• Journal of the Korean GEO-environmental Society
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• v.9 no.4
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• pp.5-13
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• 2008

Just as infill material can reduce the shear strength of a rock joint, a layer of soft material between concrete and the surrounding rock socket can reduce pile shaft resistance of drilled shafts socketed in rocks. This can also result from construction methods that leave smeared or remoulded rock or drilling fluid residue on the sides of the rock sockets after concrete placement. The nature of the interface between the concrete pile shaft and the surrounding rock is critically important to the performance of the pile, and is heavily influenced by construction practice. Characteristics of the concrete-rock interface, such as roughness and the presence of the soft materials deposited during or after construction can significantly affect the shaft resistance response of the pile. In this study, we conducted the parametric study to examine the performance characteristics of drilled shafts socketed in smeared rock under the vertical load with the code of finite difference method of FLAC 2D. As the results of the current research, the parameters that affect the settlement of the pile head and the ultimate unit shaft resistance could be identified.

• Tunnel and Underground Space
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• v.14 no.2
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• pp.97-107
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• 2004

Various kinds of rock mass properties, which can be obtained from laboratory tests as well as field tests, can be reasonably applied to the design of earth structures. An extrapolation technique can be used for this application and it generally guarantee its quality from a sufficient amount of test results because it is based on the RMR value in most cases. When the confident RMR can not be obtained because of the insufficient testing results, the Monte Carlo Simulation technique can be introduced fer deducing the proper RMR and this assessed RMR can be reused fur the major input parameters. Authors' proposed method can be verified from the comparison between the results of numerical analysis and the evidences of field site.

• Journal of the Korean Geosynthetics Society
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• v.18 no.3
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• pp.89-100
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• 2019

The purpose of this study is to analyze the characteristics of bearing capacity of pre-bored super strength PHC (SSPHC) piles socketed in rocks based on dynamic load test results. Because the SSPHC piles have high compressive concrete strengths compared with those of regular high strength PHC piles, the allowable structural strengths of the SSPHC piles were increased. For optimal design of the super strength PHC piles, the geotechnical bearing capacity of the SSPHC piles should also increased to balance the increased allowable structural strength of the SSPHC piles. Current practices of pile installation apply the same amount of driving energy on both SSPHC and high strength PHC piles. As results of analyzing factors that influence bearing strength of SSPHC piles using dynamic load test, there was no relationship between SPT-N value at pile toe and end bearing capacity. But driving energy effects on end bearing capacity. In case of skin friction, driving energy had no effects. And reasonable method verifying design bearing strength is necessary because end bearing capacity is not considered sufficiently in restrike test results.

• Tunnel and Underground Space
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• v.10 no.4
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• pp.526-532
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• 2000

The uniaxial compressive strength of rock mass is known as the major factor in the assessment of drillability and the optimum excavation design in full-face tunnel excavation by TBM. Referring to worldwide cases, TBM has been applied mostly to the rock mass within the strength range of 80~250 MPa. Recently, a water way tunnel has been constructed as a part of Milyang dam project by TBM within the rock masses where the rock type is mainly granite with some granophyre, hornfels and andesite. Their uniaxial compressive strengths in extended area are estimated higher than 260 MPa. In this paper, the relation between the penetration rate and the rock mass properties is analyzed and TBM application to the very hard rocks is discussed. As a result that three suggestions to predict the TBM net penetration rate are analyzed, NTH method seems a better approach than other methods in the extremely hard rocks. NTH prediction matches with the results of actual values with the variations of 2~20%. Hardness measurement by Schmidt hammer and RMR estimation are carried out along the L = 5.3 km entire TBM tunnel alignment. The net penetration rate measured monthly is shown to be reciprocally proportional to Schmidt rebound hardness and RMR where coefficients of correlation, $R^2$are 0.705 and 0.777 respectively. As a result, they are good quantitative indices for the prediction of TBM net penetration rate in the extremely hard rocks. Magnitude of in-situ stress has a certain effect on TBM performance, and it is required to measure the in-situ stresses in TBM excavation design.

• Tunnel and Underground Space
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• v.25 no.3
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• pp.284-292
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• 2015

The generalized Hoek-Brown failure criterion, the strength parameters of which are determined by using the GSI index, is an empirical nonlinear failure criterion of rock mass and has been widely employed in various rock engineering practices. Many rock engineering practitioners, however, are still familiar with the description of the strength of rock mass in terms of friction angle and cohesion. In addition, almost all rock mechanics softwares incorporate the simple linear Mohr-Coulomb function. Therefore, it is necessary to provide a tool to implement the Hoek-Brown function in the framework of the Mohr-Coulomb criterion. In this study, the use of upper-bound solution of limit analysis for bearing capacity of a strip footing resting on the ground surface is proposed for the estimation of the equivalent friction angle and cohesion of rock mass incorporating the generalized Hoek-Brown failure criterion. The upper-bound bearing capacity is expressed in terms of friction angle by use of the relationship between tangential friction angle and tangential cohesion implied in the generalized Hoek-Brown function. The friction angle minimizing the upper-bound bearing capacity is taken as the equivalent friction angle. Through the illustrative implementations of the proposed method, the influences of GSI, $m_i$ and D on the equivalent friction angle and cohesion are investigated.

