Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Project goals

The primary objective of the RC experiment is to

characterize the mechanical rock mass properties

of Opalinus Clay relevant for the scale of a repository

drift. Of particular interest are the mechanical

characteristics of pre-existing faults or fault zones

and their influence on the stress redistribution in

the near-field of the excavation. In 2008, a 5 m

high, horseshoe-shaped tunnel section located between

GM 94.5 and GM 127 of Gallery 08 was

used to characterize the rock mass behavior of

Opalinus Clay during and after tunnel construction

(Thoeny et al. 2010). Complementing the in-situ

experiment, a laboratory investigation program

was conducted to improve our understanding of

the failure process of intact Opalinus Clay under

different loading conditions (Amann et al. 2009

& 2010). The synthesis of geological, laboratory

and monitoring data contributes to a better understanding

of the long- and short-term excavationinduced

rock mechanical processes in the near-field

of an excavation in a transversal isotropic and heterogeneous

clay shale, thus improving our ability to

properly characterize this material for future project

requirements.

Work carried out and results

obtained

Rock-water characteristic curve

In 2012 a series of laboratory tests (Zimmer 2012)

were conducted to quantify the influence of total

suction on the mechanical properties of Opalinus

Clay. Eight desiccators were built to dry samples

under controlled environmental conditions. The

temperature in the laboratory was held constant at

22 °C, and the relative humidity in the desiccators

was controlled by supersaturated salt solutions

(Table 1). Samples were dried under these environmental

conditions to constant weight. After drying

Figure 1:

Rock water characteristic curve obtained in this study

(Zimmer 2012). For comparison the water retention curves

obtained by Ferrari and Laloui (2013), Zhang et al. (2007) and

Major et al. 2007 are shown.

Figure 2:

Relationship between Brazilian Tensile Strength and total

suction parallel and perpendicular to the bedding orientation

(Zimmer 2012).

it was assumed that the total suction in the samples

is uniform and equal to the values given in Table 1.

The data obtained for water loss and the total water

content were used to calculate the total porosity

and saturation degree. Based on these results

the rock water characteristic curve for the shaley facies

(drying path) was established according to Van

Genuchten (1980). Figure 1 shows the relationship

between saturation degree and total suction. For

comparison the water retention curves obtained by

Ferrari and Laloui (2013), Zhang et al. (2007) and

Major et al. 2007 are shown. The rock water charactistic

curve obtained in this study is consistent

with those obtained by Ferrari and Laloui (2013).

Table 1:

Supersaturated salt solution

used to establish

constant relative humidity/surface

suction

in the desiccators.

Salt Solution

(–)

K 2 SO 4

KCl

NaCl

NaNO 2

Ca(NO 3 )2*(4H 2 O)

K 2 CO 3

CaCl 2

LiCl

Relative

Humidity

(%)

97

85

75

66

52

43

31

19

Suction

(MPa)

4

22

39

56

89

115

159

226

The influence of suction on the tensile

strength of Opalinus Clay

Subsequently to drying, the test specimens were

used to obtain the relationship between the Brazilian

Tensile Strength (BTS) and the total suction. The

tests were performed at the rock mechanical laboratory

at the Chair of Engineering Geology at ETH

Zurich. A modified 2000 kN Walter and Bai servohydraulic

rock testing device with digital feedback

262

ENSI Erfahrungs- und Forschungsbericht 2012

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