| Fractured rockmass is a kind of complex medium which is existing in dam foundation engineering, slope engineering, underground chamber engineering and so on. Its mechanical characteristics, such as deformation, strength, and penetrability of groundwater, can affect the design, construction and the long term stability of rockmass engineering directly. The engineering rockmass is a part of the geologic body in the engineering area. It is existing in a certain geological environment. The formation and development of rockmass has been affected and changed by all kinds of external dynamic geological process in geological history period. So the rockmass has been cut by discontinuities, and has a certain of structure characteristic. The rockmass shows itself as heterogeneous, discontinuous and anisotropism strongly. The deformation and failure of rockmass are also controlled by its structure characteristics. The traditional mechanical analysis methods for homogeneous, continuous and isotropy medium will not be suitable to evaluate the fractured rockmass. The study of deformation and failure of rockmass based on quantitative analysis of its structure conditions is an important question for discussion of the rockmass mechanics.The mechanics of the whole rockmass is always defined by the orientations and distributing of the discontinuities in rockmass. The failure and breakage of rockmass are all begeted by the connected fracture planes which come into being of rupturing, closing and expanding of cracks in rockmass. So, the distributing of rockmass discontinuities should be studied first. And then the deformation and strength properties of fractured rockmass can be realized correctly. Based on surveing discontinuities in the field, the discontinuity network can be rebuilded on the computer with the Monte-Carlo theory. The simulation results can be consistent with the real distributing of rockmass discontinuities in a statistical rule. So we can set up the models of rockmass structure to provide a basis for the further study of the mechanics of fractured rockmass.Because of the large number of discontinuities existing in rockmass, it would be very hard to confirm the macromechanical parameters of rockmass by in-situ tests or lab tests. In recent years, with the development of nonlinear science of fracture theory, damage theory, fractal geometry, and fuzzy mathematics, many kinds of models have been adopted to estimate the macromechanical parameters of fractured rockmass. However, the reciprocity theory of discontinuities in different stress conditions and the deformation and strength properties of rockmass in complex stress conditions are still the theoretics research difficulties, and have not yet been solved very well.With the development of numerical simulation technique, numerical test method, which colligate the results of is-situ investigations, discontinuities statistic, lab tests with small samples, can simulate the rockmass fractrures and its mechanical behavior with different scales. It provides a new approach to study the mechanical properties, failure theory and confirm its macromechanical parameters of fractured rockmass. As the deformation, strength and stability of rockmass are mainly controlled by the discontinuities, the simulation of discontinuities is become the key problem of these numerical methods. Distinct Element Method, based on the Newton's second law of motion, is especially adapted for discontinue deformation analysis of fractured rock mass. At present, the study of Distinct Element Method is still at the beginning phase, and much achievement need to be applied, compared and improved.The Enshi area in the west of Hubei Province, which locats on the end of the second ladder landform of China, is a part of the expend of Yunnan and Guizhou Plateau, and nearby the Szechwan basin. The rock slopes are often in a complex geological enviroment and have complicated geological structures. Because of all kinds of internal and external dynamic geological process, the structure of Silurian fractured sandstone rockmass can be destroied easily. Joints and fractures are very dense in these sandstone rockmasses. When the Silurian fractured sandstone stratum emerged on the ground, the fractured sandstone rockmasses can be deformed and breakaged by outside force. And the geological disasters of falling, sliding and debris flow may be induced.This study is under the guide of system science methodology. In-situ investigations, engineering geological analysis, statistic analysis, lab. tests, mechanic calculations and computer simulations are adopted for the research. The geological history cause of formation and stochastic distributing rule of the discontinuities in Silurian fractured sandstone rockmass have been analyzed. Numerical tests with different scale models of different structure characteristics of fractured sandstone rockmasses have been carried out in order to look for the deformation properties, failure modes, and estimate the defromation parameters and strength parameters. And they can be applied in engineering practice. So the main task completed in this paper are as follows:(1)Study on the structure characteristics of fractured sandstone rockmass in the west area of Hubei Province. The structural system in Enshi area in the west of Hubei province is come into being by Indosinian movement, Yanshan movement and Himalayan movement in a long term. The Silurian fractured sandstone stratum are effected by N-S compressive stress, N-S compressive-shear stress, NW-SE compressive stress, E-S compressive stress from early to late. And the NW, NE, NNW and NEE are the predominance directions of discontinuities. Each group of the discontinuities azimuth submits to normal distribution. The discontinuities obliquity of the third group submits to negative exponential distribution, and the others submit to logarithm normal distribution. The half trace length and aperature of the discontinuities submit to negative exponential distribution generally. With the intersection of sandstone layer and stochastic distributing discontinuities, the structural model of fractured sandstone rockmass can be generalized into three types: layered rockmass model, rockmass model contained fracturs attribute to two groups and rockmass model contained stochastic distributing discontinuities.(2)The mechanics of sandstone are obtained by lab tests. The average value of deformation modulus is 7.4GPa, the average Poisson's ratio is 0.25, the shear strength parameter internal friction angle is 42.3°, and the cohesion is 12.9MPa.