# Experimental Study on the Permeability of Microbial-Solidified Calcareous Sand Based on MICP
## 1 Introduction  
The construction of artificial islands is in full swing all over the world due to space limitation in coastal areas [1, 2] and transfer transportation requirements in distant seas (e.g., Fiery Cross Island in the South China Sea). On the artificial islands, the storage and access of fresh water are important fundamental conditions for human living [3, 4]. In China, it has been reported that the cost of transporting high-quality tap water from the mainland to one island reaches USD 370 thousand per year [5]. How to store more fresh water on the artificial islands is of great significance to cost savings. Previous efforts have found that the permeability of island-building materials dominates the water storage on the island. General consensus has been achieved that the application of less permeable island-building materials can increase freshwater storage [6, 7]. Regarding island-building materials, calcareous sand, formed by the accumulation of coral debris and other biological debris, has been widely used as a foundation filler [8, 9, 10] due to its excellent mechanical properties and extensive availability in marine environments [11, 12]. However, the reclamation in distant seas using sand is often carried out viadredgers, and the foundations are formed using the rainbow method on the water surface or the bottom dumping method [13; 14], which are generally unfeasible to meet the required permeability [15]. This problem is especially prominent when using calcareous sand as a foundation material after a hydraulic fill. To solve this problem, island builders propose the rolling method, dynamic consolidation and vibroflotation to treat calcareous sand foundations. However, the rolling method is only suitable for low depth foundations, and dynamic consolidation and vibroflotation require large machinery, which is inconvenient and costly to transport. Large-scale mechanical operations may also cause great damage to the ecological environment of the island reef. Therefore, there is an urgent need to develop an efficient, economic and environmentally friendly technology to reduce the permeability of calcareous sand foundations. Microbial-induced calcite precipitation (MICP), an emerging soil improvement technology, uses urease-producing microorganisms to hydrolyze urea to form carbonates, and then calcite formed with calcium ions is added. It can cement loose sand bodies [16; 17; 18; 19], increasing sand strength and decreasing its permeability at the same time. This technique presents the advantages of low grouting pressure, viscosity and concentration and, thus, less disturbance and pollution to the formation compared with conventional grouting reinforcement technology [20; 21; 22; 23]. Currently, MICP has been widely used to improve the mechanical properties of sands [24; 25; 26; 27; 28; 29; 30; 31], and some field-scale applications have been successful [32; 33; 34; 31], but these studies mainly focused on common silica sand. Calcareous sand, as a kind of marine sediment, has the characteristics of fragility, high compressibility and a low bearing capacity [35], which is quite different from silica sand. Pioneering works applied MICP to such special sand and found that the mechanic strength of calcareous sand can also be improved after cementation. For instance, [PERSON] [36] found that the dynamic shear stress of cemented calcareous sand was 50% higher than that of uncemented calcareous sand. [PERSON] [37] found that artificial beach rock, made from coral sand solidified by MICP, achieved the expected unconfined compressive strength of 20 MPa. However, for calcareous sand foundations on artificial islands, in addition to the mechanical properties, its permeability that determines freshwater storage on the islands needs attention. In particular, when physical reinforcement measures are adopted to treat calcareous sand foundations, the large-size calcareous sand particles are prone to breaking under external forces [38; 39; 40; 41], leading to changes in the gradation and pore structure [42].