• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.2
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• pp.175-188
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• 2009

When a new tunnel is excavated by the drill and blast method near pre-existing underground structures or tunnels due to the region restricted condition such as urban area, the ground will be relaxed by the excavation. In this case, issues can be created in terms of stability of pre-existing underground structures. One of major factors determining the stability of pre-existing underground structures can be a separation distance between pre-existing underground structures and a newly excavated tunnel. The region of ground relaxation defined by the plastic zone due to new excavation can be varied by separation distance. In this study, in other to estimate an influence of new tunnel excavation in terms of separation distance on the stability of pre-existing large pipelines, two-dimensional scaled model tests using plaster were performed for six models which have a different separation distance, The results show that based on the analysis of induced displacement during tunnel construction, the displacement decreases as the separation distance between large pipeline and new tunnel is increased until the distance is 2.5 times of pipeline diameter. Beyond this point, however, the displacement has become stabilized.

• Journal of the Korean Society of Safety
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• v.31 no.3
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• pp.68-74
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• 2016

This paper presents a new strengthening method of underground box structures against seismic loads for anti-seismic capacity improvement. A threaded steel member with pressure devices(so called 'Pre-flexed member system') is used to improve seismic capacity of the RC box structure. The pre-flexed member system is fixed the corner of opening after chemical anchor was installed by drilling hole on the box structure. The structural performance was evaluated analytically. Two bracing types of strengthening methods were used; conventional bracing method and I-bracing pressure system. For the performance evaluation, seismic analyses were performed on moment and shear resisting structures with and without strength member system. Numerical results confirmed that the proposed pre-flexed member system can enhance the seismic capacity of the underground RC box structures.

• Journal of the Korean earth science society
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• v.44 no.4
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• pp.275-290
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• 2023

Structural inversion refers to the reverse reactivation of extensional faults that influence basin shortening accommodated by contractional faults or folds. On the Korean peninsula, Miocene inversion structures have been found, but the Cretaceous rocks on Geoje Island may have undergone inversion as early as the Upper Cretaceous. To evaluate the structural inversion on Geoje Island, located on the eastern side of South Korea, and to determine the effects of preexisting weakness zones, field-based geometric and kinematic analyses of faults were performed. The lithology of Geoje Island is dominated by hornfelsified shale, siltstone, and sandstone in the Upper-Cretaceous Seongpori formation. NE and NW-oblique normal faults, conjugate strike-slip (NW-sinistral transpressional and E-W-dextral transtensional) faults, and NE-dextral transpressional faults are the most prominent structural features in Geoje Island. Structural inversion on Geoje Island was evidenced by the sinistral and dextral transpressional reactivation of the NW and NE-trending oblique normal faults respectively, under WNW-ESE/NW-SE compression, which was the orientation of the compressive stress during the Late Cretaceous to Early Cenozoic.

• Steel and Composite Structures
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• v.20 no.4
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• pp.833-849
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• 2016

Pre-stressing of existing structures using steel cables, FRP cables or FRP laminates has been successfully tried in the past. Retrofitting of beams using pre-stressed laminates does not utilize the full strength of the FRP due to de-bonding of the laminates before the fibre fracture. In the present study attempt has been made to overcome this problem by replacing the FRP laminates by the FRP sheets. In the present paper the effect of initial damage level and pre-stress level on strength, stiffness, cracking behaviour and failure mode of girders retrofitted using pre-stressed CFRP sheets has been studied. The results indicate that rehabilitation of initially damaged girders by bonding pre-stressed CFRP sheets improves the flexural behaviour of beams appreciably. However, it has been observed that with increase in pre-stressing force the load carrying capacity of the girders increases up to a particular level up to which the mode of failure is fibre fracture. Thereafter, the mode of failure shifts from fibre fracture to de-bonding and there is no appreciable increase in load carrying capacity with further increase in pre-stressing force.

• Computers and Concrete
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• v.10 no.2
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• pp.121-133
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• 2012

Structures suffer from damages in their lifetime due to time-dependant effects, such as fatigue, creep and shrinkage, which can be expressed by concrete strains. These processes could be seen in the context of strain estimation of pre-stressed structures in two phases by using numerical methods. Their aim is checking and validating existing code procedures in determination of deformations of pre-tensioned girders by solving a system of nonlinear equations with strains as unknown parameters. This paper presents an approach based on the Newton-Raphson method for obtaining the stresses and strains in middle span section of pre-stressed girders according the equilibrium state.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.224-231
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• 2005

The ground excavation causes the deformation of the ground where the neighborhood structure is located. The ground deformation result in the vertical settlement of the neighborhood structure as well as the horizontal displacement of the temporary earth retaining structures. The decreased volume of the soil due to the ground settlement is defined as 'the ground loss quantity' or 'the ground loss'. When excavation is performed nearby existing structures, retaining walls should be designed and constructed to minimize the ground loss. Among various methods for reducing the ground loss, this study introduces the pre-loading method which has been recently developed. The reduction effect of the ground loss by pre-loading has been found to be larger as using a wall with relatively smaller rigidity.

