Soft Ground Tunneling for MRT in Urban Areas

5/26/2018 Soft Ground Tunneling for MRT in Urban Areas

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Soft Ground Tunneling for Rapid Mass Transit Systems in Urban Areas

SOFT GROUND TUNNELING FOR RAPID MASS TRANSIT SYSTEMS

IN URBAN AREAS

(Literature Review)

Muhammad Shakeel

Master Student in Asian Institute of Technology Bangkok, Thailand

1. Introduction

As the time passes, there is a significant change around the world in terms of population, urbanization and standard of

living. It demands a proper allocation and redistribution of limited urban space to the various urban functions both

existing as well as new. Underground space has been created in urban areas for traffic and utilities. It also is beingused for storage, underground stations and many other usages. It is also studied that underground space not only the

requirement of congested populated cities but it also improves the life style of people. Tunneling in urban areas is one

of the most efficient usages of underground space.

For safe tunnel excavation make its use very frequent in urban areas. Especially now a days, very fast and safe tunnelexcavation can be done by TBM without leaving excessive settlement and damages to the surface structures. TBM can

be worked every type of soil, shallow overburden, below ground water table and varying soil formations. Earth

pressure balance shield, slurry pressure balance shield and varying density shield are commonly used for soft ground

tunneling depending on the soil nature.

Conventionally open cut, cut and cover, NATM and STM can be used for tunneling according to the suitability.

The countries are innovating and implementing new technologies and constructing versatile structures. Mass Transit

Systems is one of the said projects. Underground Tunneling in soft soil is less destructive than the cut and cover and inmost of the countries the depth of the tunneling is kept shallow i.e. in the soil, which shows the necessity of the Soft

Ground Tunneling in urban area and it will also increase in future.

The increase In Underground Tunneling projects is significant in the past decades. There are large no of projects

around that involves the Underground Tunneling in Soil. These projects includes Mass Transit Systems, Hydropowergeneration Projects, Highways etc.

The soft ground is referred to as soil. The soil is mainly classified in to two main types i.e. Cohesive Soil andFrictional Soil. During the construction of the tunnels both the soils behave differently.

2. Engineering geological investigations of mechanized tunneling in soft ground

For successful mechanized tunneling, characterization of sub-surface and proper investigation of geotechnical risks

are vital important for the selection of Tunnel Boring Machine and its efficient use.

Figure 1 Underground MRT Tunnel and Station


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3 | P a g e A S I A N I N S T I T U T E O F T E C H N O L O G Y B A N G K O K , T H A I L A N D

Characterizing the ground along the tunnel and predicting the geological, hydro-geological and geotechnical

conditions with respect to mechanized tunneling are essential tasks to perform before the design and construction of atunnel. Any unforeseen adverse geological and hydro-geological condition, especially in mechanized tunneling

projects, can increase construction time and cost and cause more risks for the workers, additional environmentaldamages and more ground settlement problems (Rienzo et al., 2008). Failure of proper investigation of geological and

geotechnical risks will make the project delay, costly and disputed.

Figure 2. Flow chart of engineering geological investigation for mechanized tunneling in soft ground (Sadegh Tarigh Azali - Engineering

Geology 166 (2013) 170185)

3.Geological and Geotechnical Hazards with respect to mechanized tunneling

After the soil characterization and proper assessment of geological, geotechnical and hydrological parameters varioushazards scenarios are possible to face with varying probability. Following are the common possible hazards which

mechanized tunneling may have to face.

3.1 Stickiness and clogging of soil:

Some types of cohesive soils, especially those consisting of highly plastic clays, have the tendency to develop stickybehaviors (adhesion of clay particles to metal surfaces and/or cohesion of clay particles and their sticking to one

another), which may lead to clogging in the cutter head, working chamber, and screw conveyor of an EPB machine

and induce balling problems in the pipes and at the separation plant of a slurry TBM or obstruct the shield advance

due to friction (Marinos et al., 2008).


