An experimental study of the rheological behaviour of three natural pyroclastic soils with different depositional processes remixed with water was carried out with the help of a rotational rheometer and inclined plane tests. A homogeneous fluid-like behaviour is obtained only within a very narrow range of concentrations, typically not more than 10%. Below this range the material sedimentates rapidly; above this range it behaves like a solid. In the fluid-like range the typical rheological behaviour of these suspensions is that of a yield stress fluid exhibiting a static yield stress larger than its dynamic yield stress. This effect probably finds its origin in a ‘local’ sedimentation effect: that is, the particles sedimentate just as necessary to form a structure more jammed than the structure during flow. As a result the flow of such materials is usually unstable: they will start to flow beyond a critical stress, but just beyond this value will reach a high shear rate associated with a high flowing velocity. The static and dynamic yield stresses of these materials increase widely from very low to very large values (several orders of magnitude). Inclined-plane tests were shown to provide reasonable although still approximate values for the static and dynamic yield stresses. These results suggest that in the field a small change in solid fraction will cause a slight decrease of the static yield stress, readily inducing a rapid flow that will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of pyroclastic debris flows, which are able to flow over very long distances, even on smooth slopes

Abstract.A model able to describe all the processes involved in a debris flow can be very complex owing to the sudden changing of the material that turns from solid into liquid state. The two phases of the phenomenon are analysed separately referring to soil mechanics procedures with regard to the trigger phase, and to an equivalent fluid for the postfailure phase. The present paper is devoted to show the experimental results carried out to evaluate the behaviour assumed by a pyroclastic-derived soil during the flow. A traditional fluid tool has been utilized: a standard rotational rheometer equipped with two different geometries. The soils tested belong to deposits that cover the slopes of the Campania region, Italy, often affected by debris flows. The influence of solid concentration C v and grain size distribution was tested: the soils were destructurated, sieved and mixed with water starting from the in situ porosity. All material mixtures showed a non-Newtonian fluid behaviour with a yield stress τ y that increases with a solid volumetric concentration and decreases for an increase of sand fraction. The experimental data were fitted with standard model for fluids. A simple relation between C v and τ y was obtained. The yield stress seems to be a key parameter for describing and predicting the post-failure behaviour of debris flows. These results suggest that in the field a small change in solid fraction, due to rainfall, will cause a slight decrease of the static yield stress, readily inducing a rapid flow which will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of debris flows, which are able to flow over very long distances even on smooth slopes.

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a b s t r a c tThe earthquake response of cantilever retaining walls is explored by means of theoretical analyses and shaking table testing conducted at University of Bristol (EERC -EQUALS). The theoretical investigations employ both limit analysis and wave-propagation methods, which take into account different aspects of the problem such as inertia, strength, kinematics and compatibility of deformations. The experimental programme encompasses different combinations of retaining wall geometries, soil configurations and input ground motions. The response analysis of the systems at hand aims at shedding light onto salient features of the problem, such as: (1) the magnitude of soil thrust and its point of application; (2) the relative sliding versus rocking of the wall base and the corresponding failure modes; (3) the importance of the interplay between soil stiffness, wall dimensions and excitation characteristics, as affecting the above; (4) the importance of wall dynamics and phase differences between peak stresses and displacements. The results of the experimental investigations are in good agreement with the theoretical models and provide a better understanding on the complex mechanics of the problem.

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