The requirement to increase the capacity of a section of road tunnel first emerged at the start of the 1990s, during a study to widen a 40km section of the A1 Milan-Naples motorway between Orte and Fiano Romano, near Rome, Italy.
A 337m-long twin bore tunnel already existed in the section, in the municipality of Nazzano and various constraints were faced which restricted design choices.
Considering the volume of traffic already flowing, it was not possible to close one carriageway of the motorway, even temporarily, to divert the traffic onto the adjacent carriageway.
Conversely, the situation meant that construction of a new tunnel would be very difficult. And would mean a third bore for vehicle transit, with substantial changes to the motorway route in very close proximity to the Tevere-Farfa Nature Reserve.
The only solution was finding a way to increase the capacity of the existing tunnel by widening, but without interrupting traffic flow, and so expand it to three lanes plus an emergency lane in each direction, giving a net road profile of 14m in width and 4.5m in height.
The challenge to tunnel designers was not simple. A construction system had to be devised to allow the necessary work without affecting the traffic flow or the safety of users passing through a complex underground construction site.
The design concept of this project was developed by Pietro Lunardi during the construction of the ‘Baldo degli Ubaldi’ single vault underground station for the Rocksoil-designed extension of Line A of the Rome metro between Ottaviano and Battistini stations.
The construction system adopted to excavate the crown of the tunnel – approximately 20.5m in width by 9m in height – included two important new features:
– The use of advance reinforcement of the core-face with fibre glass structural elements combined with mechanical precutting technology and an ‘active arch’ lining;
– A high degree of works mechanisation.
Active arch lining
Active arch lining refers to a lining that produces immediate confinement of the surrounding ground, without waiting for it to deform and push down on the extrados. A lining of this type is formed by erecting an arch of prefabricated reinforced concrete segments. This is immediately rendered a load-bearing structure by two opposing flat jacks inserted in the key segments which artificially apply the axial compressive stress needed to contrast the loads to be borne.
A special machine was fabricated and used to combine the above technologies into a single, highly efficient system. It consisted of a large steel portal frame with a geometry designed to fit the profile of the crown of the station tunnel. This was supported on stabilisers set on longitudinal beams installed in the side-drifts excavated for the side walls of the tunnel in order to allow it to traverse longitudinally. In addition to the equipment needed to perform the mechanical precutting, the equipment for handling and erecting the prefabricated concrete segments of the final active arch lining was also installed within the portal.
It was observed that during operations to construct the vault (the volume of ground below the springline of the tunnel, which had a cross section similar to that of a normal motorway or mainline rail tunnel) was unaffected by the construction operations. It was determined that the Nazzano road tunnel could be widened in the same way without the need to close it to traffic, naturally as long as appropriate safety measures were taken to protect tunnel users.
It therefore clearly emerged from a study of the design and construction used for the ‘Baldo degli Ubaldi’ station on the Rome metro, that the technology existed to widen a tunnel while it remained in service, at least in terms of advance ground improvement and reinforcement and the placing of the preliminary and final linings. All that was needed was an appropriate system to protect traffic travelling on the motorway below, under the active construction site.
Traffic protection shield
To achieve separation of the construction from traffic, a steel tunnel shield approximately 60m in length, termed a ‘Traffic Protection Shield’ was constructed and placed inside the existing tunnel with an appropriate structural design.
Vehicle traffic on the motorway would therefore be able to run under the protection of the protective shield in the opposite direction to that of the advance of the widening works in order to avoid producing the irritating sensation of a narrowing field of vision for tunnel users passing from a section of widened tunnel to a section still to be widened.
The key factors in the basic idea for widening a tunnel without interrupting traffic consisted of a traffic protection system and appropriate operating technology to allow works to be performed operating exclusively inside that protection system, without interference with traffic.
Once these had been identified actual design itself of the whole works consisting of the project to widen the section of motorway running through the Nazzano Tunnel could then begin.
The first design (1996-1998)
Background
The lot consisting of the Orte-Fiano Romano section on the A1 Milan-Naples motorway was 1,200m in length running from km 522+000 to km 523+200 and also included two artificial portal tunnels for each carriage way, waterproofing works, walls to confine excavations at the portals, support walls, surveys, trenches and auxiliary works for access to working areas. The widening works also included improvements to the central reservation between the two carriageways, lighting systems and service systems to manage motorway traffic.
