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  • Author Name: 羽柴公博

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Drilling Processes with Hydraulic Percussion Rock Drills
  • Kimihiro HASHIBA, Katsunori FUKUI, Masahiro KOIZUMI
  • Journal of MMIJ
  • Vol. 133 (2017) No. 9
  • Released: September 27, 2017

Rock drills were developed about two hundred years ago, and hydraulic percussion rock drills are about half-century old. Performance and efficiency of rock drills have been increased by a number of researchers and engineers. Percussion energy was dramatically increased with changing the power source from pneumatic to hydraulic pressures; rods and rod joints were improved to endure the high percussion energy; carbide button bits were developed for hard rock drilling. This paper reviewed the previous studies and future issues on the drilling processes with hydraulic percussion rock drills. Studies on the stress wave propagation in rods and rod joints were based on theoretical and graphical methods and recently on numerical simulation. Studies on the interaction between a button bit and rock included crack propagation in rock, force-penetration relationship during drilling, and bit wear. Studies on the factors affecting drilling efficiency and drilling rate made a transition from simple to precise numerical simulations. Finally, important future issues were presented for the further progress of hydraulic percussion rock drills.

Study of Blasting Vibration Properties and Vibration Re-Production Method with Advanced Electronic Detonator
  • Keita IWANO, Junichi NAGAE, Katsunori FUKUI, Kimihiro HASHIBA
  • Journal of MMIJ
  • Vol. 133 (2017) No. 6
  • Released: June 19, 2017

Although blasting is a quite efficient and economical excavation method, it is rarely applied to tunnel projects close to residential areas due to its environmental impacts, especially with regard to vibration and noise. Two key controls used to mitigate the occurrence of noise and vibration due to blasting are the use of small instantaneous explosives charges and the use of precise delay times to initiate the blast design sequence. An advanced electronic detonator, which has its precision of 0.01% of designed delay time and has made it possible to achieve precise initiation control in blasting, was introduced and used in a tunnel construction site in Japan. Testing of the delay times during blasting was carried out, the test results revealed the specific features and performance of the detonator to control the blasting vibration especially within a short distance of the tunnel face. In previous studies, several ways of simulating blasting waveforms were used. One of these, the Monte Carlo method, in which the production wave was reproduced by superposing a single wave had the potential to give good predictions of the production waves that might occur. Therefore, in this study, a similar way of simulating production waves was carried out. Section peaks in the production waveforms correspond with the detonation of each blasthole. These section peaks were found to follow the Weibull distribution, even though the section peaks might be influenced by amount of explosives, drill alignment, and geological inhomogeneity etc. More detailed simulation with consideration of difference in travel time and the change of the Weibull parameters with distance gives further precise results compared with the real production waves. This study leads more precise method of predicting production waves and of optimum blasting design.

Effect of Water Saturation on Stress-Strain Curve of Rocks in Uniaxial Compression
  • Minami KATAOKA, Tianshu BAO, Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 133 (2017) No. 6
  • Released: June 06, 2017

The effect of water on the mechanical properties of rocks, such as strength and Young's modulus, has been investigated in many studies. To understand the mechanism of the effect of water on the mechanical behavior of rocks, not only these mechanical properties but also the stress-strain curve, which is one of the most basic data showing the mechanical behavior, should be focused on, and the change of the stress-strain curve with the different water conditions is needed to be known. In this study, the effect of water saturation on the uniaxial compressive strength and the stress-strain curve of rocks was investigated. The uniaxial compression test with alternating loading rate was performed using Sanjome andesite, Tage tuff, Kimachi sandstone and Akiyoshi marble. Inada granite was used for the uniaxial compression test with constant loading rate. The tests were conducted under various water conditions: oven-dried, vacuum-dried, air-dried, air-dried for one day, immersed in water and watersaturated conditions. The test results showed that the stress-strain curve of the andesite, tuff, sandstone and granite changed with the water saturation. On the other hand, the effect of water saturation on the stress-strain curve of the marble was not observed. From the test results, the increase in strength due to decrease in water saturation was discussed. A shift of the peak strength point with the water condition change was investigated, and the relation between the stress-strain curves under the different water conditions was considered based on it.

