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J. C. Chai
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PERFORMANCE OF VACUUM CONSOLIDATION IN A THICK CLAYEY DEPOSIT IN SHANGHAI Y. J. Wu; Y. C. Yuan; J. C. Chai
Lowland Technology International Vol 15 No 2, Dec (2013)
Publisher : International Association of Lowland Technology

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Abstract

A vacuum consolidation test was successfully carried out in a (more than 25 m) thick soft clayey deposit in Shanghai. At the site, there is a clayey silt layer located about 5.3 to 6.8 m depth with a relative higher hydraulic conductivity, and a cement deep mixing formed cut-off wall was able to prevent the vacuum leakage through this layer. Prefabricated vertical drains (PVDs) were installed with a spacing of 1.4 m and triangular pattern to a depth of 14 m. The measured results indicate that the degree of vacuum consolidation reached more that 90% in PVD improved zone for a period of about 38 days. Analysis results indicate that the vacuum pressure induced ground deformations can be calculated reasonably well by a previously proposed method. Further field monitoring results whow that one month after stopping the vacuum loading, 10 to 65% of the vacuum pressure induced lateral displacement was rebounded, and the percentage rebounding increased with the depth.
EFFECT OF BASE REINFORCEMENT ON THE BEHAVIOR OF EMBANKMENT OVER SOFT SUBSOIL C. W. Yang; J. C. Chai; N. Miura; Y. S. Ye
Lowland Technology International Vol 1 No 2, Dec (1999)
Publisher : International Association of Lowland Technology

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A case history of both reinforced and unreinforced embankments on soft subsoil built to failure is described. The results of a laboratory model test about the behavior of embankment on soft soil are referred during discussions. The effect of geotextile reinforcement on embankment behavior is discussed by comparing the field data of with and without reinforcement cases. The field data as well as analysis indicate that the reinforcement had a certain effect on embankment stability. However, at working state (normally with a factor of safety of 1.2 to 1.3), the reinforcement did not have an obvious effect on subsoil response. Only under the condition that unreinforced embankment approached to failure, the effect of reinforcement on subsoil could be noticed. The laboratory model test results indicated that if the reinforcement is strong enough, the effect of reinforcement is considerable. It is suggested that although the geotextile certainly has a beneficial effect on embankment over soft subsoil, due to the relative lower stiffness of geotextile, to achieve a substantial improvement on embankment behavior, the multi-layer geotextile reinforcements or high strength geogrid may be needed. This case history also demonstrated that the rate of lateral displacement and excess pore pressure development are sensitive indexes for the stability of embankment on soft subsoil.
FIELD PROPERTIES AND SETTLEMENT CALCULATION OF SOIL-CEMENT COLUMN IMPROVED SOFT SUBSOIL - A CASE STUDY J. C. Chai; S. Y. Liu; Y. J. Du
Lowland Technology International Vol 4 No 2, Dec (2002)
Publisher : International Association of Lowland Technology

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The unconfined compression strength, standard penetration test (SPT) results for soil-cement columns and the field loading tests results of the composite subsoil at Lian-Yun-Gang section, Xu - Lian expressway, China, were presented. The Methods for calculating the settlement of the soil-cement column improved soft subsoil were discussed. It is recommended that the stiffness of the "slab" on the top of the improved subsoil should be flexible "slab". the equilibrium method for the improved layer and the average stress method for the underlying unimproved soft layer are prefered, while in the case of a stiff "slab", the average modulus method for the improved layer and the combined method for the underlying unimproved layer are prefered. Finally, the settlements of the soil-cement column improved subsoil at Lian-Yun-Gang section were evaluated by the suggested methods and compared with the field data. The stress concentration ratio (n) is back-calculated, and it showed that n value increased with the increased of the area replacement ratio.
CETEAU PVD VACUUM SYSTEM IN SOFT BANGKOK CLAY: A CASE STUDY OF THE SUVARNABHUMI AIRPORT PROJECT J. Saowapakpiboon; D. T. Bergado; S. Hayashi; J. C. Chai; N. Kovittayanon; T. P. de Zwart
Lowland Technology International Vol 10 No 1, June (2008)
Publisher : International Association of Lowland Technology

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A new improvement technique is currently applied for soft Bangkok clay combining capped PVD with vacuum pressure and embankment loading whereby the prefabricated vertical drains (PVD) are connected by PE tubes to a vacuum pump called “CeTeau PVD Vacuum System”. The method uses a surface soil layer as a sealing layer for leakage protection and there is no need to place air-tightening geomembrane sheets on the ground surface. This method has two advantages for situations of a) high air/water permeability layer exist near the ground surface, and b) combining vacuum pressure with embankment load. An actual field project combining PVD vacuum and embankment loading has just been completed. The performance data of the system during the improvement of the section EW-4, a part of the third runway of Suvarnabhumi International Airport, Thailand are presented and interpreted. The monitored data indicated that the system mobilized -60 kPa atmospheric pressure. This allowed for unprecedented loading and settlement rates during the construction of an embankment and achieved the required degree of consolidation within the specified time period. The prediction by PVDCON FEM Software generally agreed with the observed values. As expected, increasing the Kh/Ks and OCR values resulted in lower settlement values.
Co-Authors C. W. Yang D. T. Bergado J. Saowapakpiboon N. Kovittayanon N. Miura S. Hayashi S. Y. Liu T. P. de Zwart Y. C. Yuan Y. J. Du Y. J. Wu Y. S. Ye