Double-O-Tube shield tunneling for Taoyuan International Airport Access MRT

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Technical Note

Double-O-Tube shield tunneling for Taoyuan International Airport Access MRT

Yung-Show Fang

a,⇑

, Chung-Cheng Kao

b

, Yu-Fen Shiu

a

Department of Civil Engineering, National Chiao Tung University, Hsinchu, Taiwan b_{Department of Rapid Transit Systems, Taipei City Government, Taipei, Taiwan}

a r t i c l e

i n f o

Article history:

Received 14 February 2011

Received in revised form 30 January 2012 Accepted 18 March 2012

Available online 12 April 2012 Keywords: DOT tunneling Settlement Shield Tunneling cost Tunneling duration

a b s t r a c t

From 1989 to 2010, 20 tunneling projects have been carried out with the Double-O-Tube (DOT) shield tunneling method in the world. In this paper, the DOT shield tunneling for the construction of Taoyuan International Airport Access (TIAA) Mass Rapid Transit (MRT) system is introduced. A 6.42 m-diameter, 11.62 m-wide DOT shield machine was used to build the ﬁrst DOT tunnel in Taiwan. Field data indicated that, throughout the tunneling operation, the rolling angle of the DOT shield varied between +0.23° and 0.39°, which was within the limiting design values of +0.6° and 0.6° proposed by both TIAA MRT and Shanghai Metro. For the six surface settlement troughs collected from Tokyo, Shanghai and Taipei, the ground loss due to DOT shield tunneling ranged from 0.23% to 1.30%, and the average ground loss was 0.78%. As compared with the ground loss due to single-circular Earth-Pressure-Balance (EPB) shield tun-neling in cohesive soils, the range of ground loss due to DOT shield tuntun-neling was relatively narrow, and the peak ground loss value was signiﬁcantly less. Underground excavation with the DOT tunneling method would increase the tunneling duration for about 32%. The cost per meter of tunnel constructed with a DOT shield was about 1.5 times that constructed with single-circular shields. The cost of shield machine and segment lining were 23% and 53% of the total tunneling costs respectively. The expensive DOT shield machine and the complicated manufacturing and assembly of DOT lining segments are the main reasons for higher cost of tunneling. However, it would cost a lot more budget and it would be much more risky to excavate three cross-passages between the single-circular tunnels under the river.

1. Introduction

In recent years, due to the rapid development of urban areas, a lot of public facilities such as the Mass Rapid Transit (MRT) sys-tems and underground sewerage syssys-tems have been constructed. Because of the disruptive effects of the cut-and-cover method, it has been becoming more popular to employ the shield tunneling method for passing under commercial areas with heavy trafﬁc. The Double-O-Tube (DOT) shield tunneling has a minimized sec-tion area, and enables the most efﬁcient use of underground space (Chow, 2006). As compared with circular twin-tube tunnels, the DOT shield tunnel may pass narrow underground corridors, and

the impact on nearby structures is minimized (Sterling, 1992;

Moriya, 2000).

From 1989 to 2010, as summarized inTable 1, 20 tunneling

pro-jects were constructed with DOT shield tunneling method. The ﬁrst 13 cases were carried out in Japan, the next six cases were con-ducted in Shanghai, China, and the latest one was constructed in Taipei, Taiwan. The purposes of tunneling were to excavate subway tunnels, sewer mains, and common conduits. The formation of soil

consisted of gravel, sand, silt, clay and peat. InTable 1, the

diame-ter of DOT shields varied from 4.45 to 9.36 m, and the width of shields varied from 7.65 to 15.86 m. The length of tunnel ranged from 249 to 2497 m. The minimum radius of curvature of tunnel alignment was 102 m, and the maximum tunnel gradient was

5.9%.Table 1indicated, all 20 DOT shields used up to 2010 were

made by Japanese manufactures. It is obvious that Japanese play a leading role regarding the development of DOT shield tunneling

technology. InTable 1, IHI represents Ishikawajima-Harima Heavy

Industries, MHI represents Mitsubishi Heavy Industries, and KHI represents Kawasaki Heavy Industries.

In this paper, the DOT shield tunneling for the construction of Taoyuan International Airport Access (TIAA) MRT is introduced. This is the ﬁrst DOT shield machine employed in Taiwan. The roll-ing angle of the DOT shield measured as a function of the rroll-ing number was presented and studied. Surface settlement troughs due to DOT shield tunneling in Japan, Shanghai and Taipei were collected and compared with those estimated with empirical methods.

Shen et al. (2009)stated that the disadvantages for a single-circular shield tunnel are resulted by high construction cost and long construction period. That is why the DOT shield tunneling method was proposed in Japan. However, from a practical point

⇑ Corresponding author. Tel.: +886 3 571 8636; fax: +886 3 571 6257. E-mail address:ysfang@mail.nctu.edu.tw(Y.-S. Fang).

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of view, limited information regarding the construction cost and duration for both single-circular and DOT shield tunneling meth-ods was reported in the literature. In this article, the speed and duration of DOT shield tunneling for the construction of TIAA

MRT were collected and compared with values suggested byShield

Tunneling Association of Japan (2004). A comparison of tunneling cost between single-circular twin-tube and DOT shield tunneling was also made.

