Abstract
The roughness property of rocks is significant in engineering studies due to their mechanical and hydraulic performance and the possibility of quantifying flow velocity and predicting the ...performance of wells and rock mass structures. However, the study of roughness in rocks is usually carried out through 2D linear measurements (through mechanical profilometer equipment), obtaining a coefficient that may not represent the entire rock surface. Thus, based on the hypothesis that it is possible to quantify the roughness coefficient in rock plugs reconstructed three-dimensionally by the computer vision technique, this research aims to an alternative method to determine the roughness coefficient in rock plugs. The point cloud generated from the 3D model of the photogrammetry process was used to measure the distance between each point and a calculated fit plane over the entire rock surface. The roughness was quantified using roughness parameters (
$$R_a$$
R
a
) calculated in hierarchically organized regions. In this hierarchical division, the greater the quantity of division analyzed, the greater the detail of the roughness. The main results show that obtaining the roughness coefficient over the entire surface of the three-dimensional model has peculiarities that would not be observed in the two-dimensional reading. From the 2D measurements, mean roughness values (
$$R_a$$
R
a
) of
$$0.35\,\upmu \hbox {m}$$
0.35
μ
m
and
$$0.235\,\upmu \hbox {m}$$
0.235
μ
m
were obtained for samples 1 and 2, respectively. By the same method, the results of the
$$R_a$$
R
a
coefficient applied three-dimensionally over the entire rocky surface were at most
$$0.165\,\upmu \hbox {m}$$
0.165
μ
m
and
$$0.166\,\upmu \hbox {m}$$
0.166
μ
m
, respectively, showing the difference in values along the surface and the importance of this approach.
The present study evaluates the discrete element method (DEM) as a tool for understanding the step-path failure mechanism in fractured rock masses. Initially, the study simulates crack propagation ...and coalescence in biaxial and triaxial laboratory tests. The results of this analysis show that the DEM accurately represents these processes in comparison to other studies in the technical literature. The crack propagation and coalescence processes are important in the step-path failure mechanism for slopes. Simple examples of this mechanism were modeled, and their results were compared with those of the analytical model proposed by Jennings (1970). Among the possibilities suggested by Jennings, modeling with DEM did not provide a good approximation for the case of coplanar cracks, for which failures in the intact rock bridges should only be caused by shear forces. In modeling with DEM, tensile failures occur within the sliding block, generating forces that are not considered in the Jennings model. The non-coplanar crack condition provided a better approximation, since the Jennings model formulation for this case includes the tensile failure of the rock. The main advantage of the DEM over other computational tools is its micromechanical representation of discontinuous media, which permits a better understanding of the step-path failure mechanism. However, good calibration of the macroscopic parameters of the rock and its discontinuities is necessary to obtain good results.
► No specific highlights as the modifications required were minor.
Granitic rocks can in general be easily split through three mutually orthogonal surfaces, empirically identified in the field1,2. In this regard, the present study intends to evaluate the mode I ...fracture toughness of a syenogranite, obtained from a quarry in the city of Cachoeiro de Itapemirim in the state of Espirito Santo. Fracture toughness consists of an intrinsic mechanical property of rocks and indicates the resistance to the initiation or propagation of a fracture as well as the amount of energy the rock absorbs until its fracture. In order to evaluate mode I fracture toughness anisotropy, the Cracked Chevron Notched Brazilian Disc (CCNBD) test was conducted, following ISRM3. Twenty-five (25) samples were tested considering four (4) different orientations: short transverse, arrester, divider and a random direction (inclined). In addition, the indirect tensile strength test (Brazilian test) was also conducted, in order to characterize the samples. The test was performed on fifteen (15) samples and in three (3) different orientations: short transverse, arrester and divider. The tests and sample preparation were carried out at the Structural and Materials Laboratory (LEM-DEC), and the physical properties were obtained at the Geotechnical and Environmental Laboratory (LGMA), both located at PUC-Rio. It is possible to observed that the physical properties results obtained in the present study were very similar to the results obtained by Almeida et al.4 and by Jaques5. In the Brazilian test it was possible to observe that all the mean tensile strength values are similar to each other, which indicates that there is little anisotropy in this rock. In the CCNBD test it was possible to verify that the mean fracture toughness values are similar in every direction, which indicates that there is little anisotropy in this rock as well as in the Brazilian test.
The land topography of certain mountainous regions can be highly variable. The geometry of the topography may be influenced by geological, seismic, weathering, and other processes. The creation of ...the full 3D numerical models of these topographies represents a challenge. In this work, we present a methodology that allows to create numerical material point method (MPM) models based on the contour lines of the land. The proposed methodology has three main characteristics: the construction of the MPM discrete model is based only on raster digital elevation model (DEM) data; finite element meshes are not required during the process; and the heterogeneities are defined by the DEM data of each material. The application of the proposed methodology to the Daguangbao landslide which occurred in China, in 2008, proved to be adequate for the creation of models with high topography variations and spatial heterogeneities.
