# Subsidence of Artificial Reefs with Bamboo Foundations on a Soft Seabed
## 1 Introduction: The Engineering and Environmental Problem  
Durability and serviceability are two main goals for the designers of artificial reefs. Researchers from several fields have provided important contributions in recent decades to solve engineering problems, such as those based on hydrodynamic and structural models [1], to assist with the prediction of reef behavior over its planned life. The subsidence of an artificial reef is a crucial problem involving both ecological and economic aspects, being its serviceability related to target depth intervals, which must ensure that the resident organisms receive well-defined levels of light and oxygen. In this context, geotechnical modelling is the most important tool for the simulation of seabed subsidence scenarios. The Hong Kong International Airport (HKIA) Master Plan 2030 [2] revealed that the airport would reach a saturation point sometime between 2019 and 2022. A third runway was deemed essential, to be built on land reclaimed from the sea. The AA3214R project prompted calls from local green groups and residents to mitigate potential effects on marine wildlife--particularly the Chinese white dolphin--and noise impacts in the Pearl River Delta. The Airport Authority (AA) deployed 100 artificial reef (AR) units in the western waters of Hong Kong International Airport's south runway (Figure 1) from mid-June to early July 2021. The ARs, placed near the future North Lantau Marine Park, serve as a pilot scheme to provide micro-marine habitats for enhancing the marine ecology and fishery resources [3]. In particular, the AR units were placed to develop a \"fouling\" community, including bivalves, sponges, tunicates, bryozoans, benthic algae and other associated organisms; to develop a complex habitat with numerous and varied refuges for predators, prey and/or foraging resources; and to promote the recruitment of adult and juvenile fish. As mentioned, durability in water is a factor conditioning the choice of the material used to realize reefs. However, the material should also be sustainable. Bamboo has been recently employed, for example in Malalayang Dua [4]. [PERSON] [5] quantified the benefit of bearing capacity related to bamboo reinforcement in shallow foundations on peat soil. Looking specifically at the reef foundations, a further requirement is settlement limitation. [PERSON] and [PERSON] [6] found that ARs deployed on footings made of geogrid or hybrid geogrid-bamboo mat reduced their settlement remarkably. [PERSON] and [PERSON] [7,8] prepared three reinforcement types: geogrid, geogrid-bamboo mat (GBM) and seaweed-pile mat (SPM) and performed bearing capacity tests, large size settlement tests and two-dimensional flow scour tests according to the different soil types (sand, silt, clay). They observed that the bearing capacity of the seabed with a reinforced artificial reef increased, and its settlement and scour depth reduced for all reinforcement types. [PERSON] and [PERSON] [9] performed comparative studies on clay beds reinforced with bamboo and geosynthetics. They found that the ultimate bearing capacity of the clay bed reinforced with bamboo cell and the bamboo grid was 1.3 times more than that of the clay bed reinforced with geocell and geogrid. Furthermore, a substantial reduction in the footing settlement and surface deformation was observed. The bamboo was treated chemically to increase its durability. Although the performance of bamboo in terms of the tensile strength and load-carrying capacity of the clay bed was found to be reduced by 15-20% after the chemical treatment, the treated bamboo was characterized by a better performance than that of its geosynthetic counterparts. In the present paper, the behavior of the reef foundations realized in bamboo at North Lantau, Hong Kong, has been studied. The results of a 1-year marine survey and of a Finite-Element (FE) numerical modelling are shown and discussed.