# Evaluation of Land Subsidence during Groundwater Recovery
## 4 Results and Discussions
### Soil Surface Displacement  
Only the upper soil layer, about 30 m from the ground surface, was affected by land subsidence because the minimum groundwater level was 27 m from the ground surface. Both soft clay and stiff clay were considered to evaluate soil surface displacement. We calculated the soil surface displacement by determining the period and characteristics of the groundwater level recovery to the ground surface level using the measurements by the DGR [11]. The soil surface displacement was calculated based on the one-dimensional consolidation of [PERSON]'s theory. The soil layer above the groundwater level was considered as unsaturated soil, and the soil layer below the groundwater level was considered as saturated soil. We estimated the total displacement based on the relationship between the groundwater level and the time period described by [PERSON] et al. [26]. Following a previous study, groundwater level and time are linearly related; therefore, we calculated the soil displacement for every 5 m of groundwater level change from 2001 until 2032 when the  
Figure 6: A 3D schematic of View A.  
Figure 7: A 3D schematic of View B.  
groundwater level reaches the ground surface. In conclusion, the groundwater level was 25 m below ground surface in 2001. According to our groundwater level evaluation and prediction, the groundwater level will reach the ground surface level by 2032. The eight boreholes show similar total displacement trends and displacement rates from 2001 to 2032, as shown in Table 4. Figure 8 shows the relationship between the total displacement and time of the surface displacement predictions of all the boreholes in Zone D. There is a tendency for rebound displacement due to groundwater recovery.  
As shown in Figure 9, in the same study area, Our results show soil surface behavior similar to other results, such as the results of the PLAXIS2D evaluation, which showed that soil displacement rates are equal to 0.35 cm/ year from 1997 to 2037 [18]. The results of ABAQUS software produced rebound rates equal to 0.26 to 0.32 cm/year from 1997 to 2016 [19]. However, the soil displacement monitoring data have been almost constant since 2007 at the Chulalongkorn University station, Pathum Wan [11]. It should be noted that the results of this study were evaluated using predictions of groundwater levels from previous studies. At present, no one knows when the groundwater level will reach the ground surface or how the soil surface displacement will rise during increases in the groundwater level. Thus, we tried to simplify the calculation method. The results from the simplified theoretical calculation can be applied to evaluate or predict soil surface displacement due to groundwater recovery in the future.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Borehole**} & \multicolumn{2}{c}{**Total Displacement (cm)**} & \multicolumn{2}{c}{**Displacement Rate**} \\ \cline{2-4}  & **In 2001** & **In 2032** & **(cm/year)** \\ \hline NP & \(-9.07\) & \(-17.68\) & \(-0.28\) \\ SS & \(-7.29\) & \(-13.64\) & \(-0.20\) \\ BK1 & \(-12.30\) & \(-23.50\) & \(-0.36\) \\ BK2 & \(-17.41\) & \(-33.31\) & \(-0.51\) \\ NB & \(-18.92\) & \(-35.03\) & \(-0.52\) \\ SP & \(-17.05\) & \(-28.52\) & \(-0.37\) \\ PT1 & \(-12.63\) & \(-25.90\) & \(-0.43\) \\ PT2 & \(-11.48\) & \(-20.44\) & \(-0.29\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Total displacement and displacement rate.  
Figure 8: Displacement at each borehole.