Contracting Joint Venture (ACE) handed over the completed civil works for the Campo Grande to Ameixoeira section of the new Yellow Line Extension, to the client Metropolitano de Lisboa, on 27 November 2001, 2 months ahead of schedule. This success drew not only from sound tunnelling knowledge, but also from extensive research into the interaction of EPB technologies with the local geology for the planned 2.5km route.
The Contract, which began on 28 February 2000 had an initial value of US$43.2M. The design and works inspection for the project came under the responsibility of Ferconsult (subsidiary company of the Metropolitano de Lisboa) with the contracting JV of Somague Engenharia, Bento Pedroso Construções (Brazilian group Odebrecht) and Engil Sociedade de Construção Civil in charge of construction, with Spie Batignolles TP (French group Spie) as technical managers.
The scope of the works consisted largely of the construction of a 2.5km long, 9.83m diameter, metro extension from an existing start shaft at Ameixoeira, where a 25m conventional tunnel had already been pre-constructed for TBM assembly, to the reception shaft near Campo Grande station.
Figure 1 shows the geology of the project, together with the tunnel and station locations. For the first 200m of the drive the alignment passed through the “Benfica Complex”, a hard ground composed of marlstone and sandstone with Oligocene layers. The tunnel then entered Miocene layers, the first of which was 1500m through compact ground composed of clay, sandstone and limestone and the last 800m in sand or silty sand (more or less cohesive) and recent earth filling.
The line crosses three stations, the construction of which falls under a separate contract. During construction of the tunnel, steel ring beams were installed in two of the station areas to stabilise the lining of the tunnel during future station excavation and dismantling of the rings.
The project also includes two ventilation shafts, located between the stations. One constructed as open cut, using concrete diaphragm walls, while the second was excavated underground using NATM from a 6m i.d. x 25m deep shaft, stabilised with concrete piles.
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By GlobalDataThe reception shaft (30m long x 15m wide) and a cut and cover tunnel (129m long x 8m wide) connect the new line to the existing metro.
The installation of a fire control system was also part of the scope of works.
Tunnel construction
Boring a tunnel under densely populated urban areas can potentially generate significant ground deformation and structural damage to surface buildings, especially in soft soil.
A total of approximately 30km of jet grouted columns were drilled in specific zones where buildings and surface structures came within the area of influence of the tunnel (function of ground characteristics and tunnel depth), and where calculations showed differential settlements which could cause severe damage. A re-inforcement arch was created above the crown, using high-pressure injections of water and cement, spaced 1m apart.
To further minimise this risk, an earth pressure balance technique was selected, and has proved to be a successful choice.
In order to achieve real control of the earth pressure, the main factors were:
- the TBM itself, especially the configuration of the cutterhead and ground conditioning system;
- the geological conditions;
- the level of ground cover.
The client owned a Lovat ME388 machine that was designed in 1995 to bore part of the EXPO line for the Metropolitano de Lisboa. During this first drive, the contractor installed, in addition to the supplied ground treatment system, a homemade system to inject polymer and/or bentonite.
On the Yellow line, the JV set out to devise a completely new ground conditioning system, allowing the use of polymer and/or foam agents and/or bentonite on each of the machine’s eight injection pumps.
Due to the hydraulic doors between the cutter arms, designed for safe worker access into the chamber, the open section of the cutter head is quite small (about 33%). The centre of the head is completely closed for an approximate 2m diameter, which means the ground has to flow some distance in order to enter the chamber, this requires a very high treatment rate in the central area.
Before the start of boring commenced, extensive theoretical calculations were carried out to determine the minimum pressures to be applied to the face in order to maintain and minimise settlement. These calculations were done by Spie Batignolle’s design office utilising the Murayama method (see box, p23), taking into account the geotechnical ground characteristics and cover. The results were basically as follows:
- the first half of the drive would be carried out in hard soil with considerable cover and bored using the open mode. A nominal face pressure value of 0.3 bar was recommended;
- the second half would be bored through soft soil with low cover in closed EPB mode and needed a theoretical face pressure of 0.4-0.8 bar, plus an added safety value of 0.6 bar.
