Block failure analysis

using digital joint network

characterisation

A. Buyer & W. Schubert

Institute of Rock Mechanics and Tunnelling

Graz University of Technology

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Introduction

Discontinuity Controlled Block Failure

Description

Photo: SchubertPhoto: Schubert

„Rock is heavy and tends to fall down!“

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▪Mapping and description of the joint network geometry ▪ Conventional/manual▪ Digital

▪Definition of the rock mass geometry▪ Block size▪ Block shape▪ Block orientation

▪ Joint properties

▪Stress conditions

Introduction

Discontinuity Controlled Block Failure

Prediction

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▪Analytical▪ Stereographic projection

▪ Key Block Theory

▪Numerical▪ UDEC (2D) / 3DEC (3D)

▪ DDA

▪ UnWedge

▪ Etc.

Introduction

Discontinuity Controlled Block Failure

Analysis

Pötsch (2005)

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▪ Manual mapping of the joint network geometry is insufficient in:▪ Objectivity

▪ Statistical representivity

➢Rock mass models are:➢non-deterministic

➢unspecific

Improving the geotechnical design by applying digital rock mass characterization methods

Introduction

Academic Void and Objectives

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▪Vector-based joint plane detection (JPD)

▪Pixel-based joint trace detection (JTD)

➢Highliy detailed structure map of the joint network geometry

▪Direct implementation of the structure map into 3DEC ▪ Deterministic DFN approach

➢Explizit in joint position, orientation and size

Methodology

Assessment of the Joint Network Geometry

Mapping and modelling

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▪Discrete Element Modelling using 3DEC v.5.2 (Itasca CG, Inc.)

▪Deterministic DFN modelling

▪Actual tunnel specifications

▪Rigid body movement only

▪Sensitivity analysis regarding▪ Joint friction angles

▪ Circumferential stressconditions (K)

Methodology

Numerical Analysis

Model layout

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Case Study – Foliated rock mass

Joint Mapping

Combination of JTD and JPD

Set

ID

Dip Direction

[°]

Dip Angle

[°]

SA

[°]

Concentration DoO

[%]

CoC

[°]

NoM Colour

JS1 196 72 23.5 12.6 84.2 1.9 489 Green

JS2 325 5 18.5 19.9 90.0 1.2 703 Blue

JS3 059 77 21.8 14.5 86.2 1.8 467 Red

JS4 320 40 18.1 20.6 90.3 1.4 493 Yellow

JS5 114 85 27.8 9.2 78.3 2.6 361 Orange

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Case Study – Foliated rock mass

Numerical Analysis

Impressions

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Case Study – Foliated rock mass

Numerical Analysis

Sensitivity of j and K on SVB,0.01

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▪Good match between digital and manual joint mapping

▪Very good detection of the foliation by JTD (even internal folding)

▪Good detection of joint planes by JPD

▪Statistical assessment of the joint size distribution for the DFN worked well

Case Study – Foliated rock mass

Conclusion

Joint mapping

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▪Good match between actual and modelled excavation profile▪ Block detachments localized in correlation with geological documentation

▪ Detaching volumes rather low

▪Sensitivity analyses▪ High influence of j in case of block sliding (SSS-3)

▪ Increasing influence of K in case of confined blocks in the roof section (SSS-4)

Case Study – Foliated rock mass

Conclusion

Block stability analyses

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