A decade after the opening of its first underground metro line, Bangkok is in the middle of a major upgrade to its public transport system. The first, Blue, line is being extended with both above ground elevated track and new tunnelled sections. A series of other lines are either in construction or being readied, to expand the city links over the next half a decade
"Most of the new lines are elevated" says Christian Schulz, an engineer with German consultancy firm Dorsch Consult who is currently on secondment as executive project director for the Blue Line works. "That is obviously less expensive than tunnel, though it can sometimes bring complications with land acquisition, utilities and traffic".
The city’s first ever line, the privately developed BTS Skytrain, is an elevated system and has proved highly popular since it was completed at the turn of the century. Running through the busy tourist Sumkamvit area and down to the river it has not only helped alleviate the severe traffic congestion that used to bring the city to a near standstill at times but has stimulated retail and residential development. Some extensions are being added to this. An airport rail link is also popular.
The Blue Line, built by the government’s Mass Rapid Transit Authority was the city’s first underground line however, running in a horseshoe shaped loop north to south, finishing near the central rail station. Opened in 2004 it took a while to develop its ridership in a city with a large poorer population for whom ticket costs are difficult. But there is a growing middle class and it has become well used; it is also an important contributor to city development. As the deputy director for the Mass Rapid Transit Authority of Thailand, Peerayudh Singpatanakul puts it "people are learning the habit of public transport".
The line will now be made into a complete circle, or rather an elliptical loop line, with the "missing" section completed up through the districts lying along the west bank of the Chao Phrya river. This section will be elevated, and so too will a 10.5km long westwards extension. But a 5.4km long connection across the river to the original underground at the south end will be in tunnel. There are four stations in this part, all underground.
As well as going under the river, a key reason for tunnelling here is that the alignment passes through or nearby some of the oldest and culturally most important parts of the city, including alongside major temples and museums. These are part of the royal palace district and also significant as tourist sights. Towards the rail station end of the new tunnel it also runs underneath the Chinatown quarter, another bustling and important area. Elevated line would have been obtrusive.
"The new line is split into five major contracts," says Schulz "the elevated section to the north end, which is 11km long, the elevated western extension which also includes a depot works, trackworks, and then two tunnel contracts.
"The first of these is for a connection to the existing line at Hua Lamphong just outside the main railway station. A cut and cover excavation makes the link to the existing underground station and from there twin running tunnels are driven for 2.85km. There are two underground stations, the Wat Mangkon in Chinatown and the Wang Burapha closer to the old city."
The contract was let to ITD in February 2011 on a designbuild basis and is worth THB 11,441M (USD 381M) The other contract continues the tunnels for another 2.7km and also includes two underground stations, the Sanam Chai, situated just close to the Thai museum and nearby a temple with Bangkok’s famous reclining Bhuda, and then on the far side of the river, the Itsaraphrat. It concludes with a cut and cover section with an incline for the track making the transition to elevated way.
The tunnels themselves are twin bores of 6.44m diameter, which after installation of 300mm thick segmental linings and grouting, leaves running tunnels with an internal 5.7m. They are both using earth pressure balance tunnelling machines for the work, a Kawasaki for Ch Karnchang and a Terratec, for ITD on the first contract.
Both have to pass through the very soft clay layers which underlie the city and into water bearing sand layers beneath. Bangkok’s geology is notoriously difficult for building construction, requiring major deep piling for most structures through the very soft clay layers – like toothpaste most engineers say – into the firmer sands and firmer clay layers under that. The city has also had issues in the past with water extractions from the aquifers beneath which caused significant settlement in the past, though it has slowed up following a ban on unregulated water use.
Stations are conventional diaphragm wall box constructions, but relatively constrained by the narrow street pattern and the difficult property issues. These have caused some delays on particularly the first contract. For the ITD contract it also means double stacking the lines in the stations to keep the width down, with one of the stations deepened to 29m. First of the ch Karnchang stations is also 29m deep.
The stations overall are 22m wide at the maximum point, with part of the station box, for concourses and escalator links, offset to one side and with a surface station building above that. It is the land purchases for the wider sections, which involve demolition of some buildings alongside the alignment which have held up some work. The tunnels themselves are kept below the street lines above.
There are also city imposed constraints on surface excavation for the two centre stations, one in each contract which has led to some unusual "pipejack roof" construction.
"The area is part of the ancient settlement and has possible archaeological importance," says Schulz. A ruling says that previously undisturbed ground, mostly under the street must be left intact for the top 5m and so contractors have had to find a way to support the top layer while excavating underneath.
Thailand’s two largest contractors won the work, Ital Thai Development, ITD for the Hua Lamphong section and Ch Karnchang on the continuation underneath the Chao Phrya river. Both started in March 2011 and both have had some difficulties and hold-ups with the tunnel drives but both are currently in progress. Completion to a revised schedule is due in mid-2016.
