Influence of excess pore pressures on the stability of the tunnel face

J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
<strong>Influence</strong> <strong>of</strong> <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> on the stability <strong>of</strong> the tunnel face
W. Broere
A. Broere’s Aannemingsmij. BV, Amsterdam, The Netherlands
Geotechnical Laboratory, Delft University <strong>of</strong> Technology, The Netherlands
ABSTRACT: In shield tunnelling in loose and water bearing soils, a bentonite slurry is <strong>of</strong>ten used to help support
the tunnel face. During excavation, the bentonite cake, which is intended to seal the face, is removed by the cutter
bits and subsequently slurry will infiltrate the soil. This infiltration causes <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> the
TBM, which lower the stability <strong>of</strong> the face. The effect has been investigated using a stationary model as well as
with a time-dependent groundwater flow model, linked to a limit equilibrium face stability model. This model
can be used to predict the minimal required support pressure and also the build-up <strong>of</strong> <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in
front <strong>of</strong> the TBM over time. The calculated <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> compare well with field observations.
1 INTRODUCTION
The use <strong>of</strong> tunnel boring machines to realise underground
works is still continuously extended to conditions
that were, until recently, deemed too difficult to
work in. A list <strong>of</strong> examples can be found in the Netherlands
alone, such as the tunnel beneath the Green
Heart, part <strong>of</strong> the High Speed Link South from Amsterdam
to Brussel. This almost 15 m diameter tunnel
is constructed in part in loose sand layers with a high,
confined, water table, below a thick stratum <strong>of</strong> extremely
s<strong>of</strong>t peats and clays. The soil conditions can
only be characterised as difficult and extremely sensitive
to settlements. Yet due to the presence <strong>of</strong> several
roads, railroads, dikes and pipe infrastructure at the
surface, the allowable surface settlements are limited
to only 10 mm in some locations.
Another example is the proposed construction <strong>of</strong> the
North-South light rail link in Amsterdam, where a twin
tunnel will be constructed in the vicinity <strong>of</strong> the old
wooden pile foundations supporting the historic buildings
<strong>of</strong> the inner city. In these conditions the acceptable
deformations <strong>of</strong> the subsoil are even more limited,
to avoid damage to the highly sensitive masonry
structures. For such projects, a reliable prediction <strong>of</strong>
the required support pressure and, subsequently, a reliable
control <strong>of</strong> the pressure at the tunnel face, is <strong>of</strong>
utmost importance.
2 SUPPORT PRESSURE CALCULATION
The most common tunnel boring techniques used in
s<strong>of</strong>t, water bearing soils are the slurry shield and the
earth-pressure balance (EPB) shield, as other techniques
fail to meet the rigorous settlement criteria outlined
above.
In a slurry shield a pressurised slurry is used to
stabilise the tunnel face, <strong>of</strong>ten combined with an air
bubble in the pressure chamber to limit pressure fluctuations.
This bentonite slurry is normally injected
into the working chamber at a pressure higher than the
<strong>pore</strong> water pressure in the soil. Due to the pressure
difference the slurry will infiltrate the soil and form a
filter cake at the face (Krause 1987). This filter cake
will then seal the face, help to transfer the slurry pressure
onto the soil skeleton and protect against microcollapses
at the face. Numerous models used to determine
the minimal required support pressure at the
face implicitly assume this filter cake to be a thin and
perfectly impermeable membrane, so that no slurry infiltrates
the soil, e.g. the models by Jancsecz & Steiner
(1994), Leca & Dormieux (1990) or Murayama, see
(Kanayasu et al. 1995).
Anagnostou & Kovári (1994) noticed that during
stand-still <strong>of</strong> the TBM the slurry will infiltrate the soil
and the filter cake is no longer a thin membrane. In
coarse sands and gravels, the infiltration length <strong>of</strong> the
slurry can reach several decimeters. The support pressure
is no longer transferred to the soil at the tunnel
face, but gradually over this infiltration zone. The result
is a reduced efficiency <strong>of</strong> the slurry in stabilising
the face, which may lead to collapse <strong>of</strong> the face in very
coarse soils.