Study on the Wear Process of Styli in the CERCHAR Test
  • Kimihiro HASHIBA, Katsunori FUKUI, Motoki SAKAGAMI, Shogo SANUKI
  • Journal of MMIJ
  • Vol. 132 (2016) No. 8
  • Released: August 31, 2016

In the CERCHAR abrasivity test for rocks, to understand the wear process of styli is important for the appropriate setting of test conditions and for the estimation of bit wear in in-situ rock excavation. In this study, the wear process of a stylus was estimated from both the stylus vertical displacement measured during the test and the groove depth measured after the test. The results showed that the stylus wear during the test can be traced with a simple calculation of the measured values for the two high abrasive rocks, Inada granite and Iwaki sandstone. In those tests, the styli were worn flatly. The calculation was corrected for the other medium to low abrasive eight rocks, because the measured groove depth was not thought to be completely consistent with the penetration depth of the stylus. Comparing the results of the nine rocks with the exception of the granite, the stylus wear continued for longer sliding distance in the tests for higher abrasive rock. It was found from the results of the granite that the mineral constitution affected the wear process of the stylus. In addition, this paper discussed the relation of the CAI (CERCHAR Abrasivity Index) and its variation to the stylus wear process and the rock hardness. On the basis of the results, points of attention in the CERCHAR test were described.

Effect of Bit Configuration in Impact Penetration Tests with Button Bits
  • Yingzong LIANG, Kimihiro HASHIBA, Katsunori FUKUI, Masahiro KOIZUMI, Toshio MATSUDA
  • Journal of MMIJ
  • Vol. 132 (2016) No. 8
  • Released: August 27, 2016

Computer simulation of percussive rock drilling requires appropriate numerical models for the interactions among the drill body, rod, rod joint, bit, and rock. In this study, the impact penetration of a bit into rock was investigated for the modeling of the bit-rock interaction. Impact penetration tests on Inada granite were carried out with six rod-bit configurations which were composed of four kinds of button bits and two kinds of rods. In the calculation of force-penetration curves from the measured rod strains, the bit model constructed from the acoustic impedance was simplified, and the empirical data correction method proposed by the authors was applied to all the rod-bit configurations. The force-penetration curves for the six rod-bit configurations showed that the bit force in loading phase was approximately proportional to the square of the penetration. The curves in unloading phase had a linear relation between the bit force and the penetration. The final penetration of each blow had a linear relation with the maximum penetration, and the measured borehole depth was proportional to the maximum penetration. The effect of rod diameter on the force-penetration curves was not obviously observed. On the other hand, the bit force corresponding to the same penetration increased and the specific energy decreased with an increase in bit diameter or in the number of button tips on the bit. These findings will contribute to the improvement of the accuracy in the simulation of percussive rock drilling.

Methods and Points of Attention for the Direct Tension Test on Rock
  • Kimihiro HASHIBA, Kazuo TANI, Tetsuji OKADA, Suguru SHIRASAGI, Yasuki OIKAWA, Naruki WAKABAYASHI
  • Journal of MMIJ
  • Vol. 132 (2016) No. 1
  • Released: January 16, 2016
Compared to uniaxial or triaxial compression tests on rocks, direct tension tests on rocks are not common in practice. As is well known, the reasons are because underground rock masses are usually under compressive stress state and/or because the direct tension test on rock is rather difficult and time-consuming. However, the direct tension test is important to understanding mechanical characteristics of rocks under tensile stress state. Moreover, the number of case examples requiring accurate tensile strength of rock is recently increasing in rock engineering. This paper summarized the methods and points of attention for the direct tension test on rock, based on the previous studies and the authors' experiences. The direct tension tests on rock have adopted a variety of specimen shape, loading apparatus and testing conditions. This paper presented some important factors affecting the deformation and failure of rocks under tensile stress state. The relations of the direct tensile strength to the other mechanical properties, such as the Young's modulus, the elastic wave velocity and the compressive strength, was discussed. The rock tests related to the direct tension test were explained: tension test using centrifugal action, tension test under confining pressure, dynamic tension test and in situ tension test on rock masses. This paper introduced the authors' ongoing standardization activity for the direct tension test on rock.
Consideration on a Calculation Method of Force-Penetration Curves in Impact Penetration Tests
  • Kimihiro HASHIBA, Yingzong LIANG, Katsunori FUKUI, Masahiro KOIZUMI, Toshio MATSUDA
  • Journal of MMIJ
  • Vol. 131 (2015) No. 1
  • Released: January 01, 2016
To improve the performance and efficiency of rock drills, it is essential to understand the penetration behavior of a bit into rock. However, studies on the impact penetration with a button bit, which is commonly used for percussive drilling of hard rock recently, are insufficient. In previous studies, large fluctuations were observed in a force-penetration curve calculated from rod stresses measured during percussive drilling, and therefore precious force-penetration curves could not be obtained. In this study, a calculation method was examined to preciously obtain a force-penetration curve from impact penetration tests. The results of numerical simulation showed that the fluctuations can be caused by the mismatch between the product and calculation model, and that the precious forcepenetration curve may be obtained with subtracting the bit force in a free end test from that in an impact penetration test. The suggested method was applied to the measured results of impact penetration tests with granite conducted by Fukui et al. (2010), and improved force-penetration curves were obtained. The method proposed in this study can be adopted to in-situ rock drilling, and contribute to the development of the bit suited to each rock type.
