• Journal of Advanced Marine Engineering and Technology
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• v.36 no.8
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• pp.1136-1142
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• 2012

With concern for new renewable energy sources rising, the offshore wind turbine energy market is growing. In order to install offshore wind turbines safely in a harsh environment, jack-up rigs need to be used. Recently, a WTIV(Wind Turbine Installation Vessel), which has a self-propelling system, was developed to replace traditional jack-up rigs. Since the jack-up rig works at 60 meters of depth in offshore and the seafloor is composed mostly of soft clay and sand, it is necessary to conduct a stability examination for ground subsidence before using it. This study conducts an improved numerical analysis of the effect of jack-up rig loading on the soft ground by using FLAC3D considering consolidation theory. This includes analyzing the amount of subsidence when applying different cases of surchargings. It can be concluded that the mid-loading method has an advantage as regards clay layers and the pre-loading method has an advantage as regards sand layers.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.09c
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• pp.37-46
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• 2010

Marine clays are soft soil deposits having complicated mineralogy and formation characteristics. Thus, characterization of its geotechnical behavior has been a main issue for geotechnical engineers. Nowadays, the importance and applications of geophysical exploration on marine clays are increasing significantly according to the accuracy, efficiency, and reliability of geophysical survey technology. For marine clays, seismic survey is effective for density and elasticity characterization, while electro-magnetic wave provides the information about the fluid conductivity phenomena inside soil. For practical applications, elastic wave technology can evaluate the consolidation state of natural marine clay layers and estimate important geotechnical engineering parameters of artificially reclaimed marine deposits. Electrical resistivity can provide geophysical characteristics such as particle cementation, pore geometry shape, and pore material phase condition. Furthermore, nondestructive geophysical monitoring is applicable for risk management and efficiency enhancement during natural methane gas extraction from gas hydrate-bearing sediments.

• Journal of the Korean Geotechnical Society
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• v.21 no.9
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• pp.13-23
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• 2005

The prefabricated vertical drains (PVDs) are one of the most widely used techniques to accelerate the consolidation of soft clay deposits and dredged soil. Discharge capacity is one of the factors affecting the behavior of PVDs. In the field, a PVD is confined by clay or dredged soil, which is normally remolded during PVD installation. Under field conditions, soil particles may enter the PVD drainage channels, and the consolidation settlement of the improved subsoil may cause 131ding of the PVD. These factors will affect the discharge capacity of the PVDs. In this study an experimental study was carried out to estimate the discharge capacity of three different types of PVDs by utilizing the large-scale laboratory model testing and small-scale laboratory model testing equipments. The several factors such as confinement condition (confined by soft marine clay or dredged soil) and variations of the discharge capacity were studied with time under soil specimen confinement, The test results indicated that discharge capacity decreases with increasing load, time, and hydraulic gradient. With load application, the cross-sectional area of the drainage channel of PVD decreases because the filter of PVD is pressed into the core. The discharge capacity of the soft marine clay-confined PVDs is much lower than that of the dredged soil-confined PVDs.

• Journal of the Korean GEO-environmental Society
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• v.10 no.7
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• pp.43-51
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• 2009

This paper holds three objects. The first is to analyze surveys of concerning zone and promotion department. The data were collected through an examination of construction excavated in coastal soft (marine) clay and measurements obtained during excavating construction. The second is to observe the appropriate selection and the application of support system on earth retaining wall in soft clay. Lateral deformation behavior during the excavating construction according to the differences in a soft ground pressuring degree was investigated. The third is to compare the results with those of numerical analysis. Therefore, the purpose of this study is to analyze the characteristics of lateral deformation when soft ground improvement for the expansion of infrastructure in object of study zone has been incompleted. Also, it is to identify the relationship between the degree of consolidation of soft ground and lateral deformation, in a method of displacement quantity in compliance with the numerical analysis and a quatitative analysis. In conclusion, displacement of excavated section after consolidation was fewer 60% averagely than section under consolidation.

• Journal of the Korean GEO-environmental Society
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• v.10 no.6
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• pp.69-77
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• 2009

The shortage of bearing capacity and settlement, shear deformation may occur when constructing a structure such as harbor, airport and bridge on soft ground such as marine clay, silty clay, sandy soil because it is very soft. The various ground improvement methods were applied to obtain preceding settlement of soft ground and strength increase. The vertical drain method has been used to reduce the required time for consolidation of the soft ground. Especially, the PBD (Plastic Board Drain) has been widely used among in the vertical drain method. In this study, a behavior of characteristic was evaluated by operating a compound drainage capacity test about the PBD (Plastic Board Drain) method applied in soft clay in deep depth. As a result, the settlement gradually occurred with increase of surface load. The consolidation settlement was processed with dissipation of pore pressure after surface load of $500kN/m^2$. Accordingly, it was found that change of settlement through load steps was resulted from dissipation of pore pressure. It was also found that the drainage capacity of vertical drains was considerably reduced with pressure increase and time elapse.

