Abstract

The purpose of this study is to investigate the relationship between groundwater level changes and the physical characteristics of rock mass during an earthquake. We conducted laboratory experiments to study the effect of pore pressure oscillation on the permeability of fractured sandstones and granite. The rocks used were Shirahama sandstone, which has relatively high permeability, and Kimachi sandstone, which has low permeability. Inada granite was also evaluated. The effect of the amplitude difference of pore pressure on the rock permeability was investigated. Two different drainage conditions (drained and undrained) in the downstream side were tested. For both cases, we measured the permeability through the flow pump test.

The results show that the permeability of the rocks tended to increase as a result of the oscillation of pore pressure. The permeability of the fractured rock was higher in Shirahama sandstone than in Kimachi sandstone and the difference was clearly confirmed. The increases in permeability were greater with the larger amplitude of pore pressure oscillation. The permeability of the fractured rocks was influenced by the crack width and roughness of the macroscopic cracks. The results show that the rock type influences the flowability of the macroscopic cracks.

1 Introduction

Fluctuations in groundwater levels during an earthquake are closely related to the mechanical and hydraulic characteristics of the underground rock mass and thus critical indicators for understanding changes in underground rock mass behaviour associated with earthquakes. Changes in the hydraulic properties of underground rock mass including that of aquifers resulting from earthquakes affect various geological changes in regional groundwater flow paths, effects on fault reactivation, and changes in resource productivity in oil and gas (Manga et.al 2012).

The groundwater levels during an earthquake can be classified into two types: fluctuations with shortperiod oscillation and long-term sustained fluctuations. Groundwater level changes with short-period vibrations are believed to be caused by the passage of seismic waves through the pore water of the aquifer. The most observed period of short-period oscillatory groundwater level changes is 15 to 30 s. However, sustained changes in ground water levels gradually increase or decrease over several weeks following an earthquake, and they gradually recover in some cases. According to Wang and Manga (2014), these fluctuations in groundwater levels are related to the elastic strain of porous media, undrained compaction characteristics, increase in permeability resulting from earthquakes, and their interactions. However, few examples exist in which the spatial and temporal effects of changes in hydraulic and mechanical properties of rocks during earthquakes on groundwater level fluctuations have been examined in detail. In addition, difficulties remain in observing the changes in conditions of aquifer rock mass with the passage of seismic waves. Therefore, relating the continuous groundwater level fluctuations to the physical properties of the rock mass is difficult.

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