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[HCG20-01] Geological evidences of elastic slip along the minor fault with response to the variation of water pressure: An example from the Horonobe Underground Research Laboratory
Keywords:fault, shear stiffness, variation of water pressure
It is planned that high-level radioactive wastes packed into engineered barriers (metallic canisters and surrounding bentonite buffers) are disposed at the depth of deeper than 300 m in the deep geological repository project. Thus, assessing the physical and chemical phenomena, which can exert a influence on the engineered barriers, is one of the important issues. In case that the engineered barriers are emplaced at the host rock where minor faults are existed, considering the possibility of shear displacements along the minor faults after the emplacements is important for guaranteeing function of the engineered barriers. Although shear displacements include elastic shear displacements and inelastic shear displacements (permanent strain), we will focus on the elastic shear displacements. For example, there were cases in which shear stiffness along a fault showed outstandingly low based on constant-head step-injection tests targeting on minor faults in the soft sedimentary rocks, which showed that shear displacement of a few cm was generated accompanying an increase of water pressure in the faults (Ishii, 2020, 2024). Such results imply that elastic shear displacements along the minor faults around the engineered barriers can be caused by recovery of water pressure after the emplacement of the engineered barriers in soft sedimentary rocks. Thus, it is needed to quantitatively understand the shear stiffness and the pre-existing elastic shear displacements in case engineered barriers are emplaced at the host rocks where minor faults exist. In order to develop such methods, we investigated pre-existing elastic shear displacements based on grouting, observations of boring cores and borehole television (BTV), which are conducted to understand the faults in silicious mudstone (the Wakkanai formation) in the Horonobe Underground Research Laboratory (URL).
Grouting was carried out into the fault at the depth of 480 m using borehole (66 mm in diameter) from the floor of the ventilation shaft at the depth of 380 m in the Horonobe URL, which resulted in boring cores possessing characteristic occurrence shown in the figure (uploaded figure a and b). As a result of boring core and BTV observation, the fault which originally behaved as normal fault was estimated to have been slipped back. This means that water pressure in the fault was largely decreased by the boring from the depth of 380 m while effective normal stress loading on the fault plane was increased. This enabled the increase of shear stiffness of the fault and releasing a part of the elastic shear displacements as the back slip (Ishii, 2024), which results in the displacement of boreholes between hanging wall and footwall of the fault. Since water pressure in the fault during the grouting was still lower than that of pre-boring, grout was consolidated while the back slip remains occurred. It is thought that this allowed us to obtain such boring core mentioned above (uploaded figure c). Because the horizontal displacement between the boreholes of the hanging wall and footwall was approximately 2 cm, the net slip of the fault is assumed to be a few cm. Namely, the elastic shear displacement as the normal fault caused more than a few cm slips along the fault prior to the boring, whereas it is considered that some of such elastic shear displacements with few cm slips was released by the decrease of water pressure during the boring and the grouting.
In this study, we confirmed that the elastic shear displacement of a few cm along the fault occurred with response to the decrease of water pressure. The fault is assumed that about the same shear displacements will be generated when the water pressure is recovered (increased) to its original state. These observational results are expected to contribute to developing the methods to quantitatively assess the shear stiffness and pre-existing elastic shear displacements in the phase of investigation from ground surface.
Grouting was carried out into the fault at the depth of 480 m using borehole (66 mm in diameter) from the floor of the ventilation shaft at the depth of 380 m in the Horonobe URL, which resulted in boring cores possessing characteristic occurrence shown in the figure (uploaded figure a and b). As a result of boring core and BTV observation, the fault which originally behaved as normal fault was estimated to have been slipped back. This means that water pressure in the fault was largely decreased by the boring from the depth of 380 m while effective normal stress loading on the fault plane was increased. This enabled the increase of shear stiffness of the fault and releasing a part of the elastic shear displacements as the back slip (Ishii, 2024), which results in the displacement of boreholes between hanging wall and footwall of the fault. Since water pressure in the fault during the grouting was still lower than that of pre-boring, grout was consolidated while the back slip remains occurred. It is thought that this allowed us to obtain such boring core mentioned above (uploaded figure c). Because the horizontal displacement between the boreholes of the hanging wall and footwall was approximately 2 cm, the net slip of the fault is assumed to be a few cm. Namely, the elastic shear displacement as the normal fault caused more than a few cm slips along the fault prior to the boring, whereas it is considered that some of such elastic shear displacements with few cm slips was released by the decrease of water pressure during the boring and the grouting.
In this study, we confirmed that the elastic shear displacement of a few cm along the fault occurred with response to the decrease of water pressure. The fault is assumed that about the same shear displacements will be generated when the water pressure is recovered (increased) to its original state. These observational results are expected to contribute to developing the methods to quantitatively assess the shear stiffness and pre-existing elastic shear displacements in the phase of investigation from ground surface.