Study On Mechanism And Prevention Of Frost Heaving And Frost Boiling In Seasonally Frozen Road

Seasonal freezing soil is a kind of soil frozen in winter and thawed totally in summer. In China, it distributes in the northward regions of north latitude 30°,and its area is up to 513.7×104 square kilometers that occupies about 53.5 percent of the Chinese territory. Seasonal freezing soil is a kind of special soil-water system in which there is ice crystal, and it is multiphase complex object composed of soil grain,ice,liquid water ,and gas.Highway is a kind of band construction that link among cities. Because road net crossed-interleaving, road construction is all over China, and seasonal freezing area is over half of the Chinese territory, it is impossible that road is builded in seasonal freezing area. But subgrade produces a series of frost hazard because of some natural elements in seasonal freezing area—geology, hydrology, weather and so on. In order to meet rapidly economic development, to form express-way net that capital connect metropolis, that metropolis communicates each other, that communicates main cities and important town, to build safe, comfortable, environment-protected, demonstratable express-way. It is very important for us to research frost hazard in seasonal freezing area, to provide theory bases for design and construct.Firstly, the paper systemically summarizes the theories on frost heaving and frost boiling to realize distinctly research state of it. Secondly, auther does a lot of subgrade soil tests-grain,mineral,salt ingredients, specific suface area,cation exchange and physical-mechanical characteristic, researches thermal parameters and unfrozen water contents with temperature, does capillary height test for various test methods and degree of compaction. at the same time, author disposes observation spots to monitor ground temperature, subgrade water contents,underground temperature,frost depth and underground water line.with scan electronic microscope, author does micro-fabric experiment of subgrade soil, contrasts micro-fabric of various soil and various degree of compaction, researches for flactal of micro-grain, and analyzes relativities of physical and mechanical parameters. On the premise of mounts of experimental parameters, auther studies in details mechanism of water transport, frost heaving and frost boiling. In allusion to features of water transport and frost heaving and frost boiling in test road, the paper brings forward prevention methods of subgrade frost heaving and frost boiling to instruct design and construction according to investigate and test data. It is very significative to supply credible parameters for road engineering construction. Its main jobs and results are as follows:1 In order to master adequately research state and development direction, the author previews systemically its research state and theories by means of consulting large amounts of documents on frost soil. And realizes road frost heaving and frost boiling, clarifies its harm to road construction, specifies the project importances to design and construction according to features of road construction.2 Author studies physical ingredients and physical-chemical parameters. A section of test road locates near Changchun city suburb belonging to Changchun-Siping road, Changchun-Songyuan road and Changchun-Jilin north road.subgrade soil composes of clay, silty clay and silt, granularity ingredient is mainly silt occupied up to 60 to 70 percent, and clay occupied up to 20 percent. Mineral ingredient is mainly proto-mineral, quartz is 40 to 49 percent, feldspar is 33 to 44 percent. Clay mineral contents are not much, it is mainly illite and illite-to-montmorillonite mixed layer mineral, total contents is 10~14 percent. Chemically soluble salt contents are lower, it is less than 0.1 percent, cation(Na+,Ca2+) contents are much. Cation exchange contents are 20 to 40 m·e/100g, specific surface area is 40 to 140m2/g. Thus according to mineral and chemical ingredients, subgrade soil clay mineral contents in shree test road are not much, soluble salt contents is less, cation exchange capacity and specific surface area is less, So water transport capability of subgrade soil is weak, but there are a lot of Na+, that can incrassate bound water film of soil grain, it is convenient to water transport of film water. Mechanically, subgrade soil is in over-concretion, concretion press is 158 to 316kpa, void ratio is 0.627 to 0.826.In this paper, auther does capillary height test by Kaminski capillary cube method, direct method and frozen-thaw method. Subgrade is clay and silty clay, whose capillary height is higher, capillary height is 1.0 to 1.6 meter by direct method and frozen-thaw method, capillary height is 256 to 345 centimeter by Kaminski capillary cube method, such soil easily produces all sorts of question on water transport.Soil skeleton specific heat mainly lies on mineral ingredients and organic matter contents, and correlative with tempareture, thaw-soil skeleton specific heat is a little more than frost soil, but difference is not much. Conducting heat coefficient varies less under the same temperature. Unfrozen water contents of subgrade soil decrease exponentially