Shallow Tunnelling Method (STM) for Subway Station Construction in Soft

Tunnelling and Underground Space Technology 29 (2012) 10–30

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Tunnelling and Underground Space Technology

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Shallow tunnelling method (STM) for subway station construction in soft ground ⇑ Qian Fang a,b, Dingli Zhang b, Louis Ngai Yuen Wong a, a School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore b School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China article info abstract

Article history: This paper provides an in-depth illustration of the shallow tunnelling method (STM) used for tunnelling Received 2 May 2011 in shallowly buried soft ground. Limited arching effect and limited ground strength mobilization are the Received in revised form 30 November 2011 two mechanical characteristics of the STM. The stability of the cutting face and the dry tunnelling condi- Accepted 28 December 2011 tion are the two preconditions that should be satisfied for the STM. Some ‘‘necessary’’ auxiliary methods Available online 30 January 2012 mainly served to guarantee these two preconditions are highlighted. Five principles, namely proper auxiliary methods, sequential excavation with short advance length, rigid support with quick installation, Keywords: short ring closure time and systematic deformation monitoring, which are required to follow when using Shallow tunnelling method the STM are summarized. The state-of-art of the STM is classified into five different construction Soft ground tunnelling Subway station construction approaches according to tunnelling sequences, which are adopted in the construction of the nine subway Sequential excavation stations in Beijing. The tunnelling procedures, support measures and settlement characteristics associ- Ground surface settlement ated with excavation are demonstrated. Statistical analyses of the settlement data of 342 ground surface Numerical simulation monitoring points above these nine stations are performed to illustrate the ground deformation characteristics of the STM. Numerical simulations are also employed to study the ground deformation characteristics of different construction approaches under the same geological conditions. This paper systematically demonstrates the applicability of STM in theory and practice. It is helpful in updating the database of the world tunnel projects and serving as a practical reference for future similar projects. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction possible extent by allowing a controlled ground deformation (Brown, 1981; Sauer, 1988; Will, 1989; Health and Safety Execu- The New Austrian Tunnelling Method (NATM) has become very tive, 1996). Institution of Civil Engineers (ICE) claimed that any soft popular for tunnel construction all over the world due to its tech- ground application of NATM is associated with the following nical feasibility, safety and economic competitiveness. This method principal measures: (1) Excavation stages must be sufficiently is based on the principles established by Rabcewicz (1964, 1965) short, both in terms of dimensions and duration. (2) Completion for the use of shotcrete as a support system in tunnel construction, of primary support—in particular, closure of the sprayed concrete together with observational method to determine whether the ‘‘ring’’ must not be delayed. Since these two measures are not support system is sufficient. The use of NATM in soft ground was compatible with the original NATM philosophy for soft ground, largely pioneered by Professor Müller for the Frankfurt Metro con- ICE proposed a different title of Sprayed Concrete Lining (SCL) struction in 1968 and was later further developed by London instead of NATM for soft ground tunnelling (ICE, 1996). Underground in the early 1990s. The NATM was first introduced to China at the end of 1960s. It Although the NATM has achieved remarkable successes (Tauern was widely used after the construction of the Dayaoshan railway tunnel, Arlberg tunnel, Inntal tunnel, metro Rankfurt, Schweikheim tunnel (double track, 14.3 km in length, constructed from January tunnel, Tarbela caverns) all over the world (Kolymbas, 2005), as 1981 to November 1988). This method was first adopted for soft many of the NATM’s recommendations were already in use, there ground excavation in the construction of the Jundushan railway exists some confusions and conflicts (Karakus and Fowell, 2004). tunnel (double track, 8.5 km in length, constructed from January One debate of the NATM is whether it is suitable for soft ground 1985 to August 1988). This method was first used for shallow sub- construction (Brown, 1981). It is widely accepted that one of the way tunnel construction in the Fuxingmen U-turn project (445 m major principles of the NATM is the deliberate mobilization of single track, 262 m double track, 7.0–14.9 m tunnel span, 9–12 m the strength of the ground around a tunnel to the maximum soil overburden, constructed from May 1986 to May 1987). These three projects are regarded as the milestones for the NATM used in China. During the Fuxingmen U-turn project construction, ⇑ Corresponding author. Tel.: +65 6790 5290; fax: +65 6791 0676. tunnelling in shallowly buried soft ground was found to be greatly E-mail address: [email protected] (L.N.Y. Wong). different from tunnelling in solid or fair rock, although the

Ground surface (P /σ )×100 σ i 0 0 Legend 100 O σ Legend 0: in-situ stress GRC for ur σ Pi: support pressure Bt: tunnel width natural ground 0 r: tunnel radius Pi 2r Ht: tunnel height 80 ur : radial displacement W W: arching zone width D: arching zone height A c d SCC 1 H H: tunnel overburden depth 60 SCC 2 SCC 3 D 40 C D loosening B

(Zone of arching) 20 E GRC in the presence of ground reinforcement

EF (ur/r)×100 Ht 0 ab 012345

Bt Fig. 2. Schematic representation of ground-support interaction curves.

Fig. 1. Conﬁguration of ground arch (after Terzaghi, 1946).

Although great construction achievements have been made by using the STM, limited research papers on this topic have been construction techniques adopted for these two ground conditions published in international journals (Xiang et al., 2005; Fang et al., were more or less the same. Therefore the name ‘‘shallow tunnel- 2011). In order to bridge the gap between practice and research, ling method’’ (STM) was assigned by the Ministry of Construction this paper provides an in-depth illustration of the STM. In this pa- of the People’s Republic of China in 1987 to distinguish it per, the two mechanical characteristics of the STM which distin- from the NATM. This method has been widely used in subway guish it from the NATM are ﬁrst illustrated. The two necessary construction in densely built urban areas in many cities in China preconditions of the STM are then explained. Next the auxiliary since then, such as Beijing, Shanghai, Guangzhou, Shenzhen and methods which are used to guarantee the two preconditions of Hangzhou. the STM are demonstrated. After that the underlying principles of

Table 1 Classiﬁcation of typical auxiliary methods. 12 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

8mm injection hole 0.1m

0.35m 2.5-3.5m 42mm (a) Forepoling pipe

Forepoling with grouting Forepoling pipe I 10-30° VII III I 0.5m or 0.75m III II VII Arc-shaped top heading II

0 Support . 3 Support core - core 0 . 5 m 6.3m Bench + Invert VI IV IV VI V V

6.0m Determined by monitoring 4-5m (b) Typical layout of forepoling

Fig. 3. Example of forepoling.

sults are presented to compare various sequential excavation approaches.

Arc-shaped top heading t 2. Insights into the STM n e m e ) rc In China, soft ground tunnels are generally shallowly buried, fo m Support core n 3 i = particularly common for urban subway construction in soft re L g n 2 ground. The associated tunnelling techniques dominated by using ti 4 o Φ Bench + Invert o ( manpower-excavation are collectively referred as STM. F e il p The STM is a concept or philosophy for tunnelling in soft ground rather than a set of excavation and support techniques. It is different from the NATM with regard to the design philosophy, although the former also adopts some of the widely used techniques such as sequential excavation, ground reinforcement, shotcreting, monitoring as in the NATM. Fig. 4. Typical layout of footing reinforcement pile. The critical overburden depth, which distinguishes the shallowly buried conditions from the deeply buried conditions, is the STM based on the engineering practices are elaborated. Fur- obtained by considering whether the arching effect can be ade- thermore nine subway stations using typical sequential excavation quately developed. Under deeply buried conditions, the arching approaches of the STM together with their on-site monitoring zone heights are assumed to be unchangeable. While for shallowly results are shown respectively. Finally numerical simulation re- buried conditions, the failure zones are easy to extend to the

Φ108 steel pipe

0.3-0.4m

5 °

Φ108 steel pipe Support core

Fig. 5. Typical layout of pipe roof protection. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 13

Tunnel crown Grout pipe

Primary lining

Fleece Sealing membrane Grout pipe Secondary lining

Fig. 6. Typical layout of contact grouting.

