Showing papers on "Discontinuity (geotechnical engineering) published in 2021"

Monotonic behavior of interface shear between carbonate sands and steel

Abstract: The characteristic of interface shear between carbonate sands and steel is important for the design of onshore/offshore foundations in coral reef deposit. By using an interface ring shear apparatus, the influences of particle size, normal stress, steel roughness and shear displacement on the interface shear strength were studied in this paper. During the tests, the particle movement localized in the deformation band was observed and the degree of particle breakage was evaluated. The test results showed that under the effect of large displacement interface shear (≥ 1 m), a significant particle breakage occurred in the thin zone near the interface, and the roughness of steel interface decreased accordingly. The relative smooth steel surface shows small interface friction angle without dilation phenomenon during shearing, while higher roughness leads to higher interface strength due to stress dilation. It was also found that there was a kind of particle size discontinuity in the shear zone. Particle breakage is main reason for the increase in interface friction angle in large displacement shear. A new dimensionless parameter was proposed to consider the influences of particle breakage and interface roughness on the mobilization of interface strength between sands and steel. It is recommended that the influence of fine particles embedded into steel interface should be considered in large displacement shear for carbonate sands.

A state-of-the-art review of automated extraction of rock mass discontinuity characteristics using three-dimensional surface models

Abstract: In the last two decades, significant research has been conducted in the field of automated extraction of rock mass discontinuity characteristics from three-dimensional (3D) models. This provides several methodologies for acquiring discontinuity measurements from 3D models, such as point clouds generated using laser scanning or photogrammetry. However, even with numerous automated and semi-automated methods presented in the literature, there is not one single method that can automatically characterize discontinuities accurately in a minimum of time. In this paper, we critically review all the existing methods proposed in the literature for the extraction of discontinuity characteristics such as joint sets and orientations, persistence, joint spacing, roughness and block size using point clouds, digital elevation maps, or meshes. As a result of this review, we identify the strengths and drawbacks of each method used for extracting those characteristics. We found that the approaches based on voxels and region growing are superior in extracting joint planes from 3D point clouds. Normal tensor voting with trace growth algorithm is a robust method for measuring joint trace length from 3D meshes. Spacing is estimated by calculating the perpendicular distance between joint planes. Several independent roughness indices are presented to quantify roughness from 3D surface models, but there is a need to incorporate these indices into automated methodologies. There is a lack of efficient algorithms for direct computation of block size from 3D rock mass surface models.

Influence of Tunnel Excavation on the Stability of a Bedded Rock Slope: A Case Study on the Mountainous Area in Southern Anhui, China

Abstract: Tunnel excavation has a substantial effect on the stability of rock slopes. The influence of tunnel excavation on the deformation and mechanical characteristics of a bedded slope is investigated by analysing the cumulative displacement, stress, and strain of the slope using the finite-element method (FEM) and field monitoring method. The deformation characteristics of the surrounding rock of the tunnel and slope with a supporting structure were analysed using the FEM under the conditions of support or without support. The results show that the deformation characteristics of the slope are controlled by its discontinuities. The deformation of the slope is mainly concentrated in the area above the second discontinuity; in particular, the deformation above the first discontinuity is the largest. In addition, the deformation and mechanical characteristics of the support structure are analysed. The supporting structure has influence on the deformation of the slope, which reduces the cumulative displacement, stress and strain. The shear strains of the second and third discontinuities are greatly influenced by the supporting structure. The mechanical properties of the tunnel support structure are controlled by the first discontinuity, and a stress concentration occurs near the first discontinuity. Moreover, based on the combination of the numerical and field test results, the deformation of the rock slope is closely related to the distance from the slope surface. Tunnel excavation mainly has a significant impact on the deformation of surface slope within a certain range, especially the slope deformation within the range of 4 m, which is the largest.

A New DIC-Based Method to Identify the Crack Mechanism and Applications in Fracture Analysis of Red Sandstone Containing a Single Flaw

Abstract: As a representative non-interferometric optical technique, the digital image correlation (DIC) can provide full-field displacement and strain measurement for the deformed rocks. However, the standard DIC technique has a limitation in measuring the displacements at the discontinuity and cannot be directly used for identifying the crack mechanism. Thus, a new DIC-based method is proposed for automatically tracing the discontinuities and quantitatively identifying the crack mechanism, i.e. mode I, mode II, and mixed-mode I/II. The new method involves three steps, including displacement measurement from the standard DIC technique, displacement field reconstruction at the discontinuity with the modified subset splitting technique, and post-processing for crack identification and displacement jump measurement. The effectiveness and robustness of the modified subset splitting technique and post-processing method have been verified with the synthetic images and theoretical displacement fields of mode I crack and dislocation. Then, the proposed method is utilized to locate cracks and quantitatively identify the crack mechanism of the initiated cracks in red sandstone containing a single flaw under uniaxial compression. The crack development in the flawed red sandstone specimens is analyzed and the crack types are summarized, in which wing cracks are in mode I, while horsetail cracks and anti-wing cracks are identified as mixed-mode I/II crack. It is shown that the new method avoids some ambiguous identification of crack mechanism and present more objective results.

