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Predicted and Observed Ground Movements around a Tunnel Boring

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Predicted and Observed Ground Movements around a Tunnel Boring

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									Predicted and Observed Ground Movements around a Tunnel
Boring Machine at Heathrow Airport

C. Pound, Y. S. Hsu and G R Walker
Mott MacDonald Limited, London, United Kingdom

Abstract

The Airside Road Tunnel (ART) at Heathrow Airport comprises twin 8.1m diameter tunnels
constructed by TBM in London Clay at shallow depth beneath taxiways and above an existing
rail tunnel. To assess the effect of construction of the ART on the Heathrow Express (HEX)
rail tunnel, a series of three-dimensional numerical analyses were carried out using the finite
difference program FLAC3D. The analyses adopted a non-linear model to replicate the small
strain stiffness of the over-consolidated soil stratum at tunnel horizon. Three construction
methods were considered, comprising open face, compressed air and earth pressure balance
tunnelling. The analyses demonstrated the significant contribution of closure of the ground
around the tunnel shield on the overall ground movements caused by the tunnelling and the
benefit of the use of ground support pressures in reducing the total magnitude of ground
movements. The analyses predicted that the loads and deformations induced in the
underlying HEX tunnel would be small whatever construction method was adopted and that
they would be within operating tolerances for the railway.

Instrumentation installed in the early part of the ART showed that heave was occurring at the
ground surface ahead and over the tunnel face. The analyses were rerun to investigate this
effect and the movements of the HEX tunnel were again predicted to be small. Monitoring of
the HEX tunnel as the ART TBM passed above showed small movements and no damage or
distress was caused to the tunnel or unacceptable movement of the rail tracks. The
monitoring did indicate that the tunnel settled as the TBM passed and this could not be
replicated in the numerical modelling.

Introduction

The Airside Road Tunnel (ART) at Heathrow Airport will provide an underground road link
between the Central Terminal Area and aircraft stands to the west of the airport and at a later
date with the new Terminal 5. The ART is 1265m long and comprises twin 8.1m internal
diameter bored tunnels with cut and cover portals and access ramps formed in retained cut at
the two ends.

At two locations operational rail tunnels pass beneath the ART. The Piccadilly line passes
beneath the west portal cut and cover structure with a separation of only about 5m between
the tunnel crown and the invert of the portal base slab. Also, at approximately the mid-point
of the ART bored tunnels, the Heathrow Express (HEX) tunnel passes beneath the ART with
a clearance of only around 3m between the HEX tunnel crown and the ART tunnel invert.

The tolerance of operating railway lines is such that small track movements can necessitate a
temporary speed restriction to be imposed and a subsequent realignment of the track. This
can have major implications for the rail operator. Consequently, there was significant
concern that construction of the ART could cause unacceptable movement of the existing
tunnels because of the limited tunnel separation. This paper discusses the design study
carried out to confirm the acceptability of tunnelling with such limited cover over the top of
the HEX tunnel and describes the observed behaviour as the first tunnel passed.
Ground Conditions

An extensive site investigation was carried out to determine the ground conditions along the
alignment of the ART and this supplemented the existing database of information on the
geological conditions at Heathrow Airport. Over much of the route, the investigation
identified that Terrace Gravels overlie a considerable thickness of London Clay. However,
the location where the HEX crosses the ART lies at the edge of a backfilled borrow pit where
the Terrace Gravels have been extracted. The geological profile at the location of the
crossing is as given in Table 1.

                                Stratum             Thickness (m)
                          Borrow Pit Materials /
                                                          4.3
                            Terrace Gravels
                              London Clay               55.4
                            Lambeth Group             Undefined

                      Table 1 Geological profile at HEX-ART crossing

The Terrace Gravels comprise a dense to very dense, sandy medium to coarse gravel,
becoming progressively more clayey towards the ground surface. The London Clay is a stiff
to very stiff overconsolidated fissured silty clay with an average undrained shear strength
measured in triaxial tests increasing from about 100 kPa at the London Clay surface to about
350 kPa at a depth of 30m below ground level. The surface of the London Clay is weathered,
but along the route of the ART the depth of weathering is no more than 1m. The Borrow Pit
materials typically comprise slightly sandy, slightly gravelly clay with occasional layers
and/or pockets of silty, sandy gravel. The SPT N values within the deposit vary between 2
and 9. It is believed that the Borrow Pit materials are derived from a mixture of London Clay
and Terrace Gravels.

The water table is located at 2.2m below existing ground level and the water pressures are
assumed to be hydrostatic through the Borrow Pit materials, Terrace Gravels and London
Clay.

