D.lecture 4 Face Pressure B&W Slides

BORED TUNNELS DESIGN AND CONSTRUCTION WEN Dazhi, BSc, PhD MICE, CEng, MIEAust, CPEng, PE, PE(Geo), AC(Geo)

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BORED TUNNELS DESIGN AND CONSTRUCTION

Lecture 1

: Segmental Tunnel Lining Design

Lecture 2

: Settlement due to Bored Tunnelling in Soil

Lecture 3

: Building Assessment

Lecture 4

: Tunnel Face Pressure

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BORED TUNNELS DESIGN AND CONSTRUCTION • • • • • •

Tunnel Face Pressure Control TBM Selection Planning Face Pressure – Clay Planning Face Pressure – Sand Maximum Face Pressure Method Statements for Tunnelling Works

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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 NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL Exceptional Settlement / Sinkholes High Risk Areas (Lecture 2) – 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) • Mixed faces of rock and soil (including soil grades of weathered rock) • Head access for maintenance • Long drives in abrasive ground

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TUNNEL FACE PRESSURE CONTROL • For EPB TBMs, tunnel face pressure is maintained by a combination of

• •

propulsion thrust and removal of spoil at the right rate to match the rate of advance. Spoil in the chamber and in the screw flights must form an effective “plug” to ensure no loss of pressure Foams / soil conditioning agents are often used to condition the spoil

Soil conditioning needs of EPBM in different ground types (EFNARC, 2005) NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL EPBM Increase / Lowering of Shield Advance Rate Increase / Lowering of Screw Discharge Rate

Water

Earth

Pressure NUS CE5104 DWen L4

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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 NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL No plug, material saturated and flowing

EPBM

“Plastic” nature allowing plug and control

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TUNNEL FACE PRESSURE CONTROL EPBM

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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. NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL Slurry Shields

Slurry IN

Slurry & Spoil OUT

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TUNNEL FACE PRESSURE CONTROL Slurry Shields 4

1. Submerged Wall 2. Excavation chamber 5

1 3 2

3. Regulation Chamber 4. Air Cushion 5. Pressure Bulkhead

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TUNNEL FACE PRESSURE CONTROL Slurry Shields

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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) NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL Slurry Shields – Filter Cake loose bedding Bentonite concentration: 40 kg/m3

medium dense bedding Bentonite concentration: 50 kg/m3

loose bedding Bentonite concentration: 60 kg/m3

Thickness of filter cake NUS CE5104 DWen L4

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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

soil

∆P

Pure penetration Maidl, et al (2012)

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TUNNEL FACE PRESSURE CONTROL Slurry Shields – Penetration Model • The penetration distance smax can be calculated from smax =

∆p d10 2τf

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 τf 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.

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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

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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 tunnelling

• 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 µm; 2-stage plants – coarse silt, 35 µm) • Centrifuge or press filters – to separate clay / silt particles NUS CE5104 DWen L4

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TUNNEL FACE PRESSURE CONTROL Slurry Shields – Slurry Treatment Plant

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TBM SELECTION EPB / Slurry Range

100

Clay

Fine

Silt Medium

Coarse

Fine

Sieve Size Sand Medium

Coarse

Gravel Medium

Fine

Coarse

90 80 70 60 50 40 30 20 10 0 0,001 0,002

0,006

EPB Methods

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0,02

0,06

0,2

0,6

2,0

6,0

20.0

60,0

Slurry Methods

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TBM SELECTION EPB

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Slurry

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TBM SELECTION

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TBM SELECTION 100 90 P ercent age P assin g (%)

80 70 60 50 40 30 20 10 0 0.0001

0.001

0.01

0.1

1

10

100

Particle size (mm)

GRADATION FOR F1 SANDS AT RACE COURSE ROAD NUS CE5104 DWen L4

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TBM SELECTION • Kallang Formation, Old Alluvium, Residual soils (Weathering Grade VI) or completed weathered rocks (weathering grade V): EPBM • Tunnelling through mixed face of rock and soil, in particular in Bukit Timah Granite Formation: Slurry TBM

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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

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PLANNING OF FACE PRESSURE - CLAY Stability Number, N Overburden Pressure – Tunnel Support Pressure N= Undrained Shear Strength Surcharge q

(γzO + q – σt) N= cu

C zo D

σt P

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PLANNING OF FACE PRESSURE - CLAY Stability Number at Collapse, Nc

σo = γ zo = (6 to 8)cu cu Failure surface

N = σo / cu < 6 NUS CE5104 DWen L4

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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

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2.5

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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

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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 NUS CE5104 DWen L4

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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) NUS CE5104 DWen L4

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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, γ = 18 kN/m3 P/D = 0, C/D = 2.83, Nc = 9 from Chart

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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 NUS CE5104 DWen L4

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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) NUS CE5104 DWen L4

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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 = σt – (zo – zs1)*γ + v Pressure

v v

Time NUS CE5104 DWen L4

Target Pressure

v v

where v is the max variation of face pressure due to control accuracy (+/20 kPa or 0.2 bar) and γ is the unit weight of the spoil in the chamber 37

PLANNING OF FACE PRESSURE - CLAY

Example of poor control of face pressure for a EPB shield

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PLANNING OF FACE PRESSURE – CLAY Heading Geometry Over cut (typically 10mm)

σt

L

P = 0 or P = Length of shield, L

zo

C D

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σt P

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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 NUS CE5104 DWen L4

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PLANNING OF FACE PRESSURE – CLAY Heading Geometry Ports for bentonite injection (16 No total)

After Shirlaw, J. N. NUS CE5104 DWen L4

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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. NUS CE5104 DWen L4

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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) NUS CE5104 DWen L4

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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: σ’ = F0γ’D – F1c’ +F2γ’Dh – F3c’ Dh/D σ’ = Effective pressure required to maintain equilibrium of tunnel face; γ’ = 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, φ’ and geometrical parameter, H/D and (h0-D)/D; and H = Overburden to crown of tunnel. NUS CE5104 DWen L4

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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, σ at the tunnel crown that is required to prevent a face collapse (maintaining equilibrium) can be calculated : σ = F0γ’D – F1c’ + P where P is the water pressure at the crown of tunnel based on the original ground water level. NUS CE5104 DWen L4

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PLANNING OF FACE PRESSURE - SAND

Anagnostou, G. and Kovari, K. (1996). Chart based assumption of γd / γ’ = 1.6. Since γd / γ’ = Gs/(Gs – 1), the assumption is that Gs = 2.67, which is reasonable within practical limits. NUS CE5104 DWen L4

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PLANNING OF FACE PRESSURE - SAND

Anagnostou, G. and Kovari, K. (1996). NUS CE5104 DWen L4

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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

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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 σ = F0γ’D – F1c’ + P After Shirlaw, J. N. NUS CE5104 DWen L4

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PLANNING OF FACE PRESSURE – SAND • Need to carry out an Ultimate Limit State (ULS) calculation • Partial factors: divide by tan φ’ by 1.2, c’ by 1.2 (but c’ often taken as 0). Factoring tan φ’ 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 NUS CE5104 DWen L4

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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 σ = Fγ’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