EQUIPMENT MOBILISATION AND LOGISTICS

Before beginning any project it is necessary to prepare the jobsite, paying special attention to entry and exit pits, where the pipe will be welded and then pulled into the tunnel.

PILOT DRILLING

It consists of drilling a borehole following the layout designed previously, thus connecting entry and exit pit and by means of a special directional bit and flexible rods whose orientation can be changed if required.

Bit orientation is real-time controlled by means of steering systems.

Directional bit is pushed into the ground from the entry pit. As it is connected to the train of rods through which drilling mud is injected, drilling operation keeps going on. To maximise eroding power it is necessary to increase the mud injection capacity. This complements the bits eroding capacity but also removes detritus out of the tunnel up to the entry pit.

In hard soils it is mandatory to drill using mud-motors, which powers the drilling bit when facing high-difficulty soils.

STEERING SYSTEMS

The navigation helps to ensure precisely, and every moment of the day, the location of the top of perforation, his inclination, and other information like temperature and so on.

• Radio systems: Wireless technology extremely used in HDD operations that can reach up to -15m depth, which is the maximum threshold waves can reach.
• Cable systems: Unlike the previous one, in this technology the drilling bit is connected through a cable, thus maximising the depth threshold up to -42m.
• MGS systems: In this case, the drilling bit can be detected thanks to a cable ring that is spread over the surface just above the drilling layout. An electric flow is powered through this cable, then generating an electro-magnetic field that let the steering technician follow up the bit movement not being mandatory to stand above it.
• Gyroscope

REAMING OPERATIONS

Normally performed in opposite direction by pulling the reamer into the borehole and once the pilot one has been drilled. Reamer is assembled to the train of rods at the exit pit. This process can entail more than one step, depending on the final diameter pursued.

Once the final diameter has been reached and before installing the pipeline it is usual to clean the tunnel and verify whether there are no sub-diameter points i.e. no obstacles for pipe installation.

MUD TREATMENT

Mud is made of bentonite, water and detritus mixed. It is pumped from the exit pit towards a recycling unit that separates fluid from detritus. This fluid can be used more times afterwards whereas dry detritus are thrown away.

• Detritus treatment (solid) after being separated, detritus are thrown into containers that are taken to special waste-treatment centres.
• Mud treatment (liquid)In this case, dried detritus can be either taken to special waste-treatment centres or used for other works at jobsite. Water can also be either thrown into the sewage system or used for other purposes at jobsite.

POLYETHYLENE PIPE WELDING(PE)

Butt welding is a technique used for connecting PE80/PE100 pipes and accessories whose diameters are 63mm or higher. It consists of smoothing ends down, melting them (210±10 ºC) by a heat source and pushing them against each other (DVS-2207- standardised force values).

Once the pipelines are connected inner burrs are also smoothed out before welding more pipes.

HDPE 100 has very beneficial characteristics such as:

• Optimal flexibility
• Resistant against weather agents as well as aggressive soils.
• Negligible head loss.
• Resistant against freezing.
• Neither burrs nor sediments on his inner face
• Resists any pulling force.
• Low density
• Excellent manoeuvre
• Easy to weld
• No damages throughout the time

PIPE INSTALLATION

Once all the stretches have been welded, the resulting pipeline is connected to a reamer and pulled into the borehole smoothly and continuously. Friction is minimised thanks to mud suspension. Finally, the pipeline is placed with no tensions and sustained by the mud suspension.