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Singapore Bukit Timah Granite: Slurry Quality Control for TBM Tunneling

Conference Paper · January 2010

DOI: 10.3850/978-981-08-6396-8_P122

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Singapore Bukit Timah granite: Slurry quality control for TBM tunneling

MASSIMO MAROTTALand Transport Authority, Singapore.

Marotta_Massimo@lta.gov.sg

AbstractFace pressure calculation is a critical factor when tunneling using Slurry Shield TBMs. Especially in case of low overburden, there is a very narrow band of lower and upper margins of suitable Face Pressure, as using a too high one might be as critical as using a too low one. The face pressure calculations are always done assuming that the quality of the slurry during operation can guar-anty proper rheological, physical & chemical properties able to fit the expected geological and hydro-geological conditions. Therefore, using slurry having inadequate properties can compromise the effectiveness of a correct applied face pressure and result in increasing the risks of high volume loss, high settlements and face instability. This paper will discuss the principles of Slurry Quality Control during operation with reference to Singapore contest and in particular to tunneling in mix face conditions along the Bukit Timah Granite formation.

Keywords: Tunneling; TBM; Slurry; Bukit Timah Granite

INTRODUCTION

Since 1995 the Land Transport Authority of Singapore has been constructing and planning the expansion of the underground roads and rails network.

TBM Tunneling using Earth Pressure Balance machines (EPB) has been extensively used in Singapore for theconstruction of the North East Line, East West Line extension, Circle Line and on the ongoing Downtown Line Stage 1.

Tunneling using Mix Face Slurry Shields has been first introduced in Singapore for the construction of part of the Circle Line to cope with particularly challenging mix ground conditions of the Bukit Timah Granite for-mation. Slurry quality is of fundamental importance when tunneling in mix ground within an urban context typical of the city state.

BRIEF DESCRIPTION OF THE BUKITH TIMAH FORMATION

The Bukit Timah Granite and its residual soil occupy the centre of Singapore Island extending some 8 km in the northerly direction and 7 km in the westerly direc-tion.

It is an acidic igneous rock formed during the LowerTriassic period and it is the predominantly igneous rock group in Singapore, forming the base rock of the Singa-pore Island.

The uniaxial compressive strength of fresh granite is reported in the typical range of 160 – 230 MPa, with peaks of over 300 MPa.

The residual soil of Bukit Timah Granite consists mainly of reddish brown sandy silty CLAY which is underlain by a thick layer of yellowish brown sandyclayey SILT and greenish to whitish grey silty clayey

SAND & sandy SILT. The total thickness ranges from a few meters to over 40 meters.

Relevant to this paper, the following characteristic of the Bukit Timah Granite are highlighted:

• The weathering grades of the Bukit Timah granite is based on Approach 2 in BS 5930:1999, and de-tailed in CP2004:2003. Six weathering grades are used for classification, with Grade I to III being rock grades and IV to VI being soil grades

• The weathering pattern in the Bukit Timah granite is mainly of stratified type; abrupt changes between rock and residual soil might be encountered.

Fig. 1: Sudden change between rock (Grade I) and Re-sidual Soil (Grade V).

• There is an high degree of local variation in mate-rial properties due to uneven penetration of weath-ering

• The bedrock can be very undulating and tunnelling might be often trough frequent changing and mixed ground of granite bedrock and soil, including pres-ence of boulders in soil matrix.

• The permeability of the jointed granite rock mass isvery low with average of 10-9 to 10-7 m/s, while the

Proceedings of the World Urban Transit Conference 2010 (WUTC 2010)Copyright © 2010 WUTC Organizers :: Published by Research PublishingISBN: 978-981-08-6396-8doi:10.3850/978-981-08-6396-8 P122 237

permeability of its residual soil might range be-tween 10-8 to 10-5 m/s.

• The abrasivity of the rock can be high due to its quartz content which can be higher then 10%

• Fluvial or marine deposits (as the Kallang’s forma-tion), as well artificial fills, might overlay the Bukit Timah granite formation, making tunneling even more challenging in those zones.

PRIMARY AND SECONDARY FUNCTION OF SLURRY IN TBM TUNNELING

PRIMARY FUNCTION

Stabilization of the tunnel face and control of groundwater

The fundamental function of tunneling slurry is related to its supporting capacity of tunnel face & wall and the related groundwater control.

