The aqueduct supplying water to the town of Riva del Garda is fed by a spring which rises in the mountains above the town. The water was previously piped down to the valley via an old steel conduit, now in poor condition.
The narrow diameter of the original pipe meant that the quantity of water it could supply was insufficient to meet the rising demand from the town. The supply pipe had been laid some decades earlier using open trenching adjacent to an existing path along a semi-vertical rock face.
The proposed solution to replace this system required the contractor to lay a new, larger diameter pipe with the least possible disruption to the surrounding environment, using technology that would enable the local authority to minimise costs as much as possible.
The consultant, Studio Zanetti De Iorio of Italy, proposed the use of horizontal directional drilling (HDD) for laying the new supply pipe. Two important considerations informed the designer’s choice of HDD: the minimal impact it would cause to this area of outstanding natural beauty, and the financial savings it brought in comparison with the other technologies.
The tender was then issued by the main contractor, the local council of Riva del Garda, and the contract was eventually won by the HDD specialists Trenchless Technology Italia.
Both the main high-density polyethylene pipe (PEAD) pipe of 280mm diameter and two additional reinforced control cables to regulate water flow were successfully installed in less than ten weeks. The 275m long bore had to be drilled on an almost vertical trajectory in hard local limestone and with an average pitch of 95%.
The particular problems of this project were as follows:
- The trajectory of the bore needed to have a pitch of 95% for a length of 275m. The bore would be drilled in hard limestone with a resistance of 800-1000kg/cm2, with a vertical depth gradually increasing to 105m and a gradient of 170m from start to finish.
- Access to the work site was severely limited – the lower part of the bore trajectory could be accessed by works vehicles, but the upper section could be reached only by helicopter or on foot.
- The space available for the work site was very restricted, even at the lower valley end.
- The unusual conformation of the terrain around the site meant that the degree of precision required in executing the bore only allowed for a margin of error of less than 8m to either left or right.
- The bore would be carried out from foot to summit. This would however necessitate precise checking of both the pressure and the flow of drilling fluids in the mud motor used, with regard to the type of rock encountered along the trajectory and the type of drilling rods selected.
- The entry point for the work site was to be positioned on top of the final arch, jutting out from a nearby tunnel. The loading and unloading of materials was to be carried out by an adjacent crane. The arch of the tunnel would first have to be strengthened with reinforced concrete, then the rig attached to the bare rock face by steel cables and supported beneath by a steel platform to keep it at the required upward-facing angle as it drilled.
- The maximum pitch at which the drilling rig could function effectively was less than the minimum pitch required for the bore. For this reason it was decided to begin the bore with an initial inclination of 16° rising progressively, over a bending radius of 250m, to an inclination of 45°.
- A magnetic guidance system would be used to create an electric circuit around the bore trajectory, thus preventing interference in the readings from external magnetic fields. This circuit would have to be in place from the very beginning of the bore, however, since any error in the initial direction established would be almost impossible to correct later. The system was eventually positioned on the rock face, with the help of local rock-climbing experts. This permitted the engineers a 3.5m margin of error at the bore’s exit point and a few centimetres of error in length.
- A helicopter transported pipes and other heavy materials to the summit of the rock face and was also used for the spraying of the concrete casting used to fill the space between the PEAD pipe and the walls of the surrounding bore.
The solutions to these problems were:
The rig used for the project was a Vermeer Navigator D100x120 with a pullback force of 45.3 tonnes, a torque of 16,300Nm, and a mud-pump with a maximum of 757L/m at 1100 psi. Firestick 2 type drilling rods with a specially designed internal profile to reduce the head loss are especially useful in rock drillings such as this. A recycling unit for cleaning the exiting drilling fluids was also used, along with a 5m3 mud-mixer to feed the rig’s pump.
Execution of the bore
The work was completed in spring 2004 and progressed in the following phases:
Pilot bore
A mud-motor was fitted to the rig. This consists of a rotor inside a stator similar to a drilling rod, with an angle of 2° near the front end. The stator is screwed onto a conventional drilling rod to become a single solid piece, while the internal rotor is free at the far end but fixed to the drill bit at the front end of the drill chain. When water is introduced into this adapted drilling rod the flow passing along the inside of the stator causes the rotor inside to rotate and the resulting pressure delivers the drilling power to the tricone drill bit. The drilling rods are then pulled into the pilot bore following the curved trajectory, which is set by the angled head of this stator. If a straight trajectory is preferred, the rods and then the mud-motor can be rotated as they are pushed into the bore.
Reaming
Two reaming phases were planned, the first taking the bore to a diameter of 310 cm and the second to 443cm.
Preparation and pulling of product
The PEAD pipes were welded in situ before being pulled into the bore. Two steel control cables were simultaneously laid in the bore for controlling the water flow.
Drilling fluids
Only water was used in this drilling since the flow of the waste material was determined by the trajectory of the bore itself. The wastewater was cleaned and drained as it exited the system.
Testing and fixing of the bore
Once the bore had been completed and the product laid, it was tested by introducing water into the pipe. The bore was then fixed with a cement casting introduced in two phases: first spraying the cement upwards from the foot of the rock face to its summit; then, using the helicopter, introducing the cement from above to avoid excessive pressure building up and damaging the pipe within.
The work was carried out within the timescale agreed with the contractor, apart from one interruption due to a landslide near the worksite, which held up the completion of the bore unavoidably for almost three weeks.