Much effort has been made for the earth pressure control in metal mines as well as in limestone mines where the ores are mined by the room and pillar method. To obtain the fundamental data for the earth pressure control in these mines the measurement of the variation in stress in the rock has been made by the photoelastic stressmeter under the authors' guidance. Recently the absolute stress has come to be measured by the stress-relief technique to supply more substantial data.
These measurements have been made mainly on the rock surface around underground openings, where the cracks initiated by excavation are frequently found. Although the measurement has served not a little for the earth pressure control, it is desired that the measurement of the rock stress will be made possible at the points at considerable distances apart from the wall surface by making use of boreholes. However no method to determine the stress in the rock from the measurements made in the boreholes has yet been established. The authors have therefore investigated this problem. The first part of this paper describes the results obtained.
Based on the theory of elasticity, they have deduced formulae to be used in determining the amount of stress in the rock (1) from the changes in diameter, (2) from the strains on the bottoms of boreholes and (3) from the strains on the wall of a borehole. The application of these formulae in terms of the measured values gives the observation equations, and their solution yields the estimation required of the stress in the rock in question. The minimum number of boreholes that afford the necessary data for determining the amount of stress in the rock has been discussed by examining the rank of the matrix made up of the coefficients of the observation equations.
Subsequently two practical examples of stress measurements are described. In the Nakatatsu mine, the absolute stresses on the walls of a number of drifts around the Stope 4-5 of the Nakayama pit were measured, from which it was discussed whether or not the rock around the shaft situated near the stope would fail by extracting the ore completely. The original stress state, which seems to depend more on the orogeny rather than the topography, was estimated. Judging from the stress concentration that might appear after a complete mining, it was found that complete extraction of ore would be allowed.
In the Hikawa mine, the main haulage drift had been driven through the deep strata in the Tobo district before any other excavation was made. For the purpose of obtaining the data for the design of mining, the stresses on the wall of the drift were measured along the drift. Then the original stress states were determined mathematically, which were taken into consideration in the design of the mining. On the other hand, this measurement presented the data about the stress state in the rock in a mountain district as well as about the influence of faults on the stress state.

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Experimental studies have shown that when metals and plastics are blasted with two free faces there is corner fracture produced at the junction of the two free faces, as was anticipated on the basis of stress reinforcement due to the superposition of the reflected waves on each free face, but that when brittle rocks are blasted no such fracture is produced. It is the aim of the present study to find out the conditions that are required to produce corner fracture by assuming various materials to be blasted with two free faces, and analyzing the stress distributions assumed as produced in each given material, on the equations derived from the theory of elasticity, with application of the values of the measured radial displacement and particle velocity caused to the material by the impulsive loading.
The results of the stress analysis in connection with the development of corner fracture can be summarized as follows;
(1) Pattern of the cracks presumed from the results of the above stress analysis coincided fairly well with that observed in the experiments.
(2) The stress concentration which is considered to give rise to corner fracture is chiefly affected with both Poisson's ratio of the material and the wave length of the stress wave produced in the material by an impulsive loading.
(3) No development of corner fracture is foreseeable in the material of which Poisson's ratio is less than about 2.5.
(4) When the value of Poisson's ratio of the material is larger than 2.5, the development of corner fracture is controlled by the values of C_f, and there is possibility of producing corner fracture for C_f<1.0, but there is no possibility of it for C_f>1.0, where C_f=t_r·C_L/W, and t_r is the rise time of the stress wave. C_L is the velocity of the longitudinal wave, and W is the length of the burden.

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A new failure criterion of rocks and rock-like materials is hereunder proposed in this paper. The theoretical consideration of the criterion is presented as well as its experimental verification. In this study failure is defined as collapse of the material.
The criterion is based on the assumption that the ultimate strength of the material under a compressive state of stress depends on its shearing strength, while under the tensile stress it may be collapsed by the tensile fracture which is thought to be predictable in the Griffith theory.
It has already been verified that the shearing strength of the material can be utilized as the function of the mean stress instead of being concluded as peculiar to the material itself. Theoretically, however, the shearing strength of the material may be considered in effect as dependent on the quantity of the tensile fractures induced around the crack or discontinuity, and the said quantity of the tensile fractures is, according to the Griffith theory, the function of the minimum stress. Therefore it is proposed as condition for the criterion that the ultimate octahedral shearing stress represents the function of both the mean stress and the minimum principal stress. The proposed failure condition may also be expressed in this way that the maximum elastic strain energy of distortion stored in the material depends upon both the mean stress and the minimum principal stress.
Under an extreamly high compressive stress state, failure may be caused by plastic flow, so that the failure criterion may also be proposed in this way that the maximum octahedral shearing stress is the function only of the mean stress.
As extended application of Coulomb's and Mohr's criteria to the three dimensional stress state will some of particular cases of the criterion be defined.
The experimental results obtained from rock salt, marble, sandstone, and concrete show fairly good agreement with the failure criterion proposed above.

