Chapter 10 - High Pressure for Vacuum Consolidation Method Using Air-Water Separation System

Abstract

Ground improvement by the vacuum consolidation method has been used widely over last decade. The original system, proposed by Kjellman Swedish engineer in 1952, was a prototype for further development in many parts of the world. The vacuum consolidation method was intensively developed from 1988~1992. Between 1990 and 2000, the system could produce a vacuum pressure at the pump and under an airtight sheet of around 80kPa and 60kPa, respectively. However, over time it was found that the vacuum pressure measured under an airtight sheet was gradually decreasing with time during vacuum operation, resulting in additional cost and time of construction. Therefore, the problem had to be analyzed and a resolution developed. A theoretical solution was proposed together with a practical solution known as the air-water separation system, which could be equipped with a conventional vacuum consolidation method. This chapter presents the case histories of the ground improvement by vacuum consolidation method in Japan using both a conventional system as well as an air-water separation system. A special case history with an air-water separation system is also presented, where a large consolidation settlement of approximately 10m took place. This is a good case study for verifying the effectiveness of the air-water separation concept as well as the performance of a vacuum consolidation system. Consequently, it was found that the air-water separation system could resolve the problem of vacuum pressure decreasing inside the improved area during consolidation settlement. The high vacuum pressure measured under the airtight sheet could be produced and maintained over 80kPa. Furthermore, the soil investigation after ground improvement confirmed the effectiveness of vacuum consolidation with an air-water separation system as a surcharge preloading method.