Long-Term Durability of Cement-Treated Soil Used in Offshore Airport Island Construction

In port and harbour areas, soft soils such as dredged soil can be solidified by mixing them with cement and other solidifiers in a pumping pipe. This method is known as pneumatic flow mixing. In this method, the soil and solidifiers are stirred and mixed using the turbulence effect of the plug flow generated in the pipe. The authors investigate the long-term durability of the treated soil on the artificial island where this method was first fully introduced. This paper summarises the results of five investigations on the island immediately after construction and 4, 10, 15, and 20 years after construction. No reduction in the unconfined compressive strength or needle penetration gradient was observed in the treated soil. Some degradation was observed at the top and bottom exposed surfaces of the treated soil, similar to that of soil subjected to other treatments. In addition to needle penetration and chemical tests, elemental mapping using an electron beam microanalysis was performed to determine the degree of degradation. The depth of degradation 20 years after construction was 18-25 mm. Although the amount of cement added in the pneumatic flow mixing method was relatively small, this value was within the range of degradation depths over time investigated in previous studies and did not represent a significant degradation.