• Steel and Composite Structures
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• v.48 no.1
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• pp.73-88
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• 2023

This paper presents a novel implicit level set method for topology optimization of functionally graded (FG) structures with pre-existing discontinuities (pre-cracks) using radial basis functions (RBF). The mathematical formulation of the optimization problem is developed by incorporating RBF-based nodal densities as design variables and minimizing compliance as the objective function. To accurately capture crack-tip behavior, crack-tip enrichment functions are introduced, and an eXtended Finite Element Method (X-FEM) is employed for analyzing the mechanical response of FG structures with strong discontinuities. The enforcement of boundary conditions is achieved using the Hamilton-Jacobi method. The study provides detailed mathematical expressions for topology optimization of systems with defects using FG materials. Numerical examples are presented to demonstrate the efficiency and reliability of the proposed methodology.

• Structural Engineering and Mechanics
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• v.66 no.4
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• pp.453-463
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• 2018

Three points bending flexural test was modelled numerically to study the crack propagation in the pre-cracked beams. The pre-existing double internal cracks inside the beam models were considered to investigate the crack propagation and coalescence paths within the modelled samples. Notch configuration effects on the failure stress were considered too. This numerical analysis shown that the propagation of wing cracks emanating from the tips of the pre-existing internal cracks caused the final breaking of beams specimens. It was also shown that when two notches were overlapped, they both mobilized in the failure process and the failure stress was decreased when the notches were located in centre line. However, the failure stress was increased by increasing the bridge area angle. Finally, it was shown that in all cases, there were good agreements between the discrete element method results and, the other numerical and experimental results. In this research, it is tried to improve the understanding of the crack propagation and crack coalescence phenomena in brittle materials which is of paramount importance in the stability analyses of rock and concrete structures, such as the underground openings, rock slopes and tunnel construction.

• Earthquakes and Structures
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• v.16 no.3
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• pp.279-293
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• 2019

The overall seismic performance of existing pre 1960-70s reinforced concrete (RC) structures is significantly affected by the inadequate length of columns' lap-spliced reinforcement. Due to this crucial structural deficiency, the cyclic response is dominated by premature bond - slip failure, strength and stiffness degradation, poor energy dissipation capacity and low ductility. Recent earthquakes worldwide highlighted the importance of improving the load transfer mechanism between lap-spliced bars, while it was clearly demonstrated that the failure of lap splices may result in a devastating effect on structural integrity. Extensive experimental and analytical research was carried out herein, to evaluate the effectiveness and reliability of strengthening techniques applied to RC columns with lap-spliced reinforcement and also accurately predict the columns' response during an earthquake. Ten large scale cantilever column subassemblages, representative of columns found in existing pre 1970s RC structures, were constructed and strengthened by steel or RC jacketing. The enhanced specimens were imposed to earthquake-type loading and their lateral response was evaluated with respect to the hysteresis of two original and two control subassemblages. The main variables examined were the lap splice length, the steel jacket width and the amount of additional confinement offered by the jackets. Moreover, an analytical formulation proposed by Tsonos (2007a, 2019) was modified appropriately and applied to the lap splice region, to calculate shear stress developed in the concrete and predict if yielding of reinforcement is achieved. The accuracy of the analytical method was checked against experimental results from both the literature and the experimental work included herein.

• Smart Structures and Systems
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• v.22 no.6
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• pp.663-673
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• 2018

In this study, the effect of particle size on the cracks propagation and coalescence or cracking pattern of the edge notched disc specimens are investigated. Firstly, calibration of PFC3D was performed using Brazilian experimental test output. Then micro parameters were used to build edge notched disc specimen. The horizontal wall of the assembly is let to move downward with a standard low speed of 0.016 m/s. The numerical results show that the tensile cracks are dominant failure pattern for the modeled discs. These tensile cracks initiate from the pre-existing notch tip and propagate parallel to the loading direction then interact with the upper boundary of the modeled specimen. As the size of the balls (ball diameter) decrease the number of tensile cracks increase. The tensile fracture toughness of the samples also decreases as the particle size increases. Understanding the crack propagation and crack coalescence phenomena in brittle materials such as concretes and rocks is of paramount importance in the stability analyses for engineering structures such as rock slopes, underground structures and tunneling.