Shallow embankment slopes are commonly used to support elements of transport infrastructure in seismic regions. In this thesis, the seismic performance of such slopes in non-liquefiable granular soils has been investigated and an extensive programme of centrifuge testing was conducted to quantify the improvements to seismic slope performance which can be achieved by installing a row of discretely spaced vertical precast concrete piles. This study focussed on permanent movement and dynamic response at different positions within the slope, especially 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 modelling, analytical (sliding-block) and numerical (Finite Element) models. This thesis makes three major contributions.
Firstly, an improved sliding-block (‘Newmark’) approach is developed for estimating permanent deformations of unreinforced slopes during preliminary design phases, in which the formulation of the yield acceleration is fully strain-dependent, incorporating the effects of both material hardening/softening and geometric hardening (re-grading). This is supported by the development of numerical (Finite Element) models which can additionally predict the settlement profile at the crest of the slope and also the dynamic ground motions at this point, for detailed seismic design were also developed. It is shown that these new models considerably outperform existing state-of-the art models which do not incorporate the geometric changes for the case of an earthquake on a virgin slope. It is further shown that only the improved models can correctly capture the behaviour under further earthquakes (e.g. strong aftershocks) and therefore can be used to determine the whole-life performance of a slope under a suite of representative ground motions that the slope may see during its design life, and allow improved estimates of the seismic performance of slopes beyond their design life. The finite element models can accurately replicate the settlement profile at the crest (important for highway or rail infrastructure) and quantify the dynamic motions which would be input to supported structures, though these were generally over-predicted.
Secondly, the principles of physical modelling have been used to produce realistically damageable model piles using a new model reinforced concrete (both a designed section specifically detailed to carry the bending moments induced by the slipping soil mass and a nominally reinforced section with low moment capacity). This was used to investigate how piles can stabilise slopes under earthquake events and how the permanent deformation and the dynamic response of stabilised slope are strongly influenced by the pile spacing (S/B) especially at the minimum pile spacing (i.e. S/B=3.5). This is consistent with previous suggestions made for the optimal S/B ratio for encouraging soil arching between piles at maximum spacing both under monotonic conditions, and for numerical investigations of the seismic problem. These were supported by further centrifuge tests on conventional ‘elastic’ piles which were instrumented to measure seismic soil-pile interaction. The importance of reinforcement detailing was also highlighted, with the nominally reinforced section yielding early in the earthquake; the damaged piles subsequently only offer a small (though measureable) reduction in seismic slope performance compared to the unreinforced case. It was demonstrated that both permanent deformations at the slope crest (e.g. settlement) and dynamic ground motions at the crest can be significantly reduced as pile spacing reduced.
Finally, a coupled P-y and elastic continuum approach for modelling soil-pile interaction has been used to develop a Newmark procedure applicable for pile-reinforced slopes. It was observed that the single pile resistance is mobilising at beginning of the earthquake’s time and it is strongly influenced by pile stiffness properties, pile spacing and the depth of the slip surface. It was observed also that the depth of the slip surface and pile spacing (S/B) play an important role in the determination of the permanent deformation of the slope. The results show great agreement to centrifuge test data in term of the permanent deformation (settlement at the crest of the slope) with slight differences between the measured (centrifuge) and calculated (this procedure) maximum bending moments.
|Date of Award||2013|
|Supervisor||Jonathan Knappett (Supervisor)|
- Centrifuge modelling
- Physical modelling
Student thesis: Doctoral Thesis › Doctor of Philosophy