Yii-Wen Pan

The influence of surface ruptures on building damage in the 1999 Chi-Chi earthquake: a case study in Fengyuan City

Abstract In addition to the main surface rupture along the Chelungpu fault associated with the 1999 Chi-Chi, Taiwan earthquake, numerous secondary or branch ruptures on hangingwall were also observed. These secondary surface ruptures are parallel or sub-parallel to the main rupture within a distance of a few meters to 1–2 km. The rupture length of these secondary ruptures varies from a few tens of meters up to 5 km. The surface deformation resulted in serious damages of buildings. The present work studied the features of surface deformation on the hangingwall around the Chung-Cheng Park, Fengyuan, Taichung. Three distinct surface ruptures, minor ruptures and tension cracks were observed in this area. The observed distribution and types of building damage on the hangingwall are demonstrated. Due to the difference in geological condition and complex pattern of surface deformation, the resulted building damages on the hangingwall vary. A series of site investigation including field survey, drilling, seismic prospecting, P–S logging tests and laboratory tests were carried out in the interested area. A geological structure model was proposed on the basis of the results of site investigation. Numerical simulation was carried out to model the surface deformation as well as subsurface potential damage zone of an active fault. It reasonably explains the observed pattern of surface deformation and indicates that the surface deformation zone during a catastrophic earthquake is predictable.

Time-dependent tunnel convergence—I. Formulation of the model

Abstract The understanding of the mechanism of tunnel convergence is essential for designing the required support of a tunnel. To take various time effects on the convergence-confinement relation into account, it is possible to correlate different factors with time, then to deal with their collective effects. As a result, the time-dependent tunnel deformation problem can be modelled as a viscoelastic problem. The solutions of various viscoelastic models can be obtained from the corresponding elastic solution based on a standard procedure. In this paper, a hierarchical approach, which considers: (1) the rheological properties of a rock mass; (2) the advancement of the tunnel; and (3) the tunnel-support interaction is proposed to model the excavation/construction process during the tunnelling in a rock mass with rheological properties. The time-dependent factors considered in the proposed model are discussed. Subsequently, the theoretical formulation of this model is developed.

TIME-DEPENDENT TUNNEL CONVERGENCE - II. ADVANCE RATE AND TUNNEL- SUPPORT INTERACTION

Abstract A time-dependent model of tunnel convergence for tunnelling in a viscoelastic rock mass is proposed in the accompanying paper (this issue, pp. 469–475) to model the excavation-construction process during tunnelling in a rock mass with rheological properties. In this paper, a parametric study based on this model is presented to investigate the effects of the tunnelling advancing and support installation, respectively, on the tunnel convergence and on the support-pressure. Furthermore, a non-linear optimization procedure is suggested to calibrate the required model parameters from the in situ measured tunnel-convergence data. Discussion on the proper application of the proposed model and the optimization calibration are also included.

Deriving landslide dam geometry from remote sensing images for the rapid assessment of critical parameters related to dam-breach hazards

Dam-breaches that cause outburst floods may induce downstream hazards. Because landslide dams can breach soon after they are formed, it is critical to assess the stability quickly to enable prompt action. However, dam geometry, an essential component of hazard evaluation, is not available in most cases. Our research proposes a procedure that utilizes post-landslide orthorectified remote sensing images and the pre-landslide Digital Terrain Model in the Geographic Information System to estimate the geometry of a particular dam. The procedure includes the following three modules: (1) the selection of the reference points on the dam and lake boundaries, (2) the interpolation of the dam-crest elevation, and (3) the estimation of dam-geometry parameters (i.e., the height, length, and width), the catchment area, the volumes of barrier lake and landslides dam. This procedure is demonstrated through a case study of the Namasha Landslide Dam in Taiwan. It was shown the dam-surface elevation estimated from the proposed procedure can approximate the elevation derived from profile leveling after the formation of the landslide dam. Thus, it is feasible to assess the critical parameters required for the landslide dam hazard assessment rapidly once the ortho-photo data are available. The proposed procedure is useful for quick and efficient decision making regarding hazard mitigation.

A systematic approach for the assessment of flooding hazard and risk associated with a landslide dam

Inundation caused by landslide dams may occur in the upstream and downstream of the dams. A proper flooding hazard assessment is required for reaction planning and decision-making to mitigate possible flooding hazards caused by landslide dams. Both quick and detailed procedures can be used to evaluate inundation hazards, depending on the available time and information. This paper presents a systematic approach for the assessment of inundation hazards and risks caused by landslide dam formation and breaches. The approach includes the evaluation of dam-breach probability, assessment of upstream inundation hazard, assessment of downstream inundation hazard, and the classification of flooding risk. The proposed assessment of upstream inundation estimates the potential region of inundation and predicts the overtopping time. The risk level of downstream flooding is evaluated using a joint consideration of the breach probability of a landslide dam and the level of flooding hazard, which is classified using a flooding hazard index that indicates the risk of potential inundation. This paper proposes both quick and detailed procedures for the assessments of inundation in both the upstream and downstream of a landslide dam. An example of a landslide dam case study in southern Taiwan was used to demonstrate the applicability of the systematic approach.

