M. Okamura - Academia.edu (original) (raw)
Papers by M. Okamura
SOILS AND FOUNDATIONS, 2002
SOILS AND FOUNDATIONS, 2008
Measures preventing an earthquake-induced soil liquefaction are of signiˆcant importance to mitig... more Measures preventing an earthquake-induced soil liquefaction are of signiˆcant importance to mitigate the liquefaction hazards. An air-injection technique may be a simple, inexpensive method-this leads the saturated soils to the desaturated by injecting pressurized air, resulting in a higher liquefaction strength and lower susceptibility. The objective of this study is to investigate the evolution of desaturation process during air injection into saturated soil deposits and verify the validity of a multiphase ‰ow simulator if it is capable of being applied for predicting the process as well as the distribution of degree of saturation after the air injection ceased. In this study simpliˆed model tests that simulate the air injection into saturated soils using air-injection probes, are conducted using two diŠerent sizes of soil containers. The experiments using the small container are aimed to examine the nominal rates and magnitudes of the soil desaturation driven by air injection, whilst those with the large container are performed to obtain not only the rates and magnitudes but also the distributions of the desaturated zones within the soil. The results obtained indicate, although clearly depending on the physical properties of targeted soils, that the evolution of desaturation is strongly controlled by the air pressures injected and the soil permeabilities. Numerical analyses are also conducted using a multiphase ‰ow simulator to describe the evolution of the soil desaturation, and to examine the applicability of the model as a prediction tool enabling an evolution of desaturation in situ to be followed with time and space. Predictions show a relatively good agreement with the experimental measurements regarding the rates, magnitudes, and distribution of desaturation speciˆcally for the small-container experiments although predictions of desaturated domain slightly overestimate the measurements for the large-container experiments. Thus, this study indicates that the numerical model described is applicable toˆeld problems when the soil properties in terms of ‰ow transport are well-constrained.
Doboku Gakkai Ronbunshuu C, 2010
Doboku Gakkai Ronbunshuu C, 2008
SOILS AND FOUNDATIONS, 2001
International Journal of Physical Modelling in Geotechnics, 2011
International Journal of Physical Modelling in Geotechnics, 2012
The degree of saturation significantly influences the liquefaction resistance of sandy soils. In ... more The degree of saturation significantly influences the liquefaction resistance of sandy soils. In preparation of models for dynamic centrifuge tests and 1g shaking table tests, however, the degree of saturation has not been accurately controlled. A technique to prepare fully saturated model ground for liquefaction study is summarised in this paper, focusing on how vacuum pressure and centrifugal acceleration during the saturation process affect the degree of saturation. A method to evaluate degree of saturation of models accurately is developed first. Then, a total of ten experiments were conducted in which water was introduced to dry sand deposits at different atmospheric pressures at different centrifugal accelerations. It is confirmed that increased acceleration contributes to enhance degree of saturation and reduces time for the saturation process. Two dynamic centrifuge tests on models with different degrees of saturation, one almost fully saturated and the other slightly unsaturated, with the differe...
SOILS AND FOUNDATIONS, 2003
SOILS AND FOUNDATIONS, 2002
STRUCTURAL ENGINEERING / EARTHQUAKE ENGINEERING, 2007
In the design practice of retaining walls which support road embankments, stability under the act... more In the design practice of retaining walls which support road embankments, stability under the action of strong earthquake ground motion is usually not examined. However, collapse of road embankments in mountainous area often results in cutting off road traffic and complete isolation of people in the neighborhood for a long time. The author surveyed a number of damaged and undamaged road embankments in the mountainous area where strong ground motions were recorded during the 2004 Niigataken-Chuetsu Earthquake. It was revealed that catastrophic failure of embankments constructed on sloping foundation soils were in many cases triggered by damage of retaining walls and most of such damaged retaining walls failed in the mechanism of the bearing capacity failure of the foundation soil. In this study a simple method to examine the seismic stability of existing retaining walls is developed, which evaluates a factor of safety for the bearing capacity failure of foundation on slope under combined loading in conjunction with soil strength parameters obtained by dynamic cone penetration tests using a portable testing device. A good correlation was found between the factor of safety of the surveyed walls derived from the method and the observed deformation of the walls, confirming the effectiveness of the method to identify seismically unstable walls out of existing walls.
