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International Symposium on Underground Excavation and Tunnelling2-4 February 2006, Bangkok, Thailand

Closed Face Shield Tunnelling Trends in AsiaJames D. Broomfield

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1Herrenknecht AG, Germany

ABSTRACT

During the course of the last few years Asia (excluding Japan) has experienced a dramatic increase in

the use of shield tunnelling methods in the urban environment, mostly in areas where there has been

little or no previous experience in such methods. In total, more than 100 TBMs, have been beendeliverd in Asia over the last seven years. The main focus of these activities has been in China and

Singapore but also with significant increases in Malaysia, Thailand and India. The vast majority of

these works have been in urban areas for Metro Rail Projects where, almost exclusively, Earth

Pressure Balance (EPB) type TBMs have been used for the construction of twin tube underground

railway tunnels. Some projects have pushed such technology to the economical limit with high

groundwater pressure and high permeability of the excavated material. More recently, the application

of shield tunnelling has increased in China and Malaysia where geological conditions involving

complex underwater crossings combined with much increased dimensions for road tunnels have

dictated the use of tunnelling methods using slurry face support. A similar pattern is emerging in

Singapore also where, despite the relatively successful use of approx 50 EPB machines, slurry

methods are now also being adopted for geological reasons. This paper, whilst discussing mechanised

tunnelling activities in Asia (excluding Japan), is not intended to provide a comprehensive report on all

projects throughout Asia but instead provides a broad overview of the areas of main activity.

1. BANGKOK, THAILAND

Intensive use of mechanised shield tunnelling in the Asia region (excluding Japan) began in 1998 with

delivery of a total of 8 EPB type machine for Bangkok MRT to be used for construction of the 20km

long blue line finally opened in 2004. The works were divided into two contracts, each utilising 4

EPB type machine. Kawasaki of Japan supplied six machines and the remaining two were from

Herrenknecht, Germany.

EPB methods were perfectly suited to the complex Bangkok strata comprising soft marine clays(known as Bangkok clay) and a hard clay layer with sand layers in depths ranging from 14 to 30m.

Best monthly production rates of approx 600m were achieved.

2. SINGAPORE

Whilst 67km of MRT was constructed in Singapore from 1983 to 1990 of which approx 20km was

underground, this was mostly executed using open face shields with the frequent use of compressed air

at pressures up to 2,5bar to control the water at the tunnel face. Tunnelling was generally carried out

in the Kallang and Jurong formations, with little granite encountered. Rapid changes in geology made

consistent progress difficult to achieve. Two EPB type machines were used in the latter stages of this

work (late 1980s) with some success.

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Subway tunnelling continued at great

speed from 1998 onwards with construction of

the 20km long North East Line, all of which

was underground. 10km was bored tunnel,

6km in cut + cover and 4km in stations. A

total of six separate contracts utilised 14 EPB

type machines as well as 2 open face shields

as listed in table 1. The geology to the north is

old alluvium with Kallang and Jurong

formations occurring in the south. Strong hard

granite was experienced at invert level of

some drives giving rise to difficulties at the

interface with the soft strata above. The old

alluvium also proved to be a very abrasive

stratum causing high wear rates on

cutterheads.

Figure 1. Herrenknecht EPB 6,250mm dia,

Singapore NE Line Contract 706

Table 1. North East Line Contracts, Singapore

Contract Machine Type Supplier Length

703 2 x EPB Mitsubishi 1.3 km

704 2 x EPB Lovat 2.5 km

705 2 x EPB Hitachi 2.3 km

706 2 x EPB Herrenknecht 1.6 km

708 2 x EPB2 x open shield

Hitachi 1.6 km

710 2 x EPB IHI 2.8 km

At the same time the 6km long Changi line, an extension of the existing East-West Line, utilised

two Lovat (Canada) EPB type machines for the 3,5km long bored tunnel section. The TBMs for this

project were purchased directly by the Singapore Land Transport Authority (LTA), both for

programme reasons and the desire to ensure that only high quality would be utilised as they would

have to pass under runway 1 in Changi airport. The machines were issued to the contractor by LTA

rather than allowing the use of cheaper, light duty equipment that may not be adequate for the task.

Activities in Singapore continued early 2001 with the commencement of the 48km long Deep

Tunnel Sewerage System (DTSS) seeing delivery of a further 8 TBMs, all of which were of EPB type,

of varying size in the range of 4,45 to 7,20m external diameter. These machines were utilised in six

separate contracts as listed in table 2 to bore soft ground and hard rock at depths varying from 20m up

to 50m.

