Abstract: Leakage from underground structures within water-rich sand layers readily triggers sand-water outburst disasters, with their destructive behavior closely linked to pore water pressure. Existing research predominantly focuses on single sand layers, leaving insufficient exploration of the entire outburst process and pore water pressure response patterns in composite sand layers. Drawing upon the context of metro construction accidents, this study designed ten sand-water outburst model tests. Employing a real-time pore water pressure monitoring system, it investigated the initiation, development, and stabilization processes of erosion within a “fine-grained upper layer, coarse-grained lower layer” composite sand formation, with particular emphasis on revealing the pore pressure response characteristics during outburst failure. Results indicate that following outburst initiation in composite sand formations, pore water pressure undergoes three distinct phases: a sudden drop, sustained low pressure, and subsequent recovery. The rate and magnitude of pore pressure decline correlate positively with the severity of failure. During cavity expansion, the rate of sand-water loss exceeds upstream water replenishment, inducing negative pressure within the cavity where local head differences exceed overall head differences. Significant permeating forces develop perpendicular to the cavity surface, triggering sand boiling phenomena that exacerbate permeation failure. Experiments reveal that within composite sand layers, the lower coarse sand and upper fine sand form a series structure. The coarse sand layer acts as a strong water supply conduit, concentrating permeation forces primarily on the upper fine sand. This results in longer duration, more severe extent, and broader scope of sand and water loss within composite layers compared to single-layer formations, presenting significantly higher failure risks. Enhanced identification and monitoring are recommended for engineering applications.

Key words: seepage, outburst failure, composite sand layer, model test, pore water pressure