Doboku Gakkai Ronbunshuu C, 2009
Soils and Foundations, 2013
SOILS AND FOUNDATIONS, 2010
Port and Airport Research Institute, Japan (kitazume＠pari.go.jp).
SOILS AND FOUNDATIONS, 2006
SOILS AND FOUNDATIONS, 2009
It has been pointed out that there are two possible mechanisms that enhance liquefaction resistan... more It has been pointed out that there are two possible mechanisms that enhance liquefaction resistances of unsaturated sand. Theˆrst mechanism is where air in a partially saturated sand mass plays a role of absorbing generated excess pore pressures by reducing its volume. Okamura and Soga (2005) derived the in‰uential factors of the liquefaction resistance for partially saturated sand from theoretical consideration and eŠects of the factors were examined through a series of triaxial tests on a clean sand. They found a unique relationship between liquefaction resistance ratios and the potential volumetric strain, which allows the estimation of the liquefaction resistance for partially saturated sand. The second is the matric suction of unsaturated sand which increases the eŠective stress and thus the strength of the soil mass. In this study two series of cyclic triaxial tests on non-plastic silt were carried out to observe the liquefaction resistance in both mechanisms. In theˆrst series, a top cap with an accumulator tank was used to study the eŠect of compressibility of pore ‰uid on the liquefaction resistance. The empirical relationship derived by Okamura and Soga is found to be valid even for the silt provided that the matric suction is negligible. In the second test series an ordinary cap was used. The liquefaction resistance increased linearly with the matric suction, with the increasing ratio being higher than that for the net stress. A unique linear relationship is found between the normalized liquefaction resistance and the matric suction. Results are summarized in the form which can be easily applied to evaluate the liquefaction resistance of a partially saturated soil.
Journal of Geotechnical and Geoenvironmental Engineering, 2011
ABSTRACT Desaturation of ground by air injection attracts considerable attention in recent years ... more ABSTRACT Desaturation of ground by air injection attracts considerable attention in recent years as an innovative technique for a liquefaction countermeasure. Several research programs were conducted in laboratories regarding the related topics. This paper describes an in situ air-injection test that aims to examine the effectiveness of the air injection to desaturate ground and the validity of observation techniques to monitor the evolution of the unsaturated zone. In the test, air was injected from an air injector deployed in a targeted saturated-sand layer at a depth of 6 m. Observations revealed that the air-flow rate increased linearly with increasing air-injection pressure and the desaturated zone was generated within 4 m from the injection point. A 3-dimensional electric resistivity tomography technique was effective for evaluation of the desaturated zone. The degree of saturation of the in situ soil was observed by using high quality undisturbed samples obtained by the ground freezing method. The degree of saturation ranged from 68-98%, which was low enough to almost double the liquefaction resistance of the soil at the site. Numerical analyses were also conducted with a gas-liquid two-phase flow simulator to describe the evolution of the soil desaturation. Qualitatively, predictions show a relatively good agreement with the in situ measurements of the 3D electric resistivity tomography and are quantitatively compatible with the in-field degree of saturation measured indirectly by using the frozen soil samples. Actual liquefaction resistance was evaluated utilizing the undisturbed samples by conducting a triaxial test under cyclic shear conditions, which revealed that desaturated samples were indeed less susceptible to liquefaction compared with the fully saturated samples. DOI: 10.1061/(ASCE)GT.1943-5606.0000483. (C) 2011 American Society of Civil Engineers.