Geology of the site
The ground through which the Nazzano Tunnel passes consists of yellow silty marine sands, with fine to medium granulometry and density increasing with depth. A layer of silty clays is present below the level of the tunnel invert. The overburdens range from a few metres to a maximum depth of 45m and the water table is located, net of seasonal fluctuations, at the level of the tunnel wall.
The main sandy formation showed values for an angle of friction of greater than 30° with maximum values of 37 to 38° and cohesion of 0.02MPa, in relation to the presence of the silt fraction.
The widening programme
The widening works were to be performed according to the following general programme. First one of the tunnel bores (stage one) would be widened in the presence of traffic and a temporary invert constructed in the widened section.
Then both flows of traffic, in a north and south direction would be diverted into the widened tunnel with two lanes in each direction (stage two).
At this point (stage three), the second bore would be widened free of traffic, completing it with the casting of the tunnel invert. Traffic is then moved into the second, widened bore with two lanes in each direction and the invert is cast in the first bore.
Section types and basic advance cycle
The initial design for the enlargement of the Nazzano tunnel included two tunnel section types: a principal type with advance ground reinforcement using mechanical precutting technology, and an alternative section type where the advance ground improvement consisted of a double ring of jet-grouting in advance around the cavity. The latter could be employed if problems arose with the integrity of the precut, given the predominantly sandy nature of the ground.
In both cases the section type was completed with the installation of a final lining consisting of 19 prefabricated concrete segments approximately 1m. in length longitudinally, to be assembled at a short distance from the excavation (4m).
The sequence of basic operations performed for each advance cycle consisted of the following:
– A mechanical precut (length 4.5m,thickness 300mm, overlap 1.5m);
– Assembly of the first arch of the lining with prefabricated concrete segments (length 1m);
– First advance excavation of 1m and simultaneous demolition of the existing tunnel;
– Assembly of the second arch of prefabricated concrete segments (length 1m);
– Second advance excavation of 1m and simultaneous demolition of the existing tunnel;
– Assembly of the third arch of segments of 1m in length;
– Third advance excavation of 1m and simultaneous demolition of the existing tunnel.
The distance between the face and the final lining of prefabricated concrete segments was of fundamental importance both for the stability of the system and for the definition of working space. It was to vary between 4m after the installation of the concrete segment arch and 5m at the end of the advance excavation.
Finally, the initial design involved excavating the first 5m from the tunnel portal under the protection of a concrete shield, to create an artificial tunnel, in order to pass through the portal of the existing tunnel in safety. On that basis, the design for the tender was delivered at the end of 1997 in time to put the project out to tender in the first half of 1998.
The first contract and dispute (1998-2001)
The first contract was won by a Spanish company with a bid 25 per cent lower than the specified contract value. The contractor had presented a substantial change to the design, abandoning the use of mechanical precutting and adopting a single tunnel section type using jet-grouting.
This offer was accepted by the client. However, it made final approval of this solution subject to the results of field tests to be carried out in situ after the handover of the site for the works at the end of August 1998.
After construction site yards were prepared at the portals of the tunnel to be widened, a field test was performed in an area facing a natural cavity overlooking the yard at the south portal. Results were not reassuring.
Although the performance in terms of the diameter and the strength of the columns of improved ground complied with expectations, substantial problems emerged with the migration and escape of grout through the ground even at a distance of several metres.
This phenomenon, concerning the use of jet-grouting technology in stage one of the works (widening of one bore with the adjacent bore in normal service) made the tunnel section type chosen by the contractor unusable, which resulted in a request by the client to return to mechanical precutting technology. The escaping grout could in fact have entered the roadway of the adjacent tunnel in service even though it was lined, with significant and uncontrollable risks for the safety of traffic.
While the client and the contractor disputed over payment of the costs resulting from the inability to use jetgrouting technology, the works continued intermittently on operations outside the tunnel itself.
In January 2001, after lengthy negotiations, the parties agreed to withdraw from the contract, with the traffic protection shield already positioned over the carriage way of the motorway and with the multi-function equipment, which should have performed the precut, the widening excavation and the demolition of the existing tunnel, as well as the active arch for the lining, ready at the portal of the tunnel to be widened.
On the date of the withdrawal from the contract, two years and six months had passed since the construction site opened, without a single metre of the tunnel having been widened.
The second design (2001-2002)
As soon as the administrative formalities were complete for the handing back of the site areas and the removal of the preparations already in place, a new design was prepared for a new tender.
This had to take account of external works and the important facts that had emerged for design during the first contract. This refers to the unusability of the tunnel section types with jet-grouting in a context of two adjacent tunnels in service, and the need to employ a conventional tunnel invert instead of slabs to join up with the tunnel invert of the old widened tunnel.
