• Geomechanics and Engineering
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• v.19 no.1
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• pp.61-70
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• 2019

Yieldable steel ribs have been widely applied in tunnels excavated in rheological rocks. For further understanding the influence of yieldable steel ribs on supporting effect, mechanical behavior of tunnels supported by them in rheological rocks is investigated in this paper. Taking into account the deformation characteristic of yieldable steel ribs, their deformation is divided into three stages. In order to modify the stiffness of yieldable steel ribs in different deformation stages, two stiffness correction factors are introduced in the latter two stages. Viscoelastic analytical solutions for the displacement and pressure in the rock-support interface in each deformation stage are obtained. The reliability of the theoretical analysis is verified by use of numerical simulation. It could be concluded that yieldable steel ribs are able to reduce pressure acting on them without becoming damaged through accommodating the rock deformation. The influence of stiffness correction factor in yielding deformation stage on pressure and displacement could be neglected with it remaining at a low level. Furthermore, there is a linearly descending relationship of pressure with yielding displacement in linear viscoelastic rocks.

• Geomechanics and Engineering
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• v.34 no.5
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• pp.547-559
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• 2023

We evaluated the applicability of machine learning techniques and the Kuz-Ram model for predicting the mean fragmentation size in open-pit mines. The characteristics of the in-situ rock considered here were uniaxial compressive strength, tensile strength, rock factor, and mean in-situ block size. Seventy field datasets that included these characteristics were collected to predict the mean fragmentation size. Deep neural network, support vector machine, and extreme gradient boosting (XGBoost) models were trained using the data. The performance was evaluated using the root mean squared error (RMSE) and the coefficient of determination (r²). The XGBoost model had the smallest RMSE and the highest r² value compared with the other models. Additionally, when analyzing the error rate between the measured and predicted values, XGBoost had the lowest error rate. When the Kuz-Ram model was applied, low accuracy was observed owing to the differences in the characteristics of data used for model development. Consequently, the proposed XGBoost model predicted the mean fragmentation size more accurately than other models. If its performance is improved by securing sufficient data in the future, it will be useful for improving the blasting efficiency at the target site.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.5
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• pp.487-495
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• 2014

In this study, in order to suggest the design method for supports in the room-and-pillar underground structure, the case study was carried out. In the case study, shape of rock pillar and room was mainly considered. From the analysis, a displacement at the roof, the maximum principle stress and plastic state were examined. To optimize variables in the case study, cases from the Seoul metro station were analyzed, then a target depth of the underground structure and ground conditions were determined. And the height of rock pillar and room were chosen from the assumed purpose of underground space, i.e. living/office and warehouse. Total cases of analysis was 180 cases including 3 types of ground condition, 5 types of rock pillar and 6 types of roof span. It is expected that results from analysis can be used to determine the installation of support in room-and-pillar underground structure with stability, utilization efficiency of underground space and applicability of vehicles.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.37 no.4
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• pp.637-645
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• 2017

Jeju Island is composed of irregular volcanic rock layers formed by several volcanic activities. Since structure such as the offshore wind turbine has to support considerably large over turning moment due to long distance from foundation to load point and relatively large horizontal load. Pile foundations are needed to economically support such structure even in the case of rock layer. Therefore, in this study, mechanical performances are estimated by mixing ratio of water, cement, and sand to figure out optimal mixing ration of filling material for pile penetrated to rocky layers, and outcomes of this study are compared and analyzed with results of other researches. In the same conditions, mechanical performances of the mortar (S/(S+C)=20~40%) are better than those of cement paste and soil cement. On the basis of major outcome of this study, appropriate range of mixing and a strengthening model are suggested.

• Geotechnical Engineering
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• v.10 no.3
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• pp.79-96
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• 1994

A tunnel design expert system entitled NESTED is developed using the artificial neural network. The expert system includes three neural network computer models designed for the stability assessment of underground openings and the estimation of correlation between the RMR and Q systems. The expert system consists of the three models and the computerized rock mass classification programs that could be driven under the same user interface. As the structure of the neural network, a multi -layer neural network which adopts an or ror back-propagation learning algorithm is used. To set up its knowledge base from the prior case histories, an engineering database which can control the incomplete and erroneous information by learning process is developed. A series of experiments comparing the results of the neural network with the actual field observations have demonstrated the inferring capabilities of the neural network to identify the possible failure modes and the support timing. The neural network expert system thus complements the incomplete geological data and provides suitable support recommendations for preliminary design of tunnels in rock masses.

