BORED TUNNELS
DESIGN AND CONSTRUCTION
Slides and notes courtesy of Dr WEN Dazhi
Some slides originate from Mr Nick Shirlaw
1
BORED TUNNELS
DESIGN AND CONSTRUCTION
• Tunnel Face Pressure Control
• Planning Face Pressure – Clay
• Planning Face Pressure – Sand
• Maximum Face Pressure
• Method Statements for Tunnelling
Works
2
TUNNEL FACE PRESSURE CONTROL
• A critical parameter in driving EPB
or slurry TBMs through soft ground
• Inadequate face pressure or inability
to control face pressure will cause
excessive ground movement or
collapse of tunnel face
3
TUNNEL FACE PRESSURE CONTROL
Exceptional Settlement / Sinkholes
High Risk Areas – Results of inadequate or
inability to control face pressure
•
•
•
•
•
•
Launching the shield - particularly into cohesionless soils below the
water table
Breaking out into shafts or excavations
Interfaces between strong, stable soils and weak soils (Kallang
Formation marine clay)
Mixed faces of rock and soil (including soil grades of weathered rock)
Head access for maintenance
Long drives in abrasive ground
4
TUNNEL FACE PRESSURE CONTROL
EPBM
Ideally spoils in the excavation chamber should have
the following properties:
 Good plastic ductility and pasty to soft
consistency: to ensure that the support pressure
acts on the face as uniformly as possible and the
flow into the screw conveyor is continuous
 Low internal friction to ensure the drive torque of
the cutting wheel and the screw conveyor remains
within economic limits
 Low permeability to maintain face pressure to
prevent over excavation (<10-5 m/s according to
Thewes, 2014)
5
TUNNEL FACE PRESSURE CONTROL
EPBM
No plug, material saturated and
flowing
“Plastic” nature allowing
plug and control
6
TUNNEL FACE PRESSURE CONTROL
Foams / soil conditioning agents are often used to condition the spoil
Soil conditioning needs of EPBM in different ground types (EFNARC, 2005)
7
TUNNEL FACE PRESSURE CONTROL
Foam product type for EPBM relative to different soils (FIR values indicative), EFNARC (2005)
Form Injection Ratio (FIR)
Sandy
clay - silt
Sandy
clayey silt
Clayey
gravels
FIR = 100 x Vfoam / Vsoil (%)
Vfoam : Volume of foam at
working pressure
Vsoil : Volume of in-situ soil
to be excavated
Sandy
gravels
Foam Type A: high dispersing capacity (breaking clay bonds) and / or good coating capacity to
reduce swelling effects
Foam Type B: general purpose, with medium stability
Foam Type C: high stability and anti-segregation properties to develop and maintain a cohesive
soil as impermeable as possible
8
TUNNEL FACE PRESSURE CONTROL
Slurry Shields
• For slurry TBMs, tunnel face pressure is maintained
by controlling the volume difference of the bentonite
suspension supplied to the chamber and the
suspension combined with excavated material
removed from it or by the provision of a compressed
air reservoir or bubble.
• The primary function of the slurry is to stabilise the
face. It is also required to suspend and transport the
cuttings, to lubricate and cool the cutter head and to
reduce abrasive wear of the cutting tools.
• Formation of filter cake (membrane) is critical in
maintaining the face pressure.
9
TUNNEL FACE PRESSURE CONTROL
Slurry Shields – Membrane Model
Membrane Model
slurry
soil
Filter cake formed
If the permeability of the ground is
relatively low (fine or medium
sands) and the bentonite content
is sufficient the suspension will
enter the ground under the
differential pressure and seal the
tunnel face with the solid matter
particles contained in it, thus
creating a thin but impermeable
film (filter cake) through which the
support pressure can be applied.
This process takes place in a short
time of 1 to 2 seconds.
