A review of the performance of the tunnelling for Singapore's circle line project
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World Tunnel Congress 2008 - Underground Facilities for Better Environment and Safety - India
A review of the performance of the tunnelling for Singapore’s circle line project
N.H Osborne, C. Knight Hassell, L.C. Tan & R. Wong
Land Transport Authority, Singapore
SYNOPSIS: The Circle Line Project (CCLP) is a fully underground orbital mass rapid transit line currently
under construction in Singapore, scheduled to open in 2010. It totals 33 kilometres in length and includes 29
underground stations, running under some of the busiest urban corridors within the city. The majority of the
line, 28.8 kilometres, was constructed using Tunnel Boring machines (TBMs), either earth pressure balance
or slurry. The project presented numerous challenges, not least in the great variability of the ground, with
tunnelling carried out in all the geological strata present in Singapore, ranging from soft Marine Clays,
through tropically weathered rocks, to fresh granite.
Tunnelling is now substantially complete, and the merits of the differing approaches to meeting the
challenges posed by the ground conditions can be reviewed. The risk management of the tunnelling process
and settlement control were critical elements within the project; these are studied in detail with key successes
highlighted and areas for future improvement suggested.
1. INTRODUCTION acceptable levels. One of the challenges is actually
identifying that risk. Despite the fact that Singapore
Singapore’s latest addition to its mass rapid is an island of 700 km2, it contains a wide range of
transport (MRT) system starts in the heart of the variable and rapidly changing geology, making the
urban city, under its’ famous Orchard Road, with an ground conditions difficult to predict. It has
interchange station at Dhoby Ghaut allowing tropically weathered sedimentary, low grade
transfer to the existing North East and the North metamorphic and igneous deposits incised by
South lines (Figure 1). The line then commences its channels of very soft marine clay and fluvial
orbital route to the east passing a through a further deposits. This geology, combined with the
28 stations, including 4 more interchanges, before urbanisation of the island, further highlights the
reaching its final interchange station at Harbour importance of settlement control to the project.
Front in the island’s south. This 33 kilometre route Tunnel settlement is a major, if not the most
is split into 9 contracts, with 87% of the alignment important, concern amongst the international
built by TBM. tunnelling community and this is very true in
Once operational, it will be of major benefit to Singapore. Over the last ten years there have been
commuters, significantly enhancing integration with numerous tunnelling projects in Singapore, with the
the existing lines and appreciably reducing 20km North East Line Project (NELP) for the MRT
commute time by enabling commuters to bypass and the 48km Deep Tunnel Sewerage System
city centre interchanges. It brings the MRT system (DTSS) dominating the scene. These projects and
to a large number of suburban hubs, resulting in a their problems have been well documented, for
substantial increase in MRT connectivity. NELP by, Shirlaw et al. 2003 (1), Osborne et al.
To achieve this, a large number of engineering 2002 (2) and for DTSS, Marshall et al 2007 (3). The
and logistical challenges had to be overcome, with papers reinforce the fact that the ground conditions
one of the greatest being controlling the settlement are very complex and controlling them particularly
induced during the driving of the tunnels and taxing. On both projects, a number of localised
limiting its impact upon the public. In the planning sinkholes developed and these were attributed to
stages optimum consideration was given to either inappropriate face pressure, problems during
minimising the tunnelling risk, by removing as launching/docking, or machine problems, however
much as feasibly possible. However, on a project of common to all was the fact the ground was
such a scale not all risk can be avoided, therefore problematic.
