A 13.7km long water supply tunnel with multiple curves, successfully bored by four 3.14m diameter EPBMs in five drives under Bangkok in 12 months – quite an achievement, especially considering the JV’s lead contractor was building its first EPBM driven tunnel with a segmental lining.
The project is the Minburi Water Supply Tunnel, recently completed by the SCS JV, comprising Thai leader See Sang Karn Yotah, and Chinese firms China International Water & Electric Corporation and the Shanghai Urban Construction Corporation.
The 2.3m i.d. tunnel stretches from New Ngamwongwan Road to a distributing pumping station in the city’s north eastern satellite town of Minburi, and makes up the first completed section of The Seventh Bangkok Water Supply Improvement Project, a 45km long system of transmission tunnel and associated pumping stations.
The client, the Metropolitan Waterworks Authority, Bangkok, Thailand (MWA) awarded the US$34M, 28 month, design and build ‘Contract G-MC-7B’ to the SCS JV in June 2001. By February 2002 the JV had the first of its four new Japanese Hitachi Zosen EPBMs in the ground and boring through Bangkok’s mixed ground.
Almost exactly a year later the last machine completed its drive, bringing the project in successfully, and on time. In October 2003 the JV handed the tunnel over to the MWA ready for the client to begin transferring the 700,000m³ capacity of potable water per day.
Tunnel route, geology and works
The tunnel’s alignment was bound by Thai legislation, which requires MWA pipelines to be constructed in public land. Subsequently, the route runs within the right of way of highways or roads, resulting in frequent changes in direction and a large number of small radius curves. Constructed some 18m below ground level with a fall of only 1m, the Minburi tunnel has a total of 61 curved sections; the minimum radius of curvature being 100m.
Geology-wise, beneath the alignment’s top 2m-3m of made ground, lies some 6m-14m of soft Bangkok clay overlying “medium clay”. Trial boring indicated that this medium clay layer was 2m-6m thick and was the weathered top section of the underlying stiff clay layer, which varies from 1.5m to more than 9m in thickness along the route (Figure 1).
The first true sand layer is below the stiff clay layer and consists of very dense to very loose silty sand. Observation of the ground water in the sand on contract G-MC-7B indicated a standing water level up to 4m above the invert.
The tunnel axis, 18m below ground level, is generally located in the stiff clay layer but due to variations in thickness of the different layers, soft clay, medium clay, stiff clay, sand and various mixed faces were encountered at different times during tunnel driving.
The tunnel
The client specified four EPBMs to undertake tunnelling on the project, leaving the contractor to decide on the number and length of drives. The only permanent shafts are at each end, one at New Ngamwongwan Road at a busy road junction and one within the grounds of the new pumping station being built under a separate contract at Minburi (Figure 2). Temporary shafts along the route were located in dual carriageway highways!
SCS JV opted to construct the nearly 14km long tunnel in five drives, using one TBM for two of the drives:
Shaft 3A/B to Shaft 2 – 1,817m
Tunnelling began in February 2002 at Shaft 5A/B on the 2km drive towards Shaft 4.
There was an extremely limited working space within the tunnel, but detailed planning and care during the drives ensured that each seven car backup trailer and 50m long belt conveyor could negotiate the curved sections. In fact, the drive from Shaft 1 to Shaft 2, which started in April 2002, headed straight into a 100m radius curve!
Each backup train was designed to negotiate curves down to 80m, as were the belt conveyors. As described above (refer inset), each EPBM was equipped with 2 x 160mm stroke copy cutters, one for use, and one for back up. These were set by the operator to overcut at the required position on the face depending on the steering direction. The two-part shield could articulate up to 1.9?, so the horizontal curves were negotiated by setting the articulation angle and overcutting the face on the inside curve whilst steering with the shove rams. In this way, none of the 61 curves along the alignment caused any of the four EPBMs any problems.
Over the five drives, EPB face pressures were limited to between 1.65-2 bar, although pressures of up to 3 bar were used in the mixed face conditions. The pressure cell on top of the shields also proved extremely useful when transversing sections of mixed face and very soft clay.
Removal of spoil from the discharge point from the screw conveyors was by a belt conveyor discharging into muck skips that were then transported to the shaft bottom and hoisted by crane to be emptied into temporary storage bins.
Primary lining
The pre-cast concrete bolted primary lining was designed and manufactured by See Sang. CRS of the UK provided advice on the segment design, factory set up and mould production. The lining consisted of a 1100mm wide x 150mm thick ring made up of six trapezoidal segments. Rings had a 33mm taper to facilitate steering adjustments and construction of curves. The segments were offloaded in front of the first trailer, some 10m behind the shield and picked up and installed by a ring type segment erector within the shield. Curved bolts were used at both cross and circle joints and each segment had a combined grout hole and lifting point fitted with a one way plastic valve.
This is the first use of this “European” style tunnel ring in Thailand and it proved to be very successful both in speed of erection and accuracy of build.
Secondary lining
The epoxy coated steel secondary lining is 12.7mm thick, giving the tunnel a finished i.d. of 2300mm. The annulus between the primary lining and the steel lining was filled with concrete with a 28 day strength of 20N/mm², and pressure grouted in sections as the lining was installed. Generally the lining came in 6m lengths with shorter 4m lengths used on tight curves to maintain a minimum 150mm concrete thickness between the outside of the steel lining and the concrete segments.
Manufactured in Bangkok by Able Industries Co, each length was grit blasted and the internal epoxy coating applied with the outside surface cement washed.
A temporary internal spider was fitted to the end of each lining section and the lining pipe installed and concreted in 12m sections.
To provide good ventilation during the lining work, a small shaft was bored midway along each drive. Powerful fans extracted foul air and fumes from the in-situ joint welding and coating through this shaft with fresh air being drawn down the main shafts and along the completed tunnel. Lining operations commenced at the ventilation shaft and worked back towards the main access shaft.
The 24hr lining work at up to six locations on busy roads required co-operation between all those involved in the contract, the police, and the concrete supplier. Transporting steel sections, concrete and equipment up to, in and out of the tunnel, for distances of up to 2km also required extensive co-ordination between all parties as any delays, either above, or below ground could have caused severe disruption.
After completion of the secondary lining work, risers were installed in the shafts at each end of the tunnel and bulkheads fitted. The complete tunnel was thoroughly cleaned and filled with chlorine treated water. After disinfection, a pressure test was successfully carried out.
The tunnel pipeline is now connected through a new valve chamber to an existing 3400mm diameter MWA pipeline in New Ngamwongwan Road and is presently being commissioned.
Conclusions
The SCS JV had to complete a 13.67km long tunnel, with secondary lining, in a short 28 month timescale.
Throughout the works SCS, in conjunction with the client and his consultant TEC/TCTCI/CDM, developed and implemented good working practices using engineers, foremen and operatives that were generally new to the organisation. The company introduced innovative, well planned methods for the construction of shafts, tunnel works and installation of the secondary lining. These factors, allied with the adoption of trapezoidal segmental rings, assisted in ensuring the project was handed over on time, to a satisfied client.
Related Files
Figure 1
Figure 2