This infiltration occurs not only during stand-still,
but also during the actual excavation process. As
the cutter bits constantly remove the established filter
cake, there is also a continuous infiltration <strong>of</strong> slurry
1

J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
z
x
y
C
D
Figure 1. Wedge and silo stability model.
F
B
A
ϑ
B
E
h
z = zh
C
z = z t
D
z = z b
into the soil in order to rebuild the filter cake. The
time required to establish a perfectly sealing filter cake,
however, is normally greater than the time between
subsequent passages <strong>of</strong> the cutter bits. As a result, during
excavation, there is a continuous inflow <strong>of</strong> filtrate
water into the soil. This infiltration results in <strong>excess</strong>
<strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> the TBM and these <strong>excess</strong>
<strong>pore</strong> <strong>pressures</strong> lower the effective stresses <strong>of</strong> the soil.
They also lower the effectiveness <strong>of</strong> the slurry infiltration
and the combined effect is a reduced stability <strong>of</strong>
the tunnel face.
In an EPB shield the excavated soil is used to support
the tunnel face, <strong>of</strong>ten conditioned with additives like
bentonite slurry or foam. These additives are injected
into the working chamber at <strong>pressures</strong> above the <strong>pore</strong>
water pressure and will infiltrate the soil in front <strong>of</strong> the
TBM, displacing the <strong>pore</strong> water present there. Part <strong>of</strong>
the effectiveness <strong>of</strong> the foam treatment <strong>of</strong> the soil rests
in the fact that the foam replaces part <strong>of</strong> the <strong>pore</strong> water,
lowering the water content <strong>of</strong> the soil (Maidl 1995). As
a result <strong>of</strong> this infiltration process <strong>excess</strong> <strong>pore</strong> pressure
are generated in front <strong>of</strong> the face. Although the underlying
infiltration process differs slightly, the resulting
<strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> are much the same as observed
for a slurry machine.
2.1 Wedge stability model
The aforementioned stability calculations by Jancsecz
& Steiner (1994), as well as Anagnostou & Kovári
(1994), are based on the limit equilibrium analysis <strong>of</strong>
a wedge shaped soil body in front <strong>of</strong> the TBM, as
proposed by Horn (1961). The basic wedge stability
model is a limit equilibrium analysis, in which the
collapsing soil in front <strong>of</strong> the TBM is schematised as a
triangular wedge, loaded by a soil silo (see Figure 1).
This wedge is assumed to be a rigid body, loaded its
effective weight and the overburden resulting from the
soil silo.
From the equilibrium <strong>of</strong> the forces acting on the the
wedge, the effective earth pressure acting towards the
tunnel face can be calculated. This force has to be
countered by the effective support force, which is the
difference between the total support pressure and the
<strong>pore</strong> pressure in the soil. In most models, the effective
earth pressure can only be found for a given wedge
angle θ, and as a result the required minimal support
pressure is then found by iterating over all possible
values <strong>of</strong> the angle θ and maximising the effective
earth pressure.
Anagnostou & Kovári (1996) and Broere (2001)
have shown that a wedge stability model can also be
used to determine the face <strong>pressures</strong> in EPB shield tunnelling.
Although the stability <strong>of</strong> the face is in general
not considered a problem in EPB shield tunnelling, an
accurate indication <strong>of</strong> the minimal required support
pressure is required to control the surface settlements
and to prevent a face collapse in the event <strong>of</strong> a partially
filled working chamber.
2.2 <strong>Influence</strong> <strong>of</strong> infiltration
The effects <strong>of</strong> infiltration during excavation and the
resulting <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> can be included in a
wedge stability analysis using a stationary groundwater
flow model, as shown by Broere & van Tol (2000).
This model assumes that the continuous infiltration
process results in a partial filter cake during the excavation
process, and that part <strong>of</strong> the support pressure is
transferred onto the soil skeleton by this partial filter
cake. The remaining pressurep p is taken as input for
a simple one-dimensional groundwater flow model, so
that the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> at a distance x in front
<strong>of</strong> the face are given by
( ) e − x
p(x) = p p exp , (1)
λ
wheree is the infiltration length <strong>of</strong> the partial filter cake
andλis the leakage length <strong>of</strong> the aquifer. The resulting
<strong>pore</strong> pressure distribution is sketched in Figure 2.
