Design of TBM Work Shaft and TBM Launching Chamber Singapore

8/13/2019 Design of TBM Work Shaft and TBM Launching Chamber Singapore

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Design of TBM Work Shaft and TBM Launching Chamber

O. Sigl, B. Stacherl

Geoconsult Asia Singapore, Singapore

ABSTRACT

Contract T03 of the Singapore deep tunnel sewerage system (DTSS) comprises a 6.0 m finished ID

main sewer tunnel, a number of access shafts, lateral connection tunnels as well as structures on the

surface. The main sewer tunnel was constructed by means of an earth pressure balanced shield tunnel

boring machine (EPB TBM). The TBM was launched from within a 12 m diameter, 32 m deep workshaft from a 7.6 m diameter launching chamber constructed at the bottom of the work shaft. The

launch chamber was excavated based on the principles of NATM at the bottom of the work shaft and

supported by sprayed concrete lining. The paper is focusing on the considerations in relation to the

design and construction of the large diameter TBM work shaft and the TBM launching chamber and

the successful operation of a system of groundwater re-charge wells. The design and selection of

construction method for the work shaft were significantly influenced by the proximity of the works to

an existing MRT depot and a highway interchange with depressed road, which was concurrently under

construction at the time of shaft construction. Owing to the presence of the MRT test track in the depot

area, stringent performance requirements were established with regards to construction methods and

acceptable ground settlements, requiring groundwater recharging and ground treatment in the vicinity

of the shaft. In addition, in its permanent condition the shaft is forming a junction of three main sewer

tunnels of the DTSS. Therefore the lower section of the shaft was designed considering significant

amount of space consumed by tunnel openings.

1. INTRODUCTION

Contractor Kumagai Gumi SembCorp Joint Venture chose to drive the main tunnel by a 7.16 m

diameter earth pressure balanced Kawasaki shield TBM, supported by a pre-cast concrete segmental

lining. In early 2001, the Kawasaki earth pressure balanced shield machine was launched from the

7.6m ID TBM launching chamber to complete the 5.2km long main sewer tunnel. The launch chamber

was constructed at the bottom of the about 34m deep work shaft by means of NATM (New Austrian

Tunneling Method) using sequential excavation and temporary sprayed concrete lining. Since the twoneighbor DTSS contractors will drive their TBMs into the work shaft. Together with the backshunt

tunnel for TBM operation and the de-aeration chamber, the five tunnels are forming a complicated

structure where most of the shaft lining is consumed by the tunnel openings. The bored tunnel was

operated and supplied through the work shaft and was successfully completed in December 2002.

The design and construction of the work shaft and all adjacent underground structures were apart

from the locally very poor ground conditions mainly dictated by the close proximity of the works to

the Braddell MRT Depot and the construction works for the extension of the depressed road of the

CTE Braddell Road Interchange ongoing concurrently with the works for DTSS Contract T-03. The

depressed road underpass was crossing above the DTSS Tunnel immediately in front of the TBM

launching chamber. For layout of structures around the work shaft refer to Figures 1 and 2 below.

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The major limitations imposed by the presence of the MRT depot were stringent requirements with

regard to allowable groundwater lowering, in the vicinity of the works and ground treatment for shaft

construction, which are described in more detail below.

2. GEOLOGICAL CONDITIONS

At the surface the site is generally covered with a fill material of clayey silt texture with some sandand gravel. Soil investigation carried out at the work shaft showed the presence of two mainformations, a recent alluvial deposit commonly termed as the Kallang formation overlying the OldAlluvium. The various members of the Kallang formation encountered include estuarine deposits(peaty clay) present at near the ground surface with extremely high water content.

Figure 1 Structures at the work & access shaft Plan layout with measured settlement isolines

These layers are overlying the upper and the lower marine clay, all of which are very soft to soft clays.

Embedded in between are layers of sand (F1) and sandy clay (F2). The soil types encountered at theshaft are shown in Figure 2 and typical design parameters described in Table 1 below. Old Alluvium

below the Kallang formation is generally characterised by an intermediate layer of about 1.5m to 6m

thickness of highly weathered to completely weathered material of the Old Alluvium (medium dense

sandy clay) and dense to very dense lightly cemented material with SPT N 300values of 80 or greater.

The 34 m deep work shaft had to be excavated through all of the above soil layers with knowledge of a

distinctive local history of consolidation settlements due to presence of compressible soil layers in

particular the estuarine and marine clay. Based on these conditions, two different principal initial

support systems have been chosen for the excavation of the work shaft. One system was applied for

the soft soils of the Kallang formation in the upper part of the shaft and one for the more competent

layers of the Old Alluvium in the lower section.

