High-pressure triaxial testing on the canadian reference buffer material

Engineering Geology, 28 (1990) 391-403

391

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

High-Pressure Triaxial Testing o n the Canadian Reference Buffer Material J. GRAHAM', F. SAADAT' and M.N. GRAY2

' Civil Engineering Department, University of Manitoba, Winnipeg, Man. R3T 2N2 (Canada) 2Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Ltd., Pinawa, Man. ROE 1LO (Canada)

(Accepted for publication February 14, 1989)

ABSTRACT

Graham, J., Saadat, F. and Gray, M.N., 1990. High-pressure triaxial testing on the Canadian Reference Buffer Material. Eng. Geol., 28: 391-403. Triaxial testing is being carried out on a mixture of Na-bentonite and sand to provide parameters for computer modelling of a nuclear-fuel waste disposal vault when the material is used as a sealant. Drained and undrained tests with porewater pressure measurement have been performed at confining pressures up to 6 MPa. The program is now being extended to 10 MPa and elevated temperatures. In the long term, at room temperatures, it is predicted that the material will behave like a normally consolidated clay and strain harden plastically with increasing mean effective stress. In shear, its undrained strength will decrease slightly after failure. Working relationships, based on critical state soil mechanics, have been established for the normal consolidation and failure conditions of the material. The behaviour of the material in drained conditions can be predicted from these relationships. INTRODUCTION

The C a n a d i a n N u c l e a r Fuel W a s t e M a n a g e m e n t P r o g r a m ( C N F W M P ) proposes a c o m p a c t e d " b u f f e r " m i x t u r e of sand a n d b e n t o n i t e as one of several b a r r i e r s limiting r a d i o n u c l i d e release from a used-fuel disposal vault. The c o m p o s i t i o n and e n g i n e e r i n g properties of the C a n a d i a n Reference Buffer M a t e r i a l (RBM) h a v e been reviewed by D i x o n a n d G r a y (1985). The buffer will fill a n n u l a r spaces b e t w e e n c o r r o s i o n - r e s i s t a n t fuel-waste c o n t a i n e r s a n d the walls of b o r e h o l e s drilled in the floor of r o o m s e x c a v a t e d in h a r d p l u t o n i c r o c k 500 to 1000 m b e n e a t h the g r o u n d surface. The buffer will (a) p h y s i c a l l y s u p p o r t the c o n t a i n e r in its borehole, (b) provide good h e a t t r a n s f e r c a p a b i l i t y from the c o n t a i n e r into the n e i g h b o u r i n g r o c k mass, (c) limit the r a t e of r a d i o n u c l i d e release to the geosphere, and (d) resist h y d r a u l i c pressures t h a t could f r a c t u r e the buffer. It was n o t a p p a r e n t a t the b e g i n n i n g of the p r o g r a m t h a t existing g e o t e c h n i c a l experience could be used to predict t h e b e h a v i o u r of the buffer. B e n t o n i t e has a p o t e n t i a l for swelling u n d e r low pressures and c o m p r e s s i n g u n d e r high pressures. The v o l u m e c h a n g e s result from r e l a t i v e l y 0013-7952/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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rapid movement of "free" water in larger void spaces and from slower creep movement of "bound" water. The influence of soil fabric changes on the relationships between external, interparticle and porewater pressures was not fully understood. With the aim of developing a predictive methodology for designing repositories, studies of the buffer were initiated (1) at microstructural scale to define interparticle behaviour, and (2) at macrostructural scale to develop constitutive stress--strain-time models for compacted Laboratory specimens. The purpose of these studies is to relate the properties of the buffer to a known body of geotechnical experience and to produce material properties for finite element modelling of container--buffer-rock interactions. The results of numerical analysis will be compared with comparable values measured in prototype-scale experiments planned for the Canadian Underground Research laboratory (URL) at Lac du Bonnet, Manitoba. This paper presents data from the part of the overall program done at macrostructural scale. The results come from triaxial tests at confining pressures up to 6 MPa. TESTING PROGRAM The RBM consists of a 50/50 mixture by dry mass of sand and bentonite. The sand is well graded fine crushed silica (percentage retained 0.2 mm-1 mm is 60%, uniformity coefficient 4.0). It decreases the drying shrinkage of the buffer (Dixon et al., 1985) and increases its thermal conductivity (Radhakrishna, 1984). The bentonite contains 80% sodium montmorillonite, 10% illite, and smaller amounts of quartz, feldspar, gypsum and carbonates (Quigley, 1984). The liquid limit is 250°/0 and the plasticity index 200. The triaxial tests have mostly been performed on specimens statically compacted to a dry density of 1.50±0.02 Mg/m 3 at 28.3± 1.1°/0 water content and 96.6± 1.6% saturation. This corresponds with 85~/o ASTM Modified Density. Additional tests that been done on material at 95% Modified Density (1.68 ±0.01 Mg/m 3, 22.4 ± 0.3% water content) and on specimens compacted to densities which led to small volume strains (less than 1°/0) under the required confining pressure. Specimen preparation procedures and testing techniques for (a) isotropic incremental loading, (b) incremental shear loading at constant mean pressure, and (c) undrained shear have been described by Graham et al. (1986, 1989). The following parameters have been used in interpreting the results: mean principal effective stress p'=(a't+2a'3)/3, deviator stress q = (a'l - a'3), u is porewater pressure, volume strain v = (~ + 2~3), shear strain ~:= 2(el -~:3)/3 = ~:~-v/3. Graham et al. (1989) have shown that the behaviour of sand-bentonite mixtures can be related to the specific volume V¢, that is, the volume occupied by unit volume of clay particles in the c l a y - w a t e r phase, neglecting the volume occupied by sand particles. To evaluate p' in these relationships it is necessary to understand how bentonites support stresses and transmit porewater pressures. In geotechnical engineering practice it is often assumed (see for example, Morgenstern and Balasubramonian, 1980) that the small clay particles in bentonite (having high

