cqy stn paper k27oct06

7/31/2019 CQY Stn Paper K27Oct06

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CLARKE QUAY MRT STATION - Engineering Provisions for Future

Development and subsequent Design of the Development

K Mahmood and S Mahatma

Maunsell Consultants (S) Pte Ltd

S S SohKTP Consultants Pte Ltd

Abstract

Clarke Quay Station is part of a 20km long Mass Rapid Transit (MRT) North East Line,

in Singapore. To fully exploit development potential of the underground transit station,

its design incorporated the planning guidelines required by Urban Redevelopment

Authority (URA) for the future development. The land above and adjacent to the stationwas subsequently sold as a white site parcel and is now known as CENTRAL.

This paper describes URAs requirements for the planned development at Clarke Quay

Station and the engineering provisions incorporated in planning and design of the station.

The issues discussed include the structural and foundation provision to support the

development, integration of future developments skewed grid arrangement with that of

the station and provisions for station integration with the development. As the

development partially sits above the station, it subjects the station to non-uniform loads.

The complexity of design for this varying load condition has also been discussed.

The paper also describes structural behavior of the station with the envisagedconstruction sequence for the development. The use of these provisions, in planning and

designing the development, is explained in subsequent part of the paper. Actual structural

behavior of the station, during construction of the development, is also presented for a

comparison with its analyzed behavior.

1.0 IntroductionUnderground rail transit system, in Singapores urban environment, helps avoid

sterilizing scarce land and making it available for more valuable use such as commercial

and residential developments. Transit oriented developments, especially those integrated

with the stations, also help to maximize access by transit transportation and encouragetransit ridership. It is important to make necessary provisions, where feasible, for such

developments during the early stages of transit station planning and design. This would

help materialize full commercial potential of such developments by mitigating constraintson their development.

Clarke Quay (CQY) Station is a part of 20km long Mass Rapid Transit (MRT) North East

Line (NEL), in Singapore, where Urban Redevelopment Authority (URA) required to

incorporate its planning guidelines for the site during the planning and design of the

station. Subsequently, during construction of the stations, the land above and adjacent to


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the station was sold as a White Site Parcel. The site has now been developed and is

known as Central.

Maunsell led the multi-disciplinary design team during tender, detailed design andconstruction stages of the CQY MRT Station. The station planning and design required

provision for up to a 12 storey future development above the station and a 20 storey

future development next to it, both with basements.

2.0 LocationThe 2-level underground station is located

on the south bank of the Singapore River,

opposite Clarke Quay, near the Coleman

Bridge and partially below the Eu Tong

Sen Street / New Bridge Road. During theinitial planning phase of NEL, it was

considered to be an ideal location forachieving the urban redevelopment

objectives and transportation integration

objectives. The alignment of the North

East Line, at CQY Station, resulted in a

station orientation skew to and partly

below the Eu Tong Sen Street / New

Bridge Road (Figure 1). The station is founded about 23m below the ground level.

Figure 1: Site Plan of Clarke Quay MRT Station

This space restriction and the skew orientation resulted in a layout of the station with

triangular shape enlargements at three corners. These large triangular areas were

required to accommodate the plant rooms at both concourse and platform levels.


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3.0 Requirements for Planned URA Future DevelopmentURA during the design stage of Clarke Quay Station issued their requirements for theintegrated development above and adjacent to the station. URAs plan for the

development consisted of one block of 4-storey promenade, 2 blocks of 12-storey

buildings with a basement, one block of 6 storey building with a basement and a 3-storeylink bridge. All these developments were required to be positioned over the station either

wholly or partially. Another block of 20-storey building with 2 basements was planned

just outside the station box (Figure 2).

Figure 2: Station Box Geometry and URAs Future Development

4.0 CQY Station BoxThe station box orientation and geometry, respecting the transit structure needs, followed

the orientation of the alignment. At the same time, it also incorporated the requirements

of future development with respect to road orientation. The station is about 200m long

with varying width along its length. It is 70m wide at the northern end near the river,

57m wide at the southern end near the Merchant Court Hotel and 28m wide along the

central public platform area. The station was designed as a Civil Defence Shelter.

The future development columns were generally supported on the station columns below.

However, due to different grid systems for the two structures, some of the future

development columns were supported directly on the 1950mm thick roof slab. In some

areas, the roof slab thickness was increased locally to 2500mm to transfer the future

development loads to the columns and walls below.

