Experimental study on seepage characteristics of coal under truly triaxial loading and unloading based on different minimum principal stress properties

To study the safety issues caused by coal mine excavation, self-developed simulation of earth interior atmosphere and sound test system was used to perform true triaxial loading and unloading tests of coal. An acoustic emission detection system was used to record the damage evolution trend of coal under different intermediate principal stress states. The experimental results show that in the true triaxial unloading test, as the intermediate principal stress increases, the failure state of coal changes from shear failure to partial shear tension failure, finally leading to overall yield failure. In the stress-strain curves, with the increase in intermediate principal stress, the strain in the direction of intermediate principal stress gradually changes from compression to expansion, and typical expansion occurs. The Mogi–Coulomb strength criterion better reflects the strength failure characteristics of coal during unloading. The stress-acoustic emission diagrams show that the increase in intermediate principal stress causes the internal cracks of the coal to grow unsteadily and exponentially, and the increase in intermediate principal stress makes the coal lose its ability to continue to bear the load. Studying the influence of the intermediate principal stress on the mechanical properties of coal has practical significance for coal mine safety production.

Download Full-text

Seepage Flow Properties of a Columnar Jointed Rock Mass in a True Triaxial Experiment

Geofluids ◽

10.1155/2021/9924230 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Yanxin He ◽

Zhende Zhu ◽

Wenbin Lu ◽

Yunjin Hu ◽

Xinghua Xie ◽

...

Keyword(s):

Rock Mass ◽

Principal Stress ◽

Jointed Rock ◽

Jointed Rock Mass ◽

Intermediate Principal Stress ◽

True Triaxial ◽

Minimum Principal Stress ◽

Baihetan Hydropower Station ◽

Columnar Jointed Rock Mass ◽

Seepage Characteristics

A columnar jointed rock mass is a type of rock mass with strong geometric anisotropy and high interface permeability. Its seepage characteristics pose new challenges to the construction and maintenance of the Baihetan Hydropower Station on the Jinsha River. The research object in this study is the columnar jointed rock mass (basalt) in the dam area of Baihetan Hydropower Station. Similar-material model samples of the columnar jointed rock mass with different column dip angles ( α = 0 ° ~90°) were prepared following a similar principle. A true triaxial seepage–stress coupling test was conducted to evaluate the seepage characteristics of similar-material samples with different dip angles under intermediate principal stress and minimum principal stress. The experimental results showed that the columnar jointed rock mass exhibited apparent seepage anisotropy. The relationship curve between the volume flow rate Q and the pressure gradient − d P / d L of the samples with different dip angles showed evident nonlinear seepage under intermediate principal stress, which could be well expressed using the Forchheimer equation. It shows the characteristics of a typical linear Darcy flow under minimum principal stress. The law of variations in the permeability of the samples with different dip angles under intermediate principal stress can be well expressed using the one-dimensional quadratic function equation k = a + b σ 2 + c σ 2 2 , and the law of variations in the permeability of the samples with different dip angles under minimum principal stress can be well expressed using the logarithmic function k = a + b ln σ 3 . The permeabilities of the columnar jointed rock mass with dip angles of 0°, 15°, 30°, and 60° were most sensitive to changes in stress, and the seepage characteristics increased in complexity after changes in stress.

Download Full-text

Experimental study on the mechanical of deep-buried soft rock under triaxial loading and unloading

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/861/2/022066 ◽

2021 ◽

Vol 861 (2) ◽

pp. 022066

Author(s):

Lehua Wang ◽

Guangqiang Feng ◽

Bingyi Zhang ◽

Weizheng Shi ◽

Can Chen ◽

...

Keyword(s):

Experimental Study ◽

Soft Rock ◽

Triaxial Loading ◽

Loading And Unloading

Download Full-text

Experimental Study on Mechanical Behavior of Concrete Loading along Path (Constant Stress, Constant Strain, Increasing Strain)

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.238.91 ◽

2012 ◽

Vol 238 ◽

pp. 91-95

Author(s):

Zhe Wang ◽

Jing Li ◽

Ren Jie Shang

Keyword(s):

Experimental Study ◽

Principal Stress ◽

Concrete Strength ◽

Test System ◽

Normal Concrete ◽

Peak Point ◽

Linear Relationships ◽

Minimum Principal Stress ◽

Two Stages ◽

The Relationship

Triaxial compressive experiments were performed with normal concrete by using the large triaxial test system. The loading process was divided into two stages: (1) Under the condition of keeping the three axial stresses equivalent, the compressive stress was increased to a specified value p, which was maintained for a period of time; (2) The strain in y direction was increased monotonously while the stress in x direction and the strain in z direction both remained constant. In this paper, the study on the experiments indicates: the minimum principal stress significantly influences the strength of concrete, i.e., the greater the minimum principal stress is, the greater the concrete strength is; the stress in y direction where the strain stayed constant is the middle principal stress; the constant strain influences the relationship between σ2 and σ1, they reach the peak values at the same time and approximate to linear relationships in much parts of their descending stage, curvature only appears at their ends; When ε13>8, each relationship curve of τ13-γ13 gradually becomes horizontal；Below a line AB in figure of coordinate system εx, εy the curves of εx-εy approximately parallel to each other; If the minimum principal stress is 15MPa or 20MPa, After εkk reaches the peak point, the volume gradually becomes stable after a transient decrease stage.

Download Full-text

Numerical simulation on the simultaneous stress interference of parallel multiple hydraulic fractures

Energy Exploration & Exploitation ◽

10.1177/01445987211001955 ◽

2021 ◽

pp. 014459872110019

Author(s):

Weiyong Lu ◽

Changchun He

Keyword(s):

Principal Stress ◽

Hydraulic Fracture ◽

Stress Ratio ◽

Fracture Network ◽

Hydraulic Fractures ◽

Fracture Spacing ◽

Induced Stress ◽

Minimum Principal Stress ◽

Fracturing Process ◽

Stress Ratios

During horizontal well staged fracturing, there is stress interference between multiple transverse fractures in the same perforation cluster. Theoretical analysis and numerical calculation methods are applied in this study. We analysed the mechanism of induced stress interference in a single fracture under different fracture spacings and principal stress ratios. We also investigated the hydraulic fracture morphology and synchronous expansion process under different fracture spacings and principal stress ratios. The results show that the essence of induced stress is the stress increment in the area around the hydraulic fracture. Induced stress had a dual role in the fracturing process. It created favourable ground stress conditions for the diversion of hydraulic fractures and the formation of complex fracture network systems, inhibited fracture expansion in local areas, stopped hydraulic fractures, and prevented the formation of effective fractures. The curves of the maximum principal stress, minimum principal stress, and induced principal stress difference with distance under different fracture lengths, different fracture spacings, and different principal stress ratios were consistent overall. With a small fracture spacing and a small principal stress ratio, intermediate hydraulic fractures were difficult to initiate or arrest soon after initiation, fractures did not expand easily, and the expansion speed of lateral hydraulic fractures was fast. Moreover, with a smaller fracture spacing and a smaller principal stress ratio, hydraulic fractures were more prone to steering, and even new fractures were produced in the minimum principal stress direction, which was beneficial to the fracture network communication in the reservoir. When the local stress and fracture spacing were appropriate, the intermediate fracture could expand normally, which could effectively increase the reservoir permeability.

Download Full-text