The effects of normal and shear stress wave phasing on coseismic landslide displacement

Abstract

Predictive models used to assess the magnitude of coseismic landslide strain accumulation in response to earthquake ground shaking typically consider slope-parallel ground accelerations only and ignore both the influence of coseismic slope-normal ground accelerations and the phase relationship between dynamic slope-normal and slope-parallel accelerations. We present results of a laboratory study designed to assess the significance of the phase offset between slope-normal and slope-parallel cyclic stresses on the generation of coseismic landslide displacements. Using a dynamic back-pressured shearbox that is capable of simulating variably phased slope-normal and slope-parallel dynamic loads, we subjected sediment samples to a range of dynamic loading scenarios indicative of earthquake-induced ground shaking. We detail the variations in strain accumulation observed when slope-normal and slope-parallel stresses occur independently and simultaneously, both in and out of phase, using a range of dynamic stress amplitudes. Our results show that the instantaneous phasing of dynamic stresses is critical in determining the amount of coseismic landslide displacement, which may vary by up to an order of magnitude based solely on wave-phasing effects. Instantaneous strain rate is an exponential function of the distance normal to the Mohr Coulomb failure envelope in plots of shear stress against normal effective stress. This distance is strongly controlled by the phase offset between dynamic normal and shear stresses. Our results demonstrate that conditions considered by conventional coseismic slope stability models can either overestimate or underestimate earthquake-induced landslide displacement by up to an order of magnitude. This has important implications for accurate assessment of coseismic landslide hazard.

Displacement mechanisms of slow-moving landslides in response to changes in pore water pressure and dynamic stress

“…These results are consistent with previous studies, which consider more complex wave forms (Brain et al, 2015). We infer from our results that the frictional properties of the materials we tested do not increase (strain harden) or decrease (strain 10 weaken) but remain constant during seismic excitation, in the dynamic stress ranges examined.…”

“…A suite of shear experiments was conducted in a Dynamic Back-Pressured Shear Box (DBPSB). The DBPSB is highly modified direct shear device, constructed by GDS Instruments Ltd and described in detail by Brain et al (2015) and 2017). The apparatus can function as both a conventional direct shear and back-pressured shear machine and provides both static and dynamic control of horizontal (shear) and axial (normal) force and displacement; total stress; and effective 30 stress.…”

“…Using the method proposed by Brain et al (2015), we use the average normal effective stress and the maximum shear 25 stress (Fig. 9a) to plot displacement rates against the distance normal to the failure envelope during each experiment (Fig.…”

The control of earthquake sequences on hillslope stability

“…">Methods and Experimental ApproachWe used a novel geotechnical laboratory testing approach to address our aim. We undertook our testing program using a Dynamic Back-Pressured Shearbox (DynBPS) (plan dimensions: 100 × 100 mm; depth: 20 mm), which simulates earthquake ground-shaking conditions at a landslide shear surface [Brain et al, 2015]. We completed a total of 17 tests (7 monotonic, 10 dynamic) on a silt material (see Table S1 in the supporting information), remolded to minimize variability in shear behavior resulting from sedimentary structure, stress history, and disturbance during sampling [Burland, 1990].…”

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

“…However, recent technological advances and experimental work have led to 53 significant advances in our understanding of coseismic strain accumulation in hillslopes (e.g. Schulz 54 and Wang, 2014; Brain et al, 2015). 55…”