The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan

Abstract

[1] Mudstones are low-permeability sedimentary rocks; however, when shear stresses induced by tectonic movement or nonhydrostatic stresses exceed the shear strength of the rock, brittle or ductile deformation occurs. The nature of this deformation is controlled by the brittleness of the mudstone. If brittle deformation occurs, the resulting dilatant structures may increase the permeability and change the transport properties of the strata. This paper addresses the relationships among brittleness, deformation behavior, and transport properties in mudstones at the Horonobe Underground Research Laboratory, Japan. Geological, mechanical, and hydrogeological data from borehole investigations and laboratory tests were systematically interpreted using a brittleness index (BRI), which is the ratio of the unconfined compressive strength to the effective vertical stress. For mudstones under natural strain rates and low temperatures, ductile deformation occurs when BRI <2, brittle/ductile deformation occurs when BRI is 2-8, and brittle deformation occurs when BRI >8, although semibrittle behavior may also occur at the brittle-ductile boundary. When BRI >8 and faulting is well developed, the mudstone behaves hydrogeologically as a fractured medium at the mesoscopic scale, whereas for BRI <8 the mudstone behaves hydrogeologically as a porous medium, even if faulting is extensive. The BRI concept is a useful tool for systematically characterizing the hydromechanical behavior of mudstones; for example, when assessing the effectiveness of mudstone as a long-term barrier in disposal repositories for radioactive waste.Citation: Ishii, E., H. Sanada, H. Funaki, Y. Sugita, and H. Kurikami (2011), The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan,

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…This is accompanied by dilatancy and by microfracturing of the original fabric but also by progressive decrease in porosity and pore size in the gouge with the nonclay particles embedded in reworked clay. The structure of macro-scale fracture in the samples compares well with Ishii et al (2011Ishii et al ( , 2016.…”

Far‐field stress dependency of the failure mode of damage‐zone fractures in fault zones: Results from laboratory tests and field observations of siliceous mudstone

“…According to the model of Neuzil [, ], a significant pressure anomaly (i.e., transient Darcian flow) can develop when the following equation is satisfied: where |Γ| is the forcing rate, K is hydraulic conductivity, and l is the minimum flow distance required to dissipate the anomaly. Because the hydraulic conductivity of the matrix in the siliceous mudstone is about 10 −11 m/s [ Ishii et al ., ] and the height of the specimens is 0.06 m, the given strain rate (i.e., 0.02% min −1 ) during the undrained triaxial tests may have allowed the development of transient Darcian flow in the specimens, resulting in heterogeneous pore pressure. The formation of shear bands might also affect local pore pressure at and around the shear bands [e.g., Laurich et al ., ; Takizawa and Ogawa , ]; i.e., pore pressure in excess of the observed pore pressure might be locally generated through the formation of shear bands with particle reorientation and pore collapse.…”

“…In addition, when the relationship between the DI and the failure mode of damage‐zone fractures in fault zones is applied to natural fractures, the local tectonic and burial history must also be considered, as the DI value at the time of failure is important. Previous studies have reported that fault zones in the siliceous mudstone of the present study (the Wakkanai Formation) developed during and/or after uplift and denudation, following the maximum burial caused by regional tectonic compression [ Ishii , ; Ishii et al ., , ]. During uplift and denudation, far‐field effective stress loadings in the siliceous mudstone would have decreased with decreasing thickness of overburden [ Ishii , ; Ishii et al ., ], indicating that DI values at the present burial depths are lower than those at any time in the burial history of the mudstone.…”

“…Previous studies have reported that fault zones in the siliceous mudstone of the present study (the Wakkanai Formation) developed during and/or after uplift and denudation, following the maximum burial caused by regional tectonic compression [ Ishii , ; Ishii et al ., , ]. During uplift and denudation, far‐field effective stress loadings in the siliceous mudstone would have decreased with decreasing thickness of overburden [ Ishii , ; Ishii et al ., ], indicating that DI values at the present burial depths are lower than those at any time in the burial history of the mudstone. Thus, if tensile/hybrid fractures are strongly developed in fault damage zones in the siliceous mudstone, such domains are estimated to be limited to areas where present DI values are <1 (tensile) or <2 (hybrid).…”

“…For comparisons with natural fractures observed in boreholes, two core‐logged data sets were used: one from boreholes SAB‐1 and SAB‐2 in siliceous mudstone of the Wakkanai Formation [ Suko et al ., ] and the other from boreholes HDB‐6, ‐8, ‐10, ‐11, and PB‐V01, also in siliceous mudstone of the Wakkanai Formation [ Funaki et al ., ; Ishii et al ., , ]. In the former data set, the fractures were classified into the three types based on visual observations, according to the criteria listed in Table , while in the latter data set, tensile fractures and hybrid fractures were grouped together as fractures exhibiting natural plumose structures on fracture surfaces, regardless of evidence for shearing.…”