Table 2. DTSS Contracts, Singapore

Contract Machine Type Supplier Length GeologyT01 1 x EPB Herrenknecht 5.9km old alluviumT02 1 x EPB NKK 7.7km old alluviumT03 1 x EPB Kawasaki 5.1km old alluviumT04 1 x EPB Mitsubishi 7.3km old alluvium / decomposed graniteT05 2 x EPB Herrenknecht 12.6km granite and decomposed graniteT06 2 x EPB Herrenknecht 9.7km granite and decomposed granite

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Completion of tunnel boring for the DTSS project was achieved on 24 January 2005 with the

final breakthrough of contract T-05 by a Herrenknecht EPB (figure 2) that had earlier set the record for

production rates achieved on the whole of the DTSS project achieving.

Best shift (12 hours) 22m

Best day (24 hours) 42m

Best week 210m

Best month 631m

Previous project records of 625m in one month were set by the Herrenknecht EPB on contract T-

06 and earlier, 613m in one month by the NKK machine on contract T-02.

Despite respectable record monthly performances, these EPB machines struggled at times

through difficult mixed face conditions under high groundwater pressures of up to 5bar. Modifications

were necessary on the T-02 contract machine (NKK supply) to address granite encountered over the

final portion of the tunnel drive. These were carried out in situ at a depth of 35m in a pre-grouted zone

of soil, prior to entering the granite. All of the scraper tools were replaced with disc cutters, the cutter

arms were strengthened and the screw conveyor armoured for the additional wear expected. The ratioof openings on the cutterhead was also reduced.

Figure 2. Herrenknecht EPB DTSS Contract

T05, Singapore

Figure 3. Herrenknecht EPB DTSS Contract

T06, Singapore

Similarly, the T-05 contract machine (Herrenknecht supply) at one point was lifted completely

out of the ground via a purpose built shaft for major refurbishment lasting approx 3 months. Theseworks included replacement of the cutterhead and screw conveyor, virtually destroyed by the extreme

abrasive granite rock conditions encountered. The refurbished EPB then continued to complete just

under 5km in 15 months.

Singapore tunnelling continued at a frantic pace in 2002 with commencement of the 34km long

Circle Line. Over a period of nearly 3 years, nine further tunnelling contracts were awarded utilising

more than twenty TBMs as listed in table 3.

Geological reports show the unweathered Bukit Timah granite to have compressive strength of

approximately 200MPa . The Kallang formation consists of soft clays and loose sands with loose rock

sections that can give rise to dramatic settlements during tunnelling. The old alluvium is a granular

soil in strongly compacted or cemented form with very high abrasivity. Experience of these soils from

the earlier DTSS projects revealed that cutter disc life could be less than 100m3

of excavated material

per cutter disc.

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Table 3. Circle Line Tunnelling Contracts, Singapore

Contarct Machine

Type

Manufacturer Geology

Stage 1 ~ 5.4kmC-824 2 x EPB Japanese

C-825 2 x EPB Herrenknecht

Soft and deep Kallang formation overlying the

Jurong and old alluvium formations

Stage 2 ~ 5.4km

C-822 2 x EPB Mitsubishi

C-823 2 x EPB Japanese

Soft and deep Kallang formation overlying the

Jurong and old alluvium formations

Stage 3 ~ 5.7km


C-853 2 x EPB Mitsubishi

Old alluvium and Bukit Timah granite formations,

except in valleys where soft Kallang formations

expected

Stage 4&5 ~ 17.0km

C-854 4 x slurry Kawasaki

C-855 2 x slurry

2 x EPB

Herrenknecht

Herrenknecht


Old alluvium and Bukit Timah granite formations,

except in valleys where soft Kallang formations

expected

Slurry type TBMs have now been introduced for the first time in Singapore to operate in the

difficult mixed ground conditions with the Bukit Timah granites, similar to those experienced in the

DTSS works earlier. In stages 4 & 5 of the Circle Line, the trend has been to change from EPB to

slurry face support where the above difficult geological conditions are expected. Slurry methods offer

the advantage of

a) reduced effects from the abrasivity of the excavated material due to the beneficial effects ofthe cutters being permanently immersed in a bentonite slurry and its associated lubricating

effects

b) reduced possibility of settlements in the densely built up areas due to the positive face supportprovided by slurry pressure

c) no inflows of water (and fine material particles) through EPB screw conveyors in highlypermeable areas subject to high groundwater pressures

3. KUALA LUMPUR, MALAYSIA

The Stormwater Management and Road Tunnel (SMART) is primarily a flood mitigation project in the

centre of Kuala Lumpur. The SMART system will help alleviate flooding in the city centre by

diverting large volumes of flood water. The tunnel is innovatively designed to incorporate a

stormwater channel and a road traffic highway. The highway section of the tunnel is expected to ease

traffic congestion at the southern gateway to Kuala Lumpur near Sungai Besi. This concept is

believed to be the first of its kind in the world.