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When such clogging occurs, operation need to be stopped for cleaning the machine cutter and chamber which effect

on the schedule of the project and therefore impact on the project budget. It is therefore important that in geotechnicalinvestigation such behavior of soils should be measured qualitatively and quantitatively.

Clay clogging is categorized in three types depending on consistence (Ic) and plasticity indices (PI) of the fine soil.The categories of clogging potential proposed by Thewes and Burger (2004) and the recommended changes in the

procedures of EPBM tunneling are as follows:

Soils with high clogging potential cause substantial excavation problems and require daily cleaning operations.Machine modification only leads to the reduction, not the eradication of the problem.

Soils with medium clogging potential can be handled following a number of mechanical modifications in the shieldmachine and soil transport system, along with changes in the operation of the machine.

Soils with low clogging potential require a reduction in the advance rate, but making major alterations to the EPB is

unnecessary.

The below figure shows the clogging risks which is related to plasticity index to consistency index.

Figure 3 Clogging Risks of Cohesive Soil (data points shown from the metro project in Iran)

3.2 Soil with low fine content:

The slurry shield method is applicable to a wide variety of soils, from clay to sand and gravel (EFNARC, 2005), whilethe use of EPB machines is limited to relatively soft and fine-grained soils (particles smaller than 75 m or particles

able to pass through a No. 200 sieve). Therefore, another difficult soil for EPB tunneling is coarse-grained soil with

insufficient fines combined with free water. In order for an EPB to properly control face pressure while excavating, it

must dissipate the face pressure along the length of the screw conveyor. Toothpaste is a term often used to describe

the ideal consistency of conditioned soil mixture for an EPB-TBM. The material in the screw must be a stiff viscousfluid like toothpaste in order to properly dissipate the face pressure. Some coarse-grained soils have insufficient fines

to achieve the consistency of toothpaste. Instead, they tend to drain free water and segregate it, providing undesirable

spoil characteristics for the EPB-TBM spoil (Ball et al., 2009). Coarse-grained soils that segregate and drain freewater do not behave like a viscous fluid, and could not be expected to dissipate pressure along a screw conveyor. If

fine particles are absent in the grain size distribution, they must be added artificially (bentonite, polymers, foam)(Wassmer et al., 2001). How much fines are needed is a point of discussion. In the British Tunneling Society (BTS)

guideline for closed face tunneling, a minimum value of 10% is recommended (BTS, 2005); but this would rely on the

addition of polymer. Without the addition of polymer, 20% fines is the considered minimum.

3.3 Permeability of soil:

Regarding permeability, the British Tunneling Society (BTS) and the Institution of Civil Engineers (ICE) (BTS, 2005)

indicate a ground permeability of 10E5 m/s as the point of selection between EPBMs and slurry TBMs (Marinos et

al., 2008). Typically, the use of EPB-TBM is optimal in grounds with permeability less than 10E5 m/s. If thepermeability is higher and the tunnel alignment is under a water table, pressurized water could flow into the tunnel

through the screw. Therefore, the type and quantity of conditioning agent to be added to the plenum and the screw

conveyor become relevant (Guglielmetti et al., 2007). For higher permeability values (N10E5 m/s), the use of a

slurry-TBM is more suitable than an EPB-TBM. Nevertheless, a slurry- TBM applies the face-support pressure

through the formation of a cake between the slurry and the soil. The higher the soil permeability is, the moredifficult the cake formation will be.


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3.4 Abrasiveness of soil:

The abrasiveness of soil or rock describes the wear and tear of tools. In abrasive ground the wear can occur on various

parts of the TBM, so the abrasiveness not only determine the tool wear but also the time to repair or change of the

wearing tools which ultimately effect on the project schedule and budget of the project. To see the abrasiveness of thesoil NTNU or LCPS tests should be performed.

Figure 4 Classification of abrasiveness coefficient for various types of soils (after Thuro and Ksling, 2009)

3.5 Oversize grains:

Oversize grains (cobbles and boulders), frequently found in tunnels excavated through soils, can pose major problems

for full-face TBMs in terms of advance rate reduction, cutter damage, and abrasive wear (Dowden and Robinson,

2001). Cobbles and boulders are commonly found in glacial, alluvial and residual soils (Hunt and Del Nero, 2010).

