Roughness Evaluation for Distinguishing Fresh and Sheared Rock Joint Surfaces with Different Sampling Intervals

The subtle alteration of surface geometry from a fresh surface to a sheared surface usually results in a considerable variation in the shear strength of jointed rock mass. Through profiling surfaces of the granite joints before and after the shear tests, an evaluation scheme was newly proposed by determining a desirable characteristic index and sampling interval of surface measurement in order to distinguish fresh and sheared joint surfaces quantitatively. The measured data demonstrated that although the mean Z₂ (root-mean square first derivation) values of all the profile lines were confirmed reasonable for estimating the joint roughness coefficient (JRC) value of the fresh joint surface, it could not completely evaluate the roughness of the sheared joint surfaces. Meanwhile, the distribution of slope angles, as the characteristic parameter, was proved to enable to clearly distinguish the fresh and sheared rock joint surfaces incorporating the small sampling scales (≤0.1 mm). The numerical simulations implemented in a mechanical shear model could confirm the critical effect of a slight change in surface geometry and further prove that the sampling interval of 0.1 mm could sufficiently capture the evolved waviness and unevenness of rock joint surfaces. Overall, it was confirmed that the results of our study provide new clues for evaluating the surface roughness of fresh and sheared rock joints and can be beneficial for understanding the variation of surface geometry during the shear process

Influence of longitudinal structural connectivity on seismic performance of three-hinged precast arch culverts

The hinge type of precast concrete arch culvert was introduced to Japan from France in the 1990s in consideration of the saving of labor, shortening of the construction period, and high quality control of the concrete members. However, due to the 2011 off the Pacific Coast of Tohoku Earthquake (March 11, 2011), the three-hinged precast arch culverts that had been constructed in Japan at the beginning of the period when precast arch culverts were firstly introduced, suffered damage, which spoiled their serviceability. According to the extent of the damage and the type of culverts that were damaged, the longitudinal structural connectivity of the culverts was assumed to be one of the possible reasons for the reported damage mechanism. Therefore, the objective of this paper was to clarify how strongly the longitudinal structural connectivity influenced the longitudinal seismic behavior of the three-hinged arch culverts. To achieve this objective, an elasto-plastic finite element analysis was conducted with an analytical model that could capture the characteristics of the damaged culverts. Simultaneously, a penalty method with the bi-linear spring model was applied as a solution to the contact-impact problems of the precast segmental arch members. As a result, it was found that the weaker longitudinal structural connectivity in the damaged culverts allowed the torsional displacements of the arch members to induce critical damage to the arch members, namely, edge defects in the arch crown and concrete foundation. The numerical results proved the unignorable influence of the longitudinal structural connection on the possible damage to three-hinged arch culverts

Deformation behavior and acting earth pressure of three-hinge precast arch culvert in construction process

The three-hinge precast arch culvert consists of two segmental precast units and three hinge points. It harnesses the passive resistance of an embankment by permitting deflection, resulting in a mechanically stable structure. However, the design of the three-hinge precast arch culvert differs from that of a conventional culvert, prompting the mechanical behavior of the culvert to become an important issue. In this study, therefore, 1/5 scale model tests were conducted on a three-hinge precast arch culvert to measure the changes in the inside width and earth pressure acting on the culvert at each step in order to investigate the culvert’s mechanical behavior at each construction stage. Moreover, the deflection measurement of the culvert was obtained at the in-situ construction site. The results indicate that the arch members were displaced according to the embankment depth in a similar manner to the design load. Therefore, the horizontal earth pressure, which was larger than the earth pressure at rest, acted on the culvert at the end of its construction

Numerical investigation on arching effect surrounding deep cylindrical shaft during excavation process

Predicting the inner displacements of deep vertical shafts during the excavation process has been a difficult task considering the geological, structural, and constructional influences. In fact, the two-dimensional (2D) analytical solution based on the retaining wall model remains insufficient for understanding the actual behavior during an excavation. This is because the deformation of vertical shafts becomes complicated due to the unexpected arching effect brought about by the three-dimensional (3D) flexible displacements occurring in the excavation process. Previous analytical solutions only considered the limit equilibrium. Therefore, the present study deals with a 3D soil-structure simulation by considering the displacements of a cylindrical shaft and the mechanical behavior of the surrounding soil as well as the geometry of the cylindrical structure. Moreover, this mechanical behaviors of the surrounding soil and shaft are controlled by the shaft stiffness; hence, the relationships among the shaft stiffness, mechanical behavior of the surrounding soil (in terms of earth pressure coefficient), and shaft displacement were investigated. A cylindrical model, 120 m in depth and 20 m in diameter, was positioned at the center of a sand domain, and each excavation step was performed at an interval depth of 20 m. A 3D finite difference method analysis was applied using the modified Cam-Clay (MCC) model to represent the soil behavior. As a result, the present study provides a new normalized lateral earth pressure theory for excavated shafts by considering the 3D arching effect obtained from parametric studies using various levels of shaft stiffness. From a comparison with the analytical solutions of previous studies (Terzaghi, 1943a; Prater, 1977; Cheng & Hu, 2005), it is found that the previous studies underestimated the earth pressure acting on the cylindrical shaft because they did not consider the accurate arching effect

