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J.-C. Chai
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Civil Engineering, Building, Construction & Architecture Engineering Transportation

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1D ANALYSIS OF LAND SUBSIDENCE IN SHANGHAI J.-C. Chai; S.-L. Shen; H.-H. Zhu; X.-L. Zhang
Lowland Technology International Vol 7 No 1, June (2005)
Publisher : International Association of Lowland Technology

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Land subsidence in Shanghai is investigated. The subsidence was mainly caused by excessive withdrawal of groundwater and since 1921, the measured subsidence was 2 to 3 m in the central area of the city. One-dimensional (1D) finite element consolidation analyses were conducted to simulate and predict the subsidence at Point-A, eastern part of Shanghai. The analysis result fairly simulated the field measured tendency and it indicates that the compression of the mucky clay layer, the silty clay layer in aquitard I and the third compression layer (aquitard II) contributes about 80% of the total subsidence. Also, it is shown numerically that for consolidation caused by groundwater level drawdown in an aquifer, the final state is a steady state water flow toward the aquifer, and the relative values of hydraulic conductivity of clayey layers above the aquifer have an important effect on calculated amount of settlement. Further, three possible scenarios were assumed for discussing the future subsidence. In the case of maintaining the groundwater level as it was in 2001, the predicted subsidence in 50 years is only about 2 mm. In the case of continuous drawdown of groundwater (1 m/year for aquifer IV and V, 0.5 m/year for aquifers II and III, and 0.2 m/year for aquifer I), in 50 years the predicted subsidence is about 1.25 m. If the groundwater level is recovered to zero elevation in all aquifers in the next 50 years, the predicted amount of heave is about 0.20 m.
MODELLING STRAIN-SOFTENING BEHAVIOUR OF CLAYEY SOILS J.-C. Chai; J. P. Carter; S. Hayashi
Lowland Technology International Vol 9 No 2, Dec (2007)
Publisher : International Association of Lowland Technology

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A method for modelling the strain-softening behaviour of clayey soils under undrained and/or partially drained conditions is proposed and applied to simulate the mechanical behaviour of undisturbed Ariake clay and limestabilized Ariake clay samples under undrained conditions. The proposed method is based on the Modified Cam clay (MCC) model. It is assumed that during the softening process, the strain increments can still be calculated by the MCC model, but the effective stress path is enforced to follow the projection of the critical state line (CSL) in a p’– q plot (i.e., q = M p’ where p’ is mean effective stress, q is deviator stress and M is the slope of the CSL in the p’– q plot). Therefore the method is not completely rigorous in the applied mechanics sense, rather it is a pragmatic approach. The proposed method has been incorporated into a finite element code and its performance was verified by simulating undrained triaxial compression tests. Subsequently, the method has been applied to simulate the mechanical behaviour of both natural and lime-stabilized Ariake clays. Comparing the simulated results with the test data indicates that the method simulated both the shear strain versus deviator stress curve and the effective stress path reasonably well. However, the results also showed that in the case of the lime-stabilized Ariake clay, the adoption of a high initial stiffness under lower confining stress should be considered. It is suggested that the proposed method can be used to analyze geotechnical problems involving strain-softening behaviour with reasonable accuracy.
MEASURED BEHAVIOR OF A TRIAL EMBANKMENT ON FLOATING COLUMN IMPROVED SOFT ARIAKE CLAY DEPOSIT Y. Igaya; T. Hino; J.-C. Chai
Lowland Technology International Vol 13 No 1, June (2011)
Publisher : International Association of Lowland Technology

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A 6.5m height trial embankment was constructed on soft Ariake clay deposit improved by the floating soil-cement columns. At the test site, the thickness of the soft soil was about 10 m, and the length of the column was 8.5 m and the area improvement ratio was about 31%. To check both the mechanical and geoenvironmental performance, the embankment was monitored for more than 2 years. The measured results indicate that the behavior of the trial embankment satisfies the performance requirements for constructing a highway around Ariake Sea, in Kyushu, Japan, i.e. settlement and lateral displacement at the toe of the embankment are less than 50 mm, and residual settlement is less than 0.3 m. The observed results also show that the column improvement not only reduced the settlement but also accelerated the consolidation rate of the deposit. The results of groundwater monitoring indicate that at the test site, in terms of groundwater level, flow velocity, pH value and the concentrations of some key ions, there was no effect on the groundwater quality due to the installation of soil-cement columns into the ground.
3D FEM investigation on bending failure mechanism of column inclusion under embankment load S. Shrestha; J.-C. Chai; D. T. Bergado; T. Hino; Y. Kamo
Lowland Technology International Vol 17 No 3, Dec (2015)
Publisher : International Association of Lowland Technology

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Bending failure mechanism of column inclusions in soft clay deposit under embankment loading has been investigated by three dimensional (3D) finite element analyses. Firstly the effectiveness of the numerical procedure has been verified by comparing the simulated and the measured results of a centrifuge model test reported in the literature in terms of lateral displacement, settlement, and the bending moment in the column. Then the effects of the size of the column improved area from the toe toward the center of the embankment, stiffness of the column, the length of the column on the maximum bending moment in the column have been investigated numerically. The numerical results indicate that increase the size of the improved area, reduced the bending moment in the upper part (near ground surface) of the column; increase the stiffness of the column increased the maximum bending moment; and the maximum bending moment occurred at the end of the column in the case of an end bearing column, and in the upper part of the column for a floating column. The numerical results also indicate that when the whole area under the embankment is improved by end bearing columns with an area improvement ratio of 28 % and tensile strength of the column of 100 kN/m2, the embankment load can be applied with a factor of safety of about 2 for bending failure of the columns is about 13 times of the initial undrained shear strength of the soft deposit.
Co-Authors D. T. Bergado H.-H. Zhu J. P. Carter S. Hayashi S. Shrestha S.-L. Shen T. Hino X.-L. Zhang Y. Igaya Y. Kamo