Evolution of bubble-bearing areas in shallow fine-grained sediments during land reclamation with prefabricated vertical drain improvement

Abstract

When land reclamation is conducted in a shallow offshore bubble-bearing sediment, methane bubbles can cause various risks, such as changes in the mechanical properties of soils and explosion during construction. The premise of assessing or addressing these risks is to understand the evolution of the bubble-bearing area during land reclamation. In this study, we selected an island-based land reclamation field in the southeastern part of Hangzhou Bay with a bubble-bearing area in shallow sediments. In this field, soft soil under the land reclamation fill was improved via prefabricated vertical drain (PVD). After the land reclamation, an electrical resistivity tomography (ERT) survey was conducted at the location of the bubble-bearing area and in an adjacent bubble-bearing area. The force and deformation of bubbles in the bubble-bearing area were analyzed at various construction stages of land reclamation. A conceptual model of the bubble-bearing area evolution in each construction stage was proposed based on linear elastic fracture mechanics (LEFM). The results of the ERT survey and theoretical analysis are in good agreement. In the process of land reclamation, although the gas pressure and size of the bubbles in the bubble-bearing area may change with the change in the environmental pressure, the bubble size is always smaller than the critical size for initiating rise; thus, the bubbles cannot rise. After land reclamation, there is no important horizontal/vertical evolution of the gas front in the bubble-bearing area.