Table 2 Information about Line 4, Line 5 and Line 10 of Beijing subway.

Line Year opened Length (km) Number of stations Number of stations using different construction method STM Open cut method (OCM) STM and OCM Above ground Line 4 2009 28.2 24 6 12 5 1 Line 5 2007 27.6 23 4 7 5 7 Line 10 2008 24.7 22 8 9 5 0

ground surface. According to the Code for Design on Tunnel of Rail- Method (CCM), which is a widely used tool for preliminary assess- way (TB10003-2005, 2005), the critical overburden depth is con- ment of convergence potential and support requirements for circu- sidered to be 2.5 times of the arching zone height obtained by lar tunnels in a variety of geological conditions and stress states the deeply buried conditions. In practice, this critical overburden (Carranza-Torres and Fairhust, 2000; Oreste, 2003), is conducted depth is commonly considered to be 4–6 times of the tunnel diam- to illustrate this key principle. The CCM basically consists of the eter as the arching zone height varies with different soil conditions. definition of the internal support pressure (Pi)–radial displacement The arching effect is illustrated in detail below. (ur) relationship on the boundary of a circular void that represents the tunnel. This relationship is indicated by the Ground Reaction 2.1. The mechanical characteristics of the STM Curve (GRC). The Support Characteristic Curve (SCC) can be constructed from the elastic relationship between the applied stress 2.1.1. Limited arching effect Arching effect is one of the most universal phenomena encoun- tered in soils both in the field and in the laboratory (Terzaghi, 53 Elevation (m) 1943). Arching can be best described as a transfer of stresses be- Back fill tween a yielding mass of geomaterial and the adjoining stationary 51 members induced by stress redistribution. The shearing resistance Sand tends to keep the yielding mass in its original position resulting in 49 a change of the pressure on both of the yielding part’s support and 47 the adjoining medium. The concept of arching is illustrated in Fig. 1 Silty sand (Terzaghi, 1946). Due to arching, the height of the relatively loose 45 overburden above the tunnel roof resulting from the excavation of Silt the tunnel is D (the height of the arching zone) instead of H (the 43 overburden depth). The limitation of this practice is that it lowers Silty clay 41 the strength of the rock mass and permits a significant roof conver- Clay gence, which mobilizes a zone of loosened rock mass above the Water 39 tunnel roof. Despite all these limitations, arching effect does play table Gravel an important role in reducing the load on the roof support of a 37 deep tunnel. However, for a shallow tunnel in soft ground, the Clay arching effect may not be adequately developed to enable an arch 35 5.8m to be self-supporting above the excavation. Therefore, under this condition, the support may actually carry a significant portion of Fig. 7. Typical geological profile of a tunnel section. the ground directly above the roof.

Table 3 2.1.2. Limited ground strength mobilization Physical and mechanical properties of soils. The philosophy of the NATM relies on the mobilization of the strength of the ground by allowing a controlled deformation. For Specific Elastic Poisson’s Cohesion Friction weight modulus ratio (kPa) angle (°) the STM, however, the ground settlement should be strictly con- 3 (kN/m ) (MPa) trolled instead of being readily mobilized. This is of paramount Backfill soil 18.0 10.5 0.32 20.0 14.0 importance for shallow tunnelling under densely built urban area, Sand 19.5 30.0 0.29 – 25.0 since excessive ground settlement may cause tunnel cave-in, and Silt sand 20.5 25.0 0.24 18.5 29.0 bring negative effects or even damages to the existing nearby Silt 20.0 16.0 0.28 23.0 27.0 structures and utilities. Therefore the key principle for the STM is Silty clay 20.0 35.0 0.29 32.0 17.0 to control the ground deformation in order to guarantee the tunnel Clay 21.0 30.0 0.32 40.0 20.0 Gravel 21.5 75.0 0.22 – 42.0 stability and the environmental safety. Convergence-Confinement 14 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

6.5m 6.5m

Back fill LMR Φ114 roof pipe Key

Silt 5.5m Ground settlement monitoring point Φ42 grout pipe Φ1000 cylindrical steel column Silty clay

Silt 3.6m 25cm temporary support

Water

table 3.5m 60cm secondary lining Sand

16.3m Floor slab

4.5m 30cm primary lining

Silty clay 4.7m Sand

Gravel 22.6m

Fig. 8. Cross-section of the Puhuangyu station.

1 5 5 2 6 6 3 7 7

4 8 8

(a) Stage 1 (b) Stage 2 (c) Stage 3 (d) Stage 4 (e) Stage 5

Fig. 9. MDA used in the Puhuangyu station.

and the resulting convergence for a section of the support in the longitudinal direction of the tunnel. By making a signiﬁcant and 0 Stage 3 - Stage 5 possibly debatable assumption that the support application does not change the ground response, the tunnel-support system -50 reaches equilibrium at the point where the GRC and the SCC inter- Stage 1 & Stage 2 sect. The curve GRC I in Fig. 2 represents the condition of natural -100 ground. The beginning point O of GRC I represents the condition

that for the internal pressure equal to the far-ﬁeld stress r0.As -150 L there is no change in the initial stress and strain state around the R void, the radial displacement of the wall is zero. With a decrease -200 M of the internal pressure, the radial displacement of the wall in- -250 creases. The initial deformation is linear. It becomes curvilinear Ground surface settlement (mm) 2003/11/1 2004/2/1 2004/5/1 2004/8/1 2004/11/1 2005/2/1 2005/5/1 2005/8/1 upon reaching point A, which indicates the onset of plastic deformation. Beyond point B, with excessive displacement, the disinte- Date gration of the surrounding ground may cause cave-in and hence Fig. 10. Measured ground surface settlements. disasters. Therefore, under this condition, it is necessary to take

Back fill 2.8m Lateral rainwater pipe branch Silt Longitudinal rainwater pipe branch

3m Vertical rainwater well Φ1200 rainwater pipe

Fig. 11. The rainwater pipes above the tunnel. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 15

L 0 R M Stage 3 - Stage 5 -50

-100 Stage 1 & Stage 2

-150 Ground surface settlement (mm) 2004/1/1 2004/4/1 2004/7/1 2004/10/1 2005/1/1 2005/4/1 Date

Fig. 15. Measured ground surface settlements.

