Physical model simulation of rock-support interaction for the tunnel in squeezing ground

The existence of squeezing ground conditions can lead to significant challenges in designing an adequate support system for tunnels.Numerous empirical,observational and analytical methods have been suggested over the years to design support systems in squeezing ground conditions,but all of them have some limitations.In this study,a novel experimental setup having physical model for simulating the tunnel boring machine(TBM)excavation and support installation process in squeezing clay-rich rocks is developed.The observations are made to understand better the interaction between the support and the squeezing ground.The physical model included a large true-triaxial cell,a miniature TBM,laboratoryprepared synthetic test specimen with properties similar to natural mudstone,and an instrumented cylindrical aluminum support system.Experiments were conducted at realistic in situ stress levels to study the time-dependent three-dimensional tunnel support convergence.The tunnel was excavated using the miniature TBM in the cubical rock specimen loaded in the true-triaxial cell,after which the support was installed.The confining stress was then increased in stages to values greater than the rock’s unconfined compressive strength.A model for the time-dependent longitudinal displacement profile(LDP)for the supported tunnel was proposed using the tunnel convergence measurements at different times and stress levels.The LDP formulation was then compared with the unsupported model to calculate the squeezing amount carried by the support.The increase in thrust in the support was backcalculated from an analytical solution with the assumption of linear elastic support.Based on the test results and case studies,a recommendation to optimize the support requirement for tunnels in squeezing ground is proposed.

An Extrinsic Fiber Fabry-Perot Interferometer for Dynamic Displacement Measurement

A versatile fiber interferometer was proposed for high precision measurement. The sensor exploited a double-cavity within the unique sensing arm of an extrinsic-type fiber Fabry-Perot interferometer to produce the quadrature phase-shifted interference fringes. Interference signal processing was carried out using a modified zero-crossing （fringe） counting technique to demodulate two sets of fringes. The fiber interferometer has been successfully employed for dynamic displacement measurement under different displacement profiles over a range of 0.7 p.m to 140 p.m. A dedicated computer incorporating the demodulation algorithm was next used to interpret these detected data as well as plot the displacement information with a resolution of 2/64. A commercial displacement sensor was employed for comparison purposes with the experimental data obtained from the fiber interferometer as well as to gauge its performance, resulting in the maximum error of 2.8% over the entire displacement range studied.

Effect of permeability and its horizontal anisotropy on enhanced coalbed methane recovery with CO_(2) storage:quantitative evaluation based on staged CH_(4) output inhibition

The permeability and its horizontal anisotropy induce a critical influence on staged CH_(4) output inhibition process.However,a quantitative evaluation of this influence has been rarely reported in the literature.In this work,the impact of horizontal anisotropic permeability on CO_(2)-ECBM was numerically investigated.The variation in the staged CH_(4) output inhibition was analyzed.The ideal displacement profile of the CO_(2)-ECBM process was established for the first time.Moreover,the variation in CH_(4) output of different wellbores was discussed.The results showed that 1)low-permeable or weak-anisotropic reservoirs were not conducive to enhanced CH_(4) recovery owing to long inhibition time(>1091 days)and high inhibition level(>36.9%).As permeability and anisotropy increased,due to the accelerated seepage of free water,the hysteresis time and inhibition time could decrease to as short as 5 days and 87 days,respectively,and the inhibition level could weaken to as low as 5.00%.Additionally,the CH_(4) output and CO_(2) injection could increase significantly.2)Nevertheless,high permeability and strong anisotropy easily induced CO_(2) breakthrough,resulting in lower CH_(4) production,CO_(2) injection and CO_(2) storage than expected.While maintaining high efficiency of CO_(2) storage(>99%),upregulating CO_(2) breakthrough concentration from 10%to 20%might ease the unfavorable trend.3)Along the direction of fluid flow,the ideal displacement profile consisted of CO_(2) enriched bank,CO_(2) and CH_(4) mixed bank,CH_(4) enriched bank,and water enriched bank,whereas a remarkable gap in the displacement profiles of the dominant and non-dominant seepage directions was observed.4)The potential of CH_(4) output might vary greatly among different wellbores.The producers along the dominant seepage direction held more potential for CH_(4) recovery in the short-term,while those along the non-dominant seepage direction avoided becoming invalid only if a long-time injection measure was taken for the injectors.These findings pave the way to understand fluid seepage in real complex reservoirs during CO_(2)-ECBM and conduct further field projects.