Temperature effects on the failure of deep circular tunnel under true-triaxial compression

The failure characteristics of thermal treated surrounding rocks should be studied to evaluate the stability and safety of deep ground engineering under high-ground-temperature and high-ground-stress conditions.The failure process of the inner walls of fine-grained granite specimens at different temperatures(25–600℃)was analyzed using a true-triaxial test system.The failure process,peak intensity,overall morphology(characteristics after failure),rock fragment characteristics,and acoustic emission(AE)characteristics were analyzed.The results showed that for the aforementioned type of granite specimens,the trend of the failure stress conditions changed with respect to the critical temperature(200℃).When the temperature was less than 200℃,the initial failure stress increased,final failure stress increased,and failure severity decreased.When the temperature exceeded 200℃,the initial failure stress decreased,final failure stress decreased,and failure severity increased.When the temperature was 600℃,the initial and final failure stresses of the specimens decreased by 60.93%and 19.77%compared with those at 200℃,respectively.The numerical results obtained with the software RFPA3D-Thermal were used to analyze the effect of temperature on the specimen and reveal the mechanism of the failure process in the deep tunnel surrounding rock.

Axisymmetric alternating direction explicit scheme for efficient coupled simulation of hydro-mechanical interaction in geotechnical engineering-Application to circular footing and deep tunnel in saturated ground

Explicit solution techniques have been widely used in geotechnical engineering for simulating the coupled hydro-mechanical(H-M) interaction of fluid flow and deformation induced by structures built above and under saturated ground, i.e. circular footing and deep tunnel. However, the technique is only conditionally stable and requires small time steps, portending its inefficiency for simulating large-scale H-M problems. To improve its efficiency, the unconditionally stable alternating direction explicit(ADE)scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain flow conditions. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving flow problems in nonuniform grids and under axisymmetric conditions. The new scheme is derived by performing a fourthorder finite difference(FD) approximation to the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps,giving rise to a new axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code fast Lagrangian analysis of continua(FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. Application of SEA-4-AXI for solving axisymmetric consolidation of a circular footing and of advancing tunnel in deep saturated ground shows that SEA-4-AXI reduces computer runtime up to 42%-50% that of FLAC’s basic scheme without numerical instability. In addition, it produces high numerical accuracy of the H-M solutions with average percentage difference of only 0.5%-1.8%.

Effects of mining－induced earthquakes on water table in deep wells

Based on the systematical collection and processing of data on the influence of mining-induced earthquakes on water table regime in deep well Lu-15 in Taozhuang Coal Mine since 1980, we study the characteristics of coseismic effect of water table in deep well in this paper. We have found precursory phenomena of water table in deep well before mining-induced earthquake. Here we discuss the physical mechanism of coseismic effect of mining--induced earthquake on water table in deep well.

Applications of FBG-based sensors to ground stability monitoring

The four diversion tunnels at Jinping Ⅱ hydropower station represent the deepest underground project yet conducted in China, with an overburden depth of 1500-2000 m and a maximum depth of 2525 m.The tunnel structure was subjected to a maximum external water pressure of 10.22 MPa and the maximum single-point groundwater inflow of 7.3 m^3/s. The success of the project construction was related to numerous challenging issues such as the stability of the rock mass surrounding the deep tunnels, strong rockburst prevention and control, and the treatment of high-pressure, large-volume groundwater infiltration. During the construction period, a series of new technologies was developed for the purpose of risk control in the deep tunnel project. Nondestructive sampling and in-situ measurement technologies were employed to fully characterize the formation and development of excavation damaged zones（EDZs）, and to evaluate the mechanical behaviors of deep rocks. The time effect of marble fracture propagation, the brittleeductileeplastic transition of marble, and the temporal development of rock mass fracture and damage induced by high geostress were characterized. The safe construction of deep tunnels was achieved under a high risk of strong rockburst using active measures, a support system comprised of lining, grouting, and external water pressure reduction techniques that addressed the coupled effect of high geostress, high external water pressure, and a comprehensive early-warning system. A complete set of technologies for the treatment of high-pressure and large-volume groundwater infiltration was developed. Monitoring results indicated that the Jinping II hydropower station has been generally stable since it was put into operation in 2014.

Analysis of Observation Data from New and Old Sites of National Meteorological Observing Station of Hejian City

This study aimed to conduct statistical analysis of temperature,relative humidity,wind direction,wind velocity,deep ground temperature and other related data from National Meteorological Observing Station of Hejian City in 2012.According to the results,observation data varied due to different instruments and equipments,surrounding environments and underlying surface properties.The difference value of monthly average temperature between new site and old site ranged from-0.5 to 0 ℃; the difference value of monthly average maximum temperature ranged from-0.4 to 0.2 ℃; the difference value of monthly average minimum temperature ranged from-0.8 to 0 ℃; the difference value of monthly extreme maximum temperature ranged from-1.1 to 0.6 ℃; the difference value of monthly extreme minimum temperature ranged from-1.2 to 0.3 ℃.Annual average temperature,annual average maximum temperature and annual average minimum temperature in new site were lower than those in old site; annual extreme maximum temperature in new site was higher than that in old site; annual extreme minimum temperature in new site was lower than that in old site.The difference value of monthly average relative humidity between new site and old site ranged from 2％ to 6％; the difference value of monthly minimum relative humidity ranged from-4％ to 5％.Annual minimum relative humidity in new site was consistent with that in old site.The difference value of 2 min average wind velocity between new site and old site ranged from-0.1 to 0.4 m/s; the difference value of monthly maximum wind velocity ranged from-1.2 to 2.2 m/s; the difference value of monthly extreme wind velocity ranged from-2.0 to 2.8 m/s.Annual maximum wind velocity in new site was basically consistent with that in old site; annual extreme wind velocity in new site was significantly higher than that in old site; annual wind direction frequency in new site was lower than that in old site; annual most frequent wind direction in new site was S and that in old site was SSW.The difference value of average temperature at the depth of 40 cm ranged from-1.1 to 2.5 ℃; the difference value of average temperature at the depth of 80 cm ranged from-2.4 to 2.1 ℃; the difference value of average temperature at the depth of 160 cm ranged from-2.5 to 2.7 ℃; the difference value of average temperature at the depth of 320 cm ranged from-1.6 to 1.1 ℃.Annual average temperatures at the depths of 40 and 160 cm in new site were higher than those in old site,while annual average temperatures at the depths of 80 and 320 cm in new site were lower than those in old site.This paper provided certain correction stand for the use of observation data from new and old sites.

Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Air2Land:A deep learning dataset for unmanned aerial vehicle autolanding from air to land

In this paper,a novel deep learning dataset,called Air2Land,is presented for advancing the state‐of‐the‐art object detection and pose estimation in the context of one fixed‐wing unmanned aerial vehicle autolanding scenarios.It bridges vision and control for ground‐based vision guidance systems having the multi‐modal data obtained by diverse sensors and pushes forward the development of computer vision and autopilot algorithms tar-geted at visually assisted landing of one fixed‐wing vehicle.The dataset is composed of sequential stereo images and synchronised sensor data,in terms of the flying vehicle pose and Pan‐Tilt Unit angles,simulated in various climate conditions and landing scenarios.Since real‐world automated landing data is very limited,the proposed dataset provides the necessary foundation for vision‐based tasks such as flying vehicle detection,key point localisation,pose estimation etc.Hereafter,in addition to providing plentiful and scene‐rich data,the developed dataset covers high‐risk scenarios that are hardly accessible in reality.The dataset is also open and available at https://github.com/micros‐uav/micros_air2land as well.