the remaining risk must be identified and reduced to
1497
Figure 1. CCLP route map
In addition to the ground causing settlement Marine Clay. It also contains interstitial fluvial
problems, there are issues at the other end of the sands and clays and is encountered within buried
tunnelling spectrum with the very strong and river channels, generally to the south and southeast
abrasive conditions. Again the papers highlight that of the island. The Clay can be divided into two
both tunnel advance rates and more importantly units, the Upper Marine Clay and Lower Marine
machine wear are particular issues of concern. Clay. These are generally separated by peats, fluvial
sands and clay, normally a few metres thick, but
occasionally demonstrating a more complex
2. GROUND CONDITIONS AND
interbedded relationship and over much greater
TUNNELLING CHARACTERISTICS
thicknesses. The formation is found at the surface
Due to the diverse depositional environments and and can extend to 40m below ground. It is of low
subsequent geological events the ground conditions strength, with an undrained shear strength typically
in Singapore are very complex and varied, of 15 kPa close to the surface, increasing slowly
presenting many challenges in tunnelling through with depth to maximum of 60 KPa. However, in the
them. For CCLP all five of the geological units that more recently reclaimed areas the clay is still
exist in Singapore were encountered, to varying undergoing consolidation and is significantly
degrees. (Figure 2). weaker, in the order of 15KPa at an equivalent
depth. From a tunnelling perspective, although of
low strength, it is a good tunnelling medium that
2.1 Kallang formation can be controlled by appropriate face pressure. It is
The Kallang Formation is a Holocene and late recommended that a range of 0.9-1.2 times the
Pleistocene deposit which is dominated by soft overburden pressure is used, Shirlaw et al. (1). The
1498
control of settlement becomes a major issue when extremely abrasive range, Peart et al. (5) This has
mixed face conditions are encountered, with resulted in large wear problems in previous projects,
Kallang in the crown and rock in the invert, most notably the DTSS.
balancing and controlling the earth pressure
becomes very complex particularly if fluvial sand is
2.3 Jurong formation
present.
The Jurong Formation makes up much of western
2.2 Old alluvium and south western Singapore, consisting of late
Jurassic and early Triassic sedimentary strata
The Old Alluvium, predominantly to the east and comprising mainly interbedded mudtsones,
northwest of Singapore, is an early Pleistocene siltstones and sandstones. It is a very variable
deposit compromising alluvial clayey sands, sandy stratum with large strength ranges, from 2 MPa, or
clays and occasional gravels, normally well less, to those exceeding 150MPa, and a range of
cemented. It generally behaves as a weak rock, but permeability from 1*10-5 m/s to 1*10-7 m/s,
with a tropically weathered zone extending typically Osborne et al (2). This variability is a consequence
to 8m. It is a good medium for tunnelling, providing of its deposition and then subsequent tilting, to
no stability issues for machine tunnelling, but can create thin, near vertical beds, which have
be more problematic for hand works if the undergone subsequent folding and faulting. Later,
weathered uncemented material is encountered, deep tropical weathering preferentially along the
Knight Hassel et al (4). Its abrasiveness is bedding planes has resulted in fractured rock to
frequently underestimated, with a Cerchar Arasivity considerable depth. To add further variability, some
Index (CAI) in the range of 0.45 – 2.25 for intact stratum has undergone low grade metamorphism.
rock. However tests done on broken down samples, Consequently stability during tunnelling is a major
mirroring the action of a TBM, give much higher issue, the weaker ground, if not fully supported, has
results with values up 5.0, putting it in the
Figure 2. Geological map of Singapore
1499
the potential to ravel along the weathering or minor 3. TUNNELLING APPROACH
fault planes, rapidly progressing to the surface. In
the stronger ground, problems with progress and For previous tunnelling projects in Singapore,
wear exist, with the quartzite and sandstones falling notably NELP and DTSS, the EPBM has been used
in the extremely abrasive category. This is further to drive the tunnels. As the tunnelling industry is
compounded by the steeply dipping beds and by the continuing to advance, and technological
fact that the strata can change very rapidly from improvements are made, more options are available
competent ground to very weak, with mixed face to the tunneller. As a result of a number of sink
conditions not uncommon. holes that occurred during the tunnelling on NELP,
due to a combination of hard and soft ground being
encountered, slurry TBMs were seen as a
2.4 Fort canning boulder bed potentially better tool to overcome this problem.
This is evident in the different tunnelling
This is a Pleistocence colluvial deposit comprising
approaches taken on CCLP, table 1. Of the
strong to very strong sandstone and quartzite
contracts, only one opted to use slurry TBM
boulders within a very stiff clay matrix. It is derived
exclusively, with two contracts using both slurry
from the Jurong Formation and is limited to areas
and EPBM, with the choice of slurry coinciding
within and around the Central Business District in
with the Bukit Timah Granite. Evidently one of the
Singapore. It does not pose settlement problems to
driving factors in this choice was to deal with the
tunnelling, but with the boulders having a diameter
mixed face conditions with the ability to increase
up to 7m and making up 25-30% of the strata, it
the face pressure instantaneously and control the
poses significant problems in terms of advance and
mixed face. Choosing a slurry machine does
machine wear. For CCLP this ground was only
introduce an additional potential complication to the
encountered on one contract and for a limited
tunnelling process, that of the slurry composition
length.
