Introduction

The tertiary mudstone ground of newer era is often accompanied by soft discontinuities or thin weak layers, When an important structure is built on such ground as a foundation, these non-homogeneous layers are considered to affect the stability of ground surrounding the structure. Especially when a non-homogeneous layer exists at a low angle in the ground, a Sliding surface partly consisting of discontinuities is likely to occur, necessitating seismic evaluation of the ground from this viewpoint.

As for the methods of seismic stability of the ground, a conventional seismic coefficient method has long been practiced, and, recently, a more detailed method of evaluation by means of FEM analysis - a very useful method of evaluating stability of complicated non-homogeneous ground - has also become available.

Although dynamic FEM analysis is considered most appropriate as a method of evaluation reflecting ground composition of high complexity and behavior of ground during earthquake, it is not entirely practical as a method of design, because much time and labor is required when it is used for analysis of ground of a wide domain. Static FEM analysis, on the contrary, has an advantage of enabling accurate analytical model of ground of complicated structure and non-homogeneous nature, but has a shortcoming that dynamic behavior of ground during earthquake is not reflected when an analysis is made on a wide domain. Also when static FEM analysis is made by means of conventional seismic coefficient method, it is neither conceivable nor reasonable to apply uniform seismic coefficient throughout a wide ground domain.

In this report, equivalent seismic intensity is proposed as seismic coefficient reflecting dynamic behavior of ground during earthquake to be used for static analysis on a given model.

EQUIVALENT SEISMIC INTENSITY
Concept of equivalent seismic intensity

When seismic force acts on a ground where it moves as a rigid body, acceleration at any point in the ground at a given time uniformly distributed because any point in the ground is in the same motion.

Seismic coefficient method conventionally used holds true when such conditions as mentioned above are assumed.

However, when seismic wave transmits to the depth of ground of a wide and deep rock domain, the shallower ground does not vibrate uniformly but vibration of higher degree with several nodes occurs in the ground as shown in Fig. 1.

In other words, distribution of acceleration in the ground is not uniform in the direction of depth but accompanied with difference of phase.

From this fact, it is not considered reasonable to assume uniform seismic coefficient in the direction of depth when deep ground is in question.

Horizontal shear force caused by earthquake is also not distributed in triangular distribution but in smaller stress distribution because of the effect of phase differences of acceleration as shown in Fig. 1. Noting that distribution of acceleration in the ground is not uniform and especially that phase differences occur in the direction of depth and that these effects are concentrated in shear stress of the ground,

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