Slow rock slope deformations promoting catastrophic collapses in tectonically active settings: the Tienchi case study (Taiwan)

Abstract

<p>Deep-seated gravitational slope deformations (DSGSD) have been increasingly recognized in Taiwan during the field investigations that followed the 2009 Typhoon Morakot, when over 2700 mm of rainfall in five days were recorded causing the widespread occurrence of rainfall-induced, deep-seated landslides. Nevertheless, the characteristics of DSGSD and their possible relationships with catastrophic landslides in the Taiwanese context, characterized by a tectonically-conditioned fluvial topography and a subtropical climate, have never been clarified so far also because of the dense vegetation and limited slope exposure.</p> <p>In this perspective we focused on the Tienchi rock slope (Lalong River valley, Kaohsiung County), carved in folded Miocene to Eocene meta-sandstone and slate successions which is characterized by high-energy relief and a complex structural setting with cross cutting relationship between the bedding and an axial plane cleavage. The slope is highly susceptible to deep-seated landslide collapse and regression caused by heavy precipitations and, especially since 2009, was affected by diffuse landslide events that caused severe damage and the closure of the South Cross-Island Highway 20, i.e. the most important roadway connecting the western and eastern sides of southern Taiwan.</p> <p>We carried out a detailed geomorphological investigation over an area of 3.7 km2, exploiting field surveys and remotely sensed data (i.e. 1m multitemporal LiDAR DEMs and 3D point clouds) to map the surface evidence of slope deformation and to identify the factors interplaying in its evolution. First, we recognized and mapped morpho-structural evidence (scarps, counterscarps, trenches, landslide scarps etc.) typical of deep-seated gravitational slope deformations (DSGSD), partly active at rates up to cm/year and characterized by total accumulated displacements up to tens of meters, suggesting that long-term slope deformations occurred well before catastrophic slope destabilization. Integrating surface evidence and morphostructures geometry, we could thus reconstructed the slope kinematic by means of serial cross sections. Then, we computed descriptive parameters (stream power index, amount of precipitation, density of structures) to relate the occurrence of instabilities to external predisposing and triggering factors.</p> <p>Finally, we combined the morphometric analysis results to set up multistage 2D Finite-Element models to analyze the progressive development of a deep shear zone in response to gravitational stress and water infiltration and to investigate the possible control of DSGSD on the susceptibility of different slope sectors to large-scale rainfall-induced catastrophic failures.</p> <p>The integration of combined morphometric variables, remote sensing products and 2D FEM results, suggests that the long-term progressive failure of the slopes was constrained by inherited ductile structures and favored by a subcritical stress state on the slope, promoted by the tectonically-forced steep fluvial topography.</p> <p>The evolution of a deep shear zone also caused the progressive damage of the uppermost and mid slope sector, preconditioning the location, size and mechanisms of a perched water table and the onset of landslides more prone to catastrophic failure due to intense typhoon precipitations.</p>