SHALE SYMPOSIUM
MUDSTONES AND CLAYSHALES: OBSERVATIONS FROM EXCAVATIONS IN THE
QUEENSTON FORMATION AND THE OPALINUS CLAYSHALE
2015 ISRM CONGRES
*M. A. Perras, K. Wild and F. Amann
ETH Zurich
5 Sonneggstrasse
Zurich, Switzerland 8092
(*Corresponding author: mperras@ethz.ch)
M. S. Diederichs
Queen’s University
34 Union Street
Kingston, Canada K7L 3N6
SHALE SYMPOSIUM
MUDSTONES AND CLAYSHALES: OBSERVATIONS FROM EXCAVATIONS IN THE
QUEENSTON FORMATION AND THE OPALINUS CLAYSHALE
2015 ISRM CONGRESS
ABSTRACT
The Queenston Formation and the Opalinus Clayshale are extensive sedimentary formations in North
America and Europe, respectively. These formations fall into the broader category of rocks known as
mudrocks, which are fine grained siliciclastic sedimentary rocks. They typically have a grain size of less
than 0.0625 mm. Geological and engineering classification systems have been reported in the literature to
distinguish between different types of mudrocks. Typically these systems use the laminated nature, grain
size and mineralogy as distinguishing factors. However, it can be difficult to distinguish between different
types of mudrocks in the field.
At the laboratory scale the grain size and mineralogy can be examined in more detail. It has been found
that the unconfined compressive strength has a stronger dependence on the mineralogy than the crack
initiation threshold. The evidence indicates that when the clay content is low, below ~20%, that the crack
initiation threshold increases with decreasing clay content. Above 20% clay content the crack initiation
threshold remains nearly constant. Previous laboratory studies on mudrocks demonstrated that cracks
initiate at bioclasts and propagate within the weaker clay matrix. These studies showed that at the
laboratory scale crack propagation tends to follow the path of least resistance.
At the excavation scale the path of least resistance is parallel to bedding and observations suggest that the
bedding and stress field orientation plays an important role in the failure process. At the Niagara Tunnel
Project, in Canada, the excavation was generally parallel to the strike of the bedding. With high horizontal
stresses the bedding planes are parallel to the orientation of the most likely extension crack propagation
direction in the crown and invert. The damage that begins at the crack initiation threshold migrates to these
bedding planes and exploits them for propagation and ultimate failure. Reported observations from
boreholes and excavations in the Opalinus Clayshale demonstrate the orientation of the stress field with
respect to the bedding influences the failure mechanism and ultimately the depth of failure. As the
maximum principal stress rotates from parallel to bedding to perpendicular, there is more normal stress
applied to the bedding surfaces. This increases the shear resistance on the bedding and therefore reduces
the depth of damage.
The influence of the mechanical properties and the orientation of the strike of the bedding with respect to
the excavation orientation are examined. The Queenston has an average unconfined compressive strength
of 46 MPa, crack initiation threshold of 15 MPa, and Young’s Modulus of 16 GPa, along the tunnel
alignment. The Opalinus Clayshale has an average unconfined compressive strength of 7 MPa, crack
initiation threshold of 2 MPa, and Young’s Modulus of 2 GPa. Despite the clear difference in mechanical
properties, similarities in the failure process are observed when the excavation parallels the strike of the
bedding.
From a geotechnical perspective the underground excavation response of mudstones and clayshales have
important similarities and differences. In this paper observations from excavations in the Queenston
Formation and from in the Opalinus Clayshale are used to highlight the failure mechanisms.
KEYWORDS
Queenston Formation, Opalinus clayshale, underground excavations, anisotropy