geomechanical behaviour characterisation of the - … 04.pdf · it appears that characteristics of...

Oral Session 4

Characterisation of the Geomechanical Behaviour Chairman : F. HOMAND

O-4-1

International Meeting, December 9-12, 2002, Reims, FranceClays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 49

A CONSTITUTIVE MODEL FOR EASTERNARGILLITE

J. Vaunat, A. Gens, E.E. Alonso

Dept. of Geotechnical Engineering and Geosciences, Technical University of Catalunya,C/ Jordi Girona 1-3, Building D-2, 08034 Barcelona, Spain

The construction of underground laboratory at Bure site, France, puts an emphasis on thecomplexity of the behaviour of the in situ deep argillaceous rock. It presents features such aslow porosity and high skeleton stiffness, significant content in carbonate calcium, anisotropy,elastic stiffness degradation, brittle behaviour during shearing, swelling and crack openingduring unloading (Su and Ozanam, 1999).In this paper, a model is presented which copes with more of the behavioural aspects of thisargillaceous rock. It models the rock as a bonded material and distinguishes two distinctbehaviours. The response of the clay matrix is described by the constitutive law of the claysoil, which can be in the most general case any constitutive law commonly used to representthe response of ductile materials. The response of the bonds is modelled through the semi-logarithmic damage elastic law proposed by Carol et al. (2001) for brittle materials. A methodis finally proposed to combine these two behaviours on the basis of energy equivalenceconsiderations. Description of model conceptual bases will be dealt with in the first part of thepaper. It appears that characteristics of other models for structured geomaterials such asproposed by Gens & Nova (1993) and Kavvadas & Amorosi (2000) can be recovered, withthe difference that irreversible strains in the prefailure domain are explained by bond damage.Such a model allows for reproducing within a limited complexity main aspects of Easternargillite response. Influence of Carbonate content is interpreted as a structuring effect, whoseintensity is related to bond cement characteristics and volume of void occupied by cementmaterial. Elastic degradation is associated to bond damage. Rock brittleness comes naturallyout from material destructuration resulting from bond strength loss and corresponding reportof efforts on the clay matrix. Peculiar aspects of the hydro-mechanical coupling and long-termresponse of the material can also be interpreted within the proposed model if microstructuraleffects are introduced into the constitutive description of clay matrix.A practical aspect of the model is the limited set of parameters it contains. Only twoparameters in addition to the parameters of the clay matrix, which depend on the constitutivelaw contemplated, are in fact introduced. If a simple elastoplastic model based on Hoek andBrown (1980) criterion is used for the clay matrix, only 6 parameters are necessary. They allhave a physical meaning and can be directly assessed on the basis of experimental results. Asan example, the derivation of intact rock and clayey matrix yield criterion is shown in Fig. 1.Data interpretation and parameter assessment for Eastern argillite will be described in thesecond part of the paper.A new and interesting point of the model is the ability of the model to represent distinct rockresponse depending on the state of stresses for which most of bond structuration is assumed tohave taken place during rock genesis. This effect is illustrated on the basis of a synthetic set ofsimulations for the case of a circular excavation at high depth in Eastern argillite. Numericalexamples will be presented in the third part of the paper and aspects related to rock responseduring access shaft excavation at Bure site finally discussed. It is expected that this model willprovide some insights into the complex response of clayey rock for radioactive wasteconfinement.

O-4-1

International Meeting, December 9-12, 2002, Reims, FrancePage 50 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement

0

10

20

30

40

50

60

70

80

90

100

-20 0 20 40 60 80 100

p (MPa)

q (M

Pa)

Laboratory data (peak)Undamaged yield locusFully destructured yield locus

Rc

Rc / (1 + χ0)

Fig. 1 – : Shape of the failure locus for intact and fully destructured Eastern argillite