Variation in the Shear Strength of Red Shale
  • Seisuke OKUBO, Yang TANG, Jiang XU, Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 130 (2014) No. 10_11
  • Released: October 02, 2015
Variation of rock strength has an important role in designing the various rock structures. Though strength in uniaxial stress state such as uniaxial compressive strength has been well investigated up to now, the strength in complicated stress state such as triaxial compressive or shear strength has been studied only to very limited extent because of its difficulty in experimental works. For example, variation of shear strength should be studied under various normal stresses and then many specimens are necessary. In this study, direct shear tests of red shale were conducted under the normal stress from 1 to 6 MPa. A special care was taken to make as many specimens as possible from the limited sample rock, and total number of 397 specimens were tested. It was found that the variation of Δτ, which is the difference between the shear strengths measured in testing and calculated with the regression line, did not depend on normal stress. The obtained failure criterion and variation of strength for the red shale were discussed comparing the envelope of rock stress which was reported previously and the results of other rocks in various testing conditions.
Numerical Simulation on Generalized Relaxation of Sanjome Andesite
  • Seisuke OKUBO, Hailong ZHANG, Jiang XU, Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 130 (2014) No. 8_9
  • Released: August 01, 2015
There are many research results on creep and relaxation aiming at investigating the time-dependent behavior of rock. However, rock mass around gateways in mines or underground structures deforms accompanied with both stress and strain change. Fukui et al. (1992) proposed the generalized relaxation test where both stress and strain change keeping the ratio between them constant. In this study, computer simulation was carried out to explain their results. The non-liner Maxwell model was used in the simulation and the calculated results well coincided with the experimental results except those in high stress level in which strain/stress rate were considerably larger than the calculated results.
Study on Constitutive Equations and Mechanical Behaviors of Rock in Uniaxial Tension
  • Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 130 (2014) No. 5
  • Released: May 13, 2015
To design underground structures and estimate their stability, it is essential to understand mechanical behaviors of rock and applicability of constitutive equations not only in compression but also in tension. In this study, the non-linear visco-elastic constitutive equations proposed by the authors in compression were applied to the behaviors in uniaxial tension. The deformation characteristics and mechanism in uniaxial compression and tension under dry and wet conditions were investigated by comparison of the mechanical properties and the constants in the equations. It was found that three non-linear visco-elastic constitutive equations can reproduce the stress-strain curves in uniaxial tension for Sanjome andesite in dry and wet, Kawazu tuff in dry and wet, and Inada granite in dry condition. Time-dependent behaviors of Sanjome andesite in dry condition such as loading-rate dependency and creep can be also reproduced with the same equations. Because the calculated results of unloading curves were not consistent with the testing ones, the new constitutive equation combining the previous ones was proposed. The testing results showed that strength degradation by water is larger in tension than in compression and that initial Young's modulus in tension is nearly equal to the slope of unloading curves in the pre-failure region in compression. Relationship of the constants in the constitutive equations under various testing conditions was discussed. The exponents representing the stress dependency of strain rate in transient creep were calculated with the constants in tension, and found to be consistent in the three constitutive equations under dry and wet conditions.
Mechanical properties of siliceous mudstone of the Wakkanai formation
  Diatomaceous earth and siliceous rock are widespread in the areas along the Sea of Japan in Hokkaido Prefecture, Akita Prefecture, Noto Peninsula and Oki Islands. To construct underground structures in the siliceous rock mass, it is essential to understand its mechanical properties. In this study, siliceous mudstone of the Wakkanai formation obtained from the deep underground in Horonobe, Hokkaido was applied to various laboratory tests; uniaxial compression test, Brazilian tension test, drying shrinkage test and the test to investigate the time dependent behavior and strength recovery. The testing results showed that water has a huge effect on the deformation and failure of the siliceous mudstone; when the specimen is air dried in room temperature, the axial shrinkage strain reaches 0.9% and the strength is twice larger than that in wet condition. It was found that the siliceous mudstone exhibits the same degree of time dependent behavior and strength recovery as other rocks such as tuff and sandstone.