393

TRIAXIAL TESTING OF BUFFER MATERIAL

surface charge densities) are not in physical contact, even at the quite high packing densities in these tests. Stresses in soil masses arise from external loading or body-weight forces. However, the soil is not a continuum and stresses are transferred between particles through net repulsive forces I(R-A)I resulting from the overlapping diffuse double layers round the particles. Morgenstern and Balasubramonian (1980) showed that when the specimen is in equilibrium, with the voids ratio, the ion concentration in the porefluid and the temperature all constant, then [(R - A)[ is constant. In a given soil with fixed particle chemistry and porefluid chemistry, [(R-A)[ depends on the separation of the particles. If the external loading increases, the particles move closer together and [(R- A)[ increases to carry the extra loading. Conversely, when the external loads are decreased, the particles separate under the action of osmotic forces and [(R- A)I decreases until it is again in equilibrium with the external loading. The [(R-A)[ expression is usually related to parallel particles and forces (stresses)normal to the particles.With more general orientation of the particles in the buffer, it would appear likely that the [(R-A)[ term can respond to applied stresses in any direction. With this understanding, the test data have been interpreted assuming: {a} = {I(R- A)I} + u{/}

(1)

when the particles are in volumetric equilibrium. The term u in eq.1 is taken as the porewater pressure in larger interstitial void spaces between the clay particles. Even if some physical contact (and therefore some "effective stress" in the classical sense) does exist between neighbouring particles, the principle of eq.1 is still valid: the stress system which controls the volumetric and shear behaviour of the buffer can be taken as the tensor difference between the externally applied stress and the porewater pressure in interstitial void spaces. Fig.1 presents some data collected to examine the applicability of eq.1. The 2.0

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specimen was initially consolidated isotropically to p= 0.5 MPa, uh = 0.2 MPa, that is, with p'=0.3 MPa. After 42 days the compressive consolidation straining of about 0.8°/,, had largely ceased. At this stage, p and u b were both increased by 0.2 MPa. In principle this should keep the "effective" stress constant in the specimen and no additional volume change should be observed. In fact about 0.1% compressive strain was recorded in a further period of 58 days. In a second specimen, increasing u h by 0.3 MPa produced 0.7°/~ expansive strain. In a third specimen with p'= 3 MPa, increasing the porewater pressure from 1 MPa to 7 MPa produced only 0.1¢Vo volume strain. Other tests in the authors' laboratories have shown that the volume changes associated with simultaneous equal changes in porewater pressure and total pressure (that is. with constant "effective stress") tend to be non-systematic and small. Dixon et al. (1986) presented oedometer data that provide equal evidence for the applicability of eq.1 at equilibrium. As a result of this work, "effective" stresses are interpreted in this paper as the differences between externally applied pressures and porewater pressures in interstitial water measured on instrumentation attached externally to the specimen. Care is taken to consider the "equilibrium" condition under which this assumption is acceptable. We will show that the results form a coherent. logical framework of behaviour even though the precise mechanisms of load transfer remain uncertain. RESULTS

