optimizing the tunneling schedule for the bangkok …

OPTIMIZING THE TUNNELING SCHEDULE FOR THE BANGKOK SUBWAY, THAILAND Richard Prust, Arup, Seattle, WA John Davies, Arup, Singapore Asim Gaba, Arup, London, UK Peter Thompson, Arup, Hong Kong ABSTRACT The civil construction for the 20km long mass transit underground railway in the centre of Bangkok has recently been completed. The system was built in two separate civil design and construct contracts. The Northern Section comprised approximately 10 km of twin bored tunnels, 1 km of cut and cover tunnel and nine stations constructed within slurry walls adopting top-down techniques. The tunnels were constructed using earth pressure balance machines (EPBM). The team adopted top down construction techniques for the stations which lead to the requirement to run the TBM through three of the station boxes before they were fully excavated. The design of the box and the tunnel liner along with the internal excavation report were designed at each location to accommodate the effects of the TBM bore. This paper describes the construction schedule and design issues associated with this innovative approach. RÉSUMÉ Le projet récemment terminé, consistait en la réalisation des travaux de génie civil pour la construction des 20 kilomètres du réseau souterrain du train de banlieu dans le centre de Bangkok. Ce projet a été réalisé sous deux contrats différents : le génie civil et les autres travaux de contruction. La Section Nord du projet comprenait approximativement 10km de tunel jumelé realisé par forage, 1km en tunel à tranché couverte, et neuf sections construites par injection de de boue liquide en utilisant un procédé du haut vers le bas. Les 10km de tunnel ont été accomplis en utilisant un tunelier à équilibre des pressions du sol (EPBM). La technique de construction proposée pour les stations consistait en une technique à caisson avec construction des portions superieures en premier. Cette technique nécéssitait le passage de tunelier (TBM) dans l’empreinte des trois stations avant qu’elles soient completement excavées. Le design de l’empreinte des stations, des tunels ainsi que des travaux d’excavation a été conçu en détail sur chacun des sites afin de considérer les effets du tunelier (TBM) utilisée pour le forage des tunnels. Cette présentation expose le design du project et l’échéancier de réalisation associé à cette approche innovative. 1. INTRODUCTION The MRT Chaloem Ratchamongkhon Line Mass Transit Underground Railway comprises approximately 20 kms of twin bored TBM tunnels, and 18 stations built within perimeter slurry walls. The route of the project is shown in Figure 1. The work was divided into two design and construct contracts. The northern section was awarded to the ION joint venture composed of Italian-Thai Development PCL, Obayashi Corporation and Nishimatsu Construction Co Ltd and comprises 10 kms of twin bored tunnels 9 stations and two cut and cover tunnel structures. The team’s preference for top down construction and the need to optimize the TBM schedule lead to the requirement to bore the TBM through three station boxes

Figure 1 – Project Map

SRT Future Railway

Sea to Sky Geotechnique 2006

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before they were fully excavated. The slurry walls were designed to accommodate the impacts of this construction sequence and a temporary tunnel lining was installed inside the station box and subsequently removed when the excavation inside the station reached the final formation level. 2. GROUND CONDITIONS Bangkok is situated in the valley of the Chao Praya River in the lower Central Plain region of Thailand. The Central Plain and the Gulf of Thailand to its south are located within a north-south trending structural depression that was generated by fault block tectonics during the Tertiary Period (64 million years BP to 1.8 million years BP). The exact profile of the bedrock is unknown, but is thought to be between 550m and 2000m below ground level. The tectonic basin was continuously filled during the Pliocene and Pleistocene periods (5.3 million years BP to 8,000 years BP) with the clastic sediments that make up the Bangkok Aquifers. These soils consist of alluvial sands and gravels interbedded with floodplain silts and clays in the north, progressing seaward into deltaic deposits and marine clays. In the late Pleistocene the sea invaded the central plain during which period the First Stiff Clay was deposited. The sea regressed between 45,000 and 14,000 years BP leaving the First Stiff Clay exposed to sub aerial processes of desiccation and oxidation. The surface of the First Stiff Clay was also dissected by streams that formed broad shallow valleys that indicate that some erosion has taken place, resulting in some of the overconsolidation that is known to have affected the stratum. The amount of erosion is unclear but it is probable that further overconsolidation results from the process of weathering and desiccation. The sea made its last transgression over the central plain between approximately 14,000 and 3,000 years BP during which time the Bangkok Soft Clay was deposited non-conformably over the First Stiff Clay. Since the retreat of the sea the exposed surface of the Bangkok Soft Clay has been subjected to desiccation, which has resulted in a stiffer weathered crust. The stratigraphy across the route is relatively consistent and is presented in Table 1. Ground water conditions are controlled by deep well pumping which has caused a drop of the water pressure of the various underlying aquifers and under drainage of the clay layers. At present the ground water level of the Bangkok aquifer, which was originally near to the ground surface, is currently approximately 20m below ground level. The piezometric profile is shown in Figure 2.

