ADECO full-face tunnel excavation of two 260 m2 tubes in clays with sub-horizontal jet-grouting under minimal urban cover

With a cover of 5–30 m, a 260 m2 cross-sectional area, and a 4-m wide pillar, the Cassia twin road tunnels underpass Cassia road (main road to/from Rome), the remnants of a 2000 year-old Roman villa, and two existing tunnels. Stiff silty clay dominates the alignment. According to the Analysis of Controlled DEformations (ADECO) principles, each tube was excavated full face to 260 m2. Compared to Sequential Excavation Method (SEM), this highly simplifies construction, leads to construction industrialization, and allows for full control of the ground ahead of the face, which is used as a stabilization measure. In order to pre-confine the tunnel core, Trevi roto-injection technique was used to create an umbrella of overlapping sub-horizontal reinforced jet-grouting columns ahead of the face, while avoiding up heave of the ground or emptying of the columns. Stiff preliminary lining sitting on sub-horizontal jet-grouting columns, final invert close to the tunnel face, and final lining no more than 50 m behind the face ensured that the pre-confinement ahead of the tunnel face became effective confinement around the cavity. The tunnels were finished on schedule and within budget with a mere 1 cm of settlement.