and the plant required to produce and deliver the
slurry to the tunnel face. To fully utilise the benefits
2.5 Bukit Timah granite
of slurry, the plant must be set up correctly to
The oldest formation in Singapore is the Bukit enable the appropriate slurry to be produced
Timah Granite, an early to middle Triassic igneous effectively and efficiently. Otherwise this process
strata found in central and northern areas of may introduce more problems than it actually
Singapore. It comprises a number of acidic rocks, solves.
predominantly granite, microgranite and
granodiorite, and is frequently encountered as 4. TUNNELLING PERFORMANCE
buried ridges beneath the Marine Clay and Old
Alluvium. It has a very wide range of strength, from With the different approaches taken to dealing with
less than 1MPa to in excess of 300 MPa, with the the varied ground conditions, a number of recurring
fresh granite causing major wear as it falls into the issues with regard to the tunnelling performance
extremely abrasive range. This strength range is a become evident. These can be summarised into
consequence of the severe tropical weathering, three key and crucial areas. Firstly, the detrimental
creating six weathering grades, from soil to fresh role of the abrasive ground conditions and the wear
rock, which can occur within a very short distance to the TBMs, this not only resulted in numerous,
of each other. Consequently the problem of difficult, and time consuming cutter changes, but
supporting a mixed face exists, particularly in the also had an impact on the machines’ ability to
intermediate weathering grades where intact rock is control settlement. Secondly, the interaction with
encountered in the invert and corestones within a buildings which are known risk areas within any
loose sandy or clayey matrix in the crown. This project; across CCLP these risks were identified and
situation is further exacerbated by the high mitigated using a number of different solutions. The
permeabilities in the intermediate weathering third area is that of settlement control itself. A
grades, increasing the instability by forcing the number of localised sinkholes and slurry/foam loss
ground to ‘flow’ into the TBM. to the surface occurred across the project, and it is
useful to analyse these to determine their causes in
attempt to prevent future occurrence.
1500
4.1 Wear Granite, Jurong and Old Alluvium are all very
abrasive materials as indicated by cerchar testing.
Much of the ground in Singapore is very hard and The culprit being the quartz, which resists the
abrasive, consequently tunnel advance rates and chemical weathering process to remain intact and
wear are both significant problems. It is evident has been altered to include overgrowths making it
from the comparison of estimated and actual more angular and therefore abrasive. As the
advance rates, Table 1, that frequently planned rates machine experiences wear its ability to cut the rock
could not be achieved, this is predominantly due to is reduced and it advances more slowly. To further
two factors. Firstly the strength of both the Jurong compound this, wear leads to more tool changes,
and the Granite which can easily be under further slowing progress. For one 1.25 km drive on
estimated, as although the fresh rock may only Contract 853, 5 tool changes were planned, but 10
make up a small percentage of the drive, the time required, 2 within 25m of each other, with each
taken to tunnel through this rock, in excess of change lasting up to two weeks.