This function is important in case of tunneling in full or partial presence of soil where the stability needs to be assured by the drilling fluid.

In order to achieve this:

The slurry should be able to create an impermeable membrane at the ground interface. Once the ground interface becomes impermeable, the slurry can develop an adequate confinement pressure

SECONDARY FUNCTIONS

Mucking

The viscosity of the moving slurry must enable it to be pumped at high velocity in the pipes of the slurry circuit

Lubrication of the plant

Especially when tunneling in abrasive formations, the slurry can limit the wear of plant (as cutting tools, pipes, valves, pumps, etc).

Encapsulation of the muck

The slurry must reduce the hydration of the muck in order to limit its propensity to stick and to flocculate (this is particularly important in when excavating in silty-clayed materials with a water contents below the plastic limit).

Inertness at the Separation Treatment Plant

The slurry must be designed to facilitate its treatment, i.e. the separation of slurry from the muck it conveys

Environmental inertness

Water, muck, and residual slurry must be acceptable in terms of environmental regulations.

SLURRY’S PROPERTIES

Rheology is the science devoted to the study of defor-mations and flow of matter under the effects of internal and external stresses.

In rheological terms, tunneling slurries are defined by a number of characteristics (or properties) whose meas-urement and verifications are of the greatest importance together with physical and chemical properties:

Density

Measurement of density serves to check the fines con-tents of the slurry being recycled to the TBM after sepa-ration.

In fact the density depends on the slurry components, on physical contamination by fines from the ground which are not separated and returns back to the TBM and on regeneration by fresh bentonite.

Sand content

The sand content of the slurry depends on the perform-ance of the separation plant. Any send remaining in the slurry directly affects the permeability of the cake and its stability on the ground interface.

Marsh Funnel Viscosity

The Marsh funnel viscosity is reported as the number of seconds required for a given fluid to flow 1 quart through the Marsh Funnel. The Marsh Funnel Viscositybecomes a more important parameter in HDD, where the speed of the fluid is low and the viscosity is needed for the transport of the solids particle. As indicated in the AFTES recommendation for slurry to be used in TBM tunneling, the Marsh Viscosity is less meaningful to represent the actual slurry properties.

Plastic Viscosity (PV)

PV can be defined as the contribution to fluid viscosity of a fluid under dynamic flow conditions. In a TBM slurry system, therefore, the PV tends to be lower in the pipe system where fluid run at higher speed, and higher in the cutter head and active tanks where the fluid run at a slower speed. The PV mainly depends on the size, shape, and number of particles in a moving fluid.

PH

A change to the PH of the slurry also affects its ion bal-ances and chemical properties. Beyond a PH range from 8 to 10, there is significant risk of poor slurry perform-ance in both acid environment (in contact with organic matters) and in basic environment (i.e. in contact with cement).

Fluid Loss

It is defined as a measure of the degree to which the water phase of the drilling fluid can permeate into per-meable formations. A too high figure increases the risks

238 Proceedings of the World Urban Transit Conference 2010 (WUTC 2010)

of the drill fluid migrations through micro fractures, porous formations, etc. Being the slurry a fluid, thus not compressible, it can assure the stability of tunnel face and walls once it does not permeate in the ground. Of course these in conjunctions with an adequate support pressure.

Its importance increases when tunneling in ground with a permeability of 10-7 ms or more, which might be found within the Bukit Timah residual soil.

Yield Point (YP)

It is defined as the resistance to initial flow or the stress required to start a fluid movement, measured from adynamic conditions. Yield point can be related to the carrying capacity of the fluid due to electro-chemical attractive forces within the fluid.

Reactive particles (as bentonite, reactive drilled soil and polymers), will increase the yield point.

Anyway the filtration effect on slurry penetration into the ground immediately reduces the velocity of the slurry flow into the ground.

Bentonite slurry, as a consequence of its reduction in flow velocity, gradually stiffens and set. Setting is di-rectly linked to the conventional Yield Point or Yield Value of the slurry.

Therefore yield is an important characteristic also re-lated to the confinement action of the slurry, as its con-trol can verify how the slurry set once eventually per-meating into the ground.

Filter cake

A membrane cake is obtained in low permeability ground with the use of relatively stiff slurry. The slurry does not penetrate very far into the ground and creates only surface impermeabilization of the ground surface. A filter cake should be thin and flexible.

