Experimental study on the availability of polyurethane foam used as compressible layers in subsea tunnels

The yielding support comprising a compressive layer is considered as a solution to the time-dependent damage of the lining in squeezing tunnels. The feasibility of the use of polyurethane foam (PUF) as the compressible layers in subsea tunnels was investigated in this study. Confined compression tests were conducted to investigate the compression performance degradation of PUF immersed in artificial seawater for 400 days. The yield strength and elastic modulus of PUF decreased within 180 days but then increased to over 80% of their initial values within the subsequent following 220 days, the degradation rate and strengthening rate of these parameters are of the same order of magnitude. The degradation rate of the yield strength and elastic modulus can be significantly inhibited by ions in artificial seawater, thus causing samples immersed in low-salt-concentration solutions to exhibit worse compression performance than those of samples immersed in a high-salt-concentration solution. The stress at the end of the yielding plateau and yielding plateau strain range were slightly affected by hydrolysis aging. The shape of the yielding plateau stage of the aged samples changed from a horizontal line to an inclined line owing to the variation in the yield strength. Even after immersion for 400 days, all the aged samples still exhibited compression failure and neither crumbled into pieces nor fractured into parts. The strengthening effect, unchanged failure mode, and almost constant yielding plateau strain range guarantee the safe utilization of PUF in marine environments.