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Numerical investigation on the stability of deforming fractured rocks using discrete fracture networks: a case study of underground excavation

  • Luyu Wang
  • Weizhong ChenEmail author
  • Xuyan Tan
  • Xianjun Tan
  • Jianping Yang
  • Diansen Yang
  • Xi Zhang
Original Paper
  • 175 Downloads

Abstract

The stability of a fractured rock mass around a subsurface opening is critical to tunnel excavation. The traditional underground excavation analysis is based on a continuum description of randomly distributed discrete pre-existing fractures. In contrast to this, we have developed an improved hybrid finite element method (FEM) to investigate the stability of fractured rocks around an excavation by incorporating the outputs into the FEM codes. The proposed model consists of a discrete fracture network (DFN) model and cohesive zone model (CZM). The DFN model automatically generates a fracture network with a given fracture opening distribution and provides grid generation strategy to the FEM. The CZM captures material failure and intersection and surface contacts through the fracture element with different constitutive laws. As a case study, a DFN model was formulated for the underground excavation at Jinping Hydropower Station, in light of the input requirements of our model, and its deformation analysis was performed. The comparison analysis of rock deformation was made for excavation in both intact rock mass and fractured rock mass under the same boundary conditions. The numerical results show that two different modes of rock failure exist in these two rock masses and that intense deformation at the fractures intersected by the tunnel is responsible for fractured rock mass instability. The proposed approach was verified using data from field investigations. A larger displacement can be produced if the rock mass is weakened by a key block. A sensitivity analysis was carried out to investigate the effects of different model parameters on deformation variations. This study provides an insightful understanding of the deformation of fractured rock mass during tunnel excavation.

Keywords

Fractured rocks Discrete fractures Underground excavation Stability analysis Hybrid finite element method 

Nomenclature

Latin symbols

b(i)

Fracture width

L(i)

Length of a fracture

KI,

KII Stress intensity factor of pure tensile and shear mode

Eij

Elastic modulus tensor of fracture elements

T0

Thickness of fracture elements

D

Damage factor of fracture elements

Kin

Stiffness for the spring in the Bandis–Barton law

H

Depth of the tunnel

U1

Displacement in the horizontal direction

U2

Displacement in the vertical direction

(x0(i),y0(i))

Center point coordinates of a fracture

(x1(i),y1(i))

Initial endpoint coordinates of a fracture

(x1’(i),y1’(i))

Translational endpoint coordinates of a fracture

(X,Y)

Coordinates on the sides of a fracture domain

Greek symbols

φ

Fracture geometric parameter

f(φ)

Probability density function of a fracture geometric parameter

σ

Standard variance of a fracture geometric parameter

μ

Expectation of a fracture geometric parameter

λ

Horizontal stress factor

γ

Unit weight of rock

α(i)

Dip of a fracture

σ(δ)

Traction-separation law

Γ0

Dissipated energy of material failure

σ

traction on fracture surface

δj

separation on fracture surface

\( {\sigma}_i^0 \)

peak traction of fracture elements.

\( {\delta}_i^0 \)

Critical separation at which traction reaches its peak

\( {\delta}_i^f \)

Critical separation at which traction becomes zero

δm

Effective displacement of fracture surface

δmax

Maximum deformation in the Bandis–Barton law

Notes

Acknowledgements

The authors gratefully acknowledge the support of the Chinese Fundamental Research (973) Program through the Grant No. 2015CB057906.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics Chinese Academy of SciencesWuhanChina
  2. 2.University of Chinese Academy of ScienceBeijingChina
  3. 3.Research Center of Geotechnical and Structural EngineeringShandong UniversityJinanChina
  4. 4.CSIRO EnergyClaytonAustralia

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