When a full-face machine encounters a boulder, there are a number of possibilities. If the boulder is not too large, itcan be ingested by a properly designed TBM mucking system. If the boulder is too large to be ingested, and the

ground is firm, it may be broken up by a suitably equipped machine cutter head (tunnel boring machines equipped

with disc cutters). If the soil matrix is weak, the boulder may be dislodged, and it may either be pushed radially

outward by the rotary action of the cutter head, and beyond the tunnel periphery, or it may stay in the face area andeventually block further progress of the machine until it is manually removed. Depending on the prevailing face

condition and cutter head chamber configuration and accessibility, manual breakup and removal can be relatively easy

or very time consuming.To cope with boulders and protect the machine during the excavation, the cutter head should be equipped with disc

cutters. Due to the rolling movement of the disc cutters, single pieces so-called chips are broken out of the boulder.

Otherwise, it may be necessary to manually split or remove the boulders in the tunnel face.

3.6 Groundwater Fluctuation:

As the earth pressure is directly related to the groundwater level, so the proper estimation of groundwater level and itsfluctuation is very important for the stability of tunnel face to avoid any potential in stability. Therefore, groundwatershould be monitored through boreholes or piezeometer at regular interval.

4.Tunneling Methods

Due to last certain decades the demand of transportation usage has increased tremendously due to limited space and

higher population growth. This leads to various underground excavation techniques for tunneling.

1. Open Cut tunneling method2. Cut and cover tunneling3. Mechanized tunneling

4. Sprayed concrete tunneling or NATM5. Shallow tunneling method (STM)


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5.1 Open cut tunneling method

In this method an open trench is done to the required depth, then the tunnel is constructed accordingly and open trench

is backfilled.

Figure 5 Open Cut Tunnel Method

5.2 Cut and Cover Tunneling MethodCut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over

with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Two basic

forms of cut-and-cover tunneling are available:

1-Bottom-up method:A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in

situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The

trench is then carefully back-filled and the surface is reinstated.

Figure 6 Cut and Cover Method


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2-Top-down method:Side support walls and capping beams are constructed from ground level by such methods as slurry walling, or

contiguous bored piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ

concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways,services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is

constructed.

5.3 Mechanized Tunneling Method:

As underground excavation leads to various problems and especially in urban areas such as groundwater inflow,surface settlement which may damage the surface structures, environment issues. To minimize these issues the use of

mechanized tunneling is increasing day by day especially urban tunneling works. By use mechanized tunnelingmethod even for shallow overburden all types of excavation potential can be minimized. There are various types of

tunneling boring machines are developed and selected accordingly.

5.4 Sprayed Concrete Tunneling Method (SCL or NATM)Sprayed concrete lining or New Austrian Tunneling Method was initially developed for rock tunneling but later can

also be used for soft ground tunneling. According to it, excavate the small area and then immediate apply the shotcretesupport that act as temporary support and then remaining area will be excavated and immediate support is to be

applied. Later permanent support shall be installed according to the requirements.

5.5 Shallow Tunneling Method (STM)

It is widely accepted that one of the major principles of the NATM is the deliberate mobilization of the strength of theground around a tunnel to the maximum possible extent by allowing a controlled ground deformation (Brown, 1981;

Sauer, 1988; Will, 1989; Health and Safety Executive, 1996). But for tunneling in urban afreas the ground settlemnt

should be controlled that is the contradiction of NATM.

Figure 8 NATM Tunneling Method

Figure 7 Mechanized Tunneling by TBM


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Shallow tunneling method (STM) can be adopted for shallow tunneling in densify urban areas.

