Tunnels in Swelling and Squeezing Grounds

Definition of Swelling and Squeezing Mechanisms

Empirical Values of Support for Case Tunnel (Highway)

Analytical Values of Support for Case Tunnel (Highway)

Design Values for the Support through the 2 methods

Excavation and Supports

Conclusion

Definition of Swelling and Squeezing Mechanisms

Swelling Mechanism:

- A combination of physico-chemical reaction involving

water and stress relief leading to volume increase with time

- Argillaceous soil or rock (Clay, Shale/Mudstone, Fault

gouge, and Weathering/Alteration zone)

Squeezing Mechanism:

- Time dependent shear displacement of the ground

leading to inward movement of the tunnel periphery

- Any soil or rocks as long as the ground around tunnel

creep

Ground and Tunnel Parameters

Ground = shale (cretaceous), fault zone

qu = 10 Mpa

v = 0.8 Mpa (Z=40m, r=20 KN/m3)

E = 500 Mpa, = 0.4

Material constant for shale (Hoek-Brown)

m=0.2 & s=0.0001, mr=0.01 & s=0

Max.swelling pressure = 0.2 Mpa (Oedometer test)

Initial radius of tunnel = 7 m

De = 14/1.0 = 14

Plan & Cross-sectional Views of the Case Tunnel

Empirical Method (Q-system)

1. RQD = very poor (10)

2. Jn = 3 joint set + random (12)

3. Jr = smooth planar (1.0)

4. Ja = swelling clay (montmorillonite) filling (10)

5. Jw = medium inflow or pressure (0.66)

6. SRF = mild swelling/squeezing (8)

Q = (RQD/Jn)(Jr/Ja)(Jw/SRF)

= (10/12)(1.0/10)(0.66/8) = 0.007

Category 38 (Barton’s Design Category)

= CCA (sr) 100-300 cm + B (tg) 0.5-1.0 m

Design category by Q-system

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 20 40 60 80 100

ui (mm)

pi

(Mp

a)

Steel rib Con'c lining Rockbolt Ground

Plot of Design Values by RQD

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 20 40 60 80 100

ui (mm)

pi

(Mp

a)

Steel rib Con'c lining Rockbolt Ground

Plot of Design Values by Q-system

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 20 40 60 80 100

ui (mm)

pi

(Mp

a)

Con'c lining Rockbolt Ground

Comparison of Empirical & Analytical Method

Empirical values

By RQD

- rock bolt : 0.6-0.9m spacing

- shotcrete : > 0.15m

- steel rib (very heavy)

: 0.6m spacing

By Q-system

- Cast concrete arch : 1-3m

- rock bolt (tensioned grouted)

: 0.5-1.0 m spacing

More conservative

Safer Design

Analytical values

- rock bolt (mechanically

or chemically anchored)

: 0.5m spacing

: 3m length

- concrete lining : 0.3m

- steel rib : 0.3m

: 12W65

More accurate

More Limitation

Excavation and Support

Excavation Methods

Sequential excavation

- Side drift method

- Heading and bench

Full face excavation

Side drift method

Heading and Bench

Full face excavation

Spring line side drift method

Excavation and Support

Effect of different support measures

Side drift method Heading and Bench

Additional Considerations on Supports in Tunnels in Squeezing Condition

Non-uniform distribution of the ground pressure

Main causes for the deviation of deformations from rotary symmetrical state

Additional Consideration on Supports in Tunnels in Squeezing Condition

Response of Supports to Non-uniform Distribution of Ground Pressure

Conclusion

We made a comparative study on the supports of the case tunnel in time-dependant ground(Shale) with the rock-support interaction analysis and empirical methods (RQD and Q-system).

The design values by rock-support interaction and Q-system showed the satisfactory results with respect to the ground stability than RQD for the given tunnel.

It is not easy to predict how much the swelling and squeezing occur with time at the stage of design.

Most important is the integration of design and construction including monitoring during construction and the possibility to adapt the design, if necessary.