Akira Asaoka

Super Loading Yield Surface Concept for the Saturated Structured Soils

“Superloading surface concept” is newly developed for the description of elasto-plastic behavior of highly structured soils. The superloading surface, which is similar in shape to the original Cam-clay yield surface, is located above the Cam-clay and the size ratio between two yield surfaces gives the degree of the structure of soil skeleton. The evolution law of the size ratio describes the decay of the structure as plastic deformation proceeds, and the superloading surface falls exactly on the Cam-clay yield surface at the end of the completely remolded state. When the soil is reloaded after elastic unloading, the elasto-plastic behavior of the soil is described by the Hashiguchi’s subloading surface, which is also similar in shape to the superloading surface. The size ratio between super and subloading surfaces gives the OCR.

Mechanical Behavior of Compacted Geomaterial Changed from the Dredged Soil in Nagoya Port by Mixing with Some Stabilizers

To solve the problem of accumulation of huge amount of dredged soil (DS) in Nagoya Port Island, some kinds of stabilizers are mixed with the dredged soil to improve the mechanical properties, and then the treated soils can be used as geomaterial. A series of laboratory tests were carried out to investigate the mechanical behaviors of these soils, and also SYS Cam-clay model was adopted to interpret these behaviors by evolutions of soil skeleton structure theoretically. It is suggested that DS behaves like typical clay, which is difficult to decay structure and easy to lose overconsolidation. Compared with DS, these treated soils are in highly-structured and heavily-overconsolidated state. Moreover, the treated soils change to be new materials with slow rate in both decay of structure and loss of overconsolidation. The strength of treated soils is therefore promoted compared with DS and can be used as a new kind of construction material.

Bayesian calibration of embankment safety under earthquake loading

Abstract This study provides a probabilistic procedure for the back-calculation of factors of safety of embankment slopes during seismic loading. The slope stability is analyzed by the conventional circular arc slip surface method in which the seismic load is introduced in terms of a horizontal body force. The factor of safety of an embankment is inversely estimated using information whether the embankment is still safe or not after the occurrence of an earthquake. In this back-calculation, soil strengths and seismic loads are treated as random variables. This is because, in many actual situations, these two factors are usually uncertain even after the occurrence of an earthquake. The developed procedure is applied to the case records of embankment behavior during the Niigata earthquake of 1964. Six embankments are analyzed. Four of them are damaged embankments while the others are non-damaged. The factors of safety are inversely estimated and they are compared with the state of damage described in the case records.

Undrained Creep Rupture of Normally Consolidated Clay Due to Bifurcation Mode Switching During Pore Water Migration

In the present study, we newly interpret the undrained creep rupture of normally consolidated soil as a result of the change in the initial higher bifurcation mode to lower one during pore water migration within the clay; i.e., “mode switching”. The computation is performed not as a single soil element as in the classical soil mechanics, but as a soil-water coupled initial-boundary value problem of a soil specimen, in which only the “inviscid” original Cam-clay model with the subloading surface concept and the Darcy’s law are used. As a result, just after the constant applied load, the axial strain is progressing at a very slow rate, however, the strain rate increases immediately when the strain reaches 5.7%. During the time, it is observed that the excess pore pressure tends to be homogeneous and then rises again due to the occurrence of softening in some element.

Creep-Like Delayed Failure of Clayey Ground after the End of Embankment Construction

Even in normally or lightly overconsolidated clay foundations, failures have been observed some days/weeks after the end of the embankment construction. The clay foundation stays apparently stable just after the completion of construction, but with the passage of time instability develops gradually instead of expected consolidation and it may undergo catastrophic failure in an unexpected moment. In this paper such a creep-like delayed failure of a homogeneous normally consolidated clay foundation is investigated through the soil-water coupled finite deformation computation with the use of the invicid subloading surface Cam-clay model.