Spacing and block volume estimation in discontinuous rock masses using image processing technique: a case study

Abstract: Application of the image processing techniques (IPT) to identify rock mass geometry provides more fast information about discontinuity properties used in geo-engineering characteristics. In this regard, the field survey can be improved using IPT. This study has utilised the IPT to identify the discontinuity and block volume characteristics in a discontinuous rock mass. For this purpose, a visual evaluation of the rock mass outcrop with discontinuities from a road slope cut located in the South Pars Special Zone, Assalouyeh, Iran, was considered. A three-step IPT analysis (i.e. pre-processing, main processing, and post-processing) was conducted to extract the features through the Python programming language. Regarding the IPT methodology, the studied rock mass characteristics consist of four major discontinuity sets and rock block volumes between the intersections of the discontinuities, as confirmed with a scan-line field survey. The evaluated data indicated that the maximum, minimum, and average block volumes processed by the IPT were 1.068, 0.479, and 1.055 m3, and their field measurement results were 1.092, 0.479, and 1.065 m3, respectively. Additionally, the orientations of the estimated discontinuity properties and their spacings determined by IPT for the rock mass ranging between 32 and 69.9° and 0.5 and 2.18 m, respectively. Similarly, the orientations of the field measurement results were also obtained between 33 and 71° and 0.58 and 2.25 m, respectively. The results of the IPT and the field survey were close, which revealed that the IPT is a reliable method for determining discontinuity spacing and rock block volume along large cut slopes. This approach provided rapid data processing with spatial extensions in a short period, making it possible to achieve accurate results in discontinuity network characteristics.

Improvement of DDA with a New Unified Tensile Fracture Model for Rock Fragmentation and its Application on Dynamic Seismic Landslides

Abstract: Discontinuous deformation analysis (DDA) method is a discrete element method, presenting a great advantage in modelling deformation and rigid body movements, and it is also an alternative approach for problems involving the fracturing process from continuity to discontinuity if the failure mechanism in DDA is well constituted. This paper presents a new united tensile fracture model (UTFM) for the two-dimensional DDA method to simulate the fracture behaviors of various brittle materials (e.g., rock, soil, and concrete). The new fracture model unifies four classical failure modes, including the maximum normal stress criterion, Tresca criterion, Mohr–Coulomb criterion, and the von Mises criterion, for tensile fracture. By incorporating UTFM into the original DDA frame, the improved DDA (I-DDA) can predict the crack initiation and propagation paths in Brazil disc and simulate rock fracture of various brittle materials. Numerical examples of the direct tensile test and the Brazil disc split tests are investigated to verify the accuracy and validity of the I-DDA method. The simulated results agree well with those obtained from physical tests and other numerical analyses, suggesting that the I-DDA has obvious advantage in simulating the fracture behaviors of the Brazil disc split test. Further, the I-DDA is applied to analyze the failure process of a practical earthquake-induced landslide with consideration of the tensile strength of the rock mass. The results indicate that the I-DDA is more feasible to analyze the slope failure, which can consider both the tensile and shear characteristics simultaneously compared with the original DDA.

Returns to higher education in China – evidence from the 1999 higher education expansion using a fuzzy regression discontinuity

Abstract: China experienced a 47% expansion in higher education enrolment between 1998 and 1999 and a sixfold expansion in the decade to 2008. Using a fuzzy discontinuity in the months of births, we show tha...