Fig. 12. Temporary vertical steel bar support. gravel. Two preconditions, namely stability of the cutting face suitable ground reinforcement measures before tunnelling to guar- and dry tunnelling condition, must be satisﬁed when using the STM. antee its stability. The GRC II is derived in the presence of ground reinforcement, where ‘‘detrimental loosening’’ is prevented. With 2.2.1. Stability of the cutting face the installation of ground reinforcement, the ground deformation Generally, the self-stability of the shallow tunnel in soft ground and the load on the support can be greatly reduced. Even though is very limited. It is impossible to tunnel through a cutting face the ground is reinforced, a stiffer support with installation with a very short stand-up time. Therefore, suitable measures immediately after excavation is still preferred (SCC 1 is preferred should be taken to guarantee a long enough stand-up time for to SCC 2 and SCC 3) for shallow tunnelling in soft ground, espe- the cutting face and the unsupported span before the support takes cially in the densely built areas, where strict ground deformation action. control is required. 2.2.2. Dry tunnelling condition 2.2. Preconditions for the STM For shallow tunnelling below the groundwater level or in the water-bearing ground, dry tunnelling condition should be guaran- Using speciﬁc construction techniques, the STM allows shallow teed. Dry tunnelling condition is essential in maintaining the sta- tunnelling in soft ground conditions, such as silt, clay, sand and bility of the cutting face by avoiding the mechanical properties

7m 7m

Back fill LMR Key Silt Ground settlement monitoring point Φ Φ 7.4m 42 grout pipe 108 roof pipe Φ800 cylindrical steel column Sand

25cm temporary support 2.61m Water table 60cm secondary lining 3m Floor slab Silt 30cm primary lining 3m

Silty clay 15.06m Silt 3m

Silty clay 2.91m Sand 23.78m

Fig. 13. Cross-section of the Tiantan Dongmen station.

1 3 11 11 2 4 12 12 5 7 13 13 6 8 14 14 9 10 15 15 (a) Stage 1 (b) Stage 2 (c) Stage 3 (d) Stage 4 (e) Stage 5

Fig. 14. MDA used in the Tiantan Dongmen station. 16 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

5.0m 5.0m 5.0m 5.0m 1.7m 1.7m 1.7m 1.9m 1.9m 1.8m 1.8m 1.8m 1.8m 1.9m 1.9m 1.7m 1.7m 1.7m 5.0m 5.0m 5.0m 5.0m

Back fill Key Ground settlement monitoring point

Φ Φ Silt 7.9m 32 grout pipe Φ121 roof pipe 800 cylindrical steel column

Backfill concrete Sand 30cm primary support 5.1m Gravel 60cm secondary lining Floor slab Φ800 Bored and cast-in-place pile Silt 3.55m 17.15m

Silty clay 2.4m 25cm primary support Water table 5m

Gravel

26.4m

Fig. 16. Cross-section of the Xuanwumen station.

5786

1 3 4 2

(a) Stage 1 (b) Stage 2 (c) Stage 3

(d) Stage 4 (e) Stage 5

Fig. 17. PBAA used in the Xuanwumen station. deterioration of the surrounding ground due to water inflow and objectives—crown stabilization, cutting face stabilization, footing increasing the effective stress of the surrounding ground. Further- stabilization, ground water control and ground (existing nearby more, dry tunnelling condition could also improve the underground working environment, hence increasing the working efficiency and decreasing the unsupported time of the free span be- 0 hind the cutting face. In practice, these two preconditions cannot be always satisfied. -10 Therefore, one or more auxiliary methods for the STM, which are -20 illustrated below, should be adopted. -30 Stage 1 (drift 1&drift 2) Stage 1 (drift 3&drift 4) 2.3. Auxiliary methods for the STM -40 Stage 1 (drift 5&drift 6) Stage 1 (drift 7&drift 8) -50 Stage 2 An auxiliary method is a construction method of a secondary or Stage 3 Stage 4 & Stage 5 special nature adopted to ensure tunnel construction safety and -60 Ground surface settlement (mm) surrounding environmental safety, where either conventional sup- -30 -20 -10 0 10 20 30 port patterns or sequential excavation measures do not provide Distance from centerline (m) effective solutions or where they are not advantageous (Japan Soci- Stage 1 Stage 2 Stage 3 Stage 4&5 ety of Civil Engineers, 1996). Date Sep,2004 Jan,2005 Mar,2005 Jun,2005 Feb,2006 Table 1 lists some typical auxiliary methods currently used in the STM. These auxiliary methods are classified according to their Fig. 18. Measured ground surface settlements. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 17

8.8m 8.8m

Back fill

6m Φ32 grout pipe Φ159 roof pipe

35cm primary support Silty clay

Φ1000 Bored and 6.06m cast-in-place pile

Water Φ table 800 cylindrical steel column 80cm secondary lining 4.2m

Gravel 18.47m Floor slab

3.52m 35cm primary support

Sand 4.69m Gravel 26.46m

Fig. 19. Cross-section of the Huangzhuang station in Line 10.

7 8 6 5 9 10

3 4 2 1

(a) Stage 1 (b) Stage 2 (c) Stage 3

(d) Stage 4 (e) Stage 5 (f) Stage 6

Fig. 20. DPCAA used in the Huangzhuang station in Line 10. structure) movement control. Most auxiliary methods have more than one function in the STM; and all these methods, except the drainage methods, serve the purpose of limiting ground settlement. More than one auxiliary method is often adopted simultaneously. Some auxiliary methods, such as forepoling, footing reinforcement 0 L bolt (pile), pipe roof, and contact grouting, are widely used in the R M STM due to their efﬁciency and cost-effectiveness. In this way, the -20 auxiliary methods are not only auxiliary but also necessary for the STM. These ‘‘necessary’’ auxiliary methods are further discussed -40 Stage 3 - Stage 6 below.

-60 Stage 1 & Stage 2 2.3.1. Forepoling Grouting type forepoling is commonly used in the STM. Forep-

Ground surface settlement (mm) -80 oling pipes (Fig. 3a), steel, U42 (or U32), 2.5–3.5 m length, are dri- 2005/4/1 2005/7/1 2005/10/1 2006/1/1 2006/4/1 2006/7/1 2006/10/1 ven at an angle of 10–30° with the tunnel longitudinal axis into the Date ground above the top heading arch ahead of the cutting face. The separation of the forepoling pipes in the tunnel cross section is Fig. 21. Measured ground surface settlements. 18 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

6.5m 6.5m

Back fill

Sand 8.24m Φ42 grout pipe Φ108 roof pipe 35cm primary support

Gravel 30cm temporary support Φ800 cylindrical 3.81m steel column

Silty clay 2.5m 9.91m 60cm secondary lining

Gravel 3.6m Water 22.08m table

Fig. 22. Cross-section of the Jiaomen west station.

1 3 7 7

2 4 8 8

5 6 9 9

(a) Stage 1 (b) Stage 2 (c) Stage 3

(a) Stage 4 (b) Stage 5

Fig. 23. MDA used in the Jiaomen west station.

about 0.3–0.5 m. The distance between the exposed ends of two 2.3.3. Pipe roof protection adjacent loops of forepoling pipes along the tunnel is 1–1.5 m, The pipe roof protection consists of installing, prior to the tun- which is 2 or 3 round lengths of the top heading. Fig. 3b shows a nel excavation, a series of steel pipes, parallel (or at a certain angle) typical layout of forepoling. Forepoling is very effective to enhance to the tunnel axis around the periphery of the tunnel. The steel the stability of the free span and prevent the failure from extending pipes are installed by horizontally boring or ramming at portals. to the ground surface during construction.

