## 1 Introduction  
When building civil engineering structures, soft soils are often improved using cement or other solidifiers to increase their stiffness and strength. It is also important to ensure that there is no impact to the environment, as an elevated pore water pH and leaching of hexavalent chromium have been noted in the immediate vicinity of cement-treated soil. In port and harbour areas, in addition to the deep mixing method, in which solidifiers are stirred and mixed into soft subsoil, lightweight mixing methods, in which solidifiers are mixed into soil collected from the field, are used effectively [1]. In the pneumatic flow mixing method, solidifiers are also added to the dredged soft soil in the pumping pipe, where they are pumped by a pneumatic pumping vessel using a dredger, and the dredged soil and solidifier are stirred and mixed using the turbulent effect of the plug flow generated in the pumping pipe [2]. The applications of treated soil include reclamation material, backfill material behind seawalls, and temporary embankments to demarcate reclaimed lands. It is also used directly behind quay walls to reduce the earth pressure and prevent liquefaction. This method was tested at the Port of Nagoya in Japan in 1998 and subsequently used extensively used extensively in the reclamation of airport islands at Central Japan International Airport and Tokyo International Airport. This method can continuously improve the quantities of soft soil; thus, it can effectively use large stockpiles of dredged soil in port areas and assist in rapidly constructing large-scale reclamation works such as airport islands. The use of dredged soil rather than soil transported from mountains in the pneumatic flow mixing method contributes to sustainability. Ground improvement with a little cement is also useful in terms of reducing carbon dioxide emissions. In recent years, this method has been adopted in coastal construction projects not only in Japan, but also in North America and other parts of Asia, and its use is progressing worldwide [3]. Cement treatment methods are useful because they can significantly improve the physical properties of soft soils in the short term. However, treated soils deteriorate in the long term, starting from the exposed surface [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. The pneumatic flow mixing method is no exception; the surface of the treated soil exposed to the untreated soil deteriorates, albeit slowly. The deep mixing method, which was developed in the 1970s as a pioneering solidification method, requires a large amount of cement, and the strength of the treated soil is high, at 1500 kN/m\({}^{2}\) or more (when solidifying clayey soil). Even after 50 years, no problematic deterioration has occurred in the field. In contrast, in the pneumatic flow mixing method, the mixing of soil and cement occurs via a plug flow in the pumping pipe, and the strength of the treated soil is not very high, ranging from 100 to 500 kN/m\({}^{2}\). In addition, this method has only been applied onsite for approximately 20 years, and there is no track record regarding its degradation characteristics. Therefore, it is important to understand the degree of deterioration from the perspective of the long-term service of structures. If geomaterials in civil engineering structures do not deteriorate, do not require energy or materials for repair, and can remain in service, they can offer many environmental, social, and economic benefits and consequently increase sustainability. Therefore, the condition of the treated soil should be continuously checked, and its long-term durability should be considered. The authors investigate the long-term durability of cement-treated soil on an artificial island of Central Japan International Airport, where the pneumatic flow mixing method was first fully introduced.