# Subsidence of Artificial Reefs with Bamboo Foundations on a Soft Seabed
## 4 Monitoring of the Seabed Subsidence over the First Year
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline
**Unit ID** & **01** & **03** & **07** & **08** & **11** & **14** & **16** & **20** & **22** & **23** & **27** & **45** & **47** \\ \hline Diff. E (m) & \(-1.0\) & \(-1.4\) & \(-1.7\) & \(-0.9\) & \(-0.8\) & \(-1.3\) & \(-0.6\) & \(-1.1\) & \(-1.4\) & \(-0.9\) & \(-1.3\) & \(-1.2\) & \(0.8\) \\ \hline Diff. N (m) & 0.6 & 2.1 & \(-1.0\) & 1.0 & 0.7 & \(-1.3\) & \(-3.5\) & 0.4 & 0.4 & 1.0 & \(-1.5\) & 0.8 & 1.2 \\ \hline \end{tabular}
\end{table} 
Table 2: AR Large Unit movement listing table (Diff. E and Diff. N are components of the horizontal displacement). The table shows displacements of only the units that moved during the year of observation. The measurements were N/A for Units 02, 09 and 13. The remaining Large Units underwent zero displacement. Data from the survey of 28 June 2022. Figure 10: _Cont._  
The primary rationale for correlating the observed one-year settlements solely with the reefs lies in the crater-like morphology of the subsidence profile. Indeed, the loading attributed to the recent airport land reclamation would theoretically induce a widespread and uniform subsidence across the entire region where the reefs were deployed. However, such a pattern was not discerned during the survey period. Figure 10: Perspective view plots (3D) of the bathymetric surveys. (**a**) 9 July 2021; (**b**) 5 October 2021; (**c**) 4 January 2022; (**d**) 2 April 2022; (**e**) 28 June 2022. (mPD: meters above the Hong Kong Principal Datum). One year after reef deployment, settlements of about 25 cm were mostly widespread across the area, although some areal variability was observed. Locally, settlement peaks even higher than 50 cm could be seldom observed. In Figure 11, measured settlement values, representative of the most widespread subsidence detected across the area during each survey (based on visual inspection of the survey maps; see Figure 10), are plotted, together with the final highest value that can be identified among all AR units, as deduced from the final survey output (Figure 10e). The difference between the final highest value and the most widespread one could be indicative of the geotechnical heterogeneity of the area, as said before, and, as a contributing factor, of locally different submarine dynamic actions. In the same figure, the calculated evolution with time of the secondary settlement is also shown, under the hypotheses that viscous deformations took place soon after reef deployment (i.e., nominally since the 9 July 2021, to assign equal history to all units), the creep involved the upper layer only (using \(\varsigma_{\alpha}=0.059\), Table 1) and the deformation process was one-dimensional. The vertical displacements measured during the first year fit the values calculated for creep quite well. This observation seems to corroborate the interpretation that secondary consolidation is an important source of subsidence in this problem and that to depict a reliable subsidence scenario the soil viscous deformation must be assumed to start soon after reef deployment. The FEM calculation of the foundation primary consolidation settlement will confirm this hypothesis, as shown in the next section. Instead, no attempt was made to calculate the immediate settlement. Indeed, from the survey of July 2021, i.e., soon after the reef installation, it seems that the initial settlement of the reefs was negligible: the difference in depth between their tops and the surrounding seabed (Figure 10a) is practically equal to the height of the units. This is expected for the present case due to the small static loads and because the units have been deployed on the foundation without free-fall installation (Figure 2b), so that momentum effects on the initial settlement [14] may be neglected.