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The main focus when choosing the support system in the Kallang formation was on control of ground

movements, water leakage and consolidation settlements. The F1 sand and the highly weathered upper

layers of the Old Alluvium (HWOA) are known to have potentially high groundwater permeability.

Such considerations played a significant role in the design of ground treatment measures to prevent

soil erosion and consolidation problems during excavation in this highly sensitive area. Therefore,

sheet piles were chosen to provide water tight ground support for this upper part of the shaft.

Figure 2 Section through the work shaft and depressed road

The TBM launching chamber had to be excavated at the lower shaft section in Old Alluvium with avertical cover of approximately 2 m to the slightly weathered, and about 5 m to the highly weathered

layers of the Old Alluvium. Previous experience from tunnelling in Old Alluvium along with the

interpretation of soil investigation results in the vicinity of the work shaft indicated that NATM using

sequential excavation and sprayed concrete linings was the most suitable and economic tunnelling

method for the lower shaft section and the TBM launching chamber in the ground conditions expected

to be encountered.

Permeability tests (falling and rising head tests) were conducted in the area of the shaft at various

depths for all soil layers encountered. The coefficients of permeability obtained from the tests were in

the range of 10-6

to 10-7

m/s for the F1 material and the sandy layers of HWOA and only

approximately 510-9m/s for the more clayey HWOA. Both of those values were lower than expected.

The values for Old Alluvium were between 10-8

and 610-8

m/s.

Table 1 Soil Types and Typical Geotechnical Design ParametersTypical depth

[m]Geotechnical unit USC Symbol Average

SPT N300Youngs modulus

Ec [MPa]

0.0 1.5 Fill (A) 5 4.31.5 5.0 Estuarine clay (E) OH 1 1.85.0 8.0 Marine clay (M) CH 1 2.2

8.0 10.0 Fluvial clay (F2) CH/CL 5 4.310.0 13.0 Marine clay (M) CH 1 2.213.0 15.5 Fluvial sand (F1) SM/SC 5 5.015.5 18.5 HWOA highly weathered) SM 10 8.718.5 21.5 WOA (weathered) SM/SC 24 21

21.5 27.5 OA SM >80 69> 27.5 OA (cemented) MH >100 87

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3. WORK & ACCESS Shaft

3.1 General

Due to the proximity of the shaft works to a MRT test track, a number of limitations and requirements

have been imposed on the works and were incorporated in the contract documents. These requirements

related to the limitation of displacements for the test track and had to be seen in conjunction with the

history of consolidation settlement in the area of the shaft. The works to be carried out were located

within the MRT reserve.

The main requirements and limitations for the design and construction of the structures to be built at

the shaft were as follows:

! No significant lowering of the groundwater table or reduction of pore pressure at depth

! Mandatory ground treatment to limit wall deflections in all soil layers

! Mandatory ground water recharge well system

! Water leakage into shaft to be very small for both initial support and final lining

! Limitation of displacements of the MRT test track to the rail intervention levels. See Table 2! Limitation of vibrations caused by construction equipment to 15 mm/s peak particle velocity

3.2 Initial Shaft Support

Two different support systems were adopted for the upper and lower section of the 32 m deep shaft,

namely:

! Upper section (022.5 m depth): Sheet piles and steel walers in combination with jet grouting

! Lower section (>22.5 m depth): Shotcrete and wire mesh

The initial support in the soft Kallang formation soil layers of the upper shaft section consisted of 23m

long sheet piles arranged in a circle of approx. 12m diameter and steel walers at about 2.5m vertical

spacing. The sheet piles were installed using silent piling equipment (hydraulic jacking system) in

order to limit vibration effect on adjacent MRT structures to allowable levels. In addition, high

pressure water jets at the sheet pile toe were used to facilitate driving in the more competent layers of

weathered (WOA) and lightly cemented Old Alluvium (OA). By that it was possible to install the

sheet pile into soil layers with SPT N300values of about 60.

In addition to the sheet pile wall, jet grouting was used in the soil layers of the Kallang formation and

HWOA down to a depth of 21 m below ground level. The purpose of the jet grouting treatment was to

increase the wall stiffness for reduction of wall displacements and reduction of permeability in the

surrounding soil to cut off seepage into the excavated shaft. The jet grouting zone was formed by two

intersecting rows of jet grouting columns arranged at the outside of the sheet pile wall. The design

diameter of the columns to be achieved in the Kallang formation was 1.5 m, thus forming a 2.5 mthick ring of jet grouted soil around the sheet pile wall.

