A true triaxial apparatus (TTA) was designed and fabricated at Northeastern University, Shenyang, China, by modifying the original Mogi type testing apparatus to emulate three-dimensional stress ...paths in deep mining and tunneling excavations. Such an apparatus can be used to investigate deformation and brittle failure behaviors of hard rocks as well as the cause of rockbursts. The novel TTA can capture the post-peak behavior of a 50 × 50 × 100 mm
3
specimen. Technical improvements such as a considerable increase of the stiffness of the loading frames were implemented to deal with difficulties in TTA testing. The accuracy of the volume change measurement was improved and a combined pneumatic and hydraulic technique was applied to create a “floating” vertical loading frame. The end friction effect and the loading gap effect were evaluated using a series of tests. Repeatability tests, brittle failure tests in a loading stress path and an unloading stress path (unloading of
σ
3
) were carried out on granite specimens to verify the performance of the TTA. The test results show that the apparatus achieves its original design goal.
Kong et al discuss the study on crack initiation and damage stress in sandstone under true triaxial compression. Rock masses in deep underground engineering are characterized by a complex geological ...environment and mechanical properties with frequent hazards such as excavation-induced spalling, rock burst and collapse. These failure behaviors result in a significant challenge for the construction of underground engineering and the prediction of abrupt hazards. It is also found that the crack initiation stress and crack damage stress have a relationship with the stress state in which the intermediate principal stress must be taken into consideration.
To investigate the deformation and failure mechanism of rock influenced by initial stress and unloading rates, this study presents minimum principal stress unloading tests of Jinping marble under ...true triaxial compression conditions under various initial stresses and unloading rates based on the in situ stresses measured in the China Jinping Underground Laboratory Phase II (CJPL-II). The effects of initial stresses and unloading rates on the bearing capacity and the deformation and failure characteristics of Jinping marble were analysed. The relationship between the strains in the three principal stress directions and the unloading extent of σ3 is defined using parameters such as the strain-unloading extent ratio and the unloading strain rate. With an increase in the initial σ1, the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential earlier, and sample failure occurred with a smaller unloading extent. Meanwhile, the number of acoustic emission rings and the amount of cumulative energy increased, and the sample failure changed from tensile failure to a mixture of tensile and shear failure that showed more transgranular cracks in the microscopic images. With an increase in the initial σ2 or σ3, the overall trend of the failure characteristics was similar to that with an increase in the initial σ1, but the sample failed with a larger unloading extent, and the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential later. The influence of unloading strain rate shows that as unloading strain rate increased, the sample failed with a larger unloading extent and changed from a mixture of tensile and shear to tensile failure, and more transgranular cracks were observed in the microscopic images.
•σ3 unloading tests with different unloading rates and initial stress levels under true triaxial stresses were carried out.•The bearing capacity, deformation process, and failure mode of Jinping marble under σ3 unloading condition were examined.•The relationships between strains along principal stress directions and unloading extent of σ3 (Δσ3) were established.•Failure mechanisms of Jinping marble were identified based on AE data and SEM pictures of fractures.
The purpose of this ISRM Suggested Method is to introduce a guideline on determining deformation and failure characteristics of rocks subjected to true triaxial compression on different stress path. ...The true triaxial testing apparatus was reviewed by means of the function and engineering application. Some key techniques, such as stress and strain measurements, and reduction of end effect between specimen and metal platens, preventing metal platens interference, were stated and suggested in detail. Methodology of specimen processing, specimen shape, and testing procedure are characterized. There is an explanation of the experimental data processing on stress–strain curves, strength, and fracture mode.
Energy principles, which can favorably explain the complete rock failure process, are one of the most reliable analysis approaches in rock mechanics and engineering. In this study, a strain energy ...approach under true triaxial compression (TTC) is proposed. On this basis, the energy evolution characteristics and variations of different failure behavior types (Class I, Class II and ductile failure) under TTC are analyzed. The variations of the strain energy characteristics of Beishan granite with
σ
2
and
σ
3
under TTC are studied. The results indicate that the total strain energy
U
and the elastic strain energy
U
e
of Beishan granite increase with the increasing
σ
2
or
σ
3
. The dissipated strain energy
U
d
rapidly increases when the value of
ε
1
/
ε
1peak
is approximately 0.6–0.8. The influence of
σ
3
on the rock failure mode and energy evolution characteristics is greater than that of
σ
2
. In highly brittle rocks, the tensile cracking of the rock microstructure is dominant, and the rock has a high strain energy storage capacity and a low strain energy dissipation capacity. The cumulative acoustic emission (AE) count rate curve shows the same trend as the total dissipated strain energy
U
d
curve. The research results show that the proposed strain energy analysis method for TTC can explain the macroscopic failure behaviors, microscopic failure mechanism and AE characteristics of Beishan granite under TTC, thereby providing new ideas and methods for investigating the behaviors of deep underground hard rock.
To study the difference in the failures of laboratories #7 and #8 of the China Jinping Underground Laboratory Phase II (CJPL-II), different loading and unloading stress path tests were carried out ...under a true triaxial compression condition based on the measured in situ stress of CJPL-II. The strength, deformation and failure mechanisms of Jinping marble under different stress paths were thoroughly investigated. The results showed that under the same stress state as that at sample failure, the bearing capacity and deformation of the unloading stress path were greater than those of the loading stress path. The bearing capacity and deformation in the
σ
2
and
σ
3
directions of stress path II and stress path III were similar, and the others decreased from stress path IV to stress path V under unloading conditions. The failure mode results reflect the violence of the unloading failure. The failure features and mechanisms of different stress paths were analysed though the acoustic emission (AE) ring, cumulative energy and strain energy trend. This research may provide a reference for the excavation and support of deep underground works.