"Fortunately the completion of the drives is not on the critical path" says Schulz, "which is determined by the rolling stock and signalling works contract, a sixth contract which is just at the start of prcurement."
For Ital Thai, one of the main difficulties has been simply fitting its works into the congested street junctions around the main station and coping with limited site space. It is launching its drives from the end part of a cut and cover construction needed to link the tunnels to the existing line terminus.
This requires work underneath one of Bangkok’s busy roads which has required the contractor to split its worksite into three sections, one occupying the middle of a roundabout for the site depot, the next for TBM launch and spoil mucking from an access pit to one side, in front of the railway station, and a further site 200m away occupying a narrow strip in the main road, which serves as a segment store and materials depot. Engineers have to constantly cross the roads from one to the other, and ironically for once, the often slow pace of the traffic in Bangkok is a blessing.
But it makes work organisation difficult. "We have to use a just-in-time method to bring the segments from the storage area to the TBM access pit," explains Ital-Thai project manager at the main contract site Thavesak Rungpiriyadech.
Additionally he says, part of the work is being done underneath heavy road decking for the construction of the 170m long cut and cover box linking the old Blue Line to the new. An opening 18m by 18m at the far end of this is where the 15m deep tunnel access pit is located for the for the TBM work.
Tunnelling began in January last year and somewhat before that was underway on the construction of the two station boxes, using diaphragm walls and top down excavation sequences.
But property acquisitions have held up some of the station work and recently an extension of time was granted for the contractor to allow for the difficulties.
Most of Rungpiriyadech’s attention is on the tunnelling however. The twin bores are being made in two drives, one after the other, passing under the street line and through the station boxes.
The first drive is the lower of the two alignments for the stacked formation which means the TBM passes through the station boxes at a lower level first, which aims to synchronise with the excavation level they will have reached. The TBM leaves a segment lining which is removed by later excavation.
The drive is due to finish within the first of Ch Karnchang’s stations just beyond the boundary between the two contracts. It is part of the other contractor’s remit to prepare a reception space. Here the TBM is to be dismantled and brought out by crane from above to return to the start box to set up again and carry through the second drive. The start point for that is a few metres in front of the first.
The Terratec TBM had all key parts produced in Australia and Japan and the final assembly of the TBM was done in Terratec’s facilities in China. From there, the machine was shipped to Thailand by sea and the components trucked to site.
The cramped service pit is also used for the back up and mucking facilities. ITD is using a continuously advancing conveyor system also supplied by Terratec to bring the spoil from the machine and back along the tunnel where a small lateral conveyor loads it into big 5m long skips. These are lifted by portal crane and tipped into a handling container where any water can drain out loading into trucks.
But before all that could get into operation, as the first drive began in January last year, the contractor had to wait for some ground treatment for an initial section of the drive.
The tunnel line follows a shallow path initially because of the presence of two existing and quite major service lines close to the starting point. "There is a 3.8m diameter water tunnel to cross and a 1.5m sewer line," explains Rungpiriyadech.
That means it begins its drive in the Bangkok clay "which is not even as firm as marine clay," he comments. The top clay is so soft that it needed firming up and for 380m along the first part of the tunnel line jet grouting was used to form a consolidated block through which the TBM could pass.
"We used a local company Kenber which made a 2m grid of 1.2m jet grouted columns" says Supavivat.
Once over the water tunnel the tunnel path drops down into firmer clay beneath the sand layers, although some treatment is still needed for the top of the tunnel line until machine is into the lower level completely.
There is also a point where the tunnel passes underneath one of Bangkok’s famous canals (khlongs), and here 600mm diameter soil mixing columns were used for treatment instead of jet grouting, in order to avoid water contamination.
Further jet grouting has also been done at the station boxes, this time by subcontractor Trevi, to form sealed ground blocks for the TBM entry point through the station diaphragm walls. These blocks have proved slightly problematical as they were probably not long enough says Schulz. The grouting through the shield skin may not have sealed off the annulus entirely.’ says Schulz. "The TBM did not go far enough into the block and there was some water leakage," he says.
The drive up to that point has gone well enough though more slowly than hoped because the machines has had to wait for progress on the station box itself.
The machine pushed off a conventional reaction frame in the pit and for the early drive worked at a lowish pressure, as little as 0.8bar according to Bunjerd Emnuch, a Terratec engineer onsite as advisor to the contractor on the TBM operation.
"As it has come into the waterlogged deeper sand layer this has changed and it needs to work at 2.5 bar" he says.
"This sand layer s very fine" says Emnuch "and we need a lot of polymer to treat the ground forwards." Deeper in the clay beneath there will be less needed.
