Fabio Gabrieli

Use of an up-scaled DEM model for analysing the behaviour of a shallow foundation on a model slope

This paper presents a methodological approach for the DEM modelling of geotechnical problems. The approach is based on quite general principles, which are illustrated with reference to a specific problem, i.e. the reproduction of a physical model of a foundation on a sandy slope. The approach mainly consists in the reproduction of the involved soil using a small, but statistically representative, assembly of spheres characterized by the same porosity and a slightly simplified grain-size curve. The DEM parameters are calibrated on the base of some standard compression tests on the same material utilised in the physical model. The thus calibrated DEM model is finally utilised to reproduce the tests on the model foundation, but, to limit the computational effort in this latter phase, an up-scaled grain-size curve is adopted and the corresponding DEM parameters are determined using the scaling rules provided with the approach. The performance of the numerical model in predicting the experiments is assessed by comparing both global results (foundation load–settlement curves) and local measurements (strain field). Moreover, the DEM model is finally used to test the foundation behaviour in some different loading conditions that could not be investigated in the laboratory.

Landslide Displacement Monitoring from Multi-Temporal Terrestrial Digital Images: Case of the Valoria Landslide Site

This work presents a low-cost method to measure the displacement of some points on the landslide surface. The method uses a sequence of terrestrial digital images. The Image Acquisition System (IAS) consists in a remote connected DSLR camera and controlled by software. The camera is fixed on a stable pillar, inside a transparent box and it periodically takes the pictures of the landslide. In order to rectify the image and to correct the little natural displacement of the image plane with respect to the landslide some fixed reference points are selected in the images. Moreover, some “well recognizable” optical targets are used in order to evaluate the displacement field. The image sequence is analyzed with a home-made digital image correlation code. The colour and size of the optical targets were evaluated in order to get the maximum accuracy of measurements and to improve the auto-matching function between the images. Taking pictures with bracketing function was proved to help the target searching phase for different weather conditions. The possibilities of the method are evaluated with reference to a sequence of images taken at the Valoria landslide site (Northern Apennines, Italy).

Collapse of quasi-two-dimensional wet granular columns

This paper deals with the experimental characterization of the collapse of wet granular columns in the pendular state, with the purpose of collecting data on triggering and jamming phenomena in wet granular media. The final deposit shape and the runout dynamics were studied for samples of glass beads, varying particle diameter, liquid surface tension, and liquid amount. We show how the runout distance decreases with increasing water amount (reaching a plateau for

Discrete particle simulations and experiments on the collapse of wet granular columns

w>1 \%

Collapse and runout of granular columns in pendular state

) and increases with increasing Bond number, while the top and toe angles and the final deposit height increase with increasing water amount and decrease with decreasing Bond number. Dimensional analysis allowed to discuss possible scalings for the runout length and the top and toe angles: a satisfying scaling was found, based on the combination of Bond number and liquid amount.

Influence of Mixture Composition in the Collapse of Soil Columns

Small quantities of liquid in a granular material control the flow dynamics as well as the triggering and jamming phases. In order to study this problem, some experimental collapse tests conducted in a rectangular box were reproduced with a 1:1 scale numerical model using the Discrete Element Method. In simulations the effect of the capillary bridges has been investigated implementing a mid-range attractive force between particles based on the minimum energy approach. Also a bonding-debonding mechanism was incorporated in the algorithm and the volume of each sessile drop on the particle surface was considered during its motion. The influence of some variables was investigated with respect to the final slope profiles and the runout lengths: the initial liquid content, the particle size, the solid density, the liquid surface tension, and the liquid-solid contact angle. Also the crucial effect of the confinement walls on the collapse phenomenon was investigated: wet particles adhere to the lateral walls prov...

Micromechanical modelling of erosion due to evaporation in a partially wet granular slope

It is well known that even small amounts of liquid can strongly modify the mechanical behavior of granular packings in static and dynamic conditions. This experimental work, therefore, focuses on the collapse of columns of wet granular materials in the pendular wetting regime. Different from previous studies, where idealized spherical materials (glass beads) are typically used, here experiments on irregular wet calcium carbonate particles (coarse sand) were carried out and compared with glass sphere results. Particles of different sizes (in the range 0.8-5 mm) were mixed with water from 0% to 4% w/w and poured in a rectangular box. Flow was then triggered by removing a lateral wall of the box. The measured runout distances showed marked differences between the two types of materials which could not be explained only in terms of particle shape or capillary forces. Ring shear tests and 3D tomographic reconstructions of the liquid distribution in the materials highlighted the role of additional mechanisms related to liquid spreading at the particle surface, inter-particle friction, and contact lubrication.

Capillary force and rupture of funicular liquid bridges between three spherical bodies

The collapse and consequent spreading of a column of granular or cohesive material is a simple experiment used by many research groups to study the rheology of the soils and for calibrating numerical propagation models. This paper deals with the results of a comprehensive experimental program carried out with mixtures of sand, kaolin and water: the main aim of the program is to know and understand how the mixture composition influences the collapse and run-out mechanism. In particular, the run-out length and the profile of the final deposit are the two fundamental characteristics taken into consideration to distinguish each test and to find a relation with the mixture composition. Four percentages of kaolin and water are considered for the experiments and different amounts of sand are added to these matrices. The main aim is the comprehension of the role of the coarser granular material in a cohesive collapsing mass. Finally, the dependency of the final runout on the aspect ratio of the initial column is discussed.