Tunnel Geometry

The HEX tunnel was constructed in the mid-1990s using a conventional open face shield. A
220mm thick, 1m wide, 6.115m external diameter, expanded pre-cast concrete segmental
lining comprising nine segments and a key was installed immediately behind the shield.
Review of surface settlements above the HEX tunnel showed that the volume loss was
typically 0.7% during the tunnel drive between the Central Terminal Area and Terminal 4
(Pound et al2). The rails within the HEX tunnel are fixed directly to a continuous concrete
track bed.

The ART was constructed using a 9.18m diameter earth pressure balance tunnel boring
machine. The machine had a 6.5m long main shield and a 3.7m long tail skin. The front of
the shield had a 10mm bead and the shield and tailskin taper by 20mm on radius over their
length. At the back of the tail skin the diameter was therefore 9.12m. The machine could be
operated in open mode, compressed air mode or earth pressure balance mode depending on
requirements. Around the shield bentonite could be injected to minimise ground loss prior to
installation of the lining. A 350mm thick, 1.7m wide, 8.8m external diameter pre-cast
concrete lining, comprising seven segments and a key, was installed within the tailskin.

At the location where the ART crosses the HEX, the HEX tunnel axis is 23.0m below ground
level. At the initial stage of the design the precise vertical alignment of the HEX tunnel was
not fixed and could lie between 12.0m and 12.8m below ground level. The theoretical
separation between the extrados of the two tunnel linings could therefore vary between 3.5m
and 2.7m. It was considered prudent to assess the impact of ART construction on the HEX
tunnel assuming the minimum clearance of 2.7m. The angle between the two tunnel axes is
77º and both tunnel alignments are on horizontal curves.

Design Calculations

Geotechnical Properties

In view of the weaker nature of the Borrow Pit materials compared to the Terrace Gravels it
was considered appropriate to assume in the design that the material overlying the London
Clay was Borrow Pit materials, as this was anticipated to provide an upper bound to the
ground movements.

No sophisticated in situ or laboratory testing was carried out on the Borrow Pit deposits
because of their natural heterogeneity and also because they were not considered to
significantly affect the tunnelling conditions. Therefore the properties were derived from
classification tests, SPT N values and triaxial compression tests. The parameters adopted in
the design calculations are given in Table 2. Since no small-strain stiffness data exists for
these deposits, it has been modelled as a linear elastic perfectly-plastic material with a Mohr
Coulomb yield criterion.

The basic properties of the London Clay are well understood from testing at Heathrow and
elsewhere in London. The non-linear stiffness properties were derived from stress path
triaxial tests complete with local strain measuring devices. The properties adopted for the
London Clay are presented in Table 2.

             Soil Type           γb              Porosity        E’         ν’      c’ φ’ (o)
                              (kN/m3)              (%)         (MPa)              (kPa)
       Borrow Pit Materials     19.5                30           10        0.15     0   21
     London Clay (short term)    20                 50             variable        20   25
     London Clay (long term)     20                 50             variable         8   23

                                     Table 2 Soil parameters

For excavation of both tunnels, the London Clay was modelled as an undrained material with
peak effective stress shear strength properties back-calculated based on the undrained shear
strength profile. A non-linear elastic perfectly-plastic constitutive model was implemented
into FLAC3D based on the method outlined by Jardine et al1. The tangent stiffness
expressions that describe the behaviour are as follows: -
                  3Gtan                   γ    B.α .γ . X γ −1
                        = A + B. cos(α . X ) −                 . sin(α . X γ )
                   p'                             ln 10
                                εs
                  X = log10
                                C
                  K tan                          S .δ .µ .Y µ −1
                        = R + S . cos(δ .Y µ ) −                 . sin(δ .Y µ )
                   p'                                ln 10
                               εv
                  Y = log10
                               T

where Gtan and Ktan are the tangent shear and bulk moduli respectively, p’ is the current mean
effective stress, and εs and εv are the shear and volumetric strains respectively. A, B, C, R, S,
T, α, γ, δ, and µ are constants. The shear strain, εs, is equal to half the octahedral shear strain
invariant.

Up to a specified minimum strain, the stiffness varies only with p’, but thereafter the stiffness
depends both on the current strain, ε, and the mean effective stress. The equations were
implemented using a sub-routine within FLAC3D that continually updated the shear and bulk
tangent moduli throughout the analysis. To prevent unrealistically low shear and bulk moduli
developing during the analysis, a minimum value of p’ of 50kPa was used when calculating
the moduli.