0 2.3.2. Footing reinforcement bolt and pile Footing reinforcement bolting and piling consists of installation of downward-facing small diameter steel pipes with grouting Stage 3 - Stage 5 -10 or bolts at the footing of the primary supports. Compared to foot- L ing reinforcement bolt, footing reinforcement pile is more com- R monly used in China. For the footing reinforcement piling, U42 M steel grout pipes, 2.5–3.5 m in length, are driven diagonally -20 Stage 1 & Stage 2 downward into the side ground at the foot of the top heading primary support. Fig. 4 shows a typical layout of the footing rein- Ground surface settlement (mm) forcement pile. This method is helpful to bring the support 2006/11/25 2006/12/25 2007/1/25 2007/2/25 2007/3/25 2007/4/25 2007/5/25 capability of the steel grid into full action soon after installation. Date It is also useful to prevent any potential collapse during the following bench excavation. Fig. 24. Measured ground surface settlements. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 19

7.0m 7.0m

Back fill Key Ground settlement monitoring point

Silty clay 12.52m Φ Φ42 grout pipe 159 roof pipe 35cm primary support Water table 30cm temporary support Sand 5.23m Φ800 cylindrical Gravel steel column 11.45m 3.2m 60cm secondary lining

Sand 3.02m

Gravel 24.06m

Fig. 25. Cross-section of the Huangzhuang station in the single-deck part of Line 4.

1 3 1 3 2 5 2 5 4 6 4 6

(a) Stage 1 (b) Stage 2 (c) Stage 3

(d) Stage 4 (e) Stage 5 (f) Stage 6

Fig. 26. SDA used in the Huangzhuang station in the single-deck part of Line 4.

The voids between the surrounding soils and pipes, and the pipes unintentionally created during tunnelling. As the voids induced themselves are then grouted. Commonly, U108, U114, U127 and by the STM generally occur between the primary lining and the U159 pipes are used. However, large diameter pipes (e.g. U300, surrounding ground, and between the primary lining and the sec- U600) may sometimes be adopted. An individual pipe of the ondary lining, there are two types of contact grouting. pipe roof is either a single pipe or assembled by welding or screwing multiple pipes together. The pipe roof should be longer than 10 m in view of the efﬁciency and cost-effectiveness. The maximum length of a pipe roof can be up to 150 m, which means -10 that the entire subway station is under the protection of the pipe L1 M1 R1 L2 M2 R2 L3 M3 R3 L4 roof. Fig. 5 shows a typical layout of the pipe roof protection. This -20 M4 R4 L5 M5 R5 method plays an important role in controlling the potential cave-in during excavation and is very useful in reducing both the -30 magnitude and the lateral extent of the ground settlement. This -40 method is widely used at tunnel portals, other types of tunnel junctions and for the protection of neighboring structures such -50 as buildings, pipelines, and existing tunnels. -60 Before construction Stage 1 Stage 2 Stage 3 & 4 Stage 5 & 6 2.3.4. Contact grouting -70 Ground surface settlement (mm) Grouting is used for various purposes during shallow tunnelling 2007/1/25 2007/5/3 2007/7/1 2007/7/23 2007/9/25 in soft ground. Among which, contact grouting, which plays an Date important role in controlling ground settlement, is highly recom- mended. Contact grouting is the process of ﬁlling voids that are Fig. 27. Measured ground surface settlements. 20 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m 2.0m

Back fill

Φ42 grout pipe Φ108 roof pipe Silt 7.09m 35cm primary support 30cm temporary support Silty clay Φ

3.15m 800 cylindrical Water steel column table 2.99m Silt 9.47m 60cm secondary lining

Silty clay 3.32m 21.64m

Fig. 28. Cross-section of the Liujiayao station.

1 3 7 7

2 4 8 8

5 6 9 9

(a) Stage 1 (b) Stage 2 (c) Stage 3

(d) Stage 4 (e) Stage 5

Fig. 29. MDA used in the Liujiayao station.

One type of contact grouting involves grouting between the pri- 2.4. Principles of the STM mary lining (or temporary lining) and the surrounding soils using pre-embedded pipes (Fig. 6). Since lattice girder and wire mesh Since the STM is mainly used for tunnelling under densely built are used in the primary lining, the shotcrete must be sprayed urban area, the primary aim of this method is to control the ground through the girder and the mesh. Due to uneven spraying and deformation in order to guarantee the tunnel stability as well as rebounding of some shotcrete off the bars, voids are inevitably left the environmental safety. The underlying principles of the STM behind the individual bars, especially on the tunnel crown. These based on the engineering practices are elaborated below. existing voids may extend and collapse in the course of tunnelling, which will lead to a disturbance to the ground above. Moreover, stress concentration induced around these voids may crack the pri- 0 mary lining locally. This kind of contact grouting is helpful in sta- bilizing the ground above the tunnel crown during construction -10 and ensuring the primary lining in full contact with the surrounding ground to allow an effective load transfer. -20 Another type of contact grouting involves injecting grout be- -30 tween the primary lining (waterproofing membrane, to be more accurate) and the secondary lining (Fig. 6). Due to the gravity- -40 Stage 1 (drift 1 & drift 2) Stage 1 (drift 3 & drift 4) Stage 1:Dec,2004-Apr,2005 driven flow of the casted secondary lining before hardening and Stage 1 (drift 5 & drift 6) Stage 2-5:Apr,2005-Dec,2005 -50 the shrinkage of the concrete, a crescent-shaped void is unavoid- Stage 2- Stage 5 Ground surface settlement (mm) ably created between the primary lining and the secondary lining -20 -10 0 10 20 on the tunnel crown. By a subsequent contact grouting from Distance from centerline (m) pre-embedded pipes through the secondary lining, the structural integrity and waterproofing ability of the lining can be enhanced. Fig. 30. Measured ground surface settlements. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 21

5.0m 5.0m 4.0m 3.5m 3.0m 2.0m 2.0m 3.0m 3.5m 4.0m 5.0m 5.0m

Back fill Key Ground settlement monitoring point

Silty clay 11.67m Φ32 grout pipe Φ159 roof pipe

35cm primary support

Sand 30cm temporary support Water

table 4.38m Φ950 cylindrical steel column 10.64m 2.9m Gravel 60cm secondary lining 3.36m

23.86m Silty clay

Fig. 31. Cross-section of the Zhangzizhong station.

1 3 3 1 2 4 4 2 5 6 6 5

(a) Stage 1 (b) Stage 2 (c) Stage 3

(d) Stage4 (e) Stage5

Fig. 32. SDA used in the Zhangzizhong station.

Proper auxiliary methods: Proper auxiliary methods should be as soon as possible. Sometimes, temporary invert and tempo- selected for the shallow tunnelling after carefully evaluating rary side walls are adopted for shortening the ring closure time their effectiveness, economic efﬁciency and compatibility with and enhancing the support capability. the particular conditions of the tunnel. Systematic deformation monitoring: Deformation monitoring in Sequential excavation with short advance length: The tunnel tunnelling projects should always be properly performed with should be driven in stages such that the area of each face is instruments installed or operated either from the ground sur- small enough to control. The entire excavation area is divided face or from inside the tunnel. Observations and measurements into multiple small drifts by temporary support, if necessary. Each drift can then be excavated by using full-face excavation or top heading (with support core)-bench-invert (if necessary) 0 sequential excavation. The drift height varies from about -10 2.5 m to 6 m and the height of the sequential excavation part Stage 1 (drifts 1) -20 Stage 1 (drifts 2) varies from about 1.5 m to 3 m. The advance length (free span) Stage 1 (drifts 3) should be strictly restricted to the distance between two -30 Stage 1 (drifts 4) -40 Stage 1 (drifts 5) adjacent lattice girders along the tunnel axis, which is about Stage 1 (drifts 6) 0.5–0.75 m. One of the many alternative ways of achieving this -50 Stage 2 principle is illustrated in Fig. 3. Stage 3 - Stage 5 -60 Stage 1: Jun,2004-Nov,2004 Rigid support with quick installation: Early strength sprayed Stage 2: Nov,2004-Mar,2005 concrete is preferred with two layers of welded wire mesh -70 Stage 3-5: Mar,2005-Jun,2006

and lattice girder for the primary lining. Support measures Ground surface settlement (mm) -20 -10 01020 30 40 should be adopted once available after excavation. Distance from centerline (m) Short ring closure time: It is imperative to close the primary lining to a complete ring at a short distance behind the face Fig. 33. Measured ground surface settlements. 22 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

10.0m 5.0m 5.0m 5.0m 2.5m 2.5m 5.0m 5.0m 5.0m 10.0m

Back fill Key Ground settlement monitoring point Silty clay 12.37m

Sand Φ32 grout pipe Φ159 roof pipe

35cm primary support Water table 30cm temporary support

4.59m Φ950 cylindrical Gravel steel column 10.64m 2.45m 60cm secondary lining Silty clay 3.6m

23.86m Sand

Fig. 34. Cross-section of the Dongsi station.