• Geomechanics and Engineering
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• v.37 no.4
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• pp.383-393
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• 2024

Monopile foundations of offshore wind turbines embedded in soft clay are subjected to the long-term cyclic lateral loads induced by winds, currents, and waves, the vibration of monopile leads to the accumulation of pore pressure and cyclic strains in the soil in its vicinity, which poses a threat to the safety operation of monopile. The researchers mainly focused on the hysteretic stress-strain relationship of soft clay and kinds of stiffness degradation models have been adopted, which may consume considerable computing resources and is not applicable for the long-term bearing performance analysis of monopile. In this study, a modified cyclic stiffness degradation model considering the effect of plastic strain and pore pressure change has been proposed and validated by comparing with the triaxial test results. Subsequently, the effects of cyclic load ratio, pile aspect ratio, number of load cycles, and length to embedded depth ratio on the accumulated rotation angle and pore pressure are presented. The results indicate the number of load cycles can significantly affect the accumulated rotation angle of monopile, whereas the accumulated pore pressure distribution along the pile merely changes with pile diameter, embedded length, and the number of load cycles, the stiffness of monopile can be significantly weakened by decreasing the embedded depth ratio L/H of monopile. The stiffness degradation of soil is more significant in the passive earth pressure zone, in which soil liquefaction is likely to occur. Furthermore, the suitability of the "accumulated rotation angle" and "accumulated pore pressure" design criteria for determining the required cyclic load ratio are discussed.

• Geotechnical Engineering
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• v.12 no.2
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• pp.115-132
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• 1996

Several soil improvement methods are applied to stabilize soft ground. But, their improvement effects are known to be reduced in view of strength and durability under poor conditions such as marine clay and the ground with the flow of groundwater. The soil improvement method is generally classified as mixing(high pressure) type and injection type, and in this study, for successflll'applications of gelling methods, first in case that mixing method with cement is applied to marine clay, the causes of strength inferiority of treated soil are analyzed, and the effectiveness of improvement is studied, second in case that injection method with water-glass chemical grouts is applied to the ground with the flow of groundwater, soil improvement effects and durability of grouted soil are studied.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.11a
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• pp.17-24
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• 2000

Hardening agent has been the traditional material for surface soil stabilization of soft ground. This study aims at determining optimal mixture ratio of hardening agent in accordance with the required design specifications. Hardening agent is properly mixtured with Fly ash, Gypsum, Slag and Cement for the ettringite hydrates which is effective for early stabilization of unconsolidated soil. The treated soil is the clay which are widely found here and there in Korea. In this study, preliminary tests were performed to get optimal mixture ratio of stabilizer ingredient, and marine clay in Jin-Hae was used to get physical and chemical properties. Laboratory tests of 50 stabilized soil were peformed to get optimal mixture ratio for 16 stabilizer material of 6 type, and stabilizer mixing was determined.

• Proceedings of the Korean Geotechical Society Conference
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• 2009.03a
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• pp.715-719
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• 2009

In this study, a method to predict the consolidation behavior of soft clays and marine clays was developed by combining the equation of Terzaghi's 1-dimensional consolidation and CPTu dissipation. The special attention was given to the consolidation anisotropy due to the difference between 1-D consolidation and radial consolidation of CPTu dissipation. The analysis combining two equations enables direct application of CPTu results. And above all it doesn't require to sample undisturbed specimens and determine consolidation coefficient which is both costly and time consuming and often contains measuring error. It is also advantageous that CPTu test can be carried out any position and any depth. Clays typically have a greater horizontal permeability, $k_h$, than vertical permeability, $k_v$, and the coefficient of consolidation in the horizontal direction is generally higher than the vertical direction. Various data of horizontal and vertical consolidation coefficient ratio were collected and analyzed to develop and verify the method.

• Journal of the Korean GEO-environmental Society
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• v.17 no.10
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• pp.73-81
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• 2016

In the construction on low strength and high compressible soft ground, the many problems have been occurred in recent construction project. therefore, the soil improvement have been developed to obtain high strength in relatively short period of curing time. Based on the laboratory tests using undisturbed marine clay, the effect of improvement on soft ground was estimated. Deep mixing method by cement have been virtually used for decades to improve the mechanical properties of soft ground. However, previous researches set the focus on the short term strength the about 10% of cement treated clay. In this paper, cement and Natural Soil Stabilizer (NSS) were used as the stabilizing agent to obtain trafficability and mechanical strength of the soft clay. Based on the several laboratory tests, optimum condition was proposed to ensure the mechanical strength and compressibility as the foundation soil using cement and NSS mixed soil. Finally, research data was proposed about the applicability of NSS as the stabilizing agent to soft clay to increase the mechanical strength of soil.