| In 2008, China was attacked by a historically-rare sleet weather and the south suffered severe losses, which drew widespread attention from both abroad and at home. "Low temperature and sleet weather" refers to the kind of weather featured with long-lasting low temperatures on the ground surface accompanied by continuous snowfall and sleet weather. In the extreme conditions of snow and ice, the thawing process of snow and ice causes numerous large-scale geological disasters. The freezing-thawing cycles not only affect the status of internal stress and the intensity of rock and soil mass, but also change the performance of bearing and support function of engineering systems, which leads to the degradation of materials. This phenomenon brings up serious threat towards the operation of geological disaster prevention system, and causes failure of supporting structures which results in more slides or collapse, which is especially dangerous in the populated areas, mining and construction sites, traffic routes, dam and its surrounding areas.At present, although there are abundant fruitful research on impact of freezing-thawing circles on the physical and mechanical properties of concrete, rock and soil mass both at home and abroad, the majority of the research focus on permafrost in cold regions, while very few focus on the research on the seasonal frozen ground in the non-permafrost regions, or areas with short-term freezing, or on the changes of mechanical properties of rock under the freeze-thaw cycles. There is even less research study on the impact of freeze-thaw cycles on the structural safety of geological disaster prevention projects. Therefore, it is of significant practical meaning to explore the mechanism of deformation and damages in the integration of rock-soil mass and engineered under the freeze-thaw cycles.Because of the bonding between rock-soil mass and supporting structure, the stress near the contact-surface coordinates with continuous deformation, it can be regarded as bi-material model to be studied. Given the above analysis, this article will set the integration of rock mass and spraylayer supporting structure as a new object of study to discuss. The main research contents include the following: First, the cretaceous sandstone which obtained from the project site were processed as standard specimens of 50mm in diameter and 100mm in length, and then carried out the freezing-thawing cycle test. out of which one group was water-saturated twice during melting process while the other group was not water saturated, in order to compare the impact of water on the damages in freeze-thaw cycle. Through the uniaxial compression test. According to the uniaxial compressive test, the uniaxial compressive strength of the twice-saturated samples decreased drastically with the increase of frequency of freezing and thawing, while the uniaxial compressive strength of the non-saturated sample increased gradually and was even close to the uniaxial compressive strength of dried samples. Therefore, it can be concluded that water percentage was one of the main reasons for the frost hwaving damage in sandstones. Preparation of the sandstone-concrete integration of standard samples to test their variation of the uniaxial compressive strength and deformation parameters under freezing-thawing cycles. The results showed that the concrete deforms with the rock through their contact surface, and with its high strength and minor deformation, the concrete actually played a good supporting role for the rock. The integration sample was also affected by freezing and thawing effects, but the freeze-thaw damage is much smaller than that in the sandstone, and the variation of deformation parameters was closer to that of those mediums with high strength.According to evolution mechanism obtained from the elastic modulus tests, I established a freeze-thaw damage evolution equation, and a damage evolution equation of the sandstone and sandstone-concrete integration based on damage theory of brittle materials, and therefore deducted the corresponding constitutive model for uniaxial compression. The stress-strain curves calculated with my constitutive model are proved identical to the stress-strain curves obtained with the experiments, which proves that the established freeze-thaw damage constitutive model is basically reliable.Second, sandstone and sandstone-concrete integration samples before and after freezing-thawing cycle were observed through micriscope and SEM, and analyzed from the microscopic point of view the destructions by freeze-thaw cycle on the internal structure of the samples. Due to the impact from frost, there tend to be micro-cracks in between the fillers among grains, or along the boundaries of grains; in sand-concrete sample there tend to be cracks on the contact surface between sandstone and the concrete. If they are not cemented completely, it is most prone to have the frost deterioration there in the contact surface.Then, through model test, and through the use of self-developed model to simulate the freezing-thawing cycles and the use of a static strain measurement system to monitor the deformation, it was obtained the variation mechanisms of the stress and strain on the surface of sandstone, concrete, and sandstone-concrete integration. In the lower temperature, sandstone tend to suffer frost deterioration, and stress and strain on the surface go up with the increase of the times of freezing and thawing; when completely frost, the sandstone gradually shrinks and shows less changes under the stress and strain. The concrete side mainly showed the feature of volume contraction in decreasing temperature and volume expansion in increasing temperature. The reaction towards stress and strain at the cementation surface combined all features in the performances of sandstone, concrete and cement surface. The reaction towards stress is more complex, and there tend to be more cracks. The main variation mechanism is between that of sandstone and that of concrete, where the maximum strain value increases with the times of freezing and thawing.Making the maximum principal strain brought by the two kinds of monitoring technology do the curve fitting with the times of freezing-thawing cycles, and the results generated are quite consistent. During experiments or real projects, it is possible to obtain more accurate and reliable test data if combining the conventional static strain measurement technology and the fiber Bragg grating strain testing technology.Finally, this article put forward a new test method of testing for frost-heave force, which combined the self-designed test device of frost-heave force with fiber bragg grating test system to test the frost-heaven force of the sandstone-concrete integration under the different cementation areas, internal temperature variation of the samples and the strain changes of connected facture frontend. In the integration sample, the frost force increases with the increase of times of freezing and thawing in the early phase, while in the later phases it decreases with the increase of times of freezing and thawing; and the smaller the cemented surface is, the greater the generated frost force is. According to the test results, the surface equation of the frost-heave stress was fitted which was controlled by freezing-thawing cycle times and cemented area. Then, the article summed up the three elements, which are necessary during rock mass and supporting structure at low temperatures caused failure, that is, adequate moisture, temperature that is able to freeze water in the cracks, and cracks that are open yet not too open to enable an instant drainage.Due to the temperature changes within the sample, the variation in the freezing process under low temperature can be divided into two stages:the frost and expand stage and the freeze and shrink stage. In the frost period, the water freeze into ice, yet the shrinkage of the material is not fully demonstrated. Thus the stress variation is far less than the compressive strain caused by the frost, and the overall performance of the material is to frost and heave. After the frost effect, the sample temperature has dropped to such an extent that the material begins to shrink, and the strain gradually decreased and finally stabilizes. Through analysis we concluded the main reasons for the expansion of micro-crack at the cement surface.
|
PDF Full Text Request