2. DOT shield tunneling in Taipei basin

The objective of Taoyuan International Airport Access MRT is to provide airport passengers with a safe, convenient, comfortable and high quality transit service. TIAA MRT system between Taipei

and Jhongli will link the Taoyuan International Airport to the Taiwan High Speed Rail (HSR), Taiwan railway systems, and Taipei MRT systems. The total budget for this project is approximately US$ 3.56 billion.

As illustrated in Fig. 1, the project starts from Taipei Main

Station (A1 Station), passes through Sanchong, Sinjhuang, Linkou,

Taoyuan International Airport (A12–A14a Stations), and

terminates at Jhongli Train Station (A21 Station). The route inter-sects Taiwan High Speed Rail at HSR Taoyuan Station (A18 Station). The total route length is 51.03 km, consisting of 10.92 km of

under-ground section and 40.11 km of elevated section. InFig. 1, the TIAA

MRT system has 22 stations, consisting of 7 underground and 15 elevated stations. The expected revenue service date for entire sys-tem is October 2014.

Table 1

Projects constructed with DOT shield tunneling method. Case

no.

Project name Purpose

of tunnel Geological condition External dimensions of DOT shield (m) Length of tunnel (m) Thickness of overburden (m) Minimum radius of curvature (m) Maximum gradient (%) DOT shield manufacture Period of construction

1 Rijo tunnel, 54th national route Hiroshima, Japan

Subway Clay, sand £6.09 W10.69 850 5.0–8.3 135 1.8 IHI 1989–1994

2 Kikutagawa 2nd sewer main Narashino, Chiba, Japan

Sewer main

Fine sand, clay, peat

£4.45 W7.65 703 2.15–9.0 1600 4.0 IHI 1990–1994

3 Ariakekita common conduit Tokyo, Japan Common conduit Clay, gravel £9.36 W15.86 249 14.0–17.0 1600 3.5 MHI 1990–1994

4 Underground line, coastline high speed transit, Kobe, Japan

Subway Clay,

gravel

£5.48 W9.75 304 11.5–15.5 1500 0.8 MHI 1995–1998

5 East district of Sunadahashi, 4th line high speed transit, Nagoya, Japan

Subway Sandy

gravel, silt, clay

£6.52 W11.12 752 10.31–16.6 500 2.3 IHI 1999–2002

6 Chayagasaka park district, 4th line, high speed transit, Nagoya, Japan

Subway Silt, sand £6.52 W11.12 1007 11.0–32.1 500 3.3 IHI 1999–2002

7 Yamamoto north district, 4th line, high speed transit, Nagoya, Japan

Subway Clay, sand, sandy gravel

£6.52 W11.12 1238 9.3–32.3 300 2.7 IHI 1999–2002

8 South district of Nagoya University, 4th line, high speed transit, Nagoya, Japan

Subway Clay,

sandy silt, sandy gravel

£6.52 W11.12 876 11.5–21.3 200 3.1 IHI 1999–2002

9 Yagoto north district, 4th line, high speed transit, Nagoya, Japan Subway Clay, sandy gravel £6.52 W11.12 782 19.0–24.0 180 0.9 KHI 1999–2002 10 Yamashitadori south district, 4th line, high speed transit, Nagoya, Japan

Subway Sandy

gravel

£6.52 W11.12 957 10.0–16.6 165 3.3 MHI 1999–2003

11 Yagoto south district 4th line, high speed transit, Nagoya, Japan

Subway Clay,

sandy gravel

£6.52 W11.12 1025 16.2 300 3.1 MHI 1999–2003

12 East-terrain line, 1st district, Aichi, Japan

Subway Sandy soil £6.73 W11.43 904 7.0–15.0 102 5.9 IHI NA

13 East-terrain line, Aichi, Japan

Subway Clay, sand £6.73 W11.43 123 12.0–13.0 102 0 IHI NA

14 Nenjiang Rd. St. to Xiangyin Rd. St. to Huangxing greenbelt St., line 8 Shanghai Metro, China

Subway Silty sand, silty clay, clayey silt

£6.52 W11.12 1759 5.2–12.0 495 2.8 IHI 2003–2004

15 Kairu Rd. St. to Nenjiang Rd. St., line 8, Shanghai Metro, China

Subway 929 5.2–12.0 495 2.8 MHI 2003–2004

16 Lot 9, line 6, Shanghai Metro, China

Subway 1713 4.0–21.0 300 2.7 IHI 2004–2005

Subway 2497 6.0–13.0 990 1.5 IHI 2004–2005

Subway 1096 6.0–10.0 420 2.7 IHI 2004–2006

Subway 1459 12.3–19.8 NA NA IHI 2009

20 Lot CA450A,Taoyuau International Airport Access MRT, Taiwan

MRT Silty clay,

silty sand

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2.1. DOT tunneling from Sanchong to Taipei

The shield tunneling with DOT method from Sanchong to Taipei

is introduced in this section.Fig. 2shows that the lot CA450A is

lo-cated between Station A2 of TIAA MRT and Station G14 of Taipei MRT, and the tunnel passes under Danshuei River. To avoid the dif-ﬁculties and potential risks associated with the excavation of three

cross-passages between two single-circular tunnels under

Danshuei River, the DOT shield tunneling method was selected. Excavation started from the work shaft at Sanchong, passed un-der Danshuei River, and terminated at G14 Station. As listed in

Table 1, the total length of the DOT tunnel is 1584 m. The overbur-den above the tunnel varies from 7.6 to 26.0 m. The maximum tunnel slope is 4.9% and the minimum radius of curvature of the route is 280 m. The owner of the project is the Department of Rapid

Transit Systems, Taipei City government, and the contractor is the Da-Cin/Shimizu joint venture.