Mechanical characteristics of STM:

Limited Arching effect:

Arching effect is one of the most universal phenomena encountered in soils both in the field and in the laboratory

(Terzaghi, 1943). Arching can be best described as a transfer of stresses between a yielding mass of geomaterial andthe adjoining stationary members induced by stress redistribution. The shearing resistance tends to keep the yielding

mass in its original position resulting in a change of the pressure on both of the yielding parts support and the

adjoining medium. The concept of arching is illustrated in below figure (Terzaghi, 1946). Due to arching, the height ofthe relatively loose overburden above the tunnel roof resulting from the excavation of the tunnel is D (the height of the

arching zone) instead of H (the overburden depth).

However, in shallow tunnel this effect cannot developed adequately, so tunnel support may carry significant portion of

overburden load.

Limited Ground strength mobilization:

Against the NATM, in STM ground settlement needs to be strictly controlled as the tunnel is in highly density urbanarea to avoid damages to the surface structures. Therefore the key principle for the STM is to control the ground

deformation in order to guarantee the tunnel stability and the environmental safety. Convergence confinement method

(CCM) can be used to predict initial requirement of support to controlled settlement. The schematic ground reaction

curve (GRC) is shown in above figure. Support characteristics curve (SCC1) can be used for shallow tunneling in

urban areas.

Preconditions for the STM:

Using specific construction techniques, the STM allows shallow tunneling in soft ground conditions, such as silt, clay,

sand and gravel. Two preconditions, namely stability of the cutting face and dry tunneling condition, must be satisfied

when using the STM.

Stability of the cutting face:

Generally, the self-stability of the shallow tunnel in soft ground is very limited. It is impossible to tunnel through acutting face with a very short stand-up time. Therefore, suitable measures should be taken to guarantee a long enough

stand-up time for the cutting face and the unsupported span before the support takes action.

Dry tunneling condition:

For shallow tunneling below the groundwater level or in the water-bearing ground, dry tunneling condition should be

guaranteed. Dry tunneling condition is essential in maintaining the stability of the cutting face by avoiding the

Figure 10 Configuration of Ground Arch (After Terzaghi1946) Figure 9 Schematic representation of ground reaction curve


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(D ) Typical layout of pipe roof protection

of the surrounding ground. Furthermore, dry tunneling condition could also improve the underground working

environment, hence increasing the working efficiency and decreasing the unsupported time of the free span behind thecutting face.

Auxiliary methods for the STM:

An auxiliary method is a construction method of a secondary or special nature adopted to ensure tunnel construction

safety and surrounding environmental safety, where either conventional support patterns or sequential excavation

measures do not provide effective solutions or where they are not advantageous (Japan Society of Civil Engineers,

1996).

5. Mechanized Tunneling Technique and selection of TBM for Soft ground:There are various types of tunnel boring machines are available for rock as well as soft ground tunneling and

accordin to the eolo of the site, the suitable one is selected.

( C ) Typical Layout of footing reinforcement layout

(E) Typical Layout of contact grouting


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For soft ground tunneling in urban areas following TBM are used:

5.1 Earth Pressure Balanced Shield (EPB)For soft, cohesive soils tunnel boring machines with earth pressure support are a preferred option. So called Earth

Pressure Balance Shields (EPB) turn the excavated material into a soil paste that is used as pliable, plastic support

medium. This makes it possible to balance the pressure conditions at the tunnel face, avoids uncontrolled inflow ofsoil into the machine and creates the conditions for rapidtunnelingwith minimum settlement.

The special feature of Earth Pressure Balance Shields is that they use the excavated soil directly as support medium.This method is the first choice in cohesive soils with high clay and silt contents and low water permeability. A

rotating cutting wheelequipped with tools is pressed onto the tunnel face and excavates the material. The soil enters

the excavation chamberthrough openings, where it mixes with the soil paste already there. Mixing arms on the cuttingwheel and bulkhead mix the paste until it has the required texture. The bulkhead transfers the pressure of the thrust

cylinders to the pliable soil paste. When the pressure of the soil paste in the excavation chamber equals the pressure of

the surrounding soil and groundwater, the necessary balance has been achieved.

The face pressure acting from the water and earth pressure is balanced by soil paste as shown in below figure. It issuitable for clayey soil of low permeability but it can be used for various soils by conditioning it.

