Mechanical and microstructural characteristics of calcareous sand reinforced by MICP-hollow fiber membrane treatment

Introduction: To address the engineering challenges of low strength and high crushability of calcareous sand in South China Sea islands and reefs, this study innovatively combines microbially induced calcium carbonate precipitation (MICP) technology with hollow fiber membranes. Methods: The effects of different hollow fiber membrane contents (0.2%-1%) and lengths (5-20 mm) on calcareous sand reinforcement were systematically investigated through physical property tests, mechanical property tests, and SEM analysis. Results: Results demonstrate that hollow fiber membranes (HFM) significantly enhance MICP effectiveness. Under optimal conditions (0.6% content, 10 mm length), calcium carbonate generation rate, bacterial retention rate, unconfined compressive strength, and cohesion improved by 37.7, 129.3, 48.13, and 64.49%, respectively. Microscopic analysis reveals triple reinforcement mechanisms: MICP strengthening through bridging, cementation, and coating effects; spatial constraint from hollow fiber membrane networks; and enhanced interfacial bonding from surface crystallization. Significant interactions exist between membrane content and length, longer membranes excel at low contents while shorter membranes avoid agglomeration at high contents. Discussion: Under laboratory conditions, the HFM-MICP combination improves the strength and microstructural performance of calcareous sand, indicating potential relevance to the reinforcement of calcareous soils in marine geotechnical settings.