A Ground Improvement Treatment for Two Raise Bored Elevator Shafts in a Historical Site of Central Italy-Paper

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The project created a pedestrian connection between Faul Valley and San Lorenzo Square in Viterbo, a historically significant area containing medieval buildings. It involved a 36 m base tunnel and two 25 m-high elevator shafts, each 3.13 m in diameter.<br /><br />The site’s volcanic ground consists of alternating pyroclastic soils and tuffs with variable strength, cementation, and possible cavities. To stabilize the tunnel route, engineers installed guided, sub-horizontal jet-grouted columns using directional drilling, with additional reinforcement around the tunnel and its excavation face. The shafts were also preceded by extensive jet-grouting to improve the soils to be excavated, protect nearby foundations, and support the enlarged tunnel area.<br /><br />The shafts were excavated using raise boring: pilot holes were drilled from the surface to the tunnel, then enlarged from below. This approach avoided explosives and reduced the need to transport excavated material through the historic square. Steel modular lift structures were subsequently installed in the shafts.<br /><br />A comprehensive monitoring program tracked tunnel deformation, ground movement, building settlement and tilt, façade crack widths, and vibration. Tunnel movements were small, with maximum surface settlements and horizontal displacements of about 2–3 mm. Shaft-drilling vibrations were generally 0.5–1.0 mm/s, well below the 5 mm/s limit. Building settlements remained below 1 mm, with no meaningful changes in tilt or crack widths.<br /><br />The combination of targeted ground improvement, raise boring, and continuous monitoring enabled safe construction with no practical adverse effects on the surrounding historic buildings.

Keywords

pedestrian connection

Viterbo

historical buildings

jet grouting

raise boring

elevator shafts

tunnel excavation

ground stabilization

monitoring program

surface settlement