A Consideration on Long-Term Strength of Rock
  • Seisuke OKUBO, Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • journal of mmoij Vol. 129 (2013) No. 10_11
  • Released: November 01, 2014
In the previous paper, based on the summarized experimental results, long-term strength (failure criterion at extremely low loading-rate) was proposed. The simplest form of the long-term strength was σ1 = 3 where σ1 and σ3 were major and minor principal stresses, and constant 'a' varied from 2 to 6 depending on rock. For validation of the proposed failure criterion, rock stresses were plotted on σ1 and σ3 diagram. It was found that 86 data except one were plotted under the line (σ1=4σ3). After the previous paper was published, Okubo et al. conducted push-in test in which a failed rock sample was compacted in a steel cylinder. The envelope of Mohr's stress circles through the test were approximated byτ= 0.7. Under this curve, it was considered that strength of the failed sample was recovered up to a certain extent. More than 150 data of rock stresses were newly added to examine the long-term strength. It was found that almost all Mohr's circles were plotted under the curve τ = 2σ0.7. This was the better result than the previous one, σ1=4σ3. As stated in the previous paper, it is reasonable to consider that, in relatively stable rock mass, rock stress is maintained at a certain value lower than a long-term strength. If so, this result supports the validity of the newly proposed long-term strength, τ = 2σ0.7.
Uniaxial Tension Tests and Constitutive Equations of a Coal
  • Seisuke OKUBO, Kimihiro HASHIBA, Katsunori FUKUI, Qingxin QI
  • Journal of MMIJ
  • Vol. 129 (2013) No. 8_9
  • Released: October 21, 2014
Only the very limited knowledge has been known concerning the mechanical properties of coal under uniaxial tensile stress which are very important factors in coalmines. In this study we carried out uniaxial tension tests on coal, with particular attention to find the most appropriate constitutive equation for the sample coal. Three constitutive equations were chosen and tested, among which two had been proposed by the authors and one was newly proposed in this study. Coal showed high sample-to-sample scatter in strength and Young's modulus, however, relatively small scatter in the normalized stress-strain curves in the pre-failure region. All three constitutive equations were found to represent the complete stress-strain curves in reasonable accuracy. This result presents a successful application of the authors' constitutive equations to the coal behavior under uniaxial tensile stress. Once these equations are determined, they can be incorporated into FEM software to investigate various time-dependent behaviors of coal and aid in the efficient design of coalmines and the prevention of coalmine disasters.
Study on an Index of Time-Dependency for Rock and Its Application
  • Kimihiro HASHIBA, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 129 (2013) No. 8_9
  • Released: October 21, 2014
To estimate long-term deformation and stability of underground structures, it is indispensable to understand time-dependency of rock sample and rock mass. Previous experimental and theoretical studies showed that rock strength was proportional to the 1/(n+1)-th power of loading-rate and creep lifetime was inversely proportional to the n-th power of creep stress, where n was a constant depending on rock type and testing environment. The smaller value of n, the larger time-dependency of rock, which suggests that n can be a useful index of time-dependency. This paper introduced three loading and a creep testing methods to obtain the value of n, and showed their results in uniaxial compression, uniaxial tension and Brazilian tension tests for 38 rocks in the country and overseas. Many rocks showed smaller values of n under water-saturated than air-dried condition. The value of n was proportional to strength under water-saturated condition or low confining pressure, but not under high confining pressure under which failure was more ductile. Rock mass classification systems considering time-dependency of rock were examined as an application of testing data on design or construction of underground structures. An index n was implemented in four classifications, RMR, Q, GSI and RMi, and their proposed equations indicated that rating decreased with an increase in duration of use of structures or in degree of time-dependency of rock mass.
Mechanical Properties and Specific Energy of Seafloor Hydrothermal Deposits in the Waters off Okinawa Island
Seafloor hydrothermal deposits have been found to be widespread in the waters off Okinawa Island. Since Japan currently depends heavily on foreign mineral resources, it is expected that such deposits will be developed in the near future. To assist with the design and manufacture of mining and crushing machinery and ore transportation equipment, in this study the mechanical properties of seafloor hydrothermal deposits were investigated. Tests were carried out on the strength, hardness and abrasiveness of boring cores and block samples of the deposits. The average uniaxial compressive strength was found to be between 20 and 60 MPa, the CERCHAR abrasiveness index was between 1 and 2, and the Mohs hardness was between 4 and 6. In uniaxial compression tests, in which a specimen was crushed up to 20% of its initial height, size of debris was a little smaller than tuff, andesite and sandstone. The specific energy calculated with the boring data was found to be closely related to the mechanical properties of the hydrothermal deposits, and its distribution beneath the seafloor was determined from some boring data. The results obtained in this study provide important guidelines for the development of mining techniques for seafloor hydrothermal deposits.
