Large-Scale True Triaxial Apparatus for Geophysical Studies in Fractured Rock

Pang

Advances in Civil Engineering

et al. 2020

The karst collapse pillar (KCP) is a common geological structure in the coal mines of northern China. KCPs contain many fractured coal rocks, which can easily migrate under the action of high-pressure water. The destruction or instability of the cementation structure between the rocks can directly induce coalmine water-inrush accidents. To study the seepage stability of cemented and fractured coal rock under triaxial pressures, a self-designed triaxial seepage testing system was used and the permeability k and non-Darcy factor β of the cemented and fractured coal rock were tested. Furthermore, the 1D non-Darcy seepage equations were used to calculate the evolution criteria of the seepage loss stability. The results show the following: (1) The cemented structure in the KCP under the triaxial pressures can be easily destroyed. The damaged coal and rock body mainly exists in bulk form, and the permeability depends mainly on the effective stress of the particles. (2) The seepage process in the KCP structure is a combination of pore flow, fracture flow, and pipe flow, and the transition of the seepage state is closely related to the change in the magnitude of β. (3) Under the long-term effect of confined underground water, the migration of small fractured particles in the KCP will increase the structural porosity. If the parameter βk2 reaches the threshold value, the seepage system will evolve into a pipeline flow state, eventually causing a water-inrush accident.

Section: Test Methodsmentioning

confidence: 99%

Determining the Seepage Stability of Fractured Coal Rock in the Karst Collapse Pillar

Pang

Advances in Civil Engineering

et al. 2020

“…We use a large-scale true triaxial testing frame to subject a 50 cm × 50 cm × 50 cm prefractured rock specimen to independent σ'x, σ'y, σ'z boundary stresses (Garcia et al, 2018).…”

Section: Devices and Test Proceduresmentioning

confidence: 99%

“…We use a large‐scale true triaxial testing frame to subject a 50 × 50 × 50 cm prefractured rock specimen to independent σ’ x , σ’ y , and σ’ z boundary stresses (Garcia et al., 2018). Thin hydraulic flat jacks installed on all six sides apply the principal stresses.…”

Section: Devices and Test Proceduresmentioning

confidence: 99%

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Long‐Wavelength Propagation in Fractured Rock Masses (3D Stress Field)

2022

Self Cite

Fractured rocks affect a wide range of natural processes and engineering systems. In most cases, the seismic characterization of fractured rock masses in the field involves wavelengths much longer than the fracture spacing; reproducing this condition in the laboratory is experimentally challenging. This experimental investigation explores the effect of fracture rock fabric and the 3D stress field on P wave propagation in the long‐wavelength regime using a large‐scale true triaxial device. P wave velocities increase with stress in the propagation direction and follow a power law of the form Vp = α(σ’/kPa)β; analyses and experimental results show that stress‐sensitive fracture stiffness and fracture density define the α‐factor and β‐exponent; conversely, long‐wavelength velocity versus stress data can be analyzed to identify the stress‐dependent fracture stiffness. P wave velocities exhibit hysteretic behavior caused by inelastic fracture deformation and fabric changes. During deviatoric loading, the P wave velocity decreases in the two constant‐stress directions due to the development of internal force chains and the ensuing three‐dimensional deformation. Following a load increment, time‐dependent contact deformations result in P wave velocity changes during the first several hours for the tested carbonate rocks; the asymptotic change in velocity is more pronounced for higher stress changes and stress levels. The fracture network geometry that defines the rock fabric acts as a low‐pass filter to wave propagation, so that wavelengths must be longer than two times the fracture spacing to propagate (Brillouin dispersion); the long‐wavelength velocity and the fracture spacing determine the cutoff frequency. Fabric anisotropy contributes to anisotropic low‐pass filtering effects in the rock mass.

Geotechnical Testing Journal

A High-Pressure High-Temperature Column for the Simulation of Hydrothermal Water Circulation at Laboratory Scale

Frank

Zuber

Pollak

et al. 2021

Modeling the geothermal energy production cycle of a deep geothermal system at laboratory scale is challenging because of high-temperature and pressure conditions. In this work, a high-pressure high-temperature column to simulate production, heat transfer, and reinjection of a geothermal fluid in a fractured rock system is presented. The column includes two independently heated pressure vessels, a heat exchanger, and sensors for temperatures, pressures, flow rate, electric conductivity, and pH value of the circulating fluid at different locations. The presented column enables the quantitative analysis of coupled hydro-thermo-chemical processes in fractured rock cores close to in situ geothermal conditions. Heat extraction and reinjection of geothermal fluids into fractured reservoirs can be reproduced because of the possibility of heating and cooling of the circulating fluid. Further, it is possible to inject a second fluid phase into the column to investigate additional processes, such as mineral precipitation during reinjection. In this work, we present the experimental setup of the column and first results showing the capability of the system.