Maidl, et al (2012)
10
TUNNEL FACE PRESSURE CONTROL
Slurry Shields – Penetration Model
Penetration Model
In coarse-grained more permeable
ground, a filter cake cannot always
be formed, even with a high
bentonite content. The bentonite
suspension penetrates into the
face and, due to its thixotropic
properties, transfers shear forces
into the grain skeleton.
slurry
P
soil
Pure penetration
Maidl, et al (2012)
11
TUNNEL FACE PRESSURE CONTROL
Slurry Shields – Penetration Model
The penetration distance smax can be calculated from
smax =
p d10
2f
where p is the pressure difference between
supporting fluid and the ground water; d10 is diameter
corresponding to 10% passing or finer in sieve analysis
and f is the yield strength of the slurry
 The extent of slurry penetration does not depend on the
complete particle size distribution; but rather is governed by
the finer particle fraction.
12
TUNNEL FACE PRESSURE CONTROL
Slurry Shields – Penetration Model
• The greater the penetration the lower the factor
of safety
• Below a d10 of about 0.6mm treat as membrane
• Above a d10 of about 0.6mm, slurry penetrates
into ground and factor of safety reduces
Anagnostou & Kovari, 1996
13
TUNNEL FACE PRESSURE CONTROL
Slurry Shields – Slurry Treatment Plant
• Slurry shields require slurry
treatment plants to separate
excavated materials,
prepare and control the
quality of slurry before
feeding into the TBM.
• Space will be required for
the plants – something not
required for EPB tunneling.
• Shaker or vibrating screens - to separate coarse particles (grain size > 3
•
•
to 6mm).
Desander & desilter – one or more stages of cyclones to separate
sands and silts (Single stage – medium sand, 70 to 150 m; 2-stage
plants – coarse silt, 35 m).
Centrifuge or press filters – to separate clay / silt particles.
14
PLANNING OF FACE PRESSURE - CLAY
• For tunnelling in clays, face
pressure is related to Stability
Number
• There is a relationship between face
pressure and ground settlement
15
PLANNING OF FACE PRESSURE - CLAY
Stability Number, N
Overburden Pressure – Tunnel Support Pressure
N=
Undrained Shear Strength
Surcharge q
(zO + q – t)
N=
cu
zo
C
D
t
P
16
PLANNING OF FACE PRESSURE - CLAY
Stability Number at Collapse, Nc
o = zo = (6 to 8)cu
cu
N = o / cu < 6
Failure surface
17
PLANNING OF FACE PRESSURE - CLAY
Stability Number at Collapse, Nc
For deep tunnels (C/D > 3), Nc = 9 when P/D = 0
9.0
8.6
Heading Geometry and Depth vs.
Stability Number at Collapse, Nc
2.5
18
PLANNING OF FACE PRESSURE - CLAY
Load Factor
Load Factor =
Stability Number (Working Condition)
Stability Number at Collapse
LF = N/NC
LF is the inverse of the factor of safety
19
PLANNING OF FACE PRESSURE - CLAY
Load Factor vs. Volume Loss
Load Factor =
0.49
1
=
N (Working Condition)
F
0.42
N (At Collapse)
Load Factor = 0.67 (F = 1.5) at Volume Loss = 4%
1.5
CIRIA Report 30, March 1996
20
PLANNING OF FACE PRESSURE - CLAY
Load Factor vs Volume Loss
Vl = 0.23e 4.4(LF)
For LF>0.2
Vl in percentage
LF = 0.49,
Vl = 0.23 * e (4.4*0.49) = 1.98 (approx. 2%)
Dimmock & Mair (2007)
21
PLANNING OF FACE PRESSURE - CLAY
Example
• Tunnel D = 6m @ 20m below ground, C =
•
•
17m, driven by EPBM, P = 0
cu = 50 kPa, q = 10 kPa,  = 18 kN/m3
P/D = 0, C/D = 2.83, Nc = 9 from Chart
22
PLANNING OF FACE PRESSURE - CLAY
Example
cu
50
kPa
density
18
kN/m^3
surcharge
10
kPa
depth
20
m
Nc
overburden
Face Pressure (%
of overburden)
9
370
kN/m^2
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
37
74
111
148
185
222
259
296
333
370
N
6.66
5.92
5.18
4.44
3.70
2.96
2.22
1.48
0.74
0.00
1/F
0.74
0.66
0.58
0.49
0.41
0.33
0.25
0.16
0.08
0.00
Volume Loss (%)
6.0
4.0
3.0
2.0
1.3
1.0
0.5
0.2
0.1
0
Face Pressure
Volume loss due to stress change
23
PLANNING OF FACE PRESSURE - CLAY
• Need to carry out an Ultimate Limit State (ULS)
calculation. ULS is instability leading to major loss of
ground or collapse at the tunnel face. ULS calculation
requires a partial factor of safety applied to cu: divide cu
by 1.5; then calculate face pressure for LF of 1.