ReferencesCarol, I., Rizzi, E. & Willam, K. 2001. On the formulation of anisotropic elastic degradation.I. Theory based on a pseudo-logarithmic damage tensor rate. International Journal of Solidsand Structures. 38:491-518.Gens, A. & Nova, R. 1993. Conceptual bases for a constitutive model for for bonded soils andweak rocks. Symp. on Geotechnical Engineering of Hard Soils – Soft Rocks, Athens,1 : 485-494.Hoek, E. & Brown, T. 1980. Empirical strength criterion for rock masses. J. Geotech. Engng.Div., GT9: 1013-1035.Kavvadas, M. & Amorosi, A. A constitutive model for structured soils. Géotechnique,50(3):263-273.Su, K. & Ozanam, O. 1999. Rheological model for the Eastern argillites. Summary of statusfor the rheological studies (Meuse/Haute-Marne site). Internal Report D RP AGEM99-061/A, ANDRA, Paris, France.

O-4-2


A POROMECHANICAL MODEL FOR M / H - MARGILLITE

Dashnor Hoxha 1, Albert Giraud 1, Françoise Homand 1, Clement Chavant 2

1. Laboratoire de Géomécanique, Ouvrages et Environnement, LaEGO-ENSG , Ruedu Doyen Marcel Roubault, B.P. 40, 54501 Vandoeuvre-lès-Nancy

2. EDF/DRD/IMA/MMN, Avenue du Général De Gaulle, B.P.408, 92141 ClamartCedex

The model we present in this paper is the first level of a rheological framework for modellingporo-mechanical behaviour of rocks. This framework is constituted by a multi-levelcomplexity model . Passing from a level to the next more complicated, the description ofmechanical behaviour is refined. Otherwise, this passage needs more information on thephysical mechanisms and identification of a more important number of parameters. We arenot going to give details on this framework, but only a particular form of it used in themodelling of poro-mechanical behaviour of M/H-M argillite.The analysis of active physical mechanisms in this argillite, shows that the poromechanicalbehaviour of this rock could be described using a particular form of general yield surface byfixing to zero one of the parameters. This form of yield function could be written as :

p.C.C)(g.q)q,p(f *** αθ ρτ −−= (1)

with SSJq :233 2 == δσσ kkS

31−=

3kkp σ

= ��

��

�−= 2/32

3

233

31

JJ

ArcSinθ

In the equation (1) the values with (*) are those used in writing this particular form andchosen to be all positive. The relations of these parameters with parameters used in generalform of yield surface are as follows 0CC;0;0CC *** >=>−=>= ρρττ αα ; β=0The g(θ) function is introduced in order to count for the role of Lode's angle in the overallbehaviour of the rock. Its particular form is chosen as follow :

( ).R.2

3sin).R1()R1(gs

ss θθ −++= (2)

In the plane p-q equation (1) presents a parabola The plastic flow is defined by the followingplastic potential :

( )

2ppN

2q)p,q(g

2c

2 −−+= (3)

The parameter pc is assumed to satisfy the dilatancy surface which for the rock in study, in thecompressive stress domain is of the following form :

0.),( ≡−+≡ δκδ Cpqqp cc (4)Note, that equation (3) presents in p-q plane an ellipse, with an axis on p axis and the othersemi-axis defined by (4). In fact the dilatancy surface is the geometric place of the ellipse'stop points. That means the equations (3) and (4) could describe both plastic contraction andplastic dilatation.

O-4-2

International Meeting, December 9-12, 2002, Reims, France2 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement

We suppose that internal active force of a rock at any moment of hardening is a sum offrictional and cohesive force. During the hardening and softening there is an evolution of thecontributions of each of these forces in the overall behaviour of rock. In order to count forsoftening behaviour, two strain hardening mechanisms are introduced. The first is a positivehardening mechanism due to the accumulation of plastic distortion :

( )pb aaeCCp

γγττ .. 210 ++= − (5)

The equation (5) presents the evolution of friction force. The initial value of this force, itsmaximum and its residual value are respectively :

10 aCC ini += ττ2

12 .2

0maxa

aba

ebaCC

−−+= ττ 0ττ CC res = (6)

The second hardening mechanism concerns the cohesive force : it becomes active once thedialatancy onset is reached in the history of rock loading. The following expression isproposed :