5.2 Slurry Pressure Balance Shield (SPB)SPB utilize the fluid mixture which is used to carry the excavated material outside and maintaining the front facepressure to minimize the settlement. The fluid mixture is also acting as machine coolant and also as lubricant.

Bentonite suspension in water with additives is used as fluid mixture. It form a cake on the tunnel face keeping wateron other side which then excavated and mixed with slurry and transferred to the surface.

It is suitable for coarse grain (sand) soil but it can be used for various types of soils after soil conditioning. The

pressure at the face of tunnel exerted by the water and soil is balanced by fluid mixture.

Figure 11 Earth Pressure Balanced Shield

Figure 13 EPB Principal Figure 12 Suitability of EPB


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Figure 15 Blowouts and sinkhole formation by using slurry shield in Karstic limestone

5.3 Variable Density TBM (Karstic Limestone with High Water Table)The high groundwater table and erratic Karstic feature renders EPB TBMs not suitable to tunnel in highly Karstic

limestone ( Kuala Lumpur Limestone Formation) conditions. Slurry Shied TBMs are usually used for such conditions

but lessons learnt from the SMART Project also revealed that the standard Slurry Shied TBMs not able to preventincidences of sinkhole formation and blowouts.

The Karstic environment means that the standard Slurry Shield TBM frequently encountered chambers or fissures into

which the slurry will escape resulting in high volume and pressure loss. If the fissures lead to the surface, the slurrywill escape to the surface, resulting in a blowout. On the other hand, if the slurry escapes into an underground cavity,

it will result in a loss of face pressure, thereby creating sinkholes on the surface.

To overcome these situations, the Variable Density TBM was developed which enables the density and viscosity ofthe slurry to be varied. This prevents the slurry from escaping into cavities or blowing out from fissures leading to thesurface. With this, the face pressure of the TBM is preserved, and the terrain is kept stable during the excavation

process. The variable density slurry shield enables the TBM to alter the density and viscosity of the slurry according to

soil condition when tunneling. Slurry of higher viscosity can stop slurry of lower viscosity from escaping into cavities

or blowing out from fissures leading to the surface. This in turn preserves the face pressure of the TBM, keeps theterrain stable during the excavation process, and prevents upwards flows of slurry, reducing the risk of slurry fountains

on the built-up surface.

6. Soft ground Response induced by Tunnel Excavation

Soft ground in tunnel excavation means, tunneling in cohesive (clay) or cohesionless (sand) soils. It is identified that

Figure 14 Slurry Pressure Balanced Shield (SPB)

Figure 16 Variable Density TBM in Karstic Limestone


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6.1 Undrained component of volume lossIn tunnel construction the amount of excavation is large than the tunnel volume. The ration of excavated volume to thetunnel replaced volume is called volume loss, VL. The need of extra volume is due to the advancement of TBM for

further excavation. It causes the disturbance and surface settlement and a settlement trough is developed at the surface.

The volume of settlement trough is equal to the volume loss.

Source of volume loss after Attewell (1978) includes:

1. Face loss2. Shield loss3. Tail loss4. Radial loss

Zone 1 is face loss, inward movement of ground from ahead zone of influence (f). It can be controlled byface pressure.

Shield loss occurs in Zone 2 and represented as s. During tunneling the diameter excavated is larger thanthe tunnel diameter to easy advancement of the TBM. This extra volume is called annular gap that to be

filled by grout. The soil tends to move inward radially which cause settlement.

As the annular grout not hardened the movement of soil continue that is termed as tail settlement t, occursin Zone 3.

The last component is radial loss that will remain continue due to the shortening of support lining andannular filled grout because load is transferred from one boundary to the other boundary.

6.2 Tunnel face stability (ITA/AITES Report 2006, Settlement induced by tunneling in softground)

Analyzing tunnel face stability provides an indication of the most probable failure mechanisms, as well as ofparameters to be taken into consideration in the evaluation of ground movements induced by tunneling. Based on thenature of the grounds encountered, two types of failure mechanisms may be observed.

