Excavation nearly finished at Herradura

Works are still on target for excavation to be finished this month for the 5.7km long Herradura tunnels in Spain that form a major part of the Mediterranean Highway scheme between Cadiz and Barcelona.

The Cantalobos, Calaceite and Gato sections of twin tunnel driven at 25m separation are 2,198m, 386m and 281m long respectively. Of these, 650m corresponds to sections of backfilled canopies at the portals. In addition, the Cantalobos tunnel features four cross passages for pedestrian egress in case of fire at 300m centres, two vehicular cross passages 900m apart, two 40m long 2.5m wide laybys per tube, three equipment rooms and 12 refuges per tube at 150m centres.

Contractors UTE La Herradura-Ploder, Azvi and Obras Subterráneas are constructing the tunnels to an internal width of 12.5m, with face areas of 105m2. The tunnel cross-section consists of a 6.3m radius arch in the crown, centred 1.85m above the roadbed. The 10.5m wide running surface is flanked on both sides by 0.75m wide pavements.

T&TI was told by a source close to the project that the tunnels are technically quite challenging as they are being driven through rock that is highly faulted and has experienced high tectonisation. With a rock mass rating varying from 30 to 70, the ground is characterised as shaly with some quartz and mica. Sandstone and quartzite was also be encountered along the alignment.

A 300mm thick concrete lining is cast using a metal shuttering. This is cast over the primary support that can vary depending on the ground to include: shotcrete with metal fibres or over mesh; 5m long 25mm diameter rock bolts either resin grouted or expanding friction anchors and arches.

The excavation sequence was for a top heading followed by benching for the bottom heading. The top heading is driven at a height of 5.5m and generally the cycle is for top heading, then bench, then repeated. Drill and blast as well as mechanical excavation by means of a hydraulic hammer fitted to an excavator have both been used. For face drilling, two Tamrock Axera T11 Data fully automated three-boom rigs and one Tamrock Axera T08 TCAD two-boom semi-automatic rig have been used on the project. The rigs have also been used for rockbolting and drilling the cross passages.

T&TI was told that typical advances on the project have been 12m per day in the headings based on three 8 hour shifts and 18-20m of benching.