5.0 Provisions for Future Development in Station Planning


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5.1 Four Gridline SystemFor a normal rectilinear shape station box, an orthogonal 2-grid line system is sufficient

to plan and define the station geometry. However, the future development was orientatedwith respect to the road alignment and was oblique to the station structure. The future

development columns were not located on the station grid line. Station grid also did not

satisfy URA requirement for a regular pattern grid for the future development. Thisnecessitated the need for a four gridline system for CQY station. Two grid lines matched

the station platform orientation and the other two matched the skew ends and the future

development column positions. It was considered important to arrange the two-grid

systems complementary to each other. This was to achieve an arrangement of column

locations common to both the station and the development. The odd shape layout made

the transit architecture more challenging to obtain optimal use of the space.

Fig 3: Grid Systems defining Station Box and Future Development Geometries

With the adoption of four gridline system, an alternative column grid was proposed byadjusting some of the future development column positions. The revised column grid

arrangement was developed for better structural integration of the station and

development, considering the following: Future development column grids to suit station platform edge columns

Future development columns to rest on the station diaphragm wall where theywere close to the wall,

To allow for 3.6m wide promenade along the river side

To suit the location of the vent shaft structures

5.2 Transfer of Future Development LoadsThe varying heights of future development resulted in concentrated loads of varying

magnitudes on the station structure below. In addition, part of the station box area, below


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developments were planned with columns outside the station box but in close proximity

It required installation of foundations very close to the station structure. The subsurfacesoil profile at CQY consisted of deep Kallang formation. Deep foundation elements were,

therefore, needed as a foundation system for the future development. To minimize theeffect of constructing the deep foundation element, in close proximity of an operational

station, barrettes and bored piles were provided as part of the station development.

Figure 5: Foundation Provisions outside the Station Box

5.4 Unbalanced Excavation adjacent to Station BoxThe future development consisted of a 2-level basement adjacent to the station box,

extending over a length of about 80m along the station box (Figure 2). To facilitate the

basement construction adjacent to the station box, the station design allowed for a 14mdeep unbalanced excavation (up to the concourse of the station) on west side of the

station structure.

Figure 6: Provision for Unbalanced Excavation outside the Station Box.


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5.5 Integration with the other DevelopmentsStation planning also considered its integration with other future developments in itsvicinity to allow convenient movement of people between the two. As there were no

specific details available for such future developments, provision was made in the station

external walls for knock-out panels to facilitate future connection to the developmentdirectly or by means of subway structure. The locations of knock-out panels were

selected to suit the known future development towards West and unknown development

towards East. In total, there are seven knock-out panels provided at the concourse level

in the station external walls.

6.0 Envisaged Construction SequenceFor the design of the station box and associated entrances, feasible construction methodsand sequence were considered for the future development. They included construction of

future developments basement adjacent to the station entrance subway

future connection at knock-out panels

6.1 Basement adjacent to the Station BoxA feasible construction sequence for (the basement of) the future development was

developed to minimize the effect of its construction on the station box, as follows:

The basement above the station roof in areas A & C to be constructed within thestrutted excavation. This is to be completed before the construction of the

basement outside the station box.

The basement outside the station box, Area B, could then proceed without a need

to support the station box against unbalanced loads.(Figure 7)

Figure 7: Basement Construction Sequence

B

A

C


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6.2 Entrance SubwayPart of future development basement was planned adjacent to the station entrance subwaynear the Merchant Court Hotel. The subway structure was supported on piles and located

between the bored tunnel on one side and the future basement structure on the other side.

An assessment was carried out to evaluate a feasible construction method to limit theimpact of future basement excavation on the station entrance structure and on the

adjacent tunnels. It was proposed to construct the future basement using diaphragm walls

as rigid ground support system, with pre-installed jet grouted slab, at the level of entrance

subway base.

Figure 8: Scheme for Basement Construction next to Entrance Subway

6.3 Connection at Knock-out PanelsA suitable construction scheme was developed to minimize load transfer from the futurelink to the station structure.


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Figure 9: Future Connection at Knock-Out Panel Locations

7.0 Analyzed Structural Behaviour of the Station BoxUsing 3-dimensional analysis model, 27 independent load cases and 7 load combinations

were considered to simulate various external loads acting on the station box including

those from the future development and unbalanced loads from adjacent future basements.

The lateral displacement at roof level of the station box, from unbalanced excavation,was estimated at about 6mm and was well within the allowable 15mm. Such a controlled

movement reflected realistic behaviour of the rigid station box and was only expected

from a 3-dimensional model.

