Ki-Il Song

Long-Wavelength Elastic Wave Propagation Across Naturally Fractured Rock Masses

Geophysical site investigation techniques based on elastic waves have been widely used to characterize rock masses. However, characterizing jointed rock masses by using such techniques remains challenging because of a lack of knowledge about elastic wave propagation in multi-jointed rock masses. In this paper, the roughness of naturally fractured rock joint surfaces is estimated by using a three-dimensional (3D) image-processing technique. The classification of the joint roughness coefficient (JRC) is enhanced by introducing the scan line technique. The peak-to-valley height is selected as a key indicator for JRC classification. Long-wavelength P-wave and torsional S-wave propagation across rock masses containing naturally fractured joints are simulated through the quasi-static resonant column (QSRC) test. In general, as the JRC increases, the S-wave velocity increases within the range of stress levels considered in this paper, whereas the P-wave velocity and the damping ratio of the shear wave decrease. In particular, the two-dimensional joint specimen underestimates the S-wave velocity while overestimating the P-wave velocity. This suggests that 3D joint surfaces should be implicated to obtain the reliable elastic wave velocity in jointed rock masses. The contact characteristic and degree of roughness and waviness of the joint surface are identified as a factor influencing P-wave and S-wave propagation in multi-jointed rock masses. The results indicate a need for a better understanding of the sensitivity of contact area alterations to the elastic wave velocity induced by changes in normal stress. This paper’s framework can be a reference for future research on elastic wave propagation in naturally multi-jointed rock masses.

Prediction of ultrasonic pulse velocity for enhanced peat bricks using adaptive neuro-fuzzy methodology

Ultrasonic pulse velocity is affected by defects in material structure. This study applied soft computing techniques to predict the ultrasonic pulse velocity for various peats and cement content mixtures for several curing periods. First, this investigation constructed a process to simulate the ultrasonic pulse velocity with adaptive neuro-fuzzy inference system. Then, an ANFIS network with neurons was developed. The input and output layers consisted of four and one neurons, respectively. The four inputs were cement, peat, sand content (%) and curing period (days). The simulation results showed efficient performance of the proposed system. The ANFIS and experimental results were compared through the coefficient of determination and root-mean-square error. In conclusion, use of ANFIS network enhances prediction and generation of strength. The simulation results confirmed the effectiveness of the suggested strategies.

Rheological properties of cemented tailing backfill and the construction of a prediction model

Workability is a key performance criterion for mining cemented tailing backfill, which should be defined in terms of rheological parameters such as yield stress and plastic viscosity. Cemented tailing backfill is basically composed of mill tailings, Portland cement, or blended cement with supplementary cement material (fly ash and blast furnace slag) and water, among others, and it is important to characterize relationships between paste components and rheological properties to optimize the workability of cemented tailing backfill. This study proposes a combined model for predicting rheological parameters of cemented tailing backfill based on a principal component analysis (PCA) and a back-propagation (BP) neural network. By analyzing experimental data on mix proportions and rheological parameters of cemented tailing backfill to determine the nonlinear relationships between rheological parameters (i.e., yield stress and viscosity) and mix proportions (i.e., solid concentrations, the tailing/cement ratio, the specific weight, and the slump), the study constructs a prediction model. The advantages of the combined model were as follows: First, through the PCA, original multiple variables were represented by two principal components (PCs), thereby leading to a 50% decrease in input parameters in the BP neural network model, which covered 98.634% of the original data. Second, in comparison to conventional BP neural network models, the proposed model featured a simpler network architecture, a faster training speed, and more satisfactory prediction performance. According to the test results, any error between estimated and expected output values from the combined prediction model based on the PCA and the BP neural network was within 5%, reflecting a remarkable improvement over results for BP neural network models with no PCA.

Utilization of geotextile tube for sandy and muddy coastal management: a review.

Threats to beaches have accelerated the coastal destruction. In recent decades, geotextile tubes were used around the world to prevent coastal erosion, to encourage beach nourishment, and to assist mangrove rehabilitation. However, the applications of geotextile tube in sandy and muddy coasts have different concerns as the geological settings are different. Applications of geotextile tubes in sandy beaches were mainly to prevent coastline from further erosion and to nourish the beach. However, for the muddy coasts, mangrove rehabilitation and conservation were additional concerns in coastal management schemes. The mangrove forests are natural barriers which can be found on the muddy coasts of many tropical countries. In this paper, the viability of geotextile tubes in sandy and muddy beaches was analysed. The advantages and disadvantages of the utilization of geotextile tubes in coastal management were discussed based on the experiences from the tropical countries such as Mexico, Malaysia, and Thailand. From the case studies, impressive improvements in coastal restoration after installation of geotextile tubes were shown. Based on the discussion, several recommendations to improve the application of geotextile tubes were suggested in this paper.

