• Geomechanics and Engineering
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• v.20 no.2
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• pp.165-174
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• 2020

The enzyme-induced carbonate precipitation (EICP) method has been investigated to improve the hydro-mechanical properties of natural soil deposits. This study was conducted to explore the stiffness evolution during various stress scenarios. First, the optimal concentration of urea, CaCl₂, and urease for the maximum efficiency of calcite precipitation was identified. The results show that the optimal recipe is 0.5 g/L and 0.9 g/L of urease for 0.5 M CaCl₂ and 1 M CaCl₂ solutions with a urea-CaCl₂ molar ratio of 1.5. The shear stiffness of EICP-treated sands remains constant up to debonding stresses, and further loading induces the reduction of S-wave velocity. It was also found that the debonding stress at which stiffness loss occurs depends on the void ratio, not on cementation solution. Repeated loading-unloading deteriorates the bonding quality, thereby reducing the debonding stress. Scanning electron microscopy and X-ray images reveal that higher concentrations of CaCl₂ solution facilitate heterogeneous nucleation to form larger CaCO₃ nodules and 11-12 % of CaCO₃ forms at the interparticle contact as the main contributor to the evolution of shear stiffness.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.974-981
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• 2005

Because general laboratory tests for sand compaction pile method including unit-cell test device have fixed outside diameter, as area replacement ratio increase, diameter of sand pile increase. These condition can bring about overestimation of stiffness of composite ground. In addition, existing large soil box which consist of bellows type loading plate can occur serious mistake in checking the amount of drained water because there are additional drainage along the inside wall in device. Overcoming these shortcoming, this paper developed modified large scale soil box consist of piston type load plate. In this study, using this device, series of modified large scale soil box tests were performed, and investigated the settlement and stress transportation characteristics with area replacement ratio in sand compaction pile method.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.03b
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• pp.159-166
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• 2000

Non-destructive out-hole tests, impact-echo and impact-response are widely applied to evaluate integrity of drilled shafts. In these tests, vibratory motions of drilled shafts are interpreted, which induced by impacts on the shaft head. In applying the tests to evaluating integrity of shaft, it has been attended whether the tests have resolutions enough to distinguish existence of slime at between the shaft end and a bearing soil deposit. To distinguish existence of slime by tests, modes of shaft vibrations need to be reasonably interpreted, which generally vary according to a shaft boundary condition such as, a free-free or a free-fixed condition. The boundary condition of a shaft is, however, found to be significantly affected by stiffness of soil deposits around shaft as well as penetration depths of shaft into a bearing soil deposit. Thus, these effects on the boundary condition of a shaft should be considered reasonably in interpreting test results to decide the existence of slime. To investigate the effects, in this study, vibratory motions of shafts constructed in various soil conditions and end penetration depths are examined analytically. Based on the studies, variations of boundary condition are characterized in terms of soil stiffness contrast between a shaft perimeter and a shaft end, and also the ratio of a penetration depth to a shaft length. The results can be applied to verify the applicability of tests to identify the slime.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.529-536
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• 2005

Based on an estimating method for post-cyclic strength and stiffness with cyclic triaxial tests, Direct Simple Shear (DSS) tests were carried out to confirm whether the method can be adapted to DSS test on fine-grained soils: silty clay, plastic silt, and non-plastic silt. Results from post-cyclic DSS tests were interpreted by a modified method as adopted for post-cyclic triaxial tests. In particular, influence of plasticity index for fine-grained soils was emphasised. Findings obtained from the present study are: (i) the higher the plasticity index of fine-grained soils is, the less not stiffness ratio but strength ratio decreases with increment of a normalised excess pore water pressure; and (ii) post-cyclic strength and stiffness results from DSS tests agree well with those predicted by the method modified from a procedure used for triaxial test results.

• Journal of the Korean Geotechnical Society
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• v.15 no.5
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• pp.81-98
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• 1999

The influence of tunnelling on buildings has become an important issue in urban areas. The problem is an interactive one: not only do tunnelling settlements affect existing structures, but existing structures affect tunnel-induced soil movements. In order to examine the constraint of surface settlement and the degradation of building damage parameters, 3-dimensional elasto-plastic finite element analyses are peformed. Also, in this paper, the results of the parametric studies for the variations of the damage parameters due to the ground movements are presented by utilizing 2-dimensional elasto-plastic finite element models, totally 162 models. The width of a structure, its bending and axial stiffness, its position relative to the tunnel and the depth of tunnel are considered. The interaction is shown by reference to commonly-used building damage parameters, namely angular distortion, deflection ratio, maximum building settlements, maximum differential settlements and horizontal strain. By introducing relative stiffness parameters which combine the bending and axial stiffness of the structure with its width and stiffness of soil, design curves are established. These give a guide as to the likely modification of the greenfield settlement trough caused by a surface structure. They can be used to give initial estimates of likely building damage.