Stratum Depth to top of stratum (m)

Description

Made Ground 0 Loose sands, soft clays and construction material

Bangkok Soft Clay

0.1 to 4.0 Very soft to soft dark grey inorganic clay, becoming medium stiff with depth.

First Stiff Clay 12.9 to 17.9

Stiff to hard light brownish or greenish grey silty clay.

Bangkok Aquifer

20.4 to 29.3

Alluvial and deltaic deposits consisting of medium dense to very dense sands with discontinuous clay bands

Table 1 – Generalized Stratigraphy The bored tunnels are mainly located in the stiff clay layer with the top of the tunnel at the boundary with the soft clay layer. The slurry walls of the station penetrate the stiff clay and the underlying sand layers.

Figure 2– Piezometric Profile 3. DESCRIPTION OF THE STRUCTURES

3.1 Station box The cut & cover station boxes are typically 200m long and 23m wide supported by retaining walls consisting of 1.0m thick slurry walls. The toe level of the walls were different for each station, ranging from 32.5m to a maximum depth of 37.5m. The maximum dig was approximate 22m and each station was built top down. Temporary plunge columns founded on piles were used as temporary


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supports to the slabs prior to the construction of the permanent columns (Figure 3).

Figure 3 – Typical section through a station box 3.2 Bored tunnel precast concrete lining The tunnel linings consist of precast concrete rings of six segments plus one keystone. The nominal inner diameter is 5700 mm and the segment thickness is 300 mm. The design strength of the concrete was 40 MPa. The segments were installed in a staggered arrangement and connected in the longitudinal and radial directions by curved steel bolts. In the station the top of the bored tunnel lining was generally located at approximately 14.5m below the ground surface. 4. CONSTRUCTION AND SCHEDULE ISSUES Optimizing the construction schedule was a key part of the bid stage design of the scheme. Top down construction was selected (see Figure 4). The slurry walls were constructed first, followed by excavation to the underside of the proposed roof slab.

The roof was cast and acted as the upper prop to the walls. Glory holes were left in the roof slab to allow access for the subsequent excavation below the roof slab. Excavation continued to the underside of the mezzanine slab which was cast as an intermediate prop. Excavation then continued to base slab level. The benefits of this approach are that:

• The use of the slabs as props during excavation results in a very stiff structure, minimizing ground movements and reducing impacts on adjacent structures and facilities.

• The amount of temporary propping during excavation of the station is reduced.

• The amount of temporary formwork for subsequent construction of the slabs is reduced.

• The excavation is also less constricted as a result of fewer props making working easier.

However, excavation to base slab using this method is slower, as each slab must be allowed to cure and to gain sufficient strength before excavation below can proceed. Allowing the stations excavation to reach full depth for use as launch shafts or to pull the TBMs through, in the case of mid line stations, would have had a significant impact on the TBM schedule. Implementing the top down construction method, while maintaining an aggressive TBM schedule (See Figure 5) required the use of temporary launch shafts which were excavated using conventional excavation methods with temporary strutting. Boring of the tunnels through the station boxes prior to their full excavation was adopted to increase the length of tunnel bores and to reduce the number of shafts. This technique is not common and specific design methods were develop to allow its use and to avoid detrimental impacts to the slurry walls or the tunnel lining constructed within the station.

a) Tunneling through station

b) Excavation of station

Figure 4 – Construction Sequence


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c) Completion of Station

Figure 4 – Construction Sequence

Key:

