## 1 Introduction  
Development by civil engineers often places higher priority on safety and economic feasibility than on the environment, coexistence, and the sustainability of the Earth. In particular, soft ground with mud flats that serve as purifiers of seawater have lost their sustainability due to developments by civil engineers. Soft ground is made of clay and its strength varies depending on the water content [1; 2; 3; 4]. Civil engineers remove the pore water in clay to increase its strength and then build buildings or roads on top of it. To remove pore water from clay, a large amount of sand is taken and loaded on top of it, and the water, deep under the ground, is drained through a sand spout by drilling a hole into the ground [5; 6; 7; 8; 9; 10; 11; 12]. Therefore, if a reclamation project involves the development of clay ground, it requires a supply of sand. Usually, this involves cutting a mountain, collecting sand from it, and transporting it to a reclamation-site [13; 14]. Such a development method can pose damage to mountains and roads. Instead of this, a way of removing pore water by clay heating can prevent the degradation of nature and ensure the sustainability of mud flats. The typical soft ground improvement method is preloading with prefabricated vertical drains. Excess pore water pressure is generated by the preloading of the embankment in cases of low permeability soils. The pore water excess is drained by vertical prefabricated drains [2; 15; 16; 17]. However,due to large settlements and/or filter clogging, the vertical prefabricated drain is blocked and excess pore water is not drained [18; 19]. As an alternative, other ground improvement methods such as vacuum consolidation, jet grouting, deep mixing, and ground heating can be used. The deep mixing method improves the soil by bonding and consolidating soil particles using a hardening agent [20]. It prevents the ground surface settlement and increases the soil strength. However, a precise on-site construction is required and the workplace should be large enough because of the equipment dimensions. Vacuum consolidation is a method of making the ground soft under a vacuum state and using atmospheric pressure instead of the embankment load for preloading. The advantages of this method are the fact that the consolidation effect is assured up to deep depth, the shear fracture due to preloading can be prevented by using atmospheric pressure, and the construction period and workability are good. However, maintenance of the vacuum state due to airtightness is very important, so measurement management is required. The jet grouting method is a method of injecting high-pressure injection material and air into the soft ground borehole and mixing the scoured soil and the injected material to form a solid cylindrical body. The advantage of this method is that the improvement effect is excellent. The cost of the construction is high and, in areas affected by seawater, the hardening and improvement effects are poor. The material loss is high due to the high-pressure injection and the solid cylindrical body is not uniform. Moreover, this method is not applicable to peat soils and is less effective in clayey soils. Another ground improvement technique is the ground heating method. There is an improved soft ground method utilizing ground heating. The ground heating method using fossil fuels was developed by [PERSON] [21] and [PERSON] [22], as shown in Figure 1. The ground heating method, applied in the Soviet Union and in Romania, which is rich in fossil fuels, is a method of injecting and burning gaseous fuels mixed with air into soft ground. Because the pore water of the clay dissipates by burning fossil fuel, the water content of the clay decreases and the strength increases in a short time (about 10-20 days). Additionally, the color of the soil particles changes [23; 24]. Most of the soil was non-plastic at 400-600 \({}^{\circ}\)C [25; 26; 27; 28; 29; 30; 31].