The Chaloem Ratchamongkhon Line (formerly known as the Blue Line or Initial System Project) is the first stage of an integrated transportation plan for Bangkok to be implemented in conjunction with other schemes by the Metropolitan Rapid Transit Authority (MRTA). The southern half of this line involves construction of a twin-bored underground metro from the inter-city railway terminal at Hua Lamphong near the Chao Praya River. It will run eastwards for 5km beneath the busy Rama IV Road to the Queen Sirikit National Convention Centre. It then proceeds 4.5km north beneath Asoke and Ratchadaphisek roads, ending at Rama IX Station, with a connecting line to the depot.
The northern section of the metro continues from Rama IX Station for 4.5km north along Ratchadaphisek Road to Lat Phrao Road, then turns west to Chatuchak Park and finally terminates under the Bang Su yards of the State Railway of Thailand. Tunnel axis level is typically 16-23m below the surface. Each section is approximately 11km long and includes nine underground stations (Fig 1).
Along Rama IV Road, the presence of an elevated 4-lane road necessitates stacking the two tunnels one above the other to fit within the road easement and avoid the foundations of adjacent buildings and fly-over piers. In this location, the lower south-bound tunnel will be driven first at an axis depth of 16-28m below the surface, followed by the shallower north bound tunnel at a typical axis depth of 12m.
Land constraints mean that three of the stations must have a stacked platform configuration. Two stations at Silom and Phahonyothin are located under overpasses, necessitating underpinning and reconstruction respectively. Other design and construction issues involve coping with the fact that the tunnels pass close to a high pressure water supply tunnel at two locations; removal of piled foundations which interfere with the tunnel alignment beneath five bridges over canals (known locally as khlongs); and avoidance of overpass and expressway pier foundations.
Underground stations are typically 200m long and 23m wide, located generally under the street easement with an upper retail level, a ticketing concourse and a platform area. Rail levels are typically 21m below ground level, with access provided by escalators, stairs and a lift for the disabled. Entrances are usually adjacent to the footpath on each side of the roads, with ventilation shafts within severely confined sites. The tunnels are 5.7m nominal i.d., with a 0.3m thick by 1.2m wide concrete segmental lining.
Hydrology and regional settlement
Pumping from the deeper aquifers underneath Bangkok and its environs has reduced the pore water pressures under the southern part of the Chao Praya plain by approximately 23m from the original hydrostatic profile from near ground level. Detailed piezometric studies for the Chaloem Ratchamongkhon Line provided an insight into local variations of pore water pressure along the route.
The hydrostatic conditions at the surface reduce below a depth of 8-10m, reaching a pore pressure minimum at a depth of 21-23m, depending on the location and thickness of sand layers, the result of under-drainage from the first and second sand layers (Fig 2, below). Typically, the upper few metres of the sand layers underlying the first stiff clay and the very stiff clay have been de-watered. The tunnel drives are generally located in the vicinity of this pore pressure minimum. As the moisture content of the stiff clay is close to the plastic limit, water with polymer or bentonite must be added to the face to ease the stiff clay out of the screw conveyors.
The groundwater change occasioned by deep pumping has induced surface settlements between 1933 and 1987 over the system alignment of 0.6-1.1m. Settlement rates of 100mm/year and 40mm/year in the north and south respectively were recorded in1978-1981. However, with a reduction in pumping based on the last ten years of settlement monitoring, surface settlement rates now range from 4-7mm/year in the vicinity of Hua Lamphong Station and 17-20mm/year around Mo Chit and Kamphaeng Phet stations.
Most of the surface settlement is now due to consolidation of the uppermost soft clays, the settlement rate contributions from each layer reducing with depth. However, there is sufficient variation with depth to create a potential for small differential settlements between the tunnels and the station diaphragm walls. As the Kawasaki TBMs are being used to cut through the diaphragm walls, an enlarged annulus around the lining will be cut by hand at a later date, which will be filled with a compressible packer. A watertight movement joint will be attached between the permanent segmental lining and a collar tied into the diaphragm walls. On the second, south, section the diaphragm wall will be broken out entirely by hand and the movement joint installed after the segmental lining is completed.
Tunnelling progress
The tunnelling conditions principally involve excavation in stiff to hard clay, which is self supporting, and dense, fine silty sand as the drives get deeper between stations. Although six of the eight machines are built by Kawasaki, they are operated more or less autonomously by each of the partners within the respective JVs and have slightly different specifications. They are all high-speed machines and are capable of up to 100mm/ min cutting speed.
The machines can operate in dual mode but, to date, have been operated with slurry injection using polymers or bentonite to control the plasticity of the clays. The volume of water injected varies considerably between operators, with machines 3 and 4 requiring the largest addition of water as the spoil is pumped the entire distance back to Ratchada Shaft.
Most drives to date have started with minimal cover to the soft clay. This medium, which is initially less stiff, and the slower initial advance rates due to the temporary back-up arrangements while the machines are launched has generally resulted in higher settlements near the launch shafts. Some settlements recorded near the launch shafts are shown in Fig 3.
