Satoru Ohtsuka

Spatial distribution of landslides induced by the 2004 Mid-Niigata prefecture earthquake, Japan

On October 23, 2004, an earthquake with a moment magnitude of 6.8 occurred in the Chuetsu area of Niigata prefecture in Japan. This earthquake is known as the 2004 Mid-Niigata prefecture earthquake; the event was followed by severe aftershocks and caused many types of landslides such as surficial slides, shallow slides, and deep slides. A large number of landslides occurred in the upland village of Yamakoshi, destroying the entire village; in addition, a huge number of houses collapsed in Kawaguchi town. This study investigates the correlations between each type of landslide and the bedding plane orientation and dip, and other geomorphologic conditions. The landslide occurrence ratio (LOR) is used as an index to determine the correlation between the 2004 Mid-Niigata prefecture earthquake-induced landslides and the slope angle, slope aspect, rock type, and bedding plane orientation and dip. This work also proposes a methodology to determine the geometric alignment between the topography and the orientation of geological bedding planes. The method provides an efficient means of estimating the topography/bedding plane relationship over large areas.

Examination of Caisson-Type Quay Wall for Resilient Structure by Using 1G Shaking Table Test

To investigate the resilience of structures against a large earthquake, 1G shaking table tests were performed. Three sections of caissons with different shapes were examined by incrementally increasing input acceleration. The original shape and that with bedding and a belly attachment were considered. The belly-attached section was the most resilient. The sliding, overturning, and bearing capacity were investigated using 1G shaking table tests at incremental accelerations. In the experiment, at the beginning, the original case was deformed at 350 gal, and the deformation increased in the order of bedding and belly attachment. Under the seismic intensity method, the slip safety factor of original caisson initially became less than 1.0. Then, the overturning safety factor of the bedding-type caisson decreased to less than 1.0. This expresses the experimental results well.

Ultimate Lateral Resistance of Pile Group in Clayey Soils Against Various Directions of Ground Movement

This study estimated the ultimate lateral resistance of pile group against horizontal ground movement by using two-dimensional rigid plastic finite element method (RPFEM). The effect of ground movement direction on the ultimate lateral resistance was investigated through the case studies. According to the results, it can be seen that changes in both the ultimate lateral resistance and the failure mode of ground around piles were observed. The effect of the pile spacing on load apportionment ratios was studiously investigated. Since each pile in the group affects other piles, it leads the load apportionment ratio significantly to depend on pile positions such as the front or back piles. Moreover, the load apportionment ratio of each pile went up with an increase in pile spacing, but it was obtained different remarkably among piles. The load apportionment ratio varied with the direction of the ground movement.

Identification of shear strength and seismic coefficient by back analyzing surficial slides in the 2004 Mid-Niigata prefecture earthquake

Sliding of natural and artificial slopes generally occurs during or following strong earthquakes. Such sliding is greatly affected by a combination of geological conditions and earthquake loading. Earthquake-induced landslides often cause more damage to infrastructure and human lives than the earthquake itself. Pseudo-static analysis is widely implemented as one of several design methods used in engineering practice to assess the seismic stability of natural and artificial slopes. However, the most important issue of pseudo-static analysis is to select the most appropriate method for measuring seismic coefficient. In order to investigate this, back analysis was conducted for surficial slides subjected to strong ground motion during the 2004 Mid-Niigata prefecture earthquake in Japan. This paper surveyed the stochastic properties of earthquake-induced surficial slides and clearly showed that the obtained results were applicable to back analysis of shear strength and seismic coefficient. In back analysis, soil properties such as soil strength and density and sliding depth were assumed as random variables owing to their uncertainties. Seismic coefficient is also assumed to be a random variable and varies with distance from the epicenter fault line. The analysis of 4504 recorded surficial slides clearly shows a unique relationship of landslide occurrence ratio with slope angle and distance from the epicenter fault line. This study’s results enhance the calculation of the shear strength of weathered soil covering slopes and the horizontal seismic coefficient through back analysis procedure. By considering possible stochastic properties of variables, some case studies were implemented in the back analysis.