The <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> lower the effective
stresses in the soil, thereby reducing the friction
between the collapsing soil body and the remaining
soil. This requires an increase in the support pressure
in order to stabilise the soil. Furthermore, the
effectiveness <strong>of</strong> the support is reduced, as only part <strong>of</strong>
the support pressure is transferred onto the soil skeleton.
The remainder <strong>of</strong> the support pressure drives the
groundwater flow and generates the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong>.
The combined result <strong>of</strong> these effects is that a
higher support pressure is required than in case a perfectly
sealing filter cake is assumed. The effect is most
significant in fine to medium sands, and negligible in
clays where hardly any infiltration occurs.
2

J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
∆p fc
∆s
(kPa)
50
40
30
∆s
20
∆p p
x
∆p(w(z), z))
e
s p 0
ϑ
p(x, z)
Figure 2. Schematic overview <strong>of</strong> the pressure drop over the
slurry infiltration zone and <strong>excess</strong> <strong>pore</strong> pressure distribution
in front <strong>of</strong> the face.
Table 1. Input parameters for various soil types to determine
influence <strong>of</strong> permeability on calculated support pressure.
γ φ c k d 10 a τ F
No. (kN/m 3 ) ( ◦ ) (kPa) (m/s) (µm) (s) (Pa)
1 16 15 2 10 −10 2 1 1
2 17 17.5 10 10 −9 5 1 1
3 17 17.5 5 10 −8 5 5 1
4 18 22.5 5 10 −7 10 10 5
5 18 27.5 2 10 −6 10 60 5
6 19 30 0 10 −5 50 120 5
7 20 30 0 10 −4 100 180 10
8 20 32.5 0 10 −3 500 120 10
9 20 35 0 10 −2 1000 60 15
10 20 35 0 10 −1 2000 60 20
11 20 40 0 10 0 4000 60 30
This is illustrated by a parameter analysis in Broere
(2001), where calculations have been made for several
soil types with increasing permeability. The properties
<strong>of</strong> these soils are listed in Table 1, where k is the
permeability and d 10 the characteristic grain size. The
infiltration half-timea and yield strength τ F characterise
the behaviour <strong>of</strong> the slurry in combination with the
soil type. The parameters are indicative for soils ranging
from clayey (1) to sandy (7) to gravelly (11). The
resulting minimal <strong>excess</strong> support <strong>pressures</strong> (the difference
between total support pressure and <strong>pore</strong> pressure
in rest) for a 10 m diameter TBM with a cover <strong>of</strong> 15
m are shown in Figure 3.
It can be seen that the highest <strong>excess</strong> support <strong>pressures</strong>
are needed in sandy soils with permeabilities
10 −5 < k < 10 −3 m/s. This corresponds with the
field observations by Mori (1995) that the reduction
in effective support pressure due to <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong>
can become a major problem in soils with permeabilities
within the indicated range, as well as with
field observations made during the construction <strong>of</strong> the
Second Heinenoordtunnel in the Netherlands.
10
0
1 2 3 4 5 6 7 8 9 10 11
No.
Figure 3. <strong>Influence</strong> <strong>of</strong> soil types listed in Table 1 on the
minimal <strong>excess</strong> support pressure (difference between total
support pressure and <strong>pore</strong> pressure in rest).
3 SECOND HEINENOORD
The Second Heinenoord Tunnel is a twin-tube, 8.3m
outer diameter, 950m long bored tunnel under the
River Oude Maas in the vicinity <strong>of</strong> Rotterdam, constructed
between 1996 and 1999. As the first large
bored tunnel in s<strong>of</strong>t to very s<strong>of</strong>t soils with high water
<strong>pressures</strong> in the Netherlands, the tunnel was extensively
monitored. See Bakker et al. (2000) for a description
<strong>of</strong> the soil conditions and the test programme.
