Erfahrungs- und Forschungsbericht 2012 - Ensi

Erfahrungs- und Forschungsbericht 2012 - Ensi

RC Experiment

Rock Mass Characterization

Author und Co-author(s)



Tel., E-mail, Internet address

Duration of project

R. Thoeny, F. Amann, S. Loew

ETH Zürich

Sonneggstrasse 5, 8092 Zürich

+41 (0)44 632 23 12,

4 years


In 2012 a series of laboratory tests were conducted

to investigate the water retention curve

of the clayey facies at the Mont Terri underground

research laboratory, and to establish

the relationship between the tensile and compressive

strength and total suction. The water

retention curve obtained in this study is based

on 49 data points and is consistent with published

results. Systematic strength tests utilizing

specimens with a uniform pre-defined total

suction revealed that the tensile strength for P-

and S-specimens increases linearly up to a total

suction of 50 to 60 MPa. With further increase

in total suction the tensile strength drops as a

consequence of shrinkage upon drying. Test

specimens for compressive strength tests are

currently under investigation.

Structural und geophysical characterization

of the natural fracture network around the

Gallery 08 at the Mont Terri rock laboratory

revealed important characteristics of multiscale

tectonic faults and fault zones that are

relevant for the mechanical behavior of tectonically

disturbed Opalinus Clay. It could be

shown that fault characteristics such as fault

thickness, tectonic disturbance, persistence

and fault frequency vary considerably along

the RC experiment and that these variations

significantly alter the homogeneity of the rock

mass in strength, stress and deformability. It

could be demonstrated that these variations

have a substantial impact on the spatial distribution

and the radial extent of the excavation

damaged zone (EDZ) along the Gallery 08. Systematic

analysis of the fracture network along

the tunnel invert between GM 40 and GM 127

revealed that failure mechanisms and EDZ fracture

frequencies are significantly influenced by

the pervasive bedding and the occurrence of

pre-existing shear fractures or fault zones in

the surrounding rock mass.

Based on a rigorous review of all laboratory

strength and deformability test data conducted

in the past 10 years on samples taken

at the Mont Terri underground research laboratory,

a peak failure envelope was established

for numerical modeling of the RC experiment.

Excavation procedure, monitoring setup, and

support measures were modeled as built.

Modeling of the rock mass response as consequence

of the step-wise excavation of the RC

experiment is based on a total stress approach,

assuming that the short-term behaviour is

basically undrained. First model results indicate

a heterogeneous depth distribution of the

EDZ along the RC experiment which ranges

between 0.5 and 3 m within the intact rock

mass. Within the more compliant fault zones

the model suggests a significant larger EDZ

depth up to 12 m. An increased EDZ depth in

the intact rock mass adjacent to the fault zones

suggests localized stress concentrations; as a

consequence the EDZ propagates deeper into

the rock mass. The model results are consistent

with both, the magnitude of the measured displacements

and the depth of the EDZ obtained

from seismic tomography.

Laboratory testing and numerical modeling

will continue in 2013. Final results of these

studies are expected in June 2013.

ENSI Erfahrungs- und Forschungsbericht 2012 261

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