with negative temperature, but the exponent is not same with different soil.3 Soil micro-structure is significant for engineering geology characteristics, water transport, frost heaving and frost boiling. The paper researches in details for micro-structure of subgrade soil in test road, and contrasts with subgrade soil under different degree of compaction.In the subgrade soil of test road, its grain that is more than 20μm and less than 1um is little, its grain that is 1 to 2μm is not much, main grain is 2 to 5μm and 5 to 10μm.In subgrade soil, pore whose diameter contents is more than 20μm and is 10 to 20μm is nearly zero, pore whose diameter contents is 5 to 10μm is not much, main pore diameter is in three sections-less than 1μm, 1 to 2μm and 2 to 5μm, thus it reveals that micro-pore is very strong, contact among grains is side-surface pattern,and pore connectivity is better. These provide convenient water transport passage.Soil micro-pore contents are consanguineous with degree of compaction. Pore whose diameter is 1 to 2μm is more when degree of compaction is lower(80 percent), soil fabric is relatively looser, and bound water's resistance is not strong,capillary water is easy to rise. But pore contents whose diameter is less than 1μm increase with increasement of degree of compaction, pore contents whose diameter is 1 to 2μm decrease relatively. It reveals that soil's dry density increases with increasement of degree of compaction, that soil grain shrinks tightly, that soil pore diminishes gradually, that soil resistance of internal friction and cohesion increases constantly. When soil degree of compaction increases to some extent( more than 90 percent), bound water film of neighbouring soil grain surface is interleaving each other, and blocks movement of capillary water, velocity and heght of capillary water rising will decrease. Subgrade soil of test road is flocculated-reunited fabric.Clay mineral that is mainly illite and illite-montmorillonite mixed layer forms flocculated and reunited body, cementing matter that is among fabric unit cements soil grain by bridge pattern, cementing matter is mainly clay mineral, and some clay mineral don't crystal absolutely. Linking pattern is mainly contact linking and bound water linking that varies with water content, so increase and decrease soil fabric strength. In the view of state of soil pore, the author thinks that soil fabric linking and connectivity is better, soil grain is evidently not beamed, and soil grain is basically even.4.Author selects several roads to monitor water contents, frost depth, underground temperature and underground water line during a frozen-thaw, and finds that water contents increase in 0.0 to 1.0 meter, that name roadbed, and it reveals that here is water transport area where frost heaving and frost boiling can produce. by chronically monitoring and consulting data of Jilin province observatory, weather temperature lowest time is in january to february, month average weather temperature is about -20℃.underground negative temperature is 0.0 to 1.0 meter, there is maximium in january to february, that is correlative to local weather temperature, maximium is -7 to -10℃.maximium frost depth is 2.09meter in 2000, 1.9meter in 2004, but this data is different in another monitor spot that is correlative to weather temperature and frozen time.5. Water transport in frost soil is a complex procedure that is correlative with all sorts of elements, water can transport under humidity gradient, press gradient, temperature gradient, solute concentration gradient and electric field gradient or mechanical field that produces in physical and chemical procedure. Besides capillary water, film water and gas water can also transport, but strongly bound water cann't transport under external environment because of strong attraction of soil grain surface, so strongly bound water isn't considered researching water transport. Although gas water also takes part in water transport, its contribution is very poor.Soil grain fractal value is nearly correlative to water transport by analysis.under same conditions(water, temperature), water in soil grain of big fractal value can easily transport, vice versa.6 Author analyzes mechanism of frost heaving and frost boiling for several test roads. Soil heave is produced through capillary water and film water transport, soil frost heave value sharply increase as a result that water from outside that is absorbed by frozen front freeze and form collect ice layer. Segregation frost heave mechanism includes two physical procedure:soil water transport and ice process. In several test road, collection ice layer is about 1.5meter thick, water increment is about 3.5 to 4.6 percent.subgrade soil frost heave content is about 0.74 to 1.44 percent and increasing with water content. Under coupling action of temperature, water and matter ingredient of subgrade soil, subgrade produce frost boiling by libration load.7 on the premise that author understands mechanism of subgrade soil frost heaving and frost boiling, treatment of subgrade soil frost heaving and frost boiling is provided in actual engineering, that is mainly prevention during design and is assistantly treatment during construction.