CONFINING PRESSURE EFFECT ON ACOUSTIC EMISSIONS IN ROCK FAILURE

Based on the phenomenon that acoustic emissions (AE) generated by rock mass increase suddenly because of underground excavation, time sequence of AE rate in rock failure has been discussed by using statistical damage theory. It has been demonstrated that how the influence of confining pressure on the deformation behavior and AE characteristics in rocks can be inferred from a simple mechanics model. The results show that loading confining pressure sharply brings out increasing of AE. On the other hand, few AE emits when confining pressure is loaded sharply, and AE occurs again when axial pressure keeps on increasing. These results have been well simulated with computer and show close correspondence with directly measured curves in experiments.

Confining pressure effect on dynamic response of high rockfill dam

Studies show that the dynamic properties of rockfill are strongly dependent on the confining pressure.Therefore,confining pressure effect has become a very important factor in the seismic analysis of high rockfill dam.The relationships of dynamic shear modulus versus dynamic shear strain and damping ratio versus dynamic shear strain had been improved to a certain degree on the basic of widely used Hardin-Drnevich constitutive model in this paper.Then a new model that could consider confining pressure effect has been established.Regression analysis was carried out of the dynamic triaxial experimental data of the damming materials in the Changheba hydropower station of Sichun Province,China.The results show that,the new model can fit the test data well under various confining pressures.A corresponding computational procedure was compiled and applied in the dynamic response analysis of the Changheba Dam.Comparing the calculation results between the new constitutive model and the ordinary Hardin-Drnevich model,it can be seen that the result is conservative to some extent without considering confining pressure effect.

Characterization and evaluation of brittleness of deep bedded sandstone from the perspective of the whole life-cycle evolution process

The quantitative determination and evaluation of rock brittleness are crucial for the estimation of excavation efficiency and the improvement of hydraulic fracturing efficiency.Therefore,a“three-stage”triaxial loading and unloading stress path is designed and proposed.Subsequently,six brittleness indices are selected.In addition,the evolution characteristics of the six brittleness indices selected are characterized based on the bedding effect and the effect of confining pressure.Then,the entropy weight method(EWM)is introduced to assign weight to the six brittleness indices,and the comprehensive brittleness index Bcis defined and evaluated.Next,the new brittleness classification standard is determined,and the brittleness differences between the two stress paths are quantified.Finally,compared with the previous evaluation methods,the rationality of the proposed comprehensive brittleness index Bcis also verified.These results indicate that the proposed brittleness index Bccan reflect the brittle characteristics of deep bedded sandstone from the perspective of the whole life-cycle evolution process.Accordingly,the method proposed seems to offer reliable evaluations of the brittleness of deep bedded sandstone in deep engineering practices,although further validation is necessary.

Mechanical behavior of deep sandstone under high stress-seepage coupling

The mechanical behavior evolution characteristics of sandstone are important to the application and practice of rock engineering.Therefore,a new method and concept of deep rock mechanics testing are proposed to reveal the mechanical behavior evolution mechanism of deep roadway surrounding rock after excavation with a depth over 1000 m.High stress-seepage coupling experiments of deep sandstone under various confining pressures are conducted using GCTS.Stress−strain and permeability curves are obtained.The three-stage mechanical behavior of deep sandstone is better characterized.A platform and secondary compaction phenomena are observed.With the confining pressure increasing,the platform length gradually decreases,even disappears.In the stade I,the rigid effect of deep sandstone is remarkable.In the stage II,radial deformation of deep sandstone dominates.The transient strain of confining pressure compliance is defined,which shows three-stage evolution characteristics.In the stage III,the radial deformation is greater than the axial deformation in the pre-peak stage,but the opposite trend is observed in the post-peak stage.It is found that the dynamic permeability can be more accurately characterized by the radial strain.The relations between the permeability and stress−strain curves in various stages are revealed.

CO_2 permeability of fractured coal subject to confining pressures and elevated temperature: Experiments and modeling

The CO_2 permeability of fractured coal is of great significance to both coalbed gas extraction and CO_2 storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperature on the CO_2 permeability of fractured coal with different fracture extents have not been investigated thoroughly. In this paper, the CO_2 permeability of fractured coals sampled from a Pingdingshan coal mine in China and artificially fractured to a certain extent is investigated through undrained triaxial tests. The CO_2 permeability is measured under the confining pressure with a range of 10–25 MPa, injection pressure with a range of 6–12 MPa and elevated temperature with a range of 25–70°C. A mechanistic model is then proposed to characterize the CO_2 permeability of the fractured coals. The effects of thermal expansion, temperature-induced reduction of adsorption capacity, and thermal micro-cracking on the CO_2 permeability are explored. The test results show that the CO_2 permeability of naturally fractured coal saliently increases with increasing injection pressure. The increase of confining pressure reduces the permeability of both naturally fractured coal and secondarily fractured coal. It is also observed that initial fracturing by external loads can enhance the permeability, but further fracturing reduces the permeability. The CO_2 permeability decreases with the elevation of temperature if the temperature is lower than 44°C, but the permeability increases with temperature once the temperature is beyond 44°C. The mechanistic model well describes these compaction mechanisms induced by confining pressure, injection pressure and the complex effects induced by elevated temperature.