The constants used in the analyses are given in Table 3 and were derived by curve-fitting of
small-strain triaxial test data from the project site investigation and from a review of existing
small-strain data for the London Clay.

                         A       B        C         α       γ       εsmin
                        800     700    3.0x10-6    0.96    1.0    3.1x10-6
                         R       S        T         λ       µ       εvmin
                        150     100    4.9x10-5    2.0     1.0     5.0e-5
                      Table 3 Constants for London Clay stiffness model

The ko profile for the London Clay was based on the results of pore suction measurements
taken on high quality undisturbed samples tested immediately after extraction from the
borehole. The ko profile reduced from 2.5 at the London Clay surface, to 1.5 at 20m below
the London Clay surface and to 1.2 at 40m below the London Clay surface. The ko value
adopted for the Borrow Pit materials was based on 1-sin φ’.

FLAC3D model and modelling sequence

The model developed for the analysis is shown in Figure 1. For greater numerical efficiency
symmetry on a vertical plane through the ART axis has been taken. The model is 100m long
in the direction of the tunnel axis, 50m wide and 60m deep such that the base of the model
represents the top of the Lambeth group. The model width was equivalent to about 4 times the




                                      Figure 1 FLAC3D mesh
depth of the tunnel axis which was considered sufficient to minimise boundary effects. The
model contains a total of nearly 34000 brick elements. The base is fixed against movement in
all directions whilst the vertical boundaries are fixed in the direction perpendicular to the
boundaries only. For simplicity of mesh generation, the HEX tunnel axis was taken to be
perpendicular to the ART axis.

The analysis was initially run to ensure that the initial stresses were in equilibrium and the
resulting small displacements were set to zero. The precise construction sequence for the
HEX tunnel was not considered to be critical and therefore the tunnel was excavated as a
single event. Elements representing the HEX tunnel were first removed and tractions applied
to the HEX tunnel boundary to balance the in-situ stresses. These tractions were gradually
decreased until a surface settlement corresponding to a volume of about 0.7% was achieved.
The HEX tunnel lining was then installed with the properties given in Table 4. The lining
was modelled using triangular shell elements and joints between segments were modelled by
providing pin connections between these elements. Next the remaining tractions were
removed and the model run to equilibrium. Finally, long-term pore pressures around the HEX
tunnel were allowed to develop assuming an impermeable tunnel lining.

The strains were next initialised to reset the small strain stiffness. Excavation of a 70m length
of the ART was analysed, assuming undrained conditions for the London Clay. Excavation of
the ART was modelled in 23 stages by progressively removing elements from within the
tunnel profile. As the length of the TBM shield is about 10m, the ground 10m behind the
tunnel face was allowed to deform with three different supporting conditions, namely open
face, compressed air and earth pressure balance modes. The three different support modes
were modelled as follows:

•   For the open face mode (OPFM), the 10m length behind the tunnel face was excavated
    assuming no ground support.

•   For the compressed air mode (CAM), a constant pressure of 140kPa was applied to the
    excavated surface for 10m length behind the tunnel face. This pressure is approximately
    equal to the groundwater pressure at tunnel axis.

•   For the earth pressure-balance mode (EPBM), a pressure varying linearly from 184kPa in
    the crown to 312kPa in the invert was applied to the excavated surface for the 10m length
    behind the tunnel face. This pressure variation was based on balancing the vertical stress
    at axis and assuming that the remoulded soil provides a variation of pressure given by a
    unit weight of 14 kN/m3.

For all tunnelling modes the lining elements were then installed 10m behind the tunnel face.
The TBM was advanced in stages of 1.7m representing the length of the segmental lining.
The ART lining was also modelled using triangular shell elements with properties given in
Table 4, which are considered to be realistic for the concrete lining.

                 Lining type      Thickness     E (GPa)     Poisson’s      Unit
                                    (mm)                      ratio       weight
                                                                         (kN/m3)
                HEX tunnel           220         25       0.15              23
                ART lining           350         25       0.15              23
                               Table 4 Concrete Lining Properties

Figure 2 shows the predicted surface settlement along the ART centreline relative to the ART
face position for the three construction modes. As expected, due to the 10m unsupported
length for the OPFM, the maximum ground settlement is 250mm, which occurs about 30m
behind the tunnel face. For the CAM, the maximum ground settlement is 30mm occurring
30m behind the tunnel face. For the          50