( ) pv

pv bb eCekcScC εε

ρ−− ==−= ...1. 00

00 (7)

This last mechanism gives the possibility to model brittle behaviour of rock for p positive (ornegative but moderated value), and ductile behaviour for p negative.The rate of plastic flow is defined assuming a normal dissipation. The expression for plasticdistortion and plastic volume strain are respectively :

��

��

�

∂∂

>−<−+=∂

∂=q

p).)q(pp(Nq.d

q)p,q(g.dd c

cp λλγ (8)

))q(pp(N.dp

)p,q(g.dd cpv >−<−=

∂∂= λλε (9)

Some simulations of typical laboratory tests are performed and compared with laboratorydata.

O-4-3


EXPERIMENTAL STUDY OF HYDROMECHANICALBEHAVIOUR OF THE CALLOVO - OXFORDIAN

ARGILLITESC.L. Zhang, T. Rothfuchs

Gesellschaft für Anlagen - und Reaktorsicherheit (GRS), Braunschweig, Germany

The most important MODEX-REP experiment will be conducted in the main shaft of theMeuse/Haute-Marne Underground Research Laboratory (MHM-URL) in Eastern of France inorder to investigate the hydromechanical response of the Callovo-Oxfordian argillites to shaftsinking. For modelling the coupled hydromechanical process, respective material parametersare required. An extensive laboratory test programme was performed by GRS within theMODEX-REP project, including uniaxial and triaxial compression tests for characterising theshort-term mechanical behaviour of the clay rock, uniaxial creep and relaxation tests for thelong-term behaviour, and permeability tests for characterising the hydraulic behaviour.Testing material was taken from various depths between 434 and 506 m of the boreholeEST205, drilled at the axis of the auxiliary shaft of the MHM-URL.

The uniaxial compression tests focused on investigating the effect of water content on themechanical behaviour. Figure 1 gives an example of the results. The peak strength and thefailure deformation of the air-dried samples were nearly two times higher than that of thesaturated ones. Young’s modulus measured on the unloading paths in the uniaxial testsincreases from ~5500 MPa to ~7500 MPa with an increase of the axial stress from 2 MPa to20 MPa, independent of the water content. In order to characterise damage, failure andresidual strength of the clay rock, multistage triaxial compression tests were carried out oncylindrical samples oriented perpendicular to the bedding plane in undrained conditions.Figure 2 shows an example, in which axial yield stress σ1-Y, onset of dilatancy σ1-D, peakfailure σ1-F and residual strength σ1-R are marked. The strengths increase with confiningpressure. Based on the own data and some other data obtained on the clay samples taken byANDRA from the same rheological zone, the strength parameters were estimated for the clayrock (Figure 3):

Hoek-Brown: ( ) 212

331 cc sm σσσσσ +⋅⋅+= Mohr-Coulomb: c+⋅= ϕστ tanPeak strength: s = 1, m = 2.5, σc = 25 MPa c = 9.0 MPa, ϕ = 19°.Residual strength: s = 0, m = 2.5, σc = 20 MPa c = 4.2 MPa, ϕ = 19°.

Uniaxial creep tests were conducted in creep rigs at ambient temperature. Axial load wasapplied stepwise from 2 MPa to 15 MPa on saturated samples: 6 large samples with 100 mmdiameter and 200 mm length, and 5 small samples with D/L= 45/90mm, orientedperpendicular and parallel to the bedding plane. Most of the tests lasted for about 1 year. Asan example, some creep curves of the small samples are given in Figure 4. The mainobservations are :(1) the clay rock creeps under very low loads of 2–3 MPa, indicating no low creep limit,(2) the large samples show relatively higher instantaneous elastic deformations in comparison

to the small ones for a given load. However, no significant differences in pure viscoplasticdeformation and creep rate were found for the both sizes of the samples, i.e. no significantscale-effects on the creep behaviour,

O-4-3


(3) instantaneous elastic deformations perpendicular to the bedding are larger than parallel tothe bedding. However, no significant differences in creep deformation and creep rate werefound for the both loading directions (Figure 4),

(4) over some months of creep duration, steady-state creep was reached beyond the transientcreep. A linear relationship of the steady-state creep rate to the stress was found, but thecreep parameter depending on location of the rock,

(5) desaturation of the clay samples, exposed to the air in the room under 15 MPa, leads to arapid increase of the deformation up to a peak, and beyond the peak, some reversedeformations were measured in the further creep phase, as shown in Figure 4.