Seismic response analysis of a coastal artificial reclaimed ground containing a soft layer

The seismic resistance of a coastal artificial ground situated within the Port of Nagoya against the Tokai-Tonankai-Nankai triple-segment earthquake was assessed. The analysis was carried out using the elasto-plastic constitutive equation for soils (SYS-Cam clay model) mounted on the soil-water coupled finite element deformation analysis program The model focuses attention on the action of the soil skeleton structure, and the analysis allows description of the mechanical behavior of a wide range of soils without distinguishing between static and dynamic behavior. When considering earthquake damage, attention usually tends to focus only on the phenomenon of liquefaction in sandy grounds. In this paper, finite element analysis was employed to simulate the phenomenon of liquefaction of soft sand and, in addition, to show that because of disturbance of the soil during the earthquake, there is a danger of large settlement occurring over a long period after the earthquake in the soft clay layer directly beneath the area that is under the action of vertical loads.

Numerical Analysis on Co- and Postseismic Behavior of Sandy/Clayey Soil Ground Improved by Sand Compaction Pile Method

The co- and post-seismic behaviors of typical loose sandy ground and soft clayey ground improved by the sand compaction pile (SCP) method were investigated using a soil-water coupled finite element deformation analysis program incorporating the SYS-Cam clay model. The results indicated that the deformation caused by earthquakes was suppressed in the SCP improved grounds, irrespective of whether they were sandy or clayey. Although the surrounding ground was liquefied in the case of SCP-improved sandy ground, the confining pressure was maintained in the SCP interpile zone because of the compaction effect produced by the cavity expansion of the sand piles. In the case of the SCP-improved clayey ground, negative excess pore pressures appeared in the SCP sand piles because of the hardening that results from plastic expansion and stress was concentrated on them.

Soil-Water Coupled Finite Deformation Analysis of Seismic Deformation and Failure of Embankment on Horizontal and Inclined Ground

The Noto Hanto Earthquake in 2007 caused large-scale landslide of 11 embankments in NOTO YURYO road, especially the embankment on inclined ground. In order to clarify the cause of the large-scale landslide of embankment, the laboratory tests on the embankment material, the weathering tuff and the seismic response analysis of the embankment on inclined and horizontal grounds by the static/dynamic soil-water coupled finite deformation analysis code, GEOASIA. The essential results and conclusions are follows; 1) The unconfined strength dramatically decreases by absorbing water beyond the optimum water content. 2) The landslide of the embankment on inclined ground is caused by migration of porewater during/after the earthquake. The soil element of landslide swells both elastically and plastically with structure upgradation and loss of over-consolidation after the earthquake. 3)The embankment on the horizontal ground does not slide but move a little. The soil element did not swell but consolidate after the earthquake due to the generation of the positive porewater pressure in the center of the embankment during the earthquake.

Different Failure Modes of a Clay Foundation-Embankment System Corresponding to Different Incident Seismic Waves

In recent years, damage to embankments from earthquakes has been frequently reported. In this paper, the deformation and failure behavior of an embankment constructed on a weak clay foundation during and after an earthquake is investigated using numerical analysis, GEOASIA, assuming a 40 g centrifugal field. The main conclusions obtained were as follows. 1) Although the embankment remained stable without large deformation during the earthquake, a slip plane subsequently penetrated through the embankment, resulting in delayed failure (circular arc slip). 2) The deformation behavior (failure mode) of the foundation and embankment varies greatly depending on the characteristics of the incident seismic motions.

Modeling and seismic response analysis of a reclaimed artificial ground.

Compared with natural grounds formed by sedimentation over many years, reclaimed artificial grounds are often weak. In this work, modeling of a reclaimed offshore artificial ground and its seismic response analysis were carried out, and the vulnerability of the weak reclaimed layers to seismic activity is pointed out. This study was carried out using a soil-water coupled finite deformation analysis program GEOASIA incorporating an elasto-plastic constitutive model (the SYS Cam-clay model). This model is capable of describing, within a single framework, the behaviors of sands, clays, and intermediate soils consisting of sand/clay mixtures through their soil skeleton structures (structure, overconsolidation, anisotropy) and its works. In reclaimed sand, lateral flow and upward lifting of lightweight underground structures occur because of liquefaction during an earthquake. In reclaimed clay, long-term settlement, which continues for several years after the earthquake, occurs.