2.2. Subsoil conditions

The DOT tunneling for lot CA450A was carried out in Taipei basin, which is an area of Quaternary Holocene alluvial deposits.

Woo and Moh (1990) reported, in descending order, the alluvial deposits consist of a topsoil layer (1–6 m thick), the Sungshan For-mation (40–70 m thick), the Chingmei ForFor-mation (0–140 m thick), and the Hsinchuang Formation (0–120 m thick). Most under-ground projects in Taipei were constructed less than 25.0 m below ground level; hence most construction works are carried out in the topsoil and the Sungshan Formation.

Sinjhuang Sanchong Taipei Taiwan High Speed Rail Taoyuan Jhongli Taoyuan International Airport TIAA MRT Linkou

Fig. 1. Taoyuan International Airport Access MRT system.

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Fig. 3shows the geological proﬁle interpreted from the site

investigation for Contract CA450A (Da-Cin/Shimizu, 2009). In

descending order, properties of the layers involved with this pro-ject are as follows:

(1) Layer 1 (Topsoil): A layer of surface ﬁll (indicated as SF), located at 0 to 3.0 m below ground level, with Standard Pen-etration Test (SPT) N-value ranging from 1 to 5.

(2) Layer 2 (Sungshan Formation): Inter-layers of silty clay (classiﬁed as CL, N = 4–7) and silty sand (classiﬁed as SM, N = 8–18), located at 3–50 m below ground level. Traces of decayed wood, organic matter and shell fragments were reported.

(3) Layer 3 (Chingmei Formation): A layer of silty gravel (classi-ﬁed as GM, N > 50), located at about 50 m below ground level.

The ground water table was typically 2.9–5.0 m below ground

level. It may be observed in Fig. 3 that DOT tunneling for lot

CA450A was entirely carried out in the silty sand and silty clay lay-ers of the Sungshan Formation.

2.3. DOT shield machine

The mud-injection Earth-Pressure-Balance (EPB) type DOT

shield machine adapted for this project is shown inFig. 4. This

articulated shield machine was designed and manufactured by IHI, and has the outside dimensions of £6.42 m W11.62 m. A total of 32 shield jacks were installed in the shield, including 20 upper jacks with 2000 kN capacity each, and 12 lower jacks with 2500 kN capacity each.

Fig. 4shows two cutter heads were equipped in front of the DOT shield for soil excavation. Each cutter head has four radial spokes and two auxiliary spokes. The adjacent cutter heads rotate in oppo-site directions on the same plane. The motion of the two heads is synchronized for avoiding to touch or smash each other. Each cut-ter head is equipped with two copy cutcut-ters with the stroke of

150 mm. The copy cutter shown inFig. 4was used for overcutting

of the ground at the curved section of tunneling, and also used for the correction of shield rolling. More information about cutter

head of DOT machine may be found inChow (2006).

Fig. 5shows the cross sectional area of the DOT tunnel. Each ring of tunnel lining consists of 11 segments (8A + 1B + 1C + 1D), 0 -10 -20 -30 -40 -50 10 EL. (m)

Danshuei River Taipei

Sanchong Departure Shaft Arrival Shaft DOT Tunnel G14 Station Topsoil (SF) Silty sand (SM) Silty clay (CL) Gravel (GM)

Fig. 3. Geological proﬁle for tunneling.

Radial spoke Auxiliary spoke

Copy cutter

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and the segments are connected with straight steel bolts. Due to their size and shape, the segment B is often nicknamed the large seagull and segment C is nicknamed the small seagull. To avoid creating a continuous weak plane on the tunnel lining, the large and small seagulls swap their positions in the next ring. The rein-forced concrete segments are 0.3 m thick and 1.2 m long. The assembled tunnel section is 11.4 m wide and 6.2 m high. The tail void of the DOT shield is 0.11 m thick. To reduce the ground settle-ment due to tail void closure, two grouting holes were fabricated above the upper seagull segment, and one grouting hole was

fabri-cated below the lower seagull segment as shown inFig. 6a and b.

When the lining segments were pushed out of the shield, instanta-neous grouting was automatically conducted to ﬁll the newly gen-erated tail void space. After the lining segment had been pushed out of the shield, backﬁll grouting was conducted manually

through the grouting hole on the segment as shown inFig. 7.

Fig. 8shows the south portal of the DOT shield tunnel. Note in

Fig. 8that a ﬂood-preventing gate was constructed at the entrance of the tunnel. The gate is used to isolate and protect the completed tunnel and DOT machine from a possible ﬂooding problem in the

work shaft during typhoon seasons.Fig. 9shows the DOT shield

tunnels for the construction of TIAA MRT. The passage between the two tunnels is used to establish an automatic ﬁre-protection sliding door. In case of a ﬁre emergency, the MRT passenger could escape from one tunnel to the other through the door.

