Investigation on the Load/Unload Response Ratio With Coda Q-1 as Response

The load/unload response ratio YQ with the geophysical parameter coda Q-1 of the crust as response is denned in this study.The variation in YQ-1 before and after the Northridge earthquake of January 17,1994(California)has been investigated by using the data of coda Q-1 with frequencies of 1.5,3.0,6.0,12.0,and 24.0 Hz in the Southern California from 1987 to 1994.It can be found that YQ-1 for coda waves with all frequencies,the frequency of 12.0 Hz excluded,ascended to a certain extent prior to the occurrence of the rnainshock and returned to normality after the main shock.

Physical meaning and prediction efficiency of the load/unload response ratio of rocks in strain-weakening phase before failure

Rock experiment results indicate that the load/unload response ratio (LURR) of rocks expressed via strain energy may have singular or negative value after the stress in the rock reaches its maximum before rock failure or when the rock goes into the strain-weakening phase. The universality of this phenomenon is discussed. Expressed via strain or strain energy and the travel time of P wave, the variation form of the reciprocal of LURR during the process of rock failure preparation is derived. The results show that after a sharp decrease the reciprocal of LURR reaches its minimum when the main fracture of the rock is about to appear. This feature can be taken as an indication that the rock main fracture is impending.

Variation of the Load/Unload Response Ratio Before the Main Shocks of the Reservoir-Induced Earthquakes

In this paper, the theory of the load/unload response ratio is applied to the prediction of the reservoir-induced earthquakes, and variation of the load/unload response ratio Y preceding the occurrence of main shocks of the reservoir-induced earthquakes in Xinfengjiang, Foziling, Danjiangkou, and Shenwo. The results show that the load/unload response ratio Y rises evidently prior to the main shocks.

Application of the Load/Unload Response Ratio Theory to Predictions of Moderate Earthquakes (7.0>M≥6.0)

The variation in load/unload response ratio before some moderate earthquakes is analyzed based on the theory of the load/unload response ratio.The results show that the load-unload response ratio increases noticeably before moderate earthquakes,and there are three kinds of patterns in which the load/unload response ratio varies and the duration of noticeable increase in load/unload response ratio ranges from half a year to two years.

The Experimental Simulation of Rocks on Load/Unload Response Ratio for Earthquake Prediction

The load/unload experiments on rock failure under pressure have been carried out in Material Test System (MTS) in the Laboratory for Non-linear Mechanics of Continuous Media (LNM), Institute of Mechanics, Chinese Academy of Sciences, and load/unload response ratio (LURR) values with strain as response (i.e. inverse elastic constant as response rate) have been obtained. The experimental results are in accordance with theoretical results and those in real earthquakes: LURR rises just before rock failure. So LURR can be used as the precursor of rock failure and earthquake prediction.

Experimental Research on the Load/Unload Response Ratio (LURR) Theory

Implementing acoustic emission experiments with large rock samples, LURR (Load/Unload Response Ratio) theory was studied. The loading conditions in the experiments were designed to simulate the complicated loading process of underground rocks. The damages emerging inside the rock samples were recorded by the acoustic emission technique during the loading process. The experimental results were consistent with prediction by LURR theory. Integrating the changing processes of LURR value Y and the location process of acoustic emission events showed agreement between the variation of LURR value Y and the damage evolution inside the rocks. Furthermore, the high value of Y emerged before the complete breakdown of materials. Therefore, the damage evolution of rock specimen can be quantitatively analyzed with LURR theory, thus the failure of the rock materials and the earthquake occurrence may be predicted. The experimental results gave a further verification of LURR theory.