Some results of the uniaxial relaxation tests on the saturated and air-dried samples areillustrated in Figure 5. Under the comparable conditions, the axial stress in the saturatedsamples relaxed somewhat faster than observed at the air-dried ones.

Gas permeability of the rock was measured on dry samples under confining pressures pc up to16 MPa, as shown in Figure 6. Permeability parallel to the bedding plane varied between8·10-19 and 1·10-17 m2 for pc = 2.4 – 3.0 MPa and k = 5·10-19 - 8·10-19 m2 for pc = 14.5 –16.2 MPa, while permeability perpendicular to the bedding was obtained in the range of6·10-20 - 3·10-19 m2 for pc =2.4 – 3.0 MPa and k = 4·10-20 - 1·10-19 m2 for pc =14.5 – 16.2 MPa.That means the permeability perpendicular to the bedding is about one order of magnitudelower than that parallel to the bedding in the testing range. Dependence of the permeability onconfining pressure is not very significant. The mean values of permeability for the clay rockare given as follows: ( ) n

oco ppkk −= , where po = 1 MPa, and ko = 1.5·10-19 m2 andn = 0.2 for the permeability perpendicular to the bedding; and ko = 6·10-18 m2 and n = 0.8 forthe permeability parallel to the bedding.

0

1 0

2 0

3 0

4 0

5 0

0 0,5 1 1 ,5 2

A x ia l stra in (% )

Axi

al s

tres

s (

MPa

)

sa tura te d sa m plew = 7 .1 %

air-dried s am plew = 2 .8%

σ c = 41 .5 M P ae c = 1 .9 %

σ c= 2 4.4 M P aec = 1 .0%

Fig. 1 : Uniaxial compression tests

0

10

20

30

40

50

0 4 8 12 16 20

Confining pressure σ3 (MPa)

Stre

ss d

iffer

ence

σ1 -

σ3

(MPa

)

Peak strength

Residual strength

Fig. 3 : Peak and residual strength

0

5

10

15

20

0 0,5 1 1,5 2 2,5 3 3,5

Time (day)

Stre

ss

(MPa

)

EST05677-02: w=7.1%EST05677-04: w=2.8%

Initial conditions of Relaxation I: EST05677-4: w=2.8%, ε=0.19%, σo=5.0 MPaEST05677-3: w=7.1%, ε=0.17%, σo=4.7 MPa

Initial conditions of Relaxation II: EST05677-4: w=2.8%, ε=0.53%, σo=15.0 MPaEST05677-3: w=7.1%, ε=0.49%, σo=14.6 MPa

Fig. 5 : Uniaxial relaxation tests

0

10

20

30

40

50

60

-2 ,0 -1,5 -1,0 -0,5 0 ,0 0,5 1,0 1,5 2,0 2 ,5 3 ,0 3 ,5 4,0 4,5

S tra in (% )

Axi

al s

tres

s (

MPa

)

E S T 05677 -01 -C

σ 3 3 .7

7.4

15.8

7.4

εv ε1ε3

3.71.2

1-R

σ 1-R

σ 1-R

σ 1-Y

σ 1-D

σ 1-R

σ 1-F

σ 1-F

Fig 2 : Multistage triaxial compression test

O-4-3


0,000

0,005

0,010

0,015

0 50 100 150 200 250 300 350Time [ day ]

Axi

al s

trai

n

[ - ]

EST05582-03-II EST05582-02-IIEST05630-02-= EST05671-02-=EST05671-03-=

D45mm/L90mm, T = 24oC

σ1 (MPa):

