F.S. Jeng

Characteristics of hazards induced by extremely heavy rainfall in Central Taiwan — Typhoon Herb

Abstract In 1996, Typhoon Herb devastated Taiwan, moving across the northwestern part of the island. Historically, typhoons traveling such a route have frequently caused worse damage than typhoons on other routes. Herb brought record-breaking precipitation to Taiwan, especially the central part of the island. The heavy rainfall led to more than 1315 landslides and 20 debris flows. Seventy-three lives were lost, 463 people were wounded, and property losses of about 1 billion USD were sustained. This study presents the distinct characteristics of Typhoon Herb as well as the induced hazards, including their extraordinary size and recurrent nature. The focus is on central Taiwan, where the severest hazards were encountered. Furthermore, the geological, geomorphologic and engineering factors, which may have magnified the consequences of the hazards, are also discussed. A comparison of hazards in other countries is also made and the results indicate that the extremely heavy rainfall encountered as well as the existence of a boundary fault in the study area are the most susceptible factors inducing the extraordinary hazards.

Deformational characteristics of weak sandstone and impact to tunnel deformation

Abstract In northern Taiwan, a tunnel under construction along a segment where weak sandstone, the Mushan sandstone, was encountered and an excess crown settlement (14–30 cm) has been reported. This paper studies the deformational characteristics of Mushan sandstone and its impact on tunnel deformation. To distinguish the volumetric and the shear deformation of the sandstone, experiments with controlled stress paths, including hydrostatic compression, pure shearing and conventional triaxial compression, were conducted. The measured deformations were then decomposed into elastic and plastic components further exploring the stress–strain behavior of weak sandstone. The results indicate that, similar to other soil-like geo-materials, this sandstone has plastic strain before the stress path reaches the failure envelope and significant shear dilation is induced, especially when approaching the failure envelope. Meanwhile, the distinct features of deformation have also been highlighted by comparing the experimental results to the prediction, derived from existing constitutive models that were originally developed for other geo-materials. These features include significant plastic volumetric strain at low levels of confining stress, suppression of plastic volumetric strain at higher levels of confining stress, and the fact that the actual amount of shear compression is less than that predicted by the model. Numerical analysis indicates that the weak rock leads to the greatest inward displacement, which results from the shear dilation prior to failure state.

PERFORMANCE OF TOUGHNESS INDICES FOR STEEL FIBER REINFORCED SHOTCRETE

Abstract To indicate the flexural toughness of Steel Fiber Reinforced Concrete or Shotcrete (SFRC or SFRS), several indices have been developed and some of them are now national standards. Whether these indices can properly indicate the actual toughness of sprayed shotcrete is of concern. This paper aims at assessing the performance of the indices for a variety of SFRS sprayed at job site, instead of being cast in laboratory. The mixture actually sprayed, especially the steel fiber content, may differ from the mixture before spraying and this inherently affects the toughness of SFRS, therefore, counting of steel fiber over the fracture surface of a tested beam was conducted. This is helpful in interpreting the experimental results. The indices evaluated include: toughness index (I); residual strength factor (R); equivalent flexural strength (fe); and the indices based on the residual flexure stress method, including a Norwegian index, f, and a proposed relative residual stress factor fm. The experimental results reveal that increases of steel fiber content, cement content, silica fume content and the aspect ratio of the steel fibers improve the toughness of SFRS. The performance of the indices is evaluated on the basis of the following factors: independence with regard to the testing machine as well as sufficient sensitivity to the ingredients of SFRS. The results of evaluation indicate that the indices based on the residual flexural stress method, and involving greater deflection, tend to perform better in indicating the toughness of SFRS.

The holding mechanism of under-reamed rockbolts in soft rock

Abstract The holding mechanism of under-reamed rockbolts differs from that of conventional rockbolts, in which the bonding or the friction along the element–rock interface provides the holding capacity. The under-reamed end is kinematically blocked by the surrounding rock mass and can provide a greater holding capacity, especially in soft rock, whereas the strength of a soft rock frequently controls the bonding strength of the element–rock interface. Both an experimental study and numerical analyses were performed to examine the holding mechanisms of model under-reamed rockbolts subjected to direct pull out loading and pre-tensioning. When subjected to direct pull out loading, the holding capacity originates from the capacity of the rock resisting tensile fracture. Failure is characterized by the formation of a smooth, conical region bounded by a tensile crack, which subsequently separates from the surrounding rock. Correspondingly, the holding capacity is related to the tensile strength of the rock, bolt length and size of the under-reamed end. When subjected to pre-tensioning, the holding mechanism is provided by the ability of the rock to form two conical zones between the faceplate and the under-reamed end, and to prevent subsequent indentation of the two cones. Major factors influencing the holding capacity of under-reamed bolts include the size of the under-reamed end, bolt length and properties of the rock.

Characteristics of seismic energy release of subduction zones—examples from Taiwan

Abstract The characteristics of seismic energy releasing for oceanic subduction and continental collision or subduction zones were explored by a proposed observational approach based on contour maps of seismic energy release on both horizontal and vertical planes, magnitude–energy curve and magnitude–energy–time relationship. It was found that distinct features of energy-releasing behavior exist for the two types of subduction. The oceanic subduction tends to have a more active background noise by continuously emitting small magnitude earthquakes and to release energy from a wider range of depth owing to the existence of subducted plate. On the other hand, surficial strata of a continental subduction tend to release almost all of the seismic energy through a major earthquake with relatively much less long-term noise. Meanwhile, several seismic gaps have been found based on the proposed release energy contour. Together with the tectonic framework and the regional geologic setting, these gaps potentially may involve aseismic zones (e.g., accretionary deposit) or zones awaiting to release its accumulated energy (e.g., the upper crust). The quite zones prone to yielding major earthquakes are accordingly highlighted for a further study.

Influences of Microscopic Factors on Macroscopic Strength and Stiffness of Inter-Layered Rocks — Revealed by a Bonded Particle Model

Since a conventional petrographic analysis does not allow a systematic and detailed study on how the microscopic factors affect the macroscopic behavior of inter-layered rocks, this research adopted a numerical model, the bonded particle model, to explore the micro-mechanisms associated with the strength and stiffness of inter-layered rocks. The model was first calibrated by comparing the simulations to the actual behavior until they tally with each other. Following, the microscopic factors, including the bond strength, the bond stiffness, type of bonds and friction of particles and type of bond stiffness, are varied to study their influences. As expected, the bond strength and the bond stiffness are found to have a direct and significant influence on the macroscopic uniaxial compressive strength and stiffness, respectively. Furthermore, close observations on the breaking of bonds during the loading process reveal interesting phenomena, including the transition of shear/normal bond breaking, the type of internal fracture and the factors controlling internal failure, etc. These phenomena enlighten the interpretations about the micromechanisms accounting for the macroscopic strength and stiffness of inter-layered rocks.