High-resolution photogrammetry to measure physical aperture of two separated rock fracture surfaces

Photogrammetry,reconstructing three-dimensional(3D)models from overlapping two-dimensional(2D)photos,finds application in rock mechanics and rock engineering to extract geometrical details of reconstructed objects,for example rock fractures.Fracture properties are important for determining the mechanical stability,permeability,strength,and shear behavior of the rock mass.Photogrammetry can be used to reconstruct detailed 3D models of two separated rock fracture surfaces to characterize fracture roughness and physical aperture,which controls the fluid flow,hydromechanical and shear behavior of the rock mass.This research aimed to determine the optimal number of scale bars required to produce high-precision 3D models of a fracture surface.A workflow has been developed to define the physical aperture of a fracture using photogrammetry.Three blocks of Kuru granite(25 cm×25 cm×10 cm)with an artificially induced fracture,were investigated.For scaling 3D models,321 markers were used as ground control points(GCPs)with predefined distances on each block.When the samples were wellmatched in their original positions,the entire block was photographed.Coordinate data of the GCPs were extracted from the 3D model of the blocks.Each half was surveyed separately and georeferenced by GCPs and merged into the same coordinate system.Two fracture surfaces were extracted from the 3D models and the vertical distance between the two surfaces was digitally calculated as physical aperture.Accuracy assessment of the photogrammetric reconstruction showed a 20-30 mm digital control distance accuracy when compared to known distances defined between markers.To attain this accuracy,the study found that at least 200 scale bars were required.Furthermore,photogrammetry was employed to measure changes in aperture under normal stresses.The results obtained from this approach were found to be in good agreement with those obtained using linear variable displacement transducers(LVDTs),with differences ranging from 1 mm to 8μm.

非均布荷载作用下煤岩力学强度特性试验研究

考虑不同初始垂向应力、不同初始水平应力及不同应力转移速度条件,借助于自主研发的采动应力试验系统,研究了非均布荷载作用下煤岩体的力学强度特性。研究结果表明:煤岩外部损伤残余块体对煤岩内部力学承载能力具有强化作用,当初始垂向应力、初始水平应力、应力转移速度一定时,煤岩内部力学承载能力近似为煤岩外部力学承载能力的2倍;伴随初始垂向应力的增加,煤岩整体后期力学承载能力不断降低,且处于不同初始垂向应力范围,煤岩内、外部后期力学强度递减特征不同,当处于较小初始垂向应力范围时,煤岩外部后期力学强度递减量大于煤岩内部,当处于较大初始垂向应力范围时,煤岩外部后期力学强度递减量小于煤岩内部;初始水平应力越大,煤岩体的力学承载能力越高,且无论较小还是较大初始水平应力范围,煤岩内部后期力学强度增量总大于煤岩外部;伴随应力转移速度的增加,煤岩峰值强度不断升高,应力转移速度对煤岩内部区域力学性质的影响相对较大。

Simulation of the interactions between hydraulic and natural fractures using a fracture mechanics approach

HYDROCK method aims to store thermal energy in the rock mass using hydraulically propagated fracture planes.The hydraulic fractures can interact with the pre-existing natural fractures resulting in a complex fracture network,which can influence the storage performance.This study investigates the interactions between hydraulic and natural fractures using a fracture mechanics approach.The new functionality of the fracture mechanics modelling code FRACOD that enables crossing of hydraulically driven fracture by a pre-existing fracture is presented.A series of two-dimensional numerical models is prepared to simulate the interaction at different approach angles in granitic rock of low permeability.It is demonstrated that multiple interaction mechanisms can be simulated using the fracture mechanics approach.The numerical results are in agreement with the modified Renshaw and Pollard analytical criterion for fracture crossing.The results show that for large approach angles,the hydraulic fracture crosses the natural fracture,whereas for small approach angles,the hydraulic fracture activates the natural fracture and the wing-shaped tensile fractures are propagated from its tips.Thus,the presence of fractures with low dip angles can lead to the growth of more complex fracture network that could impair the thermal performance of the HYDROCK method.

Modelling fracture propagation and failure in a rock pillar under mechanical and thermal loadings

The Aspoe Pillar Stability Experiment （APSE） was conducted to study the rock mass response in a heated rock pillar between two large boreholes. This paper summarizes the back calculations of the APSE using a two-dimensional （2D） fracture propagation code FRACOD. To be able to model all the loading phases of the APSE, including the thermal loading, the code was improved in several ways. A sequential excavation function was developed to model promptly the stepwise changing loading geometry. Prior to the mod- elling, short-term compressive strength test models were set up aiming to reproduce the stress-strain behaviour observed for the Aspoe diorite in laboratory. These models simulate both the axial and lateral strains of radial-controlled laboratory tests, The volumetric strain was calculated from the simulations and compared with the laboratory results, The pillar models include vertical and horizontal 2D models from where the stress in the pillar wall was investigated, The vertical model assesses the stability of the experimental rock volume and suggests the resultant stress below the tunnel floor in the pillar area. The horizontal model considers cross-sections of the pillar between the two large boreholes. The horizon- tal model is used to simulate the evolution of the stress in the rock mass during the excavation of the boreholes and during and the heating phase to give an estimation of the spalling strength. The modelling results suggest that the excavation-induced stresses will cause slight fracturing in the pillar walls, if the strength of the APSE pillar is set to about 123 MPa. Fracture propagation driven by thermal loading leads to minor spalling. The thermal evolution, elastic behaviour and brittle failure observed in the experiment are well reflected by the models.