2.4. Backﬁll grouting

The backﬁll grout material consisted of a mixture of grout A and

grout B as indicated inFig. 7. For every cubic meter of mixture,

grout A consisted of 250 kg of cement, 30 kg of bentonite, 3 l of sta-bilizer, and 820 l of water. Grout B consisted of 85 l of water glass. The amount of grout used per ring was determined with the back-ﬁll injection rate, which is deﬁned as follows:

Backfill injection rate ¼Volume of grout injected per ring Volume of tail void per ring 100%

ð1Þ

For lot CA450A, the tail void area of the DOT shield was 3.45 m2

and the length of the lining segment was 1.2 m. The volume of tail

void per ring was 3.45 m2_{1.2 m = 4.14 m}3_{. The backﬁll injection}

rate was assigned to be 130%. As a result, the total volume of

back-B

C

D

A

6.2 m

11.4 m

64° 46° 64° 64° 64° 58° 46° 58° 64° 64° 64° 64°

Fig. 5. Section of tunnel.

(a)

(b)

Lower tail skin

grouting hole Upper tail skin grouting holes

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ﬁll injected at tail skin and segment grout holes was 5.38 m3per ring.

For the grout to fill the tail void successfully, the backfill sure was controlled to be 100–200 kPa above ground water pres-sure. For example, at the monitored section of SSI-3 of lot CA450A, the tunnel center line was located at the depth of 20.85 m, and the ground water table was located at 2.9 m below ground level. The ground water pressure at the tunnel center line was 176 kPa. By adding 100–200 kPa to the water pressure, the backfill grouting pressure would be 276–376 kPa. The contractor used 200–400 kPa as the pressure range for both tail skin and seg-ment backfill grouting. To prevent damaging the lining segseg-ments and connecting bolts, 400 kPa was selected as the upper bound va-lue for backfill grouting pressure.

2.5. Face management

The excavated soils were mucked out of the tunnel with hydraulic pumps and pipes. Therefore, polymer slurry was injected at the face through the injection pipes on the cutter heads. Mud injection was carried out for the following reasons: (1) prevent soils sticking to the soil chamber; (2) enhance a plastic ﬂow of excavated soils; and (3) reduce the applied torque due to soil-cutter friction.

The volume of mud injection at the face was determined with the assigned face injection rate. For the DOT shield used in lot

CA450A, the cross-sectional area was 61.54 m2 and the face

injection rate was assigned to be 10%. For each ring, the volume of polymer slurry injected at the face was calculated by

61.54 m2 1.2 m 10% = 7.38 m3.

To keep the ground in a stable condition during tunneling, the EPB shield machine measured and controlled the lateral earth pres-sure in the soil chamber. The upper management earth prespres-sure

shown inFig. 10was calculated by adding 20 kPa to the earth

pres-sure at-rest at the face. To prevent heaving of structures near the tunnel, the lower management earth pressure was determined

by adding 20 kPa to the active earth pressure. InFig. 10, the upper

earth pressure changed form 110–332 kPa, and the lower pressure varied form 74–245 kPa.

The measured face support pressure was also indicted inFig. 10.

From ring 40 to 290, the DOT tunnel passed beneath an Segment backfill

grouting hole

Grout B

Grout A

Fig. 7. Segment backﬁll grouting.

Fig. 8. South portal of DOT shield tunnel.

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underpinned bridge foundation. The face pressure was controlled to be less than the lower management value to avoid damaging the bridge foundation. From ring 991, the tunnels were driven un-der existing buildings. As a result, the face pressure was controlled below the lower management pressure.

2.6. Rolling of shield

During construction, rolling is one of the characteristics of the DOT shield machine. Main reasons of rolling include asymmetry of the cross section of the DOT shield due to manufacturing error, non-balanced weight of both sides of the DOT shield machine, and difference of torques of both cutter heads due to different soil properties. The reasons of rolling and their correction measures

were summarized byZhang (2004)andShen et al. (2009).

InFig. 5, assuming the tunnel rolls 0.5° clockwise with respect to its center, which is the center of segment D, the left tunnel would rise 22.7 mm and the right tunnel would settle 22.7 mm. The occurrence of shield rolling would cause a difference in eleva-tion between two tunnels and a tilting of the center column of the lining segments. Shield rolling makes the distribution of internal forces among segments more complex and the lining assembly more difﬁcult.

Fig. 11shows the measured rolling angle of the DOT shield as a function of the ring number of the tunnel. Ring 1 was assembled on December 14, 2009. For an operator in the tunnel facing the exca-vation zone, a clockwise rolling angle is deﬁned as positive. The management target of the rolling angle was between +0.2° and 0.2°. That means correction measures are required for a mea-sured rolling angle more than plus or minus 0.2°. In the DOT tunnel design, for both TIAA MRT and Shanghai Metro, the limiting design rolling angles during construction are plus or minus 0.6°.

InFig. 11, the rolling angle became signiﬁcant at ring 117, the measured rolling angle was 0.29°. To reduce the amount of shield rolling, correction measures such as reverse-rotation of cutter heads and secondary grouting with opposite direction of rolling were carried out. The rolling angle was successfully reduced to 0.10° at ring 130. The maximum rolling angle 0.39° was mea-sured at ring 278. The contractor employed reverse-rotation of

cut-0 200 400 600 800 1000 1200 1400 Ring Number 0 100 200 300 400

Earth pressure (kPa)

Upper Management Earth Pressure Lower Management Earth Pressure Measured Earth Pressure

Fig. 10. Face support pressure.

0 200 400 600 800 1000 1200 1400 Ring Number -0.6 -0.4 -0.2 0 0.2 0.4 0.6 Rolling Angle (de g) Management range Lot CA450A, TIAA MRT Line 8, Shanghai Metro (Zhang, 2004)

Fig. 11. Rolling angle with ring number.