Tunnel through partially complete station

Launch shaft construction

Tunnel boring

Station Construction

Figure 5 - Construction schedule

S12 S13 S14 S15 S16 S18 S19 S20 S21


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The details of the schedule are shown in Figure 5, where the stationing is along the X-axis and time along the Y-Axis. The vertical bars represent the station construction schedules and the diagonal lines the tunnel boring schedule. For the longest tunnel run, to the south, a conventionally constructed launch shaft (shown as a dashed line) is built at the first station (S12) and the tunneling commences before the excavation in the remainder of the station reaches the concourse slab level. The TBMs reach the next station (S13) during its excavation to roof slab level, and the third station (S14) at the time the roof slab is being cast. The TBMs arrive at the retrieval station (S15) after it has been excavated to full depth and therefore no retrieval shaft is required. The second run which commences excavation at a similar time to the first run also from a launch shaft at station S15, is bored through the next station (S16) while it is being excavated to roof level. The relative schedules of station and tunnel construction vary in each case and therefore each station was designed separately. 5. DESIGN ISSUES

5.1 End walls panels The passage of the TBM through the station has a significant effect on the design approach to the perimeter slurry walls. The major design issues that were addressed included:

• The TBM bored through the end walls as it entered the station box. The walls were designed for the additional thrust loads from the TBM and it was assumed that the face pressure was maintained to ensure stability at all times to enable the TBM to break through the walls.

• The end walls were designed to transfer vertical loads across the tunnel opening to the adjacent load bearing panels by use of a capping beam and the intermediate floor slabs.

• where the tunnel boring machine passed through the slurry wall, ground improvement by jet grouting was carried out locally behind the wall in the zone of the tunnel openings. An upper limit of 1.2 x total overburden pressure was imposed on the lateral grouting pressure exerted on the wall during grouting.

• The traditional steel reinforcement was replaced in the area of the ‘tunnel eye’ by glass fibre reinforcement polymer (GFRP) bars. These bars could be broken by the cutter head of the EPBM without halting the machine for manual excavation.

• GFRP bars are very brittle and therefore the design was based on a limit stress approach rather than ultimate limit state. The properties of glass fibre reinforced concrete are described in the Japanese Society of Civil engineers (JSCE 1997) publication.

The end wall panels were subdivided into two types:

The panels that the TBM cut through were designed to allow for the use of glass fibre reinforcement polymer (GFRP) in the tunnel eye. Figure 6 shows the general arrangement of the reinforcement in the panel whilst Figure 7 shows a typical reinforcement cage as it was installed in the slurry wall trench.

Figure 6. Typical reinforcement layout in end wall Figure 7. GFRP reinforcement in slurry wall cage

Intermediate panels were designed to carry any additional load transferred from the less stiff tunnel eye panels via the use of waling beams to prop this part of the end wall during construction. The waling beams were also required to assist form openings in the upper slabs to allow TBM access. 5.2 Side wall panels The tunnels were constructed during the excavation for the main station box. This causes a loss of passive resistance over the zone of the tunnel. Numerical modeling was employed using the OASYS finite element program SAFE to assess the effects of the tunneling on the support to the main slurry wall panels and other station structural elements. This confirmed a significant loss of passive resistance during tunneling. The OASYS computer program FREW was used to model the slurry wall behavior throughout the proposed construction sequence. The program determines the wall


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deflections, bending moments, shear forces and earth pressures on both sides of the wall at each construction stage. In order to ensure that the computer program provides realistic results for the local ground conditions, the FREW model and associated input soil parameters were checked and “calibrated” by back-analyzing the observed behavior of other deep excavations in Bangkok using the OASYS finite element program SAFE to compare with the FREW analyses. The typical output of the vectors of soil movement from the SAFE analysis are shown in Figure 8.

Figure 8(a). Vector of soil movements due to station box excavation.

Figure 8 (b). Vector of soil movement Where the tunnel boring machines passed through the station structures in their semi completed state the station slurry walls were subjected to a reduction of the earth

pressures on the passive side of wall in the zone of the tunneling works. Figure 9 shows the overall assumptions for the passive resistance adopted in the computer modeling during and after tunneling through the station. The effect of the tunnel passing by adjacent to the wall was modeled by replacing the soil in the tunnel zone with two soil layers.

South North

Passive Resistance

provided by wedge of

soil (see Figure 6)

Total Loss of Passive

Resistance in this zone

(Tunnel Layer 1)

During Tunneling

Full Passive Resistance

in this zone


in this zone

Tunnel Layer 2


restored in this zone

Tunnel lining assumed

to act in hoop stress to

transfer support across

excavation

Stiffness of soil reduced to

allow for reduced thickness

of compressible strata

Tunnel Lining Installed



Figure 9 - Assumption for analysis of slurry wall 5.2.1 Tunnel Soil Layer 1 Total loss of passive support to the wall was modeled by a soil with the following earth pressure parameters: Ka = 0.01, Kp = 0.02, Cu = 0 and γ = 19 kN/m3.) 5.2.2 Tunnel Soil Layer 2 The lateral resistance provided by the wedge of soil between the bench of the tunnel and slurry wall was determined using simple wedge theory. In this analysis the angle of the most likely failure plane was determined by equating internal and external work for the system under the application of a horizontal force. Figure 10 shows the results from the analysis to determine the most likely failure plane and the force (Pp) required to fail the wedge for the layer.