Journal of Geotechnical and Geoenvironmental Engineering, 2006
Soils and Foundations, 2015
Soils and Foundations, 2015
SOILS AND FOUNDATIONS, 2002
SOILS AND FOUNDATIONS, 2008
Measures preventing an earthquake-induced soil liquefaction are of signiˆcant importance to mitig... more Measures preventing an earthquake-induced soil liquefaction are of signiˆcant importance to mitigate the liquefaction hazards. An air-injection technique may be a simple, inexpensive method-this leads the saturated soils to the desaturated by injecting pressurized air, resulting in a higher liquefaction strength and lower susceptibility. The objective of this study is to investigate the evolution of desaturation process during air injection into saturated soil deposits and verify the validity of a multiphase ‰ow simulator if it is capable of being applied for predicting the process as well as the distribution of degree of saturation after the air injection ceased. In this study simpliˆed model tests that simulate the air injection into saturated soils using air-injection probes, are conducted using two diŠerent sizes of soil containers. The experiments using the small container are aimed to examine the nominal rates and magnitudes of the soil desaturation driven by air injection, whilst those with the large container are performed to obtain not only the rates and magnitudes but also the distributions of the desaturated zones within the soil. The results obtained indicate, although clearly depending on the physical properties of targeted soils, that the evolution of desaturation is strongly controlled by the air pressures injected and the soil permeabilities. Numerical analyses are also conducted using a multiphase ‰ow simulator to describe the evolution of the soil desaturation, and to examine the applicability of the model as a prediction tool enabling an evolution of desaturation in situ to be followed with time and space. Predictions show a relatively good agreement with the experimental measurements regarding the rates, magnitudes, and distribution of desaturation speciˆcally for the small-container experiments although predictions of desaturated domain slightly overestimate the measurements for the large-container experiments. Thus, this study indicates that the numerical model described is applicable toˆeld problems when the soil properties in terms of ‰ow transport are well-constrained.
Doboku Gakkai Ronbunshuu C, 2010
Doboku Gakkai Ronbunshuu C, 2008
SOILS AND FOUNDATIONS, 2001
International Journal of Physical Modelling in Geotechnics, 2011
International Journal of Physical Modelling in Geotechnics, 2012
The degree of saturation significantly influences the liquefaction resistance of sandy soils. In ... more The degree of saturation significantly influences the liquefaction resistance of sandy soils. In preparation of models for dynamic centrifuge tests and 1g shaking table tests, however, the degree of saturation has not been accurately controlled. A technique to prepare fully saturated model ground for liquefaction study is summarised in this paper, focusing on how vacuum pressure and centrifugal acceleration during the saturation process affect the degree of saturation. A method to evaluate degree of saturation of models accurately is developed first. Then, a total of ten experiments were conducted in which water was introduced to dry sand deposits at different atmospheric pressures at different centrifugal accelerations. It is confirmed that increased acceleration contributes to enhance degree of saturation and reduces time for the saturation process. Two dynamic centrifuge tests on models with different degrees of saturation, one almost fully saturated and the other slightly unsaturated, with the differe...
SOILS AND FOUNDATIONS, 2003
SOILS AND FOUNDATIONS, 2002
STRUCTURAL ENGINEERING / EARTHQUAKE ENGINEERING, 2007
In the design practice of retaining walls which support road embankments, stability under the act... more In the design practice of retaining walls which support road embankments, stability under the action of strong earthquake ground motion is usually not examined. However, collapse of road embankments in mountainous area often results in cutting off road traffic and complete isolation of people in the neighborhood for a long time. The author surveyed a number of damaged and undamaged road embankments in the mountainous area where strong ground motions were recorded during the 2004 Niigataken-Chuetsu Earthquake. It was revealed that catastrophic failure of embankments constructed on sloping foundation soils were in many cases triggered by damage of retaining walls and most of such damaged retaining walls failed in the mechanism of the bearing capacity failure of the foundation soil. In this study a simple method to examine the seismic stability of existing retaining walls is developed, which evaluates a factor of safety for the bearing capacity failure of foundation on slope under combined loading in conjunction with soil strength parameters obtained by dynamic cone penetration tests using a portable testing device. A good correlation was found between the factor of safety of the surveyed walls derived from the method and the observed deformation of the walls, confirming the effectiveness of the method to identify seismically unstable walls out of existing walls.