During design the safety measures for the management of interferences between traffic and construction site were improved with the introduction of support elements for the traffic protection shield. These were in concrete, firmly cemented into the ground, with a New-Jersey barrier shape on the roadside and with dimensions sufficient to resist any type of impact from light or heavy vehicles in transit.
Finally, after around five years had passed since the first cost estimates were prepared for the works, new estimates were performed for the completion of the project based on up-to-date prices.
Completed in April 2001, the new design was approved by the client ANAS in July 2001. The new tender with a basic contract value of approximately EUR 37.5M (USD 47.6M) was launched in April 2002 with bids on unit prices. It was won by a group of companies led by Cossi Costruzioni. The site was handed over to the contractors at the end of July 2002, exactly four years after the first work began.
The second contract (2002-2008)
Work on the northern carriageway: the preliminary preparation stage
The programme of works attached to the second design estimated the time needed to perform the set of operations preparatory to the widening of the northern carriageway at seven months.
Those operations also comprised the acquisition of the multi-function equipment that would perform the tunnel advance works (mechanical precutting, active arch lining, excavation and demolition).
The machine provided by the previous contractor, which was still parked on the construction site at the north portal of the tunnel, was acquired by the new contractor who performed the necessary overhauls and idle running tests, without making any substantial modifications.
The traffic protection shield was installed on the new reinforced concrete supports over the roadway at the same time as the machine was overhauled.
When site preparation was completed, the first widening excavations commenced in January 2003.
Significant problems were immediately encountered in relation on the one hand to the poor mechanical properties of the sandy ground in the portal zone, which had been disturbed considerably by the excavation of the previous tunnel, and by functional deficits in the multi-function equipment which had not been properly prepared to produce the minimum performance required in terms of production capacity.
The former problems meant that the ground did not possess the minimum required ability to support itself during the precutting operations, not even for the few moments needed to clean the cut and fill it with grout. This resulted in frequent and unacceptable over excavation.
The problem with the multi-function equipment was that it was unable to deliver sufficient power during the erection of the concrete segments in order to be able to handle them quickly and easily.
In addition to these two problems, which were serious and not easy to solve, a series of difficulties due to with the very limited space available were encountered as advance works commenced. This made excavation, demolition and spoil removal operations particularly slow and laborious.
Faced with these problems, for many of which no precedent existed to which reference could be made to identify rapid and appropriate technical solutions, the widening works advanced at less than 0.1m per day for approximately seven months. Difficulties arose at all stages of the works.
At the end of August 2003, after having widened just a 25m length of the tunnel, the consulting engineer and the project manager, in agreement with the client, commenced a reassessment of the operating procedures in order to solve the problems that had arisen.
In order to ensure the stability of the face and that the cavity would support itself, above all during the precutting stage, ground improvement in advance was performed with selective and repeated injections of cement and chemical mixes through valved pipes.
These operations, which initially extended for 25m ahead of the face were performed for an area above the tunnel where it was possible to create a working yard without excessive earthworks.
At the same time, the contractor started a series of substantial overhauls and replacements to the multi-functional equipment to improve its ability, especially in terms of the installed power.
To deal with the confined space and the soil removal operations, the most important change to operating procedures was made. The multi-functional equipment would only perform the precutting and active arch erection operations.
Excavation at the face, demolition of the lining of the existing tunnel and spoil removal would be performed using conventional methods, with roadheaders and mechanical shovels on wheels. Even the erection of the side wall segments of the active arch was to be performed using a crane on wheels and not the multi-function equipment, given the difficulty of installing these segments, which, because of their size and weight, placed the equipment and structure of the machine under considerable stress.
In order to achieve this, a special area for protected parking was constructed outside the tunnel over the roadway where the multi-functional equipment would be able to park during excavation and demolition work; a system for transporting the machine to and from the face as rapidly as possible was designed; finally, a self-propelled steel shield was constructed under the machine to prevent oil and other material from falling onto the roadway.
The whole complex moved on rails, which were to be extended to follow the excavation face. The machine was to move on rails as far as possible to then advance to the face using its own propulsion.
In order to further improve production, the maximum distance between the face and the final lining was extended from 5m to 6.5m to significantly increase the working space, one of the most critical aspects of the system.
Finally, the tunnel advance cycle was modified by reducing the excavation and lining erection operations to two instead of the previous three.
It was expected that significant benefits would be obtained from these changes, thanks to the more streamlined and efficient management of the works, made possible by increasing the available space, reducing down times for moving and parking the machine, two movement and parking operations instead of three, and employing two sprayed concrete operations at the face instead of three.
