Hirofumi Toyota

Anisotropy of Undrained Shear Strength Induced by K0 Consolidation and Swelling in Cohesive Soils

AbstractStrength anisotropy should be introduced into stability analyses, because the shear direction differs at each sliding point. However, few experiments evaluating the effects of anisotropic consolidation on undrained shear strength have been available for practical construction problems. Therefore, torsional shear tests were conducted on specimens consolidated and swollen in different directions to evaluate the anisotropy of undrained shear strength. Furthermore, to treat two-dimensional problems, the tests were also performed under the plane strain condition, where no radial strain occurs. Results show that the anisotropy of the undrained shear strength is closely related to the differences between consolidation and shearing in terms of directions and stress conditions. Finally, slope stability analyses were conducted to demonstrate how the anisotropy of the undrained shear strength obtained from the experiments can be practically used in field analyses.

Variation of Liquefaction Strength Induced by Monotonic and Cyclic Loading Histories

AbstractOnce loose sand has liquefied, it becomes denser along with water drainage after liquefaction. Therefore, liquefied sand is regarded as more robust against liquefaction. Such sand after liq...

Technique for Undrained Triaxial Tests on Unsaturated Soils Using Active Control of Pore-Air Pressure

Triaxial testing apparatus and procedures for unsaturated soils have become common along with the development of equipment and sensors. Although many drainage conditions are related to the pore-air and pore-water phase, the drained pore-air condition is usually adopted for tests of unsaturated soils. The main reason is that it is difficult to keep the pore-air undrained because the air diffuses easily through a rubber membrane. However, pore-air and pore-water are sometimes not allowed to drain under ground deformation problems. Therefore, undrained conditions in which pore-air and pore-water are not allowed to flow in and out of the soil are achieved in the experiments described herein. First, problems of a conventional triaxial apparatus for unsaturated soils were investigated to produce the undrained conditions. Then, triaxial testing methods under the undrained conditions were developed for overcoming the problems. Finally, the performance of the proposed testing procedure was demonstrated in comparison with results obtained using conventional testing.

Detection of Elastic Region Varied by Inherent Anisotropy of Reconstituted Toyoura Sand

Inherent anisotropy affects deformation and strength characteristics of sand deposits. Accurate deformation modulus and shear strength are important parameters to design and construct soil structures such as retaining wall, foundation and tunneling. In this study, specimens having various angles of depositional plane (sedimentation angle) were artificially made using an inclined container to estimate soil inherent anisotropy. The container was designed to prepare easily a specimen with different inclined depositional plane by changing positions of its side walls. Specimens are made using Toyoura sand by air pluviation method to attain the same relative densities regardless of the depositional angle. The strength and deformation characteristic were evaluated using both bender elements (BE) method and local small strain (LSS) measurements. To compare the accuracy of acquired results, the both tests were conducted in the same specimens under drained condition of triaxial tests. Results from the experiments indicate that initial shear modulus has tendency to slightly increase with an increase of the depositional angle. However, shear strength has the obvious inverse relationship with changing of the depositional angles. Concerning elastic region, the elastic region tends to shrink with an increase of the depositional angle in spite of increasing of the initial shear modulus with the depositional angle. In comparison between BE and LSS tests, the initial shear moduli obtained from LSS tests are slightly greater than those obtained from BE tests.

Geotechnical Distinction of Landslides Induced by Near-Field Earthquakes in Niigata, Japan

Landslides triggered by near-field earthquakes with epicentres directly beneath towns have attracted intense attention since the 2004 Mid-Niigata (Niigata-ken Chuetsu) Earthquake. Hilly and mountainous areas sustained heavy damage. Social problems developed when many towns became isolated because landslides cut off traffic and public service lifelines. Soil from landslides closed river channels and formed natural dams. The natural dams submerged some towns. Emergency measures were undertaken promptly to prevent debris flows caused by natural dam breaks. Subsequently, the 2007 Mid-Niigata Offshore (Niigata-ken Chuetsu-oki) Earthquake and the 2011 Northern Nagano Earthquake struck the Niigata region. Landslides triggered by those earthquakes differed in terms of their number, scale, and location. Therefore, characteristics of the landslide sites of the respective earthquakes were examined to ascertain their topographical and geological features. Furthermore, differences in groundwater level and damage related to compound disasters were explained for discussion of the stability progress of damaged slopes.