A HIERARCHICAL MODEL OF ROUGH ROCK JOINTS BASED ON MICROMECHANICS

Abstract A rough rock joint model is proposed in this study on the basis of micromechanics concepts. In the model, the global behavior of a rough joint depends on the microfeatures of the contact planes on the joint. The contact mechanics on contact planes controls the mechanical behavior of the joint via a homogenization process. Also, the complex mechanical behavior of a joint is associated with simple microfeatures of the joint, including the frictional properties and the structure of contact planes. To capture the feature of scale dependency of joint roughness, a hierarchical representation of a joint profile is proposed in the form of a multi-level-asperity model. Moreover, the constitutive relation of the multi-level-asperity model is derived through a recursive homogenization process. The proposed models framework is general and systematic. Furthermore, major deformation mechanisms of joint asperities, i.e. interlocking, wearing, shearing-off, sliding, separation and degradation, are taken into account.

FAILURE MECHANISM AND BEARING CAPACITY OF SHALLOW FOUNDATION ON POORLY CEMENTED SANDSTONE

This paper aims to investigate the failure mechanism of a shallow foundation on poorly cemented sandstone and to propose an upper bound solution for the bearing capacity of the foundation. A series of laboratory material and load-bearing model tests with specimens made of artificial rock mimic undisturbed natural poorly cemented sandstone. Based on a series of load-bearing model tests, bearing behavior and progressive failure mechanisms are investigated. It was found that the bearing behavior on poorly cemented sandstone is distinct from the cases on hard rock or on soil, and exhibits both plasticity and brittle characteristics. It is noted that the bearing capacity formulas for a shallow foundation commonly used for soil or hard rock are not appropriate for the case of poorly cemented soft sandstone. Based on the observed failure mechanism, a simplified plastic collapse mechanism is proposed and an upper-bound solution on the basis of a multi-block translation mechanism is formulated. It is shown that the upper bound solution agrees well with the experimental bearing capacity as long as a proper non-associated flow rule is adopted.

A model of the time-dependent interaction between rock and shotcrete support in a tunnel

Abstract A viscoelastic model is developed to simulate the tunnel-support interaction of a circular tunnel in a viscoelastic rock mass. This model accounts for the time-dependent stiffness and the yielding strain of shotcrete support. This model is formulated to analyze the effects of the time-dependence of support characteristics on tunnel convergence, support pressure, and tunnel-support interaction. Results of a parametric study reveal that the time-dependence of shotcrete stiffness has a strong influence on tunnel convergence and support pressure. Ignoring the yield strain of shotcrete may lead to the underestimation of tunnel convergence and the overestimation of the ultimate capacity of shotcrete support.

Soft-Rock Scouring Processes Downstream of Weirs

Diversion weirs are typically constructed to elevate river water levels and thereby increase water supplies. Most riverbeds in the western foothills of Taiwan are composed of soft sedimentary rocks covered with an armor layer of varying thickness. Due to the low rock strength and head fall caused by weirs, rapid scouring downstream of the weir often occurs once the armor layer is worn away. To estimate scour depth and mitigate its damage to weir foundations, scouring processes must be identified correctly. Because of different mechanical behaviors and the water-jet conditions, scouring processes downstream of a weir on soft rock may not be that same as those assumed by existing models. Analytical results from a series of studies of scouring downstream of weirs on soft rock indicate that the shape of a scour hole on soft rock differs markedly from that of a scour hole on hard rock. This study identified three basic scouring processes downstream of weirs, namely, (1) plucking, (2) uniform incision, and (3) trenching incision. The suitability of existing approaches for calculating the depth of a scour hole on soft rock were also assessed.

Development of a Multiple-Purpose Borehole Testing Device for Soft Rock

The authors developed a versatile in situ testing device specifically for soft rock. The main purpose of this device was to provide design parameters for shallow or deep foundations in soft rock. The new device, referred to as the Borehole Testing Device (BTD) consisted of four radial curved platens at the top and a circular steel plate at the bottom. The BTD was designed to be used in a 200-mm-diameter borehole. Driven by a high stress pressuremeter and a hydraulic piston, the BTD can be used to perform a borehole jacking test, plate-loading test, and borehole shear test in the same borehole. A multistage testing procedure was proposed to perform the BTD test. This paper describes the design and operation of the BTD and presents a set of test data to demonstrate the capabilities of the BTD.