Lowering the tracks that cross the city of Andria is an important step in the renewal of railway transport in the northern region of Puglia in Southern Italy. The project, commissioned by Ferrotramviaria S.p.A., the company that oversees 70km+ of the railway line of the Ferrovie del Nord Barese, is part of the more ambitious project to upgrade rail links in the metropolitan area of Barletta, Andria and Trani (BAT province). When finished, the infrastructure is destined to improve mobility within the province since it will facilitate and speed up the public transport system. The project entails the excavation of a trench under the old line of the single track railway of a total length of 2,932.81m which was assigned to the company CMB.

Hard, slightly fractured rock

As per project specifications, the trench must have a depth of between -5m to -11m from the surface level, but at the level of the water collection tanks the depth reaches over -16 m. Arriving at the bottom of the trench means crossing rocky layers with a high rock hardness and density due to a relatively low degree of fracturing (average 50%-75% on the RQD index). In some areas, the grey dolomite comprises of large erratic rocks with high density and hardness that can even exceed a compressive strength of 150MPa. Given the project time frames, the excavation of the nearly 3km trench had to be completed in a 560 day time frame. Due to the site’s geological characteristics, engineers also needed a retaining wall to be built for consolidation before the start of excavation.

The presence of nearby roads, and in some points buildings close to the track, called for a solution to counteract the thrusts on the trench walls. The system consists of micro piles, 190mm or 220 mm in diameter, reinforced with HEB beams, and in some points with a row of passive soil nails. In the work sequence, excavation is performed as the retaining system advances, with the wall progressively sprayed with fibre reinforced shotcrete; finally, the excavated trench will be cast with a definitive lining.

Since blasting was impracticable due to the urban setting, CMB initially considered using milling machines and roadheaders. However, given the prevalence of red soils and the uneven stratification of the rock at the base of the trench, this was eliminated as an option. At that point, the decision was made to use hydraulic hammers, provided they be high powered and in suitable numbers to meet the site requirements.

Calling in the professionals

The size of the excavation involves the removal of over 250,000m3 of material, with the rock hardness requiring a subcontractor with the experience and equipment to manage the works and related logistics.  After a careful selection, CMB chose Multiscavi srl based in Andria, which has an extensive fleet and the suitable experience for the job. Sebastiano Liso, manager of the company along with his brother Vincenzo, said of the contract: “A trench of these proportions, and the time allowed for excavation, implies a very precise technical and economic evaluation. There are several challenges to meet on this job site, starting with the hardness of the rock, which isn’t the only aspect that we evaluated carefully; organisation and logistics have a big role in this project too.”

Reliability and productivity

The rocks along the section have a strength of 35-150 MPa, providing a test of the hammers’ power and performance. Sebastiano Liso commented: “We’ve been using Indeco hammers for a few years now. At the Andria site, we’re using an HP 7000 coupled to a CAT 340 excavator and two HP 5000 coupled to CAT 330 excavators. This application immediately presented a challenge to prove the reliability of both the machines and attachments; the hammers, especially, were constantly submitted to stress.”

Helping provide productivity were Indeco’s solutions for its hammers. The HP 7000 and the two HP 5000 used for excavating the trench feature a special hydraulic system that automatically varies energy and blows according to rock hardness. The hydraulic power delivered by the excavator is thus ensured. The dual damping system helps to minimise stresses to the boom and subject it to the least amount of vibrations possible along with providing the optimum ratio between weight and power.