| Under extreme ice and snow disaster condition, repeated freezing and thawing change the internal stress balance status of rock mass and supporting structure, the structure of geohazard prevention will under a serious threat. The supporting system failure will lead to landslides, collapses and other geological disasters, resulting in great loss of lives and property. In early 2008, a persistent large-scale extreme ice and snow disaster swept most areas of central and southern of China, the disaster caused enormous losses to the Chinese economy, and also a series of secondary geological disasters are developed. Over the years, the research at home and abroad about extreme low temperatures almost focused on the freeze-thaw questions of rock and soil mass of great depth in cold region, the surface rock mass(less than 20cm below the surface) was rarely involved in freeze-thaw studies. Therefore, it is urgent to carry out the research on the safety influence of extreme ice and snow disaster for the surface rock mass and supporting structure in central and southern regions of China.This dissertation relies on the key projects of "Eleventh Five-Year" National Science and Technology Support Plan "Safety Research on Integration of Rock and Soil Mass and Supporting Engineering under the Conditions of Extreme Snow and Ice Disasters" (No.2008BAC47B0X). On the basis of extreme climatic of ice and snow disaster, geotechnical engineering examples and relevant literature of home and abroad, the dissertation is set in the rock slope and supporting engineering in central and southern regions of China under extreme ice and snow disaster condition, and based on the theory of test mechanics, cryogenics, frozen earth mechanics, frozen rock mechanics, geotechnical engineering and damage mechanics. In this dissertation, the author carried out a series of physical, mechanical experimental research including rock acoustic tests, X-ray diffraction experiment, closed/open freeze-thaw tests, routine physical tests, before and after freeze-thaw rock and cement surface uniaxial compression test, triaxial compression test, direct shear test, concrete compression testing, SEM micro-scanning test and medium physical model test of interaction between rock mass and supporting structure, according to the rock mass and supporting structure affected by freezing and thawing. Firstly, the dissertation describes the whole independent research process of the environmental control systems that not only can simulate the extreme ice and snow disaster climatic conditions, but also can work in conjunction with large-scale rock MTS. Then, the dissertation takes the gray sandstone and common retaining walls, spray-anchor structure from the heavily-disaster area of Hubei province for instance, and the interaction between rock mass and retaining walls supporting structure, the interaction between rock mass and spray-anchor structure under extreme ice and snow disaster condition, this dissertation researched systematically on the deformation law, strength properties and failure yield criterion under the action of multiple factors such as temperature, moisture, freeze-thaw method etc. Secondly, based on triaxial compression test results, and through the introduction of statistical damage theory, the freeze-thaw damage softening statistical constitutive model for the rock, the interaction between rock and shot-crete before and after freeze-thaw are presented in this paper. Based on above results, a medium physical model test of interaction between rock mass and supporting structure under extreme ice and snow disaster condition is provided. In addition, the dissertation proposed some research methods and recommendations suitable for the physical model test of interaction between rock mass and supporting structure under extreme ice and snow disaster condition. The major conclusions got in this dissertation are as follows:(1) A set of dedicated environmental control systems of cabinet liner size 1000×1000×1000mm has been developed independently. The system has the characteristics of a large interior space, convenience for circling, a continuous freeze-thaw stable, freeze-thaw speed, the temperature can be programmed and has a dedicated loading facilities, etc. and the system can not only regular freeze-thaw test of rock and soil mass, but also can carry out a variety of mechanical tests of a medium physical model of interaction between rock mass and supporting structure under freeze-thaw condition with cooperating large-scale rock MTS. The system reach the domestic advanced level in the fields of the net volume inside, high and low temperature control performance, rock and soil mass lifting and loading convenience and so on.(2) Firstly, author carried out the acoustic and physical indicators test of gray sandstone from the 2008 disaster hit area, and carried out closed freeze-thaw cycles test in 10℃～-20℃and open freeze-thaw cycles test respectively. Then the propeties of gray sandstone's size and quality variation at ordinary temperatures of different water content (dry and saturated) and after different freeze-thaw cycles are provided. Secondly, author carried out the uniaxial, triaxial compression mechanical tests of the sandstone before and after freeze-thaw, and analysis in arriving at its damage pattern, stress-strain curves, strength yield criteria and strength indexes. Finally, the sandstone before and after freeze-thaw was analyzed by SEM micro-scanning. Experimental results shows, after the sandstone is saturated or open-freeze-thaw cycles, its uniaxial compressive strength, elastic modulus and deformation modulus decrease significantly. after closed-freeze-thaw cycles, the indicators shows that the opposite trend of changes, the data indicate that sandstone's water content is the decisive