250MPa, is proportionally much greater. Secondly
and more importantly is the wear to the TBMs. The
Table 1. TBM usage and performance on CCLP
Circle Line Stage 1 Circle Line Stage 2 Circle Line Stage 3
Contract
825 828 823 822 852 853
Contractor WSN JV NLC JV NLC EN JV WH/ SH/ APJV Taisei
Type EPBM EPBMEPBM EPBM EPBM EPBM Slurry
TBM Herrenknecht Hitachi Zosen
Hitachi Mitsubishi Herrenknecht Kawasaki
Zosen Kawasaki
CCL Drive PRM-DBG BLV- BLV- MBT- DKT- TSG- PYL- TSG- LRC- SER- BSH- MRM-
NCH MBT DKT PYL BLY MPS MPS SER BLY LRC BSH
Max Drive 1695.4 (IB) 1073 180.6 539 1005.2 704 664 795 700 1200 1513 1237
length (m) 1568.4 (OB) (IB) (IB/OB) (IB/OB) (IB/OB) (IB) (OB)
TBMs 2 2 4 2 2 2 2
proposed ( 2 from
(No.) C828)
Geology Old Kallang Kallang Kallang formation, Old Alluvium Granite Varying
Alluvium, Formation Formation Old Alluvium & & Graniite & Old grades of
Jurong Bukit Timah Granite Alluvium weathered
& FCBB Granite
TBM
Outside 6580 6630 6630 6600 6630 6680 6720
dia.(mm)
TBM Length / 7.4 / 47.6 8.6 / 74.6 8.6 / 65.4 8.8 / 88 7.6 / 70.4 9.9 / 64.1 9.9 / 74.1
Back-Up (m)
Planned Daily 4.9 7 4 6 6 6.5 7.5 7.8 6.3 6.3 5.6 4.5
Production (m)
Achieved Ave 2.8 (IB) 5.7 3.7 4.5 6.7 5.5 7.3 6.9 4.9 3.2 4.95 3.6 (OB)
Daily 2.7 (OB) 5.2 2.9 5.5 6.8 5.2 6.7 9.0 4.7 3.2 5.15 3.2 (IB)
Production (m)
Actual/Planned 56% 77% 82.5% 83% 100% 82% 93% 100% 76% 51% 89% 75%
Percentage
1501
Table 1 (contd.)
Circle Line Stage 4 Circle Line Stage 5
Contract
854 855 856
Contractor Taisei Sembawang
Type Slurry Shield Mixshield EPBM EPBM
TBM Kawasaki Herrenknecht Herrenknecht Herrenknecht
Manufacturer
CCL Drive BKB-TSN, TSN-MRM ONH-BNV-HLV- ONH-KRG-WCT PCS-PPJ, PJ-WCT
BKB-BTN, BTN-FRR FRR PCS-TLB, TLB HXO
HXO-HBF, HBF-
EVS
Max Drive 3170 5800 3000 1100
length (m)
TBMS (No.) 4 2 2 3
Geology Mainly Bukit Timah Granite ONH-HLV: Predominantly Jurong formation with
(GI – GVI) overlain by fill material Predominantly Jurong Formation overlaying F2, Marine
Jurong Formation Clay, Estuarine and
HLV-FRR: Mainly fill material
Bukit Timah Granite
Formation
Outside dia. – 6720 6630 6630 6600
cut (mm)
TBM Length / 10.1/ 110 9.6 / 95 8.7 / 74 7.9 / 70
Back-Up Length
(m)
Planned Daily 5 7 7 6
Production (m)
Achieved Ave 3.7 4.3 3.9 7
Daily Production
(m)
Actual/Planned 74% 61% 55% 116%
Percentage
The wear problem is further compounded by to cutter discs occurs when tunnelling through
other factors. Firstly the fact that to control the mixed ground. This is caused when the cutter discs
ground conditions the tunnels often needed to be move from the soft soil in the face to the very hard
driven with pressures in excess of 2 bar, and this granite resulting in the discs cracking or in some
force accelerates the wear of the machine and its cases shattering.
tools. The mixed ground also increases wear, The impact of wear on the tunnelling project
particularly the combination of strong ground and should not be underestimated. In addition to the
clay. The clay combines with the quartz minerals to above, tool change is a high risk operation. Entering
form a grinding paste which tends to clog up the the cutterhead either in free or compressed air
machine face and is difficult to remove through the contributed to several localised sinkholes on DTSS
chamber. This paste rapidly abrades components of and CCLP. Therefore, the less frequent the need to
the machine with the cutterhead and the discs most change tools the lower this risk. The use of
susceptible, the discs can easily get clogged appropriate conditioning agents, foams and slurry
preventing rotation and resulting in flats spots rheology can help to reduce this impact and avoid
forming. It is not only the cutterhead that wear, but the choice of the most appropriate agent is
experiences wear but all parts of machine that a difficult and complex science, particularly for the
encounter the spoil. Across CCLP, significant wear slurry machines. This choice is made even
of the removal pipes in slurry machines, the more difficult by the ever-changing ground
bulkhead and screw was reported. Further damage conditions.
