| This thesis investigates the strength and deformation characteristics of red sandstone at No. 5 aquifer(563 m ~ 599 m) of airshaft frozen construction zone of Meilinmiao mine using laboratory element tests, namely triaxial compression tests under negative temperature, pre-peak and post-peak unloading tests and freeze-thaw tests. The effect of various parameters, such as temperature, confining stress, loading-unloading path and freeze-thaw cycles, on its mechanical properties has been studied. In addition, numerical simulations were conducted following similitude theoretical framework to reproduce ice-saturated fractured rock masses, which enables to demonstrate the significance of fracture pattern, confining stress, temperature and fillings on the strength of rock with single/double fractures. Finally, statistical methods help in generating a network model of rock with multiple fractures to analyze its mechanical response. The main conclusions are as follows:1. Triaxial compression tests under negative temperature(1) Given a constant confining stress, the compressive strength, internal friction angle, elastic modulus and brittleness of saturated red sandstone are increasing with the decrease of temperature from 10℃ to-15℃, while cohesion c reduces.(2) Given a temperature, the compressive strength and elastic modulus are proportional to confining stress(0 MPa ~ 12 MPa), while brittleness is inversely proportional.(3) Poisson’s ratio is almost independent on the variation of confining stress, but varies with temperature significantly. Elastic modulus is sensitive to both the temperature and confining stress, and the change in temperature is more influential. Comparing to Poisson’s ratio, elastic modulus can be easily affected by environment conditions.(4) The temperature and confining stress lead to different failure mechanisms of red sandstone. Pore water is frozen under negative temperature, and this induces the increased friction force due to large contact area between rock and ice. While the increase of confining stress results in the raise of the normal force on the fractureing surface, and consequently this is accompanied by large friction force.2. Pre-peak and post-peak unloading tests(1) The strength of saturated red sandstone in pre-peak unloading tests is greater than uniaxial compressive strength, but less than triaxial compressive strength, while, the strength in post-peak unloading tests is approximately equal to triaxial compressive strength. In pre-peak unloading tests, the sample fails dramatically and the stress-strain curve has a sudden drop after peak, while a gentle/smooth response is obtained in post-peak unloading tests(i.e., constant strain, unloading).(2) In pre-peak unloading tests, the lower the temperature is applied, the lower sensitivity on confining stress the rock experiences, and the larger resistance to disturbance the rock has; while in post-peak unloading tests, the influence of temperature variation on strength is negligible. The increase of initial confining stress can reduce the sensitivity of unloading on the rock, as well as the resistance to disturbance. The strength of red sandstone in post-peak unloading tests only depends on real-time confining pressure instead of temperature.(3) Tensile-shear failure governs in pre-peak unloading tests, which is actually a tensile failure mode and dependent on the unloading path. Conversely, failure modes in post-peak unloading tests are determined by loading history, and irrelevant to unloading process.3. Freeze-thaw tests(1) The strength of red sandstone is reduced as the freeze-thaw cycles increase. This is because the cohesion is degraded by freeze-thaw cycles, which results in the reduction in strength.(2) Void ratio is increased with freeze-thaw cycles, and subsequently hydraulic conductivity is enhanced. The strength degradation of rock under negative temperature is smaller than the one under ambient temperature.(3) Under freezing-thaw cycles, confining stress reduces the strength degradation of red sandstone. In other words, the strength degradation of rock under high confining stress is less than the one under low confining stress. Temperature inhibits the development of damage within the rock mass.(4) After freezing-thaw cycles, tests under ambient temperature present a large amount of smashed small blocks, where the damage is more substantial than the tests under negative temperature. The failure modes of rock under both the ambient and negative temperature are similar under varying confining stress and freezing-thaw cycles.4. Model tests of ice-saturated rock with single fracture(1) The strength of rock with single fracture varies significantly when the dip angle is shallower than 10° and it is mainly affected by the fracture pattern. When the dip angle approximates to 90°, the strength, elastic modulus and Poisson’s ratio of fractured rock can be calculated as the intact rock. For other dip angles(10°~80°), the strength of rock conforms to Jaeger’s single discontinuity theory and can be computed by the fitting formula.(2) When the trace length is small(joint continuity degree < 10%), the strength of fractured rock can be estimated following the formula for intact rock; while the strength of fractured rock decreases significantly when the trace length is getting larger(joint continuity degree falls within 10%~25%) and it can be evaluated using the fitting formula.(3) The strength of fractured rock increases with the confining stress, and the same behavior is observed for elastic modulus under ambient temperature. However, it is found that elastic modulus is constant for various confining stress under negative temperature. Poisson’s ratio is independent of confining stress and temperature.