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Here is a brief report of improved testing methods of precise measurement of fracture strength that have been applied to experimental studies of the effect of combined stress states on rock failure. The increase of strength with the least compression stress σ₃ in triaxial compression tests is well explained by the Mohr theory among the current failure criteria. The marked effect of the intermediate principal stress σ₂ has been found by the biaxial compression test and the comparison of triaxial compression with extension tests. Strength at failure increases with σ₂, by an amount that is proportional to, although smaller than, the amount of σ₃.

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This paper contains a discussion of the macroscopic shearing strength of brittle materials in the discontinuous state such as joint systems or stratification in the materials.
The plaster-sand model tests have been performed by the application of two types of direct shear test (box shear test and rock shear test) for jointed and stratified specimens in order to investigate their failure mechanism and anisotropy of the macroscopic shear strength and the dilatancy.
These tests have made it clear that the macroscopic shear strength is affected by the failure mechanism which depends upon the movement of fractured particles after the crack formation around the joints or in the strata. The apparent resistances to the shear force for the jointed or stratified materials with positive inclination to the fictitious shear plane are less than for the materials with negative inclination.
The jointed materials possesses less sensitivity against the anisotropy of strength and dilatancy with respect to the direction of discontinuous planes compared to the stratified materials.

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A considerable difference between the fracture stresses theoretically calculated and those actually observed is usually explained by the presence of minute flaws, the so-called Griffith cracks, in the material.
Assuming that the fracture is caused by the stress concentration around Griffith cracks, a statistical theory of brittle fracture is given which could be applied to arbitrary stress field. In this theory, the distribution of strength of test piece is related to the distribution of shapes of Griffith cracks.
In this paper, it is supposed that the probability density function of the shape of Griffith cracks is
f(ξ₀)=δk^δ/ξ₀^(δ+1)·exp{-(k/ξ₀)^δ}
where ξ₀ is the index of the shape or flatness of Griffith cracks.
Based on this probability density function of the shape of Griffith cracks and some assumption concerning the stress concentration around cracks, the size effects of the uniaxial tensile and compressive strengths are calculated and given in the equations which coincide with the formula derived from Weibull's distribution function, that is,
S_t=C_t·k·s·n^-1/m
for the uniaxial tensile strength of the test piece which contains n cracks, where C_t is a constant, and s is the theoretically calculated fracture stress.
In the calculation of the uniaxial compressive strength, besides Griffith's theory, a modified Griffith theory by McClintock and Walsh is considered.
As the result of these calculations, the relations between the parameter of the distribution function of the shape of Griffith cracks δ and Weibull's coefficient of uniformity m are given in graphs. And the relations between the parameter δ and the“brittleness index”(the ratio between the uniaxial compressive and tensile strength) is given in another graph.
Using these graphs, the dispersions of the actually observed uniaxial compressive and tensile strengths of rocks are discussed.
To conclude, the following suggestions are given:
(1) The brittleness index is not always a constant, but is a function of the distribution of shapes of Griffith cracks.
(2) Besides the“inherent”dispersion discussed in this paper, there may be some experimental errors. Based on the theoretical aspect given in this paper, it is possible to evaluate the difference of experimental errors between the uniaxial compressive and tensile strength.

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It is often considered that linear relationship exists between the uniaxial compressive strength and Young's modulus of rock. There is some misunderstanding in this. Uniaxial compressive strength is sensitive to structure in property while Young's modulus is insensitive to structure in property. There is essentialy no relationship between them.
Based on the experimental data of the uniaxial compressive strength and Young's modulus of rock, and on the study of the scattering of values of both the properties, it is argued that there is no relationship between both the properties of rock.
It is conceded, however, that assumption of some indefinite relationship is possible between both the properties, but it is emphasized that such is only usable for rough estimation of rock strength.

