Hideki Ohta

FEM ANALYSIS OF HIGH AIRPORT EMBANKMENT WITH HORIZONTAL DRAINS

Abstract A fill of Noto Airport has been constructed rapidly to a height of 55xa0m, and its filling material is high water content clay. Geotextile horizontal drain is installed in the fill to accelerate consolidation. To seek reasonable method to simulate the behavior of a fill with horizontal drains, an elasto-visco-plastic FEM is applied in this paper. The result of the simulation agreed well with observed deformation and monitored stress, but horizontal deformation of the fill body measured is much smaller than estimated by FEM. It probably implies that the reinforcing effect due to friction between the soil and the geotexitile is generated in the field, but is not well-modeled in the analysis. Some trial FEM calculations are carried out for various hypothetical cases of field conditions such as filling speed and permeability of filling material. It is shown that the FEM capable of adapting complicated conditions at a given site enables rational design for embedding horizontal geotextile drains.

Statics of loose triangular embankment under Nadai’s sand hill analogy

Abstract In structural mechanics, Nadai’s sand hill analogy is the interpretation of an ultimate torque applied to a given structural member with a magnitude that is analogously twice the volume of stable sand heap which can be accommodated on a transverse cross-section basis. Nadai’s analogy is accompanied by his observation of a loose triangular embankment, based on the fact that gravitating loose earth is stable if inclined just under the angle of repose. However, Nadai’s analysis of stress distribution in a planar sand heap was found to be inaccurate because the total pressure obtained from Nadai’s solution is greater than the self-weight calculated from the heap geometry. This raises a question about the validity of his observation in relation to the analogy. To confirm his criterion, this article presents and corrects the error found in Nadai’s solution by analyzing a radially symmetric stress field for a wedge-shaped sand heap with the purpose of satisfying both force balance and Nadais closure. The fundamental equation was obtained by letting the friction state vary as a function of angular position and deduce it under the constraint that the principal stress orientation obeys Nadais closure. The theoretical solution sufficiently agreed with the past experimental measurements.

Electrostatic Properties of Polyethylene Langmuir-Blodgett films

Medium-density polyethylene (PE) thin films have been successfully prepared by the Langmuir–Blodgett technique. The absorbance obtained by the Fourier transform infrared spectroscopy (FT-IR) measurement was proportional to the number of deposited layers, indicating that the layer-by-layer deposition of PE films proceeded successfully. Using the Kelvin probe surface potential measurement, it was concluded that as-deposited PE films were positively charged due to the displacement of electrons from the film to a metal. Finally, an energy diagram of the alkane oligomer molecule C30H62 was computed based on an ab initio molecular orbital calculation, and then the possibility of positive charging of the PE film was discussed.

Elasto-Plastic Constitutive Model for Unsaturated Soils with Subloading Surface Concept

An elasto-plastic constitutive model for unsaturated soils is improved to realize numerical stability of computations at the singular point on the yield surface and an accurate prediction of the mechanical behavior of overconsolidated (or elastic state) soils in this chapter. The authors introduce the exponential contractancy model (EC model) by Ohno et al. (J Appl Mech JSCE 9:407–414, 2006) and the subloading surface model by Hashiguchi and Chen (Int J Numer Anal Method Geomech 22:197–227, 1998) to the elasto-plastic constitutive model for unsaturated soils by Ohno et al. (J JSCE 63(4):1132–1141, 2007). The applicability of the constitutive model is verified by simulating triaxial shear tests under constant net stress undrained conditions.

On the numerical implementation of hyperplasticity non-linear kinematic hardening modified cam clay model

A continuous hyperplasticity model named kinematic hardening modified cam clay (KHMCC) is a constitutive soil model based on thermodynamic principles. This model has addressed some shortcomings of the modified cam clay (MCC), specifically on small strain stiffness. Because of employing multiple surfaces plasticity, it can simulate a smooth transition from elastic to plastic behaviour as well as the effect of immediate past stress. This article aims to present some important issues on the numerical implementation of the continuous hyperplasticity non-linear KHMCC model. The incremental stress–strain response is calculated based on a rate-dependent algorithm. A significant advantage of the rate-dependent calculation is that it is not necessary to attach with the consistency condition during calculation of plastic strains. Effect of time step and number of yield surfaces in rate-dependent algorithm will be also presented. A discussion on using of numerical integration rules of hardening functions is addressed. Furthermore, model verification is performed against analytical solution of ideal undrained response which has been obtained from theoretical integration of the MCC function over the imposed stress or strain path. Finally, some numerical demonstrations are also carried out to illustrate several key features of the model.

