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Mitigazione del rischio liquefazione e stabilizzazione di pendici franose tramite sistemi drenanti realizzati con perforazioni direzionate

Vanni Daniele
Articolo Immagine
Gallerie e grandi opere sotterranee
Gallerie e grandi opere sotterranee N.118/2016

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Frane e terremoti (per quanto attiene ai danni indotti dalla liquefazione dei terreni) sono senza dubbio tra i maggiori fattori causali di dissesto geologico sia per frequenza che per pericolosità. Per entrambi i fenomeni, oltre ai fattori geotecnici e geologici predisponenti, l’elemento scatenante è riconducibile alla presenza nei terreni di un eccesso di acqua che determina per i movimenti franosi lo scivolamento reciproco di strati di terreno, nel caso della liquefazione l’annullamento della resistenza al taglio del terreno a causa delle sovra pressioni che si generano nell’acqua presente nei pori. Nel primo caso (frane), la realizzazione di un efficace sistema di drenaggio preventivo è di primaria importanza per la riduzione del rischio; per la liquefazione, invece, la formazione di un reticolo drenante nel terreno in grado di smaltire le sovra pressioni è una delle tecnologie applicabili e, nel caso di interventi al di sotto di strutture esistenti, quella meno impattante e meno costosa.

Liquefaction risk mitigation and stabilization of landslide slopes with draining systems realized by means of directional drillings 

Landslides and earthquakes (as far as damages provoked by soil liquefaction are concerned) surely are some major causes for geological instabilities both in terms of frequency and hazard. For both phenomena, besides the geotechnical and geological predisposing factors, the triggering element is ascribable to the presence of excessive water in the soil which determines, in case of landslide movements, the mutual sliding of soil layers and, in case of liquefaction, the annulation of soil shear strength due to the overpressures generating in pore water. In the first case (landslides), the creation of an effective system for preventive drainage is of major importance for risk reduction; as for liquefaction, the creation of a draining grid in the soil capable of dissipating overpressures is one of the applicable technologies which, in case of operational procedures below existing structures, have less impact and are less expensive. Over the last years, companies and designers have been devising technologies and operational procedures capable of complying with different needs, among which: interventions in areas characterized by reduced spaces, compliance with more restrictive regulations concerning safety, construction of more efficient and longlasting plants, minimization of the impact of works interfering with ongoing activities. In many cases said interventions are made possible by the use of Directional Drilling, i.e. rectilinear or curvilinear, sub-horizontal, high accuracy, directional drillings having a remarkable length. In case of landslides, systems made up of medium and large-diameter base shafts (non-draining) are depicted. Said systems are capable of capturing water from the soil (both via horizontal drains starting from the walls of the shaft itself, and via grids of vertical draining shafts connected to the base shafts by means of bottom ducts) and conveying it to other shafts of the same type and, finally, to a “delivery” point by means of sub-horizontal main ducts that may reach lengths of several hundred meters. In case of liquefaction below existing structures, an innovative method is described, which allows to obtain a draining grid thanks to horizontal tubular elements (properly dimensioned in terms of diameter and distances) installed in the liquefiable layers within cross-by, sub-horizontal, curvilinear drillings executed from outside the building. In a “normal” situation, i.e with no earthquake occurring, drains are passive and they do not affect water tables. In case of earthquake, interstitial overpressures generated by seismic waves in the liquefiable layers are dissipated through the sub-horizontal drains and then discharged into suitable arranged relief devices. The Main issue is to avoid drains foulings so as to maintain drainage capacity undiminished throwh the years. To this purpose it is not possible to use traditional pvc slotted pipes. Together with a leading world drain producer, Trevi has tested and finalized for this application special porous polyethylene well screens with a homogeneous pore structure capable of avoiding fouling. All the procedure is entirely carried out in external areas so working and residential activities performed inside said structures are not impaired. This is a quite flexible solution that fits different geometries. In the presence of pre-fab industrial buildings it is also possible to execute interventions only in the footprint area of pillars’ alignment, thus remarkably reducing time, quantities and costs. For the above-mentioned applications, this paper illustrates the technology, relevant issues, geometries and execution schemes of already performed activities, as well as of those currently being performed and studied.