The settlement of the station box under varying load was also estimated using soil-

structure interaction program FLAC. For deformation analysis, stages of construction for

the station box were modeled in FLAC. For the maximum vertical downward loads, total

estimated settlement of the raft at the most heavily loaded column was within theallowable 20mm. It was also noted from the assessment that, under the differential

loading condition, the base slab settlement gradient was within the permissible limits of 1

in 1000.


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Figure 10: Station Box Deformation from Unbalanced Loads

8.0 Design of Central8.1 Development on Station StructureThe URA white site development located directly over the station structure is a podium

with one basement, a 12-storey SOHO1 (small office home office) near to the Coleman

Bridge and a 8-storey SOHO2 in front of Merchant Court Swissotel. The podium and

basement are designed for retail & beverage outlets.

SOHO1 starts from the 5th storey whereas SOHO2 starts from 4th storey. The 25-storey

tower block that is located north of the station sits on a triangular footprint and comprises

of 4-storey of retail & beverage, carparking and offices in the upper floors. There is a

basement in the tower block and a link to the concourse level. At the time of preparation

of this paper, the podium, SOHO1 and SOHO2 had been completed whereas the tower

block was partially completed.

The loads to be supported by station and the raft foundation were given by URA based on

the number of floors and the usage of the floors. In the design development phase, theseloads were a governing factor especially for the SOHO1 block.


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Figure 11: View of SOHO1 structure over the Clarke Quay MRT Station.

Figure 12 shows the plan with the column locations and design loads for the new

development. The provision for future developments loads, in the station design,was considered sufficient as the actual column loads were less than those originallyprovided.

Figure 12: Plan showing the URA columns above the Clarke Quay Station.


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8.2 Design CriteriaIn addition to the LTAs Code of Practice for Railway Protection, there were other

specific requirements in land sale conditions for the Clarke Quay MRT Station site. Forthe station structure, a maximum deformation of 20 mm and 15mm were permitted in

vertical and horizontal directions respectively.

The complex provision of a range of column loads for the different levels of the buildings

above the station would cause uneven settlement. Likewise, the station is supported by

peripheral diaphragm wall that would settle less than the raft foundation. To minimize the

effect of the differential settlement, the structure longitudinal differential movement was

limited to 1/1000.

The complex provision of a range of column loads for the different levels of the buildings

above the station would cause uneven settlement. Likewise, the station is supported byperipheral diaphragm wall that would settle less than the raft foundation. To minimize the

effect of the differential settlement, the structure longitudinal differential movement was limited to 1/1000.

Table 1: Design criteria for station structure movement.

Type of movement LTA Contract C708

Structure total movement in vertical direction 20 mm

Structure total movement in horizontal direction 15 mm

Structure longitudinal differential movement 1/1000

Track vertical dip or peak 5 mm over 5 m chord

Track horizontal corresponding versine 6 mm over 15.8 m chord

These design criteria are applicable during construction as well as for in the long term

condition. On the completion of the station construction, the rail tracks were aligned

vertically and horizontally before the tracks were put to use.

8.3 Design Parameters for Raft Foundation AnalysisThe design parameters used in the settlement analysis was adopted from Maunsellsearlier design. These are summarized in Table 2. The 1500 mm thick raft is supported on

three soil types at the founding depth, 23 m below ground level. Beneath the raft slab, at

northern end, there was a thin layer of soft soil that was treated with suitably spaced jet

grout piles. At central zone of the station box, soil improvement was carried out with

concrete trenches.

The penetration of 1200mm thick diaphragm wall varies from about 32 m to 38 m . The

diaphragm wall is founded in S2 material of the Jurong Formation sedimentary siltstones.


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Table 2: Subgrade modulus adopted for settlement analysis.

Soil Type Subgrade Modulus

Lightly JGP treated or lean concrete trench soil 13,000 kPa

Heavy JGP treated soft soil 16,000 kPa

S4a/b Weathered rock of Jurong Formation 22,500 kPa

Figure 13: Ground improvement layout plan.

8.4 Raft Foundation AnalysisThe SAFE finite element program was used to model the entire raft including the

diaphragm wall along the periphery of the station. The various load cases and

combinations are given in Table 3.

During the course of the construction of CENTRAL, there was no instrumentation to

monitor the hydrostatic pressure beneath the station raft. Even though the Singapore

River is next to the station, the diaphragm wall around the peripheral of the station could

have maintained the hydrostatic pressure acting beneath the raft slab. The tidal

fluctuation in the Singapore River varies from RL98.5 m to RL101.5 m. The raft soffitlevel is RL 79.0 m.

