Takato Takemura

Damage growth and permeability change in triaxial compression tests of Inada granite

Microcracking (crack growth), along with accumulation of inelastic strain, takes place in crystalline rock such as granite when it is subjected to differential stress. As a result, growing cracks become interconnected, completely altering permeability. Therefore, coupling between crack growth and permeability change must be determined to fully understand the hydro-mechanical response of rocks subjected to non-hydrostatic stress. Damage growth in triaxial tests on Inada granite under confining pressures up to 140 MPa was analyzed using the crack tensor concept proposed by Oda (Soils and Foundations 22 (4) (1982) 96), and permeability change was also formulated in terms of damage growth. Transient pulse tests were carried out on the damaged samples to see if the permeability change is really related to the associated damage growth. The conclusions are summarized as follows: A permeability tensor formulated by microstructural parameters is well supported by the transient pulse tests. This is particularly true when we deal with highly damaged granite. Where crack density is low, however, the hydraulic properties must be considered by taking into account the effect of spherical pores on them. The permeability of the sample subjected to increasing stress up to failure is about two to three orders of magnitude larger than that of intact granite under the same confining pressure (140 MPa). This change is surprisingly large compared with the result by Zoback and Byerlee (J. Geophys. Res. 80 (5) (1975) 752). Permeability tensors of the damaged samples are represented as more or less isotropic tensors. Rocks under stress in the field can be regarded as isotropic porous media, in spite of the fact that cracks grow preferentially parallel to the major stress.

Preferred orientations of open microcracks in granite and their relation with anisotropic elasticity

In order to study how to deal with open microcracks in rock, anisotropic behaviors of Oshima granite were investigated by carrying out wave velocity tests and uniaxial compression tests, together with observations of microcracks under an optical microscope equipped with a universal stage. Anisotropy in the longitudinal wave velocity VL and secant deformation modulus E10 at 10% strength is caused by pre-existing open microcracks, not by pre-existing healed microcracks. The structural anisotropy formed by open microcracks, which is quantitatively represented by a second-rank tensor (called crack tensor), is in good agreement with the directional changes of E10 and VL: The mechanical, as well as structural, anisotropy shows rhombic symmetry with orthogonal symmetry axes in the directions roughly normal to the rift, grain and hardway planes, which are parallel to the major joint sets in the field. Since longitudinal wave velocity changes drastically depending on the density and orientation of open microcracks in granitic rocks, it is suggested that the crack tensor can be determined from non-destructive wave velocity tests. The elastic modulus tensor theoretically formulated in terms of the second-rank crack tensor can be used, as a first-order approximation at least, to describe the anisotropic elasticity of Oshima granite induced by pre-existing open microcracks. It is of particular importance to point out that the micro-scale structure by open microcracks is geometrically similar to the macro-scale structure by joints and faults (scale independent). This finding strongly suggests that some of the conclusions related to open microcracks are applicable to deal with macro-scale cracks in rock masses.

Effect of sedimentary facies and geological properties on thermal conductivity of Pleistocene volcanic sediments in Tokyo, central Japan

When ground source heat pump systems are installed underground, an estimate of the thermal conductivity is required to determine the desired total length of the heat exchanger (U-tube). Many large cities in Asia are built on Quaternary sediments, but the thermal conductivity of these sediments is not well understood. To measure the thermal conductivity of Pleistocene volcanic sediments in Tokyo, Japan, we discuss methods of measuring thermal conductivity and factors influencing the thermal conductivity of volcanic sediment, which has low quartz content. The results obtained from experiments using a drill core, borehole data and artificial sediment samples are as follows: (1) values of thermal conductivity predicted using water content, porosity or sand content can be underestimated in volcanic sediment or sediments with large amounts of magnetic minerals; (2) magnetic minerals have a higher thermal conductivity than quartz, so there is a relationship between magnetic susceptibility and thermal conductivity: (3) comparison of thermal conductivity measurements performed using box- and needle-type probes showed that the values measured using the former are comparatively larger. This decrease in thermal conductivity is explained by formation of air-filled cracks when the needle penetrates the sediment, as air has a lower thermal conductivity than sediment.

Effects of pH on nano-bubble stability and transport in saturated porous media

An understanding of nano-scale bubble (NB) transport in porous media is important for potential application of NBs in soil/groundwater remediation. It is expected that the solution chemistry of NB water highly influences the surface characteristics of NBs and porous media and the interaction between them, thus affecting the stability and transport characteristics of NB. In this study, in addition to stability experiments, one-dimensional column transport experiments using glass beads were conducted to investigate the effects of pH on the NB transport behavior. The results showed that the NBs were more stable under higher pH. Column transport experiments revealed that entrapment of NBs, especially larger ones, was enhanced in lower-pH water, likely suggesting pH-dependent NB attachment and physical straining, both of which are also probably influenced by bubble size. Although relatively smaller NBs were released after switching the eluting fluid to one with lower ionic strength, most of the NBs in lower-pH water were still retained in the porous media even altering the chemical condition.

Effect of Thermal Change by Ground Source Heat Pumps on Groundwater and Geoenvironment in the Late Pleistocene Terrace Area of Tokyo, Japan

Soil and underground water pollution by waste heat in urban area is linked to the development of underground commercial facilities and subways and to the installation of ground source heat pump (GSHP) systems. In this study, we carried out laboratory tests of the dependence of dissolution elements on temperature, using boring core samples taken from the central Tokyo area in order to reduce the risks to underground water pollution by installed GSHP system. Our results demonstrate that we have to be careful not to risk contaminating shallow groundwater with hazardous elements, including marine and/or volcanic sediments, and by oxidizing the environment.