Pipejacking is a useful technique for more than just service installation. On the new Blue Line metro extension in Bangkok it is being used for top down construction work, with interlinked pipes forming a roof structure 275m long below which excavation can proceed.
Jacking has been used this way in the past for making adits and underpasses in countries like China, Singapore and Germany but local contractor Ital-Thai Development (ITD), one of the country’s largest firms, thinks it is possibly the biggest project of this kind. It is certainly the first use of the "piperoof" method in Thailand says the ITD project director Pairach Supavivat.
The contractor has developed the method to solve a specific problem on the extension work for the Blue Line underground metro. The city’s first underground line was opened ten years ago with a C-shaped route on the east side of the Chao Phrya River finishing at the central railway station. Client, the Mass Rapid Transit Authority of Thailand, now wants to complete the line as a circle, filling in the western side gap in the route.
First part of the work is a 5.4km long underground extension across the river with four new stations, let in two halves.
Later an elevated line will finish the route. The new tunnels will carry the southern end of the route just 300m away from the historic buildings of the royal city areas and through the well known Chinatown district. That makes challenging work for ITD on the first 2.5km length. Although one of its deep stations can be built by conventional excavation within diaphragm walls the other, Wang Burapha, is potentially in archaeologically important ground. The city authorities have banned any excavation of the top 5m of undisturbed ground where relics might exist. The ban also helps maintain traffic in the street above, which is very important in the often congested city.
The contractor is allowed excavation to the side of the road where part of the station will replace older buildings and during the 2010 tender stage, looked at enlarged diameter tunnelling to make the station platforms under the road, but then it turned to the piperoof solution as a better option. Essentially the plan was to form a new roof under the road by jacking from the excavations to the side. With a pipe roof in place a concrete slab could then be formed on the underside, embedding the jacked pipes and below this excavation could proceed leaving the ground above untouched.
The scheme devised is for a diaphragm wall construction through Bangkok’s very soft clay ground and into stiffer clays and a sand layer below. Under the building this would be excavated to make a working space to set up a jacking rig frame tied to a central diaphragm wall. The method uses 1.3m diameter steel pipes running across the 12.8m wide space for the station tracks and platforms. It is kept this narrow to fit under the street above, with platforms one above the other. It means the twin tunnels will arrive in a stacked formation. The pipes are pushed across the gap in a parallel formation along the entire 200m length of the station. ITD, which has substantial pipejacking experience anticipated this would be relatively straightforward in the city’s notoriously soft clay ground.
"We are using two hydraulic cylinder jacks of 100t capacity each" says Pairach. "They are used with a total jacking load of about 40t."
Mucking out is straightforward using a half bucket system, with a pulley wire for drawing it back, to remove the spoil. The pipes are big enough for man entry if any obstacles occur. The most complex part of the jacking is keeping the pipes on line, not least because they have to be connected together; for this a clutch system is being used a little like that on sheet piles. The clutches are welded onto the sides of the pipes. Details for the clutch design were worked up in conjunction with a Singapore consultant GX Tunnel & Environ Services. Pairach says that he does not anticipate any great difficulties with this because the distances are relatively small. "Standard surveying methods for vertical and horizontal direction should be sufficient with adjustments by the four jacking forces."
To be sure, the company ran a trial last year at the station site itself, making a small temporary shaft when the diaphragm walls were being done and jacking two pipes. These will be incorporated into the final roof. The most complex detail has been how to make the connection at the far end to the diaphragm wall there. ITD has worked on this detail in conjunction with its designer, the Hong Kong office of consultant Arup International. The consultant is part of the design and construct team. "Initially we considered ground treatment in front of the diaphragm wall by perhaps soil mixing" says Pairach, however during the trial runs the team decided that this preparation would not be needed.
But the rest of the connection method remains as planned, using couplers embedded in the diaphragm wall to link to dowel bars which are cast into a concrete plug at the end of the steel pipes. "The ends of the pipes are jacked until they reach the diaphragm wall on the opposite side of the station" he explains. Here there is a compressive pad cast into the diaphragm wall panel and the pipes push into that to minimise the ingress of water and soil at the far side.
With that done and the ground sealed the face of the diaphragm panel can be broken away to reveal the couplers and make the links. Formwork is set up inside the pipe and a 3m length of concrete is then cast into the end of the pipe making the connection. The first, or rather the third pipe after the trial pipes, was jacked into place at the beginning of December and installation is currently proceeding along the roof length. Once that is finished the slab will be formed. To do this ground will be excavated underneath the roof and then formwork on a hydraulic jacked frame will be positioned and concrete pumped in. That forms a composite roof structure below which normal top down construction will continue to make the two tracks and platform boxes.
The TBM making the tunnels will pass through the end diaphragm walls and be hauled through the station. Meanwhile the other side of the station will also be excavated for the concourses and ticket areas. Above ground there will be a new structure and entrances.