• Journal of the Korean GEO-environmental Society
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• v.19 no.12
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• pp.25-32
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• 2018

This paper investigates the effect of rock discontinuities on a shear stress wave that is induced by earthquake or blasting and provides the result of numerical parametric studies. The numerical tests of different conditions of rock and discontinuity have been carried out after confirming that the numerical approach is valid throughout a verification analysis from which the test results were compared with a theoretical solution. In-situ stress condition was considered as a rock condition and internal friction angle and cohesive value, which are the shear strength parameters, were considered as discontinuities condition. The joint inclination angle was also taken into account as a parameter. With the various conditions of different parameters, the test results showed that a shear stress wave propagating through a mass is highly influenced by the shear strength of discontinuities and the condition of joint inclination angle as well as in-situ stress. The study results indicate that when earthquake or blasting-induced dynamic loading propagates through a jointed rock mass or a stratified soil ground the effect of in-situ stress and discontinuities including a stratum boundary should be taken into account when evaluating the dynamic effect on nearby facilities and structures.

• Tunnel and Underground Space
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• v.33 no.5
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• pp.414-424
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• 2023

In this study, a total of 326 data on a variety of geometric parameters of pick cutters and the corresponding cuttable rock conditions were collected and built as a database. Statistical analysis of the database showed that there is a significant positive correlation between the parameters that define the geometry of a pick cutter, especially between the parameters related to the length of a pick cutter and the geometry of a tungsten carbide insert. The diameter of a pick cutter shaft was also strongly correlated with the geometry of the inserts. On the other hand, it was difficult to find a clear correlation between the parameters for the rock conditions defined by the four conditions and the geometric parameters of pick cutters, which may be due to the uncertainty of the rock mass and the fact that the application of a pick cutter is presented as a range rather than a numerical single value. However, the mean values of geometric parameters of pick cutters tend to increase as a rock mass becomes harder. However, the pick length parameters are found to decrease as a rock mass becomes harder, which may be a way to reduce the moments that can occur when using long pick cutters in a hard rock condition.

• Tunnel and Underground Space
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• v.30 no.3
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• pp.214-225
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• 2020

Uniaxial compressive strength (UCS) of rock is one of the important factors to determine the advance speed during shield TBM tunnel excavation. UCS can be obtained through the Geotechnical Data Report (GDR), and it is difficult to measure UCS for all tunneling alignment. Therefore, the purpose of this study is to predict UCS by utilizing TBM machine driving data and machine learning technique. Several machine learning techniques were compared to predict UCS, and it was confirmed the stacking model has the most successful prediction performance. TBM machine data and UCS used in the analysis were obtained from the excavation of rock strata with slurry shield TBMs. The data were divided into 8:2 for training and test and pre-processed including feature selection, scaling, and outlier removal. After completing the hyper-parameter tuning, the stacking model was evaluated with the root-mean-square error (RMSE) and the determination coefficient (R²), and it was found to be 5.556 and 0.943, respectively. Based on the results, the sacking models are considered useful in predicting rock strength with TBM excavation data.

• Journal of the Korean GEO-environmental Society
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• v.11 no.1
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• pp.13-18
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• 2010

It is crucial to decide reasonably the parameters necessary for design through research on the characteristics of rock in order to analyze stability of rock structure. This article has conducted both pressure meter test, uniaxial and triaxial compression test to the areas of andesite or tuff located in local regions such as Yeosu, Gwangju, Yangsan, Busan, and Daejeon and has comparatively analyzed previously proposed estimative formulas. According to the result of estimating the deformation modulus through using the damping coefficient suggested by Nicholson & Bieniawski, when RMR is less than 60, it is desirable to use the result of the pressure meter test considering the damping coefficient. If the RMR value is over 60, however, the formula suggested by Kim Gyo-won has been proved to be the most applicable. Moreover, according to the result of comparing the RMR, adhesion, and angle of internal friction, both the adhesion and angle of internal friction best correspond to the formula proposed by Tsuchiya. Comparatively analyzing the relations between the rock deformation modulus and RMR and also suggesting the formula of calculating the shear strength parameter to use the RMR value, this study did not include the deformation modulus and shear strength parameter as factors to decide the RMR index. Since result can differ by several factors, it will be necessary afterwards to suggest practical estimative formulas applicable to the actual spots of Korea.

• Tunnel and Underground Space
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• v.27 no.2
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• pp.109-119
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• 2017

A lot of twin tunnels were modelled with different pillar widths, rock mass classes and stress ratios in order to consider various site conditions, and the stabilities of the pillars were estimated by numerical analyses and scaled model tests. The strength-stress ratios of the pillar were obtained from three different methods which were using the stresses appeared at the middle point, the whole average and the left/right edges of the pillar. The strength-stress ratio of the pillar edges showed relatively conservative values among them, and it was also practically consistent with the tunnel excavating steps comprising the construction sequence analyses which included the partial excavation and the support system. Scaled model tests were also performed to investigate the tunnel stability, where it was found that cracks were progressively generated from the pillar edges toward the middle point of the pillar. Therefore, in order to both prevent the local damage of pillar and conservatively estimate the tunnel stability, it was thought to be an appropriate method using the strength-stress ratio obtained from the left/right edges of the pillar.