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Soilmec ST- 120 and special patented jet-grouting technologies at work for the enlargement of the Bologna-Taranto highway. The transport infrastructure efficiency is today a business key factor. Especially in a country like Italy in which the geographical structure, like a boot, has compelled the construction of longs and narrows highway sections. Trevi S.p.A. has been chosen to works in the project to enlarge from two to three lanes the Adriatic highway in order to streamline the communication in one of the most congested infrastructure in Italy.
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In particular, inside this wide project, Trevi is involved in the execution of "Cavallo" tunnels. To satisfy the three lanes highway needs the tunnel has an excavation radius up to 9 m, an average 200 m2 section and extends for an overall length of 510 m.
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| Soilmec ST - 120 Tunneling Machine at Job Site |
The site geology is mainly characterized by the presence of Plio-Pleistocene cohesive deposits. From a lithological point of view, the units interested from the excavation are clayey silts and silty clay with very few presence of silty sand small strata. Consistency increases from stiff to very stiff to hard with the depth. The track is very superficial due to a cover that ranges between 8 to 20 m. The dimensions of the excavation front, the poor mechanical characteristics of the encountered soils and the reduced cover would have prudently suggested the excavation be carried out via a series of partial excavations. Approaching the problem with the ADECO-RS approach (Analysis of Controlled Deformations in Rock and Soils) and using the state of the art innovative system of soil improvement, allowed the full face excavation increasing in the same time safety, improving the schedule and reducing the costs. According to ADECO analysis» the behaviour category of the tunnel was classified as C type (unstable front), so it was necessary to stabilize the excavation front by means of proper soil improvement installed in advance to artificially produce the arch effect at the outline of the excavation.
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| Soilmec ST - 120 Tunneling Machine at Job Site |
To consolidate in advance a clayey-silt formation like the one present Is a very hard problem. Trevi has proposed (and successfully completed) a patented jetting rotoinjection technique using a steel casing which is left in place as reinforcement of the arch. Moreover to be able to perform the jetting columns with the requested size, ETJ (Enhanced Trevi Jet) system has been used in the test field and applied in the following. ETJ systems foresees special monitors and other devices to focus the jetting energy, increase the desegregation of the jetting thus obtaining column diameters bigger 40% more of the standard. Fibreglass elements were used to reinforce the advance core. The rig chosen by Trevi is the Soilmec equipment for tunneling ST-120. The ST-120 main feature is the presence of 2 different masts independently controlled and powered by two electric motors and supported by 2 telescoping cylinders. The cylinders are connected to the main frame by means of 2 slewing rings. The main frame, powered by a diesel engine, is mounted on a crawler type undercarriage complete with 4 outriggers and sliding frame for positioning the slewing rings at different heights and levels in order to meet the requirements of the tunnel to be excavated. The two masts are equipped with a telescopic foot hydraulically controlled, that can be placed against the excavating face. This allows the use of rope operated crowd system/pull back to be applied to the drilling. Once the mast has been positioned as required, the consolidation treatment can be completed. This configuration allows drilling to be carried out through 180° directly connected to One of the entrance mast suPPort- The rotoinjection technology requires the use of two strings which are of "Cavallo" tunnel, simultaneously driven by two coaxial power swivels (double rotary).
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