Uso di diaframmi in calcestruzzo per la riduzione degli spostamenti indotti dallo scavo di gallerie

Ground movements induced by tunnelling in urban areas can be reduced by pre-installing a protective barrier between the tunnel and a nearby building. Acting as a passive intervention, the diaphragm wall limits the propagation of displacements, preventing structure damages with a favourable variation of the displacement field. In this paper, the effectiveness of a continuous diaphragm, made of adjacent concrete panels, was investigated carrying out plane-strain numerical analyses, assuming the building to follow the ground movements. Simplified soil conditions of a homoge-neous deposit of sandy silt were also assumed to car-ry out the parametric study. Two values of the tunnel diameters were considered and the depth of the tun-nel axis was changed to simulate the schemes of shallow, intermediate and deep tunnel characterised by cover to diameter ratio C/Dt = 1.6, 2, 3.2 respec-tively. In all the analyses, the achievement of a no-tional volume loss at the ground surface VL = 1% was imposed; the offset of the diaphragm from the tunnel axis was equal to d = 0.8·Dt, and its length was set to L = z0 + Dt, where Dt is the tunnel diame-ter. Moreover, a fully rough interface was assumed at the soil-diaphragm contact that provides lower values of efficiency, thus representing a conservative assumption (Rampello et al., 2016a). The capacity of a protecting diaphragm to reduce the ground movements was evaluated through the comparison of the displacements fields computed beyond the wall in the presence and in absence of the intervention. A local and an integral efficiency were defined, the first providing the settlement re-duction at a given distance from the tunnel axis, while the second providing an overall index of set-tlement reduction. The highest reduction of soil set-tlement beyond the diaphragm is computed for the shallow tunnels (C/Dt = 1.6), with values of loc = 91% irrespective of tunnel diameter. Local ef-ficiency reduces with increasing distance from tun-nel axis, with settlement reduction in the range of about 60% to 80% at a distance x =2Dt from the tunnel axis. Local efficiency also decreases with in-creasing the cover to diameter ratio. This can be as-cribed to the higher level of soil strength mobilisa-tion induced into the soil to get the same volume loss for deeper tunnels. Therefore, the shear stresses mo-bilised at the soil-diaphragm interface increase, this resulting in a reduction of the efficiency. Consistently with the above observations, the in-tegral efficiency, which represents an overall index of settlement reduction, decreases about linearly as the C/Dt ratio increases, providing similar values irre-spective of the tunnel diameter: from int ≈ 85% for C/Dt = 1.6 to int ≈ 70% for C/Dt = 3.2.

La realizzazione di nuovi attraversamenti sotterranei in aree densamente urbanizzate è fortemente condizio-nata dalla necessità di limitare gli effetti indotti dalle operazioni di scavo sugli edifici posti nelle vicinanze. In questa nota si valuta l’efficacia di barriere costituite da diaframmi in calcestruzzo nella riduzione degli spo-stamenti indotti dallo scavo di gallerie. A tale scopo, sono state eseguite analisi numeriche con il metodo degli elementi finiti, confrontando gli spostamenti verticali calcolati in presenza e in assenza dell’intervento. Nelle analisi si è assunto un terreno a grana fine omogeneo e si sono ipotizzate condizioni di deformazione piana, corrispondenti a un diaframma continuo indefinitamente esteso e a una galleria interamente passata; si è inoltre assunto che l’edificio da proteggere segua gli spostamenti del terreno. L’efficacia dell’intervento è stata valu-tata mediante un’analisi parametrica, confrontando schemi caratterizzati, a parità di configurazione del dia-framma di protezione, da diversi valori del diametro e della copertura della galleria.