0-10 min Gel Strength

It can be defined as the ability, or the measure of the ability, of a colloid to form gels. Gel strength is a time dependent measurement of a fluid’s share stress under static conditions. During rotation of the cutter head the cake is several time deformed and destroyed with each sweep of a cutting tool, but reform it immediately.

It takes some time before the slurry return to its original consistency. This time should be as short as possible in order to restore the water tightness. It is characterized by the 0-10 min gel strength.

Gel strength is a time dependent measurement of a fluid’s share stress under static conditions. During rota-tion of the cutter head the cake is several time deformed and destroyed with each sweep of a cutting tool, but reform it immediately.

CHALLENGES OF MINING TROUGH MIX FACE

Tunneling in rock would generally require slurry with properties related mostly to its secondary functions (lu-brication, transport of muck).

Tunneling in soil would require slurry with properties related to both primary (face support) and secondary functions, depending on a number of geotechnical char-acteristic of the soil.

Tunneling in mix ground conditions requires slurry with specific properties which are not simply the one wewould apply to the worst soil conditions encountered at the face. There are new factors to consider, which exist only in mix face conditions, as:

• Presence of interfaces between rock and soil (typi-cally GIII/GIV), which might be connected to the present of artesian aquifers

• Potential effects on soil stability due to slow ad-vance dictated by the rock component at the face.

• Vibrations due to the impact of the cutters on the rock

• Impact of potentially higher number of stoppages for interventions

Therefore the slurry properties in a mix face might be even higher then the properties selected for the individ-ual worst ground at the face if encountered alone.

In relation to the Bukit Timah Granite formation andoverlying layers and limited to Slurry Quality Manage-ment, we can define “Tunneling in complex mix groundconditions” as tunneling in zones with either:

• A concurrent presence of rock, residual soil, and/or other loose ground (overlaying layers of flu-vial/fill/etc.) within the tunnel face.

• Tunnel face in Rock but with shallow cover to a layer of loose ground/soil.

• A zone of frequent transitions between rock and weathered rock along the tunnel’s alignment

Fig. 2. Graphical representation of typical “Mix ground condi-tions” within the Bukit Timah formation

Proceedings of the World Urban Transit Conference 2010 (WUTC 2010) 239

Figure 2 shows typical mix ground conditions often encoun-tered when mining trough the Bukit Timah formtion.

SLURRY MANAGEMENT (KPI)

Being the slurry of fundamental importance to assure a safe and smooth tunneling operation, the implementa-tion of a proper control procedure is basic.

Although not part of this paper scope, it is still impor-tant to highlight that a proper Quality Management Sys-tem can be implemented only with the presence on site of competent and experienced personnel with specific experience in Mud Engineering applied to TBM tunnel-ing.

The process shall be based on monitoring and review of the slurry performance.

In order to do so, it is possible to introduce the applica-tion of Key Performance Indicators (KPI) which shall define the required properties for typically classified tunneling conditions.

While depending on different factors including strength, permeability, etc, slurry properties shall be grouped for few typical expected tunneling conditions, to make the process of quality control and review easily implement-able on site.

Fig. 3. Slurry Quality Control process based on Key Perform-ance Indicators

An example of possible classification of primary KPI for tunneling within the Bukit Timah formation is given below.

• KPI-1: For tunneling in GI, GII, GIII, the primary KPI shall refer mostly to properties related to slurry secondary functions.

• KPI-2: For tunneling in GIV, GV, GVI, the pri-mary KPI shall refer mostly to properties related to slurry primary functions. Yield value and Fluid Loss are important properties. Density of the slurryis related to the separation efficiency of the pantand shall be included as primary KPI although is not critical in the face supporting function of theslurry.

• KPI-3: For tunneling in difficult mix ground condi-

tions, the primary KPI shall refer to properties re-lated to both primary and secondary functions of the slurry, with generally stricter requirements then in KPI-1 & KPI-2

• KPI-4: KPI for interventions shall be related to the ability of the slurry to form a proper filter cake and therefore requirements shall be set for Fluid Loss and Filter Cake test results prior to the intervention.

The secondary KPI shall include all the other properties not selected as critical for the primary supporting func-tion of the slurry.

While the excavation/interventions shall not proceed if the slurry fails to meets the required primary KPI (as it might endanger the stability of the face, a failure to reach any secondary KPI shall initialize a corrective process which might be also develop in parallel with the ongoing tunneling operations.