At the north bank <strong>of</strong> the river a monitoring site was
instrumented with piezometers placed in the path <strong>of</strong>
the TBM. At this location the tunnel was excavated in
a highly stratified soil, with Holocene fine sands and
several s<strong>of</strong>t clay layers along the top part <strong>of</strong> the tunnel
and with Pleistocene, medium to coarse, sands at the
bottom part. The tunnel axis is situated approximately
15 m below ground level and the average piezometric
head lies 12.4 m above the tunnel. A small tidal variation
can be observed in this head, slightly delayed
with respect to the tidal fluctuation observed in the
river.
The piezometer placed in the soil was constantly
monitored during the approach <strong>of</strong> the TBM, up to the
time the piezometer was excavated by the TBM and
destroyed. The <strong>pore</strong> <strong>pressures</strong> are plotted in Figure 4
as a function <strong>of</strong> the distance between the TBM and the
piezometer, and are compared with the <strong>excess</strong> <strong>pore</strong>
<strong>pressures</strong> according to (1). The <strong>excess</strong> <strong>pore</strong> pressure
increase as soon as excavation starts and rises over time
as the TBM approaches the piezometer. The vertical
downward spikes at 1.5 m intervals are caused by the
dissipation <strong>of</strong> <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> during stand-still.
The peak <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> correspond well with
the calculated pr<strong>of</strong>ile.
4 TRANSIENT GROUNDWATER FLOW
For the soil conditions at the Second Heinenoord, the
groundwater flow in front <strong>of</strong> the TBM reached stationary
conditions after approximately 5 min. And within
30 min. after the TBM was stopped, no <strong>excess</strong> <strong>pore</strong>
3

180 200
170 190
160 150 140 130 120 110
<strong>pore</strong>pressurepinkPa
0
5
Figure 4. Measured <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> TBM
at the Second Heinenoord, compared with <strong>pore</strong> pressure
according to (1) (dashed)
<strong>pressures</strong> remained. For these conditions the use <strong>of</strong> a
stationary groundwater flow model to predict the influence
<strong>of</strong> infiltration is sufficient. However, for many
cases where the infiltration has a significant effect on
the stability <strong>of</strong> the face, the permeability and storage
capacity <strong>of</strong> the aquifers are such that stationary flow
is not reached within the 30 to 60 minutes it normally
takes to excavate a lining ring.
In these cases the infiltration process can be modelled
using a transient groundwater flow model
coupled to the wedge stability calculation. A similar
transient groundwater flow model can be used to
evaluate the dissipation rate <strong>of</strong> the generated <strong>excess</strong>
<strong>pore</strong> <strong>pressures</strong>, as the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> will not
always be fully dissipated when the next excavation
cycle starts. Combining these effects in a stability
analysis allows the effects <strong>of</strong> the infiltration process on
the stability <strong>of</strong> the tunnel face to be quantified, without
overestimating their influence.
A problem with this approach forms the fact that the
solution <strong>of</strong> the basic differential equations for transient
flow can become rather complex. For the case <strong>of</strong> a
semi-confined aquifer, the basic equations are given
by Strack (1989) as
∇ 2 = λ 2 + S s
k ∂ t (2)
where the potential in the aquifer defined as =
kH(ϕ−ϕ 0 ), ϕ the piezometric head, S s the coefficient
<strong>of</strong> specific storage and k the permeability.
If the problem is simplified to a single, onedimensional
aquifer <strong>of</strong> height H , with a discharge Q
at x = 0 and has a constant head ϕ 0 at infinity, the
solution to (2) has been given by Bruggeman (1999)
as
ϕ − ϕ 0 = Qλ
4kH
[ ( xu
erfc
−erfc
√ t
2 √ t + uλ
( √ xu t
2 √ t − uλ
) ( x
exp
λ)
) (
exp − x λ
(3)
J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
TBM x gauge
withu = √ S s /k and erfc(x) the complementary error-
10distancetogaugexinm
15 20 25 30 35
) ]
function. In order to use this equation to predict the
<strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> the tunnel face, an
estimate <strong>of</strong> the discharge Q is needed. For a slurry
shield Mohkam (1985) reports that the amount <strong>of</strong> water
displaced by the infiltrating slurry is roughly equal to
the porosity <strong>of</strong> the excavated material. This leads to
an estimated discharge per unit area <strong>of</strong> the tunnel
q = −nv (4)
with n the porosity and v the advance rate <strong>of</strong> the TBM.
