## 1 Introduction  
Recently, South Korea has seriously lacked the land space because of the increase with the population density and industrialization, thus a number of constructions such as the housing complex, high-speed railway, international airport, and harbor construction are progressing in soft soil areas located in coastal area.  
Especially, organic soils are widely distributed over the world, including South Korea, Japan, Australia, Malaysia, and Canada. The area of organic soils is approximately 2.3 million km\({}^{2}\). [PERSON] [1] reported that the organic soil deposits have occurred on the Gold Coast region, Australia, because of current and past marine processes near coastal zones. Also, Malaysia has organic soil deposits observed mainly along the coastal area [2]. According to the Unified Soil Classification System [3], organic soils can be classified as organic clay (OL), organic silt (OH), and peat (Pt). Especially, organic soils can be defined as one with a minimum of 50% organic matter. Organic soils have typically low unit weight, high water contents, high content of organic matter, and high compressibility [4]. The high compressibility of organic soils leads to the increases of risk under the constructions, such as excavations, foundations, and embankments. Therefore, if the structure or embankment are constructed on the organic soils, it cannot be ensured the safety of structure because of the large deformation and low bearing capacity. Hence, it is difficult to predict the behavior of organic soils, thus encountering significant challenges in the preliminary design stage. Therefore, a proper technique for large deformation analysis is necessary to investigate the behavior and safety of the structures on very soft soil, such as organic soil.  
Many studies have been performed to solve the geotechnical problems using numerical techniques, especially the finite element method (FEM). However, it has limitations to solve those, including large deformation between soil-structure or soil-soil interactions using traditional numerical methods, since most researches have been based on the Lagrangian coordinates for the small deformation analysis. For the finite element methods based on the Lagrangian coordinates, the large deformation may lead to the problems, including the mesh and the unconvergence divergence due to severe relative displacement.  
Recently, a Coupled Eulerian-Lagrangian (CEL) technique has been developed to overcome the limitations of the small deformation analyses. This technique has been applied to various geotechnical fields, such as the penetration of offshore structure [5; 6; 7; 8; 9], the landslide and debris flow [10; 11], the pile penetration [12; 13; 14; 15; 16], and the effect on the nearby soil due to tunneling [17].  
The overall objective of this study is to investigate the behavior and safety of the structures on very soft soil such as organic soil, accounting for the application and constructability of ground improvement methods, including forced replacement method and deep mixing method. The large deformation finite element (LDFE) analyses are carried out using the Coupled Eulerian-Lagrangian technique. For CEL modelling validation, the results of CEL analyses in this study are compared with field investigations with settlement, height, and range of soil heave. Additionally, to examine the constructability and economics of each method, comprehensive reviews are performed.