Guidance for the machine is by a Japanese Enzan system, "which includes monitoring equipment and data."
The machine installs a 300mm thick concrete segment ring comprising six segments and a key. Straight and left and right tapers are used for turns and the key is kept away from the invert. The design matches the original Blue Line.
Grouting is done just behind the machine shield through the segments themselves rather than from annulus grout points on the machine. There are lifting sockets in the segments to let them be pulled onto the TBM segments delivery track and these double up as grout points.
A significant number of bolts are used, some 25 around the rings to link one to the next and two bolts between each segment in the rings. "We use banana bolts" says Emnuch "which is for stability".
Segments are made at an ITD works about 150km outside Bangkok, which has to cope not only with the three ring shapes but four different levels of reinforcement in the mixes, to vary the strength of segments according to location. They are delivered by trucks, which must negotiate the traffic and are restricted for daytime movements to between 10am and 3pm. Segments go to the holding yard as the TBM shaft head can hold only a couple of rings; these are replenished from the holding yard whenever they are lifted down to the rail cars beneath by a big Demag portal crane. A Chinese made electrical locomotive uses battery power, which is sufficient for the relatively short drives being done.
Progress has been slower than possible, steady rather than record breaking, achieving around ten of the 1.2m long rings daily. That is because of the need to wait for the station box work to progress far enough to allow the tunnelling to go through. First of the stations was passed last summer – the TBM boring through the 1.2m thick diaphragm wall.
An unusual cone shape head with hardened steel blades mounted on four radial axes has been used on the ITD machine to allow the mainly soft ground machine to cope with the hard concrete.
Terratec worked out this design in conjunction with Japan Tunnel Systems Corporation, a subsidiary of Japanese IHI Group. It is the largest head of this type yet made.
The cone, first developed for smaller microtunnelling work, allows the grinding of the 40N concrete to start centrally and work outwards, rather than requiring the full face to bore at once, imposing higher torques.
"We need to penetrate concrete at two ends of two stations and two interventions shafts, as well as the final station" says Emnuch from Terratec. "And then the same is needed on the second drive.
"On top there is the chance of meeting concrete pile obstacles that need to be cut, especially at points where we pass under bridges."
A recent hold up has been caused at an intervention shaft where the TBM was entering through a grouted plug. "There was some water ingress into the TBM chamber as the fine sands are under a considerable hydrostatic pressure," says Schulz. "Additional grouting was not successful enough and compressed air system was required."
But the TBM has now entered the shaft where the cutterhead blades were changed and has left on its way towards the final station.
Some water problems have also been experienced on the first drive for the Ch Karnchang contract. For this work the Kawasaki TBM was assembled at the far end of the contract.
"There is more space here because the transition point to the elevated section is longer and the cut and cover for the ramp downwards is on a greenfield site" says Schulz. The TBM set out and passed the first as yet unexcavated station and then continued towards the next station, passing underneath a khlong and heading towards the river. But last summer the machine stopped due to issues still yet to bvwe evaluated says Schulz. "Operational issues seem to have contributed to some water leakage in the tunnel (between segments). Settlement control (by ground treatment) and repair of the TBM’s effected electronic and hydraulic systems were time consuming.
A bulk head was built at the station meanwhile were work has proceeded. Recently the TBM backup system of conveyors and segment supply rail has been reinstalled, work in progress when Tunnels visited in late February, and the Kawasaki machine has cautiously started its drive again "and was on target to reach and intervention shaft in June".
Schulz says that despite these issues the overall schedule will accommodate it, because the sixth contract sets the critical path. Meanwhile there has been other work ongoing at the four stations. These involve primarily conventional top-down excavation with diaphragm walls but some some interesting additional works as well, including the pipejack roof works (see box) for the two centre stations, Sanam Chai on the Ch Karnchang contract and Wang Burpahpa on the ITD contract.
At Sanam Chai the contractor has also carried out a massive grouting operation to form a base slab to the station.
"The design intent was that the dense clay layer deep down would seal the base of the excavation," says Schulz, "with the diaphragm walls penetrating into it."
But as luck would have it, the clay layer at this point is not as thick usual in most of the city area "or rather it is inconsistent and quite thin at points" says Schulz "and therefore not an effective seal." Dewatering would have caused tremendous risk to the project and MRTA was concerned that use during excavation could trigger an uncontrollable threat of settlements to the neighboring Thai museum and other adjacent prominent and historic buildings.
So a Taiwanese contractor was brought in, making hundreds of holes to the base of the station area, for a systematic tube a manchette grouting operation, including a 4m thick grout slab 50m below the ground suface and a vertical cut-off wall,recently completed. recently completed.
Excavation work is proceeding downwards readying the station for the arrival of the TBMs.