Long-Term Assessment of an Innovative Mangrove Rehabilitation Project: Case Study on Carey Island, Malaysia

Wave energy and storm surges threaten coastal ecology and nearshore infrastructures. Although coastal structures are conventionally constructed to dampen the wave energy, they introduce tremendous damage to the ecology of the coast. To minimize environmental impact, ecofriendly coastal protection schemes should be introduced. In this paper, we discuss an example of an innovative mangrove rehabilitation attempt to restore the endangered mangroves on Carey Island, Malaysia. A submerged detached breakwater system was constructed to dampen the energy of wave and trap the sediments behind the structure. Further, a large number of mangrove seedlings were planted using different techniques. Further, we assess the possibility of success for a future mangrove rehabilitation project at the site in the context of sedimentology, bathymetry, and hydrogeochemistry. The assessment showed an increase in the amount of silt and clay, and the seabed was noticeably elevated. The nutrient concentration, the pH value, and the salinity index demonstrate that the site is conducive in establishing mangrove seedlings. As a result, we conclude that the site is now ready for attempts to rehabilitate the lost mangrove forest.

Evaluation of Morphodynamic Changes in the Vicinity of Low-Crested Breakwater on Cohesive Shore of Carey Island, Malaysia

There is no adequate research available on the responses of coastal morphodynamic to low-crested breakwaters along the cohesive shore. Since the shore responses to coastal structures vary widely among different case studies, it is necessary to evaluate the performance of these structures before they are chosen as a coastal protection alternative. In the present study, we investigated the bed level changes and the accretion—erosion pattern on intertidal area of the cohesive shore in Malaysia due to the presence of low-crested breakwater from the year 2009 to 2014. The bathymetry data was obtained during a project in Carey Island, Malaysia. The results showed that the existing breakwater have successfully increased the bed level behind the structure. The increase of sediment accumulation have reached 0.176m3/m2 five years after the construction of the breakwater. However, the increased bed level five years after its installation is still not sufficient to fulfill the required bed level and hydraulic regime for the mangroves survival. Thus, the bed levels need to be increased further to establish a suitable hydraulic regime for the mangrove rehabilitation project at the site.

Evolution of Joint Roughness Degradation from Cyclic Loading and Its Effect on the Elastic Wave Velocity

The application of stress to the rock joint has a significant impact on morphological and mechanical properties of the joint. In particular, the evolution of the joint condition is dramatic on the surface of the weathered rock whose material integrity is altered in a manner similar to the rock joint behavior from weathering effects (Resende et al. 2010). As indicated in Kabeya and Legge (1997), a significant change in the weathered rock joint surface can be induced by the modification of grain properties of the joint surface. The grain size distribution has been found to be strongly correlated with the shear behavior and the joint roughness coefficient (JRC). The open joint or gouge dominates in weathered joints of the rock mass structure, thereby providing access for weathering agents to increase the failure likelihood from a slickenside (Wooet al. 2010).This granular disintegration leads to surface flakes to transform the joint aperture into a wider opening. In the rock mass rating (RMR) system, the geomechanic classification method for rock masses is associated with the roughness of the rock joint surface as the discontinuity condition (Hoek 2007). Conventionally, the roughness of a rock joint surface is rated and quantified in term of the JRC, which ranges from 0 to 20 (Barton and Choubey 1977). However, the validity of techniques and the accuracy of measurement methods for identifying and classifying rock joint surfaces are generally questioned because of the subjectivity of JRC results. The undulation and unevenness of a rock joint surface are recognized by its peak-to-valley height (Hotar and Novotny 2005) to classify the rock joint surface profile. In particular, Mohd-Nordin et al. (2014) applied the JRC to naturally fractured rock surfaces using the scan line technique through the implementation of the peak-to-valley height. In this regard, the maximum peak-to-valley height (Pmax) can be used as a meaningful quantitative indicator of the degree of roughness of a rock joint surface. Previous studies have examined the propagation of elastic waves across multiple jointed rock masses by considering joint surface conditions (Mohd-Nordin et al. 2014; Huang et al. 2014a; Cha et al. 2009). One of the main findings is that the roughness and unevenness of a replicated natural rock joint surface have significant effects on the propagation of elastic waves. However, changes in & Ki-Il Song ksong@inha.ac.kr