Laboratory trials and ground treatment
In order to determine the most effective products for the local strata and in order to estimate the most effective treatment rates, preliminary laboratory trials were performed on ground coming from Ground Types C10, C8, C6, C5 and C2a by French based company Condat.
Each trial entailed measuring the torque resistance through a mix of ground and treatment products at different rates. Permeability tests were also performed to check the behaviour of the ground after treatment.
On site, one of the peculiarities of the project was the need to permanently modify the type of ground conditioning, due to the ascending profile of the tunnel against the descending slope of the complexes, each one made up from different layers. To manage this situation, a procedure of continually recording, analysing and modifying the treatment data was put in place. Conclusions drawn were as follows:
- even on sandy or silty material, priority was given to fluidification of the ground in the nose cone area. Results were similar with foam or polymer in terms of instant speed, but foam was preferred because of problems with re-compaction of the clay in the screw and obstruction of the nose by the injection hose. In comparison with laboratory trials, the effective treatment rate in the tunnel was found to be more than double the predicted figure;
- to reduce the torque and minimise wear of the tools on the cutterhead, injection of polymer on the periphery gave good results;
- foam agents and polymers from JFTec and Condat were used alternately, with no notable difference between manufacturers.
At the end of the project, unexpected dry and uncohesive sand was encountered. This kind of ground is perfect for a slurry machine, but less suited to an EPB machine. The increase of friction on the cutterhead and shield was treated with additional injections of bentonite or foam through the shield, combined with more thrust. Dense foam allowed the continued effective use of the EPBM and helped to control the settlement in these difficult ground conditions.
Lining and back-up
The tunnel has an i.d. of 8.8m with a lining consisting of precast concrete segments, 0.36m thick and 1.2m long. The universal ring (6 segments + 1 key) was manufactured by Spie Batignolles in a new ISO 2000 approved segment factory.
A cast in situ concrete raft was poured into the invert between the shield and the back-up system. The drainage system (315mm dia PVC pipe with manholes every 15-30m) was embedded in this raft.
The 9.83m diameter Lovat ME388 EPBM, utilised active shield articulation, and a cutting head configured with 1 nose cutter, 222 scraper teeth and 66 positions for ripper teeth or disc cutters. The power was delivered by 8 electro-hydraulic pumps of 225kW each and the thrust applied by 18 pairs of hydraulic thrust jacks.
The extraction of the muck was carried out using a screw conveyor (Ø1.3m) and a belt conveyor (1.2m width) feeding into two muck hoppers of 35m3 on the back-up sledges.
Muck was transported to the shaft by 5 15m3 trucks with double drive cabins. It was extracted from the shaft by a 30t KONE automatic overhead crane. Segment pallets, concrete, grout and other bulk material were transported by 4 Astra BM44 trucks and handled by a 20t travelling crane.
On the start shaft platform, a grout batching plant, ventilation system, slurry water treatment system and two concrete loading tubes were also installed.
Settlement
About 100 instrumented sections were installed throughout the length of the tunnel to check ground deformations and settlements, by means of superficial survey marks, inclinometers, extensometers and piezometers.
As shown in figure 2, settlement was less than 20mm all along the line, with exception of the final stage when encountering the conjunction of uncohesive sand, low cover and long stoppages.
Conclusions
The ground conditioning system facilitated good advance rates for the project and combined with comprehensive earth pressure calculations and extensive jet grouting techniques, helped to significantly reduce the ground deformation and settlement. The unexpected dry and uncohesive sand which was encountered towards the end of the drive, could have jeopardised ground settlement levels had the increase of friction on the cutterhead and shield not been treated with additional injections of bentonite or foam through the shield.
The research and development carried out by the JV in conjunction with Condat certainly added to the success of the project and aided the works early completion, 2 months ahead of schedule.
Final invoicing for the project, including additional works and price revisions is currently expected to result at approximately US$49.3M.
Related Files
Figure 1: Lisbon’s existing metro system
Figure 2: Geology and alignment