15024.2 Settlement control – structures house. Precondition and damage assessments of the
building further highlighted its poor structural
Tunnelling within the urban environment inevitably
condition, consequently as an additional
leads to interfacing with existing structures, either
contingency, the house was propped to avoid any
tunnelling under or adjacent to them. Despite efforts
sudden collapse (Figure 3). During tunnelling face
during the design of the alignment to minimise this
pressures were maintained in the range of 0.8 -1.0
risk by avoiding existing structures not all can be
times overburden. To try to reduce the settlement as
avoided. The only benefit is that these risks are
much as possible, maximising the potential of the
obvious and known early within the project,
grouting was identified an additional key measure.
therefore they can be mitigated. Across CCLP a
Body grouting was introduced with a bentonite mix
number of different solutions were implemented to
injected through the shoulder of the shield to fill the
successfully tunnel past structures.
overcut void and prevent the ground moving onto
The alignment for the project in the early
the shield skin, before the primary grouting at the
planning phase is in the main designed on ridership,
tailskin could take over. Although volume takes
and the most direct route to link proposed stations.
were relatively small it was important not to over
It is however beneficial to revisit the alignment
pump resulting in a temporarily increase of the pore
particularly during the design phases and examine
pressure within the Marine Clay, leading to larger
whether further adjustments can be made to further
consolidation settlements later. As the tunnel
reduce the risk. This was the case for Contract 853,
advanced, primary grouting from four ports, with 2
a contract in the Bukit Timah Granite, with a known
pipes per port to ensure redundancy, took over.
number of mixed ground interfaces. Modifications,
Here a grout mix with a gel time of 7 seconds was
both horizontal and vertical were made, reducing
injected at 120% of theoretical volume. The
the number of residential properties to be tunnelled
combination of the correctly applied face pressure
under to 57 and taking the tunnel deeper, to 45m
and the grouting to arrest the movement of the
below ground level, within the limitations of
ground on to the TBM skin proved successful. The
gradient and station position, therefore reducing the
maximum settlement recorded on the house was
number of mixed ground interfaces, Nakano et al.
less than 10mm, with immediate volume loss at 1%,
2007 (6). In addition, the tunnels were separated by
reducing to 2% after six months due to the effects of
up to 60m, to limit the maximum settlement by only
consolidation. An identical approach was taken to
having to deal with one settlement trough. These
tunnelling on the adjacent Contract 828, with a
measures, combined with a strict excavation spoil
similar TBM and similar ground conditions, this
management system, resulted in settlements in the
again proved successful to limiting settlements on
order of 10mm, a volume loss of 0.5% and no
key structures, for more details, Osborne et al 2007
disruption to the property owners.
(7).
Full face Marine Clay, although originally
On Contract 852, by mitigating one risk,
considered one of the most challenging ground
another smaller and more manageable risk was
conditions in Singapore is now viewed as a
introduced. For a 245m stretch of the tunnel drive,
preferred ground condition for tunnelling, although
the distance between the extradoses of the tunnels
this does not hold true for cut and cover
was reduced to between 4.0 - 2.3m, to avoid
excavations. Due to its uniformity and the
tunnelling beneath adjacent piled structures. The
techniques available, settlement can be controlled
ground was granite with varying weathering grades
with appropriate tunnelling. Contract 823 presents a
from grade II to V, with much of the tunnel
good example, a tunnelling project entirely in the
alignment through the soil/rock interface,
Kallang Formation, with EPB the choice of
consequently the control of ground movement was
tunnelling machine. The twin bored tunnel
difficult and the potential for the second tunnel to
alignment was at 15m below ground and passed
adversely affect the first very high. The control of
within 7m of a two storey house built in 1940 on
the spoil and achieving an effective plug through
shallow foundations. This property was in a very
these interfaces was essential to controlling the
poor condition due to 60 years of consolidation
ground movement. A number of measures were put
settlement of the Marine Clay. The option to
in place to manage this situation. The choice of
demolish was not available, therefore tunnelling had
machine was a Herrenknecht EPB with a double
to be controlled to ensure minimal impact upon the
screw conveyer configuration to control ground and
1503water inflow during spoil removal. The addition of control the settlement as early as possible. The
the second screw increased the length by 50% result was that ground movements were limited to
providing a better pressure gradient along its length. 1% volume loss and the tunnel movement of the
This, combined with independent rotation speeds of adjacent tunnel was less than 5mm. The largest
the augers and effective use of soil conditioning, movement induced on the constructed tunnel lining
enabled the formation of a solid plug and controlled was as a result of a cutter tool intervention in free
the discharge of spoil through these difficult air 3 metres away from the lining, demonstrating
interfaces. As an additional contingency, passive great control over this risk.
