Reiko Kuwano

ANISOTROPIC STIFFNESS MEASUREMENTS IN A STRESS-PATH TRIAXIAL CELL

Anisotropy plays a significant role in many geotechnical problems. This paper describes how the anisotropic stiffness properties of soils may be assessed through stress path triaxial tests. Local strain instrumentation has been optimised to identify the linear elastic region of sand without sacrificing the ability to study behavior at strains up to 15%; the system described performs equally well with sands, silts and clays. A novel technique has been developed in which multi-directional shear wave velocity measurements are combined with static tests to provide a complete description of the soils cross-anisotropic elastic properties through a simple manipulation of classical elastic theory. Results obtained in tests on a dense sand are presented to demonstrate the system capabilities, show how the theoretical approach may be applied in practice, and draw attention to some interesting features of the soils elastic anisotropy.

Statically and Dynamically Measured Poisson's Ratio of Granular Soils on Triaxial Laboratory Specimens

Poissons ratios and small-strain stiffnesses of three kinds of granular materials—fine, medium coarse, and coarse sands—were evaluated in the laboratory. Both axial and radial strains of cylindrical triaxial specimens were measured with a local deformation transducer and clip gauges. Small-strain (≈0.001 %) cyclic loadings were applied in the vertical direction at several isotropic stress states. We determined Poissons ratio statically based on the local strain variations during the cyclic loadings; we also investigated the stiffness in parallel. In conjunction with static measurements, elastic waves were propagated. By utilizing the disk transducer method, we were able to measure both compressional and shear waves in a single specimen under identical stress states. Based on the received wave velocities of compressional and shear waves, we were able to evaluate both Poissons ratio and the small-strain stiffness dynamically. We then compared elastic parameters evaluated both statically and dynamically for the geo-materials.

Effects of Excess Pore Water Pressure on the Displacement of Failed Dip Slopes in 2004 Niigata-Ken Chuetsu Earthquake

The effects of excess pore water pressure on the behavior of earthquake-induced dip slope failure were evaluated by field investigation, laboratory tests and numerical calculation. Two dip slopes which failed in the 2004 Niigata-ken Chuetsu Earthquake were investigated intensively. One is at Yokowatashi, Ojiya, and the other is at Yamakoshi Village (currently Nagaoka City). In both cases, the sliding plane was mainly along a weak thin layer. Laboratory test results showed that excess pore water pressure possibly increased during earthquake and/or during sliding. The displacement calculation was executed by using extended Newmark and Janbu method which considers the effects of irregular geometry of the sliding plane and the excess pore water pressure increase or the strain softening property of the weak layer. The calculation results showed very significant effects of excess pore pressure or strain softening.

A preliminary study on the piping erosion of soils using glucose dissolution method

Piping erosion-induced earthen structure failures can cause catastrophic consequences and great economic loss. Experimental laboratory simulation of piping plays a key role for the study of eroded soils. Up till now, piping erosion has been simulated in different ways, such as artificial slot or drilled hole. This paper is a continuation of such efforts and presents a new method of reproducing piping erosion by dissolution of preinstalled glucose column inside the tested material. The proposed method was examined through a series of triaxial compression tests under different densities and confining pressures, aiming to identify the association between stiffness loss and particle dissolution during piping erosion. With the help of local sensors, volumetric strains during piping propagation and small strain stiffness of soil at post-erosion state were obtained. Results showed significant degradation of Young’s modulus and Poisson’s ratio in soil subjected to piping erosion, while the overall shear strength of the disturbed specimens appeared to be less affected.

Investigation into the multiple recent sinkholes in Pokhara, Nepal

Since November 2013, numerous sinkholes have been forming in the Armala area of Pokhara Valley, Central Nepal, posing serious threat to local residents. In order to provide countermeasures for reducing sinkhole risk, detailed investigations into the cause and the formation mechanism of the sinkholes are crucial. Preliminary surveys were conducted in June 2014 and November 2014. Comparison of photos, taken in the two surveys, clearly indicates not only the formation of new sinkholes, but also the re-activation of filled sinkholes. By means of dynamic cone penetration tests and surface wave investigations, qualitative characterization of the soil profile was attained, and shallow weak soil layers which are believed to be the location for future sinkholes could be identified. On the basis of the preliminary field investigation, possible sinkhole formation mechanisms are considered. A risk of sinkhole does not seem to disappear as white turbid water continuously springs. It indicates that the internal erosion of white clayey silt layer is still in progress. In August 2015, a boring was carried out beside one of the largest sinkholes. The overall structure of ground layers was first revealed and a 2.5m high cavity at 7.5-10m deep from the ground surface was found within a thick white clayey silt layer. Further ground investigations including surface wave exploration were conducted in December 2015 and the results are reported.

Model tests on subsurface cavities below road pavement due to sand eruption from the liquefied ground

A significant number of subsurface cavities was found in the liquefied ground after the 2011 Great East Japan earthquake. Using the results of the radar exploration conducted along the coastal area of Tokyo Bay, characteristics of subsurface cavities were investigated. The size and shape of the cavities are larger and thinner compared to those of cavities observed in the non-liquefied ground. A series of model tests was conducted in order to understand the mechanism of sand eruption and underground cavity formation when liquefaction occurs. It was found that the flow rate at the opening seems to be the most important factor for the sand eruption.

Preliminary Field Assessment of Sinkhole Damage in, Pokhara, Nepal

Since November 2013, numerous sinkholes have been forming in the Armala area of Pokhara Valley, Central Nepal, posing serious threat to local residents. In order to provide measures aimed at reducing sinkhole risk, investigations into the cause and features of the sinkholes are crucial. This paper presents early research results based on two damage surveys conducted in June 2014 and November 2014 in the Armala area. Comparison of photos, taken in the two surveys, clearly indicates not only the formation of new sinkholes, but also the re-activation of filled sinkholes. By means of dynamic cone penetration tests and surface wave method investigations, qualitative characterization of the soil profile was attained, and shallow weak soil layers which are believed to be the location for future sinkholes could be identified. On the basis of the field investigation results, possible sinkhole formation mechanisms are identified for the Armala area. Furthermore, results of a reconnaissance survey conducted in the Armala area in early May 2015 (following the 2015 Gorkha Nepal Earthquake, which occurred on April 25th) are also reported. Although the epicentral distance to Pokhara was closer than Kathmandu, which suffered from severe damage, no major apparent effects of the earthquake were observed in the sinkhole damaged area.