure behavior of highly stressed rocks.In case of rock failure under quasi-static unloading,the rock mass ahead of working face is regarded as an elasto-brittle material,and the stress-displacement curves are used to characterize the tensile fracture of peak-stress area.It is observed that,when intensive unloading happens,there is an elastic unloading wave（perturbation wave） propagating in the rock mass.If the initial stress exceeds the critical stress,there will be a fracture wave,following the elastic unloading wave.To study the propagation feature of fracture wave,the conservation laws of mass,momentum and energy are employed.Results show that the post-peak deformation,strength and energy dissipation are essential to the failure process of highly stressed rocks.

Mechanical properties and failure characteristics of fractured sandstone with grouting and anchorage

Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fractured sandstone under different conditions of anchorage. The experimental results show that the strength and elastic modulus of fractured sandstone with different fracture angles are significantly lower than those of intact sandstone. Compared with the fractured samples without anchorage,the peak strength, residual strength, peak and ultimate axial strain of fractured sandstone under different anchorage increase by 64.5–320.0%, 62.8–493.0%, and 31.6–181.4%, respectively. The number of bolts and degree of pre-stress has certain effects on the peak strength and failure model of fractured sandstone. The peak strength of fractured sandstone under different anchorage increases to some extent, and the failure model of fractured sandstone also transforms from tensile failure to tensile–shear mixed failure with the number of bolts. The pre-stress can restrain the formation and evolution process of tensile cracks, delay the failure process of fractured sandstone under anchorage and impel the transformation of failure model from brittle failure to plastic failure.

Two-and three-dimensional stability analysis of underground storage caverns in soft rock(Cappadocia, Turkey) by finite element method

Engineering design in soft rocks and its stability analysis exerts many challenges to rock engineers. Many engineering works in Turkey’s Cappadocia region must face and tackle the existing sites covered by the soft rocks. This study is aimed to examine the stability condition of a typical underground storage cavern(USC) excavated in a soft rock in this region. For this purpose, two-and threedimensional stability analyses of the USCs were performed using the finite element method(FEM).Because of the inherent difficulty in characterizing soft/weak rock masses in the region using traditional classification systems, the stability of a typical USC was evaluated by representing the rock mass condition with two distinct scenarios in FEM analysis.While these structures were unstable according to the 2D analysis conducted in RS2 software in the worstcase scenario, they were stable in the 3D analysis using RS3 software in both scenarios. Besides,feasible cover depths were examined to assess their possible effects on the factor of safety and deformation measurements. It was found that 15 m seems to be an optimal depth for excavating a typical USC in the soft rocks exposed in the region. The 3D FEM results provide valuable information to optimize the future planning and preliminary design of USCs.

Estimation of fracturing mechanisms by the ratio of radiated energy between S and P waves of microseismic events during mining in metal mines

In the mining process of deep metal mines,diff erent types of rock mass instability failures are caused by strong mining disturbance.It is beneficial to master the fracture mechanism of rock mass in time to effectively prevent and control the ground pressure disasters.Microseismic signals are generated by the propagation and expansion of cracks inside the rock mass that contain plentiful information about the structural changes of rock mass.The ratio of the radiated energy of S and P waves(Es/Ep)of microseismic events can fast and eff ectively calculate the rock fracture mechanism,which is widely used for ground pressure hazard risk assessment.In this paper,this method was used to analyze the fracture mechanism of rock mass around deep stope in Hongtoushan copper mine and Ashele copper mine.Furthermore,the spatial distribution characteristics and proportion changes of microseismic events with diff erent fracture mechanisms along with the mining process were studied.The results show that tensile cracks play a dominant role,accounting for 62%of the total events,during non-shear fracturing of the rock mass caused by the stoping unloading eff ect,while shear cracks occupy 68%of the total events during orebody slip failure.When the physical and mechanical properties of the orebody and rock mass are signifi cantly diff erent,slip failure along their contact zone is prone to occur under blasting disturbance.During deep mining,it is necessary to control the exposed area of the roof by each stoping,especially during the earlier mining stage,to avoid tensile stress concentration.The temporal and spatial variation of tension cracks and shear cracks induced by roof damage obtained in this paper can guide the prevention and control of ground pressure disasters in deep mining eff ectively.

Rock engineering design of post-tensioned anchors for dams - A review

High-capacity, post-tensioned anchors have found wide-spread use, originally in initial dam design and construction, and more recently in the strengthening and rehabilitation of concrete dams to meet modern design and safety standards. Despite the advances that have been made in rock mechanics and rock engineering during the last 80 years in which post-tensioned anchors have been used in dam en- gineering, some aspects of the rock engineering design of high-capacity rock anchors for dams have changed relatively little over the last 30 or 40 years. This applies, in particular, to the calculations usually carried out to establish the grouted embedment lengths required for deep, post-tensioned anchors. These calculations usually make simplified assumptions about the distribution and values of rock-grout interface shear strengths, the shape of the volume of rock likely to be involved in uplift failure under the influence of a system of post-tensioned anchors, and the mechanism of that failure. The resulting designs are generally conservative. It is concluded that these aspects of the rock engineering design of large, post- tensioned rock anchors for dams can be significantly improved by making greater use of modern, comprehensive, numerical analyses in conjunction with three-dimensional （3D） models of the rock mass structure, realistic rock and rock mass properties, and the results of prototype anchor tests in the rock mass concerned.

Effect of heterogeneity on occurrence of zonal disintegration around deep underground openings

By utilizing the two numerical codes RFPA3 D and FLAC3 D, the effect of heterogeneity on failure mode and failure mechanism of rock around deep underground excavations under tri-axial stress is analyzed. It is found that zonal disintegration is a large scale shear-slip failure developed in deep surrounding rock mass under tri-axial stress, which is accompanied by a large amount of tensile failure. The distance between fractures and the number of fractures have a close correlation with the rock mass heterogeneity. With an increase of the homogeneity index of the rock mass, the distances between fractures decrease and the number of fractures increases. For an intact hard rock mass with relative high homogeneity, only failure mode characterized as v-shaped notches can be formed due to the intersection of intensively developed shear bands. None of the zonal disintegration can be formed due to the fact that with increasing homogeneity, the failure mechanism of rock mass is gradually dominated by shear failure rather than tensile failure.