# Numerical Simulation of Subdam Settlement in Ash Disposal Based on CGSW Optimization
## 5 Conclusions  
Although dry ash disposal in valley has been used and studied for more than 40 years, the soft foundation problems of the subdam are rarely encountered. With single dam foundation calculation, it is hard to evaluate the consequences of uneven settlement. Carrying out an effective and economical foundation treatment method that ensures dam stability is challenging. The primary goal of this study was to verify CGSW distribution by drilling engineering and then to initially evaluate the influence of CGSW on the proposed subdam. The orthogonal test theory was utilized for the development of various construction strategies for the possible soft foundation using numerical simulation. The finite difference code FLAC3D 6.0 was then employed to carry out stone column studies on the subdam stability to examine the effect of center distance and diameter. Based on the modeling results, the optimal construction scheme was selected after deformation and subsidence analysis. The key observations from the scheme assessment are:  
1. CGSW distribution based on borehole data can help to build a more accurate FLAC3D digital model; this is the basis of the deformation analysis and differential settlement analysis under various subsequent construction schemes. 2. If deformation or differential subsidence on top of the CGSW cannot be effectively controlled, it could cause damage to the subdam, such as shear failure or cracking. The subdam stability is dominated by the differential subsidence rather than the absolute magnitude of subsidence. The modeling results indicate that the subdam differential settlement caused by CGSW weathering may threaten the stability of the overlapping subdam-XIV, leading to tension failure in the right and middle abutment. 3. Both the replacement method and the stone column methods helped to reduce the subdam subsidence value. The displacement method could provide only a 10.615% reduction in the maximum subsidence value; the stone column methods worked better. Scheme 2, 3, 8, and 9 produced reductions of over 20%; scheme 8 provided the best reduction of 22.391%. The deformation analysis showed that scheme 3 and scheme 5 had the best effect in that they both eliminated the tension failure zone in the middle of subdam-XIV. The shear failure zone and tension failure zone in scheme 3 were reduced by 60.51% and 87.75%, respectively, compared to the original condition (scheme 0). Scheme 5 reduced the shear failure zone by 70.75%; meanwhile, it reduced the tension failure zone by 97.83%. 4. Generally, it is known that under longitudinal loading conditions three types of deformation or damage, known as \"bulge deformation\", \"shear damage\", and \"block damage\", may occur in the gravel pile. The diameter and spacing of the gravel pile both affect the settlement of the superstructure within the diameter range of 0.5-1.5. Regardless of the pile spacing value, the maximum settlement value of the top surface always exhibits an ascending trend followed by a descending trend. This may be due to the fact that as the diameter (plane replacement rate) increases, the resistance to deformation of the pile-soil system increases; so, the vertical deformation in the pile-soil system will gradually decrease, which leads to a decrease in the maximum settlement value. Conversely, an increase in diameter enlarges the mass of the pile-soil system, leading to an increase in the compressive deformation of the soil layer beneath it, which leads to an increase in the maximum settlement value. This shows that in the deep burial pile-soil system, with the enlargement of the gravel pile diameter, theincrease in its lower fly ash compression deformation is first greater than and then less than the decrement of the vertical deformation of the pile-soil system. * Subdam-III was constructed in 2016; it has been standing for 7 years since then. Its deformation has stabilized.