• Journal of the Korean Geosynthetics Society
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• v.16 no.4
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• pp.93-101
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• 2017

In this study, a subjective weighting factors were awarded based on some indication of the difficulty of assessing the preconsolidation stress using traditional methods (Casagrande, Onitsuka et al., Silva, Becker et al., Janbu and Karlsrud methods) such as those proposed by Casagrande and Janbu using undisturbed sample obtained from Gwangyang dredged clay with high plasticity located in the southern area of Korean peninsular. These numbers only assess the relative ease of finding preconsolidation stress and say nothing regarding the accuracy of the value. The data were compared with measurements of undrained shear strength using strength incremental ratio, checking where or not the values are in the range of 0.25 to 0.35 (typical values of Korean marine clay) and analyzing standard deviation(degree of variability). The measurements of undrained shear strength were obtained from unconfined compression tests (UCT). When determining preconsolidation stress of Korean marine clay, at first, the work method proposed by Becker et al. and the bilogarithmic method proposed by Onitsuka et al. should be used. In addition, preconsolidation pressure should be estimated using the traditional Casagrande method as a basic of comparison.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3C
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• pp.115-125
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• 2009

The preconsolidation pressure has been commonly determined by oedometer test. However, it can also be determined by insitu test, such as piezocone test with theoretical and(or) empirical correlations. Recently, Neural Network (NN) theory was applied and some models were proposed to estimate the preconsolidation pressure or OCR. It was already found that NN model can come over the site dependency and prediction accuracy is greatly improved when compared with present theoretical and empirical models. However, since the optimization process of synaptic weights of NN model is dependent on the initial synaptic weights, NN models which are trained with different initial weights can't avoid the variability on prediction result for new database even though they have same structure and use same transfer function. In this study, Committee Neural Network (CNN) model is proposed to improve the initial weight dependency of multi-layered neural network model on the prediction of preconsolidation pressure of soft clay from piezocone test result. Prediction results of CNN model are compared with those of conventional empirical and theoretical models and multi-layered neural network model, which has the optimized structure. It was found that even though the NN model has the optimized structure for given training data set, it still has the initial weight dependency, while the proposed CNN model can improve the initial weight dependency of the NN model and provide a consistent and precise inference result than existing NN models.

• Journal of the Korean Geotechnical Society
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• v.21 no.6
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• pp.127-135
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• 2005

In order to analyze effects of strain rate on consolidation characteristics of Busan clay, a series of constant rate of strain (CRS) consolidation tests with different strain rate and incremental loading tests (ILT) were performed. From experimental test results on Busan clay, it was found that the preconsolidation pressure was dependent on the corresponding strain rate occurring during consolidation process. Also, consolidation curves normalized with respect to preconsolidation pressure gave a unique stress-strain curve. Coefficient of consolidation and permeability estimated from CRS test had a tendency to converge to a certain value at normally consolidated range regardless of strain rate. An increase in excess pore pressure was observed after the end of loading without change of total stress on the incremental loading test, which phenomenon is called Mandel-Cryer effect. It was also found that rapid generation of excess pore pressure took place due to collapse of soil structure as effective stress approached to preconsolidation pressure.

• Journal of Industrial Technology
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• v.25 no.B
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• pp.63-71
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• 2005

This paper is an experimental result of investigating lateral soil movements at piled bridge abutments by using the centrifuge model facility. Three different centrifuge model experiments, changing the methods of ground improvement at bridge abutment on the soft clayey soil (no improvement, preconsolidation and plastic board drains (PBD), sand compaction pile (SCP) + PBD), were carried out to figure out which method is the most appropriate for resisting against the lateral soil movements. In the centrifuge modelling, construction process in field was reconstructed as close as possible. Displacements of abutment model, ground movement, vertical earth pressure, cone resistance after soil improvement and distribution of water content were monitored during and after centrifuge model tests. As results of centrifuge model experiments, preconsolidation method with PBD was found to be the most effective against the lateral soil movement by analyzing results about displacements of abutment model, ground movement and cone resistance. Increase of shear strength by preconsolidation method resulted in increasing the resistance against lateral soil movement effectively although SCP could mobilize the resistance against lateral soil movement. It was also found that installment with PBD beneath the backfill of bridge abutment induced effective drainage of excess pore water pressure during the consolidation by embanking at the back of the abutment and resulted in increasing the shear strength of clay soil foundation and eventually increasing the resistance of lateral soil movement against piles of bridge abutment.

• Journal of the Korean Geotechnical Society
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• v.20 no.6
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• pp.109-118
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• 2004

The consolidation of clay occurs with time lag, and this kind of lag can be separated into plastic lag and hydraulic lag. In this study, CRS tests were performed to research the effect of original secondary consolidation of the clay with respect to the characteristics of consolidation. Test results showed that plastic time lag was one of the key factors to get the preconsolidation pressure, and suggested the formula of the Quasi-preconsolidation pressure obtained from the relationship between consolidation time lag and consolidation pressures. In addition though the characteristics of coefficient of consolidation show a wide range of values, after passing the double preconsolidation point, it showed the tendency to converge into the constant value. The coefficient of permeability in normally consolidated state is related to its void ratio, and the permeability variables, n and $C_1$ were determined by the test results using the equation suggested by Samarasinghe. et. al. And then the equation was compared with the Kozeny-Carman's equation. Because of delayed compression caused by consolidation time lag, aging effect could be also found in the relationship between coefficient of permeability and void ratio.

