The 32 km Flaibano – Gonars pipeline has a 30 inch nominal diameter pipe. It runs through the province of Udine in the Region of Friuli Venezia Giulia, located northeast of Italy. The focus of this article is the technical solutions adopted during the design phase as well as the environmental monitoring performed during the construction phase of the microtunnelling. Design and ground conditions

During the design phase, a geognostic survey was performed to determine which drilling method was best suited to the geotechnical features of the land to be crossed. Besides the geotechnical characteristics of the soil itself, this survey also revealed two water tables, separated by limited impermeable layers.

From a technical point of view, the trenchless crossing was designed to safeguard and comply with the fauna and groundwater protection of the two areas; the site of community importance ‘Paludi di Gonars’ and the regional biotape ‘Paludi del Corno’. Microtunnelling was deemed the most suitable technique to safeguard the hydrogeological balance of the area.

The area falls in the transition zone called ‘Linea delle Risorgive’, between the ‘Upper’ and ‘Lower’ Friuli Plans.

The Upper Plain consists of alluvial permeable sediments – gravel and pebbles with sandy or silty-sandy matrices. The rainwater penetrates the subsoil and descends until it reaches the impermeable layer, which blocks its path, creating the aquifers which flow slowly toward the sea.

In the Lower Plain, alluvial sediments were found of fine sand and clay alternating with irregularly distributed gravelly-sandy impermeable sediments. The alternate permeable and impermeable layers causes the local upwelling of groundwater, and creates the characteristic ‘rogge’.

The geological and geotechnical survey provided the following information:

To the south of the town of Gonars, within the gravelly alluvional complex, there is a surface district with soils typical of a natural marsh (The Corno Wetlands);

In the SIC area the soils consist of:

1. A 6 m thick surface layer made up by clayey silts and sandy silts, at times organic; 2. An underlying layer made up of sandy-gravelly deposit containing stones (maximum diameter 8-10 cm), running to at least the maximum depth of 34 m reached.

The technical approach was based on information gathered from the geotechnical survey, aiming to avoid any possible risk of environmental pollution. A totally sealed starting pit was created to prevent any contact between the surface and deep aquifers. Plastic baffles were arranged at the points crossing through the clayey-silty layer, located above the gravelly-sandy aquifer. A variable hydraulic balance was maintained during the various phases of excavation and the drilling head operating pressure was kept in equilibrium according to the external pressure detected along the route.

During excavation, a bentonite film was created to insulate the dig. Particular attention was paid to the points of lithological changes, thus preventing the various aquifers from intermingling. The cavity between the pipeline and the microtunnel was completely plugged to prevent any possible passage of water on the inside and the entire yard was marked off with sound-proofing panels in order to prevent any disruption to the fauna present in the area. Moreover, in order to limit interference with the aquifers, drilling was completed without an exit pit and the drilling head was only recovered after the microtunnel and the soil had been fully sealed with special plastic mixtures.

The microtunnel testing ground

The microtunnel diameter was sized to allow, in the event of tunnel jacking blockage, a visual inspection and the use of a machine equipped with a ‘push-module’. The jacking shaft was sized in compliance with Snam Rete Gas recommendation. A solid contrast ground fill was performed after the construction of the starting pit was completed.

The cutting head – designed according to the characteristic particle size distribution curve – is a mixed head with special partialisation plates to ensure better control over excavation front stability. In order to prevent any risk of drilling being blocked, shifts were set to guarantee steady work 24 hours a day.

During the drilling phase groundwater monitoring was conducted in two phases:

Phase 1: Monthly piezometer level readings (June 2006 December 2007) on piezometers PZ1, PZ2 and PZ3.

In the PZ1 two levels of aquifers were identified; a superficial aquifer in the sandy-silty level and an underlying semi-artisian aquifer. With reference to PZ2 and PZ3 only a semi-artisian aquifer was verified.

Phase 2: Continuous monitoring (October 2007 – December 2007) of the water quality of both the undisturbed groundwater and the excavation area, using multiprobe units installed in wells A and B (temperature, pH, electrical conductivity, dissolved oxygen, redox potential, piezonmetric level).

