ring beam shoring system 25oct06

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INNOVATIVE RING BEAM TEMPORARY BRACING SYSTEM

Soh Seng Siong, Chen Yue Feng, Tan Rwe Yun

KTP Consultants Pte Ltd

Abstract

The CENTRAL is a landmark commercial development partially supported by the NEL

Clarke Quay MRT Station. The 1.4 hectare site is situated over the north east line tunnels

and is bounded by the Singapore River on one side.

Adjacent to the Clarke Quay MRT Station is a triangular footprint of the 25 storey office

tower. Deep excavation was carried out for the basement construction and a link

connection to the MRT concourse.

This paper presents the innovative bracing system using 2 layers of reinforced concrete

ring beams for the excavation and the implementation of the internet online realtime

monitoring system.

Introduction

The Urban Redevelopment Authority (URA) announced on 29 September 1999 the

launch of sale of the Land Parcel at Clarke Quay MRT Station. The 1.4 ha site was sold

as a "white" site to allow the successful developer maximum flexibility to decide on the

best mix of commercial, residential and hotel uses with approximately 77,000 sqm of

gross floor space.

The land parcel above the Clarke Quay MRT Station is a prime site with a long frontage

to Eu Tong Sen Street. It is situated at the section of the Singapore River where it goes

into a wide bend, thus providing the development on the land parcel panoramic views of

the river, Fort Canning Hill and the Singapore city skyline. It is one of the last few

undeveloped sites at Clarke Quay. When completed, the development on the land parcel

will be fully integrated with the future Clarke Quay MRT Station. Three of the four

entrance points to the station will be located within the boundary of the land parcel. The

new landmark development will thus enjoy a good flow of MRT commuters.

Under the construction contract for the station, LTA has to exercise the option to back-fill

the excavated area by March 2000. Hence, by deciding on the award of the land parcel

before March 2000, the excavated area may not need to be back-filled. The site could

then be handed over to the successful tenderer in an excavated condition. This will save

the government the cost of backfilling the excavated area and it will save the successful

tenderer the cost of re-excavating the site subsequently for construction purposes.

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The Development

CENTRAL is a landmark commercial development with a podium over the entire site, a

12-storey SOHO, a 8-storey SOHO and a 25-storey tower. The tower block is situated on

a triangular footprint north of the Clarke Quay MRT Station and is bounded by the

Singapore River on one side.

Figure 1. View of Central with Eu Tong Sen Street on the south and the Singapore River

on the north.

Site Geology

The site is overlain with fill, peaty clay and marine clay with varying depth from 6 m to

16 m. Beneath this layer, are the residual soil (S4) and sedimentary layers of the Jurong

Formation (S2/S1). During the installation of the diaphragm walls, some sandstone

boulders were encountered.

History of Site

The site was occupied by shop houses and godowns before the pre-war days. It is likely

that footing and bakau piles were used to support the pre-war buildings. Figure 2 shows

the Coleman Bridge which is adjacent to the site and the busy Singapore River.

Figure 2. Coleman Bridge.

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Figure 3. View of Clarke Quay in the 1970s.

During the 1970s, public housing with shops on the lower floors was developed by HDB.

Records showed that steel H-piles and bored piles were used as foundation system.

During the diaphragm wall and piling works, it was necessary to extract the steel H-piles

that were in the way of the diaphragm wall.

Figure 4. View of Clark Quay from across the Singapore River.

In the mid 1990s, river walls were re-constructed for the stretch along the Clarke Quay

area. Contiguous bored pile system with soil improvement was used for supporting the

river walls.

Site Constraints

When the site was handed over to the developer, the excavated area above the MRT

station was left in an open state as shown in figure 5. There were inclined struts, working

platforms, a temporary road crossing along part of Tew Chew Street, temporary columns

supporting the north vent shafts and part of entrance 5. Cooling towers for the station

were located in the tower area affecting the diaphragm wall and piling works.

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Figure 5. Condition of site before construction commenced.

Between the site and the Singapore River is 15 m wide promenade which also serves as a

fire engine access for the MRT station. The riverwall is also a critical retaining structure

and form one of the many constraints on the method of basement construction.

The most critical consideration affecting the engineering works is the Clarke Quay MRT

station. The NEL started operation in 2003 while the construction of the development

commenced in mid-2004. The construction works should not cause any adverse effect on

to the station structure and also should not cause any disruption to the operation and any

inconvenience to its users.

Design and Construction Considerations

Diaphragm Wall

The diaphragm wall of 1000 mm thickness was designed for the retaining system for the

construction of the basement in the tower block footprint. This area is north of the station

and is bounded by the Singapore River on one side and Tew Chew Street on the western

side. The Merchant Court Swissotel is located on the other side of Tew Chew Street.

