Fig. 3
Waterfall plot of DAS data showing spatiotemporal strain evolution during the wet-sand experiment. The experiment consists of three phases: water rise, hold, and drainage. The horizontal axis represents optical fiber length (mm), the vertical axis shows time (minutes), and the color scale indicates strain amplitude (µε). Colored numbers and double-headed arrows mark the positions of the six fiber segments, while colored letters A–E denote the five fiber support points. The fiber start and end points are labeled O and P. For each segment, fiber length also corresponds to the depth of the vertical fiber (mm). Synchronized tank water levels (cm) are annotated along the time axis, segmented into the three experimental phases. Arrowed solid lines, color-coded by fiber segment (#1–#6), trace the downward migration of water–sand interaction zones; their slopes represent average infiltration rates. Arrowed dashed lines, also color-coded, show decreasing strain amplitudes during the drainage phase. Strain responses at the three bottom supports (A, C, and E) and strain changes at the two top supports (B and D) are highlighted with solid and dashed rectangles, respectively.