The trend in the depth of the groundwater (Piezometer Pz1 A) and the piezometric level of the underlying alluvial aquifer (Piezometers Pz1 B, Pz2 and Pz3) are reported in the graph below left.

The graph illustrates the seasonal fluctuation of the three principal aquifers. From a maximum recorded on June 2006 there was a steady drop in the piezometric level until the minimum recorded in November 2006. After that the piezometric level of the main aquifer started increasing again. Likewise the groundwater aquifer (recorded at piezometer Pz1 A) showed a general, seasonal trend similar to that of the underlying aquifer. However, a peak situation – that is the minimum aquifer depth – was recorded in the month of February 2007 (-1.3 m below ground level) and anomalous piezometric levels were seen in March 2007 (-1.54 m bgl) and in April 2007 (-1.89 m bgl).

To check for any significant variations in the chemical-physical properties of the underground waters, during October 2007 two PVC monitoring wells ‘well A’ and ‘well B’ were created at a distance of 5 m from the drilling axis, downstream of the underwater runoff component. With continuous monitoring, the team was able to verify the interference of the digging equipment and its progress with regard to the hydrogeological and chemical-physical conditions at the beginning of works, during, and at the final phase.

The undisturbed groundwater in well A had a temperature of about 14.08 degrees Celsius. During the boring machine passage, on 7 November, the temperature increased from 14.08 to 14.15 degrees. From 9 November until the end of measurements on 22 November, the temperature gradually increased again to nearly 14.35 degrees Celsius. This increase in recorded temperature is due to the phenomenon of heat radiating from the reinforced concrete pipes of the microtunnel, which are heated by electric motors and equipment surrounding the microtunnel under works.

At a depth of 20 m, the groundwater maintains an almost constant temperature; well B is approximately 14.16 degrees. From 27 November, the temperature increased to 14.25 degrees, then fluctuating between 14.24 and 14.22 degrees until 4 December. At well B, the increase in temperature, due to the heat flow induced by the reinforced concrete piping heated by the drilling equipment, was lower than at well A: ∆T=0.09 degrees Celsius. The drilling works finished on 8 December.

Twelve days after drilling was completed, the temperature was still 0.02 degrees higher than the original because the microtunnel was still being affected by internal works to remove the equipment.

For well A the values of electrical conductivity were maintained at constant levels – in the undisturbed groundwater – fluctuating between 790 and 800 μS/cm. From 27 to 28 October, values in the range of 804 – 814 μS/cm were registered (natural variations). A slight increase in the conductibility (from 801-803 μS/cm) was seen after the cutting head passage, however the values stabilise on 8 November at 793-795 μS/cm. There were however, no marked or abnormal fluctuation values.

For well B, the μS/cm values maintained a constant – in the undisturbed groundwater – fluctuating between 670 and 678 μS/cm; from 26 to 28 November fluctuating values of 670 to 688 μS/cm were registered. The successive values remain in the range of 671 to 675 μS/cm until the 6 December and they then become constant at 672–673μS/cm. There was no marked or abnormal fluctuation values in this case either.

There was no anomalous situations found in pH during the mechanised drilling and during the cutting head passage there was no marked variation in dissolved oxygen values.

Conclusions

The study showed that none of the parameters were permanently affected by the drilling. The parameters monitored tended to return to their initial condition in only a few days.

Ongoing monitoring

The installation of multiprobe units in adequately built monitoring wells set at 5 m from the drilling axis showed an absence of any significant variations in the chemical-physical parameters of the waters. No anomalous situations were found due to contamination and/or alteration induced by the mechanical drilling.

In regard to the outside environment at the surface, the precautions adopted were able to prevent any disruptions and the works were performed without objections from the control authorities. Moreover, the pre-existing hydrological system was perfectly intact when the works were completed.

Thus the drilling method adopted, together with the insights applied during execution, ensured full respect for the surrounding territory.

This is an edited version of a paper prepared by Snam Rete Gas, Enereco and ICOP at No-Dig 2009 entitled A microtunnel crossing of a highly environmental impact area (Italy). For more detailed information, references and acknowledgements please refer to the paper.