The two lines M5 subway underpass at cross point of the four lines underground Domodossola Railway Station 

The execution of the new twin galleries of the M5 under the four lines of Domodossola station of Ferrovie Nord Milano (F.N.M.), already operating, represented probably the most significant and hard point of the whole M5 line. The new galleries were located at a short distance from the rails of F.N.M. trains beacause it was necessary to keep on the track elevation and also to avoid that the tunnel section would be totally submerged by the water table. At the moment of the works, the water table (108,05 m a.s.l.) submerged the lower half of the tunnels. The crown excavation reinforcing, by means of double overlapping sub horizontal mono fluid jet grouting columns and umbrella arch steel pipes, resulted the proper solution. This method brought about relevant time saving, if compared to the first solution envisaged by means of grouting from two small drifts, executed in advance. Actually the drifts should have been consolidated beforehand and the grouting mixture at such a short distance from the rails of FNM ( 4 meters in the crown area) would have created various problems of leakages in the rail ballast. The jet grouting mono fluid system, in spite of the initial hesitations, has definitely solved this difficult excavation under railways normally operating. It hasn’t’ been easy and the success was obtained by some important factors such as: – the monitoring system of the rails and of the FNM structures that directed, step by step, the sequence of the jet grouting, the excavations behavior and setting of temporary linings; – the very accurate preliminary tests able to identify the most proper jet parameters; – a rigorous timing of the execution of the columns, organized at a proper elapse of time, with a maximum limit for each day, so as to reduce the uplift of the rails; – the prewashing with water under pressure (about 250 bar with a withdrawal rate of 4 cm each 3 seconds of time) in the upper crown columns, with the purpose of removing the finest grain size of the soil; – an accurate control of the spoil return flow (weight density and bleeding); – the immediate stop of the jet grouting operations in case of lack of spoil return, followed by a cleaning of the hole; – the good team spirit arisen among the general contractor, the specialized firm, the designer and the consultants. Uprisings during the jetting operations and successive settlements during the excavations were carefully and continuously monitored: they were contained within the limits imposed by Ferrovie Nord Milano. So the railway traffic of the 4 lines (Malpensa Express and Suburban Transport Services) was always operating at the prefixed speed (80 Km/h). The only one slowing down, imposed by an overtaking of the alert zone, lasted just one morning: it was necessary to tamp the ballast of the Malpensa Express line no 1. The maximum uprisings, obtained during the jetting of upper columns around the crown zone, reached values higher than 10 mm, but the values of the uprisings and the twists set by F.N.M. never exceeded ( except that only one event above mentioned). The lower zones of the tunnels under the water table were treated with traditional system of grouting by injecting in the ground cement and silicate mixtures through manchette pipes. The drilling and grouting were executed from an elevation of 30÷50 cm above the water table (108,05 m a.s.l.). The result of that treatment was good, without upraises neither water leakages. The invert excavation did not produce significant settlements. At the end of the work, the rails of F.N.M. were replaced almost at the original elevation, as showed on the diagrams of the paper.