1 1 4 2 2 5 3 3 6 (a) Stage 1 (b) Stage 2 (c) Stage 3 (d) Stage 4 (e) Stage 5 (f) Stage 6

7 7 8 8 9 9

(g) Stage 7 (h) Stage 8 (i) Stage 9

Fig. 35. DCA used in the Dongsi station.

are used to: (1) Assess the stability of the tunnel and existing Hundreds of kilometers of new lines and extension lines are cur- nearby structures. (2) Verify the assumptions made during the rently under construction. By 2015, 19 lines with 561 km tracks design period. (3) Adjust the construction techniques, mostly will be in operation. auxiliary methods. (4) Improve the ground model for further In this study, some typical stations of Line 4, Line 5 and Line 10 calculation. (5) Accumulate experiences for the following simi- in Beijing using the STM are introduced. These three lines are lar projects.

0 It can be concluded from this section that the major difference between the philosophy of NATM and STM is whether the strength -5 of the ground should be mobilized by allowing a controlled deformation. The STM utilizes some NATM construction techniques, but -10 not necessarily NATM philosophy. Some typical subway stations in

Beijing using the STM are shown below. Stage 1 (drifts 1) -15 Stage 1 (drifts 2) Stage 1 (drifts 3) 3. STM used in Beijing for subway station construction Stage 2 -20 Stage 3 Stage 4 Stage 5 - Stage 9 3.1. Overview of the Beijing subway -25 Ground surface settlement (mm) -30 -20 -10 0 10 20 30 Beijing subway is a rapid transit rail network that serves the ur- Distance from centerline (m) ban and suburban districts of Beijing municipality. The subway’s ﬁrst line (Line 1) was opened in 1971. The network has 14 lines, Stage 1 Stage 2 Stage 3 Stage 4 Stage 5-9 Date Apr,2004Jul,2004 Sep,2004 Oct,2004 Nov,2004 Oct,2005 198 stations (if stations linked with transfers are counted sepa- rately), and 339 km of tracks in operation by January 2011. Fig. 36. Measured ground surface settlements. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 23

Table 4 Statistical data of the ground surface settlement of the above-mentioned Beijing subway stations.

Station Line Construction Number Number Overburden Width (m) Number of Monitored ground surface settlement approach of decks of arches thickness (m) height (m) monitoring points Max Mean Standard deviation (mm) (mm) (mm)

Puhuangyu 5 MDA 2 1 5.5 22.6 16.3 42 241.8 97.7 61.7 Tiantan Dongmen 5 MDA 2 3 7.4 23.8 15.1 64 200.9 93.8 47.6 Xuanwumen 4 PBAA 2 3 7.9 26.4 17.2 75 86.7 45.0 18.0 Huangzhuang 10 DPCAA 2 3 6.0 26.5 18.5 39 72.5 48.8 15.1 Jiaomen west 4 MDA 1 3 8.2 21.7 10.0 12 31.9 21.7 5.4 Huangzhuang 4 SDA 1 3 12.5 23.0 11.5 15 57.2 39.3 8.5 Liujiayao 5 MDA 1 1 7.1 21.6 9.5 13 53.0 26.1 16.8 Zhangzizhong 5 SDA 1 1 11.7 23.9 10.6 32 63.5 40.4 12.8 Dongsi 5 DCA 1 1 12.4 23.9 10.6 50 24.6 16.8 5.6

120

110

100

20 Number of settlement monitoring points 10

0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Ground surface settlement (mm)

Fig. 37. The histogram of the final ground surface settlements of the selected points. located in the Beijing downtown area. The STM deems to be more 3.2. Construction approaches for a double-deck station suitable in many stations as the heavy traffic above will not be interrupted by the subway construction. Table 2 lists some detailed 3.2.1. MDA for the single-arch–double-span station information of these 3 Lines. MDA was adopted for the Puhuangyu station, which was a sin- The geological and hydrogeological conditions of Beijing sub- gle-arch–double-span–double-deck station. The typical design way are very complicated, which are enormously different among cross-section is shown in Fig. 8. The MDA for the Puhuangyu different stations. A typical geological profile of the Beijing subway station construction is shown in Fig. 9. A total of five stages are is shown in Fig. 7. It reveals that the ground is typically composed required to accomplish the excavation of the entire cross-section. of backfill soil, sand, silty sand, silt, silty clay, clay, gravel, etc. Gen- It is noted that, for any two consecutive stages, the commencement erally the soil layers are not continuous and are interbedded with of a later stage does not necessarily wait until the earlier stage each other. The typical physical and mechanical properties of soils is completed in order to improve the construction efficiency. are shown in Table 3. The ground water level, which varies with the geographic position and seasons, is about 10–25 m below the ground surface. 10m According to the construction characteristics and especially the

construction sequences, the STM used in subway station construc- 35m tion in Beijing is categorized into five ‘‘approaches’’, which are γ =20kN/m3 E =35MPa µ =0.3 c =25kPa ϕ =30° named as Middle Drift Approach (MDA), Side Drift Approach 1 1 1 1 1 (SDA), Drift Column Approach (DCA), Pile Beam Arch Approach (PBAA) and Drift Pile Column Arch Approach (DPCAA), respectively. γ 3 ϕ 2=20kN/m E2=50MPa µ2=0.3 c2=35kPa 2=35° These approaches are used for the construction of both double-deck 35m subway stations and single-deck subway stations. A particular approach may be adopted with some variations in different sta- 160m tions, while one station may use different approaches according to its specific conditions. These five approaches will be discussed Fig. 38. Geometrical conditions and material properties used in the numerical in detail below. study. 24 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

Therefore, when mentioning the time–deformation curves in the 2 excavation, drift 3 excavation and drift 4 excavation respectively. following paper, the consecutive stages that are not distinctly Stage 3 is composed of drifts 5 excavation, drifts 6 excavation, separated are grouped together. drifts 7 excavation and drifts 8 excavation respectively. These The Arabic numbers labeled in the drifts indicate the designed explanations are also applicable to the following various stations. excavation sequence. Any two drifts labeled with the same Arabic According to the monitoring data, the ground surface settlement number will be excavated simultaneously. For the convenience of was mainly induced in the excavation stages (stage 1 and stage 3). In the following description, the labeled Arabic numbers are also re- stage 1, the excavation of the upper two drifts accounted for about ferred as the identiﬁcation of the drifts. For example, stage 1 in 80% of the settlement in this stage. The average ground surface set- Fig. 9 is composed of four substeps, namely drift 1 excavation, drift tlement above the middle drifts in this stage was about 40 mm. In

