Scaled analogue model experiments are an excellent technique to examine underground geometry of geologic structures and its development processes, thus have been widely applied to the petroleum exploration. The experimental results are strongly affected by the physical properties of the materials, thus this paper measured the grain size and the internal friction angle of popular granular materials and compared them with the model results by a series of shortening experiments. The shear-box-test results showed that micro glass beads have a smaller internal friction angle than that of sand grains, and the grain size has little effect on the internal friction angle of micro beads. The results of the analogue experiments suggested that the internal friction angle defined by the grain shape affected the thrust fault angle as well as the surface inclination of the uplifted wedge-shaped structure. The material of a higher friction angle extended the fault spacing and increased the displacement of each fault. The grain size of the material has an effect of strain localization, and the number of faults was increased in the experiments of smaller grains.
Before the underground structures such as tunnels are constructed, electrical prospecting is often applied, because of its sensitivity for the water included in the rock mass. However, the relationship between resistivity and the physical property of the rock mass has not quantitatively been clarified. Especially, in the jointed rock mass, it is important to grasp the influence of the joint on the resistivity. In this paper, two kinds of jointed rock mass model are used. One is the discontinuity model, which is made to change the open width of discontinuity by inserting various number of filter papers. The other is the porous model, in which micro opening cracks are generated inside the rock by heating with the electricity muffle furnace. Using these models, the characteristics of resistivity for the jointed rock mass are examined by comparing with the field data obtained by electrical prospecting. As the result, it is shown the possibility to evaluate the porosity of in-situ rock mass from the relationship between porosity and resistivity of the porous model.
It is well-known that encounter of difficult geological condition such as fault and fractured zones in underground construction projects leads to cost overrun and delay of construction term. Particularly, water inrushing from excavated face is a typical example of geotechnical risk factors which cause serious damage. However, cost variation caused by geotechnical risk factors mentioned above has not been investigated sufficiently until now, since it has been born by construction owners with deep pocket. From such a viewpoint, this paper focuses on estimation of cost variation caused by water inrushing from excavated face in a tunnel. In detail, this study adopts DFN (Discrete Fracture Network) model, which is often used for seepage analysis in discontinuous media, to simulate groundwater behavior due to tunnel excavation. Seepage analysis based on Monte Carlo Simulation, which select analytical parameters concerning information on fractures involved in DFN model as random variables, is conducted to evaluate discharge along excavated tunnel. And, in accordance with plan, which prescribes cost of countermeasures against water inrushing, the relationship between cost of countermeasures and corresponding exceedance probability, which is so-called Risk Curve, is obtained. Furthermore, Value at Risk (VaR), which is a popular index to evaluate the occurrence of unwanted events and/or losses based on Risk Curve in financial engineering, is adopted as an index to evaluate cost of countermeasures. The adoption of an index in accordance with financial engineering helps project participants, whose major is not engineering, to make decision on execution of underground construction projects. Furthermore, the methodology proposed in this paper is applied to cost estimation of tunnel countermeasures due to water inrushing in a tunnel to be excavated by TBM method in underground research facility. Finally, results make it clear that methodology proposed in this paper is very effective to investigate cost variation risk caused by encounter of fault and fractured zones during construction.
In this paper, simulation analysis for shearing mechanism of rock joint by Distinct Element Method is performed. The purpose of this study is to analyze the sliding and shearing behavior of discontinuity surface in the shear process and destruction locations of the roughness is tried to be clarified by numerical simulation. The characterization of this simulation model can be applied on bonding force between particles. As the results of this study, it is recognized that the shearing and sliding behavior of discontinuous plane using this simulation model correspond with the experimental results and that this simulation model are useful. The distribution of internal stress during shear on discontinuity plane can be visualized.
Underground excavation causes crack initiation and propagation, and Excavation Disturbed Zone (EDZ) could be formed around the cavern. Especially in highly stressed rock at great depth (e.g. high-level waste repository case), the formation of EDZ is strongly influenced by stress change in rock mass. Therefore, it is very important to monitor EDZ with stress change. For this purpose, AE measurement would be a suitable in both of scientific and practical point of views, and this will allows us to perform the rational observational design and construction. In order to realize this system, the relationship between AE, stress change, and fracturing is examined through the triaxial test and DEM analysis.
An in-situ triaxial test method was invented for accurate evaluation of stress and strain relationships of rock masses in the field. A series of proof tests were conducted at an abandoned quarry of Ohya stone. Comparison was made between the strength characteristics of Ohya stone evaluated by the in-situ triaxial tests conducted in the field and those by laboratory triaxial tests on core samples retrieved by rotary drilling from the same site. The results show that the in-situ triaxial test method on a large specimen is most reasonable and appropriate to evaluate mechanical characteristics of rock masses. The influence of heterogeneity was found significant for small specimens, while the influences of drainage condition and scale effect seem to be rather small.
