| In norhern China, surface water bodies, such as rivers, lakes and reserviors, are usually characterized by the seasonal growth and decay of ice covers. Static grown ice covers take signifficant impacts on the aquatic environment and engineering infrastructures. On one hand, ice cover obstructs the mass exchange and weakens the heat exchange between air and water, changing the aquatic ecology. For instance, ice cover cut off the gas exchange between atmosphere and water, which can cause a hypoxia. The irradiance tranmissivity and heat conductivity of ice cover takes the charge of the thermal structure of under-ice water, which can influence the activities of hydrobios. On the other hand, constraited by the banks and hydrostructures, thermal forces can be produced in form of compression and shear due to ice temperature variation, and be exerted on or even destroy the infrastructures, such as dams, revetments and water intakes. Therefore, the knowledge of basic physical properties, thermal and mechanical parameters of ice has many implications on ice-infested environment and engineering. The present paper conducted the continuous filed investigations and lab tests of static lake ice during several winters. Its purpose is to reveal the growth and decay process and physical structure of lake ice, to determine the thermal conductivity, shear and flexural strengths, and to discuss the effects of microstructure on the thermal and mechanical properties.Firstly, the investigations of lake ice processes and microstructure indicated that,(1) lake ice covers are the results of local meteorological processes in high-latitude Northeastern plain (NEP) and in high-altitude Qinghai-Tibet Plateau (QTP). The surface ice sublimation and thaw is rigorous after the beginning of melt period in NEP and during the whole ice period of QTP.(2) Lake ice is mainly consituted of the upper granular-grained ice layer (P1or P3ice) and the middle and bottom columnar-grained ice layer (S1or S2ice). The granular layer accounts for less than40%of the maximum thickness. Gas inclussion within lake ice is complicated, with the shape of sphere, slim cylinder, dotted-line and rachis. The gas inclussion is also characterized by site-specitic. QTP lake ice trapped much larger gas content than those reported previously.(3) Lake ice crystal type and size do not alter temporally. However, as the ice cover grows, the spheric bubble band, clear ice and slim cylinder bubble band emerge subsequently. As the ice starts to melt, the expanding and connecting of gas bubbles increases the bubbles sizes and contents. Annual variations of ice crystalline and gas structure are controlled by the changing local hydrology and meteorology.(4) The relationships of lake ice growth rate with ice crystal size and gas content were developed.(5) The microstructural intercomparisons of freshwater lake ice, Bohai Sea sea ice and summer Arctic sea ice were evaluated, and it is proved that the position of voids incorporated in ice is not related signifficantly to ice crystal boundaries.Secondly, the thermal conductivities of lake ice in high latitude and altitude were measured several times, and the effects of microstructure on conductivity were analyzed. The results showed that,(1) the size and orientation of ice crystal have minor effects on the thermal conductivity. Horizontal thermal conductivity of columnar ice is slightly smaller than vertical one by～5%. Gas inclusion does not take effective impacts on the conductivity of NEP high-latitude lake ice, due to its too minor gas content (<3%).(2) Previous porous medium models for thermal conductivity calculation were validated. Considering the effects of gas content, size and structure, a new synthetical model was formulated to determine the lake ice thermal conductivity.(3) A tempt to measure thermal conductivity of "warm" ice was conducted. For warm ice, as the ice temperature increases near the melt point, the thermal conductivity decreases drastically. A new curve was proposed to estimate the thermal conductivity of warm lake ice.Thirdly, a great number of single-plane shear tests and three-point singly supported beam tests were carried out to determine the shear and flexural strengths of NEP high latitude lake ice. The effects of microstructure were also analyzed on lake ice strengths. The findings are as follows:(1) a large quantities of shear tests were precursively done for natural lake ice. The granular-and columnar-grained ice have an isotropic and anisotropic shear behavior. The cross-crystal force, cohesion friction of crystal boundary and crack propagation were introduced to account for the failure modes and anisotorpy of ice specimens.(2) A large group of simple beam tests were conducted. The flexural strength of columnar ice has not been reported previously. The directional differences of flexural strengths is similar to those of shear tests for granular and columnar ice. The crystal effects were discussed from the aspects of the compression and tension crack formation and propagation.(3) Although the gas content difference is quite small, it seems to cause15%depress in flexural strength of bubbly ice specimen. This is largely attributed to gas structure that acts as the original crack in tension stress field.(4) The effects of the strain rate and ice temperature were estimated synthetically. Lake ice strengths increase with the decreasing temperature. The peak strengths takes up at strain rate of2×10-5～5×10-4/s (i.e. ductile-brittle transition section).Generally, the present thesis thoroughly revealed the features, and the inter-and intra-annual variations of lake ice crystal and gas pore structure, and formulated the relationship between the ice growth rate and microstructure. The effects of microstructure on the thermal conductivity, shear and flexural strengths were also discussed essentially. The findings of this thesis would provide good grounds for the running and management of wintertime lakes and reservoirs, and for the anti-ice design of hydro-infrastructures, and also have important implications for assessing the responses of the ice-infested engineerings and equatic environment to the climate change. |
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