Isotropic consolidation The relationship between p', V,, v and time are examined in Figs.2, 3, 4a. The data come from specimens given access to water and allowed to compress or swell under a chosen p' (cell pressure minus back-pressure) for 10 to 20 days. The selected load durations were a compromise between (a) ensuring the equilibrium required for eq.l by using long increment durations, and (b) minimizing leakage and osmosis through the membranes. The speciments were arbitrarily assumed to have reached equilibrium when the rate of volume straining was less than 0.1°/,,/day. The volume change during consolidation was then combined with measured water contents to show how V, varied with p' at the end of consolidation. The line given by eq.2 in Fig.3 has been regressed in ln(p'), ln(V,)-space through data points from specimens that tended to compress during the subsequent shear phase of testing: ln(V~) = -0.116 ln(p')+ 1.727

(R2 = 0.89, p' in kPa)

(2)

Eq.2 represents a hardening law fbr the buffer. The remaining data come from specimens that tended to dilate during shearing. As expected, the V~ data for these specimens are generally less than those predicted by eq.2. Fig.4a shows data from a typical specimen subjected to incremental isotropic loading with a load increment r a t i o = 1.32, ending in increment 5 with p ' = a .... = 3 MPa. The increments lasted between five and seven days. Volume

TRIAXIAL TESTING OF BUFFER MATERIAL

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strains were largest during the first increment due to installation procedures, specifically some swelling that was allowed as water was flushed between the specimen and the membrane to flush out trapped air. The strains were lowest in the second increment, and increased systematically in succeeding increments. Fig.4a also shows the volumetric straining rate 0.1%/day used as the criterion for acceptable equilibrium. The degree of consolidation evaluated using Sridharan and Rao (1973) was generally higher than 85%. The results of incremental isotropic consolidation tests summarized in Fig.5a show a consistent and repeatable relationship between p' and v for the buffer. The figure has been drawn using the volume strains v accumulated during the first three days

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of each increment (Graham low confining stresses and have been approximated by suggest the bulk stiffness pressure.

et al., 1983a). It shows that the material expands at compresses at higher stresses. In Fig.6, the data K = Ap'/Av in the final increment of each test. They is approximately proportional to consolidation

Incremental drained shear at constant mean pressure p'

Specimens in the second series of tests were initially consolidated isotropically until they reached nominal equilibrium (~<0.1~/o/day). They were then sheared by increasing a' 1 incrementally and decreasing a' 3 so that p' remained constant. Each increment was sustained for four to five days. Small volume strains recorded during this process indicate some coupling between shear stresses and volume strains, and suggest anisotropy in the buffer fabric caused by 1-D compaction in rigid moulds. Fig.4b shows typical ~x-(time) relationships from a specimen with a .... = 1.2 MPa. Generally, ~1 decreased steadily in all increments except the one (increment 5 in Fig.4b) leading to failure at qw~. Fig.Sb summarizes the q-e behaviour of the buffer. To accommodate extra stiffening caused by increased a ..... q has been normalized by qm~. The strains in Fig.Sb are again the accumulated shear strains after three days in each increment. Apart from some initial differences caused by systematic testing procedures, Fig.5b shows consistent results from tests with 0.2 MPa < aco,, < 3.0 MPa (see also Graham et al., 1989). Fig.6 also shows that the normalized tangent shear modulus Gso/a .... at q = qm,,/2 increases less rapidly than the confining pressure. As in Critical State soil mechanics (Wroth and Houlsby,

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1985), this implies that the Poisson ratio v varies with consolidation pressure. Such behaviour would be different from natural clay (Graham et al., 1983b) but is expected in a swelling clay mixture which is markedly more ductile at high pressures than at low pressures. Undrained shear

