Construction of the 17.5km long, 2.8m i.d. EPBM driven tunnel on Contract G-MC-7C of Thailand’s Seventh Bangkok Water Supply Improvement Project has seen Thai contractor See Sang Karn Yotah triumph against a tight 26 month programme – with just over a 12 month window for tunnelling.

It’s all the more impressive when you consider this was only the contractor’s second experience with a trapezoidal lining, whilst it juggled the logistics of running five machines through a mixed face alignment with 63 curves of a minimum 93m radius.

See Sang Karn Yotah was awarded the US$43.8M construction contract for the 20-25m deep tunnel and seven shafts by the Metropolitan Waterworks Authority, Bangkok, Thailand (MWA) in April 2004 with a start date of 19 May 2004.

A fairly comprehensive design was issued with the tenders, although the contractor could re-arrange drive lengths and temporary shaft positions to best suit the timetable. The client appointed TEC Thai Engineering Consultants as project engineer, to check the contractor’s re-designs, drawings, and supervise the works.

The outline design saw the tunnel split into five drives (figure 1), with five EPBMs stipulated to bore each drive through the medium to very stiff clay, sandy clay and sand. The alignment for the most part runs through the stiff clay with a UCS of 75-150kPa (figure 2).

Forward thinking pays off

See Sang Karn Yotah had a real advantage during the tender for G-MC-7C, as it had just completed the MWA’s adjacent 13.7km long, 2.3m i.d. Contract G-MC-7B tunnel (T&TI, May ‘04), on the same water improvement project in July 2003. For this first tunnel, the contractor had bought four 3.14m diameter Hitachi Zosen EPBMs to bore through very similar geology and curves.

Having ensured from the very start that these four EPBMs could be re-engineered for possible use on Contract G-MC-7C gave See Sang Karn Yotah a competitive edge, as its project manager for the contract, Bob Moncrieff explained, “It was known that the next project (Contract G-MC-7C) would be 500mm greater in diameter and would require up to five shield machines. The company had invested in four new machines for Contract G-MC-7B and it was an easy decision to make to ensure that these machines could be upsized and re-used.”

After completing Contract G-MC-7B, and after confirmation of the next contract award, the four machines were dismantled and shipped back to Japan in May 2004. In the Hitachi Zosen factory, the trailing back up, the hydraulic equipment and the main drive units were re-cycled and new shields, erectors and cutter wheels manufactured. The original drive units were designed to accept additional motors and the screw conveyors and belt conveyors specified with additional capacity so they didn’t require modification when the shields were upsized. Delivery of the machines back to site began just three months later in August 2004.

But there was still the matter of the fifth shield that was vital if there was any chance of sticking to the schedule.

“One of the long drives required a completely new shield machine that could negotiate curves down to 50m radius. With a manufacturing time of over eight months eating into the 26 month programme, this 4km drive and secondary lining activities were on the critical path. A delay to this shield would have delayed the whole of the works and contract completion. At some risk to both parties, we set in place a pre-contract agreement with the shield manufacturer. This enabled Hitachi Zosen to order critical materials and components to ensure its delivery schedule could be achieved, and it was!” said Moncrieff. EPBM 5, arrived on site in November 2004.

Early works and changes

Whilst awaiting delivery of the EPBMs, the contractor got on with the two, 10m diameter x 32m deep permanent shafts, and the five temporary shafts, four of which were 9m diameter. A 9m x 5.5m “capsule” shaft was installed in a busy main road where working space was restricted. Activities on site began in June 2004 after a complete re-design of the client’s proposed shaft construction method, as Chana Erbkamol, See Sang Karn Yotah’s senior design engineer explained, “Originally, the client had both temporary, and permanent shafts designed as open caissons, cast in-situ and sunk in stages as each succeeding lift was constructed. This would have taken about four months per shaft! Instead, pre-casting in large elements streamlined the concrete work and ensured that the caissons could be sunk in a controlled manner in a continuous process, shortening the construction time of each shaft by around six weeks”.

The change paid off – the first working shaft was finished within 10 weeks.

The flexibility and open mindedness of the client coupled with the contractors will to minimise construction time was to pay off again, but this time regarding the tunnel’s primary lining. The client’s original design consisted of 5 segments + key, with steel bolt boxes at segment joints and curved bolts at the circle joint. As well as straight rings, different tapers were specified for negotiating the various curves and See Sang Karn Yotah was expected to use timber packing for normal steering adjustments.

“Apart from the difficulties of manufacturing many different types of segments, the logistics of installing this system to a good standard in five rapidly advancing tunnel drives, up to 4km long was daunting,” said the contractor’s deputy project manager Teerawat Teeranuwatkul, “a “universal” ring with a fixed taper consisting of six nominally equal trapezoidal segments was the answer. On Contract G-MC-7B, which had been a design and construct project, we had introduced trapezoidal segments and successfully demonstrated how well the system worked. The client had no reservations about adopting our proposal to change the lining system.”

The nominal ring width was upsized from 1000mm to 1100mm, with a fixed taper to allow the rolling of succeeding rings in a fixed sequence on curves down to 100m. For tighter curve radii the rings were made to the same taper but with a ring width reduced to 700mm. The EPBMs’ segment erectors, trailing back-ups, grouting systems and rail system had final adjustments made to accommodate the new ring design and allow for easy segment installation and grouting. The steel segment moulds were manufactured in Thailand by BNK Ltd, with all segments made at See Sang Karn Yotah’s purpose built factory at Saraburi, some 100km north of Bangkok.