In a foam-conditioned EPB machine the amount <strong>of</strong> water
displaced depends on the foam injection rate, but
as the amount <strong>of</strong> foam injected is <strong>of</strong>ten <strong>of</strong> the same
order as the porosity <strong>of</strong> the excavated material, and
displaces the <strong>pore</strong> water originally present, the above
relation can also be used to get an estimate <strong>of</strong> the discharge
in this case.
The calculated piezometric head can be used instead
<strong>of</strong> (1) in the wedge stability analysis. In that
way the minimal required support pressure, at time t
after boring has started, can be calculated, as well as
the <strong>pore</strong> <strong>pressures</strong> generated in front <strong>of</strong> the face. If the
discharge Q has been estimated correctly, the <strong>excess</strong>
<strong>pore</strong> <strong>pressures</strong> predicted by (3) will approach those
given by (1) over time.
When the <strong>excess</strong> <strong>pore</strong> pressure distribution at the
time the TBM stops are known, these can be used as
input to calculate the dissipation over time. This will,
however, lead to a rather complex solution or require
the use <strong>of</strong> numerical solution methods. Assuming that
stationary flow conditions according to (1) have been
reached as the excavation ends, simplifies the problem
somewhat. In that case the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong>
remaining at time t after boring has stopped can be
estimated from (see Bruggeman 1999)
ϕ − ϕ 0 = p [ ( √ )
p xu t
( x
erfc
2γ w 2 √ t + exp
uλ λ)
(
+erfc − xu √ ) t
(
2 √ t + exp − x uλ λ) ] ,
(5)
which is remarkably similar to (3).
From the equations (3) and (5) it can be found that,
in those cases where the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> do not
reach equilibrium shortly after boring has started and
a transient flow model is needed to accurately describe
the build-up <strong>of</strong> <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong>, the time required
for the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> to fully dissipate will also
be large. If in an undisturbed boring process the standstill
period, between the boring <strong>of</strong> two subsequent tunnel
rings, is <strong>of</strong> the same order as the time needed to
excavate a tunnel ring, part <strong>of</strong> the <strong>pore</strong> <strong>pressures</strong> generated
during the first excavation period will remain
when the boring <strong>of</strong> the next ring is started. This will
be illustrated using an example.
4

J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
∆p(kPa)
140
120
∆p(kPa)
140
120
100
80
60
40
20
0
0 2 4 6 8 10 12
t (h)
Figure 5. Excess <strong>pore</strong> <strong>pressures</strong> at the face due to infiltration
and dissipation over time.
4.1 Prediction <strong>of</strong> Pore Pressures at the Face
Alternating (3) and (5) during boring and stand-still,
the time-dependant <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in an aquifer
resulting from the infiltration <strong>of</strong> slurry or foam can be
predicted. For a fine, silty sand layer with λ = 9m,
S s = 7 · 10 −4 m −1 , k = 10 −5 m/s and n = 0.4
this is illustrated in figure 5. The average advance
rate <strong>of</strong> the TBM has been chosen v = 5cm/min and
H/D = 1.33, with D the diameter <strong>of</strong> the TBM. It is
further assumed that the excavation <strong>of</strong> a single tunnel
ring takes 45 minutes and the stand-still between
subsequent excavation periods is 75 minutes. For two
consecutive rings the <strong>excess</strong> <strong>pore</strong> pressure at the face
has been plotted as a function <strong>of</strong> time. It can be clearly
seen that the time needed for the generated <strong>excess</strong>
<strong>pore</strong> <strong>pressures</strong> to fully dissipate is longer than the time
between subsequent excavation periods. At the start
<strong>of</strong> the second excavation approximately 20% <strong>of</strong> the
generated <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> remains.