5.2m Ground surface 6.6m Ground surface 7.7m Ground surface L M L M L M

(a) MDA for (b) MDA for (c) PBAA for double-deck-single-arch station double-deck-triple-arch station double-deck-triple-arch station

9.1m Ground surface Ground surface L M 7.9m 7.6m Ground surface L M L M

(d) DPCAA for (e) MDA for (f) SDA for double-deck-triple-arch station single-deck-triple-arch station single-deck-triple-arch station

7.5m Ground surface 6.9m Ground surface 6.1m Ground surface L MML L M

(g) MDA for (h) SDA for (i) DCA for single-deck-single-arch station single-deck-single-arch station single-deck-single-arch station

Fig. 39. The detailed mesh shape around the tunnel in the simulation. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 25 stage 3, the excavation of the upper four drifts (drifts 5 and drifts 6) was about 90 mm, while that above the right side drifts was about accounted for about 80% of the settlement induced in this stage. The 125 mm. The largest ground surface settlement value, which ex- average ground surface settlement above the side drifts on the left ceeded 235 mm, was reported at point R (Fig. 8) of one section.

0 0

-20 -20 stage 1 stage 2 stage 1

-40 stage 2 -40 -60 -60 -80 M M -80 stage 3 stage 4 -100 L L stage 5 stage 3 stage 4 -120 stage 5 -100

-140 -120 Ground surface settlement (mm) Ground surface settlement (mm) 0 2 4 6 8 10121416 0 2 4 6 8 1012141618202224 Construction sequence Construction sequence (a) MDA for double-deck-single-arch station (b) MDA for double-deck-triple-arch station

0 0

-20

-20 -40

-60 -40 -80

-100 -60 Ground surface settlement (mm) Ground surface settlement (mm) 024681012 024681012 Construction sequence Construction sequence (c) PBAA for double-deck-triple-arch station (d) DPCAA for double-deck-triple-arch station

0 0 M M L L stage 1

stage 2 -20

-20 stage 1 stage 2 stage 3 stage 4 stage 5 -40 stage 6

-40 stage 3 stage 4 stage 5

-60 -60 Ground surface settlement (mm) Ground surface settlement (mm) 0246810121416 02468101214 Construction sequence Construction sequence (e) MDA for single-deck-triple-arch station (f) SDA for single-deck-triple-arch station

0 0 M M -20 L L stage 1 stage 2 -20

-40 stage 1 stage 3 stage 4 stage 5 -60 stage 2 stage 3 stage 4 stage 5 -80 -40

-100

Ground surface settlement (mm) -60 02468101214 Ground surface settlement (mm) 02468101214 Construction sequence Construction sequence (g) MDA for single-deck-single-arch station (h) SDA for single-deck-single-arch station

Fig. 40. Simulated ground surface settlements. 26 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

and accounted for about 35–45% in stage 2 and stage 3. The typical 0 measured ground surface settlement trough derived by the selected monitoring points (Fig. 16) of different construction stages -10 are shown in Fig. 18.

3.2.4. DPCAA for triple-arch–triple-span station -20 DPCAA was adopted for the Huangzhuang station, which was a stage 1 stage7

stage 2 triple-arch–triple-span–double-deck station, in Line 10. The typical stage 8 stage 9

stage 3 design cross-section is shown in Fig. 19. The DPCAA for the M -30 Huangzhuang station construction is shown in Fig. 20. A total of

L stage 4 stage 5 stage 6 six stages are required to accomplish the excavation of the entire Ground surface settlement (mm) 0246810 12 14 16 cross-section. Construction sequence According to the monitoring data, the ground surface settlement was mainly induced in the excavation stages (stage 1 and (i) DCA for single-deck-single-arch station stage 3). Generally, ground surface settlements accounted for Fig. 40 (continued) about 55–65% of the final settlements in stage 1. This value was about 5–15% for stage 2, 10–30% for stage 3 and stage 4, and 2–5% for the remaining two stages. The typical measured ground The measured ground surface settlement curves of the three moni- surface settlement curves of the three monitoring points (Fig. 19) toring points in this section are shown in Fig. 10. The abnormally are shown in Fig. 21. large settlement value was mainly caused by the leakage of the nearby rainwater pipes. According to the site survey, there existed 3.3. Construction approaches for single-deck station one U1200 rainwater pipe, 3 m above the right drift, which was con- nected with one rainwater pipe branch in the longitudinal direction 3.3.1. MDA for triple-arch–triple-span station and another rainwater pipe branch in the lateral direction by a ver- MDA was adopted for the Jiaomen west station, which was a tical rainwater well (Fig. 11). The two rainwater pipe branches had triple-arch–triple-span–single-deck station, in Line 4. The typical very serious leakage problems, which liquified the surrounding silt. design cross-section is shown in Fig. 22. The MDA for the Jiaomen So remedial measures, including temporary vertical steel bar sup- west station construction is shown in Fig. 23. A total of five stages port (Fig. 12), installation of additional plastic pipes inside the leak- are required to accomplish the excavation of the entire cross- age pipes, and compensation grouting on tunnel crown, were section. adopted after a certain length of drift 5 and drift 6 on the right site According to the monitoring data, ground surface settlement were excavated. These measures were proved to be effective for was mainly induced in stage 1 and stage 3. The typical measured subsequent construction. ground surface settlement curves of the three monitoring points (Fig. 22) are shown in Fig. 24. Ground surface settlements ac- 3.2.2. MDA for the triple-arch–triple-span station counted for about 55–85% of the final settlements in stage 1 and MDA was adopted for the Tiantan Dongmen station, which was stage 2, and about 15–45% in the remaining three stages. a triple-arch–triple-span–double-deck station, in Line 5. The typical design cross-section is shown in Fig. 13. The MDA for the Tian- 3.3.2. SDA for triple-arch–triple-span station tan Dongmen station construction is shown in Fig. 14. A total of SDA was adopted for the Huangzhuang station, which was a five stages are required to accomplish the excavation of the entire triple-arch–triple-span–single-deck station, in Line 4. The typical cross-section. design cross-section is shown in Fig. 25. The SDA for the According to the monitoring data, the ground surface settle- Huangzhuang station construction is shown in Fig. 26. A total of ment was mainly induced in the excavation stages (stage 1 and six stages are required to accomplish the excavation of the entire stage 3). The average final ground surface deformations above cross-section. the middle drifts and the side drifts were about 123 mm and As the single-deck part of the Huangzhuang station in Line 4 97 mm respectively. Ground surface settlements above the middle was excavated after the construction of the double-deck parts of drifts accounted for about 70% of the final settlement in stage 1 and this station, ground surface settlement above the single-deck part stage 2, while ground surface settlements above the side drifts was mainly induced by the former double-deck construction (Fang accounted for about 50% of the final settlement in stage 1 and stage et al., 2011). The ground surface settlement curves of the three 2. The typical measured ground surface settlement curves of the monitoring points (Fig. 25) in five different cross sections are three monitoring points (Fig. 13) are shown in Fig. 15. shown in Fig. 27. The average ground surface settlement before construction accounted for about 74% of the average final settle- 3.2.3. PBAA for triple-arch–triple-span station ments. Excluding the pre-settlement value, the average ground PBAA was adopted for the Xuanwumen station, which was a tri- surface settlement in stage 1, stage 2, stage 3 and stage 4, and ple-arch–triple-span–double-deck station, in Line 4. The typical remaining two stages accounted for about 73%, 9%, 16%, and 3% design cross-section is shown in Fig. 16. The PBAA for the Xuanw- of the average net settlement induced by the single-deck part exca- umen station construction is shown in Fig. 17. A total of five stages vation respectively. are required to accomplish the excavation of the entire cross- section. 3.3.3. MDA for single-arch–triple-span station According to the monitoring data, ground surface settlement MDA was adopted for the Liujiayao station, which was a single- was mainly induced in the first three stages. After stage 3, with arch–triple-span–single-deck station, in Line 5. The typical design the protection of the concrete arch above the tunnel crown, even cross-section is shown in Fig. 28. The MDA for the Liujiayao station though the remaining excavation area was about two times that construction is shown in Fig. 29. A total of five stages are required of the former excavation area, limited ground surface settlement to accomplish the excavation of the entire cross-section. (less than 10 mm) was induced. The ground surface settlements According to the monitoring data, ground surface settlement in stage 1 accounted for about 45–55% of the final settlements was mainly induced in stage 1 and stage 3. Three cross sections Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 27 installed with 21 ground surface settlement monitoring points final settlements in stage 1, about 10–20% in stage 2, and about (Fig. 28) were chosen for detailed monitoring. The largest ground 15–30% in the remaining three stages. surface settlement was reported to be about 53 mm in one of these three sections. The ground surface settlement troughs for different 3.3.5. DCA for single-arch–triple-span station construction stages of this section are shown in Fig. 30. The ground DCA was adopted for the Dongsi station, which was a single- surface settlements accounted for about 65–80% of the final arch–triple-span–single-deck station, in Line 5. The typical design settlements in stage 1, and about 20–35% in the remaining four cross-section is shown in Fig. 34. The DCA for the Dongsi station stages. construction is shown in Fig. 35. A total of nine stages are required to accomplish the excavation of the entire cross-section. 3.3.4. SDA for single-arch–triple-span station According to the monitoring data, ground surface settlement SDA was adopted for the Zhangzizhong station, which was a was mainly induced in stage 1, stage 3 and stage 7. Two cross sec- single-arch–triple-span–single-deck station, in Line 5. The typical tions installed with eleven ground surface settlement monitoring design cross-section is shown in Fig. 31. The SDA for the Zhangziz- points (Fig. 34) were chosen for detailed monitoring. The largest hong station construction is shown in Fig. 32. A total of five stages ground surface settlement was reported to be about 26 mm in are required to accomplish the excavation of the entire cross- one of these two sections. The ground surface settlement troughs section. for different construction stages of this section are shown in According to the monitoring data, ground surface settlement Fig. 36. The ground surface settlements accounted for about 60– was mainly induced in stage 1 and stage 3. Three cross sections in- 70% of the final settlements in stage 1, about 10–20% in stage 2, stalled with thirteen ground surface settlement monitoring points and about 20%-30% in stage 7. installed (Fig. 31) were chosen for detailed monitoring. The largest ground surface settlement was reported to be nearly 70 mm in one 3.4. Statistical analysis of the ground surface settlement of these three sections. The ground surface settlement troughs for different construction stages of this section are shown in Fig. 33. A total of 342 ground surface settlement monitoring points with Ground surface settlements accounted for about 55–65% of the final monitored settlements of the nine above-mentioned stations