Development of a Portable Triaxial Testing Apparatus—Smart Triaxial

Reported in this paper are the design concept and performance of a light-weight triaxial apparatus that is portable, being contained in carrying bags and is nicknamed Smart Triaxial. It works with power supplied through AC 100 V or DC 24 V. The apparatus consists of (1) triaxial chamber combined with a loading system, (2) piston pumps supplying cell pressure and back pressure, and (3) control and measuring units. The size of specimens is 50 mm in diameter and 100 mm in height. The drainage line coming up from the upper end of the specimen goes outside through the centers of the top cap and load cell and is therefore invisible from outside. The test program is fully automatic and needs no manual adjustments after a test starts. This makes it possible to produce test results of reasonably high standard even when the operator is not a specialist at soil testing. A series of tests is carried out to check the performance of the apparatus and to make sure that the apparatus works in a satisfactory manner.

Stress–Strain Relationship for the Singular Point on the Yield Surface of the Elasto-Plastic Constitutive Model and Quantification of Metastability

In this chapter, the stress–strain relationship on the singular point of the yield surface of the original Cam clay model and the Sekiguchi-Ohta’s model is derived in order to compute the volume change correctly. Metastable space, the range of increment of strain in which effective stress does not get away from the singular point of yield surface, was theoretically derived. In metastable space, the increment of volumetric strain is larger than the increment of deviatoric strain.

Possibility of Four Metre Deep Flooding in Densely Populated Low-Land Area of Tokyo Induced by a Mega-Earthquake

In this chapter, the authors describe the historical evidence of how the ground conditions in Tokyo have changed over the past 150 years and those changes would influence the area if a mega-earthquake hits Tokyo in the near future. In the recent 150 years, Tokyo was developed as a highly industrialized city protected from flooding by dykes, flood-gates and reclaimed land which have been built in progressive manners. Water needed in industrialized low-land area of Tokyo was partly obtained by deep-well pumping of the groundwater during a period of 100 years from 1873 to 1973. The excessive groundwater withdrawal resulted in lowered groundwater level down to a maximum depth of about 60 m below sea water level, which led to serious land subsidence in the low-land area particularly during a period of 1960–1973 when the withdrawal was extremely heavy. The latest big earthquake struck Tokyo area in 1923 when the altitude of Tokyo low-land area was still higher than the sea water level. Since then Tokyo low-land area has experienced the following three major changes: (i) land subsidence (max: about 4.5 m) due to the excessive groundwater pumping, (ii) construction of extremely extensive underground networks of lifelines, railways, roads and shopping areas, and (iii) increasing danger of failure of the riverine levees due to the recent increase in the torrential rainfalls typically accompanied by typhoons that strike Japan several times a year because of increasing typhoon-activation power supplied from the sea water 1 or 2° (Celsius) warmer than before. These changes made the low-land area of Tokyo much more susceptible to flooding than the time of 1923. In case that the sea water of Tokyo Bay flows onto the low-land area of Tokyo through possible breakage of some part of the existing seawalls and/or the flood gates, the depth of the water in major part of the area is expected to reach about 4 m at the deepest area and about 1.5 m on average. This may cause unacceptably serious situation in which huge number of people will suffer from the submersion of subways when the sea water flows into the underground facilities. About two million local residents in the low-land area submerged by flood water need to move to higher areas because almost all lifeline systems become out of service in the low-land area. Many of submerged underground facilities will not work anymore even after drying off and therefore will have to be replaced by new ones. About ten million people currently using subways in their daily commuting, business trips etc. every day will have practically no alternative means of transport. In the worst scenario, this will give extremely serious effects on enormous numbers of economic activities not only in Tokyo but also in the entire Japan.

Elasto-Plastic FEM Analysis and Safety Evaluation of Large Rockfill Dams During Reservoir Filling

This chapter suggests a method to evaluate the safety of large rockfill dams during first filling. The stress–deformation behavior of large rockfill dams during construction and first filling were simulated using soil–water coupled elasto-plastic analysis. The validity was confirmed by comparing with actual measurements during first filling and laboratory hydraulic fracturing tests using embankment material. A tentative plan for the pre-estimation of the embankment’s hydraulic stability and first filling safety control was suggested and put into practice at an actual dam.

Analysis of Earth Pressure Problems by Upper and Lower Equilibrium Methods

The problem of active and passive earth pressure acting on a rigid retaining wall has been studied ever since Coulomb formulated the limit equilibrium solutions in 1776. It has long been recognized that such solutions greatly overestimate the passive pressure. With the development of the plasticity theory, many problems have been solved by much more logical methods such as the slip line method and limit analysis.