(3)Study on the constitutive model and mechanical parameters of sandstone discontinuities. The power function model can represent the deformation properties of discontinuities fairly well. According to the results of normal cyclic loading tests, a linear-arc curve model is adopted. The linear model is adopted to fit the loading curve and an arc curve model is adopted to fit the unloading curve. The linear-arc curve model not only represent the sclerotic properties during the normal cyclic loading process of discontinuities, but aslo reproduce the hysteresis loops in cyclic loading tests. Besides, a half-logarithm function model is adopted to represent the shear deformation curves of sandstone discontinuities. In the low stress conditions, the the three parameters in three-parameter Mohr-Coulomb strength criterion can represent the shear strength properties of the discontinuities.(4)Study on the numerical tests of rock blocks and discontiuities. Through the numerical tests of rock blocks and discontinuities, the results of numerical compress tests of rock blocks and shear tests of discontinuities are consistent with the lab tests. It shows that the study of rockmass mechanical actions by numerical tests with Distinct Element Mehtod is reliable.(5)Study on the deformation properties and its deformation parameters of fractured sandstone rockmass. The compressive defromation properties and equivalent deformation parameters have a close relation to these factors: structure model of rockmass, network of discontinuities, analysis scale of rockmass, deformation parameters of rock blocks and discontinuities and so on. The macro equivalent deformation modulus of layered sandstone rockmass change from 3.91GPa to 7.36GPa, the equivalent Poisson's ratios change from 0.13 to 0.34; The macro equivalent deformation modulus of sandstone rockmass contained fractures attribute to two groups change from 3.25GPa to 4.86GPa, the equivalent Poisson's ratios change from 0.12 to 0.34; The macro equivalent deformation modulus of sandstone rockmass contained stochastic distributing discontinuities change from 1.68GPa to 2.75GPa, the equivalent Poisson's ratios change from 0.18 to 0.29. The deformation parameters of layered sandstone rockmass and sandstone rockmass contained fractures attribute to two groups are symmetrical with layer direction and its normal direction. The deformation parameters of sandstone rockmass contained stochastic distributing discontinuities are symmetrical with layer direction. Besides, the anisotropy of equivalent deformation parameters of layered sandstone rockmass is the most conspicuous of all, but its scale effect is the most unconspicuous; the anisotropy of equivalent deformation parameters of sandstone rockmass contained stochastic distributing discontinuities is the least conspicuous of all, but its scale effect is the most conspicuous; and the anisotropy and scale effect of sandstone rockmass contained fractures attribute to two groups are between the layered sandstone rockmass and sandstone rockmass contained stochastic distributing discontinuities.(6)Study on the failure modes and its strength parameters of fractured sandstone rockmasses. The failure properties and shear strength parameters have a close relation to these factors too, such as structure model of rockmass, network of discontinuities, analysis scale of rockmass, deformation parameters of rock blocks and discontinuities and so on. With the quantity of discontinuities increasing, the equivalent shear strength parameters of fractured rockmass will minish gradually. And the quantity of discontinuities is bigger, the anisotropy of the strength parameters is less conspicuous, but its scale effect will be more conspicuous. The shear strength parameter internal friction angles of layered sandstones change from 29.7°to 42.3°, and the average value is 38.6°; the cohesions change from 0.1 to 12.9MPa, and the average value is 7.2MPa. The shear strength parameter internal friction angles of sandstones contained fractures attribute to two groups change from 29.5°to 42.1°, and the average value is 35°; the cohesions change from 0.2 to 12.6MPa, and the average value is 6.1MPa. The shear strength parameter internal friction angles of sandstones contained stochastic distributing discontinuities change from 28.3°to 38.1°, and the average value is 32.5°; the cohesions change from 0.2 to 13.5MPa, and the average value is 2.65MPa. The failure mode of layered sandstone has a close relation to the principal stress directions. With the angles between the layer direction and the principal stress directions increasing, three different failure modes will happen: shear yield of rock block, mixed failure of rock block and disncontinuities and slide failure along the discontinuities. When the principal stress parallel to the layer of rockmass or the angle between them is on the small side, holistic failure caused by the yield of rock block material will happen in the fractured sandstone rockmass contained fractures attribute to two groups. When this angle become bigger, the failure mode will be changed to another one, combining failure surface of local rock block yield surface and discontinuities with steep dip angles. When this angle is between 45°and 60°more or less, the main failure mode is slide along the discontinuities. However, the failure modes of fractured rockmass contained stochastic distributing discontinuities not only have a relation to the principal directions, but also have a relation to the confining pressures and the scale of rockmass models. And the failure paths are mostly the combined surfaces of the discontinuities which have an angle of 30°to 45°to the maximum principal stress.(7)Discussion on the influences of the intermittent fractures to the mechanical parameters of rockmasses. For rockmass engineering in shallow surface, the influences of these intermittent fractures to the equivalent deformation parameters can be ignored, which have small angles with the principal stress directions. However, for these intermittent fractures, which have large angles with the principal stress directions, its influences should be considered with the deformation standard of the whole engineering rockmass. However, with the expanding and cracking of the intermittent fractures in sandstone rockmass, the vertical compressive stress inflicted on the rockmass should be more greater. So if the stress of engineering rockmass is low, before the intermittent fractures are cracked by the maximum principal stress, a failure along a combined path of transfixed discontinuities may happen in the fractured rockmass. In this condition, the intermittent fractures have little influence to the failure of rockmass. The failure of rockmass is controlled by the transfixed discontinuities, and the influence of the intermittent fractures to the strength parameters of fractured rockmass can be ignored.
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