Extension of Constitutive Equation of Variable Compliance Type and its Validation based on Creep Data
  • Seisuke OKUBO, Katsunori FUKUI, Kimihiro HASHIBA
  • Shigen-to-Sozai
  • Vol. 118 (2002) No. 12
  • Released: October 29, 2013
A constitutive equation of variable compliance type had been proposed by Okubo in 1987. The equation is simple and its parameters are relatively easy to obtain. The equation can be applicable to creep of rock, however, experimental result in primary creep region in which strain rate is decreasing with time does not conform well to the calculated result.
Recently Okubo and Fukui (2002) proposed a constitutive equation especially focusing on low stress level creep. In this study, old one proposed in 1987 and new one proposed in 2002 are combined to extend an applicable range of the constitutive equation of variable compliance type.
For validation of the proposed constitutive equation, calculated results are compared with the experimental data by Okubo and Nishimatsu (1986) and Fukui et al. (1992, 1993), and it can be said that experimental results in uniaxial compression can be well simulated by the proposed equation. The equation is also tested against the creep performed in uniaxial tension and possible applicability of the equation to tensile stress field is discussed.
Development and Preliminary Use of a Transparent Vessel for Tri-axial Compression Test of Rock
  • Seisuke OKUBO, Katsunori FUKUI, Kimihiro HASHIBA
  • Shigen-to-Sozai
  • Vol. 118 (2002) No. 3,4
  • Released: October 29, 2013
In soil mechanics, a transparent tri-axial vessel is frequently used, however, in rock mechanics, a metal vessel has been used to withstand higher confining pressure. In this study, a transparent vessel made of an acrylic acid resin with tensile strength of 75 MPa was designed and manufactured for tri-axial compression test under confining pressure up to 10 MPa.
Every component of the vessel was designed with safety factor more than four. Paying special attention to safety, confining pressure was increased over the designed value until its failure. The confining pressure at the failure was more than 55 MPa and daily use at 10 MPa was considered to be safe. It was also revealed that an annular transparent tube made of an acrylic acid resin did not break into pieces, but only a vertical crack was extended.
Preliminary tri-axial tests were conducted for Tage tuff and Dotan that were deformed easily under the confining pressure. Many photographs were taken by a digital camera, and vertical and lateral deformations / strains were read out successfully.
Uniaxial Tensile Strength and Irrecoverable Strain of Rock Under Air-Dried and Water-Saturated Conditions
  • Kimihiro HASHIBA, Seisuke OKUBO, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 127 (2011) No. 12
  • Released: November 25, 2012
Moisture is one of the important factors influencing mechanical properties of rock. Uniaxial compressive and indirect tensile strengths have been well known to vary from air-dried to water-saturated condition. However, in a uniaxial tension test, which has been difficult to be conducted with a water-saturated rock specimen, knowledge about the effects of moisture is insufficient; for example a complete stress-strain curve has not been obtained. In this study, the uniaxial tensile testing for an air-dried specimen was modified to obtain a complete stress-strain curve for a water-saturated specimen. Especially, bonding method between a specimen and loading platens was carefully reexamined and modified. By the modified testing method, complete stress-strain curves of two andesites, a granite and a tuff were obtained under water-saturated condition. Experimental results under the water saturated condition showed larger inelastic strain at peak strength than that under the air-dried condition. With a two-dimensional specimen model, crack extension in uniaxial tensile test was numerically simulated. Calculated results showed more cracks extension and larger inelastic strain under water-saturated condition as in the experiments.
Constitutive Equations for Time-Dependent Behavior of Rock in the Pre-Failure Region and Parameter Acquisition
  • Kimihiro HASHIBA, Seisuke OKUBO, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 126 (2010) No. 10_11
  • Released: September 25, 2011
Authors have proposed constitutive equations for rock based on the theory of non-linear visco-elasticity. The constitutive equations are grouped into two types: compliance or irrecoverable-strain monotonically increases with stress. In earlier studies, it was reported that both types of constitutive equations are applicable at least to the post-failure region.
In this study, applicability of the constitutive equations to the pre-failure region was examined. At first, parameters were estimated from the results of unloading-reloading and alternating loading-rate tests under uniaxial compression. The calculated results with the estimated parameters were compared with the experimental results under various loading conditions.
