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LABORATORY HYDRO-MECHANICALCHARACTERISATION OF BOOM CLAY AT ESSEN

AND MOL

Y. F. Deng1, 2, A. M. Tang2, Y. J. Cui2, X. P. Nguyen2, X. L. Li3, L. Wouters4

1. Southeast University, Institute of Geotechnical Engineering, Transportation College, Nanjing,China (noden@163.com)

2. Ecole des Ponts ParisTech, Navier/CERMES, Marne-la-Vallée, France (cui@cermes.enpc.fr)3. Euridice Group, c/o SCK/CEN, Mol, Belgium (xli@sckcen.be)4. ONDRAF/NIRAS, Belgium (l.wouters@nirond.be)

Boom clay has been selected as a potential host hock formation for the geological disposal of radioactivewaste in Belgium. At Mol site, an Underground Research Laboratory (URL) was built at a depth of about223 m to study the feasibility to dispose the High Level Waste (HLW) in Boom clay formation. Since theconstruction of the URL about 30 years ago, many thermo-hydro-mechanical experimentations have beencarried out around the excavations (Bernier et al., 2007). The Essen site is located in the north-east ofBelgium, about 60 km far from Mol and has been considering as an alternative site (De Craen et al., 2006).Borehole was drilled at Essen site in order to make fundamental geological, hydraulic, geochemicalinvestigation of Boom clay at that site. Soil cores at different depths were taken for geo-mechanicalcharacterisation. In the present work, the hydro-mechanical behaviour of Boom clay taken from the Essensite was investigated in laboratory and the results were compared with that of the Boom clay taken fromthe URL at Mol.

Four cores of Boom clay were taken at various depths. The identification physical-geotechnical propertiessuch as specific gravity Gs, liquid limit wL, plastic limit wP, plasticity index IP, blue methylene value VBS,and carbonate content (CaCO3) are shown in Table 1. In addition, some properties corresponding to itsinitial state (water content wi and void ratio e0) are equally shown. The corresponding properties of Boomclay from Mol (according to the data collected by Francois et al., 2009) are presented in Table 1 as well.It can be noted that the soils from the two sites present similar physical-geotechnical properties. The onlydifference lies in the initial void ratio: the Boom clay at Essen presents a larger void ratio.

Different mechanical tests have been run. First, oedometer tests were performed in order to study thecompressibility and the permeability of the soil. Loading and unloading paths were followed in steps fora wide range of vertical stress (from 0.05 to 32 MPa). The compression index Cc and the swelling indexCs as well as the pre-consolidation stress σv0 were determined and compared to those of Boom clay at Molsite. The hydraulic conductivity was determined using Casagrande method; It was found that the Boomclay at Essen site and that at Mol site present similar fundamental hydraulic behaviour: the hydraulic

Table 1: Basic properties of Boom clay at Essen and at Mol

Core No. Depth (m) Member GswL

(%)wp

(%)Ip

(%)wi e0

VBS(g/100g)

CaCO3(g/kg)

Ess75 218.91-219.91 Putte 2.65 78 33 45 29.7 0.785 6.47 9.1

Ess83 226.65-227.65 Putte 2.64 70 33 37 27.2 0.730 6.67 7.6

Ess96 239.62-240.62 Terhagen 2.68 69 33 36 26.5 0.715 6.20 2.4

Ess104 247.90-248.91 Terhagen 2.68 68 29 39 27.7 0.700 6.67 43.6

Mol site 223 Putte 2.67 59-76 20-26 - 20-40 0.43-0.66 - -

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conductivity varies from 10-13 to 5 × 10-12 (m/s) over a range of void ratio from 0.3 to 0.8 which is quitesimilar to that of Boom clay at Mol site. Secondly, for each soil core, three triaxial compression tests atvarious confining pressures were performed. Compression tests in oedometer and in triaxial cell wereequally carried out. The compression tests were used for determining the compressibility behaviour and,also the soil permeability. The triaxial tests were used for determining the shear strength characteristics(effective cohesion and internal friction angle). On the other hand, the peak strength values, the isotropicpre-consolidation pressure and the oedometer pre-consolidation stress were gathered in order to analyse theyielding and failure behaviour of the soils.

Comparison between Boom clay from Essen and that from Mol shows that, on the whole, the initial voidratio of Boom clay from Mol is lower than that at Essen; the relationship between void ratio andpermeability of the two sites are similar. As far as the compression behaviour is concerned, Figure 1 showsthe loading-unloading- reloading curve of the sample Ess83 from oedometer test. The identifiedpreconsolidation stress is 2.28 MPa, equal to the in-situ effective vertical stress estimated by consideringthe soil density and depth. This suggests a normally consolidated state of Boom clay from Essen with aoverconsolidation ratio OCR = 1. Note the results from isotropic compression test confirm this point. ForBoom clay from Mol, Francois et al., (2009) reported an OCR value of 2.

The preliminary laboratory test results on the Boom clay cores from Essen site and the comparison withthose of Boom clay at Mol site can already provide important information to the transferability ofknowledge on Boom clay at different sites, taking into account the fact that most investigations werecarried out in URL at Mol site.

References:Bernier, F., Li, X. L., Bastiaens, W., 2007. Twenty-five years’ geotechnical observation and testing in the

Tertiary Boom clay formation. Géotechnique, 57(2) 229-237.

De Craen, M., Wemaere, I., Labat, S., Van Geet, M., 2006. Geochemical analyses of Boom Clay porewater and underlying aquifers in the Essen-1 borehole. External report, SCK.CEN-ER-19, 06/MDC/P-47, Belgium

Francois, B., Laloui, L., Laurent, C., 2009. Thermo-hydro-mechanical simulation of ATLAS in situ largescale test in Boom Clay. Computers and Geotechnics 36(4): 626-640.

Figure 1: e - logσv curve of Ess83.