The main problems which had affected production had been addressed and tunnel advance works resumed in March 2004 using the new operating procedures. The results were positive and immediately visible to the team.
Advance beneath improved ground was carried out immediately, with production rates increasing by five-fold compared to the previous operating procedures, from 0.1m per day to 0.5m per day.
Construction of the north bore: improvements to the design
After a few tens of metres of advance without problems for traffic below or accidents to site personnel, caused by the sudden fall-in of loose material, tunnel widening advance finally reached constant production rates of close to 0.7m per day.
The results of the ground improvement in advance were positive, with the elimination of all problems relating to the integrity of the cut performed.
Georadar surveys carried out in the section of tunnel still to be widened to detect the presence of cavities adjacent to the precut zone were negative.
As a consequence it was decided to introduce a further improvement by replacing the precut blade with a length of 4.5m with another 5.5m in length, to be able to increase the length of each advance cycle from 3m to 4m, while maintaining the overlap between successive shells and the maximum distance between the face and the last lining ring erected unchanged.
This produced substantial benefits in terms of reducing down times required to move and park the multi-functional equipment. In fact since there was little difference in the time taken to create both 4.5m and 5.5m precut shells, production rates increased by almost 25 per cent.
The remaining part of the tunnel (approximately 200m) was completed without incident, in approximately nine months, between February and November 2005, with production rates varying between 0.7m per day and 0.8m per day.
Out of seven years since the site opened, 75 per cent of the widening of the north bore was completed in one year.
Construction of the south bore
The works to widen the north bore were completed successfully. They had proceeded at very acceptable and regular advance rates, once the problems that had emerged in the first 80m of tunnel advance had been solved.
This persuaded the client to perform an in-depth assessment of the advance methods to be used for the widening of the south bore of the tunnel, with a view to minimising inconvenience to road users travelling on the only point on the Orte-Fiano Romano section of the motorway not yet widened to three lanes.
The initial proposal, to widen the second bore in the absence of traffic by deviating traffic flow onto the four lanes of the bore already widened, had been formulated ten years earlier, when the volumes of traffic present could still be dealt with in this manner without running into serious problems.
Ten years later, however, the situation had changed considerably and the capacity to absorb traffic provided by just four lanes appeared totally inadequate, especially during the summer and the major public holidays in spring and autumn.
It was therefore decided to apply this method to widen the south bore, while keeping it in service.
Four lanes in the north carriage way (three running north and one south) and two lanes in the south carriage way would always be open to traffic, to give a total of three lanes in each direction and a traffic flow capacity of approximately 4,000 vehicles per hour in each direction instead of approximately 2,600 to 2,800 vehicles per hour.
The validity of the decision to widen the south bore while traffic was flowing, with a considerable improvement in the functional use of the tunnel during construction, was borne out by a further study of the project, necessary because the original contract specifications concerning the operational procedures to be followed to widen the second bore had undergone a number of substantial change.
After approximately three months (January-March 2006) spent moving construction plant and operations to the south bore and installing identical safety measures for traffic, works commenced.
Experience acquired during the widening of the first bore was used to immediately achieve even faster average production rates of close to 0.9m/day, with peaks of around 1m per day, which enabled widening of the entire bore to be completed by May 2007, just 15 months.
The entire works were completed without any significant disturbance to traffic.
Completion of the two carriageways
In order to maintain at least two lanes open to traffic on the carriageway affected by the works, concrete was poured in longitudinal sections with the consequent adoption of connecting couplers between reinforcement rods, while the safety of traffic and site workers was constantly guaranteed by appropriate methods to separate the in-service road area from the adjacent construction site.
The work to cast the tunnel inverts, which first began in the south bore and then continued in the north bore, lasted for around six months from June until December of 2007.
Overall distribution of construction times
The period of eight years and nine months between September 1998 and May 2007, from the start to the finish of works was spent as follows:
– Some 44 per cent on the difficulties of the first tender and the formulation of the procedures necessary for the second tender;
– Some 25 per cent on preparation of the construction site for the north bore after the second handover of the site from the first contract to the second contract, and problems with the fine tuning of the construction systems to overcome difficulties related to the characteristics of the ground and the first time ever use of the operating methodology;
– Only 31 per cent of the remaining time was spent on excavation to widen the two bores of the motorway tunnel.
There is probably no more valid evidence to the success of the method than the one hundred percent reliability record in terms of public safety.
