Aftershocks and the whole-life seismic performance of granular slopes

Abstract

Shallow embankment slopes are commonly used to support elements of transport infrastructure in seismic regions. In this paper, the seismic performance of such slopes in non-liquefiable granular soils is considered, focusing on permanent movement and dynamic motion at the crest, which would form key inputs into the aseismic design of supported infrastructure. In contrast to previous studies, the evolution of this behaviour under multiple sequential strong ground motions is studied through dynamic centrifuge, numerical (finite-element, FE) and analytical (sliding-block) modelling, the centrifuge tests being used to validate the two non-physical approaches. The FE models focus on the specification of model parameters for existing non-linear constitutive models using routine site investigation data, allowing them to be used routinely in design and analysis. Soil-specific constitutive parameters are derived from shearbox and oedometer test data, and are found to significantly outperform existing empirical correlations based on relative density, highlighting the importance of specifying a suitably detailed site investigation. An improved sliding-block ('Newmark') approach is also developed for estimating permanent deformations during preliminary design, in which the formulation of the yield acceleration is fully strain-dependent, incorporating both material hardening/ softening and geometric hardening (re-grading). The site-specific (improved) FE models and the new sliding-block approach are shown to outperform considerably existing FE parameters and sliding-block models in capturing the permanent deformations of the slope under virgin conditions, and further, only the improved FE and sliding-block models are found to capture correctly the behaviour of the 'damaged' slope under subsequent earthquakes (e.g. strong aftershocks). The FE models can additionally accurately replicate the settlement profile at the crest and quantify the dynamic motions that would be input to supported structures, although these were generally overpredicted. The FE procedures and sliding-block models are therefore complementary, the latter being useful for preliminary design and the former for later detailed design and analysis.

Modelling the seismic performance of rooted slopes from individual root–soil interaction to global slope behaviour

“…For both root-reinforced and unreinforced slopes, a decreasing trend of settlement was observed when the slope was subjected to successive identical motions. This can be associated with the slope geometry change (re-grading), as previously proposed for fallow slopes by Al-Defae et al (2013). Dynamic motions observed from the centrifuge tests will be discussed alongside results from numerical simulation later in the paper.…”

“…The slopes were constructed as a uniform deposit at a relative density of I D ¼ 55-60%, and had a height of 2·4 m from toe to crest, with a further 0·8 m of soil underneath. The slope angle was 27°(1:2), and it has been shown by Al-Defae et al (2013) that a slope of similar angle was statically stable, yet with a low enough factor of safety (or yield acceleration) to maximise the magnitude of slip displacement during strong ground motion. The model configuration and instrumentation are shown in Fig.…”

“…In this way, the performance of the boundary deformation in the ESB container can be controlled by the adjacent soil. This same procedure has previously been used by Al-Defae et al (2013).…”

“…This specific constitutive model has been verified to be effective at simulating the dynamic behaviour of HST 95 sand (Al-Defae et al, 2013). Model parameters used in the analyses are summarised in Table 4.…”

“…These smeared properties are finally used within a fully dynamic FE model in the time domain to simulate the global seismic response of the slope. This will build on procedures for the seismic response of fallow slopes presented by Al-Defae et al (2013). The BNWF soil-root macro-elements are validated against a series of large direct shear tests with stress conditions mimicking those in the centrifuge models, and the whole procedure (macro-elements to smeared properties to dynamic FE modelling) will be validated against the centrifuge test data.…”