Estimating rockmass properties based on DFN methods Caroline

Estimating rockmass properties based on DFN methods Caroline Darcel, Romain Le Goc, Etienne Lavoine, Diane Doolaeghe Itasca Consultants, France Philippe Davy, Univ Rennes, CNRS, France Diego Mas Ivars, SKB, KTH, Sweden ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 1

Objective: Introduce Discrete Fracture Network methods for rockmass modelling DFN modeling Relevant complexity • Anisotropy • Scale effect • Integration from cm to km Modeling purpose • Stress, strain, hydraulic, transport, HM, thermal • Available data • Modeling as DEM, discrete or equivalent continuous modeling Define which metric of a DFN is the controlling factor of rockmass elastic prop ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 2

Classical approach: fractured rock as a block assembly (Hoek and Brown, 2018) ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 3

Concepts for fractured rock Block Assembly population of individual fractures ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 4

Fractured rock – block Assembly vs Network of fracture population of individual fractures ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 5

Fractured rock – block Assembly vs Network of fracture population of individual fractures ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 6

Fractured rock – block Assembly vs Network of fracture population of individual fractures Need to assess the density vs size ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 7

Description of the fractured rock with DFN Originally applied to § Crystalline rocks § Potential host for nuclear waste storage § Connectivity, flow and transport modeling ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 8

Observation and mapping • Mapping conditions: Resolution and Censoring • Physical boundaries of the distribution, min and max sizes, not directly accessible ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 9

A Fracture, what is it 10 m A fracture in the model is an ensemble of fracture segments that define a consistent plane (mechanical coherence). ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 10

A Fracture, what is it North East Roughly Planar discontinuity Lateral dimensions >> thickness Resulting from rock failure controlled by physical processes and field conditions 2 D planar object Can be cracks, joints, faults, shear zones, bedding planes Flow, transport, mechanical properties Position, size, orientation, shape … to the fracture population (Discrete Fracture Network) ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 11

[SKB report P-04 -35] ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 12

Example – 2 D trace size distribution • Power-law trend may be identified • from a limited observation range • parameters independent from observation range ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 13

From data to size distributions – Laxemar site ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 14

DFN in the Rockmass ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 15

Scale, resolution, size, density Resolution increase Start by one single fracture System dimension increase Resolution decrease ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 16

Predicting equivalent elastic properties ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 17

Mechanical model - single fracture Frictionless fracture ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 18

Single fracture isolated ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 19

Frictionless isolated fracture Displacement profile Stress and displacement at fracture shear disp. ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 20

Frictional isolated fracture Fracture friction, cohesion and stiffness terms [Davy et al, 2018] Stress and displacement at fracture Displacement profile shear disp. ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 21

Stress perturbation around a fracture 0 1 ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 22

Two regimes for the shear displacement Fracture sizes are critical ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 23

Shear regime Slip regime Critically stressed fractures Critically stress regime threshold is size dependent Fracture sizes are critical ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 24

From single fracture to DFN and rock mass Rock mass with DFN ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 25

Fracture network to rock mass strain Davy et al. , 2018, Elastic properties of fractured rock masses with frictional properties and power‐law fracture size distributions: JGR, v. 123, p. 6521 ‐ 6539. ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 26

Comparison to numerical simulations Numerical simulation s Theoretical calculations ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 27

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Application to realistic multiscale DFN ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 29

Application to site conditions – Forsmark case § § Input : generated DFN Input : Intact rock properties Input : stress state Output: § Compliance tensor § Scale effect § Level of anisotropy ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 30

Application - DFN and rock conditions SKB Forsmark site, Sweden DFN (FFM 01 unit) Mechanical properties 5 10 15 20 25 No critically stressed fractures ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 31

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20 -25% 12% ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 34

SUMMARY § DFN representation of rockmass help to integrate multiscale fracture distribution § Rockmass effective properties can be derived and controlling factors – as a combination between mechanical, geometrical and scale - identified § Extent of scale effect and anisotropy can be quantified § Tool (DFN. lab) to integrate DFN in geomechanical models ITASCA SYMPOSIUM, VIENNA 17 -21 FEBRUARY 2020 35

DFN. lab Numerical platform for modelling fractured media Expertis Mechanic s e Data Flow Statistical analysis Generatio n model Expertis e 36