Sealing Mechanism of Foamable Polymer for Rehabilitation of Underground Structures

Polymers play a crucial role in modern construction, particularly in the context of underground engineering structures. Foamable polymer (FP) is commonly used for grouting to remediate civil engineering construction. The foaming expansion process produced different microstructures based on the surrounding geological conditions, resulting in the appearance of a crystal structure and cellular structure at the interface in turn. When the crystal structure was located at the interface of the concrete or rock, it was known as a molded skin. To study the water-plugging effect of FP applied in underground structures, this study visualized water intrusion into FP by using nuclear magnetic resonance (NMR) analysis and determined the sealing pressure threshold. Two empirical formulas were obtained to verify the sealing pressures of FP. The water infiltration process was observed as “microfoamed molded skin to polymer cellular structure to partial foamed cracks to microfoamed molded skin.” When there was preexisting damage on the polymer surface, water directly contacted the cellular structure and accumulated on the molded skin, decreasing the sealing pressure threshold. The maximum sealing threshold pressure of the polymer was approximately 1.50 MPa, increasing exponentially with density, while foaming reduced density. In simulated crack grouting tests, that threshold of polymers increased from 0.74 MPa with the increased polymer density. This study provided a means of verifying the water-plugging effect of grouting in geological engineering structure disasters.