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ter heads, secondary grouting, and over-excavation by the copy cutter to reduce the amount of rolling to a small positive angle less than +0.10°. For a comparison, the rolling angle measured at line 8

of Shanghi Metro is also indicated inFig. 11. It is clear in the ﬁgure

that, throughout the tunneling of lot CA450A, the rolling angle was controlled by varying between +0.23° and 0.39° approximately, which was within the limiting values of +0.6° and 0.6° proposed by both TIAA MRT and Shanghai Metro.

2.7. Drift woods

In Taipei basin, drift woods had been a major problem for tun-neling with single-circular shield machines. Taipei basin is covered with a 40–70 m-thick alluvial Sungshan Formation. Large-size tree trunks were encountered in the alluvial stratum during

under-ground excavation for MRT stations and building basements.Ju

et al. (2009) reported, during shield tunneling for Banqiao Line lot CP261 of Taipei MRT, a 0.9 m-long 0.3 m-diameter drift-wood piece was recovered in the soil chamber of an EPB shield. For the next 40 m of tunnel excavation for lot CP261, drift woods were encountered for 10 times.

As seen in theFig. 2, the DOT shield was used for excavation

un-der Danshuei River. It is highly possible that drift woods could be

encountered in the alluvial riverbed. For lot CA450A,Fig. 4shows

different types of steel cutting-bits were attached to spokes of the cutter heads. It is expected that large pieces of drift wood could be chopped off by the cutting-bits to small pieces and then mucked out. For this project, drift woods were frequently encountered dur-ing tunneldur-ing. Cloggdur-ing of the screw conveyor due to drift woods during excavation was reported. Manual removal of the clogged drift-wood pieces from the screw conveyor was conducted and

tunnel excavation was successfully completed.Fig. 12a and b show

the drift wood pieces excavated by the DOT shield. 3. Settlement due to DOT tunneling

In this section, formations of ground settlement troughs due to DOT shield tunneling measured in the ﬁeld are reported. Since there are only 20 cases of DOT tunneling in the world up to 2010, the settlement data available are limited. In this study, six sets of surface settlement data recorded in Tokyo, Shanghai and Taipei were collected and studied. Only surface settlement is

dis-cussed in this paper. Based on the ﬁndings of Peck (1969), the

empirical equal-area method and superposition method are brieﬂy

introduced and used to estimate the surface subsidence due to DOT shield excavation. On the basis of experience gained through ﬁeld measurements, the empirical method provides simple and practi-cal alternatives to numeripracti-cal solutions.

3.1. Peck’s method

Based on ﬁeld data,Peck (1969)suggested that the surface

set-tlement trough over a single circular tunnel can usually be approx-imated by the error function or normal probability curve as follows:

SðyÞ ¼ Smax exp

y2

2i2

ð2Þ where S(y) is the settlement at offset distance y from the tunnel

center line (tunnel axis), Smax is the maximum settlement above

tunnel center line, and i is the distance from the inﬂection point of the trough to the center line. The parameter i is commonly used to represent the width of the settlement trough. Peck presented a dimensionless relationship between the width of settlement trough i/R versus depth of tunnel Z/2R for tunnels driven through different materials, where R is the radius of the tunnel, and Z is the center line depth. Based on error function, the maximum surface

settle-ment Smaxand the volume of surface settlement trough per linear

length Vshas the following relationship:

Vs¼

ffiffiffiffiffiffiffi 2

p

iSmax 2:5iSmax ð3Þ

Ground loss due to tunneling is deﬁned as: Ground loss ¼Vs

Vt 100% ð4Þ

where Vtis the volume of tunnel excavated per linear length.

3.2. Equal area method

InFig. 13,Zhang (2007)suggested that the ground settlement due to DOT shield tunneling can be approximated with that due to a single-circular tunnel which has the same cross-sectional area as the DOT tunnel. The volume of settlement trough per linear

length due to DOT tunneling Vs,DOTcan be monitored in the ﬁeld.

The volume of settlement trough per linear length due to the

sin-gle-circular tunnel with a radius Req is assumed to be equal to

Vs,DOT. 0 2.5i i Z 2R G.L. 2R Smax eq

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The next step is to determine the ground settlement curve due

to the single-circular tunnel with a radius Req. In Eq.(3), with the

volume of settlement trough Vs,DOTmonitored in the ﬁeld and the

trough width parameter i determined with the dimensionless

rela-tionship proposed by Peck, the maximum surface settlement Smax

can be obtained. By substitute Smaxand i into Eq.(2), the

distribu-tion of settlement due to the tunneling of an equal-area single-circular shield can be calculated. This settlement trough is assumed to be equal to that due to DOT shield tunneling.

Assuming the single circular tunnel with a radius Req has the

same cross-section area as the DOT tunnel. For each linear length of tunnel driving, exactly the same volume of soil would be excavated by the single circular tunnel and the DOT tunnel. This is the reason why the settlement trough was assumed to be equal.

3.3. Superposition method

For a pair of parallel tunnels, if the interaction between the

tun-nels can be neglected,Fang et al. (1994)suggested that principle of

superposition may be used to estimate ground subsidence

associ-ated with the construction of parallel tunnels. In Fig. 14, the

settlement distribution due to the construction of tunnel 1 and 2

is illustrated as curve S1(y) and S2(y), respectively. The total

settle-ment St(y) due to the two circular tunnels is calculated with the

principle of superposition: St(y) = S1(y) + S2(y).