• Structural Engineering and Mechanics
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• v.57 no.3
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• pp.403-423
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• 2016

The challenges involved with fatigue damage assessment of steel catenary riser (SCR) in the touchdown zone (TDZ) are primarily due to the non-linear behaviour of the SCR-seabed interaction, considerable uncertainty in SCR-seabed interaction modelling and geotechnical parameters. The issue of fatigue damage induced by the cyclic movements of the SCR with the seabed has acquired prominence with the touch down point (TDP) interaction in the TDZ. Therefore, the SCR-seabed response is critical for reliable estimation of fatigue life in the TDZ. Various design approaches pertaining to the lateral pipe-soil resistance model are discussed. These techniques have been applied in the finite element model that can be used to analyse the lateral SCR-seabed interaction under hydrodynamic loading. This study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. In this study, global analyses are performed to assess the influence of vertical linear seabed springs, the lateral seabed model and the non-linear seabed model, including trench evolution into seabed, seabed normalised stiffness, re-penetration offset parameter and soil suction resistance ratio, on the fatigue life of SCRs in the TDZ.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.33-40
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• 2005

Impact-echo test, a kind of simple and economical method to evaluate the integrity of drilled piles has some limitations to use because the stress wave can be generated only on the head of a pile and the wave propagation in the pile with surrounding soils are very complicated. Numerical analyses and model tests in the laboratory have shown that both the ratio of length to diameter of a pile and the stiffness ratio of pile to soil have influence on the resolution of testing results. Full scale testing piles which have artificial defects were used to verify the capability of impact-echo technique as a tool for the pile integrity evaluation. Behaviour of the reflected signal of stress wave was investigated according to the type of defects. Elastic modulus of the pile was calculated using the wave velocity in the pile and the unconfined strength of concrete specimen. Influences of the stiffness difference between the pile and the ground on the characteristics of a wave signal were also examined.

• Proceedings of the Korean Geotechical Society Conference
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• 2002.03a
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• pp.41-48
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• 2002

The fact that nonlinearity and anisotropy of soil should be considered for the proper estimation of soil deformation has been recongnized for a long time. In this study, a new stiffness model which can reflect both nonlinearity and anisotropy is proposed. Nonlinearity is simulated by Ramberg-Osgood model and anisotropy is modeled with the cross-anisotropic elasticity. Analysis results with the developed model compared with those from analyses using linear isotropic model, linear anisotropic model, and nonlinear isotropic model. In the triaxial compression like condition, the effects of nonlinearity on the vertical strain are significant, but soil anisotropy does not affect the vertical strain. In 1-dimensional deformation condition, however, both nonlinearity and anisotropy of soil influence the final magnitude of the vertical strain. Also the increase of poisson's ratio magnifies the effect of anisotropy on the vertical strain in this condition.

• Geomechanics and Engineering
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• v.29 no.4
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• pp.471-486
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• 2022

The effect of segmental joints is one of main importance for the segmental lining design when tunnels are excavated by a mechanized process. In this paper, segmental tunnel linings are analyzed by two numerical methods, namely the Beam-Spring Method (BSM) and the Solid-Interface Method (SIM). For this purpose, the Tehran Subway Line 6 Tunnel is considered to be the reference case. Comprehensive 2D numerical simulations are performed considering the soil's calibrated plastic hardening model (PH). Also, an advanced 3D numerical model was used to obtain the stress relaxation value. The SIM numerical model is conducted to calculate the average rotational stiffness of the longitudinal joints considering the joints bending moment distribution and joints openings. Then, based on the BSM, a sensitivity analysis was performed to investigate the influence of the ground rigidity, depth to diameter ratios, slippage between the segment and ground, segment thickness, number of segments and pattern of joints. The findings indicate that when the longitudinal joints are flexible, the soil-segment interaction effect is significant. The joint rotational stiffness effect becomes remarkable with increasing the segment thickness, segment number, and tunnel depth. The pattern of longitudinal joints, in addition to the joint stiffness ratio and number of segments, also depends on the placement of longitudinal joints of the key segment in the tunnel crown (similar to patterns B and B').

• Tunnel and Underground Space
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• v.11 no.4
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• pp.339-343
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• 2001

Shield tunnel having circular section located in the soil or soft rock layer is liable to deform in such a way that its two diagonal diameters crossing each other expand and contract alternately during earthquakes. Based on this knowledge, the ground-tunnel interaction effect for this particular vibration mode is investigated. The ground surrounding a tunnel is assumed to be a homogeneous elastic medium. The bonded boundary condition on the ground-tunnel interface is considered. This suggests a firm bond between the ground and the tunnel lining. As Poisson's ratio and stiffness of the ground increases, the strain induced within the tunnel lining increases.