Toshinori Kawabata

Seismic analysis of sliding wedge: extended Francais–Culmann's analysis

Abstract Pseudo-static analysis is commonly used to design earth structures. Most pseudo-static methods of analysis require a computer program. This paper presents a simple closed form solution of seismic stability analysis by extending Francais–Culmanns analysis. The analysis is valid for a slope with the most critical planar mechanism and under the influence of horizontal and vertical earthquake accelerations. The resulting permanent displacement is discussed and demonstrated using several real earthquake records. The simplicity of extended Francais–Culmanns solution allows it to be used for preliminary design and classroom instruction.

Finite Element Analysis of Pipe Buried in Saturated Soil Deposit Subject to Earthquake Loading

A coupled stress-flow finite element procedure, based on dynamic Biot equations, was used to analyze the behavior of pipe buried in liquefiable soil. The governing equations, soil constitutive model, finite element discretization and solutions were described. The results of analysis were compared with two cases of dynamic centrifuge test of soil deposit and pipe conducted at 30 g acceleration field. The horizontal soil deposit was analyzed followed by the deposit having a buried pipe of diameter 10 cm (3 m in prototype). The deposit was composed of loose Nevada sand that was saturated with a viscous solution in satisfying the similitude rules of time for the dynamic event and diffusion phenomena. The response of the ground, such as acceleration and excess pore water pressure, and the earth pressure and uplifting of the pipe, were presented and compared. The results of analysis indicated that a coupled stress-flow finite element procedure where the soil was expressed by Pastor–Zienkiewicz Mark-III model was able to simulate the dynamic response of the soil and pipe up to the stage of liquefaction. Several other issues related to the analysis were discussed.

Earth Pressure Distribution for Buried Pipe Bend Subject to Internal Pressure

The behavior of a buried pipeline is significantly influenced by its interaction with the surrounding ground as well as the backfill material. The thrust force generated by the action of internal water pressure tends to move the bend of underground pipeline to the back side. This thrust force is supported by the passive soil pressure that affects the back ground. The concrete block is also used at the bend to minimize the thrust. There is a lack of study on the magnitude of passive soil pressure and distribution of a pipe bend. Such information are required for design. In the current design standard for irrigation pipeline in Japan, back passive soil pressure is assumed to increase in depth with a trapezoid distribution. In other words, when the bend is buried without the thrust block, a trapezoidal passive earth pressure is assumed by projecting it laterally. However, the earth pressure distribution is not readily known when the pipe moves laterally in the ground. In addition, it is not clear about the relationship between pipe intrusion resistance and the interface of the pipe and ground. In this paper, pit tests were conducted using a model pipe having a diameter of 260 mm that was equipped with 20 bi-axial load cells. In addition, similar tests were conducted using a plate with bi-axial load cells. As the pipe moved horizontally in the ground, the distribution of the back earth pressure acting on the bend and plate, and development of slip surface in the backfill sand were measured and investigated.

Field Measurement and Numerical Analysis for Buried Large Diameter Steel Pipes

This paper presents an overview of field tests for buried pipe, which were conducted using steel pipes with 3500 mm diameter and 26 mm thickness. In this full-scale test, five kinds of construction site were employed. The depth of soil cover was approximately 3.0 m in every case. Sheet pile method was employed in Case-5, and open-cut method was conducted in the other cases. Displacement transducers were installed in the pipe and strain gauges were circumferentially attached to the inner of the pipe. The deformation of pipe was observed for over a year by those measures. Therefore, it is found that the behavior of the buried pipe was strongly influenced by backfilling methods. In addition, FEM analyses were conducted to confirm the behavior of soil-pipe interaction. Thus, it is found that the inhomogeneous strength of backfill materials brings local deformation of the pipe.

Lateral force–displacement prediction for buried pipe under different effective stress condition

The aim of the present study is to elucidate the soil–pipe interaction in saturated sand with different effective stress. A force–displacement curve that took into account the variation of the effective stress was formulated based on the results of lateral loading experiments on a model pipe. To reproduce liquefaction experimentally, the effective stress of the soil bed was controlled by the upward seepage induced by a hydraulic gradient. The model pipe was pulled laterally under either displacement or load control. The experimental results indicate that the normalised force–displacement relationships can be well approximated by hyperbolic curves under each hydraulic gradient. When the variation in effective stress is reflected in the submerged unit weight of the soil, the two coefficients of the hyperbolic curves also show a hyperbolic dependence on the unit weight, and the ultimate resistive force increases in proportion to the unit weight. The predicted force–displacement curve fits relatively well to the experimental results.

