Shunsuke Sakurai

Lessons learned from field measurements in tunnelling

Abstract Over the past two decades, various numerical methods of analysis have become popular in the field of geotechnical engineering. However, the accuracy of numerical analyses varies considerably, primarily because of the uncertainties involved in modelling ocmplex geological formations with the complex geomechanical characteristics of soils and rocks. Field measurements carried out during construction can be used to overcome this difficulty. The author reviews ways of using measurement results to improve numerical analyses, including the determination of a “hazard warning level” for each measurement item prior to the start of construction, the use of back analysis. The importance of choosing a proper model is also discussed.

BACK ANALYSIS FOR TUNNEL ENGINEERING AS A MODERN OBSERVATIONAL METHOD

As complexity and unpredictability exist in nature, careful observations and interpretations of what can be measured in the field become prerequisites for geotechnical engineers to conduct safe and economical construction works. Observational methods have evolved from basic visual procedures, conducted on site, to sets of sophisticated procedures using modernized measuring instruments and computer-based back analysis techniques. From the wide range of procedures available for modern tunneling engineering in this field, the present paper tries to address a series of back analysis procedures in which the identification of strain distribution is sought as the primary goal in order to achieve a solid and reliable routine of observations and data interpretations. The discussion starts by identifying structures and flows of forward and back analyses; and it is then expanded to cover several back analysis procedures, including application examples, formulated for linear and non-linear material behaviors. The current status and limitations of the procedures available are also discussed.

Characteristic of the vertical seismic waves associated with the 1995 Hyogo‐ken Nanbu (Kobe), Japan earthquake estimated from the failure of the Daikai Underground Station

The dynamic behaviour of underground structures built by cut-and-cover methods is discussed. A simple model analysis shows that a column supporting the overburden at midspan (central column) can resonate upon incidence of an elastic wave of a specific frequency. The analytical results indicate that not only the size and material properties of the column, but also the static load acting on the column (overburden) is a decisive factor that influences the resonant frequency. Based on the results obtained by the analysis, the mechanism of the failure at the Daikai Underground Station in Kobe caused by the 1995 Hyogo-ken Nanbu, Japan, earthquake is investigated. It is shown that the wave-induced damage to underground structures can concentrate on the sections with specific overburden, and from the induced damage, it is possible to estimate the frequency characteristics of the associated seismic waves. Copyright

Characterization of non-elastic ground behavior of a large underground power house cavern by back analysis

Abstract This paper presents results of a characterization procedure, based on back analysis, of a nonlinear deformation behavior of a discontinuous rock mass during excavation of a large underground power house cavern. The objective is to introduce the concept of isolating effects of nonlinear joint deformation to be back-analyzed from measured displacements and to demonstrate the effectiveness of this method for identifying strain distribution and deformational mechanism within a jointed rock mass.

Influence functions for stress and displacement discontinuity elements in an anisotropic medium

Influence functions, that permit us to determine stresses and displacements at an arbitrary point in an infinite, homogeneous, linear elastic, anisotropic medium due to different three-dimensional (3-D) stress or displacement discontinuities distributed on infinite, flat, band-type elements, are presented. Any straight-line segment on the band, which is perpendicular to its infinite side, has the same distribution of the discontinuities. Along with the functions, their Taylor series approximations are also provided. The last can be useful to analyse stresses and displacements at points distant from the elements. The functions allow us to avoid procedures of numerical integration in the Indirect Boundary Element Method and/or the Displacement Discontinuity Method computer codes that are able to solve complete plane-strain problems with 3-D boundary conditions for an elastic, anisotropic medium.

Dynamic tensile cracking in slopes possibly induced by Rayleigh surface waves

The 1978 Miyagi-ken-oki earthquake generated crack openings in fill slopes in Sendai, Japan. Although the slopes were reinforced with steel pipe piles after this event, tensile cracking was induced again in the same slopes by the 2011 off the Pacific coast of Tohoku earthquake. This clearly indicates conventional countermeasures against dynamic slope failures, usually based on body wave interaction with slopes, are not always effective. Considering that similar open cracks in the top surfaces of slopes were also found in New Zealand and California, we may need to clarify the generation mechanism of such cracks with some new insights. Recently, it has been indicated that the effect of Rayleigh surface wave propagation on dynamic slope stability may become significant. In this contribution, therefore, by performing two-dimensional elastodynamic analyses of Rayleigh/body wave interaction with a geometrically simple model slope, we show Rayleigh waves may play a more dominant role than body waves in generating the open cracks, and emphasise the importance of taking into account the dynamic slope failures induced by Rayleigh waves. Surface waves may be produced also by nearby blasting, etc., and therefore, the results obtained here may be of crucial importance in comprehending the dynamic stability of slopes in general.

The Generation of Seaquakes and Its Impact on Floating Bodies

A seaquake is a strong vertical shock experienced on board a floating body at sea (e.g. ship) during a seismic event, but its generation mechanism and effects on structures have not been fully understood yet. Here, in order to better comprehend the physical process associated with seaquakes, dynamic wave propagation in a seabed-seawater system is studied. From the linear elastodynamic and nonlinear hydrodynamic wave equations, the time-dependent displacements or velocities in the system are quantitatively evaluated. It is analytically shown, for instance, that in a one-dimensional framework, wave interaction may generate resonance of the solid-liquid system or a nonlinear wave of velocity discontinuity (vertical shock) may propagate in liquid. Earlier hydrodynamic analyses of motion responses of a floating body to a seaquake usually assume a potential flow, but such an assumption may not be able to offer a physically more precise description of the generation of seaquakes and their impact on structures.

Development of a system for earthwork progress measurements by using the global positioning system.

人工衛星を利用した測量システムであるGPSを用いた土工事の出来形測定システムの開発を行った. 本研究では, 特にアンテナ自動水平保持装置とリンケージプログラムを開発した. 本論文では, このシステムの詳細について述べ, 実際の測定に適用して, 従来法と比較し本システムの有効性を確認した.

INVESTIGATION ON COLLAPSE MECHANISM OF SUPER-DEEP VERTICAL SHAFT

超大深度地下開発のアプローチとなる超大深度立坑は，その重要性を再認識されつつある．しかし，国内石炭産業の縮小に伴い，深度1,000m級の立坑はほぼ20年以上施工されていない．筆者らは超大深度立坑技術の継承と，定量的な立坑設計技術の確立を目的として，過去に施工された超大深度立坑の技術文献調査，およびかつて施工に従事された技術者からの聞き取り調査を実施し，立坑工事において発生する蓋然性の高い崩壊形態を調査した．その結果，立坑における崩壊のほとんどは高抜けと異常地圧による覆工破損の二つに分類されることが明らかとなった．さらに，その発生状況を検証したところ，この二つの現象は同じ原因により発生するものと判断された．

A STUDY ON TUNNEL FACE STABILIZATION BY TANK TEST

トンネルを合理的に施工する工法として, 先受と加背割りとを併用した新しいトンネル施工法について, トンネル切羽の挙動や安定性への影響の評価を試みた. 本論では, 要素実験や数値解析で得られた結果を検証, 補間するとともに, 実際の地盤中での効果や影響を定量的に評価するため, 大型の土槽を用いて, トンネル切羽近傍地盤の3次元的挙動を把握する実験を実施した. その結果, 切羽応力の解放に伴う地盤の3次元変形特性を把握すると共に, 切羽安定に必要な限界支持力等を推定できること確認した. また, 筆者らが過去に要素実験を実施した切羽形状による切羽安定性の比較を行い, 先受や加背割りが切羽安定に効果的であることを確認した.