Application of Load/Unload Response Ratio in the Study of Seismicity in the Region of Iran

The variation of Load/Unload Response Ratio (LURR) against time for earthquakes in Kerman Province, Iran, on February 22, 2005, M6.4 and in Lorestan Province, Iran, on March 31, 2006, M6.1, has been calculated and analysed in this paper. The tempo-spatial scanning of LURR in the region of Iran during January 1, 2003 to March 31, 2006 has been conducted, with 1 year as a time-window, 1 month as a time-step, and the comparison of the LURR anomalous regions in 2004 with the actual earthquakes with M≥5.0 in the next year (2005) is also given, which shows that 11 earthquakes with M≥5.0 occurred in LURR anomalous regions while 12 earthquakes with M≥5.0 in LURR regions in 2005. Furthermore, the seismicity in this region is studied by investigating the evolvement of the anomalous LURR regions.

In-situ observations of damage-fracture evolution in surrounding rock upon unloading in 2400-m-deep tunnels

The damage-fracture evolution of deep rock mass has obvious particularity,which is revealed in 2400-mdeep tunnels by field tests.The evolution of the excavation damaged zone depth is consistent with that of the fractured zone depth.The ratio of the excavation damaged zone depth to the excavation fractured zone depth is greater than 2.0 in a rock mass with both high strength and good integrity,but less than1.5 in a rock mass with lower strength or poor integrity.Zonal disintegration in a rock mass with high strength and fair integrity is more likely to occur when it contains more than two groups of primary fractures in damaged zones.Fractures develop outward in zonal disintegration but are totally different from the single-zone fracture,in which the fractures develop inward,and it is the starting position of the fractured zone when the excavation surface of the middle pilot is 7–9 m close to the pre-set borehole and it stops after the excavation surface of the baseplate is 11–14 m away.The most intense evolution occurs around 2–4 m from the pre-set borehole in the sidewall expansion stage.The research results provide a reference for the monitoring scheme and support design of CJPL-Ⅲin its future construction.

Experimental study on unloading induced shear performances of 3D saw-tooth rock fractures

A fractal model governing saw-tooth fractures was first introduced to replicate sandstone samples containing an inclined 3D penetrating rough fracture surface with various joint roughness coefficients(JRC).In conventional triaxial compression,the peak strength for fractured samples increased with both confining pressure and JRC.During the unloading confining pressure process,the normal stress of fractures declined but the shear stress increased,resulting in shear sliding of fractures.The shear displacement of fractures exponentially increased,and the positive normal displacement decreased gradually to negative values under coupling effects of shear contraction caused by normal stress and shear dilation due to climbing effects of fractures.Transition from quasi-static to dynamic sliding of the fractures was identified.The sliding resistance duration increased with confining pressure but decreased with JRC.After prepeak unloading,the fracture surfaces presented a more significant surface wear response and JRC values decreased by 1.70%–59.20%due to more remarkable asperity degradation compared with those after conventional triaxial compression.The theoretical model for shear strength of fractures was established through improving the Ladanyi&Archambault model by introducing the relations between normal stress and surface wear ratios of fractures,which agreed well with the experimental results.

Rockburst process and strength-weakening effect of the high-stress circular tunnel under internal unloading

To investigate the influence of unloading effect of a circular tunnel face on rockburst process,by innovatively combining rock drilling unloading devices and triaxial systems,the strain rockburst simulation under the entire stress path of“high initial stressþinternal unloadingþstress adjustment”(HUS test)was realized for the intact cubic red sandstone samples(100 mm×100 mm×100 mm).Comparative tests were conducted on cubic red sandstone samples with prefabricated circular holes(425 mm)under the stress path of“prefabricated circular hole+þhigh initial stress+stress adjustment”(PHS test),thereby highlighting the influence of internal unloading on rockburst failure.The test results revealed that with an increase in vertical stress,the sidewalls in both the HUS and PHS tests suffered strain rockburst failure.Compared with the PHS test,the initial failure stress in the HUS test is lower,and it is easier to induce sidewall rockbursts.This indicates that the internal unloading influences the sidewall failure,causing an obvious strength-weakening effect,which becomes more significant with an increase in buried depth.The strain rockburst failure was more severe in the HUS test owing to the influence of internal unloading.V-shaped rockburst pits were formed in the HUS tests,whereas in the PHS test,arcshaped rockburst pits were produced.It was also found that strain rockburst failure may occur only when the rock has a certain degree of rockburst proneness.