support to the first tunnel was installed, combining One of the most challenging risks on the CCLP
with permanent glass fibre dowels through occurred on Contract 856. Tunnelling for a length
the lining into the soil pillar between the tunnels of 90m twin bore, 16m below two rows of occupied
(Figure 4). 40 year old shop houses, which were in a poor
Again grouting into the annulus between the condition and founded on shallow footings. The
TBM shield and the ground was carried out to
Figure 3. Building in poor state of repair to be tunnelled and machine
Figure 4. C852 Passive support with the tunnels & fibreglass dowels through the lining
1504ground consisted of mixed face of Kallang and increasing available options of the soil conditioning,
Jurong Formations, with a high proportion of the replacing the centre double cutters with ripper tools
Kallang comprising fluvial sand (Figure 5). The to reduce plugging and improvements to the
first tunnel passed to the side of the shop houses and primary grouting making it more efficient. A
although there were some relatively large geotechnical engineer was permanently present on
settlements these were limited to the road and had the TBM during excavation, monitoring the ground
minimal impact on the shop houses. The second conditions and checking excavated muck volumes.
tunnel was directly under the shop houses, The shop houses themselves were monitored every
consequently a much larger risk. Ground treatment 3 hours and a series of passive props were installed
to the fluvial sands was considered but not regarded as an additional contingency. The whole process
as a viable option as it had the potential to cause was overseen by a special task force who reviewed
large settlement during the drilling. A number of and approved the tunnelling parameters that were
other measures were employed to ensure that the presented in a tunnel look ahead report, prior to
shop houses were not impacted. The EPBM was commencement. The tunnelling was a success; the
enhanced to include a minimum face pressure settlements were controlled and although some
alarm, the capability to inject bentonite directly into minor cracking occurred to the shop houses this was
the face during stoppages, doubling the capacity and non structural.
Shop houses – 90m
Figure 5. Constantly changing ground beneath shop houses on Contract 856
1505Not every high risk solution required specific above the complex ground conditions, can be
measures to control the settlement. For Contract 825 identified as contributory causes for these events.
the ground compromised of three different Although preferable to a sinkhole, discharge of
formations over 500m, the Old Alluvium, the Fort conditioning agent to the surface is a problem as it
Canning Boulder Bed and the Jurong Formation. is a hazard to the public, particularly the road user.
Two major risks to tunnelling were identified; This happened on a number of occasions across
tunnelling through two piles for an underground car more than one contract. On each occasion one of
park link which could not be avoided and extended three factors was involved. With the slurry
below tunnel crown level; and tunnelling 4m below machines blockages occurred as the weathered
an existing live MRT tunnel. For both cases the Jurong broke down to a sticky clay, blocking the
ground condition was very good and this was suction entry gate area, whilst fresh slurry was
established and verified by extensive soil delivered to the excavation chamber. This resulted
investigation. For the piles, within the Old in pressure spikes occurring before the operator
Alluvium, the option was taken to cut the piles could react and forcing slurry to the surface,
using hand excavation from the machine face and Shirlaw et al (8). Further problems were
support them on specifically designed tunnel encountered when air leaked through the TBMs
segments as the tunnel continued its drive. This was submerged wall into the excavation chamber
achieved with no discernible settlement to the car reaching the air cushion in the plenum chamber,
park link. Tunnelling under the MRT tunnel was causing air spikes and slurry discharge at the
again carried out in very competent ground, Fort surface. On other occasions slurry managed to find
Canning Boulder Bed. As the tunnel approached a path to the surface through either through left in,
settlements were monitored both at the surface and or poorly backfiled, sheet piles, or poorly backfilled
the level of the MRT to verify that settlements were site investigation boreholes. The third factor can be
small. The twin tunnels passed beneath the MRT, attributed to the sensitivity of face pressure
with a high level of instrumentation, and a calculations. When tunnelling through particularly
maximum recorded settlement of 3mm. difficult ground there is little room for error in these
calculations and it is preferable to err on the side of
caution. Consequently a small variation in the input
4.3 Ground settlements
parameters, be it a localised reduction in pore water
The ground conditions in Singapore make the pressure or a reduction in ground level for a storm
control of settlement very challenging, in particular drain, can result in the face pressure calculation
the mixed face conditions which occur all too being too high and slurry or foam can force its way
frequently. They are very difficult to identify with to the surface.
