INTRODUCTION

The deformability and strength of a rock mass are affected by the hardness and fracturability of the rock constituents, as well as by the number and orientation of joints and fractures in the rock. It is standard practice to classify rock according to these characteristics in order to estimate the engineering properties of the rock. It is difficult, however, to judge rock by these characteristics because the characteristics vary significantly within a given rock formation. Frequently, one or two of these characteristics are sufficiently pronounced to exert a predominant influence on the engineering properties of the rock.

In this paper, the deformability and strength of fractured Cretaceous sandstone and shale, as measured by the results of Menard pressuremeter tests, are compared with the grossly observable characteristics of core samples of the sandstone and shale. It is suggested that there are correlations between the deformability of fractured rock and the hardness of the rock, the distribution of fractures within it, and, especially, the number of fractures in it.

CORE SAMPLE STUDY

The rock formation which is the subject of this study consists of alternate layers of Cretaceous sandstone and shale and is called

(Table in full paper)

Izumi Formation. The strike and dip of the Izumi Formation are N40–50E and 40–50 SE, respectively, and its compressive strength is 500–1500 kg/cm2. The rock mass is permeated by extensive faults and joints due to its proximity to a tectonic line. Therefore, the formation can be considered as fractured rock.

About 40 core boring were carried out using NX-size rods with double core tubes, and 692 samples were taken from the borings. The sample recovery ratios were, on the average, over 90 percent. The core samples obtained were examined and classified according to the system of classifying rock core samples described in Table 1. Of the four classification criteria used, the hardness of rock fragments, the interval between adjacent fractures, and the fracture condition are independent criteria. The criterion of RQD is an amalgamation of the other three criteria, used because it facilitates the quantitative description of rock samples and because it serves as a convenient index of comparison with the three independent criteria.

PRESSUREMETER TESTS

The pressuremeter developed by Louis Menard is used to perform a type of loading test. The pressuremeter applies radial pressure on the walls of a borehole when a rubber tube, 25 cm in length, is inflated with water. Water pressure is usually increased by increments of 1–3 kg/cm2, and each increment is maintained constant for two minutes. Deformation of the borehole wall is measured by the volume of water which enters the rubber tube. Fig. 1 shows a typical result of a pressuremeter test.

The curve is called a P-V curve, and, from the straight portion of the curve, the deformation modulus, Esp, is calculated using the following: There is an inverse relationship between the deformation modulus and the deformability of rock.

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