The construction of the tunnel Kennedy in Santiago de Chile. A major challenge of an urban tunnel in soils

Costanera Norte is an urban expressway concessionary in Santiago de Chile connecting its Western and Eastern sides. To enhance this connectivity several measures were necessary, including four new tunnels. The most singular one is 'Tunnel Kennedy', that has a length of 1,166 m with two sections (4 and 5 lanes), 17.45m and 20.95m wide, and 11.8m height. The tunnel was excavated in alluvial gravels, partially under the water table. The main challenges were: its section over 250m 2 , the ground conditions (requiring SEM), its urban environment and low overburden (10 to 16m). The support consists in a 3cm sealing, HEB steel arches spaced 1.0m and 27cm shotcrete. Systematically buttress and occasional canopy tubes have been used. A concrete lining (20cm to 40 cm, depending on the water column), was added, and a third layer 10cm with polypropylene fibers for fire protection. The excavation took place from October 2014 to February, 2017. The typical cross section of the tunnel provides four lanes, 3.5 m wide each one, and two shoulders of 0.75 m respectively. Below the pavement a maintenance gallery is provided. The inner dimensions with respect to the lining are 17.45 m wide and 11.30 m height. At the beginning of the tunnel, there is a connection lane so for around 50 m, a special section of five lanes have been constructed. For this section, the inner dimensions are 21.0 m width and 11.8 m height. An additional section was also envisaged, which was called “4.5 lanes”, designed in order to soften the transition between the sections of five and four lanes. Figure 2 shows the geometrical definition of the two existing sections of the tunnel. Figure 2. Functional sections (4-lanes and 5-lanes). Kennedy Tunnel. As mentioned before, three emergency exits have been constructed for the evacuation plan of the tunnel. The first one is located close to the access structures at the portal. The second and third ones were solved by shafts of 8,3 m diameter from surface and pedestrian gallery connections. 2 GEOLOGICAL AND GEOTECHNICAL CHARACTERISTICS The entire tunnel has been excavated in the well-known alluvial materials composed by gravels and pebbles of the second and first deposition of the Mapocho River. In the lower part of the tunnel, the presence of weathered andesites and lutites of Abanico Formation, was expected between Km 1,046 and Km 1,050 in the invert of the tunnel section. However, just some andesite blocks have been excavated In terms of the soil parameters, Table 1 shows strength and deformational parameters considered for the support calculations. Table 1 Strength and deformational parameters of the soil in the Kennedy Tunnel. SOIL PARAMETERS GEOTECHNICAL UNIT ANTHROPIC FILL 2nd DEPOSITION 1st DEPOSITION γ (kN/m 3 ) 17,0 22,5 22,5 Cmax (kPa) 10 25 35 φm (°) 33 45 48 Ec (kPa) 20.000 42.000xZ0,5(Z<17) 42.000xZ0,55(Z<17) (*) 55.000xZ0,53(Z>17) v 0,30 0,25 0,25 Ψ (°) 0 12 12 K0 0,43 0,70 0,9-0,0533X(Z-6)(6