• Geomechanics and Engineering
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• v.16 no.6
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• pp.569-575
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• 2018

The stability of shallow cavities with an arbitrary profile is a difficult issue in geotechnical engineering. This paper investigates this problem on the basis of the upper bound theorem of limit analysis and the Hoek-Brown failure criterion. The influence of pore pressure is taken into consideration by regarding it as an external force acting on rock skeleton. An objective function is constructed by equating the internal energy dissipation to the external force work. Then the Lagrange variation approach is used to solve this function. The validity of the proposed method is demonstrated by comparing the analytical solutions with the published research. The relations between shallow and deep cavity are revealed as well. The detaching curve of cavity roof with elliptical profile is obtained. In order to facilitate the application of engineering practice, the numerical results are tabulated, which play an important role in tunnel design and stability analysis of roof. The influential factors on potential collapse are taken into consideration. From the results, the impact of various factors on the extent of detaching is seen intuitively.

• Geomechanics and Engineering
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• v.33 no.4
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• pp.393-400
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• 2023

Displacements near crack and stress intensity factor (SIF) are key parameters to solve rock failure issue when using fracture mechanics. In order to study the horizontal displacement and stress intensity factor of the mode I fracture, a series of three-point bending tests of granite specimens with central notch were carried out. The evolution of horizontal displacements of precast notch and crack tip opening displacements (CTOD) were analyzed based on the digital image correlation (DIC) method. Stress intensity factors for three-point bending beams with arbitrary span-to-width ratios(S/W) were calculated by using the WU-Carlsson analytical weight function for edge-crack finite width plate and the analytical solution of un-cracked stress by Filon. The present study provides a high efficient and accurate method for fracture mechanics analysis of the three-point bending granite beams.

• The Journal of Engineering Geology
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• v.17 no.1 s.50
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• pp.101-113
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• 2007

A landslide was occurred due to soil cutting for construction to expand the Donghae express highway in Dong-hae-City, Korea. The total area of the landslide was about $9,550m^2$ with 100 m of width and 87m of height. The landslide was occurred due to the internal factor of the unstable geological structure including the clay layer and the external factor of continuous heavy rainfalls. As the result of field instrumentation during the landslide, the horizontal displacement of the slope ground increases with increasing the accumulated rainfall by continuous rainfall during the rainy season. Also, the depth of sliding failure was decided by the horizontal displacement distribution during landslide occurrence. It makes sure that the horizontal displacement starts from the depth of sliding failure and the depth of sliding failure matches well with the location of the clay layer. As the slope stability analysis using Bishop's Simplified Method at the landslide area, the safety factor of slope during the rainy season was 0.53. This safety factor of slope was enough to trigger the landslide at this area. The depth of sliding failure obtained by analytical method matches well with the depth of the clay layer.

• Tunnel and Underground Space
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• v.22 no.3
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• pp.205-213
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• 2012

In this study, scaled model tests were performed to investigate the stability of twin tunnels constructed in anisotropic rocks with $30^{\circ}$ inclined bedding planes under the condition of lateral pressure ratio, 2. Five types of test models which had respectively different pillar widths and shapes of tunnel sections were experimented, where both crack initiating pressures and deformation behaviors around tunnels were investigated. The models with shallower pillar width showed shear failure of pillar according to the existing bedding planes and they were cracked under lower pressure than the models with thicker pillar width. In order to find the effect of tunnel sectional shape on stability, the models with four centered arch section, circular section and semi-circular arch section were experimented. As results of the comparison of the crack initiating pressures and the deformation behaviors around tunnels, the semi-circular arched tunnel model was the most unstable whereas the circular tunnel model was the most stable among them. Furthermore, the results of FLAC analysis were qualitatively coincident with the experimental results.

• Tunnel and Underground Space
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• v.14 no.6 s.53
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• pp.411-422
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• 2004

Sinkhole subsidence occurs over abandoned mine workings and can be detrimental to human lives, damage to properties and other surface structures. In this study, simulation of sinkhole development process is performed using special finite element procedure. Especially, creation of mine voids due to roof falls and generation of goaf from broken rocks are simulated using active-passive-active finite elements. An active or solid element can be made passive or void once the tensile failure criterion is satisfied in the specified sinkhole formation zone. Upon completion of sinkhole development process, these passive elements in again be made active to simulate goal region. Several finite element models are analyzed to evaluate the relationships between sinkhole formation with width of gallery. depth of mine, roof condition and bulking factor of roof rocks. This study demonstrates that the concept of passive elements in numerical analysis can be used effectively for analyzing sinkhole formation or roof fall phenomenon in general.