The disturbances experienced by the soil due to the pile installation and dynamic soil–structure interaction still present major challenges to foundation engineers. These phenomena exhibit complex ...behaviors, difficult to measure in physical tests and to reproduce in numerical models. Due to the simplified approach used by the discrete element method (DEM) to simulate large deformations and nonlinear stress–dilatancy behavior of granular soils, the DEM consists of an excellent tool to investigate these processes. This study presents a sensitivity analysis of the effects of introducing a single pile using the PFC2D software developed by Itasca Co. The different scales investigated in these simulations include point and shaft resistance, alterations in porosity and stress fields and particles displacement. Several simulations were conducted in order to investigate the effects of different numerical approaches showing indications that the method of installation and particle rotation could influence greatly in the conditions around the numerical pile. Minor effects were also noted due to change in penetration velocity and pile–soil friction. The difference in behavior of a moving and a stationary pile shows good qualitative agreement with previous experimental results indicating the necessity of realizing a force equilibrium process prior to any load-test to be simulated.
Previous studies have indicated that many rock slope failures in Rio de Janeiro, Brazil, occur during relatively dry periods when there is little, if any, rainfall. In the present paper, we describe ...these events and propose possible mechanisms for their occurrence. It is believed that daily fluctuations in temperature may create thermally induced stresses of enough magnitude to propagate fractures already existing in the rock mass. Laboratory experiments were conducted in order to measure temperature variations in a fractured rock system. Using the experimentally obtained data, we carried out numerical analyses in order to verify whether temperature-related stresses were indeed able to propagate fractures existing in the rock mass. The results obtained indicate that cycles of temperature change over time, especially during the colder months of the year can indeed create stress conditions in the rock mass able to propagate existing non-persistent joints, and ultimately lead to failure of the rock slope.
During the life cycle of an oil well, the annular cement sheath will be exposed to load combinations that can result in failure. To enhance the structural integrity and ductility of the cement paste, ...and to improve the resistance to tensile crack propagation, fiber reinforcement can be used. In the present work, the effects of different polyvinyl alcohol (PVA) fiber concentrations on the rheological and mechanical properties of a class G oil well cement paste was investigated. The study focuses on impacts of fibers on the cement paste viscosity, its confined and unconfined stress–strain responses, and resistance to shear and bending loads. An increase in the effective viscosity of the cement paste with increasing fiber content was observed, although the impact was minor for the smallest (semi-dilute) fiber concentration. All cement paste specimens showed a modest frictional strengthening with increasing confining pressure, with the fiber reinforced samples exhibiting improved post-peak load-bearing capacity compared to the base formulation with no fiber additive. Fibers were found to significantly improve cement paste resistance to shear and bending loads, to enhance the toughness and to arrest tensile crack growth. Despite all the benefits already known about adding fibers, for application in the oil and gas industry, it is essential to evaluate the rheology of the mixture that will directly impact pumpability and placement. In this work, flow curves were measured only for concentrations ranging from 0.1% to 0.5% since the other cases approached the concentrated suspension regime with the fluid behaving like a plug. Still, the results elucidate the potential benefits of adding even a relatively small concentration of high-aspect-ratio PVA fibers to oil well cement pastes, particularly for enhancing early-age cement’s shear and bending strength.
•Effects of adding high-aspect ratio fibers to class G cement paste.•Increasing fiber concentration results in modest viscosification of slurry.•Fibers improve post-peak load-carrying capacity of cement.•PVA fibers improve the plastic behavior of the material under triaxial loading.
Risk analysis of existing slopes in catchment areas requires quantification of their stability. This quantification becomes particularly difficult when dealing with larger areas under 3D conditions ...and including saturated and unsaturated water flow. This paper proposes the use of an effective numerical procedure to solve three-dimensional slope stability problems in large areas subjected to pore pressure effects. This numerical approach, numerical limit analysis, utilizes the finite element method and mathematical programming techniques. Mathematical programming is needed because the basic plasticity theorems for limit analysis can be cast as optimization problems. The generated optimization problem is formulated under a second-order cone programming framework, which is known to solve large-scale problems with great computational efficiency. The main objective of this work was to determine the slope safety factor and the collapse mechanism of soils governed by the Drucker–Prager yield criterion for large-scale 3D problems including pore pressure effects. This approach is applied to an experimental catchment in the Oregon Coast Range that failed after an intense rainfall. The results were compared with a previous stability analysis of the area available in the literature that used a novel 3D limit equilibrium method.
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