Short- and Long-Term Observations of Fracture Permeability in Granite by Flow-Through Tests and Comparative Observation by X-Ray CT

Having a grasp of the variation in the fracture contact area is a kernel in the understanding of the permeability evolution of fractured rocks. However, the number of studies that focus on measuring the long-term variation in the fracture contact area under different conditions is insufficient. In this study, a series of short- and long-term permeability tests under coupled conditions is performed to check the performance of permeability. The results reveal that the permeability measured in the short-term tests shows reversible behavior and a dependence on the applied confining pressures and temperature. In contrast, the permeability in the long-term tests displays irreversible behavior and an irregular change under the constant confining pressure. In order to verify the evolution of permeability, microfocus X-ray computed tomography (CT) is developed to observe the changes in the internal fracture structure under the same conditions as those in long-term permeability tests by assembling a triaxial cell with heating capability. The fracture aperture and the fracture contact-area ratio are calculated by a CT image analysis technique. The image analysis results show that the estimated aperture is seen to decrease with an increase in the confining pressure and to also decrease with time under a constant confining pressure. Moreover, the increase in the fracture contact area under the constant confining pressure observed by X-ray CT is confirmed. This also corresponds to a decrease in permeability in long-term tests. The hydraulic aperture calculated from the permeability tests and that evaluated from the CT observation have a similar decreasing trend. Therefore, the CT observation can better capture the evolution of the internal fracture contact area. These experiments underscore the importance of mechanical compaction and/or mineral dissolution at contacts in determining the rates and the magnitude of permeability evolution within rock fractures

Atmospheric polycyclic aromatic hydrocarbons in air samples of Hanoi

Joint Research on Environmental Science and Technology for the Eart

Multi-physics numerical analyses for predicting the alterations in permeability and reactive transport behavior within single rock fractures depending on temperature, stress, and fluid pH conditions

The aim of the current study was to establish a validated numerical model for addressing the changes in permeability and reactive transport behavior within rock fractures based on the fluid pH under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions. Firstly, a multi-physics reactive transport model was proposed, considering the geochemical reactions that depend on the temperature, stress, and fluid chemistry conditions (e.g., fluid pH and solute concentrations), as well as the changes in permeability in the rock fractures driven by these reactions, after which the correctness of the model implementation was verified by solving the 1D reactive transport problem as a fundamental benchmark. Secondly, the validity of the model against actual rock fractures was investigated by utilizing the model to replicate the measurements of the evolving permeability and the effluent element concentrations in single granite fractures obtained by means of two flow-through experiments using deionized water (pH ∼ 6) and a NaOH aqueous solution (pH ∼ 11) as permeants under stressed, temperature-elevated conditions. The model predictions efficiently followed the changes in fracture permeability over time measured by both experiments. Additionally, the observed difference in the changing rates, which may contribute to the difference in the fluid pH between the two experiments, was also captured exactly by the predictions. Moreover, in terms of the effluent element concentrations, among all the elements targeted for measurement, the concentrations of most elements were replicated by the model within one order of discrepancy. Overall, it can be concluded that the developed model should be valid for estimating the changes in permeability and reactive transport behavior within rock fractures induced by geochemical reactions which depend on the fluid pH under coupled THMC conditions

Discussion on the mechanism of ground improvement method at the excavation of shallow overburden tunnel in difficult ground

Tunnel construction opportunities involving shallow overburdens under difficult (e.g., soft, unconsolidated) grounds have been increasing in Japan. Various auxiliary methods for excavating mountain tunnels have been developed and can satisfy stringent construction requirements. The ground improvement method, which is one of the auxiliary methods for shallow overburden tunnels, has demonstrated its ability to effectively control the amount of settlement under soft ground. However, the mechanism of the ground improvement method has not been clarified, nor has a suitable design code been established for it. Therefore, because the strength of the improved ground and the suitable length and width of the improved area have not been fully understood, an empirical design has been applied in every case. In this paper, the mechanical behavior during the excavation, including that of the stabilized ground, is evaluated through trapdoor experiments and numerical analyses. In addition, the enhancement of tunnel stability resulting from the application of the ground improvement method is discussed

Pollution of Polyaromatic Hydrocarbons in the Airborne Particles in the Developing Countries in Asia Region

Joint Research on Environmental Science and Technology for the Eart

DISTRIBUTION OF PERSISTENT ORGANIC POLLUTANTS AND POLYCYCLIC AROMATIC HYDROCARBONS IN SEDIMENT SAMPLES OF VIETNAM

Joint Research on Environmental Science and Technology for the Eart