traditional site investigation as the ground can Large settlements or sinkholes also occurred
change from very competent to mixed face over a across a number of the contracts, these were limited
meter. Even with boreholes spaced at 25m apart, to the road or side table, and on no occasion was
only 0.5% of the ground to be tunnelled through is any individual injured, nor property damaged. All
sampled, therefore it is not surprising that not every these incidents occurred in the known problematic
mixed ground interface is identified. Tunnelling ground conditions, either the mixed face conditions
close to or under structures is an obvious and in the granite, the weathered Jurong or mixed face
identifiable risk and measures can be put in place to of Jurong and Kallang Formations. It is interesting
mitigate this risk. However with much of the tunnel to note that this ground risk was not always fully
alignments actually avoiding structures and identified by the site investigation and its
following roads the likelihood of coming across subsequent interpretation. Consequently the wrong
difficult ground unexpectedly is high. Both NELP face pressure was applied. As the tunnel advances
and DTSS experienced this problem with sinkholes whilst excavating the harder ground in the invert the
appearing at the surface, 20 recorded for NELP, soft or loose ground in the crown is not properly
Shirlaw et al.(1) and more than 5 on DTSS, controlled. This results at worst large over
Marshall et al (3). CCLP experienced a similar excavation, or at best small but significant over
problem with sinkholes but at the other extreme an excavation per ring. On all projects spoil excavation
additional problem with slurry/foam discharge at control was being practised, but with the varying
the surface. A number of common factors, over and ground conditions this is not easy. If the wrong
1506ground is identified, the wrong bulking factor is ground treatment carried out to mitigate this risk
then applied; this can result in a difference between when it was warranted.
actual and theoretical volumes and any over Linked to the tool change problem is the
excavation is not identified. The difference between recurring theme of machine wear. This was a
Jurong and a fluvial sand can be in excess of 5m3 contributory factor in several sinkholes. As various
per ring, which quickly becomes significant over components of the machine wear, the ability to
several rings and can result in a large settlement. In control settlements is impacted in one of two ways.
the granite, there is an even larger range in dry soil Firstly advance slows taking a longer time to
volumes excavated per ring, these can vary by as excavate through a difficult ground interface and
much as 17m3 or 40% per advance, across the increasing the possibility of over excavation. Also,
varying weathering grades, Nakano et al (6). It is particular to EPBM, the ability to control the face
therefore very important that even if the changing can be compromised when components such as the
ground is not identified in the soil investigation, a screw are worn compromising the machines ability
constant review of the excavated material should be to exert the required face pressure.
undertaken and bulking factors revised by
experienced personnel on a very regular basis. In
5. CONCLUSION
the case of the slurry machines it would seem
sensible to introduce various points in the system As with all the biggest engineering challenges the
where the amount of material excavated can be first issue is to understand the ground and its
reconciled. For example the amount of material geotechnical model, as this allows the potential
excavated at the face needs to be monitored risks to be realised. The ground in Singapore is a
immediately at the TBM and then as it flows particular challenge to tunnelling with the mixed
through the slurry processing plant and finally if interfaces of the granite, Jurong or these grounds
practical the plant should treat each TBM separately combined with the Kallang, and the frequency with
so that the resulting muck from each TBM can be which they occur. The next issue is to identify
identified in the respective muck pit. where these interfaces exist, so that particular
There appear three other broad categories attention can be paid to dealing with them. Given
which contributed to more than one sinkhole. The the variability of the ground and the site
first category can be defined as man made investigation this is a very demanding task. It is
obstructions. On two occasions during tunnelling suggested that more attention needs to be paid to the
sheet piles, not shown on any drawings were site investigation in both the location of the
encountered, once directly in the face and once at boreholes and exploring alternative options,
crown level. This allowed a passage to the surface inclined boreholes, seismic studies, for example.
to develop quickly, pulling ground into the The interpretation of the site investigation should
excavation chamber during excavation, resulting in always be carried out by an experienced geologist.