In the following sample, two tables show the allowable and actual tested properties for a classified “mix ground conditions”, during a hypothetical test done prior to start a new round of excavation.

Table 1. Sample of primary KPI for Mix ground conditions

S/N

Primary KPI U.M. Allow-

able

Range Actual

1 Filtrate Loss cc < 30 25

2 Filter Cake mm 1 - 4 mm 2.5

3 Yield Point lb/100sq.ft > 10 14

Table 2. Sample of secondary KPI for Mix ground conditions

S/N Secondary

KPI U.M.

Allow-able

Range Actual

1 Density g/cm³ 1.04 – 1.25

1.23 OK

2 Sand Content % < 5% 0.6 % OK

3 Marsh Viscos-

ity sec > 40

38

NOT OK

4 Plastic Viscos-

ity cp 8 - 12

7

NOT OK

5 10 Min Gel

strength lb/100sq.ft 5 - 15

6

OK

6 PH 7 – 11 9 OK

From the table 1 and 2, we can see that the slurry prop-erties related to the primary KPI match the require-ments, while few properties related to secondary KPI fail the requirement.

CLASSIFICATION OF THE EXPECTED GEOLOGICAL CONDITIONS

SELECTION OF PRIMARY AND SECONDARYK.P.I

VERIFICATION & REVIEW PROCESS

240 Proceedings of the World Urban Transit Conference 2010 (WUTC 2010)

A procedure shall establish the action to be taken in such situation.

The test reflects anyway a situation in which the deci-sion becomes purely operational as the fundamental supporting function of the slurry is not affected.

In the following sample (table 3) a hypothetical test done prior to start a new round of excavation show slurry having primary KPI properties lower then re-quirement.

Table 3. Sample of primary KPI for Mix ground conditions

S/N Primary KPI U.M. Allow-

able

Range

Actual

1 Filtrate Loss cc < 35 45 NOT OK

2 Filter Cake mm 1- 4 mm 5 NOT OK

3 Yield Point lb/100sq.ft > 10 10 OK

A procedure shall establish the action to be taken in such situation, which in this case shall basically results in improv-ing/recovering the slurry’s properties prior to start a new ex-cavation which might be otherwise seriously affected.

Monitoring of slurry performance

The frequency of testing shall be at least at the begin-ning of every mining cycle, in order to verify the prop-erties for the shoving, and at the end of each mining in order to verify the properties and plan the measure to eventually increase them to meet the specified KPI.

SLURRY TESTING EQUIPMENT AND METHODS

A site laboratory shall be equipped with all testing fa-cilities in order to have a proper and fast control of the slurry properties. The main tests and the related testing equipments are shown in Table 4.

Table 4. Tests & Testing equipments

S/N Test Testing Equipment

1 Density Mud Balance

2 Sand Contents Sand content test Kit

3 Marsh Viscosity Marsh Funnel Vis-cometer

4 Plastic Viscosity Fann rehometer

5 Yield Point Fann rehometer

6 Gel Strength (10 sec, 10 min) Fann rehometer

7 Filtrate Loss Filter Press

8 Filter Cake Filter Press

9 PH pH meter

Fluid Loss test

Among the various tests, the Filtrate check (Fluid Loss) is the one requiring the longest time.

In fact it shall take 30 minutes. In those conditions when the Filtrate Loss is one of the KPI and therefore it has to be verified in order to start a shoving, waiting the end of the test result might postpone the start of each new mining.

It is possible, and accepted as testing method, to refer to the Filtrate Loss after 7.5 minutes which shall be the 50% of the one at 30 minutes.

The chart in figure 4 shows a test done on slurry with a measured Filtrate Loss of 16cc and 31cc after 7.5 and 30 minute respectively.

This confirms with some minimal approximation that the 7.5 minutes test is a reliable method to have a quick verification of this important property.

0

10

20

30

40

0 5 10 15 20 25 30Time (min.)

Filt

rate

Los

s (c

c)

Fig 4. Filtrate Loss Test

TYPICAL CHALLENGES OF TUNNELING IN MIX GROUND

HANDLING OF FINES

During excavation solids are removed and sent to the Separation Treatment Plant.

The capacity of the STP should fit the expected ground conditions and as well the TBM expected performances.