Mechanical behavior of sandstone during post-peak cyclic loading and unloading under hydromechanical coupling

This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unloading subjected to hydromechanical coupling effect, confirming the peak and residual strengths reduction laws of sandstone with water pressure, and revealing the influence of water pressure on the upper limit stress and deformation characteristics of sandstone during post-peak cyclic loading and unloading.Regarding the rock strength, the experimental study confirms that the peak strength σ_(p) and residual strength σ_(r) decrease as water pressure P increases. Especially, the normalized strength parameters σ_(p)/σ_(pk) and σ_(r)/σ_(re) was negatively and linearly correlated with the P/σ_(3). Moreover, the Hoek-Brown strength criterion can be applied to describe the relationship between effective peak strength and effective confining stress. During post-peak cyclic loading and unloading, both the upper limit stress σ_(p(i)) and crack damage threshold stress σ_(cd(i)) of each cycle tend to decrease with the increasing cycle number. A hysteresis loop exists among the loading and unloading stress–strain curves, indicating the unloading deformation modulus E_(unload) is larger than the loading deformation modulus E_(load). Based on experimental results,a post-peak strength prediction model related to water pressure and plastic shear strain is established.

Mechanical and hydraulic properties of fault rocks under multi‑stage cyclic loading and unloading

The rock mass in fault zones is frequently subjected to cyclic loading and unloading during deep resource exploitation and tunnel excavation.Research on the mechanical and hydraulic characteristics of fault rock during the cyclic loading and unloading is of great signifcance for revealing the formation mechanism of water-conducting pathways in fault and preventing water inrush disasters.In this study,the mechanical and seepage tests of fault rock under the multi-stage cyclic loading and unloading of axial compression were carried out by using the fuid–solid coupling triaxial experimental device.The hysteresis loop of the stress–strain curve,peak strain rate,secant Young's modulus,and permeability of fault rock were obtained,and the evolution law of the dissipated energy of fault rock with the cyclic number of load and unloading was discussed.The experimental results show that with an increase in the cyclic number of loading and unloading,several changes occur.The hysteresis loop of the stress–strain curve of the fault rock shifts towards higher levels of strain.Additionally,both the peak strain rate and the secant Young's modulus of the fault rock increase,resulting in an increase in the secant Young's modulus of the fault rock mass.However,the growth rate of the secant Young's modulus gradually slows down with the increase of cyclic number of loading and unloading.The permeability evolution of fault rock under the multi-stage cyclic loading and unloading of axial compression can be divided into three stages:steady increase stage,cyclic decrease stage,and rapid increase stage.Besides,the calculation model of dissipated energy of fault rock considering the efective stress was established.The calculation results show that the relationship between the dissipated energy of fault rock and the cyclic number of loading and unloading conforms to an exponential function.

Morphological evolution and flow conduction characteristics of fracture channels in fractured sandstone under cyclic loading and unloading

In coal mining,rock strata are fractured under cyclic loading and unloading to form fracture channels.Fracture channels are the main flow narrows for gas.Therefore,expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata.In this regard,graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels.Then,the three-dimensional models for fracture channels in sandstones were established.Finally,the fracture channel percentages were used to reflect the flow conductivity of fracture channels.The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels.It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels.The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels.When the proportion of fracture channels varies gently,fluids flow evenly through those channels.However,if the proportion of fracture channels varies significantly,it can greatly affect the flow rate of fluids.The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.