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Rocks in instability under stress are prone to produce micro-seismic noise, and it is considered that that has something to do with destruction of rocks.
In order to find a suitable method for foreseeing roof falls in tunnel construction works or slope failures in excavation works by detection of the micro-seismic noise, the necessary studies of fundamental facts are being made in the laboratory.
The present paper describes the results obtained from the experiments of the micro-seismic noise during destruction of rock samples under the laboratory condition.
The main results are summarized as follows.
The relationship between the occurrence of micro-seismic noise and the load depends on the properties of the rock. The frequency distribution of number of the micro-seismic noise with respect to the energy possesses a statistical regularity, and it is expressed by the following equation:
n(e)=k·e^-m
where e is the energy of the micro-seismic noise, n(e) is the number of the micro-seismic noise having energy e, and k and m are constants.
In the above equation, the value of m has relation to the modulus of elasticity of the rock and depends on the deformation condition during the process of destruction.
It is suggested that the value of m is an important index for predicting rock failures.

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A crank-lever type fatigue testing machine has been designed for the study of fatigue property of rocks. By means of this testing machine, the pulsating compression test has been undertaken for three kinds of rocks: the“Iwaki”sandstone, the“Kimachi”sandstone, and the“Tako”sandstone.
From the test, the following results have been obtained.
(1) When the S-N curve of the rock begins to be independent of the number of cycles to failure, the number of cycles to failure at that time lies in the order of 10⁴ to 10⁵. This order is much less than the order in the case of steel, which is between 10⁶ and 10⁷.
(2) The width of stress-strain hysteresis loop of the rock under the pulsating compression has a tendency to narrow as the number of cycles increases, and when the rock approaches to failure, the width of hysteresis loop widens again.
(3) Young's modulus of the rock has a tendency to decrease as the number of cycles increases.

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Hereunder is presented a brief report of investigation made of the elastic anisotropy in rocks by the measurements of compressional wave velocity in the ultrasonic wave experiments. The velocities of compressional waves were measured at ordinary atomospheric pressure to 5 kilobars for Sanbagawa metamorphic rocks collected from various places in Shikoku, which were believed to be important constituents of the earth's crust. Three cylindrical specimens of 15mm in diameter and 45 to 50mm in length were cored from a block of rock samples by means of a diamond-impregnated boring machine. The orientations of the three specimens were mutually at right angles so as to coincide respectively with the three tectonic axes: a, b and c.
The principal factors contributing to the wave-velocities in these rock samples of crystalline schists were investigated. The velocity variations due to pressure were empirically obtained as
V(p)=V_f-AP^-n
in which A, n were constant paramaters indicating the speed of decrease in the porosity effect on wave velocity and also indicating elastic anisotropy. V_f was the velocity at pore-free condition at high pressures.
The wave velocity along the b-axis was the largest and that along c-axis was the smallest. The largest difference in these wave velocities amounted to about 40% of the velocity along a-axis at the ordinary pressure and to about 10% at high pressures. The elastic anisotropy was great at lower pressure, decreasing with the increase of pressure. The relationship between wave velocity and density has been found to be V=α+βρ and β=2.20∼3.75 for crystalline schists.

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This paper is a continuation of the first one entitled“Method to Find Secular Bending of Big Granite Beams and Results Obtained for the First Seven Years”, Journ. Soc. Materi. Sci. Janan, Vol. 14, No. 141, pp. 507∼519, 1965.
In order that the secondary creep (steady flow) can be experimentally confirmed, the creep-test should be continued for a period that equals the relaxation time of the material tested. From our experiments the relaxation time of granite is estimated to be 30∼300 years.
The deflection curve of the test-piece undergoing bending is assumed to be represented by y=T (t)·X(x), where T(t)is a function of the time t that has the reciprocal dimension of Young's modulus E and T(0)=1/E. Plotting the values of T(t) found from our experiments against t, the mean curves of T(t) for both the center-loaded and unloaded beams that are represented by broken lines in Fig. 4 strongly oscillate along some monotone and slightly increasing curves (not represented in the figure) during the first 4 to 5 years, and afterwards the oscillation for the center-loaded beam becomes markedly weak and that for the unloaded remains similar yet strong. It is noticed that the oscillations for both the two test-pieces run in the general trend parallel to the annual variation of humidity inside the laboratory. Elimination of the effect of humidity on the observed values of T(t) is one of our tasks that will be done next.
In the later part of this paper an interpretation of the above mentioned strong oscillations of T(t) is made to suggest that the observed values of T(t) would be apparent and the variation of the true values of T(t), corrected for humidity, would be monotone and slightly increasing with respect to time.
The conclusions obtained in this paper are: (1) Granite makes viscous flow or plastic one that has a very small yield stress beyond which the flow takes place. Since the unloaded beam has been flowing and its maximum stress is 12.8kg/cm², the above mentioned yield stress, if it existed, would be much smaller than this value. (2) Assuming T(t)=1/E+t/3η, the viscosity η of granite is calculated to be 10²⁰∼10²¹ poises, of which the greater value equals one tenth to one hundredth of the viscosity of the earth's upper layer under continents computed from the post-glacial uplift of Fenno-Scandia and equals that from the Pleistocene lake uplift in Utah, U.S.A.

