Metodi per la valutazione della stabilità interna dei terreni granulari

Recenti studi teorici e sperimentali hanno evidenziato come la progettazione delle transizioni filtranti sia particolarmente complessa nel caso di terreni di base granulari internamente instabili. In tali condizioni i criteri attuali di dimensionamento dei filtri non risultano sempre affidabili. La valutazione della stabilità interna dei terreni granulari diventa quindi uno dei punti fondamentali nella progettazione dei filtri. Nell’articolo, al fine di verificare l’instabilità interna di terreni granulari tramite i criteri di Kezdi, Sherard e Kenney e Lau trasformati in pendenze secanti limite da Chapuis, è stato proposto un metodo sperimentale consistente in prove di filtrazione a lungo termine eseguite tramite un permeametro a parete rigida utilizzando un filtro di geotessile nontessuto a porometria molto piccola

Valutazione della stabilità interna dei terreni granulari

The evaluation of the internal stability of a granular soil in contact with a filter is a very important aspect for the design of a filtering system. The erosion of the base soil takes place when the size of the constrictions of the filtering transition are too large and therefore are not able to retain the particles of the base soil entrained in the flow. This phenomenon produces significant volume changes within the soil and the resulting deformation may not be admissible with the limit state of the structure. In some cases, the phenomenon can lead to failure of the granular structure in contact with the filter with the consequent associated damage. In the paper, the main methods available in the literature to evaluate the internal stability of granular soils are described and critically discussed. Therefore, the recent research developments on internal stability achieved by the authors are illustrated. In particular, two methods, considering the grain size distribution of granular base soil, both validated by long term filtration tests, are described in detail

A new theoretical method to evaluate the internal stability of granular soils

The geotextile filter design is particularly complex when granular base soils are internally unstable. In these conditions, the design criteria available in literature are not always reliable. This paper deals with a new theoretical method developed to evaluate the internal stability of granular soils. To simulate, theoretically, the filtration process inside these soils, a set of spherical particles and different soil relative densities have been considered. The soil has been represented by means of a sequence of parallel layers, containing constrictions and particles, placed upon each other at a distance, in the direction of hydraulic flow, which is a function of the soil relative density. The movement of the fine particles through the different soil layers has been simulated by means of a mechanism that compares each particle contained in the i layer with the constrictions contained in the next i + 1 layer. The results of the numerical simulations were used to evaluate the internal stability of the analyzed granular soil and the corresponding critical diameter of suffusion, Dc. Finally, the reliability of the proposed theoretical method was evaluated by means of the results of experimental long-term filtration tests performed using a rigid-wall permeameter on different unstable granular soils

A new theoretical method to evaluate the upper limit of the retention ratio for the design of geotextile filters in contact with broadly granular soils

The geotextile filters design is based on the retention and the permeability criteria. In particular, the retention criterion, that a filter must satisfy in comparison to the base soil, is commonly expressed as OF D85, where OF is the geotextile characteristic opening size and D85 is the soil particle diameter corresponding to the 85% of the passing soil mass grain size distribution. As consequence, the filter should retain only the larger particles of the base soil. This criterion works if the larger particles retain the smaller particles and this condition is verified when these particles form the solid skeleton (constituted by interconnected particles that transfer the stresses). When the granular soil has a broadly grain size distribution, with uniformity coefficient greater than 3, the larger particles generally do not belong to the solid skeleton but they are “immersed” in the smaller particles matrix that constitutes the solid skeleton (Giroud, 2010). So, for broadly granular soils, if the retention criterion previously defined is satisfied, this condition does not guarantee that the whole base soil is retained. In fact, the base soil could be subjected to an internal erosion phenomenon if the geotextile filter characteristic opening size is too large and the larger particles retained by the filter are not able to retain the smaller particles of the base soil and a hydraulic flow of dragging exists. The paper focuses on the development of a new theoretical method that, starting from the base soil mass grain size distribution and from its relative density, determines the upper limit value of the geotextile filter characteristic opening size, OF, to be used in the retention criterion in order to avoid the internal erosion of broadly granular base soi