This extended abstract presents an interpretation of the response of a mudstone
during the excavation of a shaft at high depths (500m). Instrument layout and numerical model are described. A very good agreement is observed between the field measurements and computed values.Peer ReviewedPostprint (published version

Garitte, Benoit

Gens Solé, Antonio

Vaunat, Jean

English

UPCommons. Portal del coneixement obert de la UPC

X International Conference on Computational Plasticity COMPLAS X E. Oñate and D. R. J. Owen (Eds)  CIMNE, Barcelona, 2009  MODELLING OF A DEEP EXCAVATION IN A STIFF CLAY Jean Vaunat*, Benoit Garitte† and Antonio Gens* * Department of Geotechnical Engineering and Geosciences Universidad Politécnica de Cataluña Campus Norte UPC, 08034 Barcelona, Spain e-mail: jean.vaunat@upc.edu e-mail: antonio.gens@upc.edu  † International Center for Numerical Methods in Engineering (CIMNE) Universidad Politécnica de Cataluña Campus Norte UPC, 08034 Barcelona, Spain e-mail: benoit.garitte@upc.edu Key words: Excavation, Callovo-Oxfordian mudstone, numerical modeling, coupled damage elastoplasticity. Summary. This extended abstract presents an interpretation of the response of a mudstone during the excavation of a shaft at high depths (500m). Instrument layout and numerical model are described. A very good agreement is observed between the field measurements and computed values.    1 INTRODUCTION Within the research program developed to study the safety of storing radioactive waste at large depth in the underground, a laboratory has been built at a depth of 500 m in a formation of stiff clay located in Northern France. For a better understanding of material field behaviour, the response of the formation has been followed during the works realized to build the laboratory. Particularly, the excavation of the main access shaft has been followed by an intense instrumentation between depth 460 and 480 m1.  2 FIELD MEASUREMENTS A layout of the instrumentation is shown in Figure 1.a. Three boreholes equipped with extensometers (red points), one borehole with inclinometer (red squares), four boreholes with pore pressure transducers (blue points) and four boreholes with strain gauge cells (green points) were installed from a lateral niche previous to the excavation of the shaft at the considered depth. Instrumentation campaign is completed by wave velocity survey, hydraulic conductivity measurement by means of pulse tests and installation of radial extensometers after excavation1.  Jean Vaunat, Benoit Garitte and Antonio Gens  2  a) b)  Figure 1: a) experimental shaft and instrumentation layout ; b) Geometry and spatial discretization considered in the numerical model.  3 MODEL DEVELOPED Excavation of the access shaft has been modelled using Code_bright, a Finite Element code that solves 3D thermo-hydro-mechanical problems in geological media. In the analyses presented, the equations for stress equilibrium, water mass balance and solid mass balance were solved simultaneously, resulting in a hydro-mechanical coupled formulation of a Biot type. The mesh used is depicted in Fig. 1.b: it consists of a 2D axisymmetric mesh where the shaft and the lateral niche are represented (the latter in an approximate manner due to the assumption of axisymmetry). A damage/elastoplastic law that couples degradation of both elastic modulus and strength with increasing load has been implemented and the resulting parameters have been calibrated using laboratory tests2.   4 RESULTS AND INTERPRETATION  Values of convergence, displacements and pore pressure computed by the model at a number of points are compared with field measurements in Figure 2, 3, 4 and 5, respectively. A very good agreement can be observed. The examination of a number of modelling features demonstrate the importance of the following points to achieve a good reproduction of field data: vertical and horizontal values of permeability, value of Young modulus, low damage threshold, damage evolution controlled by the energy input to the material (which takes into account the damage in both compression and shear), increase of permeability with damage, and modelling of the upward drainage due to the presence of the lateral niche.  Jean Vaunat, Benoit Garitte and Antonio Gens  3 -5-4-3-2-101234567891011121317-feb-05 08-abr-05 28-may-05 17-jul-05 05-sep-05 25-oct-05Time (days)Convergence (mm)-40-30-20-1001020Relative front elevation (m)z= -467.40 m; r= 3.18 mBase 1-8Base 3-8Base 3-9Base 4-10Base 5-11Base 1-7Base 6-12Shaft relative Debris relative advanceLining relative advance Figure 2: Shaft convergence: comparison between measurements and numerical results.  -1.50-1.00-0.500.000.501.001.502.0017/02/2005 08/04/2005 28/05/2005 17/07/2005 05/09/2005 25/10/2005Time (days)Angular variation (mm/m)-40-35-30-25-20-15-10-50510Relative front elevation (m)REP_2208_ICL_01 z= -460.79 m; r= 6.83 mAngular variation (mm/m)Shaft relative Debris relative advanceLining relative advance Figure 3: Deformation close to the shaft: comparison between measurements and numerical results. Jean Vaunat, Benoit Garitte and Antonio Gens  4 010203040506017-feb-05 08-abr-05 28-may-05 17-jul-05 05-sep-05 25-oct-05Time (days)Pore pressure (bars)-50-40-30-20-10010Relative front elevation (m)REP2103_PRE_01:z= -462.67 m; r=5.16 mFront relative distanceDebris relative distanceLining relative distance-7-6-5-4-3-2-10123456-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6REP2103_PRE15 Figure 4: Pore pressure close to the shaft: comparison between measurements and numerical results.  REP_2104_0205101520253035404517-Feb-05 8-Apr-05 28-May-05 17-Jul-05 5-Sep-05 25-Oct-05Time (days)Pore pressure (bars)-35-30-25-20-15-10-50510Relative front elevation (m)Front relative distance Figure 5: Pore pressure far from the shaft: comparison between measurements and numerical results.  REFERENCES [1] G. Armand, K. Su, Y. Wileveau and J. Delay, J. “Expérimentation REP Résultats des mesures après la phase de fonçage du puits entre les cotes -454 m et -480 m”. ANDRA report DRPADPE050855A, Paris, France. (2005).  [2] K. Su. “Development and validation of the constitutive hydromechanical models for Callovo-Oxfordian Argillites: the MODEX-REP Project”. ANDRA report, Paris, France. (2007). 

Modelling of a deep excavation in a stiff clay

This extended abstract presents an interpretation of the response of a mudstone

during the excavation of a shaft at high depths (500m). Instrument layout and numerical model are described. A very good agreement is observed between the field measurements and computed values.Peer Reviewe

UPCommons

https://upcommons.upc.edu/bitstream/2117/9635/1/2009%20COMPLASX%20Vaunat%20at%20al%20MODEX%20REP%20p659.pdf

Modelling of a deep excavation in a stiff clay

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