EPBM, the ground settles slightly in          0

front of the tunnel face and heaves to




                                                                                                                                  Vertical displacement (mm)
                                            -50

a maximum of about 20mm 20m
                                           -100
behind the tunnel face. Based on
these displacements, the estimated         -150


volume loss of the open face               -200

(OPFM), compressed air (CAM) and           -250
                                                                                      Open face mode
                                                                                      Compressed air mode
earth pressure balancing (EPBM)                                                       Earth Pressure Balance mode
                                           -300
modes is 15.5%, 3.1% and 0.25%                  -40 -30 -20 -10        0       10        20       30        40    50

respectively. The deflections for the                          Distance from ART face (m)


OPFM       and     CAM      obviously Figure 2 Surface settlement on ART centre-line.
overestimate the actual ground
movements as the analysis ignores the presence of the shield which would limit the actual
ground convergence to about 30mm around the tunnel profile. A uniform inward movement
of the excavated surface of 30mm would represent a volume loss of about 1.3%.

Figure 3a shows the vertical displacement of the HEX tunnel crown at the point below the
centre-line of the ART relative to the ART face position. For the OPFM, the HEX crown
initially settles as the ART tunnel face approaches the crossover point and then starts to heave
when the ART face passes with a maximum heave of 21mm occurring 5m behind the ART
tunnel face. For both the CAM and EPBM, there is only slight crown movement as the ART
tunnel approaches but after the ART face has passed, crown heave occurs. For the CAM, a
maximum HEX crown heave of 9mm occurs 7m behind the ART face. For the EPBM, a
maximum HEX crown heave of 2mm occurs about 17m behind the ART face. It is interesting
to note that although the OPFM and CAM analyses predicted very large surface settlements,
the predicted ground movements below the tunnel were significantly smaller and these
analyses were therefore useful in providing an upper bound assessment of the movement of
the HEX tunnel.

Figure 3b shows the HEX tunnel crown horizontal displacement beneath the ART centre-line
plotted relative to the ART face position. Positive values indicate that the displacement is in
the same direction as the ART tunnel drive. For all modes, the crown moves towards the ART
face as the ART approaches and when the ART passes the centre-line of the HEX, the crown
starts to move in the direction of the ART drive.

A review of the predicted movements at track level, allowing for the effect of the tunnel
shield, suggested that the track would undergo heave of between 2 and 5mm depending on the
tunnelling method employed.

                                            25                                                                                                                                                               15
                                                                                                                                                               Horizontal displacement of HEX crown (mm)..
Vertical displacement of HEX crown (mm)..




                                                       Open face mode                                                                                                                                                    Open face mode
                                                       Compressed Air mode                                                                                                                                               Compressed Air mode
                                            20
                                                       Earth Pressure Balance mode
                                                                                                                                                                                                             10          Earth Pressure Balance mode
                                            15
                                                                                                                                                                                                              5
                                            10

                                             5                                                                                                                                                                0

                                             0
                                                                                                                                                                                                              -5
                                            -5
                                                                                                                                                                                                             -10
                                       -10

                                       -15                                                                                                                                                                   -15
                                                 -25   -20       -15     -10      -5      0        5      10      15         20             25                                                                     -25   -20    -15      -10     -5     0       5       10      15         20   25
                                                             ART face position relative to the HEX-ART crossover point (m)                                                                                                 ART face position relative to the HEX-ART crossover point (m)

                                            a)                                                                                                                                                                     b)
                                                                  Figure 3 Vertical and horizontal movement of the HEX tunnel
Instrumentation Layout and Monitoring System

Instrumentation installed above and around ART

The ART drive was monitored for surface settlement from the launch of the TBM. Surface
monitoring typically comprised manual precise levelling in front of and over the TBM
position as well as on the completed bore behind the TBM. Due to the presence of extensive
airfield pavement areas, much of the surface monitoring utilised 1.2 m deep monitoring pins
that allowed comparison between the pavement settlement and the movement of the
underlying ground. Typically the monitoring points were set out either in perpendicular
arrays spaced every 100 m, extending 25 m either side of the centreline or as points along the
centreline spaced every 10 m to 25 m depending on the sensitivity of the locality to
settlement. Monitoring intervals varied depending on accessibility of the TBM position, but
in general was between 12 and 24 hours.
Where considered appropriate the precise levelling has been augmented by the use of manual
or real time monitoring techniques including a robotic total station, in-place electrolevel
inclinometer and rod-extensometers.

In the vicinity of the HEX Tunnel, surface monitoring comprised manual levelling and
manually read extensometers and inclinometers.

Instrumentation installed in the HEX

Monitoring of the HEX tunnel was undertaken to provide Heathrow Express Ltd with
assurance that the track and tunnel remained within operational tolerances permitting the safe
transit of passenger carrying trains whilst the TBM continued to work above. To provide the
robustness necessary for a safety critical system, primary and secondary real time methods
were used. The remote data was confirmed by manual observations taken during Engineering
Hours (hours during which the trains were not operating), which provided a tertiary
monitoring system.