North section 1
“Both tunnels have passed beneath two fly-over bridges across the alignment with no adverse effect on the foundations and piers” |
Nishimatsu launched two TBMs from a temporary start shaft inside Thiam Ruam Mit Station in April 1999. Back-up equipment was initially fitted inside the upper two levels of the launch shaft, with umbilical cables and hoses linked to the machine. Each drive progressed 60 rings before the back-up equipment was transferred to the tunnel. These drives started in stiff clay, which changed to largely dense fine sand as the depth between stations increased.
The machines are being operated in EPB mode with injection of water into the cutter chamber to maintain consistency within the screw conveyer. Face pressures of typically 0.5 bar are being used, resulting in surface settlements from both machines of 50mm initially in soft clay but now typically 30mm from both TBMs. The spoil is transported by skip to a tipping hopper at the launch shaft. A vertical conveyer is used within the shaft.
Before completion of the drives to the south end of Ratchada Station, excavation of the Thiam Ruam Mit Station will have been completed and the base slab cast. The connecting diaphragm wall will be broken out in October to ease the logistics in the shaft area. This shaft will then serve the drives between Thiam Ruam Mit Station and Rama 9 Station and the depot approach, with muck trains travelling through the station. At the end of August 1999, the tunnel drives were approximately 21% complete.
North section 2
Obayashi began the southbound (SB) tunnel drive in February 1999 from a launch shaft formed from a partitioned section within the Ratchada Station box. A second machine started the northbound (NB) drive approximately 100m behind the first in March. The back-up equipment was assembled on the surface, with umbilical hoses and cables powering the initial drive of the SB tunnel of 98 rings to beyond the underpinned Khlong Naem Kaew Bridge piles. While the cutters were being inspected after cutting through the 300mm thick driven piles, the back-up equipment was shifted to the tunnel.
The initial drive of the NB tunnel was limited to 53 rings, a minimum requirement for installing the back-up equipment in the tunnel and launch shaft. The Kawasaki EPB machines were designed to grind through the fibreglass rod reinforced concrete ‘soft eyes’ of the diaphragm wall and the small diameter redundant bridge piles at Khlong Naem Keaw. They had also to contend with the bored pile foundations of a pier of the Lat Phrao Road fly-over. The machines then continued through Lat Phrao Station. Before each intersection with the diaphragm wall, Obayashi provided a small jet grout layer beneath which the head was inspected and the cutters changed where necessary.
A 450mm diameter pipe was installed inside each end of the station box in a borehole from the surface to provide face ventilation during cutter pick inspection, to facilitate a wire drop survey of the alignment and to provide future access for invert concrete supply.
The initial advance on launching was typically up to 40 rings/week. After installation of the back-up equipment, Obayashi has consistently achieved 80 rings/week. The machines have been operated entirely in EPB mode, with polymer injected into the cutter chamber to condition the generally stiff clays for pumping back to the Ratchada Shaft. These two machines will be retrieved from a reception shaft being prepared inside the end of the incomplete Phahonyothin Station shell for subsequent re-installation at Bang Su Station. From Bang Su Station, the machines will start the drive to Kamphaeng Phet Station in December.
South section 1
Kumagai Gumi launched two Kawasaki EPB machines from a completed Rama 9 Station shell in June 1999, heading south towards Petchaburi Station. The NB machine cut through the Petchaburi Station diaphragm wall at the end of August and was followed soon after by the machine on the SB drive. Both machines successfully negotiated the backfilled holes left at one underpinned khlong bridge after extraction of numerous driven piles. Typically, progress rates of 80 rings/week were achieved, with a best week of 111 rings from one TBM.
The machines, complete with back-up equipment, were installed in the Rama 9 Station shell soon after the base slab was cast, permitting the drives to progress without interruption. Initial surface settlements of approximately 30mm, recorded close to the station, were reduced when face pressure was increased. Typical face pressures of 1.5-2 bar result in a minimum surface settlement of typically 10-15mm. Both tunnels have passed beneath two fly-over bridges across the alignment with no adverse effect on the foundations and piers. These Kawasaki machines differ slightly from the Nishimatsu and Obayashi machines with respect to the shove rams, a fourth cutter motor, a ring former and a double man lock as a contingency for compressed air working.
South section 2
At Sirikit Station, Bilfinger+Berger has launched two re-conditioned Herrenknecht EPB machines previously used on the Taipei MRT project. The back-up equipment for these machines was fully installed within the station box. The SB TBM was launched from the south end of the partially completed station in July this year and the NB tunnel drive in late August. Both machines began in a 200m radius curve on a 3% down gradient in order to pass close underneath a 2.5m diameter water supply tunnel in Rama IV Road.
The machines initially pumped the spoil back to the shaft but this method was changed a little earlier than scheduled to skips, due to the harder than expected nature of the clays encountered after about 150 rings. The tunnels pass beside the bored piled foundations of a new fly-over being constructed by the Bangkok Municipal Authority at the Ratchadaphisek Rama IV intersection. These machines will be skidded through completed station boxes at Bon Kai, Lumphini, Silom and Samyan, and retrieved at Hua Lamphong.
From mid-way between Lumphini and Bon Kai to near Hau Lamphong, the NB tunnel is stacked above the SB tunnel and penetrates the medium clay at the bottom of the soft clay.
Related Files
Surface Settlement
Ground Conditions
Location Route Map