localised settlement at the ground surface. A similar Once the risk has been identified there are
occurrence occurred on another occasion with a many tunnelling options available to mitigating it,
poorly backfilled borehole the culprit. this has been demonstrated on CCLP by the
As already identified tool changes are a high successful tunnelling in proximity to numerous
risk operation, these contributed to the sinkholes. structures. From the tunnelling experience on CCLP
Ideally changes should always be carried out in there are several areas that have resulted in
good ground, but with the combined problems of problems. In the variable ground the management of
identifying the good ground and the high rate of the spoil is important if over excavation occurs this
wear this is not always possible. Consequently on must be identified as early as possible to enable
one occasion during a long tool change small reaction and to prevent the development of any
volumes of material were lost into the chamber. As sinkhole. The importance of planning ahead and
the change lasted a period of weeks this volume having contingencies, such as ground treatment in
accumulated sufficiently that upon tunnel restart a place to minimise the impact of any over excavation
sinkhole developed. On the whole tool changes is also crucial. The ground is very abrasive and
were carried out successfully; they were carefully wear is a recurring problem. It slows progress,
planned with additional site investigation to identify impacts the ability of the machine to function to its
the risk and on a number of occasions specific full potential and results in the high risk operation
1507of tool change. Consequently the wear must be kept 2. Osborne N.H., Williams O.I. and Lim W.B. (2001)
to a minimum and the use of effective conditioning The Jurong Formation, a Variable Ground Condition
agents and slurry should be practised, even if this – its’ Tunnelling characteristics. Underground
Singapore 2001.
costs more initially, there should be long term
benefits. 3. Marshall R.H., Flanagan R.F., Singapore’s Deep
Tunnelling in Singapore is continually Tunnel Sewerage System – Experiences and
evolving and lessons are being learnt from past Challenges. RETC 2007.
projects. On the NELP 25% of all sinkholes 4. Knight Hassell C. K., Rosser H.B., Eng W. C.,
occurred during launching or docking, for CCLP Difficult ground conditions Encountered during
this risk was clearly identified and through ground Construction of a Cross Passage. Underground
treatment eradicated. The problematic ground Singapore 2001.
conditions are known, but when encountered
5. Peart M., Poh J., Kho C. M., A revised Interpretation
unexpectedly sometimes difficult to deal with. On of Old Alluvium Formation based on Ground
CCLP both EPBM and slurry machines were investigations for Changi Airport Line. Underground
utilised to try to control settlements generally with Singapore 2001.
great success, but with occasional sinkholes. The
6. Nakano A., Sahabdeen M. M., Kulaindran A., Seah
introduction of the slurry machines has resulted in a
T. P., Excavation Management for Slurry TBMs
new learning curve, with new problems due to the Tunnelling under Residental Houses at C853.
additional level of sophistication these machines Underground Singapore 2007.
bring, to utilise them to their potential they need to
be operated properly by people who understand 7. Osborne N. H. Lim W. B. The Realignment of Nicoll
Highway Tunnels, A risk Management Approach.
them.
Underground Singapore 2007.
Despite the numerous challenges, tunnelling
for CCLP can be regarded as a success. The key 8. Shirlaw J. N., Hulme T. W., Risk Mitigation for
risks were identified and mitigated by employing a Slurry TBMs Tunnels and Tunnelling April 2008.
number of different techniques. More lessons for
BIOGRAPHICAL DETAILS OF THE AUTHOR
the future have been learnt, leaving the tunnelling
industry in Singapore in a stronger position to face Nick Osborne graduated with a degree
the upcoming challenges for the new Down Town in Geology and followed it with an
MRT project, which is just about to commence. MSc in Engineering Geology at
Imperial College, London. He has
REFERENCES worked on a number of tunnelling jobs
in the UK, including the Channel
1. Shirlaw, J.N., Ong, J.C.W., Rosser, H.B., Tan, C.G., tunnel and the Jubilee Line extension.
Osborne, N.H., Heslop, P.E, 2001. Local In Singapore he has worked on the
Settlements and Sinkholes due to EPB Tunnelling. North east line and Circle line projects
Proceedings of the Institution of Civil Engineers Vol and currently works on the new down town line.
156 Issue GE4 pp193- 211.
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