factor of freeze-thaw damage. Accordingly, two kinds of failure modes of damage after sandstone was subjected to closed/open freeze-thaw cycles are proposed:internal micro-damage mode and weakening pattern along the bedding plane. After closed/open freeze-thaw cycles, the uniaxial failure modes of sandstone are different, but they are the essence of shear failure. After open freeze-thaw cycles, the uniaxial compression strength of sandstone decreases significantly, the strain increases and it shows significant rheological behavior in unloading stage. Under low confining pressure, dry or saturated sandstone shows a typical diagonal shear failure, the stress-strain curve evident that a transformation process of brittle to ductile, and appears a stress drop during the unloading process. With water content increases, the cohesion, internal friction angle and shear strength of sandstone will reduce. SEM micro-scanning analysis shows that, because of the freeze-thaw cycles, the pores within sandstone and the voids between particles increase, the squeeze between the particles and the relative dislocation lead to gradual intrusion of water and damage inside the rock.(3) Choosing the interaction between rock mass and retaining wall supporting structure as research object, the interaction sample made up of gray sandstone and C20 normal pouring concrete, the cement plane sample's size and quality variation at ordinary temperatures and after different freeze-thaw cycles is produced. Secondly, the direct shear test of cement plane samples before and after open freeze-thaw cycles was carried out, and the sample's shear failure strength criterion and influencing factors is provided. Experimental results shows, with freeze-thaw cycles increases, the height and quality of samples are increasing, the diameter of sandstone and concrete part are also increasing, and with the loss of strength, cement planes breaks off after 5-10 cycles. With axial pressure increases, the shear stress of cement plane increases. In addition, as freeze-thaw cycles increases, the peak value of horizontal stress drops significantly, the phenomenon of different levels of shear rheology appears after cement plane shear yield and some samples appears the phenomenon of intermittent stress falling. After 3 cycles, the cohesion of cement planes will reduce and its friction angle will increase.(4) Choosing the interaction body between rock mass and anchor-shotcrete supporting structure as research object, the interaction sample made up of gray sandstone and C20 shotcrete, and the cement plane sample's size and quality variation at ordinary temperatures and after different freeze-thaw cycles are produced. Secondly, author carried out the uniaxial compression test, triaxial compression test under medium/low levels of confining pressure, direct shear test under medium/low levels of axial pressure of cement plane samples before and after open freeze-thaw cycles, and analysis for its damage pattern, stress-strain curves, strength yield criteria and strength indexes. Finally, the cement plane samples before and after freeze-thaw was analyzed by SEM micro-scanning. Experimental results shows, in ordinary temperature or after freeze-thaw cycles, all of the failure modes of the cement plane samples are split along the axial tensile in uniaxial compression test, and does not occur the sliding trend of shear from the cementation surface. As water content increases, cement plane's strength decrease significantly. With freeze-thaw cycles increases, elastic modulus and deformation modulus decreases linearly. Under low confining pressure, most of dry or saturated samples show a right angle to the crack failure, and as water content increases, cohesion, friction angle and peak value of strength will reduce. Under moderate confining pressure, after 3 open freeze-thaw cycles, samples' damage in the form of part of the crushed concrete in triaxial compression test, some minor cracking of the rock shows significant ductility. Author believes that the overall increase in intensity is limited by the concrete's brittle failure. In addition, the cohesion of samples does not change significantly than that of low confining pressure and before freeze-thaw, but the friction angle reduces substantially. With freeze-thaw cycles increases, the average peak value of shear stress drops significantly, and the creep phenomenon obviously appears in the unloading stage. With axial stress increases, the peak value of shear stress increases. Under low axial pressure, an increase in the number of freeze-thaw cycles leads to the cohesion of cement plane decreases significantly, but the friction angle increases slightly. Under high axial pressure, both of the cohesion and the friction angle of cement plane decrease significantly. SEM micro-scanning analysis shows that, after the open freeze-thaw cycles, the particles of sandstone surface become fragmented, and the cement among particles of concrete surface become weaker, and dislocation and displacement occurs in the particles. But the fundamental reason of cement plane damage is that the width of the gap between cement and the two kinds of material increases after freeze-thaw cycles.(5) According to the triaxial compression test results under middle and low confining pressure, the dissertation introduced the statistical damage mechanics theory based on Weibull distribution statistical law and the concept of micro-element strength firstly. Secondly, author built up two kind of three-dimensional damage softening statistical constitutive model for sandstone and the interaction between sandstone and shotcrete of different water content (dry and saturated) in ordinary temperatures. In addition, through the fitting of the relationship among peak stress, peak strain and confining pressure, the dissertation extended it to the damage softening statistical constitutive model suitable for arbitrary confining pressure. Then, the one-dimensional damage softening statistical constitutive model for interaction between sandstone and shotcrete afer 3 open freeze-thaw cycles are provided. Finally, according to the above constitutive model, a comparison was carried out between the stress-strain curves based on model calculations and based on experimentations. The result shows that the damage softening statistical constitutive models are reliable.