Due to symmetry, it is assumed that each single-circular tunnel would share 50% of the total ground loss. That means the volume of settlement trough induced by each circular tunnel was assumed to

equal 0.5Vs,DOT. For each circular tunnel, the trough width

parame-ter i could be deparame-termined with the dimensionless relationship

proposed by Peck. The maximum surface settlement Smaxfor each

tunnel could be obtained with Eq.(3), and the settlement

distribu-tion curve S1and S2for each circular tunnel could be calculated

with Eq.(2). The total settlement curve St is the summation of

settlement curves S1and S2.

It should be mentioned that the displacements and strains associated with tunneling will always create a disturbed zone

sur-rounding the tunnel. Hoyaux and Ladanyi (1970) analyzed the

stress distribution surrounding tunnels driven through soft soils with the finite element method. Their finding indicated that, among many other factors, the width of the plastic zone is mainly influenced by the sensitivity of the soil deposit and the diameter and depth of the tunnel. It is quite obvious that many assumptions and uncertainties are involved in the proposed empirical methods.

3.4. Settlement troughs monitored in the ﬁeld

Six settlement troughs monitored at Ariakekita common con-duit in Tokyo, Shanghai Metro, and Taoyuan International Airport

Access MRT are illustrated inFig. 15a–f. For comparison purposes,

the settlement curves estimated with the empirical equal-area and superposition methods are also included in these ﬁgures.

The ground settlement data due to tunneling for Ariakekita

common conduct are shown inFig. 15a.Oda and Yonei (1993)

re-ported the DOT shield driven through clay and gravel deposits was 9.36 m-diameter and 15.86 m-wide. At the monitoring section, the depth of tunnel center-line was 20.68 m and the induced ground loss was 0.23%. This was the smallest ground loss among the six

settlement troughs reported inFig. 15. The maximum settlement

estimated with the equal-area method was greater than the ﬁeld data.

The surface movements monitored at the 80th ring, line 8 of

Shanghai Metro are presented in Fig. 15b. Zhou et al. (2005)

reported ground heaving was observed due to the earth-pressure-balance shield tunneling. The settlement data monitored at the 460th ring, Line 8, of Shanghai Metro are presented in

Fig. 15c. The ground loss induced by DOT tunneling was 0.67%.

Field data inFig. 15c exhibit a narrow and W-shaped settlement

trough, which is quite different from all other measured settlement

curves shown inFig. 15, and those estimated with the empirical

methods.

Fig. 15d shows the settlement trough recorded at the 100th ring, lot 9, line 6 of Shanghai Metro. The ﬁeld settlement data are in fairly good agreement with settlement curves estimated with both empirical methods. A similar ﬁnding could be observed in

Fig. 15e.

The settlement trough measured at SSI-3 section, lot CA450A of

Taoyuan International Airport Access MRT is shown inFig. 15f. The

measured settlement trough was not symmetric. The monitored settlement trough was wider than the estimated troughs, and the monitored maximum settlement was apparently smaller than the estimated values. The ground loss due to DOT tunneling was 1.30%, which is the largest one among the six values reported in this study.

InFig. 15, the ground loss due to DOT shield tunneling varied from 0.23% to 1.30%, and the average ground loss was 0.78%. Based

on ﬁeld data,Chang (2007)reported that, for a single-circular EPB

shield machine tunneling through cohesive soils, the induced ground loss varied from 0.12% to 7.48%. It may be concluded that, as compared to that due to single-circular EPB shield tunneling, the range of ground loss due to DOT shield tunneling would be narrow,

Center 2R z G.L.

Tunnel

1 Tunnel

2

S S S 2 1 t

(10)

-40 -20 0 20 40 Distance from Center of Tunnel, y (m) 60 40 20 0 Surface Settlement, S (mm) Observed Estimated (superposition) Estimated (equal area) Z = 20.68 m

9.36 m*W15.86 m Ariakekita Common Conduit Tokyo, Japan

(Oda and Yonei 1993) Clay and gravel

Ground loss = 0.23 %

-40 -20 0 20 40

Distance from Center of Tunnel, y (m) Observed Estimated (superposition) Estimated (equal area)

60 40 20 0 Surface Settlement, S (mm) Z = 14.36 m 6.52 m*W11.12 m 100th Ring, Lot 9, Line 6 Shanghai Metro (Zhang 2007) Silty clay and silty sand

-40 -20 0 20 40

Distance from Center of Tunnel, y (m) Observed Estimated (superposition) Estimated (equal area) 60 40 20 0 Surface Settlement, S (mm) _{Z = 14.86 m} 6.52 m*W11.12 m 130th Ring, Lot 9, Line 6 Shanghai Metro (Zhang 2007) Silty clay and silty sand

Ground loss = 0.8 %

-40 -20 0 20 40

60 40 20 0 Surface Settlement, S (mm) Z = 20.85 m 6.42 m*W11.62 m SSI-3 section Lot CA450A

Taoyuan International Airport Access MRT, Taiwan Silty clay and silty sand

-40 -20 0 20 40

60 40 20 0 Surface Settlement, S (mm) Z = 10.76 m 6.52 m*W11.12 m 80th Ring, Line 8