The primary monitoring system used in the HEX tunnel comprised an optical technique using
a high specification fully automatic total station instrument. A secondary real time system
was designed using discretely placed electrolevel tiltmeters. This allowed a remote
assessment to be made of changes in track cant and track gradient and thus, could be used to
confirm the movements observed using the primary system.

Manual observation of tunnel deformation formed the tertiary system and was carried out by
precise levelling on levelling studs, gauge and cant surveys using a standard track gauge,
together with alignment and versine surveys carried out using standard total station
techniques. The manual data concentrated on the track-bed as the size of the HEX tunnel and
the presence of the OHLE significantly increased the risk of using conventional steel tape
extensometers or inverted levelling off the crown. Again the manual survey was used to
confirm the data recorded by the robotic total station instruments, but was also the preferred
method for assessing changes in rail cant and gauge as it was not considered possible to affix
a target prism directly to the rails themselves.
                           MP


                                               MP             MP – Mini-Prism Target
       MP                                                     Lvl – Level Target
                                                              TM – Tiltmeter
                                                              RTS – Robotic Total Station
                        OHLE Catenary
             RTS        MP

MP                                      RTS           MP




  MP                TM
                                                    Lvl

       Lvl
                                          MP
                                                           Figure 4 HEX tunnel instrumentation
                   MP

The monitoring scheme required the installation of some 112 mini-prism targets, 21
electrolevel tiltmeters and 40 manual levelling points (as the ART comprised two separate
tunnels drives, the initial deployment of instruments was designed to accommodate, as far as
possible, both crossings; the second drive requiring only a small number of prisms to be
relocated). HEX identified that the structural envelope was most critical at the edge of the
emergency walkway and that movement of the overhead line electrification (OHLE) catenary
supports would also be critical. Consequently, a mini-prism was located on the walkway edge
on full arrays to give an assessment of the change in clearance of the structural envelope and a
prism was bolted to the catenary support arm. A diagrammatic representation of a full array
is shown in Figure 4; the robotic total stations are shown to indicate their position within the
tunnel circumference. The arrays were spaced nominally at between 3 m and 5 m intervals
along the HEX tunnel.

Construction

Optimisation of the tunnel alignment during the design period meant that the final design axis
level of the ART at the point where it crossed the HEX was 12.0m below ground level
resulting in a theoretical clearance of 3.5m between the two tunnels.

Excavation of the ART tunnel commenced on 21st June 2002. The machine was generally
operated with air pressure applied at the face and with bentonite injected around the shield.
Prior to the TBM entering the zone of influence of the HEX, defined as a zone 40m either
side of the HEX tunnel centre-line, the preventative maintenance of the TBM was carried out
to ensure that tunnelling over the HEX tunnel could be carried out as a continuous operation.
The ART crossed the centre-line of the HEX on 18 October 2002. Within the zone of
influence the average rate of progress was 18.9m/day.

Pressures were measured continuously in the TBM. The thrust applied by the TBM averaged
between 12MN and 14MN during the period while the TBM was in the zone of influence.
The face air pressure and bentonite pressure was maintained at 50 kPa and 100 kPa
respectively during this same period. The grout pressure recorded at the tunnel crown varied
between 180 kPa and 270 kPa, although as the HEX was crossed the crown grout pressure
was maintained at about 200 kPa.

The face pressure of 50 kPa if applied over the whole face area of 66.2 m2 represents a thrust
of only 3.3 MN. Thus the TBM applied an additional force of about 10 MN directly to the
ground through the cutterhead.
The weight of the TBM in full operating mode is approximately 853 tonnes whereas the
weight of ground of equivalent volume to the TBM is about 1300 tonnes and therefore the
TBM is lighter than the ground excavated. Although the cutter head provided a significant
proportion of the overall weight, the screw conveyor is cantilevered off the back of the
machine and therefore it is considered that the weight of the TBM is relatively evenly
distributed. Assuming that the TBM is in contact with the ground over the lower quarter of
the tunnel circumference, the bearing pressure exerted by the TBM is only 130 kPa which is
significantly lower than the original vertical total stress at tunnel invert level of about 330 kPa
existing prior to tunnelling.