If a constitutive equation was properly selected, the equation well simulated the various time-dependent behaviors in the pre-failure region: loading-rate dependency of Young's modulus, creep and generalized relaxation where a set of parameter values was used in not only pre-failure but also post-failure region.
Simulation of Compaction Test with Fractured Rock
  • Ming LEI, Seisuke OKUBO, Katsunori FUKUI, Kimihiro HASHIBA
  • Journal of MMIJ
  • Vol. 126 (2010) No. 3
  • Released: April 29, 2011
In order to express the behaviour of fractured rock in compaction test with a constitutive equation, some basic discussions were attempted. The variable-compliance-type constitutive equation was employed in this study. The constitutive equation can express the failure process of rock, which has been well proved heretofore. In the constitutive equation, it is assumed that the rock fails at a rate which depends on stress severity.
In this study, the concept of stress severity Sv was expanded to the stress state where Sv took negative value and healing parameter Hp = -Sv was introduced. The constitutive equation was also modified assuming that fractured rock is gradually compacted or healed at a rate which depends on healing parameter. According to the results in this study, it can be said that the equation expresses the behaviour of the fractured rock when the compliance becomes smaller in the compaction test. In addition, to some extent, it was also possible to explain the increase of the principal stresses with time. Though further discussions are essential in the future definitely, it can be said that the modified variable-compliance-type constitutive equation showed much potential for simulating the behaviour of fractured rock in compaction test and interpreting strength recovery of rock in loosen zone or EDZ around an underground structure.
Strength Recovery and Loading-Rate Dependency of Fractured Rock
  • Kimihiro HASHIBA, Ming LEI, Seisuke OKUBO, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 125 (2009) No. 9
  • Released: February 26, 2011
Rock around an underground opening may be damaged with excavation. Recently, it was pointed out that damaged zone or fractured rock might be compacted and recovered in strength under a proper condition. Strength recovery will, however, not contribute to long-term stability of an underground structure if time-dependency of the strength-recovered zone is so large and deformation of the region is increasing with elapsed time.
As a first step to understand time-dependent behavior of a strength-recovered or compacted specimen, in this study, loading-rate dependency of the specimen in uniaxial compression was investigated. A rock specimen was, at first, completely fractured and then compacted in a steel cylinder. After pushed out of the cylinder, the compacted rock sample was loaded at alternating loading-rates to obtain loading-rate dependency accurately.
Strength recovery was remarkable for Tage tuff, Kimachi sandstone and Kawazu tuff, but poor for Sanjome andesite. In the alternating loading-rate test, stress fluctuated clearly corresponding to up and down of the loading-rate in both pre- and post-failure regions. For the four rocks, loading-rate dependency of uniaxial compressive strength (peak strength) was found to be similar to the ordinary rock specimens. The result indicated that damaged zone or fractured rock around an underground opening might show time-dependency similar to the surrounding rock mass.
Loading-Rate Dependency of Stress-Strain Curve for Rocks in the Post-Failure Region
  • Kimihiro HASHIBA, Ming LEI, Seisuke OKUBO, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 125 (2009) No. 3
  • Released: February 19, 2011
Loading-rate dependency of rocks in the post-failure region has been investigated by many researchers, but knowledge obtained in the previous studies is insufficient compared to that for the peak strength. In this study, loading-rate dependency of stress-strain curve at very low stress level in the post-failure region was investigated with three testing methods.
A testing method, in which a single rock sample is loaded at alternating loading-rate, was applied to andesite and granite. Loading-rate dependency of stress-strain curve was clearly observed up to the very low stress level in the post-failure region, where failure progressed considerably. Next, unloading-reloading cycles were repeated in constant loading-rate test with these two rocks, and unloading curves at many points along stress-strain curve were obtained. Stress-strain curve at slow loading-rate obtained in alternating loading-rate test was shifted along the unloading curves, and then it overlapped on stress-strain curve at fast loading-rate. And percentage of change of stress in post-failure region was almost equal to that of the peak strength.
A new testing method, in which alternating two loading-rates and unloading-reloading cycles were repeated with a single rock sample, was proposed and applied to andesite, granite and tuff. It was found that a new testing method is easy to conduct and suitable for practical use.
Ten-Year Creep Test of Tage Tuff
  • Kimihiro HASHIBA, Seisuke OKUBO, Katsunori FUKUI
  • Journal of MMIJ
  • Vol. 124 (2008) No. 12
  • Released: February 15, 2011
For a couple of decades, creep of rock has been one of the research topics, but little knowledge has been obtained concerning long-term creep behavior under low stress level. To estimate the long-term stability of underground structures, it is important to know how creep strain rate will change for a long time, and to know whether or not creep strain will asymptotically approach a constant value under low stress level.