5 BOTLEK RAIL TUNNEL
The Botlek rail tunnel is a 9.65m diameter twin-tunnel
bored through Holocene and Pleistocene sand, clay
and peat layers in the vicinity <strong>of</strong> Rotterdam, the Netherlands.
The soil conditions are roughly similar to
those at the Second Heinenoord, s<strong>of</strong>t Holocene clays
and sands over a Pleistocene sand layer. The Holocene
sand layer in which the first part <strong>of</strong> the tunnel
was constructed can be simplified as in the example
in §4.1. The tunnel has been constructed using a
foam-conditioned earth-pressure balance shield and at
several points along the tunnel alignment piezometers
have been placed in the projected path <strong>of</strong> the TBM, as
part <strong>of</strong> an extensive research program (COB 2001).
The <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> measured at location
MQ1 have been plotted against time in figure 6. The
100
80
60
40
20
0
0 1 2 3 4 5 6 7
t (day)
Figure 6. Pore pressure measurements at Botlek Rail, MQ1
as function <strong>of</strong> time (BTC/NS-RIB 2000).
peak <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> are measured during the actual
excavation periods and the subsequent drops with
the periods <strong>of</strong> ring building and maintenance. The
measurements made at day 4 show clearly that the
<strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> are not fully dissipated as the
excavation <strong>of</strong> the next tunnel ring starts. Only during
the stop on day 5 the TBM is halted long enough for
the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> to fully dissipate.
The measurements are also plotted in figure 7 as a
function <strong>of</strong> the distance between the piezometer and
the TBM. In this way they can be compared with the
predicted <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> from (3) at 45 minutes
after boring has started, and from (5) 90 minutes after
boring has stopped. It should be noted that the <strong>excess</strong>
<strong>pore</strong> pressure distribution in the first few metres
from the face has the greatest influence on the face
stability. Even with the rather simplified groundwater
flow model, the comparison between prediction and
measurements is reasonable.
6 CONCLUSIONS
During excavation with a slurry shield or a foamconditioned
EPB shield in a medium or fine sand layer,
the infiltration <strong>of</strong> bentonite slurry or foam into the
soil will generate <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> the
face. These <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> lower the effective
stresses in the soil as well as the effectiveness <strong>of</strong> the
support medium. These effects lower the stability <strong>of</strong>
the face and a significantly higher effective support
pressure may be required to prevent a face collapse.
The effect can be included in a wedge stability analysis
using a simplified analytical groundwater flow
model. The influence on the minimal required support
pressure is greatest in fine sands with a permeability
in the order <strong>of</strong> 10 −5 < k < 10 −3 m/s. Furthermore,
depending on the geohydrological conditions, a transient
flow model may be needed to accurately predict the
<strong>pore</strong> <strong>pressures</strong> in front <strong>of</strong> the TBM during excavation,
5

J. Saveur (ed.) (Re)Claiming the Underground Space, ITA, Amsterdam, 2003, pp. 759–765.
∆p (kPa)
140
120
100
80
60
40
20
0
(5)
(3)
0 10 20 30 40 50
x (m)
Figure 7. Excess <strong>pore</strong> pressure pr<strong>of</strong>iles according to (3) at
t = 0.75h and (5) at t = 1.5h, compared with measurements
at Botlek Rail, MQ1.
as the time required to reach equilibrium flow can be
larger than the time needed to excavate a single tunnel
ring. Using a stationary flow model overpredicts the
influence in these cases.
Transient flow calculations show that close to the
face the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> will increase quickly
after excavating has started and also that it generally
takes longer for the <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong> to completely
dissipate than the average ring-building time.
For a presented case approximately 20% <strong>of</strong> the <strong>excess</strong>
<strong>pore</strong> pressure generated remained after 75 minutes
<strong>of</strong> stand-still. The calculated <strong>excess</strong> <strong>pore</strong> <strong>pressures</strong>
are comparable to those observed in front <strong>of</strong> a foamconditioned
earth pressure balance shield in a fine
sandy soil during construction <strong>of</strong> the Botlek Rail tunnel.
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