Although there is only one deep basement, the final excavation depth to the base of the

pilecaps is about 10.0 m below ground level. There is also an escalator link to the

concourse level that requires localized excavation to a depth of 16.0 m. The diaphragm

wall for this link is 600 mm thick.

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Figure 6. Diaphragm wall layout plan.

Finite Element Analysis

A finite element analysis was carried out to determine the effects of the excavation on

the MRT station structure as well as the effects on the river wall. The model simulates the

station structure, ring beams, diaphragm walls and river walls.

The expected lateral displacement of the Clarke Quay MRT station towards the

excavation was 14.4 mm. This prediction is based on lower soil parameters and without

the effect of the presence of existing barrettes, old bored piles and the newly installed

bored piles.

Figure 7. Analysis showing effects of excavation.

The predicted maximum lateral movement of the new diaphragm wall is 74.9mm while

the maximum settlement behind the diaphragm wall is 51mm. These values were adopted

in the criteria levels for the emergency and action plans.

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Modelling, Analysis and Design of Ring Beam System

The bracing system consisted of two layers of G40 reinforced concrete ring beam of

1000mm width by 500mm depth. The 50 m diameter ring beams and the slabs were

modeled with supports on the diaphragm wall and temporary king post embedded in the

bored piles. The slab on the 1st layer was designed to support excavator loads. Figure 8

shows the structural modeling of the ring beam. Openings in the slab were provided to

facilitate the core wall and column construction as well as access for excavation.

Figure 8. Structural modeling of the ring beam.

Figure 9 shows the stresses in the ring beam. It was observed from the graphical output

that the stresses developed are not uniform. The compressive stresses in the ring beams

tend to be larger near to the diaphragm support.

Figure 9. Stresses in ring beam.

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This ring beam provided a rigid bracing for the temporary retaining structure and against

the diaphragm wall along the side of Clark Quay Station. This effectively minimized the

movement of the wall and also station structure. It also allow for relatively unrestricted

access for excavation and construction activities.

Instrumentation

A comprehensive ground monitoring system consisting of 7 clusters of instrumentation

was implemented since September 2004 before the commencement of diaphragm wall

and piling works. Each cluster has an inclinometer, a piezometer and water standpipe. In

addition, there are extensive ground settlement markers as well as building settlement

markers. Inclinometers into the diaphragm wall were also installed. Tiltmeters and

vibration transducers were also installed at critical locations. Figure.10 shows the

instrumentation layout plan.

Figure 10. Instrumentation plan.

Automatic Tunnel Monitoring System

In addition to the ground instrumentation, an automatic tunnel monitoring system was

implemented in the station and tunnels. The system measures the movement in the

vertical and horizontal direction at the crown, sides and the track levels. The readings are

sent to various parties every 8 hours by email and by SMS alarm message when critical

levels are reached. Figure 11 shows the location of the prisms in the north and south

bound tunnels.

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Figure 11. Prism locations in station and tunnels.

Strain Gauges in Ring Beam

For the 2 layers of ring beam, there were 6 clusters of 4 strain gauges in each ring beam.

Altogether, there are 48 strain gauges in the 2 ring beams. At each location, there are 4

numbers of gauges placed parallel to the reinforcement bar near to each corner.

Figure 12.

The system used for the monitoring the strain is through a web base access to database

with password protection. It can be accessed when the users are online in the internet at

any time of the day. The system is designed with seamless integration and is very easy to

use while providing highly efficient monitoring.

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SysEng Real Time Monitoring automatically measures the vibrating wire strain gages

(VW sensors) in the ring beams every 10 minutes and then send data via wireless GPRS

to a central server. The system automatically processed the data into Engineering

information. SMS alerts will be automatically send out if their limits are exceeded. The

entire system is fully automatic and man-less. This makes the system more robust and

reliable as it removes the unreliable manual chain out of the system when it has been

operating continuously 24 hours a day and 7 days a week.

Construction Sequence

Figure 13 shows the construction sequence from the simultaneous construction of the

basement above the station and the foundation works to the completion of the 1st storey

plan over the tower area.

The 1st stage of the sequence was the construction of the basement floor above the MRT

station. This was followed by the 1st storey floor to form a stiffer structural form together

with the station box. The superstructure over the station was constructed next. While the

works over the station roof was ongoing, diaphragm wall and bored piling works on the

25-storey tower were carried out at the same time. The excavation for the basement

started when the podium was completed.

Figure 13. Construction Sequence.

Figure 14 shows the section across the excavation from the station to the Singapore

River. There is a long pile cap next to the station that transferred the columns of the 25-

storey tower to the series of barrettes. The depth of the pilecap is 4 m deep.

