Abstract
<title>Abstract</title> <p> The stability of unsaturated soil structures during seismic events remains a critical challenge in geotechnical engineering. Traditional failure criteria, based on stress-point thresholds, often fail to capture the progressive structural degradation occurring under cyclic loading. This study investigates the impact of pore-air exhaust conditions on the cyclic behavior of unsaturated silt (DL clay) and proposes a novel framework for failure detection based on the dynamics of stress increment trajectories ( <italic>d,p/dtdq/dt</italic> ). Through a series of cyclic triaxial tests at various suctions ( <italic>s</italic> = 0,10,30,60 kPa), we identified a distinct mechanical "inversion" at a critical suction threshold of 30 kPa. Below this threshold, non-exhausted (NE-UD) conditions exhibit higher resistance; above it, exhausted (E-UD) conditions prove more stable. To elucidate the underlying physics, we applied Principal Component Analysis (PCA) to define "directional coherence" via the eigenvalue ratio ( <italic>I</italic> ). We demonstrate that failure onset is not a discrete point on the stress path but a regime shift characterized by the collapse of trajectory structure. The proposed model-independent algorithm allows for the early detection of instability, providing a robust theoretical basis for monitoring and predicting geomaterial failure. </p>