Table 3: Load case and load combination for the settlement analysis.

Ref. Load Case/ Combination Load Type

1 Load case 1 Station (ST)

2 Load case 2 Development Load (DEV)


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3 Load case 3 Hydrostatic (UPLIFT)

4 Load combination 1 ST+ UPLIFT

5 Load combination 2 DEV + UPLIFT

6 Load combination 3 ST + DEV

7 Load combination 4 ST + DEV + UPLIFT

The governing load combination is the development load. The settlement analysis was

also carried out with three different ground water level conditions, i.e at 96.0 RL, 101.0

RL and 92.5 RL to assimilate simulate the minimum uplift, the maximum uplift and the

temporary condition during the end of excavation. The maximum settlement under theeffects of full development load is estimated asat 17.5 mm.

Figure 14: The settlement plot for load combination with the development loads and

hyhdrostatic uplift.

The figure 14 shows the contour plot of the raft foundation under the full development

load. The settlement is not uniform due to varying column loads. The geometry also

affects the contour of the settlement. As shown in the settlement contour, the maximum

settlement occurs at the location where the width in the vertical direction is greatest. Thislocation is also directly under the SOHO1 and is close to the south-bound tunnel line.

8.5 Comparison of Predicted & Observed SettlementThere were no available measurements of the settlement during the construction of the

station and the period between the completion of the station and the commencement of

the new development over the station. However, the observed settlement can be

accurately obtained from the automatic tunnel monitoring system (ATMS) of the south

bound tunnel instead of the north-bound tunnel that is too close to the diaphragm wall on

the northern side.


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Figure 15: Lateral and vertical displacement of South-bound tunnel at the crown level.

The ATMS was installed before any engineering works commenced and showed amaximum settlement of 13 mm when the construction of podium, SOHO1 and SOHO2were completed. It is assumed that the maximum settlement would occur when the floors

are fully loaded. This scenario is not easily simulated and monitoring over a period of

time during heavy human traffic and having all outlets fully utilized is necessary to obtain

the maximum settlement.

For the lateral displacement, the ATMS for the north-bound tunnel and the south-bound

tunnel hardlydid not register any movement towards the excavation in the tower block.

Table 4: Comparison of predicated settlement against observed settlement.

As of 15 Oct 06 Predicted Observed LTA

Settlement at fullsuperimposed dead

load (80% of full load)

14 mm13 mm

(from ATMS)

16 mm(80% of max

permissible)

Settlement at fullsuperimposed dead

load + live load17.5 mm not available

20 mmmax permissible

The table 4 shows the comparison between the predicted and observed settlement of the

station structure when loaded to about 80% of the full design load and the permitted

settlement by LTA.


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The recorded settlement is based on the south-bound tunnel crown level and it also shows

the differential settlement at 5 m intervals. The measured settlement of 13 mm comparesvery favourably with the predicted settlement of 14 mm.

The construction of the basement above the station roof commenced in November 2004

and the roof of SOHO1 and SOHO2 was completed on June 2006 and September 2006

respectively.

9.0 Interfacing Details Vent Shafts & Entrances9.1 Hacking of Column Stumps by Darda Splitter Method

During the construction of the MRT station, starter bars extending from the roof were

covered by concrete stumps and were to be exposed for lapping with the reinforcement

for the development columns. There were 55 column stumps on the station roof that

requires breaking with minimal vibration and disturbance to the users and operators in theMRT station. The contractor proposed the Darda method to be used during the daytime

and pneumatic breakers is unbearable during the train non-operating hours.

Figure 16: Column stumps above the station roof.

Darda splitting cylinders are handheld demolition devices, which controllably split

material with the use of hydraulic pressure. Larger conventional demolition devices are

ruled-out because they produce dust, flying debris, vibrations, noise and possibly exhaust

fumes.

Figure 17: Silent demolition of column stump by Darda hydraulic splitter.


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9.2 North and South Vent shafts

The north vent shaft is located near to the river promenade. URA land sale requirementdoes not permit the ventilation discharge at the covered walkway level. Due to the space

constraints and the ventilation requirements, a 4 storey high vent shaft was integrated tothe faade of the podium facing the Singapore River. The engineering solution was to

design an enclosure that would enable an interfacing without disrupting the station andtunnel operation.

The south vent shaft facing Eu Tong Sen Street and Merchant Court Swissotel had a

similar method for the interfacing. The new ventilation intake and discharge are located

on the 1st

storey with the openings away from the covered walkway and the 2nd

storey

levels.