Doboku Gakkai Ronbunshuu C, 2009
Soils and Foundations, 2013
SOILS AND FOUNDATIONS, 2010
Port and Airport Research Institute, Japan (kitazume＠pari.go.jp).
SOILS AND FOUNDATIONS, 2006
SOILS AND FOUNDATIONS, 2009
It has been pointed out that there are two possible mechanisms that enhance liquefaction resistan... more It has been pointed out that there are two possible mechanisms that enhance liquefaction resistances of unsaturated sand. Theˆrst mechanism is where air in a partially saturated sand mass plays a role of absorbing generated excess pore pressures by reducing its volume. Okamura and Soga (2005) derived the in‰uential factors of the liquefaction resistance for partially saturated sand from theoretical consideration and eŠects of the factors were examined through a series of triaxial tests on a clean sand. They found a unique relationship between liquefaction resistance ratios and the potential volumetric strain, which allows the estimation of the liquefaction resistance for partially saturated sand. The second is the matric suction of unsaturated sand which increases the eŠective stress and thus the strength of the soil mass. In this study two series of cyclic triaxial tests on non-plastic silt were carried out to observe the liquefaction resistance in both mechanisms. In theˆrst series, a top cap with an accumulator tank was used to study the eŠect of compressibility of pore ‰uid on the liquefaction resistance. The empirical relationship derived by Okamura and Soga is found to be valid even for the silt provided that the matric suction is negligible. In the second test series an ordinary cap was used. The liquefaction resistance increased linearly with the matric suction, with the increasing ratio being higher than that for the net stress. A unique linear relationship is found between the normalized liquefaction resistance and the matric suction. Results are summarized in the form which can be easily applied to evaluate the liquefaction resistance of a partially saturated soil.
Journal of Geotechnical and Geoenvironmental Engineering, 2011
ABSTRACT Desaturation of ground by air injection attracts considerable attention in recent years ... more ABSTRACT Desaturation of ground by air injection attracts considerable attention in recent years as an innovative technique for a liquefaction countermeasure. Several research programs were conducted in laboratories regarding the related topics. This paper describes an in situ air-injection test that aims to examine the effectiveness of the air injection to desaturate ground and the validity of observation techniques to monitor the evolution of the unsaturated zone. In the test, air was injected from an air injector deployed in a targeted saturated-sand layer at a depth of 6 m. Observations revealed that the air-flow rate increased linearly with increasing air-injection pressure and the desaturated zone was generated within 4 m from the injection point. A 3-dimensional electric resistivity tomography technique was effective for evaluation of the desaturated zone. The degree of saturation of the in situ soil was observed by using high quality undisturbed samples obtained by the ground freezing method. The degree of saturation ranged from 68-98%, which was low enough to almost double the liquefaction resistance of the soil at the site. Numerical analyses were also conducted with a gas-liquid two-phase flow simulator to describe the evolution of the soil desaturation. Qualitatively, predictions show a relatively good agreement with the in situ measurements of the 3D electric resistivity tomography and are quantitatively compatible with the in-field degree of saturation measured indirectly by using the frozen soil samples. Actual liquefaction resistance was evaluated utilizing the undisturbed samples by conducting a triaxial test under cyclic shear conditions, which revealed that desaturated samples were indeed less susceptible to liquefaction compared with the fully saturated samples. DOI: 10.1061/(ASCE)GT.1943-5606.0000483. (C) 2011 American Society of Civil Engineers.
Journal of Geotechnical and Geoenvironmental Engineering, 2006
Soils and Foundations, 2015
Soils and Foundations, 2015