5

8

12

perpendicular to the bedding plane

parallel to the bedding plane

15

T=18°C

desaturation

Fig. 4 : Uniaxial creep tests1E-20

1E-19

1E-18

1E-17

1E-16

0 2 4 6 8 10 12 14 16 18

Confining pressure (MPa)

Perm

eabi

lity

(m

2 )

05547/02-= 05547/03-= 05547/04-=05547/01-= 05751/02-II 05751/03-II05547/05-II average (perpendicular) average (parallel)

perpendicular to the bedding plane

parallel to the bedding plane

Fig. 6 : Gas permeability tests

O-4-4


DILATANCY EVOLUTION AND FAILURE OF THEOPALINUS CLAY

Udo Hunsche, Fritz Walter, Hajo Schnier

Federal Institute for Geosciences and Natural Resources, D-30655 Hannover, Germany

Constructing openings in the deep underground rock causes the stress field to change. Creepdeformation, dilatancy due to micro-cracking, permeability increase, and perhaps failure willtake place in the excavation disturbed zone around the opening. Elastic deformation, stressredistribution and long term creep will occur in an outer zone.

Argillaceous rocks contain liquid inside the pores (free water) and liquid bond to the “phyllosilicates” (adsorbed water). Exposing such rocks to mechanical and thermal loads may causechanges in pore volume and pore pressure as well as water content and suction pressure.

To understand the on-going micro-mechanical processes in the rock and to predict itsmechanical behavior through numerical simulation, the rock must be investigated and aconstitutive geo-mechanical model has to be developed.

Within the Mt. Terri project BGR has investigated the deformation behavior of the OpalinusClay in laboratory tests on cubic and cylindrical samples. True triaxial compression andextension tests have been performed to determine the dilatancy boundary and the dilatancyevolution in the stress domain below the failure envelop. The test matrix included differentmean stresses, temperatures, water contents, and angels between the direction of the majorprincipal stress and the foliation plane.

The results confirm the distinct anisotropy of the material and suggest a dilatancy boundary atshear stresses significantly lower than the failure boundary. Dilatancy evolution and failureare influenced by the loading angle, water content, and temperature of the material.

In strain rate controlled tests on cylindrical samples, failure and post failure strength weredetermined at different strain rates, temperatures, and water content. The results reveal thematerial strength as function of this conditions.

In the future, material damage will be observed during the tests by ultrasonic velocity andattenuation measurements.

O-4-5


PRELIMINARY RESULTS ON THE MECHANICALAND PHYSICAL PROPERTIES OF SEDIMENTARY

ROCKS IN THE HORONOBE URL PROJECTHiroya Matsui, Shunichi Miyanoma

Japan Nuclear Cycle Development Institute, 4-33, Muramatsu, Tokai, Ibaraki,319-1194, Japan

Horonobe URL site is located at the northernmost part of Japan. This is an undergroundresearch laboratory in a tertiary sedimentary rock. Japan Nuclear Cycle Development Institute(JNC) is planning to build it at a depth of about 500m for the geological science andgeological waste isolation studies. The research themes at the site are outlined in theaccompanying paper.

The URL project is divided into three stages. Currently it is in the first (surface investigation)stage. Various mechanical, physical, and geochemical tests have been carried out on rockspecimens retrieved from two-700m deep boreholes. Geophysical logging and hydrologicaltests have been performed in the boreholes, together with in-situ stress measurements byhydraulic fracturing method at selected depths.

In this paper, test results on the mechanical and physical properties on rock cores are mainlypresented. Uniaxial compression tests, consolidated-drained and consolidated-undrainedtriaxial compression test, and splitting tensile strength tests have been carried out to obtainmechanical properties of the rock cores. The tests results conducted so far appear consistentwith those of tertiary sedimentary rocks found in other areas of Japan. In general, boreholeHDB-1 shows gradual increase in strengths and elastic wave velocities with depth, whereasborehole HDB-2 shows occasional portions with low elastic wave velocities, suggesting thatthere is strong heterogeneity with depth in the rock mass properties around the boreholeHDB-2.

As to the physical properties, both boreholes show rather high porosity values (as high as60%) and high water absorption rates, compared with the data on other tertiary sedimentaryrocks in Japan. These features have to be carefully incorporated in the design and constructionof the access vertical shafts and the underground test facilities.