A New Approach to Represent Impact of Discontinuity Spacing and Rock Mass Description on the Median Fragment Size of Blasted Rocks Using Image Analysis of Rock Mass

Abstract: Several in-situ rock mass properties and blasthole parameters can affect the rock fragmentation. Because of the complexity of the variables affecting the fragmentation results of blasted rocks, to predict a proper value of the median fragment size has long been a difficult task. The blastability index (BI) represents the effect of five parameters of rock mass description (RMD), joint plane spacing (JPS), joint plane orientation, specific gravity and uniaxial compressive strength on the rock fragmentation. The median discontinuity spacing significantly varies with varying the scanline direction and an acceptable value of the median discontinuity spacing will not be expected in practice. The JPS rating also has a constant value of 20 for a wide range of joint spacing values between 0.1 and 1 m, whereas joint spacing can be in this range for most cases. A new method using image analysis of the in-situ rock mass was applied to represent JPS and RMD (belonging to one of the cases: friable, blocky and massive) as an alternative solution. The images contain the details of all individual discontinuities and interlocked in-situ small and large rock blocks. BI, rock strength factor, blasthole parameters, powder factor, fragment size distribution of blasted rock and in-situ block size distribution using image analysis technique were assessed in 15 zones of Sungun open pit copper mine, Angouran lead and zinc open pit mine, Bonab silica mine, Soufian limestone mine and Rashakan limestone mine. The results for rock mass properties and blasthole patterns cover a wide range using different mines. The fragment size distribution was assessed by Split Desktop program with proper delineating images using the Pixler software and blasting was carried out with electric delay detonators. The relations between fragment size and parameters such as in-situ block size (F50), σc, rating of joint plane orientation, powder factor (q), ϕh, Q and Lc were analyzed. The relations with high correlations were achieved by applying the new approach for the defined conditions. Not only the problem of assessing discontinuity spacing has been improved using this method but also the lower number of parameters that properly represent the factors affecting the rock fragmentation have been used. The results were also analyzed by the Sanchidrian and Ouchterlony model and modified Kuz–Ram models. The fragment size obtained by the new method in this study, Sanchidrian and Ouchterlony model and extended modified Kuz–Ram model by Cunningham (in: Proceedings of 3rd world conference on explosives and blasting, Brighton, 2005) after using correction factor [c(A)] significantly better fitted to the results than the modified Kuz–Ram models by Cunningham (in: Fourney, Dick (eds) Proceedings of 2nd international symposium on rock fragmentation by blasting, Keystone, 1987) and Gheibie et al. (Int J Rock Mech Min 46(6):967–973, 2009).

Characterization of discontinuity surface morphology based on 3D fractal dimension by integrating laser scanning with ArcGIS

Abstract: The fractal geometry method has been employed to quantitatively characterize the roughness of a rock discontinuity, which is one of the key factors affecting its shear strength and the seepage characteristics of a rock mass. However, the current fractal methods involving the three-dimensional discontinuity morphology suffer from one or more problems, such as a complicated calculation procedure, an inaccurate calculation result and an inability to characterize the undulation and anisotropy of a discontinuity. To cope with these problems, the discontinuities in artificial granite samples with irregular and undulating surfaces were taken as examples, and a quantitative study on the discontinuity morphology was conducted based on the method of three-dimensional laser scanning in combination with ArcGIS data processing, geographical research, theoretical calculations and regression analysis. After performing systematic research, we proposed an extensive 3D fractal dimension including three discontinuity morphological parameters, i.e. the fractal dimension of discontinuity morphology, the ratio between the maximal undulating amplitude and the discontinuity length, and the average value of all the apparent dip angles of the discontinuity surfaces dipping opposite the shear direction. The extensive 3D fractal dimension can comprehensively characterize the roughness, undulation and anisotropy of the discontinuity morphology. A set of theoretical calculation methods were then developed to determine the three discontinuity morphological parameters of the extensive 3D fractal dimension based on ArcGIS. We finally established a mathematical expression of the extensive 3D fractal dimension. Compared with the current fractal methods, the extensive 3D fractal dimension can effectively compensate for the inability to characterize the undulation and anisotropy of the discontinuity morphology. Its calculation methods have the advantages of simplification, low-time consumption and high precision.

Physical and numerical investigations of bedding adhesion strength on stratified rock roof fracture with longwall coal mining

Abstract: The weak bedding sandwiched between rock layers is one of the crucial factors influencing the overburden failure with the extraction of coal resources in longwall mining. In this article, the physical model tests and discrete element modelling (DEM) were carried out to examine the effects of bedding adhesion strength (BAS) on the roof fracture with longwall coal mining. A novel model generation method in the universal distinct element code was proposed for the investigation of a propagating crack interaction with existing discontinuities. It is found that crack trajectory changes from deflection to penetration with the strength increase of a discontinuity. The stair-stepping fracture of overburden cantilever structure is more prominent for the rock layers with weaker BAS, which is in coincidence with the triangular block-based DEM result. Moreover, the mechanism of failure pattern influenced by BAS was clarified by the cantilever beam theory and fracture criterion of crack competition. With the outcomes above, the longwall coal mining-induced stratified roof fracture characteristics in engineering scale were revealed. Three quantitative regions were divided in terms of the strength ratio between bedding and rock layer. Finally, three aspects, the heights of the caved zone and fractured zone, the subsidence of ground surface, and the abutment pressure of roof affected by the BAS were discussed, respectively, with some suggestions recommended for the stability controlling of roof and safety production of coal mining.

