Resolving Thermo-Hydro-Mechanical Coupling and Progressive Damage in a Metastable Toppling Rock Slope using Integrated Fiber-Optic Monitoring

The behaviour of slowly moving rock slope instabilities in alpine regions is governed by the interaction between inherited structural discontinuities and externally imposed environmental forcing, yet the mechanisms linking these controls across scales remain poorly constrained due to the lack of continuous subsurface observations. Rock slope toppling represents a typical form of such structure-controlled deformation. Here, we present a comprehensive investigation of a structurally complex toppling rock slope in pre-Variscan metamorphic units in the Bedretto Valley (Swiss Alps). This landslide is intersected by the unlined Bedretto Tunnel over a length of approximately 450 m. The presence of the tunnel and the associated facilities of the Bedretto Underground Laboratory provides a unique opportunity to resolve spatially variable deformation.

To understand the controls on the motion of this landslide, we installed a multi-parameter monitoring network, which integrates surface and subsurface measurements. It combines meteorological stations covering the toppling crown and toe, sectional groundwater pressure monitoring, and a 2-km-long distributed fiber optic sensing (DFOS) cable anchored along the Bedretto Tunnel. This configuration provides a unique internal view of deformation within the rockmass, enabling continuous, decimeter-scale observations of microstrain and temperature beneath up to 1500 m of overburden.

The measurements reveal two distinct deformation responses of the landslide. First, reversible, centimeter-scale strain oscillations correlate with surface temperature fluctuations but exhibit anomalously high amplitudes and penetration depths, which cannot be explained solely by conductive heat transfer. This points to a non-local thermoelastic response, whereby far-field thermal stresses are generated and anisotropically transmitted through the fracture network within the rockmass. Superimposed on this cyclic thermoelastic background, the data reveal discrete, irreversible strain steps near critical fracture zones. These steps temporally coincide with major seasonal hydrologic events including sustained snowmelt and intense rainfall when a two-layer bucket model predicts corresponding peaks in groundwater storage and pressure transients. This correlation provides direct evidence for hydro-mechanically driven, progressive damage within the fracture network of the slope.

Multivariate decomposition of the deformation time series isolates not only the dominant thermo-hydraulically driven cyclic signal, but also residual components characterized by spatially variable, and locally opposing, monotonic strain trends. These opposing trends are partially explained by the mechanical and geometrical heterogeneity of the fracture network and reveal the accumulation of progressive inelastic deformation. Beyond direct thermal or hydraulic forcing, such components suggest a creep-like weakening mechanism of the rockmass under quasi-static gravitational stress.

These findings reveal the dynamics of a coupled thermo-hydro-mechanical system, in which seasonal forcing drives both reversible deformation and irreversible damage. The study thus highlights the critical role of discontinuities in controlling slope behavior, showing how transient hydrology and thermal cycling progressively degrade rockmass strength along pre-existing fractures and joints, ultimately weakening large rock slope failures.