The energy evolution processes and mechanisms between three hard rocks are studied using a strain energy analysis method under true triaxial compression (TTC). Using Beishan granite as an example, ...the change in the energy storage limit Umaxe, strain energy ratio and strain energy conversion rate for different σ2 and σ3 values is investigated. The research results indicate that within the scope of this study, the energy evolution processes and characteristics are largely similar at pre-peak, and notably different at post-peak. The energy storage limit Umaxe of the Beishan granite indicates an approximately slow increase with an increasing σ2, whereas its mean value shows a favorably linear increase with an increasing σ3. The elastic strain energy ratio shows a reverse relationship with that of the dissipated strain energy during rock failure. For an increasing σ2, the conversion rates of the total elastic strain energy Ue and the total dissipated strain energy Ud of the Beishan granite demonstrate a poor relationship at both pre- and post-peak. For an increasing σ3, the mean values indicate a favorable linear change at both pre- and post-peak. The total strain energy and total elastic energy of the different hard rocks increase with increasing σ2 or σ3 values at pre-peak, whereas the total dissipated strain energy increases with an increasing σ3 and decreases with an increasing σ2 at pre-peak. When the external conditions are not considered, the difference in the elastic and plastic deformation capacities, which influence the energy difference in different rocks, is influenced by the mineral compositions and microstructures of the different rocks.
•The strain energy evolution process and difference of three hard rocks are studied under true triaxial compression.•The change of energy storage limit Umaxe, strain energy ratio and strain energy conversion rate with different σ2 and σ3 are investigated.•The energy evolution mechanism under different σ2 and σ3 levels and the energy difference of different hard rocks are analyzed
With excavation during deep underground engineering, the rock mass near the sidewall changes from ductile behavior to brittle behavior. However, the ductility of the rock mass at a certain distance ...from the sidewall may increase due to the change in stress. Over time, the failure mechanism of the rock mass is not necessarily consistent. The aim of this study is to obtain a better understanding of creep mechanisms under 3-D stress states. To achieve this goal, six brittle creep and two ductile creep experiments on Jinping marble were conducted under true triaxial stress conditions (σ1 > σ2 ≥ σ3). The test results show that creep deformation of rock decreases with increasing intermediate principal stress. Many micro-cracks were observed by scanning electron microscopy in the samples failed under the ductile creep. However, brittle creep and ductile creep present different crack expansion modes in microscopic terms, resulting in significant differences in macroscopic deformation behavior. The steady creep rate increases slowly with increasing σ1-σ3 when brittle creep occurs. Meanwhile, when the stress reaches a certain value during ductile creep, the steady creep rate suddenly increases exponentially from 10−6 h−1 to 10−5 h−1. From the test results, we can deduce that the true triaxial stress and time effects play key roles in the creep behavior of rock masses in deep engineering projects.
•Crack propagation modes are different during ductile and brittle creep.•Crack time-dependent expansion in microscopic terms, resulting in differences in macroscopic deformation.•The σ2 can inhibit creep deformation.
A new, high-efficiency technology for fracturing and breaking rocks is required. Due to various advantages including high efficiency, energy-saving, and having no secondary pollution, the technology ...of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Aiming at the realisation of two engineering applications: microwave-assisted mechanical rock breaking and stress release from rock masses in deep underground engineering works to prevent geological disasters caused by high-stress concentrations such as rockbursts, a novel (open-type) microwave-induced fracturing apparatus (OMWFA) for fracturing hard rocks was developed. On this basis, the two modes of microwave-induced subsurface fracturing and microwave-induced borehole fracturing of hard rocks were proposed. Due to removal of the restraint of the microwave cavity, OMWFA can be used to fracture large-size rock samples and engineering-scale rock masses. Using the apparatus, the fracturing effects of the two fracturing modes on different dimensions of cuboidal basalt samples were investigated. By combining the microwave-induced fracturing apparatus with a press machine to explore the influence of unidirectional stress on the fracturing effect of microwave treatment on basalt. Moreover, field tests were carried out on rock masses encountered in underground engineering works at Baihetan Hydropower Station in Sichuan Province, China, and the fracturing effects were evaluated by applying a digital borehole televiewer and conducting acoustic wave testing. The results show that the apparatus had favourable fracturing effects on the subsurface and borehole samples of basalt. When no stress was applied, the cracks radially expanded from the approximate centre of the radiant surface and unidirectional stress promoted fracturing. The number and depth of cracks increased with prolonged microwave exposure. After microwave treatment, the P-wave velocity of the samples declined, and the longer the microwave exposure, the more significant the reduction in P-wave velocity was. The results of field test reveal that borehole fracturing can exhibit a favourable effect around boreholes. The sound velocity around the borehole and between the boreholes both declined to some extent. Microwave-induced hard rock fracturing offers guiding significance to those exploring and developing new rock breaking and tunnelling methods, and generally enhances construction safety in deep underground engineering works.
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