Influence of Initial Stress and Deformation States on the Shear Creep Behavior of Rock Discontinuities with Different Joint Roughness Coefficients

Abstract: The sliding of rock blocks along rock discontinuities is caused by accumulated deformation and can result in disasters such as rock bursts and earthquakes. Creep along rock discontinuities leads to the accumulation of sliding deformation and random instability over time. The initial stress and deformation are important factors that strongly influence creep behavior. To investigate the influence of the initial state on the creep behavior of rock discontinuities, shear creep tests with loading–unloading shear pre-stress paths and shear creep tests with different initial stress and deformation states were conducted on artificial rock discontinuity samples prepared according to Barton’s standard roughness profiles. The creep behavior characteristics were investigated in detail according to the initial stress and deformation conditions. The results show that the initial stress and deformation states, which are related to crack development and plastic deformation accumulation, strongly influence creep behavior. Larger initial stress and deformation produce lower creep deformation, creep rates, and accelerated velocities. Samples with higher joint roughness coefficients produce more noticeable creep because more asperities provide additional space for creep to occur. A limit curve is also proven to exist, which allows the stress–deformation coordinate system to be divided into stable and unstable regions. When the current stress state of a sample is insufficient to drive the formation of further cracks or deformation in the rock discontinuity, the deformation and stress remain stable and no further creep is observed.

Jets from MRC 0600-399 bent by magnetic fields in the cluster Abell 3376.

Abstract: Galaxy clusters are known to harbour magnetic fields, the nature of which remains unresolved. Intra-cluster magnetic fields can be observed at the density contact discontinuity formed by cool and dense plasma running into hot ambient plasma1,2, and the discontinuity exists3 near the second-brightest galaxy4, MRC 0600-399, in the merging galaxy cluster Abell 3376 (redshift 0.0461). Elongated X-ray emission in the east–west direction shows a comet-like structure that reaches the megaparsec scale5. Previous radio observations6,7 detected the bent jets from MRC 0600-399, moving in same direction as the sub-cluster, against ram pressure. Here we report radio8,9 observations of MRC 0600-399 that have 3.4 and 11 times higher resolution and sensitivity, respectively, than the previous results6. In contrast to typical jets10,11, MRC 0600-399 shows a 90-degree bend at the contact discontinuity, and the collimated jets extend over 100 kiloparsecs from the point of the bend. We see diffuse, elongated emission that we name ‘double-scythe’ structures. The spectral index flattens downstream of the bend point, indicating cosmic-ray reacceleration. High-resolution numerical simulations reveal that the ordered magnetic field along the discontinuity has an important role in the change of jet direction. The morphology of the double-scythe jets is consistent with the simulations. Our results provide insights into the effect of magnetic fields on the evolution of the member galaxies and intra-cluster medium of galaxy clusters. Radio observations of the cluster Abell 3376, combined with numerical modelling, attribute the bent jets associated with the second-brightest galaxy in the cluster to an ordered magnetic field at the discontinuity.

Laboratory-Scale Investigation of the Slippage of a Natural Fracture Resulting from an Approaching Hydraulic Fracture

Abstract: The interaction between a natural fracture (NF) and a hydraulic fracture (HF) has been studied extensively, both experimentally and numerically, to better understand the potential for crossing and arrest of a hydraulic fracture intersecting a natural fracture. However, the actual mechanical interaction between a hydraulic and a natural fracture or a bedding plane has not been studied, particularly under triaxial stress and injection conditions. Analysis of field microseismic data recorded during hydraulic fracturing shows that the bedding plane could slip due to the approaching hydraulic fracture. In this paper, we present the results of some lab-scale experimental work, demonstrating HF/NF interaction with an emphasis on the slippage of a discontinuity surface. Injection pressure, stress applied, and the sample deformation are monitored during the tests. Acoustic Emission (AE) technology is employed to record the AE signals generated during fracture initiation, propagation, and during the sliding of the joint. In addition, strain gauges are used to measure the slippage on the natural fracture. The tests are carried out on 101.6 mm diameter cylindrical samples of PMMA, shale, and granite. The calculated displacement based on the recorded strain clearly shows a jump at the breakdown point, which is accompanied by increased AE activity and stress drop. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip and friction angle.