Specimens not used for the constant-p' tests were sheared in strain-controlled CIU tests. Typical normalized stress strain curves of q, Au and q/p' are shown in Fig.7 for a .... -values of 0.2 MPa and 3.0 MPa. In each case the behaviour is slightly strain softening in shear, with q, p' and Au tending to constant values at the end of the tests. Fig.7a shows porewater pressures decreasing quite markedly towards the end of the test, whereas in Fig.7b, the porewater pressures increase slowly until the end of testing. In common clays, Fig.7a would represent "dilative" or "lightly overconsolidated" behaviour, while Fig.7b would represent "compressive" or "normally consolidated" behaviour. This suggests the behaviour of the buffer can be related to a known body of experience and totally new models of behaviour need not be developed. Fig.8 is a normalized plot of A u / a . . . . versus A p / a . . . . for tests with ~ .... = 0.2 MPa, 1.2 MPa and 3.0 MPa. In an isotropic elastic material the Au vs. Ap relationsip is straight, with slope m = 1.0, Fig.8 shows relatively linear behaviour, but the average m from 21 tests is 2.03_+0.49. This supports the earlier ( 0 ) CTcons =0 2 MPo

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observation that the compacted buffer specimens are anisotropic. There is no statistical difference between m-values from dilative or compressive specimens (2.00_ 0.38 and 2.04_ 0.57, respectively). However, when porewater pressures at failure are examined in terms of the parameter A = Au/Aq then dilative specimens have lower A-values (0.43_+0.17) than compressive specimens (0.77_ 0.15). Fig.7 showed that p', q and Au were changing only slowly at the ends of the tests. When these "end-of-shear" results are plotted (Fig.9a), compressive specimens produce the failure envelope c'= O, ~'= 14 °. This differs by only a small amount (Graham et al., 1989) from corresponding results at peak failure qf. End-of-shear results from dilative specimens tend to lie just above the line in Fig.9a. The peak failure results from these specimens produce a higher envelope c' = 40 kPa, ~b'= 14° (not shown in the figure). Mesri and Olson (1970) indicated a markedly curved effective stress envelope for montmorillonite. In contrast, these tests suggest only a slight curve in the envelope. At stresses up to 1.0 MPa, Graham et al. (1986) proposed ~b'= 16° rather than the value of 14° proposed here from tests up to 3 MPa. The envelope in Fig.9a conforms with the strength expected from the clay fraction of the buffer, and not with the strength of the sand fraction (Graham et al., 1989). SYNTHESIS OF DATA

The purpose of this work was to use laboratory tests at macrostructural scale to develop stress-strain models for computer analysis of container-

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buffer rock interactions in the Canadian nuclear-fuel waste program. Close attention was paid to measuring volume strains and final water contents in the specimens. This has allowed V¢ to be determined at any stage of testing. Figs.2 and 3 (and eq.2) showed p'- V¢ relationships at the end of consolidation which are an extension of the more usual e-log p curve commonly used in soil mechanics. The same approach can be used to develop p'-V¢ relationships at qmax and at the end-of-shear. The best-fit line in Fig.9b through the end-of-shear data from compressive specimens is: ln(V¢) = -0.111 ln(p')+ 1.668

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(3)

Data points from dilative specimens tend to lie below this line in Fig.9b. Again there are strong similarities between these buffer results and the behaviour of lightly overconsolidated clays (Graham et al., 1983b). The stress-strain curves in Fig.7 show that q, Au and q]p' are changing only slowly at the end-of-shear. The end-of-shear values must therefore lie close to the classical Critical State condition dq/d~ = dp'/d~= du/d~= dv/d~=0 (Wroth and Houlsby 1985). When dp'/dr and du/de are zero, then the equilibrium condition in eq.1 is satisfied. On this basis, taking ( p - u) as the mean effective stress p' in the specimen, then the end-of-shear q vs. p' relationship in Fig.9a, and the log(Vc) vs. log(p') relationship in Fig.9b and eq.3 represent different projections of a potential Critical State Line (CSL) in p', q, V¢ space. This line

