# Subsidence of Artificial Reefs with Bamboo Foundations on a Soft Seabed
## 4 Monitoring of the Seabed Subsidence over the First Year  
The subsidence related to the settlements of the foundations over the entire area was monitored using a Multibeam Echo Sounder System (by Engineering Surveys Limited, Hong Kong, China). The depth measurements refer to the Hong Kong Principal Datum. Cross lines were run to verify the quality of the survey data. The monitoring campaign consisted of six surveys, carried out on the following dates: 30 April 2021, 9 July 2021, 5 October 2021, 4 January 2022, 2 April 2022 and 28 June 2022. The zero reading (date: 30 April 2021, Figure 8) was of reference to assess the rate of bathymetric changes related only to the sedimentation: looking at the survey performed on 9 July 2021 (Figure 9), the average depth of the seabed in area surrounding the reefs seems to be increased, compared to that of the zero reading, by about 10 cm. Some changes in the bathymetry during the year were expected since the Pearl River estuary is dominated by sedimentary accretion. The 2021 July survey was considered to represent, approximately for all units, the initial conditions soon after deployment, which began on 7 June and ended on 11 July of the same year. It is reasonable that the results of this survey are indicative mainly of the immediate settlement and, eventually, of an amount of primary consolidation of the soil. As already mentioned, this study focused on the area of the AR Large Units. The data concerning the horizontal displacements of the units indicate that for most units both northward and eastward movement were zero, confirming the resistance to the submarine dynamic actions exerted by the small piles, i.e., the bamboo legs. In Table 2, the movement undergone by AR Large Units at the end of the monitoring period is shown (see their position in Figure 4). Due to marine activities, some AR units have their original location displaced with increasing amount of Seabed Scars (likely caused by Anchor drag). Thirteen Large Units exhibited, on average, displacements of more than 1 m (Table 2). However, as already mentioned, most units did not undergo significant horizontal drift, confirming the efficacy of the foundation legs. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Soil Layer (or Material)** & **Thickness (m)** & **Total Unit Weight, (kN/m)** & **Void Ratio** & **Secondary Consolidation Coefficient, \(\mathbf{C\alpha}\)** & **Oedometric Modulus, \(\mathbf{E_{med}}\)** & **Compressive Modulus of Elasticity (GP)** & **Friction Angle** & **Poisson’s Ratio** \\ \hline Very soft to soft clay & 2 & 14.0 & 3.1 & 0.029-0.059 & 73 & 27 & 0.3 \\ \hline Firm clay & Undefined & 15.5 & 1.9 & - & 185 & 30 & 0.3 \\ \hline Bamboo & & 3.83 (1) & & & 16 (1) & (soil-pile) & 0.3 (1) \\ \hline \hline \end{tabular} \({}^{(\mathrm{I})}\) Based on data from [12], \({}^{(\mathrm{I})}\) Based on material similarity, using data from [13].  
\end{table} 
Table 1: Geotechnical model assumed for the calculation, based on data from [10,11] pertaining to similar soil (except for values with apex). Figure 8: Pre-deployment bathymetric survey (date: 30 April 2021) (mPD: meters above the Hong Kong Principal Datum). Figure 9: Post-deployment bathymetric survey: profile evolution with time of cross-section 1 (AR Large Units). (mPD: meters above the Hong Kong Principal Datum). In Figure 9, where the variation of the bathymetric profile for Section 1 over the entire year of the survey is shown, the reef units are recognizable in terms of topographic heights. An initial depth reduction soon after the installation can be locally observed; this is probably due to the seabed humping just around the foundation, as an effect of undrained soil distortion. In Figure 10a-e, the 3D perspective view plots of the subsidence measured during the monitoring period are shown. The seabed hosting the reefs gradually and locally deepens.