Crossing beneath the tram tracks by horizontal directional drilling

Objective

A new underground medium-voltage line had to run along the streets near Jaén station and cross the roundabout at Plaza Jaén por la Paz. The tram platform runs through the middle of the roundabout. Opening a trench over the tracks was not an option: the line had to cross beneath them.

Difficulties

• The tram tracks. The crossing could not interrupt or damage the tram platform, and the bore had to pass well below it.
• A crowded subsoil. Sewerage, water, gas, telecommunications and electricity networks run beneath the roundabout and its approaches. The route had to avoid them in plan and in elevation.
• No ground data. The cost and feasibility of a directional bore depend on how hard the ground is, and there were no tests at the roundabout.
• Traffic and space. The roundabout is a busy junction, and the launch and reception pits had to fit into the green areas and pavements at the sides.

Solution

Before designing the crossing we located the services with ground-penetrating radar and radio detection, and in June 2021 we carried out three DPSH dynamic penetration tests at the roundabout itself to establish the ground conditions.

With that data the pilot profile was defined: the bore enters from the green area on the west side at a gradient of −27 %, drops to about 6 m deep, runs some 70 m at constant level beneath the roundabout and the tracks, and rises to the reception pit on the east side. In all, 114 m.

After the pilot bore, the hole was reamed to 710 mm with bentonite mud and a bundle of four 200 mm HDPE pipes and a 63 mm guide was pulled through. Inspection chambers were built at the launch and reception pits to connect with the trenched ductwork.

The rest of the line was laid in trenches along the surrounding streets. Every service found on the route was protected or reinstated, and where necessary trial pits were opened to find the best way through.

Result

The line crosses beneath the roundabout and the tram tracks through the four pipes installed by directional drilling, without any trench being opened over the tram platform. The pipes are ready to take the medium-voltage cables and are connected to the trenched ductwork on both sides.