Table 2. Displacement Limits for MRT Test TrackCriterion ValueMax. settlement 15mmTwist 1:1000Increase in rail gauge +15 mmDecrease in rail gauge -2 mmLateral displacement 14 mmLongitudinal level 1:2500

The ground treatment works were performed using a triple tube jet grouting system. A comprehensive

testing programme for the treated ground was established in order to verify compliance with thedesign requirements.

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The requirements set by the design for soil treated by jet grouting were as follows:

! Unconfined compressive strength (UCS): 500 kPa

! Undrained Youngs modulus: 90 MPa (initial loading) and 220 MPa (un/re-loading)

! Coefficient of permeability: kf#1$10-8

m/s

Testing program included determination of density, moisture content, unconfined compressive

strength, stress/strain relationship and included packer tests to determine the in-situ permeability of the

treated ground. The requirements were achieved with comfortable margins.

After the test program confirmed the success of the ground treatment measures, excavation within the

circular sheet pile wall was carried out in stages dictated by the spacing of the walers. Excavation near

to the toe of the sheet piles was carried out in 1 m stages with concurrent installation of stiffening

rings of reinforced shotcrete installed directly against the sheet pile wall. The sheet pile toe was

supported by a 1.35 m wide cast in-situ concrete footing, to provide stability during the critical stage

of excavation right underneath the sheet pile toe.

Owing to the number of openings at the bottom of the shaft, a thorough study of the most feasibleground support system was carried out. Originally, the shaft bottom initial support was intended to be

supported by reinforced cast in-situ concrete. However, owing to the complicated geometrical

conditions related to the intersection of many tunnels of different size, the use of shotcrete was found

to be more feasible. Since the shotcrete lining offered savings and speeding up of the construction

program, it was chosen to provide ground support for the lower shaft section. Therefore the initial

ground support of the lower shaft portion consisted of shotcrete reinforced with wire mesh with

additional rebars where required. The presence of the five large openings at the shaft bottom required

a 1m thick stiffening ring of cast in-situ reinforced concrete installed in the shaft above the openings,

(see Figure 3). Excavation of the lower shaft portion was carried out in stages of 1.0 1.3 m depth

with concurrent installation of shotcrete and wire mesh.

Figure 3. Initial support, upper and lower shaft section with ring beam

Soft eyes with a reduced shotcrete thickness were provided in the regions of future openings, which

are theTBM launch chamber, the backshunt tunnel, the de-aeration chamber and the openings for

TBM break-in of two adjacent contracts (T-04 and T-06).

The backshunt tunnel opposite of the TBM launch chamber was constructed due to requirementsrelated to the marshalling of muck cars during TBM operation and was backfilled at a later stage

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before starting excavation of the de-aeration chamber. The shotcrete lining around the openings was

reinforced by rings of increased shotcrete thickness (up to 1m) with increased rebar reinforcement.

Until the arrival of the TBMs of the adjacent contracts there were only two openings in the shaft lining

at a time. Once the other two TBMs have broken in, the shaft will accommodate a total of four

openings at the bottom of the shaft ranging from about 2.5 to 7.7 m in diameter (see Figure 2).

3.3 Recharge Well System

The contract documents specified mandatory installation of a recharge well system around the work

shaft in order to control and recover any unintended groundwater drawdown due to shaft and tunnel

excavation. The layout of the recharge system was designed based on the results of the in-situ

permeability tests carried out as part of the additional soil investigation. A total number of 11 wells

were installed around the shaft and along the site boundary to the MRT test track area. The recharge

wells were 300 mm in diameter and approximately 23 m deep. The screened section of the wells

extended from 12 m depth all the way down into the firm Old Alluvium, but effective recharging was

considered to take place only in the F1 and HWOA layers. After installation of the recharge wells,

pumping tests were conducted in order to verify the effectiveness of the system, to confirm the designassumptions and the need for installation of additional wells prior to shaft excavation.

Monitoring of adjacent wells, water standpipes and piezometers was carried out during the tests and

crucial parameters back-calculated such as permeability, well efficiency and the radius of influence of

each well. The pumping tests showed that the recharging capacity of the system was adequate to

recover short-term groundwater drawdown and to avoid propagation of such drawdown towards the

MRT test track. The recharging capacity of the system was estimated at approximately 4 l/min based

on a recharging water head of 1 m, which set the limits for acceptable leakage into the excavation.