TRIAXIAL TESTING OF B U F F E R M A T E R I A L

401

is similar in principle to the strength envelope for normally consolidated specimens of common clays. The data for the end-of-consolidation conditions in Fig.3 also define a series of p', q, Vc states where the specimen is essentially at equilibrium (~<0.1°/o/day; q=0). Eq.1 can again be assumed with only small error. The log(V c) vs. log (p') relationship in Fig.3 and eq.2 therefore approximates the condition where the specimen is neither swelling or compressing under the applied stresses. That is, eq.2 in Fig.gb represents the Swelling Equilibrium Line (SEL) for the specimen. In many ways, this is similar to the Normal Consolidation Line (NCL) in Critical State soil mechanics. There is one obvious difference. It can be expected (Graham et al., 1989) that volume changes accompanying stress changes would be reversible along the S E L provided sufficienttime were given for eq.1 to be fully applicable. Elastic-plastic soil models (of which Critical State is a particular example) require formulation of: (1) the behaviour in the elastic, pre-yield range of stresses; (2) the state boundary surface at which yielding takes place; (3) the hardening law which describes post-yield elastic-plasticvolume changes; (4) the flow rule that relates plastic shear strains with plastic volume changes; and (5) a definition of steady-state failure. From this list,Fig.8 suggests that the "elastic" behaviour is fairly linear, but anisotropic. The testing has only used monotonically increasing stresses:no reversibilityhas been proved. Some sense of the shape of the yield surface can be obtained from the normalized q/a'co,, vs. p'/a'co,scurves from compressive specimens shown in Fig.10. It should however be remembered that these undrained, constant volume tests involve positive (compressive) plastic volume strains which equal the negative (expansive) elastic strains accompanying the decreasing p'-values in the figure. Thus the undrained stress paths do not conform exactly with the shape of the normalized yield locus which at this time remains unknown. Differences between the undrained stress path and the yield locus are not usually great I

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(Graham et al., 1983b). Fig.3 and eq.2 show a hardening law in In(V~) vs. ln(p') space. No evidence is yet available to define the flow rule for the material. Research on natural clays (Graham et al., 1983b) suggests t hat an "associated" flow rule may be an acceptable approximation. Failure is defined by M = 0.53 (~b'=14) in Fig.9a and by eq.3 in Fig.9b. In accordance with the overall objective of developing a constitutive model for the buffer, the foregoing analysis of the data suggests t ha t a Critical State model extended to include swelling effects and anisotropy may be an acceptable approximation. Figs.5 and l0 show functional, pressure-dependent relationships for the p' t', q-~: and q-p' behaviour of the buffer. In addition to the elastic-plastic framework that has been laid out in this paper, other work has been done (Yin et al., 1988) in developing the data into a three-function hypoelastic constitutive model. The method takes account of (1) volume straining during consolidation, (2) shear stress shear strain behaviour, and (3) dilatancy and anisotropy.

CONCLUSIONS The testing program has shown t hat the properties of the buffer are controlled by the behaviour of the clay fraction. In long-term applications, the buffer will behave like a compressive, normally consolidated clay. It is plastic strain hardening when considering increases in mean effective pressure. In shear, its undrained strength decreases slightly after failure has occurred. Relationships have been determined in p', q and V¢ terms for consolidation (or swelling equilibrium) and end-of-shearing. These relationships are similar in nature to those identified in Critical State soil mechanics used in natural clay soils. Specimens tested at low consolidation pressures or with shorter test durations exhibit dilative behaviour. They produce Vc-values lower than corresponding longer-term results, and q-values above the Critical State line in q p' space. It is appreciated that this interpretation of the data relies on the assumptions inherent in eq.1 t ha t further work is needed to evaluate its applicability. ACKNOWLEI)GEMENTS The work has been supported by the Natural Sciences and Engineering Research Council of Canada. A.W.-L. Wan contributed to the program, particularly to the tests for the effective stress principle. Ingrid Trestrail. Allison Conn, Narong Piamsalee and Prabir Mitra provided technical support. REFERENCES

Dixon, D.A. and (;ray, M.N., 1985. 3'he engineering properties of buffer material research at Whiteshell Nuclear Research Establishment. Proc. 19th Information Meeting of the Nuclear Waste Management Program, AECL Tech. Rep., TR-350, Vol.3, pp.513-530.

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