• Proceedings of the Korean Geotechical Society Conference
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• 2006.10a
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• pp.334-408
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• 2006

In Hong Kong, large-diameter (${\ge}600mm$) bored piles and large-section excavated rectangular barrettes are commonly used to support tall buildings to resist both vertical and horizontal loads. These piles and barrettes penetrate through and may found in saprolitic soils and decomposed rocks. Generally, the design of these large bored piles and barrettes involves considerable amount of uncertainty and design parameters must usually be verified by field tests. In this paper, over 50 full-scale load tests on large-diameter bored piles and over 15 large-section of rectangular barrettes in Hong Kong are reviewed and interpreted critically, in particular the degree of mobilisation of side shear resistance using a mobilization rating (MR) factor and a displacement index (DI) for floating bored piles and barrettes and rock-socketed piles, respectively. The author was heavily involved with many of these load tests. The diameter of the bored piles tested ranges from 0.6m to 1.8m and the depth varies from 12m to 75m. Sizes of barrettes critically reviewed include $2.2m{\times}0.6m,\;2.2m{\times}0.8m,\;2.8m{\times}0.8m\;and\;2.8m{\times}1.0m$ (on plan) and the depth varies from 36m and 63m. Based on these field tests, a new failure load criterion for large-diameter bored piles and barrettes is developed and proposed. The side shear resistance of the bored piles and barrettes is quantitatively analyzed with respect to local displacements, standard penetration tests, unconfined compressive strength (UCS) for rock sockets and using the effective stress principle. In addition, the effects of construction including post-grouting, construction time, side scraping and excavation tools on side shear resistance are investigated and reported.

• Tunnel and Underground Space
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• v.28 no.3
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• pp.277-291
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• 2018

LSTM (Long Short-Term Memory) algorithm which is a kind of recurrent neural network was used to establish a model to predict the stress-strain curve of an material under uniaxial compression. The model was established from the stress-strain data from uniaxial compression tests of silica-gypsum specimens. After training the model, it can predict the behavior of the material up to the failure state by using an early stage of stress-strain curve whose stress is very low. Because the LSTM neural network predict a value by using the previous state of data and proceed forward step by step, a higher error was found at the prediction of higher stress state due to the accumulation of error. However, this model generally predict the stress-strain curve with high accuracy. The accuracy of both LSTM and tangential prediction models increased with increased length of input data, while a difference in performance between them decreased as the amount of input data increased. LSTM model showed relatively superior performance to the tangential prediction when only few input data was given, which enhanced the necessity for application of the model.

• Journal of Korean Tunnelling and Underground Space Association
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• v.2 no.3
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• pp.47-57
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• 2000

It is difficult to calculate factor of safety of a tunnel by applying any analytical method based upon limit equilibrium method since the shape of failure plane in tunnel analysis can not be easily assumed in advance. To cope with this shortcoming, a method is suggested to calculate safety factor of a tunnel by numerical analysis using strength reduction technique. A circular tunnel excavated in a homogeneous rock was selected as an example problem and factors of safety were calculated for no-supported, partly-supported, and completely-supported cases respectively. Meshes with 3 different sizes were examined for a sensitivity analysis. For the verification of the proposed method, a limit equilibrium analysis was conducted and compared with the numerical analysis. The proposed method herein can be used to calculate factor of safety of a tunnel regardless of tunnel shape or geological conditions, and thus can contribute for the improved design and stability assessment of tunnels.

• Geomechanics and Engineering
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• v.15 no.2
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• pp.721-728
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• 2018

The surrounding rock mass contains cracks and joints which are distributed randomly around tunnels, and in the process of tunnel blasting excavation, radial cracks could also be induced in the surrounding rock mass. In order to clearly understand the impact of radial cracks on tunnel stability, tunnel model tests and finite element numerical analysis were implemented in this paper. Two kinds of materials: cement mortar and sandstone, were used to make tunnel models, which were loaded vertically and confined horizontally. The tunnel failure pattern was simulated by using RFPA2D code, and the Tresca stresses and the stress intensity factors were calculated by using ABAQUS code, which were applied to the analysis of tunnel model test results. The numerical results generally agree with the model test results, and the mode II stress intensity factors calculated by ABAQUS code can well explain the model test results. It can be seen that for tunnels with a radial crack emanating from three points on tunnel edge, i.e., the middle point between tunnel spandrel and its top with a dip angle $45^{\circ}$, the tunnel foot with a dip angle $127^{\circ}$, and the tunnel spandrel with $135^{\circ}$ with tunnel wall, the tunnel model strength is about a half of the regular tunnel model strength, and the corresponding tunnel stability decreases largely.

• Tunnel and Underground Space
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• v.17 no.5
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• pp.372-380
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• 2007

Surface damage due to subsidence is an inevitable consequence of underground mining, which may be immediate or delayed for many years. The surface damage due to abandoned underground mine is observed to be two subsidence types such as simple sinkhole or trough formation to a large scale sliding of the ground from with in the subsided area. An evaluation of the risk of a subsidence occurrence is vital in the areas affected by mining subsidence. For a subsidence prediction or a risk evaluation, there has been used various methods using empirical models, profile functions, influence functions and numerical models. In this study, a simple but efficient evaluation method of subsidence hazard is suggested, which is based on a diffusion theory and uses just information about geometry of caving and topography. The diffusion model has an analogous relationship with granular model which can explain a mechanism of subsidence. The diffusion model is applied for the evaluation of subsidence hazard in abandoned metal and coal mines. The model is found to be a simple but efficient tool because it needs information of geometry of caving and gangway and the topography.