Sensing optical cables were installed on the surface of a steel spiral case and into the surrounding concrete to monitor the strain distribution during a water-filling test and a preloading-unloading program based on PPP-BOTDA distributed optical fiber sensing technology.The monitored strain was close to the calculated strain.When the water-filling pressures were 2.6 MPa and 4.1 MPa

the average hoop strains of the spiral case were about 330με and 590με

respectively.With sensing cables

it was the first time that the stress adjustment of the spiral case was monitored when inner pressure increased to 1 MPa and the residual strain of-20με was obtained after preloading.Meanwhile

the shrinkage of the surrounding concrete was effectively monitored during depressurization.The measured hoop strain of concrete was 70με-100με

according to which the width of the gap between the steel plate and concrete was estimated to be 0.5-1.4 mm.The monitoring results by PPP-BOTDA provide references for the optimal design and safety monitoring of the spiral case structure.