• Tunnelling in urban environment requires stringent
settlement control. It is necessary to carry out a
Serviceability Limit State (SLS) calculation. First, assess
Load Factor to achieve the allowable settlement, then
calculate face pressure to achieve that LF, using partial
factor of safety of 1.
• To control Volume Loss to less than 2%, load factor
should not be greater than of 0.4 (corresponding F = 2.5).
• In settlement sensitive areas, need to apply face pressure
close to full overburden pressure (LF = 0 to 0.25)
24
PLANNING OF FACE PRESSURE - CLAY
The calculated pressure is the
average pressure at the tunnel face,
which can be taken as the pressure
at the axis level. The target pressure
at sensor 1 is:
P1 = t – (zo – zs1)* + v
Pressure
Target
Pressure
v v
where v is the max variation of face
pressure due to control accuracy (+/20 kPa or 0.2 bar) and  is the unit
weight of the spoil in the chamber
v v
Time
25
PLANNING OF FACE PRESSURE - CLAY
Example of
poor control
of face
pressure for a
EPB shield
26
PLANNING OF FACE PRESSURE – CLAY
Heading Geometry
Over cut (typically 10mm)
t
L
P = 0 or
P = Length of shield, L
zo
C
D
t
P
27
PLANNING OF FACE PRESSURE – CLAY
Heading Geometry
• Cut diameter is bigger than the shield skin
• Until the overcut gap closes, the area around the
shield is unsupported
• Once the over-cut closes, the ground is supported
on the shield skin
• For EPB shields the face pressure is not
transmitted around the skin – but bentonite can be
injected around skin using special ports
• For slurry shields the slurry pressure is transmitted
around the skin
28
PLANNING OF FACE PRESSURE – CLAY
Heading Geometry
Ports for bentonite
injection (16 No
total)
After Shirlaw, J. N.
29
PLANNING OF FACE PRESSURE – CLAY
Loss at Tail Void
• With good grouting typically about 15% to 20% of
theoretical volume of the tail void will close in soft
or loose soils
• Figure may reduce with improved technology and
construction control
After Shirlaw, J. N.
30
PLANNING OF FACE PRESSURE - SAND
• Assumed failure surface
for tunnels in sand,
based on limit
equilibrium methods
developed by Anagnostu
and Kovari (1996)
Anagnostou and Kovari (1996)
31
PLANNING OF FACE PRESSURE - SAND
For fully saturated, homogeneous and isotropic sands under
drained condition, the effective pressure required to maintain
equilibrium of the tunnel face is:
’ = F0’D – F1c’ +F2’Dh – F3c’ Dh/D
’ = Effective pressure required to maintain equilibrium of tunnel
face;
’ = Submerged unit weight of soil;
D = Diameter of tunnel;
c’ = Drained cohesion of soil;
Dh = Water head difference between the ground water table level,
h0 and the piezometric head in the excavation chamber, hf ;
F0, F1, F2 & F3 – Dimensionless coefficients that depend on the
drained friction angle, ’ and geometrical parameter, H/D and
(h0-D)/D; and
H = Overburden to crown of tunnel.
32
PLANNING OF FACE PRESSURE - SAND
If the piezometric head in the excavation
chamber is maintained to balance the water
pressure due to the ground water, i.e. Dh = 0
(membrane model), then the minimum face
pressure,  at the tunnel crown that is required
to prevent a face collapse (maintaining
equilibrium) can be calculated :
 = F0’D – F1c’ + P
where P is the water pressure at the crown of
tunnel based on the original ground water level.
