Yoshiyuki Mohri

Soil slopes under combined horizontal and vertical seismic accelerations

Conventional methods of designing earth structures are based on pseudo-static stability analysis employing a horizontal seismic coefficient. This paper discusses the stability and permanent displacement of a slope subject to combined horizontal and vertical accelerations. A log-spiral failure mechanism is used. It is shown that seismic force has a significant effect on stability and permanent displacement of slopes. The parametric study reveals that vertical acceleration may play an important role on stability and permanent displacement if the corresponding horizontal acceleration is large.

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.

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.

Lift-up of Cover-Blocks of Coastal Dikes by Overflowing Tsunami Current and Counter-measure

In order to build robust coastal dike against giant tsunamis, it is important to prevent tearing off of cover-blocks by lift forces produced by over-flowing tsunami. The stability of cover-blocks is affected by overflow conditions, cover-block’s weight and slope gradient etc. In this study, we predict the distribution of lift force acting on the top of slope, and evaluate the relationship between safety-factor of cover-block and overflow depth. As a result, in the conventional dike, the coverblock tears off around at overflow depth 5.7m. On the other hand, the newly proposed type, which connected the coverblock and the crest concrete, has a high resist to the overflow depth of over 16.0m. Because this newly type increases the resistant moment of cover-block with accompanying overflowing water’s weight on the crest in increase of overflow depth.

BEHAVIOR OF SHALLOWLY BURIED PIPE USING GEOGRID FOR LATERAL LOADING

近年，老朽開水路の改修やコスト縮減の観点から，パイプラインの浅埋設工法の適用が進展しているが，圧力管路水平屈曲部に作用するスラスト力に対する当工法の有効性は未解明のままである．そこで本研究では，ジオグリッドを用いた浅埋設工法を対象とした小型模型実験を行い，その有効性について検討した．実験結果より，浅埋設工法がスラスト防護工法に有効であることが明らかとなった．また，受働地盤の抵抗メカニズムについて，2次元個別要素法から検討した結果，水平抵抗力の増加について，ジオグリッドを用いて補強した領域が一体化することに起因することが明らかになった．

DYNAMIC BEHAVIOR FOR SPILLWAY WITH GEOSYNTHETICS SUBJECTED TO SIMPLE SHEAR

1995年の兵庫県南部地震では，多くのため池洪水吐が被災した．一般的に重量構造物である洪水吐は，地震時に大きな慣性力を受けることで破壊・変位を生ずることがその主因とされている．筆者らは，ジオグリッドを取り付けた耐震性を有する軽量洪水吐を提案し，振動台実験によりその有効性を明らかにしたが，メカニズムは未だ解明されていない．本研究では，地震時の地盤のせん断変形に着目し，提案工法に対して繰返し単純せん断試験を行った．アルミ棒積層体を用いた模型実験，および個別要素法（DEM）を用いた数値解析の双方から，その挙動メカニズムを検討した．実験および解析の結果から，洪水吐側壁に取り付けた補強材の引抜き抵抗により，洪水吐の変位が抑制されることが明らかになった．

Bending test for liquefied stabilized soil with reinforcement for improvement the brittleness

農業用パイプライン等の圧力管曲部には，その曲角度と内圧の大きさに応じてスラスト力が発生する．現行の設計基準では，大きなスラスト力が発生する場合，曲管部にコンクリートブロックを設けることでスラスト力を支持するが，コンクリートブロックは重量構造物であるため，地震時の位相差による継手離脱などの被災原因にもなっている．筆者らはスラスト力の対策工法として，ジオグリッドを用いた軽量なスラスト防護工法を提案し，検討を進めている．本研究では，スラスト防護工法として流動化処理土を用いる際の各種補強材の効果について，曲げ試験を行い，さらにPIV(Particle Image Velocimetry:粒子画像流速測定法)による画像解析を行うことにより検討を加えた．

Field Test for Buried Large Steel Pipes with Thin Wall

Field buried pipe tests were conducted using steel pipes with 3500mm-diameter and 26mm-thickness as a part of Irrigation Pipeline Project. In current design for irrigation pipeline in Japan, there are no design criteria for pipeline with diameter larger than 3000 mm. In the field tests, two cases of backfilling methods were employed. In Case 1, soil-cement was used as backfill material to a height of half the diameter measured from the bottom of the pipe. In Case 2, soil-cement was used as backfill to a height of quarter diameter measured from the bottom of the pipe. The mixed rock, which was recycled crushed concrete and crushed gravel, was backfilled above soil-cement to the top of the pipe. The pipe was buried under a depth of approximately 3.0 m in both cases. Displacement transducers were installed in the pipes and strain gauges were attached to the inner of the pipe. Earth pressure transducers and pore water pressure transducers were installed in the backfill materials around the pipes. The deformations in each of the backfilling stage were measured. It is found that the behavior of the buried pipe was strongly influenced by backfilling methods and external force.