Soletanche-Bachy of France and its sister company CSM Bessac, tunnelling specialist, combined with Conconcreto, a local Colombian company were awarded on June 28th last year the contract to design and construct the Interceptor Rio Bogotá, a 3m-dia, 10.3 km-long tunnel.
This sewer, initially designed as an open trench construction was modified to a trenchless technique in the last month before bidding date. This reduced the planned work duration from three to two years.
Contract
The object of the project is to collect the waste water from most of the north part of Bogotá (the districts of Usaquen and Suba) and transfer it to a new separation plant, whereas until now this was sent directly to either swamps or to the Bogotá river.
The client, the Empresa de Acueducto y Alcantarillado de Bogotá (EAAB), financed the project itself. The turnkey contract involves the whole collection of wastewater from a point near the north motorway to the swamp Juan Amarillo, 500m away from the new treatment plant of El Salitre. The contract value is around US$35M.
As part of the contract the Soletanche-Bachy design office chose the appropriate diameters for various sections of the tunnel. Consequently the project was divided into five sections.
The interceptor has been designed to work using gravity flow but the difference of the invert level between the first shaft and the last one is only 10m.
By the year 2020 the flow arriving in the first shaft is forecast to be 2950 litre/s and will increase with the network that discharges along the length of the tunnel to reach, at the end of the interceptor, an average of 7650 litre/s.
Ground
Excavated ground is mainly soft clay of the Sabana de Bogotá and is relatively homogenous. Its characteristics are:
Obviously, this ground is very favourable for the use of an EPB machine, and there is no need to feed any additives through the head.
One difficulty is the transition zone between soft ground and rock. This zone requires a special method as it has been decided not to use the TBM due to a high probability of encountering blocks during the excavation. An open trench with a diaphragm wall has been designed but construction has not yet started.
Method choices
The working (drive) shafts (seven in total) and exit shafts (also seven in number) are designed with a circular diaphragm wall of 800mm and 600mm thickness respectively. The walls are about 24m deep but the shafts are only excavated down to 13m.
The inside diameter of a working shaft is 12m giving room to jack two 2.50m long pipes at the same time. This saves time during the pipe installation. The inside diameter of exit shafts is 8.00m which is just enough to allow the TBM to be taken out of the shaft.
The design of the shafts has been carried out by the design office of Soletanche-Bachy in France and its sister company Soletanche-Bachy Cimas of Colombia, carried out the design for shaft construction.
An innovative system for transferring the jacking efforts was used in order to avoid constructing a reaction wall, utilising the shaft wall itself. The forces are transferred through two huge props to a concrete beam anchored to the slab of the shaft, thus transmitting the loads to the diaphragm wall itself.
To date seven shafts have been completed out of the total of twelve.
In addition to the above, eleven intermediate shafts also have to be realised so that a maximum length of 500m between two shafts is not exceeded. Moreover they will also be used as connections between the existing and future sewage collectors. The first stage of their construction is the installation of a diaphragm wall made of weak concrete before the TBM arrives to meet the shaft. The TBM will then excavate through the shaft wall and final civils work will start at the end of the drive. The size of intermediate shafts varies from 3.70m to 4.25m internal diameter.
The maximum length of a EPB TBM drive is 1000m which requires the installation of four intermediate jacking stations, although, until now, 600m long drives could be jacked from the main jacking station only. The shortest drive is 330m long. Three drives have a curve ranging from a 800m radius to 1200m.
In order to save time and to complete the project within the very tight schedule, Soletanche-Bachy decided to jack two pipes at the same time, as allowed by the shaft diameter (above). Although this increased excavation the time saved was essential to stay within the schedule.
Excavation
One of the characteristics of the excavation is that the excavated material is pumped direct without having to be mixed with any additives. A pump, directly connected to the screw, takes the material as a solid to an area located outside, near the shaft.
In order to avoid adding a second pump, the diameter of the pipes was increased to 8in. (200mm), instead of the initially designed 6in. and 7in. (150 & 180mm) ones.
Equipment
Two Herrenknecht TBMs are used, of the earth pressure balance (EPB) type designed to excavate very soft ground. A smaller machine is going to be used to bore in both 2200 and 2450mm diameters. For this it has been supplied with an upsizing kit to increase its diameter; a second cutter wheel was designed as well.
Both machines are fed with a 6,000 volt supply from a transformer installed close behind the TBM. The small TBM (2970mm od) is 7.14m long, weighs 54 tonne (with the upsizing kit) and has a total capacity of 280kW. It is equipped with a Putzmeister pump (85 bar – 88 m³/h) for spoil transport. The cutter head is electrically driven.
The bigger machine (3320mm od) is 10.80m long, weighs 74tonne and has a total capacity of 380kW. This TBM is equipped with a Schwing pump (with the same characteristics as the Putzmeister pump). Its cutter head is hydraulically driven.
For both TBMs all operational controls are inside the machine according to the choice of Soletanche-Bachy.
To steer the machine through quite long drives, the selected guidance system is the SLS-RV from the VMT company. This system is based on the concept that the pipes are at the same position as the TBM. Then it deduces both the co-ordinates of the reference prisms in the tunnel and the position of the theodolite.
The geometrical tolerances of the project are the main criteria of the excavation: the vertical allowance is ±35mm and the horizontal one is ±100mm.
The main jacking station has a total capacity of 1,400 tonnef. It consists of four telescopic cylinders of 5.50m stroke and 350 tonnef each. The intermediate jacking stations are made of 16 cylinders with 85 tonnef capacity each and a 700mm-stroke. The first intermediate jacking station is located 70m behind the TBM with the other stations located 250m behind each previous station.
Bentonite is used to lubricate the jacking pipes. The grout injection takes place on every third pipe and is manually controlled. This bentonite grouting has proved so efficient that a 600m drive could be jacked using only 75 % of the capacity of the main jacking station.
Pipes
The Soletanche-Bachy design office also designed the jacking pipes which are manufactured locally by the Titan Company to comply with British Standard BS5911 Part 120. Titan make the pipes using the vertical casting process in a plant supplied by Pedershabb. Actual production is 22 pipes per day using only two moulds and working 24h a day.
Schedule & performance
The first TBM arrived on the construct site in December last year and started excavating at the beginning of 2001. The second machine arrived mid January and started work at the beginning of February. On March 15th 1.5 km of pipe installation had already been completed making the project already more than one month ahead of schedule.
The first drive of 600m was completed in five weeks, including the training period, and the second drive (525m long) in less than four weeks.
All 10km of tunnelling are assumed to be completed within a year.
The normal working time is from Monday 07:00 until Saturday 19:00 non stop each week. The best weekly progress has been 260m with one machine and the best daily production 71m. Instantaneous advance of the TBM is approximately 100mm/min, while the addition of two new pipes takes around 45 minutes.
This means that two 2.50m pipes can be installed and jacked forward in less than two hours.
The Soletanche-Bachy/Conconcreto Consorcio tunnelling workforce comprises 76 direct employees of whom seven are expatriates (all of them are tunnel specialists mainly from the CSM Bessac company).
Conclusion
To date, the advance rates and the way the project is going are in accordance with what had been expected before the start. Despite some delays in getting all necessary authorisations, dealing with local public services to deviate services (electricity, gas, etc) all the works are on schedule and the completion date should be met to the entire satisfaction of the Empresa de Acueducto y Alcantarillado de Bogotá which is convinced to carry on using trenchless techniques in their future projects.