The authors have proposed and discussed temporary storage of heated water in openings excavated in rock moun-tain from the viewpoints of multiple-utilization of land, environmental safeguards, energy conservation, etc. When used for heated-water storage, the rock mass around the openings will receive the effects of the heated water. In this study, for obtaining physical properties used for analysis of thermal behavior of rock mass around openings, the effect of the difference between Wet (the water-retaining condition) and Dry and physical properties on strength, deformation characteristics and thermal properties at high temperatures were examined using 5 kinds of rock (granite, andesite, sandstone, tuff and mudstone). In addition, a uniaxial compression creep test at high temperatures of the rock was also carried out, then compared with the creep property at room temperature. The calculation of various constants on the creep was tried using Burgers model. In outline, the results show the strength of Wet rock is smaller than that of Dry, and strength lowers with the rise in the temperature, the lowering proportion being larger in Wet. The elastic modulus of tuff and mudstone are smaller than those of granite, andesite and sandstone. In particular, the lowering proportion at high temperatures is also large in Wet. Thermal property can be considered almost constant for practical use, and the effect of the temperature is small. As the difference of Dry and Wet is large, it seems that the effect of pore water is large. Especially, the tendency is remarkable in tuff and mudstone in which porosity is large. Temperature influences the uniaxial compression creep property of rock.
In Japan, it is planned to dispose a part of low-level radioactive wastes in stable geological formations. Taking the long-term behaviors of the formations into account, a mathematical model is of prime importance. As far as the long-term behaviors of materials, the Norton-Baily creep model is so common usage. However, the value of creep strain is not converged as time advances. This is because it is a power function for time. Therefore, authors modified the Norton-Baily creep model so as to remove its demerit and proposed a creep model for soft rock mass with considering both a time hardening and a strain hardening. The modification brought about numerical results close to real behaviors of the rock mass. In this study, the proposed model is employed to analyze the long-term behaviors of underground cavern surrounded by soft rock mass and to discuss the effect of shotcrete since the effect of shotcrete on the long-term behaviors of the rock mass has not been well discussed. It is turned that the installation of shotcrete affects not only short-term behaviors of the rock but also long-term ones. The important role that the shotcrete for underground cavern plays in long-term behaviors should be clarified in the consideration of radioactive waste disposal.
Various kinds of measurements were carried out during excavation of two large underground caverns to estimate rock behavior as a discontinuous material. As a result, rock behavior as a discontinuous material was estimated in detail, such as characteristics of crack propagation, displacement and stress redistribution. In particular, it was found that characteristics of displacement and stress redistribution were very remarkable in some areas and that crack propagation was also remarkable in the area. Through the measurements, it was suggested that the sliding of discontinuities were related to the discontinuous behavior. So with a simulation in which the discontinuities were modeled, the influence of the sliding of the discontinuity to crack propagation, displacement and stress redistribution is discussed.
Back analysis of a ground deformational behavior involving nonlinear behavior is discussed. It is of primary importance to make reliable prediction of deformational behavior for shallow tunnels in soft ground. However, predictions made often prove to be incorrect due to complexity of constitutive law and other relevant factors. Back analysis therefore becomes more important, for it may be used to interpret measured displacement to derive non-linear material characteristics. The paper shows some example in which a deformational mechanism is studied in the light of inhomogeneous distribution of Young's module, from which a logic is derived to identify two different types of non-linear constitutive relationships.
A large scale rock slope failure, with an estimated collapse volume of 2,700,000m3, occurred at a site where the deformational behavior had been monitored over several years. The characteristics of the behavior were in that the deformation was recorded not only during excavation of the slope, but also between excavations. This paper presents a summary of an investigation on the deformational mechanism both during and between excavations. In particular, a finite element procedure used herein has introduced a new method of modeling deformation between excavations as that associated with a reduction of material strength parameters. The results of this investigation suggest that a sign of a large scale collapse might have been seen in a relatively early stage of the slope excavation procedure.
To monitor the stability of the tunnel and the underground oil caverns, a continuous ground tilt observation was carried out using highly precise down-hole tiltmeters. The ground tilt due to earth tides, atmospheric pressure variation, and earthquakes could be measured clearly. These tilt components were separated from raw data, and the resumption to the former position after tilting by short-term events were observed by applying this analysis. It was confirmed that variation of the tendency to secular fluctuation of the ground tilt is possible by down-hole tiltmeters.