Rational design of jet grouted bottom plugs: The example of Tribunale station of Naples Metro Line 1

When planning an open excavation in granular soils below ground water level, one of the main design concerns refers to water seepage from the bottom. Such a seepage may cause local piping, subsidence of the surrounding area with undesired settlements of nearby buildings, or a water inflow which may interfere with constructional operations. The adoption of continuous jet grouted bottom plugs is by far the most effective way to solve this critical design issue, and its use is constantly increasing in underground works. In this paper, the example of Tribunale station is reported and discussed. It is shown that rational design can be best carried out using a deterministic approach to cope with code indications on equi-librium (i.e. uplift and structural cracking), while the choice of the spacing among columns cannot be but probabilistic. By using Jetplug, a numerical code developed at the University of Naples Federico II to simulate probabilistically the behavior of a plug made of any number of columns, it was possible to estimate the water inflow through the plug as a function of the spacing among columns. By assigning the maximum tolerable inflow, the spacing could be selected on a cost effectiveness basis, assuming a risk compatible with Eurocode’s indications.