第 12 回国際岩の力学会議（北京）を参加して-重要構造物の基礎岩盤の動的特性評価生産科学研究科 システム科学専攻 博士後期課程 2 年
楊 磊（派遣期間：平成 23 年 10 月 17 日～平成 23 年 10 月 20 日）
I took part in the 12th International Congress on Rock Mechanics (ISRM 2011) held in Beijing,
P.R. China from 18th to 20th Oct. 2011, and made a post presentation titled as “Evaluation ofdynamic behaviors of bedrock foundation based on FEM and DEM simulations”. In thisinternational congress, many famous professors and experts in the world, e.g. Profs. J.A. Hudsonand T. Brown (the former presidents of ISRM), Prof. X. Feng (the selected-president of ISRM),
Prof. N. Barton, et al, gave their lectures on the new development of rock mechanics and introduced
their particular research fields. This congress provided a good opportunity to learn about the newresearch fields and methods in the aspect of rock mechanics, and more importantly, to exchangewith other scholars on my research topic, from which I can acquire lots of good suggestions.
１．国際会議の概要
The 12th ISRM International Congress on RockMechanics with the topic of “Harmonizing Rock Mechanics
and the Environment”, organized by the International
Society for Rock Mechanics (ISRM), was opened on
October 18 at China National Convention Center, Beijing.
More than one thousand of experts and scholars in rockmechanics field from about 50 countries and regions
participated in this Congress. In the congress, 15
world-widely famous professors delivered keynote lectures,
including the Rocha lecture and the Müller lecture. About
20 parallel sessions were held, e.g. Analysis and Design
Methods, Dynamics and Blasting, Field Measurement andSite Investigation, and Laboratory Testing and Rock
Properties, etc. These sessions demonstrated the latest
achievements in international rock mechanics andengineering field in recent years, and provided a good
opportunity for participants to have extensive discussions on
the presentations with each other.

China National Convention Center

Communication with scholars

２．発表内容と成果（日本語による概要）重要構造物(原子力発電所等)の基礎岩盤の選定において地震による断層を含めた基礎岩盤の強度、変形が岩盤構造物に与える影響を評価する必要がある。本研究では、一般的に動的解析に用いられる有限要素法(FEM)と不連続面のせん断・大変形36

挙動を表現できる個別要素法(DEM)による動的数値シミュレーションを実施して、基礎岩盤の変形挙動を比較検討する。動的解析では、新潟県中越沖地震を対象地震とし、観測地点
NIG019小千谷の地震動(水平加速度のEW成分)を選定した。地震動とは地震波が地下深部より伝達することにより生じるものだから、一次元応答解析(k-SHAKE)によって対象岩盤基盤面(深度200m)に入力する入力地震動を算出した。
有限要素法と個別要素法の解析を比較するため(a) FEM model(b) DEM resultsに、対象領域の条件をほぼ同等にして、ある実際の原子力発電所の基礎岩盤条件をもとに、基Fig. 1. FEM and DEM models
本モデルを作成する(Fig.1)。
FEMとDEMの両解析結果においては、同等の 0.4
A(0m)
2B(30m)
地震動が起きているかを確かめるために基礎岩 0.3 Velocity (m/s )C(100m)盤において任意の四点を選定し、その地点での 0.2 (a) FEM resultsD(190m)0.1
波形の比較検討を行った。まずは、地震波が入0力されている基盤面から近い地点である地下 -0.1 0 1 2 3 4 5 6 7 8 9 10Time (s)190mのFEM・DEM解析結果から分かるように、 -0.2
ほぼ同じ速度波形が得られており、両解析方法 0.4A(0m)Velocity (m/s2)