Xiangyin Road St.to Huangxing greenbelt St. Shanghai Metro

(Zhou et al. 2005) Silty clay and silty sand

-40 -20 0 20 40

60 40 20 0 Surface Settlement, S (mm) Z = 15.26 m_{6.52 m*W11.12 m} 460th Ring, Line 8

Xiangyin Road St. to Huangxing Greenbelt St. Shanghai Metro

(Zhou et al. 2005) Silty clay and silty sand

(a)

(b)

(c)

(d)

(e)

(f)

(11)

and the peak ground loss value would be signiﬁcantly less. Except

the segment backﬁll grouting,Fig. 6shows instantaneous grouting

was conducted at the upper and lower skin-tail grouting holes. The tail void space generated when the lining segments were pushed out of the shield was backﬁlled automatically and instantaneously. This is probably the main reason why the ground loss due to DOT shield tunneling was lower.

4. Duration and cost of DOT tunneling

Duration, cost, quality and safety of construction are the four major factors considered for civil engineering projects. In this section, the duration and cost of DOT shield tunneling for the construction of lot CA450A of Taoyuan International Airport Access MRT are reported.

4.1. DOT tunneling duration

The speed of shield tunneling is mainly inﬂuenced by the tunnel diameter (2R), tunneling slope, radius of curvature (r) of alignment, initial or normal stage of excavation, and subsoil conditions. The

Shield Tunneling Association of Japan (2004)reported the tunnel-ing speed decreases with increastunnel-ing tunnel diameter. As illustrated inFig. 16, for the DOT shield with a diameter 2R = 2.35 m, the nor-mal-stage (300 m < r, nearly a straight-line) tunneling speed is

7.6 m/day. For a larger DOT shield with the diameter 2R = 9.2 m, the normal-stage tunneling speed is reduced to 5.1 m/day. It is also reported the tunnel speed in the initial-excavation stage is only about 50% of that in the normal stage. The tunneling speed for ini-tial-stage excavation as a function of tunnel diameter is also

indi-cated inFig. 16.

TheShield Tunneling Association of Japan (2004)reported that tunneling speed would slow down for excavation along curved

alignment. The tunneling speed for curved excavation Lc(m/day)

can be evaluated with the following equation:

Lc¼

a

Ls ð5Þ

where Lsis the tunneling speed in normal-stage (straight line

por-tion), and

a

is the reduction factor. The variation of

a

factor as a

function of the radius of curvature r of tunnel alignment is indicated inFig. 16. If the radius of curvature is greater than 300 m,

a

is equal to 1.0. If the radius of curvature is less than 60 m, the tunneling speed at the sharp turn is only 30% of that for the straight-line

por-tion. InFig. 16, the tunneling speed is expressed as a function tunnel

diameter, radius of curvature, and initial or normal-stage of advancing.

Fig. 5shows, for lot CA450A of TIAA MRT, the DOT tunnel has a

diameter of 6.2 m. InFig. 13, the corresponding straight-line

nor-mal-stage tunneling speed Ls= 6.2 m/day. The minimum radius of

curvature is 280 m, and the associated reduction factor

a

= 0.8.

Based on Eq.(5), the tunnel speed in the curved portion is

approx-imately Lc= 4.96 m/day. Assuming the tunneling crew works

25 days per month (30 days), the tunneling speed would be

4.1 m per calendar day. However,Fig. 2indicates the middle and

ﬁnal portions of the tunnel are not curved, thus the tunneling speed for the normal-stage excavation was estimated to be about

4.3 m/day as indicated inTable 2.

For the initial and ﬁnal stages of excavation, the tunneling speed was reduced 50% from 6.2 m/day down to 3.1 m/day. A steep 4.9% downhill slope was encountered at the initial stage of excava-tion, and the expected tunneling speed was further reduced to 2.4 m/day. Assuming the tunneling crew works 25 days per month, the tunneling speed for the initial-stage becomes 2.0 m each

calen-dar day as listed inTable 2.

A comparison of tunneling duration for the construction for lot CA450A with both the single-circular twin-tube and DOT shield

tunneling methods are summarized in Table 2. With the DOT

shield, it is expected to take 400 days to complete the tunneling assignment. In the table, since the assembly of the circular lining segments is less complicated, it is estimated to take only 273 days to construct the twin-tunnels with the single-circular shield machines.

The DOT tunneling for lot CA450A stared on December 11, 2009 and it was completed on December 5, 2010. For the initial stage, the measured tunneling speed 2.5 m/day was slightly faster than the estimated speed 2.0 m/day. For the normal-stage, the actual tunneling speed 4.55 m/day was also faster than the expected

0 2 4 6 8 10 Tunnel Diameter 2R, (m) 0 4 8 12 DO T T

unneling Speed (m/day)

Normal excavation (300 m < r) 200m < r < 300 m (α = 0.80) 150m < r < 200 m (α = 0.70) 100m < r < 150 m (α = 0.65) 60m < r < 100 m (α = 0.55) r < 60 m (α = 0.30)

Initial excavation (Departure section) (data after Shield Tunneling Association of Japan (2004))

Fig. 16. DOT tunneling speed with tunnel diameter 2R and radius of curvature r.

Table 2

Comparison of tunneling speed and duration.