In the case of cohesive soils face failure involves a large volume of ground ahead of the working front. This

mechanism leads to the formation of a sinkhole at the ground surface with a width larger than one tunnel diameter.

Figure 18 Soil Movement around tunnel in Clay(after Kimura and Mair, 1981)

Figure 17 Soil Movement around tunnelin sand (after Potts, 1976)

Figure 19 Volume loss components


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In the case of cohesionless soils, failure tends to propagate along a chimney like mechanism above the tunnel face.

Both mechanisms have been evidenced in centrifuge tests carried out in clays and dry sand as shown in below figures.

6.3 Propagation of Movements towards the surfaceGround movements initiated at the tunnel opening will tend to propagate towards the ground surface. The extent and

time scale of this phenomenon will typically be dependent upon the geotechnical and geometrical conditions, as well

as construction methods used on the site.

Two propagation modes have been identified, based on the conclusions of in situ measurements and observations.

These modes can be used to evaluate, in a transverse plane, the degree of propagation of displacements initiated at the

opening. They will be referred to, in the following, as primary mode and secondary mode (Pantet, 1991).

The primary mode occurs as ground stresses are released at the face. It is characterized by the formation of a zone of

loosened ground above the excavation. The height of this zone is typically 11.5 times the tunnel diameter and aboutone diameter wide. Two compression zones develop laterally along the vertical direction. For deeper tunnels

(C/D>2.5), the observed tunneling impact at the ground surface is generally limited (Cording and Hansmire, l975;

Leblais and Bochon, 1991; Pantet, 1991).

The secondary mode may occur subsequently, when the tunnel is located close to the surface (C/D < 2.5) andinsufficient confining support exists. These conditions result in the formation of a rigid ground block, bounded by

two single or multiple shear planes extending from the tunnel to the surface. Displacements at the ground surface

above the opening are of the same order of magnitude as those generated at the opening.

These ground response mechanisms typically lead to vertical and horizontal displacements that tend to develop at theground surface as excavation proceeds; this results in what is referred to as the settlement trough.

For practical purposes, the observed three-dimensional trough is conventionally characterized by means of a

transverse trough and a longitudinal trough along the tunnel

center-plane.

Figure 21 Face Collapse in granular dry soilFigure 20 Face collapse in clay soil

Figure 23 3D Settlement troughFigure 22 Soil movement (a) Primary (b) Secondary mode


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6.4 Effect of Ground WaterNumerous examples can be found of difficulties and accidents in underground works that were caused by

groundwater. It must be emphasized that groundwater control is a prerequisite for the successful completion of

underground works.Settlements induced by groundwater typically fall under two categories. The first category refersto the occurrence of settlements almost concurrently with construction. Lowering of the groundwater table, prior to

excavation (through drainage) or as a consequence of tunneling, may cause immediate settlements to occur in layers or

lenses of compressible soils, as well as in weathered rocky materials. The impact of such lowering of the groundwater

table varies in proportion to its magnitude and radius of influence:

when localized, induced deformations are often prone to generate large differential settlementsthat can be damaging to the surrounding buildings;

when widely spread, their consequences are generally less severe

The occurrence of groundwater at the tunnel face may induce settlements as a result of:

the hydraulic gradient weakening the mechanical conditions at the face and on the tunnel wallsthereby increasing ground deformations;

worsening effects on preexisting mechanical instabilities (washed out karsts, etc);

worsening of the mechanical properties of the ground in the invert, particularly when thesequential method is used, with the risk for punching of the foundation ground by the temporary

support due to loss of confinement.

The second category refers to delayed settlements that are typically observed in soft compressible grounds. As a result

of the tunneling works, the ground can be locally subjected to stress increase and subsequently excess pore pressures.Similar mechanisms can develop at a larger scale with fully pressurized shield tunneling. Moreover, as a result of

seepage towards the tunnel walls that inevitably occurs during and/or after construction, either along the more

pervious materials present around the opening or through the tunnel liner, consolidation will take place within theentire ground mass. The magnitude of consolidation settlements will be larger in areas experiencing higher reductions

in pore pressures.