MDA fo triple-arch station 10 0 SDA for triple-arch station 0 -20 -10 -40 -20 -60 -30 -80 -40 -100 MDA for single-arch station -50 MDA for triple-arch station

-120 PBAA for triple-arch station Ground surface settlement (mm) -60 Ground surface settlement (mm) DPCAA for triple-arch station -140 -70 -60 -40 -20 0 20 40 60 -30 -20 -10 0102030 Distance from centerline (m) Distance from centerline (m) (a) Double-deck station (b) Single-deck-triple-arch station 20 MDA for single-arch station SDA for single-arch station 0 DCA for single-arch station

-20

-40

-60

-80

Ground surface settlement (mm) -100

-30 -20 -10 0 10 20 30 Distance from centerline (m) (c) Single-deck-single-arch station

Fig. 41. Simulated ground surface settlement troughs by different approaches. 28 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 are chosen for further statistical analysis. These selected points tions prior to the support installation are calculated by using were mainly within the boundary projection of the tunnel cross- the hypothetical modulus of elastic (HME) soft lining method section to the ground surface. Fig. 37 shows the histogram of the (Powell et al., 1997; Karakus and Fowell, 2005). The primary sup- final ground surface settlements of the 342 points. Table 4 shows ports, modeled by using continuum elements, are installed imme- the statistical data of the ground surface settlements of the diately after excavation with their elastic modulus reduced to above-mentioned stations. According to the data, it can be con- 0.2 GPa. cluded that: (1) The ground surface settlement values mainly fall The simulations of the double-deck stations use the cross into the range between 0 and 80 mm. A total of 282 points, which sections of the above-mentioned double-deck stations. In order account for about 82.5% of the total points, are distributed in this to obtain a better comparison, the simulated two cross sections range. The tail of the histogram extends further to the right due of single-deck stations with triple-arch are chosen based on the to a relatively small number of large settlements. (2) Ground single-deck part of the Huangzhuang station in Line 4. In addition, surface settlements varied greatly among different stations. The the simulated three cross sections of single-deck stations with sin- maximum max-settlement (241.8 mm in the Puhuangyu station) gle-arch are chosen based on the single-deck part of the Zhangziz- is about 10 times the minimum max-settlement (24.6 mm in the hong station in Line 5. The detailed mesh shape and corresponding Dongsi station). The maximum mean-settlement (97.7 mm in the ground surface settlement monitoring points for the stations using Puhuangyu station) is about 6 times the minimum mean-settle- different construction approaches are shown in Fig. 39. ment (16.8 mm in the Dongsi station). (3) Generally, the value of ground surface settlement due to the double-deck station con- 4.2. Numerical simulation results struction is larger than that due to the single-deck station construction. (4) The monitored ground surface settlements of all The simulated ground surface settlements of two particular stations, except for the Dongsi station, exceeded 30 mm, which is points (point M and point L, Fig. 39) for different construction a universally-adopted ground surface settlement control standard approaches used in different kinds of stations are shown in in China. In fact, the construction and environmental safety of Fig. 40. The simulated final ground surface settlement troughs for these stations were guaranteed even though the ground surface these approaches are shown in Fig. 41. settlement was far larger than 30 mm. On the contrary, tunnel According to these numerical simulation results, the following cave-in and pipeline collapse were induced in tunnels with a conclusions can be obtained. ground surface settlement less than 30 mm. Therefore, further study should be performed to evaluate the applicability of this (1) The ground surface settlements induced by tunnelling vary 30 mm control standard. It is more preferable to develop specific according to different construction approaches used for dif- standards for different stations and even different parts of one sta- ferent tunnel cross sections. Even for the same cross section, tion considering the particular local conditions. different construction approaches lead to different settlements. It implies that the construction approaches do have 4. Numerical simulations of different STM approaches a significant effect on the ground surface settlements. The influence of excavation sequences on ground settlements 4.1. Assumptions and descriptions of the numerical models must be taken into account, particularly in the case of a large tunnel span with a limited overburden depth. Due to the distinctly different geological conditions, construc- (2) Generally, it is difficult to decide which construction tion techniques and construction qualities associated with differ- approach is better with regard to the ground settlement con- ent stations, it is unscientific to conclude which approach is better trol without considering its construction characteristics. For simply based on the monitoring data. Numerical simulation, using example, for the construction of the single-deck station with FLAC3D, a finite difference computer program, is employed to triple-arch and triple-span, the ground surface settlement evaluate all these approaches by assuming these approaches are value induced by MDA (maximum value: 58.8 mm) is used under the same conditions. The dimensions of the numerical slightly smaller than that induced by SDA (maximum value: simulation models, the overburden depth and the surrounding 62.0 mm). On the contrary, for the construction of the soil parameters values are shown in Fig. 38. As the main purpose single-deck station with single-arch and triple-span, the of the numerical analyses is to compare these different STM con- ground surface settlement value induced by MDA (maxi- struction approaches by evaluating their ground surface deforma- mum value: 94.2 mm) is much larger than that induced by tion, only one meshed block is considered in the longitudinal SDA (maximum value: 51.8 mm). It shows that the optimum direction to increase the computational efficiency. In these pseudo construction approach with regard to the ground settlement three-dimensional numerical simulations, the ground deforma- control is dependent on its particular construction situation. 15.2m 14.6m 16.9m