The complete stormwater tunnel is 9,7km long and has a double deck highway within it for a

length of 3km. Construction of the 11,8m internal finished diameter tunnel began mid 2004 using two

13,200mm outside diameter slurry type TBM (Mixshield) as shown in figure 4, delivered by

Herrenknecht, Germany.

Kuala Lumpur is located in an area with Karstic limestone geology with a high groundwater table.

The features of karstic limestone include cliffs, pinnacles, cavities, collapsed cavities and sinkholes.Overlying this karstic limestone is loose alluvium from previous tin mining activities. Due to the

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nature of the soil strata, much thought and planning was channelled towards the selection of tunnelling

method that would have minimal negative impact on the geological condition of the soil to protect the

natural groundwater conditions and prevent drainage of karst fissures and topsoils, in turn, avoiding

development of surface settlements and the appearance of surface sinkholes. The tunnel cover is on

average only 1 to 1,5 times the shield diameter.

The medium to strong (60-110 Mpa) limestone, fissured limestone and alluvium topsoil are

subject to a high groundwater table. Use of Mixshield (slurry) technology in water-bearing rock

conditions in comparison to EPB technology is more able to prevent groundwater drawdown and is

able to react more quickly to sudden changes in operating pressure. The first tunnel drive, 737m long,

was completed in just 24 weeks (see figure 5).

Figure. 4 Herrenknecht Mixshields SMART

Tunnel Kuala Lumpur/ Malaysia

Figure 5. SMART Tunnel KL ~ Breakthrough in Limestone

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4. NEW DELHI, INDIA

Delhi Metro Line 2 project cuts across some of the highest density areas of the city and is entirely

underground in lengths of both soft ground and hard rock tunnels. Thus, for most of the route of the

7km long MC1B contract, bored tunnels were specified and excavated using three EPB type machines

supplied by Herrenknecht, Germany (see figures 6 & 7).

Figure 6. Launch of Herrenknecht EPB in New Delhi

The area of Delhi beneath which Line 2 passes is built on the water-bearing silts of the Yumana River

flood plain that overlies a highly variable, highly weathered interface with very old, very foldedquartzite interlaced with which there are veins of hard mica schist up to 2m thick, and between 100

250MPa in compressive strength.

Figure 7. Herrenknecht EPB with disc cutters forquartzite Metro New Delhi / India

At an average depth of 15m below the surface,

Line 2 excavations pass through variable, often

mixed face conditions with a groundwater level

some 4 to 6,5m below the surface. The

underground alignment passes below and

adjacent to several protected buildings and

national monuments as well as directly beneath

the New Delhi and Delhi Main railway stations

For TBM tunnelling, this presented on occasion

full faces of extremely abrasive quartzite but

more often the extremes of both hard rock and

soft ground conditions in one face. Whilst

advance rates in the soft Delhi Silt were quite

good, advance rates in the quartzite and mixed

face conditions were quite slow. Frequent cutter

changes were necessary, at times changing four

or five disc cutters after a single advance stroke

(1,200mm) of the TBM resulting in high

downtime for cutter change.

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The extremely abrasive conditions also meant that it was necessary to have a further 12 weeks

downtime mid drive to take measures to counteract the extreme wear due to the abrasivity of the

material.

Tunnel boring commenced mid 2002 with final breakthrough in September 2004.

5. CHINA

Intensive activities started in China for construction of MRT systems from the end of 2000 onwards.

The cities of Beijing, Guangzhou, Nanjing, Shenzen, Tianjin and Shanghai have all embarked upon

ambitious schemes over the last six or so years.

By the end of 2006, in Guangzhou alone, Herrenknecht will have delivered 22 TBMs at

6,250mm diameter of which 20 are of EPB type and only 2 are of are of slurry (Mixshield) type. Soil

conditions in the densely populated areas of Guangzhou Lines 2 & 8 are silty fine sand with a

weathered layer of muddy, silty sandstone (maximum compressive strength 20MPa) were considered

to be better handled by slurry techniques. In total, Herrenknecht will have delivered 38 full face

TBMs to China by the end of 2006. No exact figures are available for deliveries to China by othermanufacturers but it can be safely assumed that the total will be well in excess of fifty.

Figure 8. Breakthrough of Herrenknecht EPB in Nanjing / China

Of the sixteen Herrenknecht TBMs delivered outside of Guangzhou, three of these are of slurry

(Mixshield) type. These machines have been used exclusively for river crossings, namely the already

completed Yangtze River crossing at Chongqing (6,570mm diameter) and the future Chongming road

tunnels in Shanghai.