33
PLANNING OF FACE PRESSURE - SAND
0.3
27.5
Anagnostou, G. and Kovari, K. (1996).
Chart based assumption of d / ’ = 1.6. Since d / ’ = Gs/(Gs – 1), the assumption is that Gs =
2.67, which is reasonable within practical limits.
34
PLANNING OF FACE PRESSURE - SAND
2.54
27.5
Anagnostou, G. and Kovari, K. (1996).
35
PLANNING OF FACE PRESSURE – SAND
Water Pressure
• For EPBM: Unless the sand has significant
cementation, additives have to be used to reduce
the permeability of the sand, such that the
piezometric head at crown is close to that based
on the original ground water level.
• For slurry TBM: The process of TBM tunnelling
increases the groundwater pressure in the face,
and the initial water pressure has to be increased
to compensate for this
36
PLANNING OF FACE PRESSURE – SAND
Water Pressure
• Documented results from slurry shield tunnelling
show an increase of up to 50kPa (5m head) of
pressure just ahead of the face
• Effects on stability are both adverse (increased
water pressure) and beneficial (outward seepage
from face)
• Net effect shown to be equivalent to an increase of
water pressure of 20 to 30 kPa in sand. This effect
should be added to the initial piezometric pressure
in deriving P in  = F0’D – F1c’ + P
After Shirlaw, J. N.
37
PLANNING OF FACE PRESSURE – SAND
• Need to carry out an Ultimate Limit State (ULS) calculation
• Partial factors: divide by tan ’ by 1.2, c’ by 1.2 (but c’
often taken as 0). Factoring tan ’ affects FO and F1
• Water pressure: use the most onerous (highest) likely
pressure. Need to allow for the tunnelling effect on the
water pressure. Need to consider the change of water
pressure when tunnelling through changes in ground level.
• If settlement is an issue, need to carry out a Serviceability
Limit State (SLS) calculation. Use partial factors of 1 for
SLS calculation
38
PLANNING OF FACE PRESSURE – SAND
• Limit equilibrium methods give pressure required
to avoid failure, not to control settlement. Need
higher pressure to control settlement.
• Limited theoretical basis: To target 1% Volume
Loss
 = F’D + P
where:
F = 0.25 for Dense Sand (SPT >30)
F = 0.4 for Medium Sand (SPT 10-30)
F = 0.55 for Loose Sand (SPT <10)
HK GEO Report No. 249 – Ground Control for Slurry TBM Tunnelling . Dec 2009.
39
PLANNING OF FACE PRESSURE – SAND
Pressure at Control Sensor
• The pressure at the
control sensor
required to achieve
the target pressure
at the crown
(S1) = + SL(ZS1-C) + v
Where SL is the unit weight of the slurry and C is the cover to tunnel crown
(expressed as H in Anagnostou and Kovari Chart)
40
PLANNING OF FACE PRESSURE - SAND
Tunnel excavated diameter: 6 m
Depth of tunnel to tunnel crown: 15 m
Water table: 2 m below ground level
c’ = 5 kPa (factored c’ = 4.27)
’ = 32o (factored ’ = 27.5o )
(unit weight of soil) = 20 kN/m3
F0 = 0.3, F1 = 2.5
 = 0.3 x 10 x 6 – 2.5 x 4.17 + (130+20)
= 158 kPa (ULS at tunnel crown)
2m
15m
D = 6m
 = F’D + P (where F = 0.4 for medium sands)
= 0.4 x 10 x 6 + (130+20)
= 174 kPa (SLS at tunnel crown for approx.
1% volume loss)
Target pressure at crown, considering a
variation of 20 kPa: 194 kPa, or 2 bars.
41
MAXIMUM FACE PRESSURE
• To avoid excessive ground heave, need to
define maximum face pressure
• Two cases:
 If undisturbed ground, pressure required to
heave the ground = Total overburden
pressure + a factor based on shear strength
of ground.
