The effect of dynamic loading on the shear strength of pyroclastic Ash Deposits and implications for landslide hazard: The case of Pudahuel Ignimbrite, Chile

• The effect of cyclic loading on the shear strength of volcanic soils is studied.
• Cohesion is destroyed by cyclic loading while the internal friction angle rises.
• Strength decrease at low normal stresses favours generation of shallow landslides.

The co-seismic and post-seismic behaviour of pyroclastic ash deposits and its influence on slope stability remains as a challenging subject in engineering geology. Case studies in volcanic areas of the world suggest that soil structural changes caused by seismic shaking results in landslide activity. It is critical to constrain how this kind of soil behaves during coseismic ground shaking, as well as the effects of dynamic loading on shear strength parameters after shaking. Direct shear tests carried out on cineritic volcanic materials from the Pudahuel Ignimbrite formation in central Chile show a direct effect of cyclic loading on the shear strength and in a minor extent on the rheology. A high apparent cohesion found in monotonic shear tests, likely attributed to suction and cementation, is destroyed by dynamic loading. At the same time, the internal friction angle rises. This defines a differential post-dynamic behaviour depending on normal effective stress conditions, which favour the occurrence of shallow landslides. These results show how the use of shear strength parameters obtained from standard monotonic direct shear tests may produce misleading results when analysing seismic slope stability in this type of soils.