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Rock Mechanics and Rock Engineering

, Volume 46, Issue 5, pp 923–944 | Cite as

Experimental Investigations into the Mechanical Behaviour of the Breccias Around the Proposed Gibraltar Strait Tunnel

  • W. Dong
  • E. PimentelEmail author
  • G. Anagnostou
Original Paper

Abstract

The proposed Gibraltar Strait tunnel will cross two zones with breccia consisting of a chaotic mixture of blocks and stones embedded in a clay matrix. The breccia is saturated, has a high porosity and exhibits poor mechanical properties in the range between hard soils and weak rocks. The overburden and high in situ pore pressures in combination with the low strength of the breccia may lead to heavy squeezing. The crossing of the breccia zones thus represents one of the key challenges in the construction of the tunnel. In order to improve our understanding of the mechanical behaviour of the breccias, a series of triaxial compressions tests were carried out. Standard rock mechanics test equipment was not adequate for this purpose, because it does not provide pore pressure control, which is important in the case of saturated porous materials. Pore pressure control is routine in soil mechanics tests, but standard soil mechanics equipment allows only for relatively low nominal loads and pressures. In addition, the low hydraulic conductivity of the breccias demands extremely low loading rates and a long test duration. For these reasons, we re-designed several components of the test apparatus to investigate the mechanical behaviour of the breccia by means of consolidated drained and undrained tests. The tests provided important results concerning the strength, volumetric behaviour, consolidation state and hydraulic conductivity of the breccias. The present paper describes the test equipment and procedures, provides an overview of the test results and discusses features of the mechanical behaviour of the breccias which make them qualitatively different from other weak rocks such as kakirites—a typical squeezing rock in alpine tunnelling. The paper also demonstrates the practical importance of the experimental findings for tunnelling in general. More specifically, it investigates the short-term ground response to tunnel excavation from the perspective of elasto-plastic behaviour with the Mohr–Coulomb yield criterion. The computational results indicate that the breccias will probably experience very large deformations already around the advancing tunnel heading, which can be reduced considerably, however, by advance drainage. The analyses additionally show that plastic dilatancy is favourable with respect to the short-term response, thus highlighting the importance of the constitutive model when it comes to theoretical predictions.

Keywords

Triaxial test Pore pressure Breccia Gibraltar Strait tunnel Squeezing ground Ground response 

List of symbols

A

Pore pressure parameter A

a

Tunnel radius

Af

Pore pressure parameter A at failure

B

Pore pressure parameter B

Cs

Compressibility of specimen skeleton

c

Effective cohesion of ground

cv

Coefficient of consolidation

Cw

Compressibility of water

D

Diameter of sample

Do

Diameter of the oil pressure amplifier cylinder

Dp

Diameter of the axial loading piston

Dw

Diameter of the pore water pressure device cylinder

E

Young’s modulus

H

Height of sample

k

Hydraulic conductivity

n

Porosity

pw

Pore pressure

p

Effective isotropic stress

pw,0

Initial pore pressure

q

Effective deviatoric stress

su

Undrained shear strength

T

Temperature

t

Time

tf

Time taken to reach 95 % dissipation of excess pore pressure at failure

ua

Tunnel wall displacement

Vo

Volume of oil in triaxial system

Vs

Volume of specimen

Vw

Volume of water in triaxial system

w

Water content

z

Coordinate in vertical direction

Greek symbols

αo

Thermal expansion coefficient of oil

αw

Thermal expansion coefficient of water

γw

Unit weight of water

Δho

Displacement of the cylinder of the oil pressure amplifier

\( \Updelta h_{\text{o}}^{\text{temp}} \)

Temperature induced displacement of the cylinder of the oil pressure amplifier

Δhp

Displacement of the axial loading piston

Δhw

Displacement of the cylinder of the pore water pressure device

\( \Updelta h_{\text{w}}^{\text{temp}} \)

Temperature induced displacement of the cylinder of the pore water pressure device

Δpw

Increment of pore pressure

Δσ1

Increment of axial stress

Δσ3

Increment of radial stress

ΔT

Temperature change

ε1

Axial strain

εvol

Volumetric strain

\( \varepsilon_{\text{vol}}^{\text{o}} \)

Volumetric strain (determined via oil volume change)

\( \varepsilon_{\text{vol}}^{{{\text{o}},{\text{corr}}}} \)

Volumetric strain (corrected via oil volume change)

\( \varepsilon_{\text{vol}}^{{{\text{o}},{\text{err}}}} \)

Temperature induced volumetric strain error (via oil volume change)

\( \varepsilon_{\text{vol}}^{\text{w}} \)

Volumetric strain (determined via water volume change)

\( \varepsilon_{\text{vol}}^{{{\text{w}},{\text{corr}}}} \)

Volumetric strain (corrected via water volume change)

\( \varepsilon_{\text{vol}}^{{{\text{w}},{\text{err}}}} \)

Temperature induced volumetric strain error (via water volume change)

η

Constant depending on the drainage conditions

ν

Poisson’s ratio

σ

Total stress

σ1

Axial stress

σ3

Radial stress

σ

Effective stress

\( \sigma_{0}^{'} \)

Initial effective stress

\( \sigma_{1}^{'} \)

Effective axial stress

\( \sigma_{3}^{'} \)

Effective radial stress

\( \sigma_{{{\text{a}},{\text{DR}}}}^{'} \)

Effective stress after advance drainage

σ0

Initial total stress

σa

Support pressure at excavation boundary

σa,DR

Total stress after advance drainage

ϕ

Effective friction angle

ψ

Dilatancy angle

Notes

Acknowledgments

The authors are very glad to thank SECEGSA and SNED for the permission to publish the test results and for allowing the presentation of Figs. 1 and 2. The authors also wish to express their gratitude to Mr. Roca and Mr. Sandoval from SECEGSA and Mr. Bensaid and Mr. Bahmad from SNED for their support for the research project. The research was funded by a Grant from the Swiss National Science Foundation (SNF Grant No. 200021-137888).

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Copyright information

© Springer-Verlag Wien 2013

Authors and Affiliations

  1. 1.ETH ZurichZurichSwitzerland

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