TUNNEL

Ken Prabowo B.D.S. / 12010062

Outline

• Definition• Background• Role of Engineering Geology• Tunnel Hazards• Stress• Methods of Excavation• Failure Preventions• Simple Case Study

Engineering Geology Modeling

What’s tunnel?

Tunnel is an artificial underground hollow structure.

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Why do we need tunnel?

• Limited space on surface• Underground mining• Economic reasons ex : Gotthard trans tunnel.

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Why am I interested in tunnel engineering?

Imagine Indonesia in 5, 10, 20, and 100 years….• How will our transportation system develops?• Where will we build any development?• What will our industry needs?• What will we need to develop our economy?• Where will we find our natural resources?

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GO UNDERGROUND!

Where does geological engineer gets involved?

Mostly assessing geological condition of the construction site. But on long tunnel project geologist continuously map the rocks on the projected path of tunnel.

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Where does geological engineer gets involved?

The aim of investigation by moving is to spot weaker area (soft rock) so we can prevent problems that might arise such as ground collapse that might causes TBM to be stuck.

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(a) Boring machine for cutting sample. (b) Investigating cutting samples.

Tunnel Hazard

In general there are certain things to watch out in a tunnel project :

a. Heterogenous geological conditionb. Ground water depressionc. Groundlossd. Ovalization

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Geological Feature

(a) Heterogeneous geological featureSoft rocks are bad news for tunnel engineers.

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Geological Feature

Discontinuity such as fault or fractures also lower the RMR.

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Groundwater

(b) Groundwater depressionSettlements near a tunnel may causes cone

depression

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Pore pressure and Effective Stress

How does it affect the tunnel?Increases effective stress that threaten tunnel

structure.

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Tunnel Deformation

(c) Ground loss(d) Ovalization (includes failure)

(a) (b)

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Ground Loss & Ovalization

Ground collapse caused by excavated tunnel beneath the surface.

Ex : Malaysian SMART Tunnel Project

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GroundLoss

Ground LossStress around tunnel are compressive creating

purely radial displacement

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Ground Loss Model

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Ground Loss Model

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OvalizationUnlike ground loss, the stress that causes

ovalization has vertical compressive stress.

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Ovalization Model

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Ovalization Model

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Ground Loss & Ovalization

Ground loss and ovalization differs by the direction of the stress.

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What caused them?

Stress

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StressThere are two kinds of stress :• In situ stress• Man induced stress

The vertical stress could be formulated as below :

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In Situ StressCaused by the weight of the overlying

natural material/overburden.

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In Situ Stress

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Man Induced StressCould be caused by settlement above

surface or by poor tunnel planning.

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Stress from Settlement

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i = KZo

Methods of ExcavationEngineering Geology Modeling

How do we excavate a tunnel?

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Methods of tunnel excavation. From traditional to high-tech TBM.

How do we excavate a tunnel?• Mechanical

For a big construction project, we would usually use Tunnel Boring Machine. Don’t use spoon to dig a tunnel.

• BlastingUse of explosive to crumble the rocks and carry them out by dump truck or conveyor. Usually used on underground mining.

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Tunnel Boring Machine

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Tunnel Boring Machine

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How does TBM works?

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How does TBM works?

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How does TBM works?

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How does TBM works?

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How does TBM works?

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Blasting

Detonator

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Blasting

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Blasting

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Blasting

Excavation by blasting.Ex : Gabi 2 Rescue Operation on Gotthard Tunnel

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Methods of SupportEngineering Geology Modeling

Failure Prevention

How do you prevent failure from engineering and natural process?

Internal : Ground improvementExternal : Supporting structure

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Ground Improvement

Stabilize the existing ground during construction and then, perhaps with some modification.

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Jet Grouting

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Dewatering

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Supporting Structure

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Gotthard Strassen Tunnel Breakthrough

Shortcrete

Case study

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3 m

5 m

Other info :•Unit weight = 25 kN m-3

•Concrete strenght = 20 MN m-2

Case study

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3 m

5 m

Vertical Stress (a)2.000 m* 25 kN m-3 = 50 MNm-2

Force per 1 meter tunnel (F)5 m2 * 50 MNm-2 = 250 MN

Area needed for the concrete250 MN / 20 MN m-2 = 12,5 m2

However, the only available area for one meter length is 5 m2 . Therefore it’s physically impossible for this tunnel to maintain its structure with the concrete support.

Area per 1 meter tunnel (A)5 m * 1 m = 5 m2

Questions

1. Does the length of the tunnel has any effect on the structural strenght? Why do we always model a tunnel in half plane?

2. What about horizontal compressive stress? Tectonic? Does it have any significant effect on tunnel structure?

3. How do we decide which method of failure prevention to use?

4. How does shear stress works on tunnel?

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References

http://www.fhwa.dot.gov/bridge/tunnel/pubs/nhi09010/07a.cfm

Price, D.G., 2009 ,Engineering Geology : Principles & Practice, Springer, Berlin.

Verrujidt, A & Booker, J.R., 1996, Surface settlements due to deformation of a tunnel in an elastic half plane, Geotecnique 46 No. 4, p. 753 – 756.

Verrujidt, A, 1997, A Complex Variable Solution for a Deforming Circular Tunnel in an Elastic Half-plane, International Journal for Numerical and Analytical Methods in Geomechanics Vol 21, p. 77-89.

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