Instrumentation Results

Approximately 400m before the location of the HEX-ART crossing a full monitoring section
was installed comprising surface settlement pins, inclinometers and magnetic extensometers.
The settlement of a point above the centre-line of the tunnel is shown in Figure 5, with
positive movements indicating heave. The point was monitored using an automated
theodolite and therefore continuous readings were obtained as the TBM passed. The data
shows that very little ground surface movement occurred until the tunnel was about 5m from
the monitoring point. Thereafter the surface gradually heaved reaching a maximum of about
9mm approximately 8m behind the tunnel face. Then the direction of movement reversed and
settlement occurred reaching about 7.5mm about 20m behind the face. Other manually
surveyed surface settlement pins located above and to the side of the tunnel showed a similar
pattern with the maximum surface heave of between 15 and 20mm. Pins to the side of the
tunnel alignment showed a comparatively smaller initial heave and final settlement. Those
pins showing the largest initial heave showed a net heave after the TBM had passed.

The maximum horizontal movement of an inclinometer located 6m from the tunnel centre-
line or approximately 1.5m from the tunnel sidewall is shown in Figure 6. Positive values
indicate movement away from the tunnel.         12.5
The inclinometer shows very little                10

movement ahead of the TBM, but as the
                                                               Vertical displacements (mm)..




                                                 7.5

TBM passes the ground moves away from              5

                                                 2.5
the tunnel by up to about 7mm and then             0
after the shield has passed, the ground         -2.5
moves back towards the tunnel resulting in a      -5

net inward movement of about 2.5mm. The         -7.5

point of maximum lateral movement                -10

                                               -12.5
occurred approximately 2m above the tunnel           -20 -10          0         10           20        30          40 50
axis.                                                        Monitoring Point Position relative to Tunnel Face (m)



                                                                                                Figure 5 Vertical displacement above
The movements recorded in these
                                                                                                ART centre-line
instruments suggested that the thrust against
the tunnel face approximately balanced the                                                     8

ground stresses so that very little ground                                                     6
                                                             Horizontal movement (mm)..




movement ahead of the face occurred. The
                                                                                               4
heave and horizontal movement away from
the tunnel suggested that the bentonite                                                        2

pressure around the tunnel exceeded the
                                                                                               0
vertical and horizontal ground stresses. The
net settlement above the tunnel and the                                                        -2

inward movement observed in the                                                                -4
inclinometer after the shield had passed                                                            -2      0       2           4           6           8        10   12
                                                                                                                Inclinometer position relative to ART face (m)
suggested that the grouting allowed some
ground relaxation around the segmental                                                                   Figure 6 Horizontal movement of
lining.                                                                                                  inclinometer
The vertical deflection of the HEX tunnel crown as the ART TBM passed above is shown in
Figure 7a. Also shown in this figure is the position of the ART tunnel face relative to the
HEX tunnel centre-line. The data indicates that there was an initial upward movement of the
HEX crown when the TBM was just 3 m away from the HEX centreline. However the
direction of movement was reversed when the TBM was 2 m past the HEX centreline and the
maximum crown settlement of 4.7 mm occurred when the TBM was some 6.5 m beyond the
centreline of the HEX. This settlement recovered rapidly and when the TBM face was 12m
beyond the HEX centre-line the HEX crown settlement was less than 0.5mm. Movement of
the HEX crown immediately after the crossing indicated a second phase of crown settlement
of up to 0.7 mm. This is considered to be due to loading from the TBM back-up within the
ART. As the ART progressed, this movement recovered rapidly. There has subsequently
been a gradual long-term heave of the HEX tunnel crown of around 0.8mm.

Horizontal displacement of the HEX crown also occurred during the crossing as shown in
Figure 7b. The monitoring points 25 m north and south of the ART crossing data have been
taken as base readings when plotting the data. Profiles are shown corresponding to ART face
distances to or from the HEX centreline. First movements occurred when the ART face was
around 1 m of the HEX centreline position and rapidly developed to a maximum value of
1.6 mm in the direction of tunnelling when the TBM was passed the HEX centreline by
2.3 m. Thereafter, the displacement reversed and at 9 m past the centreline position a
maximum movement of 1 mm in the opposite direction had occurred. As the TBM
progressed beyond the influence zone, the crown prisms recovered slowly and returned to
within 0.2 mm of their original positions.

Tunnel convergence confirmed the nature of the displacement experienced by the HEX
tunnel, and indicated an initial phase of deformation that commenced when the ART TBM
was still some distance from the HEX tunnel centreline. As the TBM approached there was a
small amount of ovalisation in the HEX tunnel with about a 0.5 mm increase in vertical
diameter and a 0.5 mm decrease in horizontal diameter. As the TBM passed directly over the
HEX tunnel, the vertical diameter reduced rapidly by some 5 mm, at the same time the
horizontal diameter increased some 3 mm. Both these responses recovered once the shield
had cleared the HEX tunnel.