O-4-6


THE EXTENSION OF THE URF HADES :DEMONSTRATION OF THE FEASIBILITY OF

INDUSTRIAL EXCAVATION IN DEEP CLAY LAYERSMarc Demarche, Frédéric Bernier

ESV EURIDICE GIE, Boeretang 200, 2400 Mol, Belgium

In 1995, SCK•CEN and NIRAS/ONDRAF founded the Economic Interest Grouping (EIG)PRACLAY. December 18, 2000 the name of the E.I.G. PRACLAY changed in E.I.G.EURIDICE. One of the main objectives of this grouping is to contribute to the demonstrationof the feasibility of the deep disposal of radioactive waste in a geological clay formation.Another main task of EURIDICE is the extension, operation and management of the URFHADES situated at a depth of 223 m in the Boom Clay layer.

The URF HADES needed to be extended by an 80 m long gallery connecting the existingURF HADES with the new build second access shaft. The E.I.G. EURIDICE decided that thisextension must be considered as an important milestone in the demonstration programme. It isindeed the first time that an industrial technique is used for the realisation of a gallery inBoom Clay at such depth. Furthermore a large instrumentation programme (CLIPEX ECproject) in and around the gallery was set up in order to improve our understanding of thehydro-mechanical response of the rock during and after its construction.

On the 7th of March 2002 the connection between the second shaft and the existing URFHADES was a fact. The excavation technique consist in the excavation of the clay rock with aroadheader under protection of a shield and the construction of the lining, using the "WedgeBlock System", behind the shield (cf. Fig. 1). The main advantages of this tunnellingtechnique is that it causes a minimal disturbance of the clay massif, it allows a very goodcontrol of the excavation parameters and high excavation rate can be reached. To apply the"Wedge Block System" however, the "instantaneous convergence” of the clay massif had tobe assessed quite precisely. This was one of the main challenge of the project because of theprevious tunnelling works in HADES were realised manually without protection of a shield sothat it was not possible to get direct measurements of the instantaneous convergence.

The construction of the connecting gallery has demonstrated that it was possible to excavategalleries in Boom Clay at great depth using industrial techniques with high excavation rate.The excavation parameters were accurately controlled. The measurements performed duringexcavation has allowed to fully characterise the instantaneous hydro-mechanical behaviour ofBoom Clay in terms of pore water pressure and displacements. Models have allowed topredict correctly the radial displacements but are not able to predict the pore water pressurevariation in the far field (at more than 50 m of the excavation). This will make the object offuture research.

In the current paper the excavation technique and the instrumentation programme will bedetailed. Results and lessons learned will be given and recommendations for future will bediscussed.

O-4-6


AcknowlegmentThe authors wish to acknowledge the EIG EURIDICE team as well as the other partners ofthe project: Design : Tractebel Development Engineering (Belgatom); Supervising : SECO;Contractor : JV SCM (Smet-Tunnelling • Wayss & Freytag AG • Deilmann-Haniel GmbH).The CLIPEX EC instrumentation programme was supported by the European Commission,ANDRA, ENRESA and EIG PRACLAY under contract FI4W-CT96-0028. This support isalso gratefully acknowledged, together with the collaboration of the other partners G3S, UPMand GEOCONTROL.

Fig. 1 : Tunnel machine used for the excavation of the connecting gallery in Mol at 223 mdepth in Boom Clay

Agence nationale pour la gestion des déchets radioactifs

Parc de la Croix Blanche -1/7, rue Jean Monnet - 92298 Châtenay-Malabry Cedex Tél. : (33)1 46 11 80 00 - http://www.andra.fr

©An

dra

- 205

- N

ovem

ber 2

002

- cré

dits

pho

tos

: Offi

ce d

u To

uris

me

de R

eim

s - c

once

ptio

n gr

aphi

que

: ada

msk

ides

igns

.com

geomechanical behaviour characterisation of the - … 04.pdf · it appears that characteristics of...

Documents