Tunneling method Item Departure section Normal excavation Arrival section Total

Single-circular twin-tube tunneling(estimated) Tunnel length (m) 60 1464 60 1584

Tunneling speed (m/day) 2.5 6.5 2.5 4.9

Duration of construction (day) 24 225 24 273

DOT shield tunneling (estimated) Tunnel length (m) 60 1464 60 1584

Duration of construction (day) 30 340 30 400

DOT shield tunneling (measured) Tunnel length (m) 60 1464 60 1584

(12)

speed 4.3 m/day. As a result, the actual duration of tunneling was 360 days, which was 40 days shorter than the estimated duration. The length of completed tunnel as a function of the time of

con-struction is shown inFig. 17. In the ﬁrst 200 days of construction,

the slower tunneling speed was most probable due to the initial steep downhill-slope of tunneling, small radius of curvature, and inexperience of the tunneling crew. After the ﬁrst 200 days, most difﬁculties had been overcome and the crew became more skillful. The tunneling speed increased and the duration of construction was reduced. As compared with single-circular twin-tube tunnel-ing method, underground excavation with the DOT method would increase the tunneling duration for about 32%.

4.2. DOT tunneling cost

The cost of DOT shield tunneling for lot CA450A is presented in this section. Since this was the ﬁrst time that DOT shield tunneling was conducted in Taiwan, the owner of the project asked ﬁve expe-rienced Japanese contractors to evaluate the cost of DOT tunneling

in Taipei.Table 3shows the cost per meter of DOT tunneling

pro-posed by the ﬁve contractors varies from US$ 19,520 to 42,330. On the average, each meter of DOT tunnel would cost US$ 28,810. Note

that the tunneling cost listed inTable 3was only valid in Taiwan in

2008.

For comparison purposes, the cost of construction of

single-circular twin-tube shield tunneling is also listed in Table 3. The

cost per meter of tunneling varied from US$ 14,710 to 23,230. The average cost per meter of tunneling was US$ 18,890. It may

be concluded fromTable 3that the cost per meter of tunnel

con-structed with a DOT shield is about 1.5 times that concon-structed with

single-circular shields. Department of Rapid Transit Systems

(2008) reported that the DOT shield machine alone would cost US$ 10.57 million, which consists of 23% of the total tunneling cost. The DOT tunnel lining segments would cost US$ 24.2 million, which is about 53% of the total tunneling cost. The expensive DOT shield machine and the complicated manufacturing and assembly of DOT lining segments are the main reasons for the higher cost of tunneling.

Department of Rapid Transit Systems (2008)stated, based on the tunneling experience in Japan, the cost for DOT tunneling is about 1.3 times that for single-circular twin-tube shield tunneling. For the tunneling of lot CA450A in Taiwan, the DOT shield machine was constructed and transported from Japan, Japanese engineers and technicians would cost more to operate in a foreign country, and the lining segment molds and associated construction parts were used only for this project. These were the reasons why the cost of tunneling for lot CA450A of TIAA MRT was higher. However, it would cost a lot more budget and it would be much more risky to excavate three cross-passages between the single-circular tunnels under the river.

5. Conclusions

Based on the DOT shield tunneling for lot CN450A of Taoyuan International Airport Access MRT, the following conclusions can be drawn. Throughout the tunneling operation, the measured roll-ing angle of the DOT shield was controlled to vary between +0.23° and 0.39°, which were within the limiting design range of ±0.6° proposed by both TIAA MRT and Shanghai Metro.

For the six surface settlement troughs collected from Tokyo, Shanghai and Taipei, the ground loss due to DOT shield tunneling varied form 0.23–1.30%, and the average ground loss was 0.78%. As compared with the ground loss due to single-circular EPB shield tunneling, the range of ground loss due to DOT shield tunneling was relatively narrow, and the peak ground loss value was signif-icantly less.

At the initial stage of excavation, the measured tunneling speed 2.5 m/day was faster than the estimated speed 2.0 m/day. At the normal-stage, the actual tunneling speed 4.55 m/day was also slightly faster than the expected value 4.3 m/day. As a result, the actual tunneling duration of 360 days was 40 days shorter than the estimated 400 days. As compared with single-circular twin-tube tunneling method, underground excavation with the DOT method would increase the tunneling duration for about 32%.

The cost per meter of tunnel constructed with a DOT shield was about 1.5 times that constructed with single-circular shields. The cost of shield machine and segment lining were 23% and 53% of the total tunneling costs respectively. The expensive DOT shield machine and the complicated manufacturing and assembly of lin-ing segments are the main reasons for the higher cost of tunnellin-ing. However, it would cost a lot more budget and it would be much more risky to excavate three cross-passages between the single-circular tunnels under the river.

Fig. 17. Duration of DOT tunnel construction.

Table 3

Comparison of proposed tunneling cost (afterDepartment of Rapid Transit Systems, 2008).

Contractor D K N O S Average

Cost per meter of tunnel 103

(US$) Single-circular twin-tube 21.10 20.13 15.29 23.23 14.71 18.89

(13)

Acknowledgements

The authors thank Mr. Pei-Jeen Wu and Mr. Chien-Hsu Chen, North District Project Ofﬁce, Department of Rapid Transit Systems, Taipei City Government, for providing valuable information and support. The assistance of Mr. Heng-Tzu Lin, CECI Engineering Con-sultants is gratefully acknowledged. Special Thanks are extended to Mr. Min-Yi Huang, NCTU for his kind assistance.

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