In late 1998 the Porto Municipality undertook a commitment to upgrade its existing railway network to a fully functional, integrated metropolitan transport system. As a result, a 70km, 66 station, light metro extension is currently under construction, 7km of which is in tunnel with 10 underground stations.
Metro do Porto SA, a public company, is implementing the project with a concession period of 50 years. Upon tendering, the design, construction and operation concession was awarded to Normetro, a JV including civil contractors, equipment and system manufacturers and an operator. The civil works design and construction was awarded, for an initial approximate value of US$401M, to Transmetro, a JV of Soares da Costa, Somague and Impregilo.
Project Description
The complex topography of the Porto area, combined with environmental and budgeting constraints, posed a host of challenges to the project design. The 7km section that crosses the town centre has been forced underground, as the picturesque area is a UNESCO world heritage site. The underground drives pertain to Line C, which stretches 2,350m from Campanhã to Trindade with three stations and a maximum overburden of 32m, and Line S, a 3,950m long drive from Salgueiros to São Bento with seven stations and a maximum overburden of 21m (figure 1).
The tunnel will be a bi-directional, 7.8m i.d, single bore structure with a tight turning radius of 200m, required to adhere to the planned alignment. Of the ten underground stations, six will be built by traditional mining methods and four by cut and cover.
The project poses challenging boring conditions, due to the presence of old buildings with shallow foundations in conjunction with poor geotechnical conditions.
Geology
The tunnel alignments pass through a granitic batholith, which intruded into the Porto-Tomar regional fault in the late Hercinian period. The Porto Granite shows deep weathering, especially in faults and joints, resulting in a very irregular profile. The alteration grades range from residual soil (W6) to fresh granite (W1); these various terms represent important chemical-physical variations with differences in density, permeability and geo-mechanical parameters. The granite is crossed randomly by aplitic/pegmatitic dykes, which display much less weathering and form an almost skeletal like structure in the decomposed granitic mass.
The area’s various weathering stages, from a geo-mechanical point of view, make the ground behave like cohesive sandy clay (residual soil), to clayey sand and sandy gravel. During the boring process this strata behaves differently depending on the specific composition (W6–W1) present at the face.
Mention must also be given to the local hydrogeology where water flows are a function of the granite-weathering grade. In the early alteration grades (W2-W3) the flow can be associated with water carrying fractures, while in the more weathered ground it can be associated with that of a porous medium. This combination results in a very variable rock/soil mass permeability following the alteration patterns. A particular feature associated with the local conditions of the Porto area, characterised by year round rainfall and a gentle sloping topography towards the sea and the Douro River, is the frequent occurrence of wells connected by drainage galleries that, in the past, ensured the population’s water supply. Long-term exploitation of these wells has led to the washing out of fines, increased permeability and generation of an unstable soil structure. This can be severely affected by tunnel excavation and lead to serious settlement when the fragile equilibrium around the wells is disturbed.
The variable geological conditions along the Line C tunnel can be seen in figure 2, with several soil/rock transitions at various depths. The water table is located between 10m-25m above the tunnel crown.
Equipment selection
Due to the variable conditions and high water table, the JV decided to use a single Herrenknecht EPB machine equipped with cutting disks to excavate the underground sections; firstly the Line C drive from Campanhã to Trindade followed by the Line S drive from Salgueiros to São Bento. The 8.7m diameter machine features some impressive statistics with 70,000kN of thrust, 12,900kNm of torque and an installed cutterhead power of 2,400kW.
This state of the art machine has a considerable torque/diameter ratio to allow EPB boring in cases of mixed soil conditions at the face. It is also equipped with a soil conditioning system capable of injecting foam, polymer and/or bentonite slurry to the face to cope with the expected coarse sandy gravel.
A continuous belt conveyor has also been provided for muck transport from the back-up to the tunnel portal, prior to loading and transportation to a disposal area. It was predicted that the difficult geology would require constant checking of the excavated weight to verify in real time the balance between the stroke and the extracted material. Two systems have been implemented to fulfil this task. The first was installed in the second conveyor belt in the back up and is composed of two redundant belt scales, which independently weigh the material carried by the belt, giving an average result. The second was installed outside the tunnel and is based on an optical device, which scans the material carried on the belt and gives an average volume by integrating the scanned belt sections. In operation the first system has proved very useful, while the second system was found to be unreliable and was abandoned early in the project.
The lining is a pre-cast concrete, six segment and key ring, 1.4m wide and 30cm thick. The innovative segment design includes three dowel type connectors in the radial joints and guidance rods in the longitudinal joints without bolts. The segments are delivered to the TBM by trucks, adapted to run on the curved invert, the same trucks also transport materials and mortar along the tunnel to the back up. The universal type ring has a total taper of 72mm, vital to manage the planned curve radius. It was found that the use of an 800mm conveyor belt posed some problems in the tight radius curves, where belt rotation caused muck to fall off the belt onto the tunnel invert.
Tunnel drives
In August 2000 the JV started boring the 2,350m drive from Campanhã towards Trindade. It soon proved extremely difficult to maintain the necessary face pressure required to bore through the granite that was in various stages of weathering. The presence of hard granite at the bottom of the face, and sugar-like, non-cohesive gravel/sand in the crown made for a troublesome, if not impossible, control of excavation. Moreover, the peculiar geomechanical characteristics of the soil resulted in few or no warning signals of collapse with the brittle behaviour of the ground leading to unforeseeable failures.
