Mitigazione degli spostamenti indotti da uno scavo profondo mediante pareti sacrificali

Improvement of underground infrastructures is a key factor to enhance liveability of densely populated cities. However, the construction environment of deep excavations can be very complex because of the presence of pre-existing buildings, often of historical or architectural relevance. To ensure that nearby structures would not be damaged by ground movements induced by deep excavation, sacrificial cross-walls can be adopted as a mitigation technique. They are installed between the perimeter diaphragm walls before the start of excavation and are removed during excavation together with the soil. The cross walls are expected to reduce the lateral wall deflection and ground settlement induced by excavation due to their high compressive strength and axial stiffness. In this paper, a 3D finite element study is presented in which the retaining walls and the cross-walls are modelled using tetrahedral elements. In the analyses, a portion of the box excavation was modelled extending between the centre line of two adjacent cross-walls and the plane of symmetry of a cross-wall. Excavations were modelled with and without sacrificial walls of fixed length, installed at different spacing. To account for the effects of the excavation time, the excess pore water pressures were partially allowed to dissipate after each excavation phase: a 2 months consolidation stage followed each phase, except for the last one, for which the consolidation time was set equal to 3 months. Influence of cross-walls spacing in reducing the ground movements is evaluated for a 42 m deep excavation, assuming a soil profile typical of central Rome. Analyses were carried out in terms of effective stresses. The results show that sacrificial cross-walls induce a substantial reduction of diaphragm wall deflections, ground surface settlements behind the excavation and bottom heave. The efficiency of the system, which provides a direct measure of displacement reduction, increases with decreasing cross-wall spacing: the maximum efficiency computed for the horizontal displacement of the perimeter diaphragms is equal to about 74% for s/B = 2.8 and 58% for s/B =4.7, while ground surface settlements are reduced of as much as 77 to 90%. This latter reduction extends to a distance of about 1.4 times the maximum excavation depth behind the diaphragm wall. Heave of the bottom of the excavation was also seen to reduce, in the range of 25 to 40%. Cross-walls are also beneficial in increasing the available shear strength below the bottom of excavation, thus reducing the risk of basal heave failure. Finally, in the presence of cross-walls, long term effects due to the dissipation of the negative excess pore water pressures induced by the excavation are sensibly reduced.

Per lo sviluppo delle infrastrutture è sempre più frequente il ricorso ad opere in sotterraneo, specialmente in area urbana, dove la densità del costruito rende difficile la realizzazione di nuovi attraversamenti in superficie. Tuttavia, l’esecuzione di scavi profondi in contesti fortemente urbanizzati è condizionata dalla necessità di limitare gli spostamenti indotti negli edifici preesistenti, specie se di rilevanza storico-monumentale. Tale obiettivo può essere perseguito mediante l’impiego di pareti sacrificali (cross-wall), che agiscono come puntoni preinstallati e che vengono costruiti tra le paratie perimetrali prima dell’inizio delle fasi di scavo. In questa nota, si discutono i risultati di alcune analisi numeriche, svolte in condizioni 3D con il metodo degli elementi finiti, per valutare la riduzione degli spostamenti indotti da uno scavo profondo in presenza di sistemi di pareti sacrificali di fissata lunghezza, ma installati a diverso interasse. Nelle analisi, eseguite in termini di tensioni efficaci, si è tenuto conto dell’effetto dei tempi di scavo, assumendo che a ciascuna fase di scavo non drenata segua un processo di consolidazione per intervalli di tempo definiti e compresi tra due e tre mesi.