Influence of volume compression on the unloading deformation behavior of red sandstone under damage-controlled cyclic triaxial loading

A reasonable evaluation of unloading deformation characteristics is of great significance for the effective analysis of deformation and stability of surrounding rocks after underground excavation.In this study,the damage-controlled cyclic triaxial loading tests were conducted to investigate the pore compaction mechanism and its influences on the unloading deformation behavior of red sandstone,including Young’s modulus,Poisson’s ratio,volumetric strain,and irreversible strain.The experimental results show that the increases of volumetric and irreversible strains of rocks can be attributed to the compaction mechanism,which almost dominates the entire pre-peak deformation process.The unloading deformation consists of the reversible linear and nonlinear strains,and the irreversible strain under the influence of the porous grain structure.The pre-peak Young’s modulus tends to increase and then decrease due to the influence of the unloading irreversible strain.However,it hardly changes with the increasing volumetric strain compaction under the influence of reversible nonlinear strain.Instead,the initial unloading tangent modulus is highly related to the volumetric strain,and clearly reflects the compaction state of red sandstone.Furthermore,both the reversible nonlinear and irreversible unloading deformations are independent of confining pressure.This study is beneficial for the theoretical modeling and prediction of cyclic unloading deformation behavior of red sandstone.

Unloading-induced permeability recovery in rock fractures

Underground space creation and energy extraction, which induce unloading on rock fractures, commonly occur in various rock engineering projects, and rock engineering projects are subjected to high temperatures with increasing depth. Fluid flow behavior of rock fractures is a critical issue in many subsurface rock engineering projects. Previous studies have extensively considered permeability evolution in rock fractures under loading phase, whereas changes in fracture permeability under unloading phase have not been fully understood. To examine the unloading-induced changes in fracture permeability under different temperatures, we performed water flow-through tests on fractured rock samples subjected to decreasing confining pressures and different temperatures. The experimental results show that the permeability of fracture increases with unloading of confining pressure but decreases with loading-unloading cycles. Temperature may affect fracture permeability when it is higher than a certain threshold. An empirical model of fracture hydraulic aperture including two material parameters of initial normal stiffness and maximum normal closure can well describe the permeability changes in rough rock fracture subjected to loading-unloading cycles and heating. A coupled thermo-mechanical model considering asperity damage is finally used to understand the influences of stress paths and temperatures on fracture permeability.

Unloading damage patterns of rock slopes in open pit mines and analyses of their mechanisms

The stability of slopes is essential for ensuring safe production in open-pit mines.Analyzing and managing the deformation and failure of the slope rock mass becomes more challenging as the slope height increases.To investigate the damage patterns of slopes with varying heights,three slope models were developed based on a rock slope in Dagushan,China.The deformation failure processes of slopes under the influence of excavation and unloading were analyzed using the base friction test method in combination with digital image technology contrasting.The results supported the following findings:(1)Unloading tensile stress caused lateral partitioning in the slope.Both the foot and top of the slope underwent initial tensile cracks.(2)The destabilization mechanism of unloading deformation in slopes of different heights involved a combination of traction at the foot of the slope or pushing at the top of the slope,followed by accelerated deformation,deceleration creep,and overall destabilization.(3)The unloading damage patterns of slopes at different heights were summarized as follows:compression tension cracking,traction,and slip damage for medium and low slopes;compression tension cracking,traction,and slip failure for the upper part of high slopes;and relaxation tension cracking,pushing,traction,and slip failure for the lower part.Moreover,the upper part of ultra-high slopes exhibited compression tension cracking,traction,and slip failure,while the middle and lower parts displayed relaxation tension cracking,pushing,traction,and slip patterns.Finally,numerical simulations were conducted to verify the results of the test analyses,which demonstrated good consistency.These research results were of great engineering value for proposing effective safety management measures for high slopes.