NUMERICAL SIMULATIONS WITH CUSHIONING MATERIAL INSTALLED BEHIND THE RETAINING WALL FOR RESILIENT STRUCTURES

Abstract. Following the Hyogoken Nanbu earthquake in 1995, the design earthquake wave was larger. After the Tohoku chihou Taiheiyouoki earthquake in 2011, the tsunami was focused after earthquake. In the future large earthquakes are expected to occur in Japan. We are required to design more resilient structures that can withstand these events. We conducted a numerical simulation of the reduction of earth pressure in a sandy backfill. When a cushioning material was placed behind the retaining wall, the earth pressure was reduced, because the backfill sand moved to the active earth pressure state. It is possible that the design section of the gravity caisson became smaller. In the dynamic approach, shaking table tests were performed and the earth pressure acting on the retaining wall was seen to reduce. However, the mechanism of earth pressure reduction was not clear in the dynamic tests. Therefore in this study, the mechanism of earth pressure reduction during an earthquake was clarified using a numerical simulation. From the simulation results, the residual horizontal displacement in the case of sand is larger than that in the case of cushioning material. It is cleared that in the cushioning material the inertial force of backfill sand was absorbed for the friction force toward to the foundation ground beside the effect of quasi-active state.

Prediction of spatial distribution on soil surveying values using geostastics methods

Recently, land subsidence and liquefaction are becoming evident. But neither the countermeasure nor research technique have not been established. In order to determine the cause, more detailed comprehension of soil properties is essential; therefore, in this research new-typed Swedish sounding testing machine (the NSWS testing machine), capable of measuring more detailed physical properties of in-ground was utilized and conducted a subsurface investigation at a narrow detailed house at which land subsurface have occurred. Based on the result of the investigation presented physical properties of the in-ground in the plane manner using Kriging method, one of geostatistics methods. Also, the comparison of converted N-value measured by the NSWS and converted N-value estimated by Kriging method is presented to examine the composite capability and benefit of Kriging method and NSWS for simplified on-site verification and re-measurement for reaffirmation (diagonal measurement) in the confined detached house ground.

Life cycle assessment on recycling of construction sludges in geotechnical engineering fields

Although waste recycling has been promoted in response to increasing environmental awareness in Japan, its marketability is being questioned due to the recycling cost. The ultimate goal of waste recycling is to reduce the environmental load. In this paper, we examined the evaluation method for social environmental efficiency to socially evaluate waste recycling, by incorporating environmental load as an environmental cost in addition to the direct cost. The social environmental efficiency evaluation including consideration of uncertainties is conducted, because waste recycling involves various uncertain elements. As the results, the social environmental significance of construction sludge recycling can be evaluated quantitatively while focusing on the particular construction sludge with a lower recycling rate.

ULTIMATE BEARING CAPACITY ANALYSIS OF GROUND AGAINST INCLINED LOAD BY TAKING ACCOUNT OF NONLINEAR PROPERTY OF SHEAR STRENGTH

In the assessment of bearing capacity of footing, the bearing capacity formula proposed by The Architectural Institute of Japan (AIJ) has been widely used in Japan for the design of building foundation. However, this formula is limited to simple conditions like simple footing shape, flat ground and uniform material property. Although the rigid plastic finite element method (RPFEM) can solve this complex problem, it has not taken into account the size effect in assessment of bearing capacity. This study newly develops the RPFEM by introducing the nonlinear shear strength property against the confining stress and proposes the rigid plastic constitutive equation of parabolic yield function, basing on the change in the angle of shear resistance of Toyoura sand. The developed method and the bearing capacity formula (established by empirical approach) provided identical results for various footing sizes showing good estimation for wider range of footing size.

Shear Behavior of Clay in Slope for Pore Water Pressure Increase

Shear behavior of clay in slope for pore water pressure increase was investigated with both triaxial compression and ring shear apparatuses. Not only shear strength but also shear deformation was examined. Pore water pressure loading test was conducted with tiaxial compression apparatus. Yield and failure points for pore water pressure increase were obtained. It was shown that yield point was on the critical state line described by internal friction angle of φ’. It means the design method with φ’ indicates the design to prevent slope from instability and deformation occurrence. Shear displacement in pore water pressure loading test revealed time dependent behavior as creep. It was made clear to be caused by water migration and successive progressive failure. Creep behavior of landslide was discussed from the viewpoint of coupling theory in continuum.