With the lining design finalised, the shafts in place and the machines ready to go, it was time to start tunnelling.

Tunnelling arrangements

All the drives, except from the Bang Plee Riser to Shaft 4, are in opposite directions from the back to back double shafts (figure 1). This was the same system as used on G-MC-7B and was devised to minimise tunnelling start time, as Erbkamol explained, “the double shaft concept allows tunnel drives to be carried out back to back using common resources and speeds the launch of the second machine. After a section of tunnel is complete it can be isolated and lined from both ends with no interference to work in the adjacent section of tunnel”.

EPBM 1 was launched, with the back-up trailers initially on surface, on 27 September 2004 from Shaft 2A through 2B and on towards reception Shaft 3. In the first 100 days, 1,400 rings were installed. EPBM 3 was then launched on 10 October in the opposite direction from Shaft 2A with the back-up assembled in the connection tunnel between Shaft 2A and Shaft 2B. Average rates of 18.5 rings/day were achieved for the first 1,400 rings.

By January 2005 all five machines were in the ground and making good progress.

Tunnelling experiences

The MWA’s tunnel, due to Thai legislation, had to be constructed in public land resulting in the alignment following busy highways and roads. This left the fore-mentioned frequent curves along the route to avoid the multitude of piles supported the road bridges, viaducts and other structures.

“In some instances we passed as close as 250mm to piles and had to co-operate with many authorities, including the Department of Highways, Express Transport Authority, Bangkok Metropolitan Authority, the Royal Irrigation Department and Bang Kaew and Bang Plee local authorities. Structures were surveyed and the pile locations plotted at the planning stage. During tunnel driving the alignment and shield attitude was rigidly controlled with the face pressure maintained within strict limits. Careful back grouting, concurrent with excavation, and secondary grouting after the back up had passed ensured there was very little settlement or ground movement. No structures were disturbed,” assured Yuthana Kulapantha, the contractor’s deputy project manager.

To handle the curves the machines were equipped with 2 x 160mm stroke copy cutters, one as back up, and two-part shields that could articulate to 1.9°. For the horizontal curves, articulation angles were set and the face over-cut on the inside of the curves, whilst steering off the shove rams.

EPB pressures were generally limited to 1.5 – 2.5 bar, although this was increased through the occasional mixed face areas, with spoil discharged from the screw conveyor to a belt conveyor and then on to muck skips for rail removal from the tunnel.

As with the machines on G-MC-7B, the fishtail at the front of the cutter wheel centre and one of the spokes was fitted with an injector for water, polymer based soil conditioning, with more additive outlets installed in the mixing chamber bulkhead and screw conveyor casing. The polymer/water injection was fully automatic and recorded by the tunnel management system (TMS). With sand and a mixed sand/clay face likely on G-MC-7C, two-channel foam injection equipment was also fitted on the five EPBMs.

“As well as helping maintain earth pressure in mixed face conditions, foam injection improved the advance rate of the shield and reduced the amount of liquid required for soil conditioning. Muck coming out of the screw conveyor was easier to handle, resulting in a cleaner tunnel,” confirmed Moncrieff.

The segments were offloaded from the unit train in front of the first trailer and picked up by crane for transportation to the tail skin and installation by ring type erector. Grouting was then carried out through the segment lifting eyes as an automatic process with all data again collected via the TMS. The TMS provided by Hitachi Zosen, which also managed the gyro guidance system on each machine, allowed the operator, and the engineer at the surface, to view real time information on visual displays showing the machine attitude, position and performance.

One area of tunnel that had to be dealt with differently was the 53m long connection between Shaft 1 and the Thub Chang valve chamber. EPBM 3 bored this stretch using umbilical connections. The alignment passes under a main rail line, and, as planned the machine had to cut through several 200mm x 200mm piles supporting the track base. Before tunnelling commenced, the ground over the tunnel crown and surrounding the piles was improved using jet grouting. The primary lining was a concrete filled, steel segmental bolted ring to protect the tunnel. As space was tight in the valve chamber the shield, after breakthrough, had to be dismantled from the inside with the outer skin left in-situ.

Wrapping up

The final breakthrough of TBM 5 into Shaft 4 on the 24 December 2005 heralded the completion of boring of 17.5km of tunnel in just over a year. Impressive drive rates were consistently achieved with best days of 33 rings for TBM 1; 30 rings for TBM 2; 29 rings for TBM 3; 30 rings for TBM 4; and 25 rings for TBM 5. Despite the ground conditions and curves, the team still made an average advance of some 12 rings per calendar day.

A 15mm thick secondary steel lining is now being installed in 6m and 4m lengths. Following completion, and installation of the riser, valves and fittings, the tunnel will be filled with water and pressure tested.

“On 4 April 2006, three months ahead of schedule, the first 9.5km of lined tunnel will be handed over to a satisfied client. The remaining 8km of tunnel connecting the system to a new reservoir and pumping station at Bang Plee will be commissioned in June 2006. The successful completion of this important infrastructure project is due to our dedicated, experienced and hard working construction team, and the client’s and the engineer’s staff, sustaining a high level of performance and co-operation throughout,” concluded Montree Chantarakulpongsa, See Sang Karn Yotah’s project director.

See Sang Karn Yotah, MWA, TEC Thai Engineering Consultants, and all involved can look back with pride at a job well done when the tunnel is transferring daily up to 800,000m3 of much needed clean water from Bang Khen treatment plant to the city’s north to Bang Plee in the Samut Prakarn province by the middle of this year.