 If there is an old borehole or other open
zone, slurry can escape to surface (Slurry
Shields).
42
MAXIMUM FACE PRESSURE
Slurry blowing
out through an
old borehole.
Grouting of
boreholes using
cement grout
important.
43
MAXIMUM FACE PRESSURE
• Practically if the pressure is kept at or below the
total vertical overburden pressure, there should
be no risk of excessive heave or loss of slurry to
the surface in intact ground.
• In special cases, such as relatively shallow
tunnels it may be found that the maximum
design face pressure exceeds the overburden
pressure. In this case a more detailed calculation
can be carried out, allowing the resistance
provided by the shear strength of the ground.
44
MAXIMUM FACE PRESSURE
• If there is an old borehole or other open zone,
slurry can escape to surface under a pressure of
slZS1 , where sl is the unit weight of the slurry
(typically 1.1 t/m3 for slurry shields and ZS1 is the
depth of the reference sensor).
• Identify as far as practicable any likely open
paths and grout them in advance of tunneling.
• Maintain continuous surface watch during slurry
TBM tunnelling and implement control measures
if a loss of slurry is observed.
45
FACE PRESSURE
Kallang Formation
• In Kallang Formation (marine clay), maintain
between 0.9 and 1.2 x total overburden for
<2% Volume Loss.
• Need to apply this pressure when cover to
Kallang formation is less than about 3m careful consideration needed in build-up of
pressure entering Kallang filled valleys, also
in reducing pressure when leaving such
valleys.
46
FACE PRESSURE
Stiff / Hard Soils
• The Fort Canning Boulder Bed and the
residual soil of Bukit Timah Granite
Formation and Jurong Formation can
generally be tunnelled without a face
pressure, but some face pressure will be
necessary to minimise settlements,
particularly in the lower strength (low SPT)
residual soils.
47
FACE PRESSURE
Variable Strata
• Bukit Timah Granite Formation, Jurong Formation
and Old Alluvium contain zones, beds or lenses of
material which act like a sand.
• In these areas a face pressure of about one half
of overburden pressure is required for stability.
• The strata are variable and it can be difficult to
identify these conditions in advance, so it is
necessary to tunnel on the assumption the
conditions are the most adverse.
48
FACE PRESSURE
Variable Strata
• Face pressure needed to control the ground
is almost identical to that which would allow
slurry to escape up a disturbed zone /
opening to the ground surface
• All known open piezometers and boreholes
must be sealed by grouting
• A close watch on slurry escape and a
contingency plan to contain escape now
common in urban tunnelling
49
EXCAVATION MANAGEMENT CONTROL
•
•
•
•
Establish net dry weight of soil removed and compare with
theoretical dry weight
Accuracy typically +/- 5 to 10%
Not accurate enough to control SLS case, but gives warning of
possible ULS (very large settlement or sinkhole)
Face pressure is the primary control, EMC is a valuable back-up
control
Flow
meter
Slurry TBM – flow and
density, in and out
Belt weigher for EPB
– use two, for crosschecking
50
LASER BELT SCANNER – FOR EPBM
51
METHOD STATEMENTS
• Method statements have to give assurance
that tunnelling will be carried out in a way
that minimises the risk to the structures /
the public.
• The method statements must cover the
major areas of risk, and provide realistic
contingency measures.
52
METHOD STATEMENTS
Areas to be Covered
• Ground interpretation, particularly identification of
interfaces, ground behaviour
• Machine configuration
• Lining method
• Face stability (pressure and volume control)
• Additives
• Grouting - mixes, pressure and volume control
• Break-in (launch)
53
METHOD STATEMENTS
Areas to be Covered
•
•
•
•
•
•
•
•
Break-out (docking)
Head access
Maintenance
Personnel
Site organisation and levels of authority
Monitoring - layout and review levels
Risk Assessment
Contingency planning
54
METHOD STATEMENTS
Ground Interpretation
• Enough information is required to make a sensible
interpretation. It is easy to miss a valley of soft ground
or an area of rock if the boreholes are widely spaced.