After some 400m the advance was severely hindered when a coincidence of several adverse factors lead to an incident (T&TI, November, p7) and a resulting stoppage that was to last approximately nine months (T&TI, June, p10). During this period an intensive consolidation programme was implemented, from the surface and inside the tunnel, to consolidate the ground around and ahead of the drive.
Meanwhile, a detailed investigation ahead of the machine was undertaken to detect geological anomalies and the presence of wells or drains.
In order to re-start the advance, very strict boring control measures were implemented to avoid further disruption. The weighing system (belt scale) was fully integrated with a programmable logic controller (PLC), in order to check the real time ratio between the extracted material, the boring stroke and the volume of injected conditioning agents (foam, bentonite, and water). Any difference in the measured and calculated volumes would start an alarm and stop the machine. An emergency bentonite system was also installed on the TBM, consisting of four pressurised slurry tanks connected to the front shield chamber. Should the EPB pressure drop below a minimum level (set in the PLC) the TBM would stop and the system automatically inject the slurry to bring the pressure at the face back to within the established limit.
Additional alarms were integrated in the machine’s PLC, and an indirect check of the filling of the working chamber was implemented, controlling the muck density by comparing readings from the pressure cells of the chamber bulkhead. If the apparent density of the muck drops below a defined value, an alarm is activated.
Parallel to the outflow gate of the screw conveyor, a large double piston concrete pump was installed, which also deposited muck onto the belt conveyor. This was considered particularly suitable for muck with a high water content (almost liquid muck), the flow of which could have been very difficult to control through the screw gate.
From the procedural point of view, the designer set a working EPB range of 1.2-2.5 bars in the crown. If the pressure drops below this level the machine stops and, in the case of lower pressures, the immediate intervention of the bentonite emergency system is activated (figure 3). This bentonite system has proven to be very effective during TBM stoppages for ring erection, to re-establish and maintain the support pressure as air and water trapped in the chamber is lost through the ground.
To keep up with the overall programme it was decided that a second machine, similar in specification to the first, was needed. This was also ordered from Herrenknecht. With this decision came the necessary rescheduling of the tunnels’ excavation. It was planned that the second machine would start from Salgueiros and bore approximately 3km of tunnel to Trindade thereby completing the majority of Line S. The first machine would complete its drive on Line C to Trindade and then bore the remaining 1km of Line S from Trindade to São Bento.
On the 18 September 2001 the first machine restarted in mixed ground conditions, in EPB mode with a pressure ranging from 1.5 to 2.0 bars in the crown. Advancing in EPB mode proved to be demanding for the machine, with very slow rates in the range of 10 mm/min, or up to 2.5 hours for a single ring shove. This generated problems for the machine, with the main hydraulic circuits and other components overheating, while the cutterhead and tools experienced excessive wear. The disk cutters in particular were literally ground down by the crushed rock paste, forcing long daily inspections and changes in a compressed air environment. The average daily advance stabilised in the range of 4-5 rings/day, well below the initial planned production rate. The implementation of strict measures, however, allowed excavation to proceed without further mishap resulting in a successful breakthrough at Trindade station on 28 October 2002.
Ground treatment and monitoring
As explained above, after boring restarting, EPB pressure was systematically maintained in a well-defined range to minimise settlement. Great emphasis was also placed on ground monitoring and, in the final section of the Campanhã to Trindade drive, a sophisticated settlement monitoring system was employed in conjunction with ground treatment.
Along the alignment, settlement has been extensively monitored using multipoint extensometres, inclinometers and settlement benchmarks on the surface. Particular attention has been taken to monitor any fluctuations in the water table. The area’s complex urbanisation somewhat restricted the available layout of the instrumentation, but a follow-up of the drive was carried out as close as possible to the tunnel crown. It was therefore possible to be confident that the readings in the machine were correct. Despite this, it was still considered vital to implement additional measures to protect the important buildings in the Old Town.
In particular, in the final stretch of Line C, approaching Trindade Station, a micropile based ground treatment was adopted, with the possibility of carrying out compensation grouting with multipoint injection pipes. The area was particularly sensitive, as the TBM had to pass below a group of old buildings, founded in residual soil. The distance between the basements of the buildings and the top of the tunnel is in the range of 3m.
The treatment was carried out from two shafts (with depths of 6m and 10m) excavated beside the buildings. From the shafts several layers of micropiles and tube à manchette were installed to aid ground support between the tunnel and the buildings. Following injection only minor heave occurred.
The works in the area have been monitored continuously in real time, from the start of treatment to the completion of the TBM drive, using total stations installed in the vicinity. A complete scan was carried out every 15 minutes. Furthermore, horizontal inclinometers were installed in the treatment area. This allowed the ground treatment to be carried out as soon as any movement developed.
The final settlement of the area was negligible and Trindade Station was reached successfully without causing any damage to buildings.
Conclusions
Following a tough learning curve in the challenging ground conditions of the Porto subsoil, strict control of the EPB shield boring parameters and the implementation of additional measures to maintain the face stability allowed for successful completion of the first 2.65km Line C drive in October. The lessons learnt will be useful for the completion of the second and third drives and also for projects around the world where similar conditions are expected.
Related Files
Figure 3 – The TBM’s, PLC activated, slurry stabilisation system
Figure 1 – Map and alignment of the Porto light metro extension
Figure 4 – An example of the mixed face conditions
Figure 2 – Longitudinal section of the Campanha to Trindade drive