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Figure 10. Passive Force vs Angle of Failure Plane 5.3 Tunnel Lining As the station is excavated the overburden pressure on the tunnel crown reduces while the lateral pressures on the tunnel sides are maintained, leading to ovaling of the tunnel lining occurring in a vertical direction. A safe excavation depth was determined such that the station could be excavated without collapse of the tunnelling. Excavation below this was carried out in a staged sequence such that the lower parts of the station excavation could be carried out and the tunnelling broken out in a safe manner. Two different approaches were used for the modeling of the precast concrete tunnel lining’s structural behaviour:

a) beam elements, for the assessment of the section

forces in the lining (Structural Design) b) continuum elements, for the analysis of the overall

lining stability (Stability Analysis). The analysis indicated that the convergence of the tunnel lining under these conditions would be of the order of 50mm. 6. MONITORING The wall movements were monitored during the excavation using inclinometers and typical results are shown in Figure 11. The wall movements were approximately 50% greater than those observed where the stations were not “tunneled through”. As has been described in Davies et al (2001) the actual overall movements were less than originally estimated. The likely reason for this was the higher soil stiffness in the field compared with that used in the analysis as a result of low strain levels. The measured convergence of the tunnel was of the order of 20mm, again significantly less than that predicted by the analysis which again is likely due to the high stiffness of the ground at low strains.

The tunneling was completed successfully with no damage either to the station box or perimeter structures. All movements were well within the acceptable movement limits. Figure 12 shows the partial excavation of the tunnel lining within the station box.

Figure 11. Comparison of wall deflections for stations with tunnelling through and without.

Figure 12. Partial excavation of tunnel lining. 7. CONCLUSIONS

1) The tunneling through the stations allowed a fast track schedule to be developed for the overall schedule. 2) The tunneling through the three stations was carried out successfully with no major problems. Similarly the excavation of the station box and the removal of the temporary segment lining was achieved without any problems of instability of the lining. 3) The use of GFRP in the walls allowed the smooth passage of the TBM through the station without the need to halt to cut the reinforcement. 4) The design approach adopted for the wall ensured that the stability of the walls was maintained at all stages of construction. 5) The overall ground movements outside of the station boxes where the TBM’s “tunneled through” were larger than for the stations where the basement box was constructed prior to the arrival of the TBM, although still within acceptable limits.

170

180

190

200

210

220

230

240

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Angle of Failure Plane (degrees)

Pa

ssiv

e F

orce (

kN

/m)

Cw/Cu=0.5 Cw/Cu=0

0

10

20

30

40

0 10 20 30 40

Wall Deflection/mm

Dep

th/m

Station Tunneled Through Station with No Tunnel


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8. ACKNOWLEDGEMENTS

The authors would like to express their gratitude to all involved in the project, the client MRTA, the contractor ION and our colleagues in Ove Arup and Geoconsult for the close working cooperation which has made the project so successful. REFERENCES

Davies J.A. & Prinzl F. (1999). Design of underground structures north of the Bangkok Initial Subway System, 3rd Asia Tunneling Summit.

Davies J.A., Thompson P. & Young S. (2001). A comparison between tender, detailed design and the field performance of slurry walls in Bangkok, 14th South East Asia Geotechnical Conference.

Japanese Society of Civil Engineers (JSCE) (1997). .Recommendations for design and cons of concrete structures using continuous fibre reinforced materials JSCE Concrete engineering Series no 23 Oct 1997

Prinzl F. & Gomes A.R.A.Grove (2001). TBM tunnel construction for the Bangkok MRT: tunneling through station Boo, Singapore conference.

Prust R.E., Davies J & Gaba A (2001). Self boring pressuremeter testing in Bangkok subsoil, 14th South East Asia Conference Hong Kong.

Yeow H.C., Gaba A.R. and Pillai A.K. (2004). Concurrent Tunnelling and Station Excavation at Bangkok MRTA North, 15th South East Asian Geotechnical Conference, Bangkok.


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