A separation plant typically consists of a primary and secondary separation level.

The primary screening handles the separation of coarser material, while a desander handles the separation of the sand particle. A desilter (by a series of hydro cyclones) shall be further used to handle the silt particle.

Dimensioning of the STP separation capacity of fines is mainly related to the d50 point, which represents the grain size which has equal chances to go either to the overflow or to the underflow of a cyclone. This shall be directly related to the geological information to be care-fully studied during the planning stage.

The d50 point is commonly referred as the cut off point.

Proceedings of the World Urban Transit Conference 2010 (WUTC 2010) 241

Particles smaller then the cut off point will not be sepa-rated and will return to the system, causing an increase in the slurry density. Therefore, more the particleswhich are separated, less the density of the slurry will increase.

Limits on the separation of clay particles

The first consideration is that the cut off point of sepa-ration plant is always greater then the particle size of the “good” bentonite which shall be retained in the system.

Separation Plants are primarily designed to separate all but and to retain the bentonite important to maintain the rheological properties of the slurry.

Therefore, together with the bentonite particles, the separation system retains also clay particles eventually coming from the excavated material, causing an in-crease in the density of the slurry.

Limits on the separation of silt particles

The desanding stage is usually not critical, as hydrocyc-lones used in the STP can easily achieve the related cut off point.

The desilting stage can be done either by hydrocyclones running in desanding & desilting modes or with a sepa-rate stage of smaller cyclones running only as desilters.

The dewatering stage of the underflow of the cyclones can help in increasing the separation of fines.

Theoretically it is possible to achieve very small cut off point but a higher number of small smaller diameters cyclones is required to maintain the same system flow capacity.

In order to have a general idea, we shall replace each 10”diam. cyclone having a theoretical cut off point of 40-50 μm with 8 numbers 4” diam. cyclones to maintain the flow capacity of the system and having a cut off point reduced to 15-20 μm, or with 33 numbers 2” diam. cyclones to maintain the flow capacity of the sys-tem and having a cut off point of 10-20 μm.

Equal flow capacity curve in relation to Size & Numbers of hydrocylones

0

5

10

15

20

25

30

35

0246810Diameter of Cyclones (inches)

Num

ber

of C

yclo

nes

Fig 5. Sample of Equal flow capacity curve

There are therefore a number of constrains (mainly op-erational), which limit the actual use of an extensive number of smaller hydrocyclones.

The cut off point depends also on other factors as the viscosity and the density of the slurry.

Higher is the viscosity & the solids content, lower is the separation capacity of a cyclone (coarser cut off point). This means having a worst efficiency in a situation which would require (during tunneling) a better effi-ciency.

This unavoidable contradiction makes the viscosity &density control of the slurry a key parameter to optimize the separation plant efficiency.

Handling high presence of fines

The chart in figure 6 shows a Particle Size Distribution (P.S.D.) diagram for solids at the STP inlet, which might be within the range encountered in the Bukit Ti-mah formation.

The chart also shows the finer and coarser separation capacity of a typical Separation Plant:

Fig 6. Particle Size distribution & separation efficiency

If tunneling in conditions represented by the curve A, it is clear that both the primary and most of the desending stages of the plant would not contribute to the separa-tion, being 100% of the particle sizes smaller then the cut off point at those stages.

The cut off point of the desilting stage would allow the plant to separate (on a theoretical base) only 11% of the solids. This would result in a massive return of fines to the system.

Assuming that for such ground conditions (Low perme-ability) it is not necessary to use slurry with particularly high rheological properties, the main problem would be limited to maintain the slurry density.

With reference to the Bukit Timah Formation, this ap-proach can be very risky with potential sudden geologi-cal changes along the drive, where slurry with goodproperties might be required at any time.

Tunneling in Mix Ground Conditions

With reference to the chart in Fig. 6, curve C might rep-resent both tunneling in a residual soil and/or in a Mix face condition. In fact the P S.D. shall reflect the slurry

242 Proceedings of the World Urban Transit Conference 2010 (WUTC 2010)

Proceedings of the World Urban Transit Conference 2010 (WUTC 2010) 243

maintain the presence of the additive in the system it can be added from the separation plant.

The addition of clay encap, as well of any other sub-stance to active slurry, shall be carefully evaluated.

Special mixing facilities, flow control equipment and a tight handling procedure might be often required.