B(30m)とも同じ地震動を再現できていることがわかる。 0.3
C(100m)D(190m)0.2
次に、地表面・地下30mと地下100m地点のFEM(b) DEM results
0.1とDEMの解析結果の比較を行う(Fig.2)。水平速
0度がほぼ一致しているが、その最大値について、 -0.1 0 1 2 3 4 5 6 7 8 9 10Time (s)FEMの方がDEMより大きくなっている。これは -0.2
Fig. 2. FEM and DEM results
断層に対する影響の受け方が違うためと考えられる。
これらの任意の地点の速度波形の比較より、両手法による地震動はほぼ同じであるが、地震動の表現仕方の違いにより、水平速度の最大値において違いが生じた。特に、DEM 解析では、節理や断層を不連続体としてモデル化し、振動の減衰効果を働かせているため、FEM より実際に近い挙動を再現している。また、地震波入力基盤面から地表に行くにしたがってより顕著に断層に対する影響を受け、速度が減衰する地点もあることを判明した。
この結果より、重要構造物の基礎岩盤の動的挙動を評価するために、岩盤内の既存節理や断層のせん断ずれ・開口を忠実に表現できる DEM 解析方法を用いるべきであり、それを用いた実現場の基礎岩盤の動的挙動の予測評価と考察を行った。
(English abstract) The seismic behaviors of the bedrock foundation concern the stability andsafety of nuclear plants. In this study, both the FEM and the DEM were adopted to investigate theseismic response behaviors of the bedrock under a nuclear power plant located in Japan. The
difference between those two methods in seismic simulations was investigated. The seismic wave

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obtained from the Ojiya observation site during the Niigata earthquake was adopted in this study.
The code of k-SHAKE was used to investigate the seismic wave at the deep rock formation with adepth of 200 m. The corresponding velocity and acceleration waves were input into the DEM and FEM
models, respectively. The response waves at different locations from FEM simulations exhibit goodconsistency. With seismic waves propagating from the deep rock formation to the ground surface,
the amplitudes at waves’ peaks and troughs increase. However, according to the DEM simulations,the amplitudes of seismic waves decrease with the upward propagation of seismic waves. Inaddition, the DEM simulations show obvious divergences of waveforms. The comparison ofresponse waves at the same locations indicates that the wave amplitudes obtained by the DEM
simulations are smaller than those of the waves by the FEM simulations. Moreover, the differencein wave amplitudes becomes obvious with the decrease of depth of monitoring points. The
differences between the FEM and DEM simulations were resulted from the treatment ofdiscontinuities. The FEM can not present the large deformation behaviors of faults, underestimating
the fault’s weakening effect on the propagation of seismic waves. Relatively, the DEM with thecapability of simulating the sliding and separation of faults can evaluate the effect of discontinuities
well in seismic simulations. Therefore, the DEM can be regarded as a better method than FEM inseismic response evaluations of bedrock with discontinuities. The main results may provide
suggestions to the numerical studies and safety evaluations of bedrock subjected to seismic loads.
３．今後の展望と感想
The seismic behavior of rock foundation remains as one of the most promising and fundamental
problems in the academic and engineering aspects. However, it is still not well understood atpresent, due to the complexity of rock structure and the uncertainty of seismic waves. There are amass of discontinuities existing in the rock mass, which principally governs the deformation andfailure behavior of rock mass. Evaluating the effect of discontinuities properly is the key issue inthe analyses of seismic responses of rock foundation. The geometric distribution of discontinuities
in rock masses, such as the dip angle, the spacing and the density of discontinuities, influences thedynamic characteristics of rock foundation to a great extent. Therefore, more numerical simulationsof DEM will be carried out in the future, to investigate the influences of geometric factors ofdiscontinuities on the seismic behavior of rock foundation. Moreover, the mechanical properties ofdiscontinuities like the normal and shear stiffness also have obvious effects on the scatting ofseismic waves, which subsequently affects the reflection and transmission of seismic waves. In thefuture, DEM simulations will be performed on the effects of the mechanical properties ofdiscontinuities.
Through taking apart in the 12th ISRM International Congress on Rock Mechanics, I learned
much about the new research methods and theories for dynamic analyses of rock mass. Especially, I
had a good opportunity to communicate with some scholar form other countries when I made mypost presentation, and received good suggestions on the dynamic simulation, which would be very
helpful for my future studies.

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