(4) The strength of fractured rock is raised when the temperature is reduced. This is due to the fact that the voids and fractures are filled with ice, so that more contacts increase both the cohesion and friction force within the rock. Elastic modulus increases with the decrease of temperature, and becomes an approximate constant value under negative temperature, which is higher than the one under ambient temperature. Poisson’s ratio is proportional to the temperature.(5) The volume of pore water expands when it is frozen to ice, and this does not indicate the expanding force in an open environment. Hence, the experimental measured strength of rock does not take into account the expanding force. Ice could potentially enhance the strength of rock.5. Model tests of ice-saturated rock with double parallel fractures(1) The strength, elastic modulus and Poisson’s ratio of ice-saturated rock with double parallel fractures are not varying with the distance between fractures, and the strength is equivalent to the one obtained for the rock with single fracture(same dip angle and trace length). The failure of the sample initiates from one of the fractures, and the propagation of this fracture leads to the break of rock bridge. The other fracture will be impacted and the whole sample fails consequently.(2) The calculation of strength, elastic modulus and Poisson’s ratio of rock with double parallel fractures is the same as the rock with single fracture when the dip angle is larger than 30°. For small dip angle case(less than 30°), the mechanical properties of rock with double parallel fractures change dramatically and should be determined through laboratory tests. The failure mechanism is closely related to dip angle and the rock bridge between fractures is often damaged first. Therefore, an appropriate protection of rock bridge should be implemented in practice.(3) The mechanical properties and failure modes of rock with double fractures parallel are influenced by the trace length significantly. The strength degradation coefficient is found to follow an exponential function with the trace length. A fitting function is proposed to estimate the mechanical properties of rock with double fractures parallel for different trace length. One can expect that the failure modes are related to the trace length and the ratio of trace length. For the rock with small trace length and the ratio of trace length, the failure modes are not affected by fractures. When the trace length is long, but the ratio of trace length is relatively small, the failure modes are dependent on the fracture with a longer trace length. For the rock with a moderate trace length( joint continuity degree falls between 25%~75%), and the ratio of trace length is approximately 1, the failure modes are controlled by two fractures together.6. Model tests of ice-saturated rock with double crossed fractures(1) The strength of ice-saturated rock with double crossed fractures is significantly influenced by the dip angle of two fractures. The measured strength of QQ16 sample is only 21.25 MPa, which is only 35.3% of the value of intact rock sample. The variation of the strength of rock with double fractures with dip angle cannot be estimated following the method for the single fractured rock, but can be evaluated approximately through the figure(Figure 4.38) in Section 4.3.1. The failure modes are categorized and summarized in Section 4.3.1 as well.(2) The strength, elastic modulus, Poisson’s ratio and failure modes of ice-saturated rock with double crossed fractures are affected by the trace length significantly. The longer the trace length is, the lower the strength and elastic modulus the rock has. This phenomena is more obvious for the rock with large trace length and the ratio of trace length around 1 that the strength is much smaller than the one of intact rock. As the increase of the trace length, the failure modes of rock become simpler with a lot of small smashed blocks.(3) The spacing influences the strength and deformation characteristics of fractured rock significantly. With the increase of the spacing, the strength of rock with double crossed fractures first decreases and then increases, and finally reaches the one for the rock with single fracture. When the intersecting point of two fractures is within the rock sample, the strength is generally small and its failure is catastrophic with large broken blocks.7. Numerical study of rock with multiple fractures(1) The strength of rock with single fracture follows a quadratic polynomial function of dip angle and is a negative exponential function of the trace length. The exact number can be evaluated through a fitting function using differentiating method. The strength of rock with single fracture decreases with the increase of fracture width. However, once the fracture width reaches a certain number, the strength does not change. The stability of rock mass will become an important evaluation index.(2) When the spacing between two fractures is close to the trance length, the rock has the minimum strength. The strength of rock with double crossed fractures is approximately equal to the one of rock with single fracture when the spacing between two fractures is two times larger than the trace length.(3) A network model of rock with multiple fractures is proposed following statistic methods. A calculation example shows that the major fracture determines the strength and failure modes of rock and the influence becomes negligible for other fractures. This new model can reproduce the rock mass with multiple fractures in practice and analyze its mechanical response effectively. |
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