The movement of the HEX track-bed during construction of the ART was considerably less
than the crown movements and were considered to be very minor with maximum vertical
displacements not exceeding 1.5 mm and horizontal displacements showing no discernable
trends. The vertical displacements described above were confirmed by measurements taken
using the electrolevel tiltmeters, both systems agreeing to within 0.5 mm.


                               3                                       40                                                                      30
                                                                                                                                                                                              Initial - 10 Oct 02
                                                                             TBM Progress (m +/- HEX CL).




                                        Settlement data
Vertical displacement (mm)..




                                                                                                                                                                                              -4.0m
                               2        HEX centre-line                30
                                                                                                            Offset from ART centre-line (m).




                                                                                                                                               20                                             -0.8m
                                        TBM Progress                                                                                                                                          +2.3m
                               1                                       20                                                                                                                     +5.5m
                                                                                                                                               10                                             +8.7m
                               0                                       10                                                                                                                     +11.9m
                                                                                                                                                                                              +15.1m
                               -1                                      0
                                                                                                                                                0                                             +24.6m
                               -2                                      -10
                               -3                                      -20                                                                     -10

                               -4                                      -30
                                                                                                                                               -20
                               -5                                      -40
                               17-Oct           18-Oct    19-Oct   20-Oct                                                                      -30
                                                                                                                                                     -2   -1         0              1         2                     3
                                                                                                                                                               Horizontal displacement (mm)
                               a)                                                                                                               b)

                                        Figure 7 Vertical and horizontal displacement of HEX tunnel crown
Discussion of observed ground movements

The small ground movements ahead of the tunnel suggest that the pressure applied to the
tunnel face through a combination of air pressure and direct thrust through the cutter-head
was approximately equal to the in situ horizontal stress. The ground surface heave over the
TBM shield suggested that the bentonite pressure exceeded the vertical ground stress in the
crown of the tunnel and the horizontal movement of the ground away from the tunnel at axis
level again suggested that the bentonite pressure exceeded the horizontal ground stress. This
would suggest that the horizontal stress cannot be significantly greater than the vertical stress.
The ground surface heave indicated that the ground was not in contact with the top of the
shield although the annulus would be filled with bentonite.

The settlement of the HEX tunnel crown as the ART TBM passed above suggested both a
high bentonite grout pressure combined with the direct load due to the weight of the TBM.
Calculations indicate that weight of bentonite displaced by the TBM shield is slightly less
than the weight of the TBM and therefore the TBM is probably sliding along the invert of the
excavation. Lateral movement of the HEX tunnel crown in the direction of the ART tunnel
drive is probably due to friction from the shield as it slides on the excavated invert. However,
the magnitude of this movement is higher than expected based on the buoyant weight of the
TBM.

The monitoring data appears to demonstrate that despite backgrouting of the tunnel segments
some ground relaxation occurs around the segmental lining.

Back Analysis

The settlement of the HEX tunnel as the ART TBM shield passed above had not been
predicted in the design calculations. Based on the construction information, two further
FLAC3D analyses were carried out taking into account the measured TBM thrust of 13MN,
the weight of the TBM, the bentonite pressure around the shield and the grout pressure around
the lining. The total TBM thrust of 13MN equates to a uniform face pressure of about
200kPa. The TBM shield is fully submerged in bentonite and therefore only the submerged
weight of the TBM acts on the tunnel invert. The bentonite pressure was taken to vary from
100 kPa in the crown to 200 kPa in the invert based on a unit weight of 11 kN/m3. The
submerged weight of the TBM is only 120 tonnes if the displaced volume of the shield and
tailskin is considered, but increases to 360 tonnes if only the displaced volume of the shield is
considered. This submerged weight of the TBM was applied as an additional pressure to that
of the bentonite over a width of about 6.5m in the excavation invert. The grout pressure was
taken to be applied over the first segment behind the shield and varied from 200 kPa in the
crown to 350 kPa in the invert.

Back analysis 1 assumed that the submerged TBM weight was applied only over the length of
the shield and the additional pressure applied was 80 kPa. In back analysis 2 the submerged
weight of the TBM was applied over the full length of the shield and tailskin as a uniform
pressure of 17 kPa. In both cases the vertical stress applied to the invert of the tunnel beneath
the TBM was less than original vertical stress of 330 kPa at invert level.

