Mechanics of the earthquake-induced Hongshiyan landslide in the 2014 Mw 6.2 Ludian earthquake, Yunnan, China

Abstract

The Hongshiyan landslide, the largest landslide (volume ~ 12.24 Mm3) triggered by 2014 Mw 6.2 Ludian earthquake, blocked the Niulan river forming a rockslide dam; the failure potential of the dam created an elevated risk to population and infrastructure downstream. We provide insight into the failure mechanics of the Hongshiyan landslide using data obtained by means of remote sensing techniques and traditional in-situ surveys. Geological data obtained by these methods was then used for kinematic and numerical analyses. Our study shows that failure of the Hongshiyan landslide involved a high rock slope with an average slope of 55° and aspect of 185° SSW, in which the geological setting consists of an upper strong limestone slab and an underlying weak silty mudstone. The geological setting allowed the landslide to develop, as follows: i) the silty mudstone, due to its poor mechanical properties, was subject to ductile deformation under the compressive loading from the overlying strong limestone slabs; ii) a large number of release fractures in the limestone developed as a consequence of pre-failure progressive deformation in the underlying weak mudstone; iii) the oxidation and solution along steep joint walls in the upper limestone indicates that the joints are pathways that allow water ingress into the mudstone, further promoting the degradation of shear strength in this layer. Distinct element modelling (UDEC) was used to back analyse the failure mechanism and develop the geological model that best reproduced the Hongshiyan failure geometry. Results show that the failure surface consisted of two elements. The upper steeply dipping release surface formed along the slope-parallel steep cross joints defining a jointed, strong limestone slab, and the toe of the failure formed a curved failure surface through the ductile weak silty mudstone, underlying the limestone slab, independent of any discontinuities within it. The pre-failure observations and numerical analysis suggest that the Hongshiyan slope was in a state of marginal static stability prior to the earthquake as a result of pre-failure progressive deformation controlled by the shear strength of the underlying mudstone. Overall, the marginal pre-earthquake stability of the Hongshiyan slope, coupled with the strongest seismic loading, which the Hongshiyan slope has been historically subjected to, are the reasons why this moderate earthquake induced a large-scale catastrophic rock slope failure.