In this study, creep testing machine and high-accuracy measuring system were developed for long-term accurate testing. In 1994 uniaxial compressive creep test with Tage tuff was started under the stress level of 30% (ratio of creep stress to strength) in wet condition, but in 1996 this test was halted due to measuring equipment fault. Then the countermeasure was examined to continue the test when equipment fault may happen. In 1997 creep test with another specimen of Tage tuff was started, and has continued for ten years without any problems.
Experimental results showed that creep strain of Tage tuff has been continuously increasing for ten years under such a low stress level. Creep strain rate, however, has been rapidly decreasing in proportion to about -0.9th power of elapsed time. Experimental results of long-term tests in this study and short-term tests in earlier studies were compared with the aid of non-linear visco-elastic theory, and it was found that both of them had many similar features.
FEM Analysis of Time-Dependent Behavior and Failure Process of Deep Shaft
  • Kimihiro HASHIBA, Toshinori SATO
  • Journal of MMIJ
  • Vol. 124 (2008) No. 3
  • Released: February 01, 2011
Shaft plays an important role in accessing or ventilating to deep underground, so it is necessary to assess its long-term stability. But little knowledge about the long-term behavior of shaft has been obtained from in-situ measurement, theoretical study and numerical simulation. In this study, time-dependent behavior and failure process of deep shaft was simulated with the aid of non-linear rheological constitutive equation of rock. This simulation focused on the effect of gravity and the deformation behavior parallel to the direction of excavation, which has hardly been examined with horizontal roadway or tunnel.
Simulation with the model of 1000m deep shaft was carried out, and the effect of rock properties and rock stress conditions was examined. It was found that time-dependency, ductility, Poisson's ratio of rock mass and the ratio between horizontal and vertical rock stresses had an effect on failure process around the bottom of shaft.
In addition, simulation with vertically and horizontally loaded disc-shaped model was carried out to investigate the effect of vertical rock stress and gravity in detail. It was found that the vertical deformation of shaft wall was much smaller than the radial one just before failure, but that the vertical rock stress and gravity had a large effect on failure process and long-term stability of shaft.
Recent Studies on Time-Dependent Behavior of Rock and the Effects of Confining Pressure
  • Kimihiro HASHIBA
  • Journal of MMIJ
  • Vol. 123 (2007) No. 1
  • Released: November 03, 2009
To estimate long-term stability of deep underground structures, it is important to investigate the time-dependent behavior of rock. Loading rate dependency, creep and relaxation have been well examined under uniaxial stress state conditions, however, less in known of the effects of confining pressure. In this paper, recent experimental and theoretical studies on the time-dependency of rock were reviewed primarily focusing on the effects of confining pressure.
Firstly, time-dependent behavior under confining pressure was compared to the uniaxial stress state, and the effects of confining pressure were discussed. Loading rate dependency, creep and the relation between them were examined with the aid of recent theoretical studies. The close relations among time-dependency, probability distribution of rock properties and scale effect were explained theoretically. Finally, equipment and practical testing methods used to investigate the time-dependent behavior under confining pressure were reviewed.
Triaxial-Compression Testing Method Developed for Small Rock Specimens
Estimation of physical properties of rock is essential to design rock structures and to evaluate the stability of rock slopes. It is well known that rock properties often vary from site to site, and even from specimen to specimen. So, in many cases, it requires many rock samples, much effort and cost to obtain the reliable data. In this study, an efficient testing method with small rock specimens was proposed and examined. At first, the preparation process for a cylindrical specimen, 10mm in diameter and 20mm in length, was developed. In the experimental work, the transparent triaxial vessel, developed by Okubo et al., was modified and used for the small specimen. Triaxial compression tests with constant loading rate were conducted with small specimens of Tage tuff, and it was found that cohesion, C, and angle of internal friction, φ, can be precisely obtained. By this test, the loading rate dependency of a stress-strain curve can be also obtained by just one small specimen with the loading rate alternately switching during a test. Small specimens are easy to carry and it is effective especially for geophysical researches in foreign countries. So, in this study, triaxial compression tests, noted above, were also conducted with rocks obtained around the Three Gorges Reservoir in China.