6.5 Effect of worksite conditionsThis includes the settlements induced by the general worksite conditions, especially vibrations induced by boringwhether with the sequential or shielded method and muck removal operations. Settlements of this type have been

observed in soft ground conditions, or in good ground with poor surface backfill material.

7. Potential Problems in Soft Ground Tunneling in Urban Area

There are many potential problems associated with tunneling in urban areas; the most important ones are as follows:

o Surface settlement trough leads to buildings (tilting or distortion) and roads settlementStructure located in the vicinity of a tunnel under construction will experience the following movements:

uniform settlement (or heave)

differential settlement (or heave) between supports

overall or differential rotation

overall horizontal displacement

differential horizontal displacement in compression or extension.o Collapse due to instability problemo Groundwater fluctuation

Figure 24 Building Tilting and sinkhole formation


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Causes of Potential Damages:

The common causes of potential damages due to tunnel excavation are:

Weak ground

Uncontrolled groundwater

Wrong selection of TBM

Varying geological conditions

Inexperience workers

Unforeseen buried obstructions

8. Conclusions & Recommendations

This paper illustrates the brief description of soft ground tunneling for MRT in urban areas. For the successful

completion of the project and to minimize the potential problems to the adjacent structures engineering geological,

geotechnical and hydrological investigation is prime important. After proper completion of investigation and sub

surface characterization the soil classification and hydrological conditions must be defined. On the basis of that

method of construction and approach will be adopted. For mechanized tunneling, SPB is preferred to use for coarsegrain soil while EPB can be used for cohesive soil. Variable density TBM is preferred to deploy if the substrata consist

of highly Karstic Limestone underneath overburden to avoid blowout and sinkhole formation at the surface.Forconventional method NATM is not preferred for shallow tunneling in urban areas because it allows deformation to use

Figure 25 Typical Idealized building response (after Attewell 1986)


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the ground strength. Shallow tunneling method (STM) can be adopted for shallow tunneling in urban areas with some

auxiliary measures to control the surface settlement.

As the tunneling for MRT is in highly congested areas so the surface settlement and its monitoring during function are

very important for the safety of surface structure as well as tunnel working itself.

9. References

Shallow tunneling method (STM) for subway station construction in soft ground (Tunneling and Underground Space

Technology 29 (2012) 1030)Qian Fang, Dingli Zhang, Louis Ngai Yuen Wong

Ground movements in EPB shield tunneling of Bangkok subway project and impacts on adjacent buildings (Tunnelingand Underground Space Technology 30 (2012) 1024)

A. Sirivachiraporn , N. Phienwej

Mechanized tunneling in urban areas Design methodology and construction control Engineering geological

investigations of mechanized tunneling in soft ground: A case study, EastWest lot of line 7, Tehran Metro, Iran(Engineering Geology 166 (2013) 170185)

Sadegh Tarigh Azali, Mohammad Ghafoori , Gholam Reza Lashkaripour, Jafar Hassanpour

Predicted and measured tunnel face behaviour during shield tunneling in soft ground (Tunneling and Underground

Space Technology 21 (2006) 264)

S.H. Kim, G.H. Jeong, J.S. Kim

Geotechnical Aspects of the Bangkok MRT Blue Line project by Chanaton Surarak (Phd Thesis)

ITA/AITES Report 2006 on Settlements induced by tunneling in Soft Ground (Tunneling and Underground SpaceTechnology 22 (2007) 119149)

http://www.mymrt.com.my/en/underground-works (MRT project in Kuala Lumpur Karstic limestone)

http://www.tunneltalk.com/Kuala-Lumpur-MRT-Nov12-Worlds-first-Variable-Density-TBM-ready-for-Klang-Valley.php

Some experience from the soft ground tunneling in urban area

Seung-Ryull Kim, ESCO Consultant & Engineers Company Ltd Seoul, Korea

Documents

Soft Ground Tunneling for MRT in Urban Areas