• 기술발표회
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• s.2006
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• pp.261-269
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• 2006

Recently, piezocone penetration test is frequently used in order to estimate the characteristics of soft ground with standard penetration test; generally used in the past In this study, standard penetration test, piezocone penetration test, driving resistance of vertical drain were used in order to increase the confidence for determination of soft ground depth. And the compressible layer was determined by the comparison between the preconsolidation pressure and the designed increase pressure. As the results, the relation between standard penetration test and piezocone penetration test shows $q_c$=(1.09~1.63)N at the soft ground, determined by 5/30 N value. And $q_c$(1.21~1.98)N was shown at the point of compressible layer, evaluated by the preconsolidation pressure. And driving resistance of vertical drain is 70 f/$cm^2$ which is equal to 10kgf/$cm^2$ cone penetration resistance.

• Journal of the Korean GEO-environmental Society
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• v.15 no.5
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• pp.67-74
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• 2014

In this study, a constant rate of strain test (CRS) and oedometer test was performed in order to study the consolidation characteristics to the liquid limit using a re-shaped clay which was preconsolidated at a constant pressure. Consolidation samples were made of kaolinite which was mixed with bentonite of 6 %, 9 %, 12 % and 15 % of it by weight and the test value of liquid limit of samples were appeared in 77 %, 84 %, 88 % and 91 % respectively. And then consolidation samples which were agitated sufficiently adding distilled water 2 times of liquid limit were preconsolidated in the condition of a constant pressure of 0.2 MPa. The oedometer test which is commonly used recently and the consolidation test of constant rate of strain which were applied in 0.001 %/min, 0.004 %/min, 0.01 %/min rate of strain according to ASTM, D4186-82 were performed and the preconsolidation pressures were compared and analyzed at 0.2 MPa preconsolidated pressure. As a result, in the case of low value of liquid limit, preconsolidation pressure was appeared same as 0.2 MPa preconsolidated pressure at the high speed strain rate, and in the case of high value of liquid limit, preconsolidation pressure was appeared same as 0.2 MPa preconsolidated pressure at the low speed strain rate.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.02a
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• pp.71-78
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• 2000

SHANSEP method involves the consolidation to stresses in excess of the preconsolidation pressure in order to overcome sample disturbance effect. The concept of SHANSEP is based on an approach to laboratory test which attempts to reproduce the in-situ conditions more closely than is possible in routine tests and evaluates normalized strength parameters for the soil as a function of OCR. But SHANSEP method can be applied only to fairly uniform clay deposits, and is unsuitable for a random deposit. In this study, CK/sub o/U triaxial compression test and incremental loading consolidation test were performed for the application of SHANSEP method on Yangsan clay. During the K/sub o/-consolidation, triaxial specimens were consolidated to stress equal to two times the in-situ vertical effective stress. And for overconsolidated condition, the specimens were swelled to a known vertical effective stress in order to have the desired OCR. With the results of CK/sub o/U triaxial compression test using the block samples, the relationship between c/sub u//σ/sub vc/' and OCR on Yangsan clay was established. For evaluating the undrained shear strength of Yangsan clay with depth, CK/sub o/U triaxial compression test was performed using the piston samples taken from Yangsan site. And also undrained shear strength was analyzed from the in-situ test such as Cone Penetration Test(CPT), Dilatometer Test(DMT), and Field Vane Test(FVT) and was compared with that of CK/sub o/U triaxial compression test.

• Journal of The Korean Society of Agricultural Engineers
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• v.58 no.6
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• pp.71-77
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• 2016

This study was to evaluate the feasibility of pre-consolidation pressure distribution characteristic of western and southern coastal region, using correlation of unconfined compressive strength and preceding research equation. Pre-consolidation of western and southern region showed similar trends undrained shear strength and pre-consolidation pressure in proportion to unconfined compressive strength. Predicted results of U.S. NAVY. (1982) equation revealed a small error western 9.7 % and southern 0.4 %. Prediction correlation results of pre-consolidation using unconfined compressive strength revealed an error western 16.8 % and southern 0.7 %. It was reported that less than 20 percent of pre-consolidation pressure prediction result of Casagrande forecasting error. Estimates of pre-consolidation pressure are possible, before the standard consolidation test, because it was reported that less than 20 % of the forecasting errors of Casagrande.

• Journal of the Korean Geotechnical Society
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• v.24 no.8
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• pp.61-79
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• 2008

Consolidation settlements on marine dredged clays are often greatly and potentially damaging to structures. Currently, large-scale projects are in planning or progressing in Korea. These projects have been performed on thick and soft clay layers. So, the evaluation of consolidation characteristics for dredged and reclaimed ground is very important in design and construction. Therefore, in this study, a series of conventional consolidation tests were performed to investigate the consolidation characteristics of marine dredged clays near Gwang-yang Port. Preconsolidation pressures were evaluated by applying previously proposed 8 methods for the conventional tests results in order to evaluate the legitimacy of these methods. In these methods, when estimating maximum past pressures for dredged and reclaimed ground, it was proved that Becker (1987), Silva (1970), Sridharan (1991)'s methods are excellent in legitimacy.