Failure mechanisms and mobilization processes of coastal landslides in sensitive soils

Abstract

Landslides are widespread along coasts worldwide. Understanding initiation, type, and areas affected by such landslides, is thus one of the primary concerns for coastal communities and infrastructure projects, like harbor constructions and residential settlements. Important short-term, high-energy impacts that may trigger a landslide are earthquakes and heavy rainfall events. Weak zones within the depositional succession comprising the slope are another important factor contributing not only to landslide initiation, but also to post-failure landsliding. Assessing the landslide hazard in coastal regions therefore requires a good understanding of the complex interrelations between the various high-energy external impacts and the internal mechanical characteristics of the slope material. Some of the most damaging landslides in the past occurred in soil with high sensitivity, a measure of the post-failure strength loss in the failure zone during landsliding. Such soil exhibits very low shear strength after failure, predisposing highly mobile landslides with long runout distance and dimensions difficult to predict. High sensitivities have been described for post-glacial sediments in Norway, as well as for altered tephra in New Zealand. In both regions, it is of common interest to better understand the weakening and mobilization processes in sensitive slope forming soil subjected to cyclic loading such as earthquake shaking. Furthermore, the processes that lead to high sensitivities in altered tephra are still poorly understood. This doctoral thesis aims to broaden the understanding of failure mechanisms and mobilization processes in landslides at the interface between land and water. Two landslides were investigated, that occurred in sensitive soil and affected society, economy, and natural environment in coastal regions: (1) The coastal submarine Orkdalsfjord landslide in postglacial silt, Norway, and (2) the coastal subaerial Omokoroa flow slide in altered tephra, New Zealand. The vulnerability of the Orkdalsfjord landslide to cyclic loading was studied by using in situ vibratory cone penetration tests and laboratory cyclic triaxial testing. Very coarse silt layers, interbedded in the post-glacial silt unit overlying the failure surface, is more vulnerable to cyclic loading compared to surrounding finer silts. Accordingly, the very coarse silt layers may have contributed to the weakening and mobilization of the Orkdalsfjord landslide in case cyclic loading occurred during landsliding. The cyclic loading behavior of altered tephra from the Omokoroa flow slide was analyzed by monotonic and cyclic triaxial testing. The altered tephra experiences brittle failure and has higher friction coefficients than normally consolidated clay, being similar to granular soil. Comparing the cyclic shear strength of altered tephra with that of marine clays shows that for altered tephra the number of loading cycles to shear failure depends more strongly on the level of shear stress applied and that altered tephra is more resistant to small cyclic loading but fails within a narrower range of shear stresses. The development of high sensitivities in altered tephra was analyzed by scanning electron microscopy and laboratory vane shear measurements along a drill core comprising the intact tephra succession of the Omokoroa flow slide. The secondary clay mineral halloysite dominates the Pahoia Tephra, a sequence that was involved in the Omokoroa flow slide. The halloysite particle morphologies are highly variable with depth. While tubular morphologies are prevalent in the upper tephra successions, the lower Pahoia Tephra sequence is dominated by spheroidal halloysite. This change in halloysite morphology coincides with an increase in sensitivity with depth. Therefore, spheroidal halloysite is likely the key in the development of sensitivity in altered tephra from New Zealand and potentially elsewhere in regions of similar volcanic origin. In the failure surface of the Omokoroa flow slide, a new open-sided spheroidal halloysite particle shape in the form of a mushroom capsa is recognized for the first time that governs the development of high mobility in the failure surface during landsliding. Based on a new a attraction-detachmenta model, it is suggested that the rearrangement in the halloysite texture during the failure process reduces the attractions between the particles at nanoscale dimensions and thus predisposes flow sliding.