Influence of liquefaction on scour behind coastal dikes due to tsunami overflow

In the present study, hydraulic model experiments were conducted to clarify the influence of liquefaction on the scour behind coastal dikes due to tsunami overflow. In particular, the variation of the scour profile (scour depth, scour length and scour area) due to liquefaction was discussed. Degree of liquefaction was controlled by upward seepage, which was induced by the head difference between a water tank and a sand bed. Six hydraulic gradients were applied to the sand bed. The experimental results showed that liquefaction had a significant impact on the scour profile. At high hydraulic gradient, slope failures occurred due to reduction of effective stress of the sand bed. As a result, scour length was increased by the slope failures. Furthermore, the maximum scour depth and scour area for high hydraulic gradient became smaller than that for low hydraulic gradient.

Model experiments and simulations on influence of liquefaction on scour at a landward toe of coastal dykes due to tsunami overflow

In the 2011 off the Pacific Coast of Tohoku Earthquake, massive tsunami caused serious damage to coastal dykes. One of the failure mechanisms of a coastal dyke has been considered as the local scour at the landward toe of a coastal dyke. Furthermore, there is a possibility that liquefaction occurs around a coastal dyke by aftershocks when tsunami strikes. In this paper, flume experiments were conducted to clarify the influence of liquefaction on the scour at a landward toe of coastal dykes due to tsunami over flow. Moreover, simulations were conducted to evaluate the influence of the upward seepage on the scour. In the experiments, scour process and scour profile is different according to the degree of liquefaction. In the simulations, the vertical velocity at the bottom of scour hole increases by the upward seepage. However, there is little influence of the vertical velocity on the scour because the uplift force of the vertical velocity is small.

Shaking Table Test for Axial Behavior of Buried Inner Rehabilitated Pipes Affected by Aging Pipes in Liquefied Ground

In Japan, the main irrigation pipeline network is increasingly getting older and losing its function due to deterioration. The concept of in-situ rehabilitation for aging pipelines, which can improve the performance of a damaged pipe by installing new pipeline inside of the existing aging pipeline, has gained increasing attention. However, the seismic mechanical behavior of pipeline rehabilitated by this method is not substantially clear. In this study, to clarify the effect of damaged outer pipe on the inner pipe, shaking table tests were conducted for the buried pipe in liquefied ground. To model the inner pipe, polyvinyl-chloride (PVC) and polyethylene (PE) pipe were used. To model the outer aging pipe, different types of concrete pipes in length were used. The test results indicated that the amplitude of the bending strain in PVC pipe sharply increased due to the stress concentration at the gap between outer pipes. Additionally, the deflection mode of the pipe was categorized into two main types; bow-shaped deformation and pendulum-shaped deformation.

The Influence of Non-uniform Bedding on the Mechanical Behavior of Buried Flexible Pipe

The soil around the buried pipe may become loose locally because of large earthquake and torrential rain, and the bedding may become non-uniform. However, the effect of non-uniform bedding on buried pipe remains to be clarified. In this study, experiments were conducted in order to clarify the behavior of buried pipe on the non-uniform bedding. The experimental pit has 64 holes (100 mm×100 mm) with valves at the bottom to create the non-uniform bedding. Polyvinyl chloride pipe having a diameter of 600 mm and 3 measuring sections (Section-A, B, and C) was used. The distance between sections was 200 mm. The test showed that when the sand was removed from the pipe bottom level around Section-A and Section-C, the pipe moved asymmetrically. As a result, when the ground becomes loose locally, large tensile strain acted on a Section-A (C) of the pipe closest to loosened ground but maximum shear strain decreased. On the other hand, at the mid section (SectionB) between the sections which was close to loosened ground, circumferential tensile strain hardly occurred but principal tensile strain acted almost in a pipe axial direction. Maximum shear strain at the mid section increased to have a risk of yield failure.

Dynamic Behavior of Buried Flexible Pipes of Varying Thickness Using the Shaking Table Test

In designing a pipeline, deflection and stress of the pipe are calculated through structural analysis based on bending ring stiffness EI/D 3 . The pipes with equivalent bending ring stiffness would have the same behavior in design. However, it is easily understandable that the behavior of pipe may be influenced not only by EI/D 3 , but also by the relationships between E and I. In this paper, shaking table test for buried flexible pipes were conducted to evaluate the effects of pipe thickness on the dynamic behavior. Some pipes having approximately equivalent bending ring stiffness and different thickness were used. In this experiment, these pipes were buried in the laminar box and shaken by horizontal sine wave. As a result, it was found that the larger bending strain and radial stress of the pipe occurred with the thinner wall. This result suggests that the probability of the buckling of the pipe is higher on thinner pipes. It is evident that the pipelines for irrigation should be designed in consideration of this fact.