Figure 8 shows the vertical and horizontal displacement of the HEX tunnel crown relative to
the ART face position for the two back analyses and for the actual monitoring data. The
pattern and magnitude of the displacement profiles are very similar, however, the observed
HEX tunnel movement is of opposite direction to the movements predicted in the two back
analyses.
                                            6                                                                                                                                      2




                                                                                                                                     Horizontal displacement of HEX Crown (mm)..
                                                             Actual data                                                                                                                       Actual data
Vertical displacement of HEX crown (mm)..   5                Back analysis 1                                                                                                                   Back analysis 1
                                                                                                                                                                                               Back analysis 2
                                                             Back analysis 2
                                            4
                                                                                                                                                                                   1
                                            3
                                            2

                                            1
                                                                                                                                                                                   0
                                            0
                                            -1

                                            -2                                                                                                                                     -1
                                            -3

                                            -4

                                            -5                                                                                                                                     -2
                                                 -25   -20      -15     -10        -5        0      5     10      15       20   25                                                      -25   -20      -15     -10        -5        0      5     10      15       20   25
                                                             ART face position relative to the HEX-ART crossover point (m)                                                                          ART face position relative to the HEX-ART crossover point (m)


                                                 a)                                                                                                                                      b)
                                                               Figure 8 Back analysed vertical and horizontal displacement of HEX
                                                               tunnel crown

     The vertical displacement at the ground surface is similar in the two analyses with between 20
     and 25mm of settlement occurring above the shield reducing to around 15mm over the
     completed tunnel. Field observations suggested that heave occurred above the shield with a
     reduced amount of heave or even settlement occurring over the completed tunnel. To achieve
     these ground movements in the analysis the bentonite pressure around the shield would need
     to be increased above that measured during tunnelling. However, although analyses indicated
     that the bentonite pressure could be increased sufficiently to cause heave, it was still not
     possible to cause the ground to settle beneath the TBM. Even when the bentonite pressure
     was increased in the analysis sufficiently to cause heave above the ART tunnel far in excess
     of that observed, the lateral pressure was not sufficient to cause outward movement of the
     ground at axis level and the downward pressure was insufficient to cause settlement beneath
     the TBM.

     The reason for the discrepancy between observed ground movements and the back analyses is
     not clear, but it is thought that the in situ horizontal stresses used in the analysis may be
     higher than those present in the ground. A lower in situ stress profile would result in smaller
     movement of the ground ahead of the face and the possibility of outward movement of the
     ground under a reasonable bentonite pressure. The settlement of the ground beneath the ART
     tunnel is even more surprising and would imply that the TBM is heavier than envisaged.
     Since the TBM is sitting on the invert it is possible that bentonite is absent in the invert.
     However, this would have no effect on the effective downward pressure on the tunnel invert
     as the increase in submerged weight of the TBM is exactly balanced by the reduction of the
     bentonite pressure acting on the invert. One possible explanation is that the shove rams force
     the TBM forwards, but may also force the rear of the shield downwards resulting in an
     increased vertical stress on the tunnel invert towards the rear of the shield. Further study of
     all the monitoring data is required to confirm whether this is indeed the case.

     Conclusions

     Initial design studies showed that the magnitude of the ground movements was sensitive to
     the type of TBM mode employed. Nevertheless the analysis showed that movements below
     the ART would be considerably smaller than the movements above the tunnel and that
     unacceptable movement of the HEX tunnel was very unlikely even adopting open-faced
     tunnelling techniques.

     To limit ground movements along the whole route a pressurised TBM was used and this was
     found to cause heave above the shield, outward movement beside the shield and settlement
     below the shield. Despite the unexpected direction of the ground movements, the movement
of the trackwork within the HEX tunnel was of similar magnitude to that predicted and
sufficiently small that rail operations were unaffected.

Back analysis of the tunnelling process using measured tunnelling pressures could not
reproduce the observed ground movements. It is concluded that the bentonite pressures
around the TBM are higher than measured and the horizontal in situ ground stress is lower
than assessed from the site investigation data.

This case study indicates the difficulty of accurately predicting in advance the magnitude and
direction of ground movements associated with tunnelling with a pressurised TBM.

Acknowledgements

The authors would like to thank their colleagues for stimulating discussions on the operation
of the tunnel boring machine and how this might influence the ground movements around the
tunnel. The authors would also like to thank BAA plc for permission to publish this paper.

References

1. R.J. JARDINE, D.M. POTTS, A.B. FOURIE and J.B. BURLAND: “Studies of the
influence of non-linear stress strain characteristics in soil-structure interaction”,
Géotechnique, 1986, 36 (3), 377-396.

2. C. POUND and J.P. BEVERIDGE: “Recent experiences of the measurement of tunnelling
induced ground movements”, In press. 2003.

								
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