Wellbore stability model in shale formation under the synergistic effect of stress unloading-hydration

According to the transversely isotropic theory and weak plane criterion, and considering the mechanical damages due to stress unloading and hydration during drilling, a shale wellbore stability model with the influence of stress unloading and hydration was established using triaxial test and shear test. Then, factors influencing the wellbore stability in shale were analyzed. The results indicate that stress unloading occurs during drilling in shale. The larger the confining pressure and axial stress, the more remarkable weakening of shale strength caused by stress unloading. The stress unloading range is positively correlated with the weakening degree of shale strength. Shale with a higher development degree of bedding is more prone to damage along bedding. In this case, during stress unloading, the synergistic effect of weak structural plane and stress unloading happens, leading to a higher weakening degree of shale strength and poorer mechanical stability, which brings a higher risk of wellbore instability. Fluid tends to invade shale through bedding, promoting the shale hydration. Hydration also can weaken shale mechanical stability, causing the decline of wellbore stability. Influence of stress unloading on collapse pressure of shale mainly occurs at the early stage of drilling, while the influence of hydration on wellbore stability mainly happens at the late stage of drilling. Bedding, stress unloading and hydration jointly affect the wellbore stability in shale. The presented shale wellbore stability model with the influence of stress unloading and hydration considers the influences of the three factors. Field application demonstrates that the prediction results of the model agree with the actual drilling results, verifying the reliability of the model.

Experimental study on failure characteristics of single-sided unloading rock under different intermediate principal stress conditions

Investigation of unloading rock failure under differentσ_(2)facilitates the control mechanism of excavation surrounding rock.This study focused on single-sided unloading tests of granite specimens under true triaxial conditions.The strength and failure characteristics were studied with micro-camera and acoustic emission(AE)monitoring.Furthermore,the choice of test path and the effect ofσ_(2)on fracture of unloading rock were discussed.Results show that the increasedσ_(2)can strengthen the stability of single-sided unloading rock.After unloading,the rock’s free surface underwent five phases,namely,inoculation,particle ejection,buckling rupture,stable failure,and unstable rockburst phases.Moreover,atσ_(2)≤30 MPa,the b value shows the following variation tendency:rising,dropping,significant fluctuation,and dropping,with dispersed damages signal.Atσ_(2)≥40 MPa,the tendency shows:a rise,a decrease,a slight fluctuation,and final drop,with concentrated damages signal.After unloading,AE energy is mainly concentrated in the micro-energy range.With the increasedσ_(2),the micro-energy ratio rises.In contrast,low,medium and large energy ratios drop gradually.The increased tensile fractures and decreased shear fractures indicate that the failure mode of the unloading rock gradually changes from tensile-shear mode to tensile-split one.The fractional dimension of the rock fragments first increases and then decreases with an inflection point at 20 MPa.The distribution of SIF on the planes changes asσ_(2)increases,resulting in strengthening and then weakening of the rock bearing capacity.

Deformation and damage properties of rock-like materials subjected to multi-level loading-unloading cycles

In the process of engineering construction such as tunnels and slopes,rock mass is frequently subjected to multiple levels of loading and unloading,while previous research ignores the impact of unloading rate on the stability of rock mass.A number of uniaxial multi-level cyclic loading-unloading experiments were conducted to better understand the effect of unloading rate on the deformation behavior,energy evolution,and damage properties of rock-like material.The experimental results demonstrated that the unloading rate and relative cyclic number clearly influence the deformation behavior and energy evo-lution of rock-like samples.In particular,as the relative cyclic number rises,the total strain and reversible strain both increase linearly,while the total energy density,elastic energy density,and dissipated energy density all rise nonlinearly.In contrast,the irreversible strain first decreases quickly,then stabilizes,and finally rises slowly.As the unloading rate increases,the total strain and reversible strain both increase,while the irreversible strain decreases.The dissipated energy damage was examined in light of the aforementioned experimental findings.The accuracy of the proposed damage model,which takes into account the impact of the unloading rate and relative cyclic number,is then confirmed by examining the consistency between the model predicted and the experimental results.The proposed damage model will make it easier to foresee how the multi-level loading-unloading cycles will affect the rock-like materials.