Singapore Bukit Timah granite: Slurry quality control for TBM .Singapore Bukit Timah granite: Slurry

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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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    Massimo Marotta

    Singapore Land Transport Authority

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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 Kallangs 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.

    SLURRYS 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 conta