China will see delivery of the worlds two biggest TBMs at 15,440mm diameter during 2006 for

construction of a road tunnel to link the city of Shanghai to Chongming island. The 13,8m finished

internal diameter subsea tunnel will accommodate six lanes of traffic (see figure 9). Extremely

difficult soil conditions consisting of soft deposits of clay, silt and sand combined with high

hydrostatic pressure and low cover over the crown of the tunnel dictates the use of a slurry type TBM.

The two slurry type TBMs (Mixshield) to be supplied by Herrenknecht of Germany are deigned for a

pressure of 6bar at the shield axis to support earth and water pressure during the 8km long drives.

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The combination of high water pressure and low cover to the seabed means that there will be

sections along the tunnel alignment where it will not be possible to change cutter tools by conventional

methods of lowering the slurry level in the excavation chamber and supporting the face by the use of

compressed air. The required compressed air pressure for face support exceeds the maximum

permissible pressure for man to work under. Therefore, design of the machine allows for changing the

66 scraper tools under atmospheric conditions by means of a specially developed back loading system.

Man access to the cutting tools is achieved by going inside the six main spokes of the cutting wheel

structure, in free air. Each cutting tool can be disassembled from the cutting wheel by means of the

special housing and powered closure gate. Thus the face conditions are mechanically closed from the

tool changing operations with only approximately 25 litres of water coming in from the housing area

around the cutting tool. Whilst offering a much safer working environment for cutting tool change, a

tremendous improvement is also achieved in the time necessary to change such cutters under

compressed air conditions when working in theactual face / excavation chamber.

Figure 9. Tunnel cross section Shanghai Chongming River Crossing

Figure 10. Connection between Shanghai and Chongming Island

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6. DIFFERENCE BETWEEN CONVENTIONAL SLURRY TBM (JAPANESE) AND

MIXSHIELD

The Mixshield design utilises face support by bentonite slurry, which in turn is pressurised by a

compressed air bubble behind the partial bulkhead.

A conventional slurry shield (single bulkhead) relies solely on the slurry pumping rates to control

the pressure of the bentonite supporting fluid, whereas the Mixshield (double bulkhead) utilises

compressed air to control the bentonite slurry pressure. Compressed air being an extremely

controllable substance, can quickly be either exhausted or admitted to the air chamber or bubble and

therefore maintain constant slurry pressure independent of the rising / falling level of the slurry due to

the pumping rates.

Figure 11a.

A conventional slurry shield with a single

bulkhead relies on the rates of pumping slurry in

and out of the face to control the face pressure.As such, face pressure is difficult to control and

large deviations from design face pressure can

occur.

Figure 11b.

With the Mixshield, however, compressed air is

used to control the pressure of the supporting

fluid. As the pumping rates vary and thus the

slurry level between the bulkheads, it is more

easy to accurately control the pressure of the airbubble with a very quick response time thereby

having much smaller deviations from design

pressure.

Figure 11c.

Comparing the two curves (See Figure 11c) for a

normal slurry and a Mixshield operation

highlights the much more constant face pressure

achieved with the Mixshield. Compressed air is

an extremely controllable medium and bycontrolling compressed air pressure rather than

slurry pressure directly a more stable face

pressure can be maintained.

CONCLUSIONS

The vast increase of closed face shield tunnelling activities in Asia over recent years has utilised earth

pressure balance (EPB) methods in over 90% of cases. The TBM and its suitability have tremendous

cost and schedule impacts to a project and, as such, pose a significant risk to any tunnelling contract.

Both the tunnel contractor and the TBM supplier are in competitive bidding situations, which can forcecompromise with respect to TBM selection and features. Some of the examples listed above have

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clearly pushed the use of EPB methods to, or even beyond, their economic limits. There is now

evidence of a clear trend towards the use of slurry type TBM in both Singapore and China after their

extensive experience in the use of EPB type equipment.

The choice of type of closed face TBM is a critical decision for any soft ground tunnelling

project. The decision is normally guided by assessment of the ground conditions as well as particular

experience of the projects contractor, the logistics and configuration of the works, and requirements

of the contract as a means to ensure that the clients minimum specification is met. The overriding

decision must be made on the basis of which type of machine is best able to provide stability of the

ground during excavation.

If both types of machine can provide optimum face stability, as is often the case, then other

factors such as diameter, length and alignment of the tunnel, increased cutter wear associated with

EPB methods, the work site area and location, and spoil disposal regulations are taken into

consideration.

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