• Rock/soil interfaces do not work in simple straight
lines
• Permeability is the key to soil behaviour during
tunnelling. There is no correlation between SPT and
permeability
• Depending on the face pressure to be used, may need
to be supplemented by probing
55
METHOD STATEMENTS
Machine Configuration
• A section through the actual machine to be
used
• Show the key features - head configuration
(cutting tool type and locations, openings),
pressure monitoring locations, conditioning
ports, screw conveyor or slurry transport
arrangement, manlocks, probing
arrangements and possible locations, tail
seals, grouting ports.
56
METHOD STATEMENTS
Lining Type
• The type of lining to be used i.e. segmental,
sprayed concrete, pipe.
• A GA for the lining showing jointing systems
and grout ports.
57
METHOD STATEMENTS
Face Stability
• A detailed plan for the face pressures to be used,
justified by calculation and previous experience.
The plan must be for the whole drive, and
prepared prior to starting tunnelling
• A procedure for changing the target face
pressure from that planned.
• Method(s) for measuring the volume of material
removed
• Procedure for identifying over-excavation during
advance, after taking into account the volume of
conditioner used
58
METHOD STATEMENTS
Conditioning Agent
• Types of conditioning agents to be used
• Typical mixes for conditioning agents
proposed
59
METHOD STATEMENTS
Grouting of Lining
• How the lining is to be grouted, and when in the
tunnelling process
• Grout mixes to be used
• Pressure AND volume controls (Min. Volume, Min.
Pressure, Max. Pressure)
60
METHOD STATEMENTS
TBM Break-in / Launching
• Requirements for ground treatment
and/or probing in the launching area
• Sequence of ground
treatment/probing/tunnel eye removal
• Details of seal to be used at eye
• Procedure for Break-in, particularly
when the annulus around the lining can
be grouted
61
METHOD STATEMENTS
TBM Break-out / Docking
• Requirements for ground treatment
and/or probing in the break-out area
• Sequence of ground
treatment/probing/tunnel eye removal
• Details of seal to be used at eye
• Procedure for Break-out, particularly
when the annulus around the lining can
be grouted
62
METHOD STATEMENTS
Head Access
• If compressed air is to be used for head
access, the planned air pressure if an
intervention is required, for the whole
drive
• If compressed air is not proposed, how
the stability of the ground will be verified
prior to head access
• Procedure for head access
63
METHOD STATEMENTS
Maintenance
• Statement as to how often the cutting tools,
main bulkhead, screw conveyor/slurry pipes
and tail seals will be checked for wear.
64
METHOD STATEMENTS
Personnel / Organisation Chart
• Organisation chart - Identify key personnel
(Project Manager, QP, engineers, machine
operators)
• Give CVs for each of the key personnel
• If the QP is providing representative(s) to
supervise the work, name(s) and CV(s) of the
representative(s)
• Identify what levels of authority each key person
has to vary the operating parameters and
implement contingency measures.
65
METHOD STATEMENTS
Monitoring
• Provide location map for monitoring
instruments. Tip levels for piezometers,
inclinometers, extensometers
• Reading frequency
• Review levels, and actions on exceedance
66
METHOD STATEMENTS
Risk Assessment
• A statement that the use of an EPB or slurry
shield, with experienced operator and pressure
controls results in ‘negligible’ risk of damage will
NOT be accepted
• The risk assessment needs to consider,
separately, all of the ‘high risk’ areas identified
previously, as well as general tunnelling in each
of the ground conditions present
67
METHOD STATEMENTS
Contingency Measures
• Accept that there is a need for contingency
planning in case a sudden loss of ground is
detected
• Need contingency plans that ensure the safety of
the public and minimise disruption to road users
68
BORED TUNNELS
DESIGN AND CONSTRUCTION
Summary
•
•
•
•
•
•
Tunnel Face Pressure Control
TBM Selection
Planning Face Pressure – Clay
Planning Face Pressure – Sand
Maximum Face Pressure
Method Statements for Tunnelling Works
69