Compatibility among different additives shall be tested and it might result in an easier operation limiting to a few number the types of additives to be used within the same plant.

Handling fines at the source

If tunneling in clay, especially sticky clay, the TBM advance speed shall be controlled.

The TBM shall mines at a rate in which it is still able to cut the clay.

With excessively fast advance rates the cutter headmight "push" into the soil and clay will just extrude trough the cutter head’s openings.

Those large chunks of clay might cause blockages or eventually a sudden ingress of fines in the slurry system causing a “peak” which cannot be handled by the Sepa-ration Plant, causing the density to suddenly increase and resulting in a longer time to restore the slurry to acceptable quality.

The TBM operators and engineers shall be sensible to signs of potential problems derived by a high presence of fine.

Cases of frequent blockages as well recorded under-excavations might be a sign of large chunks of clayey material been trapped inside the cutter head where a controlled advance speed as well the aid of a clay en-capsulator might result very effective.

SOLIDS CONTAMINATIONS: SAND

The sand content of slurry depends upon the perform-ance of the separation plant. Any sand remaining in the slurry directly affects the permeability of the cake and its stability on the ground interface, and therefore the sand content should be kept as lower as possible.

CEMENT CONTAMINATION

Characteristics

During TBM operation it is possible that slurry might be contaminated by cement present along the drive in areas which soil has been treated with ground improve-ment.

A contamination by cement is recognizable by the changes in the following slurry properties:

• Increase in PH • Increase in filtrate loss to very high values (> 50

cc) • Increase in filter cake thickness to several millime-

ters ( 5 or more )

The following chart a sample test on cement contami-nated slurry:

CEMENT CONTAMINATION

0

20

40

60

80

100

120

0 2 4 6 8grams/liters of cement added to fresh bentonite

API fluid loss (cc) Yield pointMarsh Viscosity (sec) Filter cake (mm)PH

Fig 9. Effects of cement contamination on a slurry sample

The test shows that the main variations are on FluidLoss and Filter Cake, which are two important KPI.

Recovery

Recovery can be done with the addition of additives in the slurry in order to recover its properties to an accept-able level, without disposing/replacing it with fresh one. In the test, polymer based additives are added, in differ-ent proportions, to a cement contaminated slurry.

CEMENT RECOVERY USING 2 DIFFERENT

TYPES OF POLYMERS

0

20

40

60

80

100

120

0 2 4 6grams/liters of polymers added to fresh bentonite

API fluid loss (cc) Filter cake (mm)PH Marsh Viscosity (sec)Yield point

Fig 10. Effects of additives in a cement contaminated slurry

The addition of fluid loss reducer in a ratio of 2.5g per liter of slurry shown to be already effective to bring back Fluid Loss and Filter Cake to an acceptable value.

Further addition of another general viscofier (up to 5g/liter) was able to increase also in the Yield Value.

The use of similar additives might be implemented tomaintain certain slurry properties during normal tunnel-ing operations and not just to handle exceptionally con-taminated slurry.

244 Proceedings of the World Urban Transit Conference 2010 (WUTC 2010)

CONCLUSIONS

The complexity of tunneling in mix ground conditionsrequires a comprehensive Quality Control Plan in which TBM and Separation Treatment plant shall be consid-ered and managed as a unique system.

TBM and STP shall be designed to meet each others requirements so to achieve a high level of efficiency. It is important the selection of good quality bentonite and of other additives which might help in control-ling/maintain the slurry’s properties, in combination with regular regeneration with fresh bentonite.

During operation, flow of information and proper com-munication among the tunneling team and the separa-tion plant team is of vital importance.

REFERENCES

[1] P. Longchamp, AFTES Recommendations Con-cerning Slurry for use in Slurry Shield TBM, (Tun-nels et Ouvrages Souterrains – Hors-Serie N.1, 2005)

[2] J. N. Shirlaw, Z. Yan, X. C. Xiao, Assessing face pressures for slurry shield tunneling through par-tially dewatered weathered gneiss, (Underground Singapore 2009)

[3] N.H Osborne, C. Knight Hassell, L.C. Tan & R. Wong, A review of the performance of the tunnel-ling for Singapore’s circle line project (World Tun-nel Congress 2008 - Underground Facilities for Better Environment and Safety – India)

Proceedings of the World Urban Transit Conference 2010 (WUTC 2010) 245

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