A rigid true triaxial apparatus for analyses of deformation and failure features of deep weak rock under excavation stress paths

The squeezing scenario in deep weak rock tunnels can hinder underground construction.However,due to the limitations of test technologies at hand,the real excavation stress path cannot be mimicked in the laboratory.Thus,the large deformation mechanism of deep weak rocks still remains unclear.For this,a true triaxial apparatus(TTA)to investigate the mechanical responses of deep weak rock under excavation stress paths in field and reveal the squeezing mechanism of deep tunnels is assembled and developed at Northeastern University,China.The apparatus can perform instantaneous unloading in s3 direction based on electromagnetism technology.In addition,uniform loading and deformation measurements can be carried out based on the proposed linked interlocking clamp and antifriction device,even if the sample has a strong dilatation deformation performance.Next,a bore trepanning is designed to capture noiseless acoustic emission(AE)signals for deep weak rock at a low threshold.Finally,two tests were are conducted using this instrument to preliminarily understand the failure and deformation features of deep weak rock based on fractured marble.The results show that the complete stressestrain curves of fractured marble have the characteristics of low strengths and large deformations,and the larger deformation and the more serious failure occur when the fractured marble enters the post-peak state after excavation.The results show that the developed apparatus is likely to be applicable for deep weak rock engineering.

Experimental investigation on hard rock fragmentation of inserted tooth cutter using a newly designed indentation testing apparatus

This investigation aims to explore the effects of stress conditions and rock cutting rates on hard rock fragmentation through indentation tests on a newly designed triaxial testing apparatus.This apparatus was designed to realize a triaxial loading and indentation test of cylindrical specimens using inserted tooth cutter.The boreability and crushing efficiency of granite rock was investigated by analyzing the change rules of the thrusting force,penetration depth,characteristics of chippings and failure patterns.Several quantitative indexes were used to evaluate rock boreability in this investigation.The granite rock samples all had a chiselled pit and a crushed rock core.Under initial stress conditions,only flat-shape chippings were stripped from the rock surface when the thrusting force reached 20 kN.The rock cutting special energy had a close correlation with the initial stress conditions and inserted tooth shape.Moreover,a thrusting force prediction model was proposed in this paper.The contribution of this study is that for the first time the influence mechanism of the initial triaxial stress conditions on rock fragmentation is investigated using an inserted tooth and the newly designed testing apparatus.This study has a crucial importance for practical underground hard rock crushing in geoengineering.

A novel true triaxial test system for microwave-induced fracturing of hard rocks

This study introduces a test system for microwave-induced fracturing of hard rocks under true triaxial stress.The test system comprises a true triaxial stress loading system,an open-ended microwaveinduced fracturing system,a data acquisition system,an acoustic emission(AE)monitoring system,and an auxiliary specimen loading system.Microwave-induced surface and borehole fracturing tests under true triaxial stress were fulfilled for the first time,which overcomes the problem of microwave leakage in the coupling loading of true triaxial stress and microwave.By developing the dynamic monitoring system,the thermal response and fracture evolution were obtained during microwave irradiation.The monitoring system includes the infrared thermometry technique for monitoring rock surface temperature,the distributed optic fiber sensing technique for monitoring temperature in borehole in rock,the AE technique and two-dimensional digital speckle correlation technique for monitoring the evolution of thermal damage and the rock fracturing process.To validate the advantages of the test system and investigate the characteristics of microwave-induced fracturing of hard rocks,the study demonstrates the experimental methods and results for microwave-induced surface and borehole fracturing under true triaxial stress.The results show that thermal cracking presented intermittent characteristics(calm eactiveecalm)during microwave-induced surface and borehole fracturing of basalt.In addition,true triaxial stress can inhibit the development and distribution of thermal cracks during microwave-induced surface fracturing.When microwave-induced borehole fracturing occurs,it promotes the distribution of thermal cracks in rock,but inhibits the width of cracks.The results also prove the reliability of the test system.

Advances in development of shear-flow testing apparatuses and methods for rock fractures:A review

Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration,design,construction,operation,and maintenance of many rock engineering projects such as hydropower development,oil and gas extraction,and underground waste disposal.With the rapid advancement of global and national strategies such as the“Paris Agreement”and the“Belt and Road Initiative”,more and more projects are developed in the complex geological environment with varying geological structures.Shear failure and rock instability are prone to occur in fractured rock masses under the coupled effects of high stress,high pore pressure,and engineering disturbance,which are main sources for engineering disasters such as roof collapse and caving,water and mud inrushes,and induced earthquakes.To solve these problems,extensive research on the coupled shear-flow behavior of fractures has been conducted.However,due to the complex mechanical,hydraulic and geometrical characteristics of single fractures and fracture networks,a large number of outstanding issues related to the impact of the coupled processes on the engineering characteristics of rock masses are still unsolved.The relevant experimental apparatuses and methods remain to be further developed.Therefore,in this review,we analyze and summarize the existing shear-flow experimental apparatuses,classify apparatus configurations,specimen shapes,and testing principles,and compare their advantages and disadvantages.We also summarize the main scientific findings obtained from various experimental apparatuses,aiming to provide a reference for developing new shear-flow experimental apparatuses and conducting related scientific research in the future.

A laboratory method to simulate seismic waves induced by underground explosions

The seismic waves induced by underground explosions generate geological hazards affecting deep buried tunnels such as rockbursts and engineering-induced earthquakes. This issue is difficult to study through full-scale testing due to the expense and unpredictable danger. To solve this problem, the authors developed experimental apparatus and presented a laboratory method to simulate seismic waves induced by underground explosions. In this apparatus, a combined structure of a diffusive-shaped water capsule and a special-shaped oil capsule was designed. This structure can provide an applied confining stress and freely transmit the stress wave generated by external impact. Therefore, the coupled loading of in situ stress and seismic waves induced by underground explosions in the deep rock mass was simulated. The positive pressure time and peak value of the stress wave could be adjusted by changing the pulse-shaper and the initial impact energy. The obtained stress waves in the experiments correspond to that generated by 0.15-120 kt of TNT equivalent explosion at a scaled distance of 89.9-207.44 m/kt.

APPLICATION OF SUDDEN CHANGE PRESAGES TO IMMINENT EARTHQUAKE PREDICTIONS

To observe imminent earthquake presages of base rock strain, base rock earth electricity, electro-magnetic radiation envelope and meteorology, laser lock-in strain seismometers, base rock earth electrometers, auto-electrometers and meteorological apparatus are used simultaneously. From experience we know that suddden change presages play an important role in imminent earthquake prediction, and they have the following properties: diversity, globalism,