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The interpretation of deformations, strains and stresses in rocks on the assumption that the rock behaves as a medium homogeneous, isotropic and linearly elastic, is not always practically valid.
The paper describes a theoretical analysis of stresses and strains in an orthotropic linearly elastic rock, and proposes a method for practical determination of initial or variational stresses by using the stress-relief technique. The determination of initial stresses by the aid of photoelastic plug is worked out in detail.

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It is indispensable in the continuous observation of crustal deformations to estimate the volume of the deformed region. The author has been engaged in performing the observation of the array of linear strains by means of several highly sensitive extensometers and ones of some other types at Osakayama observatory since 1960.
By analyzing the result of this observation, he has been enabled to obtain the diameter of permanently deformed parts of the crust within the recent years, the total change of the elastic energy contained in the part and the expectant magnitude of the earthquake which may break out with in the limit of the straining. The author corraborates moreover that the strains caused by such as the tidal forces have almost world-wide effects, but the ones caused by pseudo-crustal movement have various minor regional effects.

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This report is concerned with an experimental study of the behaviour of stress waves induced in steel or rock by the detonation of an explosive. A capacitance gauge developed by the authors was used to measure the dynamic stress level.
The shapes of the stress waves induced in steel, quartzite and magnetite, by an explosion of Pentolite or Belite A, and their propagation velocity together with the stress attenuation of the waves with distance from the shot point were determined. The volumetric strain under impulsive high pressure was also obtained in this series of experiments.
Considering the change in the propagation velocity observed, it has been concluded that a plastic wave zone is found over a limited region close to the explosive charge. Adjacent to this plastic wave zone, there is a shock wave zone which is replaced by an elastic wave zone at a short distance from the charge.
It has been deduced that the stress attenuation of the waves propagated through the specimen may be represented by the following equation:
σ=α(r/3√W)^-β
where σ is the stress level, r the distance from the centre of the spherical charge and W the weight of the charge. Both α and β are constants which depend upon the properties of the specimen and the explosive used. The attenuation coefficient, β, seems to be smaller in the sequence of magnetite, quartzite and steel.
Moreover, it has been found that the relations between the impulsive high pressure and the volumetric strain of the rock differ from relatively low to relatively high pressure levels.

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The methods that are commonly used in civil engineering for the purposes of locating sites for construction of dams and reservoirs, of locating ground water, of determining the thickness of weathering layers, of locating fault zones, of locating sites for tunnel construction, and for keeping precise well logs, are by electrical prospecting.
In the present paper, a new electrical method to measure the deformation and the fracture of rocks caused in consequence of the blasting, is proposed and explained as experimented in the cases of the Nagano Dam, Electric Power Development Co., Ltd. (Figs. 3, 4 and 5), the Kiso Tunnel, Kansai Electric Power Co., Ltd. (Figs. 6 and 7) and the Kubiki Tunnel, Japanese National Railways (Figs. from 8 to 13).
In blasting high pressure compressive waves are produced which propagate themselves through the earth structure surrounding the blasting point. By these waves deformation and fracture are generated in the earth structure and so variation of earth resistivity takes place. There are two kinds of the variation of the resistivity. One of these, known as the seismic electric effect, is the variation generated by the deformation only during the presence of the waves. The other one is the variation generated by the fracture which does not return to the initial earth structural condition.
The variation of the resistivity of the first kind is shown in Fig. 4. The variation of the resistivity of the second kind is shown in Fig. 5, 6 and 7.
The relations between the resistivity and porosity are shown in Eqs. (2) and (3).
This electrical method is valid in investigating the blasting effects in civil engineering.

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There seem to be three types of failures as essential factors when rock is blasted. We have conducted observation of the actual bench and coyote blasting in the field in order to study the differences of blasting effect caused by these types of explosives and drilling patterns.
These three types of failures take place almost at the same time. Generally speaking, however, only one out of these three appears prominently on most occasions.
In other words, it has been observed quite often that shear type failure causes the highest percentage of blasting action. Accordingly, when bench blasting is designed, it is recommended in most cases to keep the hole as deep as possible in the field circumstances. Thus, it is expected that the lower part of the bench is to be blasted by the shock effect and the upper part by the sliding effect.
There remains slight difference in the charged volume, however, due to the particular circumstances of the field. In general, the conventional application of Livingston formulae can be recommended in most cases.

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