DEA optimization for operating tunnel back analysis

ABSTRACT: Estimation of the stability of an operating tunnel through a back analysis is a difficult concept to analyze. Specially, when a relatively thick lining is constructed as in case of a subsea tunnel, there will be a limit to the use of displacement-based tunnel back analysis because the corresponding displacement is too small. In this study, DEA is adopted for tunnel back analysis and the feasibility of DEA for back analysis is evaluated. It is implemented in the finite difference code FLAC3D using its built-in FISH language. In addition, the stability of a tunnel lining will be evaluated from the development of displacement-based algorithm and its expanded algorithm with conformity of several parameters such as stress measurements.Keywords: Differential Evolution Algorithm(DEA) Optimization, Back Analysis, FLAC3D, Operating Tunnel 초 록: 운영 중 터널의 역해석을 통한 안정성 평가는 다루기 어려운 개념이다. 특히, 해저터널처럼 라이닝이 상대적으로 두껍게 시공된 경우에 발생하는 내공변위가 작기 때문에, 일반적으로 터널 역해석에 사용되는 변위 기반의 터널 역해석에는 한계가 있을 것으로 판단된다. 여기서는 FLAC3D 프로그램에 내장된 FISH 언어를 사용해서, 다양한 역해석 알고리즘 중에 차분진화 알고리즘의 구축을 통한 터널 역해석에 대한 적용성을 확인하였다. 또한, 변위 기반의 알고리즘 구축과 더불어 응력 계측값 등 다양한 인자의 적합성을 확인하여 터널 라이닝의 안정성을 평가하고자 한다.주요어: 차분진화 알고리즘(DEA) 최적화, 역해석, FLAC3D, 운영 중 터널Copyright ⓒ2016, Korean Tunnelling and Underground Space Association

Analysis on the behavior of shield TBM cable tunnel: The effect of the distance of backfill grout injection from the end of skin plate

3 Power System Laboratory, KEPCO Research Institute, Senior Researcher ABSTRACT: Recently, tunnelling with TBM is getting popular for the construction of cable tunnel in urban area. Mechanized tunnelling method using shield TBM has various advantages such as minimization of ground settlement and prevention of vibration induced by blasting that should be accompanied by conventional tunnelling. In Korea, earth pressure balance (EPB) type shield TBM has been mainly used. Despite the popularity of EPB shield TBM for cable tunnel construction, study on the mechanical behavior of cable tunnel driven by shield TBM is insufficient. Especially, the effect of backfill grout injection on the behavior of cable tunnel driven by shield TBM is investigated in this study. Tunnelling with shield TBM is simulated using 3D FEM. The distance of backfill grout injection from the end of shield skin varies. Sectional forces such as axial force, shear force and bending moment are monitored. Vertical displacement at the ground surface is measured. Futhermore, the relation between volume loss and the distance of backfill grout injection from the end of skin plate is derived. Based on the stability analysis with the results obtained from the numerical analysis, the most appropriate injection distance can be obtained.

EVALUATION OF THE TIME-DEPENDENT CHARACTERISTICS OF GROUTED SAND USING AN ELASTIC WAVE

For a better evaluation of a grouted zone during and after tunnel construction involving weak soil layers, it is necessary to estimate the characteristics of grouted zone effectively. This study suggests a method that can be used for characterizing the time-dependent behavior of pre-reinforced zones around a large section of tunnel in soft ground using elastic waves. Experimental tests were performed to characterize the time-dependent behavior of the pre-reinforced zone. Experimental results show that shear strengths as well as elastic wave velocities increase with the curing time. Thus, shear strength or strength parameters can be uniquely correlated to elastic wave velocities. It is possible to characterize grouted soils around tunnel using elastic waves. Time-dependent strength and stiffness parameters in the experimental tests were applied in a numerical modeling of a large-section tunnel in soft ground, taking into account its construction sequence. According to the results of the numerical modeling, displacement results for fewer than 2~3 days of constant time boundary conditions are nearly identical to the analysis results of the time-dependent condition. The proposed analysis method, which combines experimental and numerical procedures while considering the time-dependent effect of the pre-reinforced zone on the tunnel behavior, will provide a reliable and practical design basis and a means of analysis for large-section tunnels in soft ground.