Figure 4. Pore water pressure measurement results

Operation of the recharge well system during shaft and tunnel excavation confirmed the recharging

capacity and showed that the system is capable to recover drawdown that occurred during the works.Figure 4 is showing the distinctive drop in porewater pressures after switching off the recharge well

101

102

103

9/4/01 16/4/01 23/4/01 30/4/01 7/5/01

Date

WaterLevel(m,

ReducedLevel)

Recharge WellsNOT in Operation

during this period

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system for about three days and subsequent recovery when the system was in operation again.

Referring to Figure 1, the maximum settlements monitored at the MRT test track were in the order of

10mm.

3.5 Design Approach for Initial Ground Support

The main objectives for the design of the initial shaft support were minimisation of displacements and

control of seepage and consolidation. The design paid particular attention to these key factors. The

initial support design has been based on the results of numerical analyses performed for each, the

upper section where sheet piles were used as well as the lower section with sprayed concrete lining.

The analysis for the upper section was carried out based on a two-dimensional finite element model.

The model was based on plain strain conditions and considered the fact that actual construction

tolerances along with other factors may result in considerable deviation from an ideal axisymmetric

state of stresses. All excavation stages, sheet piles, walers and ground treatment were incorporated in

the model. The structural design of the polygonal walers was based on a separate plane frame analysis.

The finite element analysis was also used to investigate consolidation effects due to shaft excavation.Surface settlements due to excavation of the upper shaft portion were directly obtained from the finite

element analysis and were the basis for a detailed construction impact assessment.

The lower section of the shaft was designed with the help of a three-dimensional finite difference

analysis model using the FLAC3D programme, modelling the creation of openings in the shotcrete

lining in accordance with construction stages and associated tunnel excavations, (See Figure 6). The

model also incorporated volume elements to model the surrounding ground, using a simple elastic

constitutive law. The main focus in this analysis was on the structural effects of the openings and

associated tunnelling activities upon the initial lining of the shaft.

Figure 5. Section through FLAC3D Figure 6. Soft eyes in the lower

analysis model shaft section

4. TBM Launch Chamber

4.1 Excavation and Initial Support

It is one of the main principles of the NATM to tolerate a certain level of ground displacements,

thereby utilizing the contribution of the ground to the support of the excavated cavity. However, giventhe stringent displacement limits of the MRT test track, minimization of displacements was one of the

Backshunt

T06

T04Backshunt

T06

Launch

Chamber

T04

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major objectives for the initial support design. The Old Alluvium is considered very suitable groundfor NATM tunnel construction, provided that excavation sequencing is carried out extremely carefully,since the soil is very sensitive to excessive shear strain and prolonged exposure prior to installation ofsupport, resulting in significant strength decrease. The about 8.5m diameter excavation was carried outin three separate stages (top heading, bench and invert), which followed each other at closest possible

distance in order to minimize ground movements.

Figure 7. TBM launch chamber, excavated bench with wire mesh laps

Checking face stability conditions was an integral part of the design of excavation sequence and sub-division of headings and was carried out for drained and undrained conditions. Since the tunnels are

below the groundwater table, seepage pressures may cause instability of the working face due toreduction of shear strength by pore water pressures. Therefore, drainage drillings were carried out andmaintained ahead of the face by at least 3 m at any one time to reduce pore pressures.

Support elements consisted of sprayed concrete, applied immediately after excavation, wire meshreinforcement and lattice girders. The lattice girders were mainly used as profile control, but were alsoin place if depending on ground conditions forepoling would be required. Based on the structuralanalysis results a 350 mm thick shotcrete lining with wire mesh (T8 at 150 mm centers) was requiredfor the large TBM launching chamber.

5. CONCLUSIONS

Both, the work shaft as well as the TBM launch chamber have been completed successfully inSingapore Old Alluvium.

The following particular methods have been employed successfully:

! Jet grouting to control sheet pile wall deflections in advance of the shaft excavation.

! Ground water recharge well system to control and reverse porewater pressure drops due to

excavation and eventually successfully control and limit short-term as well as long-term

settlements.

! Sequentially installed reinforced sprayed concrete lining for a large diameter TBM work shaft and

TBM launching chamber in Singapore Old Alluvium resulting in only relatively small ground

surface settlements.

! Use of reinforced sprayed concrete to provide reinforcement around tunnel eyes, where the

geometry of intersection allows only very limited space for support structures between the tunnel

openings.

! Owing to selective groundwater recharging, focussed ground treatment and thorough excavation

sequencing, maximum ground surface settlements around the shaft were in the range of 40 to45mm and about 10mm at the MRT test track.

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Documents

Design of TBM Work Shaft and TBM Launching Chamber Singapore