(6) Choosing C20 shotcrete and gray sandstone, the physical model of interaction between rock mass sandstone and shotcrete supporting structure is produced, and the freeze-thaw cycles test with periodic water-supplied under the conditions of extreme snow and ice disasters are presented. During the experiment, for the strain caused by surface temperature change of rock, concrete, surface and inside of the cement plane between rock and concrete, measured by electrical measuring method and measurement of fiber gratings, the dissertation researched systematically on the principal strain, principal stress, maximum shear stress, principal stress direction angle on model surface, and the formation and the variation of strain within the model. Experimental result shows, because the rock can not be reached saturation water content and the poor absorption of concrete after solidification, there will not be idealized frost heave and thawing settlement in the supporting engineering under ice and snow after many freeze-thaw cycles, in fact, the supporting engineering will show the intermediate state between the state of frost heave and thawing settlement and the state of expanding with heat and contracting with cold. When the freeze-thaw cycle is 24 hours, elastic deformation stage is gradually into the plastic deformation stage on the model surface. But after the freeze-thaw cycle changes to 48 hours, temperature operates more fully to surface and inside of the model, the model shows elastic deformation properties again. In view of that, to simulate the actual temperature change and take full account of the negative impact by cumulative plastic deformation, the freeze-thaw cycle of physical model test should not more than 24 hours in experimental technique specifications. During the freeze-thaw cycles, deformation is sensitive to the cooling process in the cement plane, and compressive stress, compressive strain and maximum shear stress increase significantly. In contrast, deformation is more sensitive to the heating process in the perpendicular direction to the plane (including the surface), and tensile stress and tensile strain increase significantly. When temperature rises, the maximum shear force on model surface reduces, and when temperature drops, the maximum shear force increases. In addition, whether on the surfaces of rock, concrete or lateral surface of the cement plane, principal stress direction angles are concentrated in a 90 degree of the quadrant. Even to the two-point very near on the lateral surface of the cement plane, the principal stress directions of them are still different, but their ranges have some overlap. In the horizontal direction to the cement plane of internal model, during temperature changes, more pronounced increase in tensile strain and the change fluctuation of tensile strain is the smallest, the phenomenon of frost heave and thawing settlement appears. In the perpendicular direction to the cement plane, the strain of materials is affected most dramatic by temperature change and the change fluctuation of is the largest strain. In the cement plane, the compressive strain increases more significantly and the change fluctuation of strain is large during temperature changes.The innovation points are as follows:(1) A set of dedicated environmental control systems with cooperating large-scale rock MTS has been developed independently. The system has the characteristics of a large interior space, convenience for cycling, a continuous freeze-thaw stable, freeze-thaw speed, the temperature can be programmed and has a dedicated loading facilities, etc. The system reach the domestic advanced level in the fields of the net volume inside, high and low temperature control performance, rock and soil mass lifting and loading convenience and so on.(2) The damage pattern, stress-strain curves, strength yield criteria and strength indexes of rock, interaction body made up of rock mass and retaining wall supporting structure and interaction body made up of rock mass and anchor-shotcrete supporting structure under the action of multiple factors such as temperature, moisture, freeze-thaw method etc are presented under extreme ice and snow disaster condition. Based on the above results, two kind of three-dimensional damage softening statistical constitutive model for sandstone and interaction between sandstone and shotcrete in ordinary temperatures are provided. In addition, the one-dimensional damage softening statistical constitutive model for interaction between sandstone and shotcrete after 3 open freeze-thaw cycles has been built up at the same time. The result shows that the damage softening statistical constitutive models are reliable. (3) A medium physical model test of interaction between rock mass and supporting structure under extreme ice and snow disaster condition was presented. This dissertation researches and works out the dynamic response law and failure mechanism of interaction between two kinds of material. In addition, the dissertation proposed some researches on methods and recommendations useable for the physical model test of interaction between rock mass and supporting structure under extreme ice and snow disaster condition. |
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