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Figure 14. Section across the excavation.

Timeline for Construction of Ring Beams

Figure 15. Construction period for ring beams and excavation.

Figure 15 shows the as-built program for the two ring beams. It is of note that the

construction of the 50 m diameter ring beams was completed in 2 months. This period

includes the time taken for the hacking of more than 20 numbers of barrettes that had

been overcast way above the cut-off level.

Construction of The Ring Beams and Excavation.

Figures 16 to 20 show the sequence on the construction of the 2 layers of ring beams and

the excavation for the construction of the basement. The ring beam system enabled the

bulk of the excavation to be carried out without the obstruction of steel struts in a

conventional bracing system.

The peaty clay layer and the marine clay layer beneath the top fill layer were easily

excavated. During the excavation, visual checks were carried out on the ring beam in

addition to the real-time monitoring.

Jun-05

27 4 11 18 25 1 8 15 22 29 5 12

Completed 2 July 2005

Temp slab started 2 July 2005

Completed 29 July 2005

Excavation started 5 Aug 2005

Completed 3 Sept 2005

Started 5 Sept 2005

Sep-05

1st Layer Excavation,

Capping Beam &

Temp. Slab

Bored Pile &

Diaphragm Wall

2nd Layer Excavation

& Temporary Slab

Final Layer

Excavation

Aug-05Activities

Jul-05

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Figure 16. Completion of 1st layer of ring beam.

Figure 17. Excavation as of 25 Aug 2005.

Figure 18. Ring beam system facilitate ease of excavation.

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Figure 19. Excavation to base of pilecaps.

Figure 20. Pilecap cast on 28 Oct 2005.

Field Observations

On the measured readings of the strain gages, it was observed that the changes in strain

tend to peak in the faster rate in the later part of the afternoon. This was caused by the

ambient temperature changes. The falling rate was slower and occured from the early

evening to the morning.

Figure 21 shows the changes in the strain readings for the various gages in both the 1st

layer and 2nd layer ring beams. The changes in strain could also be observed at the

various stages of excavation. It was of note that the higher changes of strain occurred

where the ring beams connect to the diaphragm wall. The monitoring also showed the

non-uniform stresses throughout the length of the ring beams.

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03/12 to 09/12/2005

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Time

Ch

an

ge o

f S

train

SG-11A (UE) SG-11B (UE) SG-11C (UE) SG-11D (UE) SG-12A (UE) SG-12B (UE) SG-12C (UE) SG-12D (UE)



Figure 21. Changes in strain over time.

Instrumentation Readings

The observed reading from the inclinometers I-3 shows a maximum lateral movement of

25 mm towards the excavated face. I-3 is located at the promenade area and is about 1.5

m away from the diaphragm wall. I-3 was installed before the installation of the

diaphragm wall.

For the inclinometer IW-4 that is installed in the diaphragm wall, the maximum lateral

movement is 16 mm at a depth of 10.5 m. IW-4 is located at the most critical location and

would monitor the worst possible movement. The observed movement is much less than

the predicted value of 75 mm,

The inclinometer I-6 located at Eu Tong Sen Street at about 1 m from the Clarke Quay

MRT station indicated the lateral movement of 9.34 mm at the top and 3 mm at a dept of

9 m. The top lateral movement was caused by the localized excavation for the

construction of capping beam on the diaphragm wall. The movement of 3 mm at the

depth of 9 m gives a better indication of the effect of the basement construction on the

station structure. Figure 22 shows the lateral displacement from I-3, I-6 and IW-4.

Several reasons for the much reduced observed movement could be due to the sequence

of the construction, the arching action, reduced surcharge at the promenade and the

presence of existing piles and new piles. The sequence of construction was changed

slightly by casting the temporary corner slabs to introduce diagonal strutting and the

localized excavation for the casting of the ring beams.

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Figure 22. Inclinometer readings from I-3, I-6 and IW-4.

With this relatively small deflection of the diaphragm wall, there is consequently very

little displacement in the lateral direction of the station box.

Conclusions

This project has successfully demonstrated the advantages of constructing the reinforced

concrete ring beam system as temporary bracing for the basement excavation. The online

real-time monitoring of the strain gauges with its seamless integration is very highly

efficient and provides much useful information on the behaviour of the ring beam system.

The successful completion of such a complex project depends on the close cooperation of

all involved. In this regard, the writer appreciated the permission extended by the

developer, Riverhub Pte Ltd, the cooperation and construction work by Hexacon

Construction Pte Ltd together with the efforts of the LTA and SBST in the expeditious

processing of the submissions to make the introduction of the ring beam system possible.

ring beam shoring system 25oct06

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