• Journal of Korean Tunnelling and Underground Space Association
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• v.17 no.1
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• pp.33-47
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• 2015

Concrete lining in tunnel construction is used as secondary support for downward loads when primary support decays. The use of concrete lining varies greatly depending on the intentions of engineer and/or client. An engineer uses much smaller deformation modulus which determines the concrete lining thickness than of a pattern 3, when supporting patterns 4 and 5 are used in a weathered rock and soil. Considering these conditions, this study intends to suggest optimal construction procedures through a back analysis using a computer program(MIDAS-civil). Cases of Seoul Subway System line${\bigcirc}{\bigcirc}$ zone${\bigcirc}{\bigcirc}$ were selected to be examined for this study. The results show that it is possible to reduce the thickness of concrete lining. When results from this study were applied to Seoul Metropolitan subway construction projects, it is expected to bring economic benefits.

• Journal of the Korean Geosynthetics Society
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• v.21 no.3
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• pp.49-59
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• 2022

This study was conducted to find out the rational and appropriate design of drilled shaft. In other words, in order to find out the variation of ultimate bearing capacity according to the change in the support layer of drilled shaft, back analysis was performed using the bi-directional pile load test performed on drilled shaft. Based on the back-analyzed data, numerical analysis of the pile head load was performed, and the ultimate bearing capacity in the target ground was evaluated using the Davisson method. As a result of numerical analysis of one case where the end of the pile was seated on the top of the weathered rock layer, and three cases where the end of the pile was embedded at different locations in the weathered soil, it was found that sufficient ultimate bearing capacity was secured in all cases. In other words, the case where the end of the pile is seated on the top of the weathered rock layer, not embedded the weathered rock, and the drilled shaft embedded into the weathered soil also have sufficient bearing capacity, so it can be used as a support layer for drilled shaft.

• Geotechnical Engineering
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• v.14 no.6
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• pp.17-32
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• 1998

The Bakun hydroelectric project includes the construction of a hydroelectric power plant with an installed capacity of 2,520MW and a power transmission system connecting to the existing transmission networks in Sarawak and Western Malaysia, The power station will consist of a 210m height concrete faced rockfill dam. During the construction of the dam and the power facilities the Balui river has to be diverted by three diversion tunnels with a length of some 1,400m each. The inner diameter of the tunnels is 12m and the tunnel width is 16m at the portal area. This paper describes the stability analysis and design methods for the open cut rock slopes in the inlet and outlet area of the diversion tunnels. The geotechnical parameters employed in stability calculations were given as a function of four. defined Rock Mass Types (RMT) which were based on RMR system from Bieniawski. The stability calculations procedure of the rock slopes are divided into two stages. In the first stage, it is calculated for the stability of each 'global' slope without any rock support and shotcrete system. In the second stage, it is calculated for each 'local'slope stability with berms and supported with rock bolts and shotcrete. The monitoring instrumentation was performed continuously and some of the design modification was carried out in order to increase the safety of failed area based on the unforeseen geological risks during the open cut excavation.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.3
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• pp.221-232
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• 2008

Rock bolts have been installed into rockmass as a main support system. In order to evaluate the rock bolt integrity using non-destructive technique, the transmission method of the guided ultrasonic wave has been successfully performed. For the transmission method, however, the source for the generation o# guided ultrasonic waves should be installed at the end of the steel bar during construction of the rock bolt in the field. The purpose of this study is to suggest a reflection method that the source and the receiver are installed on the head of the steel bar. The reflection method is compared with the transmission method using non-embedded rock bolts and rock bolts embedded in concrete block. In this experiment, the piezo disc element is used as the source and the AE sensor is used as the receiver. The wavelet transform is applied to determine the energy velocity. The experimental studies show that the reflection method produces almost identical value of the transmission method, and the energy velocity increases with the defect ratio. This study suggests that the reflection method of the guided ultrasonic wave may be a suitable method fur the rock bolt integrity evaluation in the field.

• Journal of the Korean Geotechnical Society
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• v.25 no.12
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• pp.5-12
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• 2009

As rock bolts become one of the main support systems in tunnels and underground structures, the integrity of the rock bolts affects the safety of these structures. The purpose of this study is the evaluation of rock bolt integrity using wavelet transforms of the guided ultrasonic waves by using transmission test in the field. After several rock bolts with various defect ratios are embedded into a large scale concrete block and rock mass, guided waves are generated by a piezo disk element and measured by an acoustic emission (AE) sensor. The captured signals are analyzed in the time-frequency domain using the wavelet transform based on a Gabor wavelet. Peak values in the time-frequency domain represent the interval of travel time of each echo. The energy velocities of the guided waves increase with an increase in the defect ratio. The suitable curing time for the evergy velocity analysis is proposed by the laboratory test, and in-situ tests are performed in two tunnelling sites to verify the applicability of rock bolt integrity tests performed after proposed curing time. This study proves that time-frequency domain analysis is an effective tool for the evaluation of the rock bolt integrity.