Loading Rate Dependency of Peak and Residual Strengths of Rocks
  • Kimihiro HASHIBA, Seisuke OKUBO, Katsunori FUKUI
  • Shigen-to-Sozai
  • Vol. 121 (2005) No. 1
  • Released: February 24, 2007
Uniaxial and triaxial compression tests were conducted for five rocks with the strain rate alternately switching between C1 and C2 at every predetermined strain interval. It was found that the strain interval had to be appropriately selected according to the confining pressure and the ratio, C2 / C1, in order to obtain the loading rate dependency precisely. Two stress-strain curves corresponding to the strain rates C1 and C2 were obtained applying the spline interpolation to the experimental data for just one specimen.
Concerning Tage tuff, Sanjome andesite and Akiyoshi marble, the loading rate dependency of peak strength estimated by the proposed method was reasonably well coincided with the published data. So it can be said that the proposed method is useful to evaluate the loading rate dependency of peak strength.
Loading rate dependency of residual strength was discussed for Tage tuff, Kimachi sandstone and Horonobe mudstone. Under the confining pressure, the ratio, (stress at the strain rate C2) / (stress at C1), was almost constant in the post-failure region. It is the most important finding of this study.
Time-dependent properties of the rock are indispensable to estimate the long-term stability of underground structures. In this study, the accurate and easy method was proposed to obtain the loading rate dependency. The advantage of the method is that a few rock specimens are enough for testing.
Observation of The Tri-Axial Compressive Creep of Tage Tuff Placed Within a Transparent Vessel
  • Kimihiro HASHIBA, Seisuke OKUBO, Katsunori FUKUI
  • Shigen-to-Sozai
  • Vol. 120 (2004) No. 4,5
  • Released: April 20, 2006
It is essential for underground development to understand the failure process of rock subjected to tri-axial compression. Okubo et al. developed a transparent tri-axial vessel made of an acrylic acid resin, and succeeded in observing the failure process of rock specimens in tri-axial compression test. In this study, tri-axial creep tests of Tage tuff were conducted within the transparent vessel.
A creep test usually requires longer time than a compression test and it is not feasible to take a photograph continuously at every predetermined time interval. Therefore, in this study, the photographing system was developed which cooperated with the existing loading and measuring apparatuses. This system made it possible to take a series of photographs of a specimen in the tertiary creep region. Photographs taken in a tri-axial test were processed to obtain axial and lateral deformations by the computer program developed in this study.
After careful examination of axial and lateral deformations, it was found that the creep strain (axial strain) just before failure was nearly equal to the cross sectional width of the stress-strain curve at the creep stress level, and that not only axial but also lateral strain-rates in the tertiary creep region were inversely proportional to the residual time up to the failure. And it was also found that ratio of the lateral and axial strain increases in the tertiary creep region were similar to those of the tri-axial compression test just after the strength failure point.
A Consideration on Probability Distribution of Strength and Creep Life of Rocks
  • Seisuke OKUBO, Katsunori FUKUI, Kimihiro HASHIBA, Koichi SHIN
  • Shigen-to-Sozai
  • Vol. 119 (2003) No. 12
  • Released: April 06, 2006
Probability distribution of strength under uniaxial stress was examined on the basis of published data. It was found that strength followed the Weibull distribution. Shape parameter or coefficient of variation in air-dried condition gave good agreement with that in water-saturated condition.
Probability distribution of failure time in creep testing was also examined on the basis of published data. Failure time also followed the Weibull distribution. Shape parameter of creep life was found to agree well with that of strength.
Probability distribution of strength under triaxial stress field is considered to be important, however, only a few studies have been published. In this study, a tuff and a mudstone were tested under triaxial stress. It was found that strength followed the Weibull distribution. Shape parameters of tuff by this study and andesite by Yamaguchi did not change so much with confining pressure, however, that of mudstone increased notably.
The experimental data were discussed from not only deterministic but also stochastic points of view.
A constitutive equation of variable compliance type has been proposed by Okubo et al. in 1987. The equation is simple and its parameters are relatively easy to obtain. The equation can be applicable to an extensive range of loading conditions including creep, however, experimental result in primary creep region in which strain rate is decreasing with time does not conform well to the calculated result.
Okubo & Fukui proposed a constitutive equation especially focusing on low stress level creep and primary creep region. Recently, old one proposed in 1987 and new one were combined to extend an applicable range, and calculated results were compared with the experimental data of creep for validation of the proposed constitutive equation.
In this study, the constitutive equation proposed by Okubo et al. is tested against results of uniaxial compression and tension tests. It can be said that experimental results of uniaxial compression tests including the loading rate dependency of compressive strength can be well simulated by the proposed equation. The equation is also tested against the results of uniaxial tension tests and possible applicability of the equation to tensile stress field is discussed.