Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract:increase the overall factor of safety (Frayssines and Hantz, 2009;Jennings, 1970). Field bridges within individual rockfall scars in this inventory in order to understand how they 102 determine the type, mode and location of failure. 103 104 105 Figure 1: a) Simplified profile view of a rockfall held to a rockslope by rock bridges and a pre-106 existing yet not fully formed discontinuity. The incipient rockfall requires the rock bridges 107 separating the discontinuities to be broken before failure can occur. b) Example high 108 resolution photograph of a siltstone rockfall scar, from North Yorkshire coastal cliffs, U.K. 109The scar contains discontinuities of varying persistence, plus three separate broken rock 110 bridges that have been variously weathered, as indicated by the surface colour. Analysis of 111 the age of the features, as indicated by their weathering, suggests the order of failure, with 112 the discontinuity surfaces forming first, before fracturing and weathering of rock bridges, and 113 the final fracture of a freshly exposed rock bridge. 114 115 Study Site 116We monitored a 200 m section of near-vertical cliffs at Staithes, North Yorkshire, UK over a 117 13-month period to document and characterise rockfall activity (Figure 2). The rock portion of 118 the cliffs is ~60 m in height, and located on a storm-dominated macro-tidal coastal 119 environment. The 200 m survey section contains a lower shale unit (~10 m high, extending 120 from the cliff toe at mean high water level), an upper shale unit (~32 m high) and an 121 interbedded siltstone and sandstone unit (~12 m high), capped by a glacial till (Figure 2c). 122

No abstract

No abstract

Weathering reduces the strength of rocks and so is a key control on the stability of rock slopes. Recent research suggests that the geotechnical response of rocks to weathering varies with ambient stress conditions resulting from overburden loading and/or stress concentrations driven by near‐surface topography. In addition, the stress history experienced by the rock can influence the degree to which current weathering processes cause rock breakdown. To address the combined effect of these potential controls, we conducted a set of weathering experiments on two sedimentary lithologies in laboratory and field conditions. We firstly defined the baseline geotechnical behaviour of each lithology, characterising surface hardness and stress–strain behaviour in unconfined compression. Weathering significantly reduced intact rock strength, but this was not evident in measurements of surface hardness. The ambient compressive stress applied to samples throughout the experiments did not cause any observable differences in the geotechnical behaviour of the samples. We created a stress history effect in sub‐sets of samples by generating a population of microcracks that could be exploited by weathering processes. We also geometrically modified groups of samples to cause near‐surface stress concentrations that may allow greater weathering efficacy. However, even these pronounced sample modifications resulted in insignificant changes in geotechnical behaviour when compared to unmodified samples. The observed reduction in rock strength changed the nature of failure of the samples, which developed post‐peak strength and underwent multiple stages of brittle failure. Although weakened, these samples could sustain greater stress and strain following exceedance of peak strength. On this basis, the multi‐stage failure style exhibited by weaker weathered rock may permit smaller‐magnitude, higher‐frequency events to trigger fracture through intact rock bridges as well as influencing the characteristics of pre‐failure deformation. These findings are consistent with patterns of behaviour observed in field monitoring results. © 2019 John Wiley & Sons, Ltd.

The effects of strong ground shaking on hillslope stability can persist for many years after a large earthquake, leading to an increase in the rates of post earthquake land sliding. The factors that control the rate of post‐earthquake land sliding are poorly constrained, hindering our ability to reliably forecast how landscapes and landslide hazards and risk evolve. To address this, we use a unique data set comprising high‐resolution terrestrial laser scans and airborne lidar captured during and after the 2010–2011 Canterbury Earthquake Sequence, New Zealand. This earthquake sequence triggered thousands of rock falls, and rock and debris avalanches (collectively referred to as “rockfall”), resulting in loss‐of‐life and damage to residential dwellings, commercial buildings and other infrastructure in the Port Hills of Christchurch, New Zealand. This unique data set spans 5 years and includes five significant earthquakes. We used these data to (a) quantify the regional‐scale “rockfall” rates in response to these earthquakes and the postearthquake decay in rockfall rates with time; and (b) investigate the site‐specific factors controlling the location of seismic and nonseismic rockfalls using frequency ratios and logistic regression techniques. We found that rockfall rates increased significantly in response to the initial earthquake that generated the strongest shaking in the sequence—The MW 6.2 22 February 2011 event—Compared to the long‐term background rates derived from the dating of pre‐2010 talus piles at the toe of the slopes. Non seismic rockfall rates also increased immediately after the 22 February 2011 earthquake and decayed with time following a power‐law trend. About 50% of the decay back to the pre‐earthquake rockfall rates occurred within 1–5 years after the 22 February 2011 earthquake. Our results show that the short‐term decay in rockfall rates over time, after the initial earthquake, was attributed to the subsequent erosion of seismically damaged rock mass materials caused by environmental processes such as rain. For earthquake‐induced rockfall at the regional‐scale, the peak ground accelerations is the most significant variable in forecasting rockfall volume, followed by the relative height above the base of the slope. For both earthquake and non‐seismic conditions at the site‐specific scale, the probability of rockfall increases when the adjacent areas have failed previously, indicating that accrued damage preconditions localized areas of the slope for subsequent failure. Such preconditioning is a crucial factor driving subsequent rockfalls; that is, future rockfalls are likely to cluster near areas that failed in the past.