9.3 Precast System over MRT Entrances

The use of precast beams and prestressed hollow core slabs over the MRT entrances were

introduced for the construction of 2nd

storey floors. The access to the station is not to be

disrupted during the train operation hours. As a mandatory precautionary measure, the

launching of the precast beams and the hollow core slabs were carried out during non-

operating hours of the NEL.

9.4 Cutting of Diaphragm Wall Openings by Wire Saw System

There are three large openings for the interfacing from the development area to the

station concourse and the staircase area as well as eight other large openings in the

diaphragm wall above the station roof level.

The Hilti DS-WS15 wire saw system was used to cut the 1200 mm diaphragm wall at the

knock-out panels for the openings. Due to its small size, cutting even the most difficultsituation such as the less accessible location at the lower end of the escalator to the

concourse was completed efficiently. Smooth surface was achieved quietly without

distressful vibration.

Figure 18: Diaphragm wall opening by wire saw method.


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10.0 Effects of Excavation on Station

The design of the station permitted various sequences of engineering works such as theexcavation for the basement construction up to a depth of 14 m without a bracing system.

For the construction of the development, the substructure and superstructure commenced

simultaneously with the installation of the diaphragm wall and piling works.

The excavation for the basement beneath the tower block commenced after the

completion of the basement slab and the podium above the station roof. The effect of the

excavation on the station structure was negligibleminimal due to the stiffness of the

station box. Also, there was no trend to show the effects of the excavation on the

settlement of the station box while it was being loaded by the superstructure works of

SOHO1 and SOHO2.

11.0 Monitoring Works during Construction

The automatic tunnel monitoring system (ATMS) for north and south bound tunnel gives

accurate measurement for the settlement and lateral movement due to vertical loading and

excavation works. Prisms were installed at the crown, side wall and the rack levels in the

station and tunnel. The readings are taken every eight hours and sent by email to the

various parties including LTA. When the alert level or work suspension level is reached,

the system would send the data by sms to the key persons.

Ground instrumentation monitoring was implemented before the piling and diaphragm

wall works. The instrumentation includes water standpipes, piezometers, vibrationtransducers, inclinometers, ground settlement markers, building settlement markers amd

tiltmeters. Building settlement monitoring was carried out on a regular frequency and

compared with the readings from ATMS.

During the excavation stage of the construction, monitoring meeting was convened

weekly. Contingency measures and action plans together with the criteria levels wereestablished before the engineering works began.

12.0 Engineering Issues and Solutions

The engineering issues for the construction of the CENTRAL over the Clarke Quay MRT

Station are:

i. To ensure that there are no adverse effects on the station structure due tounbalanced loading on the station during excavation works.

ii. To minimize vibration for the hacking of column stumps, diaphragm wall openings.iii. To ensure that there is no disruption to users and operator during the interfacing

works.

iv. To ensure that fire engine access to station is not affected.


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v. To keep dust free operation near the vent shafts and MRT entrances.vi. In view of the engineering, environmental and safety issues, solutions specific to

the situation have been implemented and in compliance with the regulatory

authorities and the operators of the NEL. The assistance by the LTA authorities andSBST (operator) in guiding the implementation was also critical to resolving these

issues.

13.0 Conclusions.For seamless integration of transit oriented developments, it is essential to make

provisions, for such developments, during initial planning and design stages of a station.

Provisions for URAs future development requirements were integrated in the design of

CQY MRT Station. Provision of such requirements presented challenges in planning and

design of the station. Many of these challenges resulted from constrained geometry of the

station box, its orientation being different from that of future development anddevelopment of suitable interfaces between the column grids of the two developments.

The difficulties of station design were overcome by incorporating suitable engineeringprovisions in the station design. It also included suitable considerations for construction

schemes and sequence for the future development. Such considerations were documented

and conveyed to the developer of future development, through a Development Interface

Report (DIR) for his incorporation during design and construction of future development.

For the design of CENTRAL, station design / deformation limitations were carefully

studied and appropriate design parameters were suitably implemented in the analysis ofthe development. Deformation behavior of the station was monitored through an

appropriate monitoring system. The resulting station deformations / settlements arewithin acceptable limits.

References

1) Integrated Planning and Development of Clarke Quay Station [Ong Gin Lip, PaulBroom]

2) Clarke Quay MRT Station: Civil Engineering Challenges [G Pughazendhi, KMahmood, S Mahatma, P Broom and P Sebastian]

cqy stn paper k27oct06

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