A Pyroclastic flow deposit Shirasu, widely diStributed in southern Kyushu of Japan, is of properties like a weak rock in its undisturbed state and there has never arisen any rotational shear-sliding failure in cut slopes. In dealing with its cut slopes, two kinds of problems could never been avoidable in the traditional steep slopes; One is the gully erosion due to heavy rains and the other is the tensile failure. However, the former damage has in fact been prevented now by applying scrupulous drainage arrangements while the latter has not so made clear on its prevention design.

The authors have published an analytical Study by means of the finite element method with particular reference to the tensile failure in cut slopes of Shirasu Only in case of statical condition (Yamanouchi et al 1975). Also they have Published a study of the failure mechanism on, undisturbed samples of Shirasu applying the rock mechanics approach (Yamanouchi & Murata 1979).

This paper describes the analysis and design of cut slopes for preventing the tensile failure incorporating the past basic studies into both cases of ordinary and earthquake times. In carrying out the analysis the finite element method and the Seismic coefficient method are jointly used besides adopting the Griffith criterion as the tensile failure criterion. Thereby the authors showed not only a useful explanation on the actually causing tensile failure but also the desirable design method of the cut slopes. These prove the rationality of the Kyushu Expressway recently built in the Shirasu area in which 45 degree slope angle was adopted with scrupulous drainage works.


The authors have carried out static and dynamic finite element analyses by assuming the natural Shirasu ground with a cut-off slope to be an isotropic and homogeneous elastic body. The cut-off slope as discussed in the present analysis was of the gentle slope of 45°the intermediate slope of 60°and the steep slopes of 80°and 90° The analysis domain for the finite element method is given as shown by Figure 1. Moreover, the element size was made relatively small in the vicinity of the slope taking into consideration of the possibility of stress concentration. In addition, the boundaries to the domain were given far enough from the slope in order to avoid their interference with the concerned area at present. Besides these, the lateral movement was confined at the lateral boundary and the lower boundary was perfectly fixed

(Figure in full paper)

The analyses were carried out on the three kinds of Shirasu identified and-classified with their hardness (Yamanouchietal 1980); soft Shirasu, semi-hard Shirasu and hard Shirasu, Elastic constants of these materials are given in Table 1 depending on the results obtained from the triaxial compression test for static values and by the seismic method for dynamic ones, respectively. Table 1 also shows the unit weight, shear strength constants and tensile strength of each sort of the Shirasu material.

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