22.1m 12.8m 24.7m

Fig. 42. Some variations of PBAA. Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30 29

Suitable numerical simulations should be performed to assess construction based on the STM in China. Nine typical sta- the competitiveness of different construction approaches tions in Beijing using these approaches together with the with respect to the ground settlement control during the field monitoring data are used to illustrate the construction design phase. procedures, support measures and settlement characteris- (3) In most cases, the simulated ground surface settlement tics associated with excavation. It provides useful references trends, or the ratio of the settlement induced in one partic- for similar projects in future. ular stage to the final settlement, agree well with the mon- (3) The statistical analysis of the final settlement value of 342 itored ground surface settlement trends for the particular ground surface settlement monitoring points of the nine construction approach in different construction stages, for stations reveals that the ground surface settlement values example, Tiantan Dongmen station using MDA, Xuanwumen mainly fall into the range between 0 and 80 mm. The station using PBAA, Huangzhuang station using DPCAA, etc. 30 mm ground surface settlement control standard is not It indicates that the numerical simulation is very effective applicable to subway station construction using the STM in in predicting the settlement trends induced by different Beijing. Ground surface settlements varied greatly among construction approaches, even though the overburden depth different stations. The value of ground surface settlement and the soil mechanical parameters adopted in the simula- due to the double-deck station construction is generally tion may differ from those of the reality. Moreover, with larger than that due to the single-deck station construction. the availability of some monitoring data collected from the (4) Numerical simulations were conducted based on the assump- early construction stages and using some numerical back tion that these approaches are adopted under the same geo- analysis techniques, both the settlement trends and the set- logical conditions and overburden depth. The numerical tlement values can be more appropriately predicted. results reveal that the ground surface settlements induced (4) Generally, the ground surface settlement value associated by tunnelling vary according to the construction approaches with a double-deck station construction is larger than that used for different tunnel cross sections. Even for the same associated with a single-deck station construction. However, cross section, different construction approaches lead to differ- the ground surface settlement induced by PBAA used in a ent settlements. The optimum approach in ground settlement double-deck station construction is comparable to that due control should be judged by reasonable numerical simula- to the construction of some single-deck stations, and is tions in view of particular tunnelling conditions. Comparing about half of that due to the construction of the double-deck the numerical simulation results with the field monitoring stations using other approaches. Considering the construc- results, one can conclude that numerical simulation is an tion procedures (Fig. 17) and the predicted ground surface effective means in predicting the ground surface settlement. settlement curves in various stages (Fig. 40c), the ground The PBAA approach is proved to be better than other surface settlement of PBAA is mainly due to the construction approaches at ground settlement control as revealed both of the first three stages. After that, with the protection of the by numerical simulation and field monitoring. firm arch-pile-column support system, limited ground surface settlement is induced in the remaining two stages, even though the remaining excavation area is about two times Acknowledgments that of the former excavation. Due to the merit of PBAA in controlling ground surface settlement, many variations of The authors gratefully acknowledge the financial support by the this approach have been proposed and put into practice National Basic Research Program of China under Grant (Fig. 42). DPCAA and DCA can also be deemed as two varia- 2010CB732100 and the National Natural Science Foundation of tions of PBAA. China under Grant 51108020.

References 5. Conclusions Brown, E.T., 1981. Putting the NATM into perspective. Tunnels and Tunnelling 13, This paper illustrates the use of the STM for tunnelling in the 13–17. Carranza-Torres, C., Fairhust, C., 2000. Application of the convergence-confinement shallowly buried soft ground in China. According to the study, method of tunnel design to rock masses that satisfy the Hoek–Brown failure the following conclusions can be drawn: criterion. Tunnelling and Underground Space Technology 15, 187–213. Fang, Q., Zhang, D.L., Wong, L.N.Y., 2011. Environmental risk management for a cross interchange subway station construction in China. Tunnelling and (1) The STM is a philosophy for tunnelling in the shallowly bur- Underground Space Technology 26, 750–763. ied soft ground. Limited arching effect and limited ground Health and Safety Executive (HSE), 1996. Safety of New Austrian Tunnelling Method strength mobilization are the two mechanical characteristics (NATM) Tunnels, A Review of Sprayed Concrete Lined Tunnels with Particular Reference to London Clay, HSE Books. of this method. The stability of the cutting face and the dry Institution of Civil Engineers (ICE), 1996. Sprayed Concrete Linings (NATM) for tunnelling condition are the two preconditions that must Tunnels in Soft Ground, ICE Design and Practice Guide. Thomas Telford, London. be satisfied for this method. Auxiliary methods, which are Japan Society of Civil Engineers (JSCE), 1996. Standard Specification of Tunnel (Mountain Tunneling Tunnel) and Explanation. commonly adopted in the STM to guarantee these two pre- Karakus, M., Fowell, R.J., 2004. An insight into the new Austrian tunnelling method conditions, are categorized according to their objectives. (NATM). KAYAMEK02004-VII. In: Bölgesel Kaya Mekanig˘i Sempozyumu/ Some ‘‘necessary’’ auxiliary methods, such as forepoling, ROCKMEC0 2004 – VIIth Regional Rock Mechanics Symposium, 2004, Sivas, footing reinforcement pile, pipe roof protection and contact Türkiye. Karakus, M., Fowell, R.J., 2005. Back analysis for tunnelling induced ground grouting, are highlighted. Proper auxiliary methods, sequen- movements and stress redistribution. Tunnelling and Underground Space tial excavation with short advance length, rigid support with Technology 20 (6), 514–524. quick installation, short ring closure time and systematic Kolymbas, D., 2005. Tunnelling and Tunnel Mechanics. Springer-Verlag, Berlin, Heidelberg. deformation monitoring are summarized as five principles Oreste, P.P., 2003. Analysis of structural interaction in tunnels using the required to follow when using the STM. covergence–confinement approach. Tunnelling and Underground Space (2) MDA, SDA, DCA, PBAA, and DPCAA are summarized as the Technology 18, 347–363. Powell, D.B., Sigl, O., Beveridge, J.P., 1997. Heathrow-Express-design and five major approaches, which are categorized mainly due performance of platform tunnels at Terminal 4. In: Tunnelling’97. IMM, to the construction sequences, adopted in subway station London, pp. 565–593. 30 Q. Fang et al. / Tunnelling and Underground Space Technology 29 (2012) 10–30

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