Study On Consolidation Settlement And Stability Of Wetland Soft Soil Foundation In The Cold Region

The existing engineering geological problem to build high-grade highways on soft soil foundations mainly is settlement and stability of the soft soil foundations. Soft soil will have different engineering characteristics because of the different formation of it. In the soft soil area along the sea of the south China, there are many successful experiences and treatment measures in construction of high-grade highways, and whether these experiences and measures are suitable for marsh filled up soft soil constantly encountered in highway construction of Heilongjiang province still needs researched and demonstrated. Therefore, this article will research into the engineering characteristics of marsh soft soil in cold area, short construction period of highways, rapid load increasing due to filling action and the effect of temperature, as well as improving the method of prediction of consolidation settlement and stability appraisal of soft soil foundations. The main work includes tests for engineering characteristics of cold area marsh soft soil, site monitoring data analysis, settlement calculation, comparison of settlement fitting curves and stability data analysis, provide the suggestions of changing the calculation parameters in consolidation settlement prediction and stability assessment with embankment filling step by step.The test research uncovers the changing rule of void ratio, compression factor and compression modulus of cold area marsh soft soil with changing of consolidation compressive stress: compression factor will reduce with increasing of consolidation compressive stress, compression modulus will increase with increasing of consolidation compressive stress; the changing rule of the main consolidation factor and secondary consolidation factor with changing of consolidation compressive stress and different loading ratio: the main consolidation factors Cv of two layers of soft soil both reduce with increasing of consolidation compressive stress, and have good relativity with consolidation compressive stress. The secondary consolidation factor of the first layer soft soil is greater, i.e. the creeping feature of the first layer soft soil is stronger than that of the second layer soft soil, the loading ratio ?p p> 1, the secondary consolidation factor of soft soil increases with increasing of consolidation compressive stress, appearing obvious rule; under different consolidation compressive stress and different degree of consolidation, the relationship between the main stress difference and the axial strain: when adopting no-consolidation and no-drainage shearing, the main stress difference of the first layer soft soil is obviously higher than that of the second layer soft soil, for a greater surround compressive stress, the maximum deflective stress of the two layers of soft soil is similar, during shearing process of the first layer soft soil, the stress-strain relationship appears typical processing hardening characteristics, and the second layer soft soil approximates ideal plasticity. Under the same strain, with increasing of consolidation compressive stress and degree of consolidation, the deflective stress of two layers of marsh soft soil in cold area gradually increases; under different consolidation compressive stress, the relationship between void water pressure and axial strain during shearing process: during shearing process, when the strain is small, the void water pressure in the two layers of soft soil increases rapidly, the increasing speed of void water pressure gradually slows down with increasing of strain until becoming stable, and the second layer soft soil shows more clearly; the changing rule of soft soil strength index and shear resistance strength with consolidation pressure and degree of consolidation: when degree of consolidation U < 20%, the cohesive force c of cold area marsh soft soil reduces with increasing of degree of consolidation, when degree of consolidation U > 20%, it will increase with increasing of degree of consolidation. Inner friction angle ? increases with increasing of degree of consolidation, but the rule is not obvious. In general, the shear resistance strength of marsh soft soil increases with increasing of consolidation pressure and degree of consolidation, in several cases such as when consolidation compressive stress is 50 kPa and degree of consolidation U < 20%, cohesive force will play main part in shear resistance strength, at this moment, the shear resistance strength will reduce with increasing of degree of consolidation; through comparison and analysis, the stratum thickness of marsh soft soil is smaller than that of coastal area soft soil, on the whole, the engineering characteristics is better than that of coastal area soft soil. This conclusion provides basis for directly using the successful treatment measures of coastal area soft soil, and the test results supply calculation parameters for calculation of consolidation settlement and stability of cold area marsh soft soil under the action of various steps of loads when filling embankment.In this article, the site monitoring data analysis result of consolidation settlement and stability of cold area marsh soft soil foundations indicates that during embankment fill loading process, the settlement of soft soil foundations obviously increases, and the settlement becomes greater and greater in places increasingly near the surface of the soft soil foundations. The settlement of soft soil accounts for above 80% of the total settlement. In late period, the settlement curve of the soft soil foundations treated by bag-filled sand well is more gentle than that of the soft soil foundations treated by plastic drain board, thus more favorable for the drainage consolidation of soft soil foundations and reduction of the final settlement; the side direction displacement and maximum displacement depth of soft soil foundations increase with increasing of load and thickness of the soft soil. With greater filling speed, the side direction displacement of soft soil foundations increases more rapidly. During freezing process of embankment fill, where near the median divider, soil pressure gradually becomes smaller, and where near side slope of the embankment, soil pressure gradually becomes greater. During melting process of embankment fill, where near the median divider, soil pressure gradually becomes greater, and where near side slope of the road embankment, soil pressure gradually becomes smaller. With increasing of construction load, the reinforcing effect of geotextile grids becomes more clear, which play the tuck net part and make foundation base pressure distribute evenly.Based on indoor tests and site monitoring data, in this article, adopt loading-by-steps degree of consolidation theory, set forth the idea of changing calculation parameters step by step with loading in settlement calculation of soft soil foundation, and supply the calculation general expression. Through verification and calculation, the obtained settlement curve is similar to the actual observation settlement curve; for bag-filled sand well and plastic drain board reinforced soft soil foundation in cold and marsh area, the settlement curve fitted section by section by hyperbola method is similar to the observation result; and the final settlement obtained by various methods to calculate is consistent.In this article, use finite element method to analyze and appraise stability of highway soft soil foundation, according to the test result of indoor soft soil shear resistance strength index changing with consolidation compressive stress and degree of consolidation and the result of compression tests, and after adding loads by steps, change corresponding calculation parameter step by step, the calculation result of the vertical displacement and horizontal displacement is similar to the actual measured result. During late period of loading process, the soft soil foundation has no obvious yield area. It is consistent with the engineering fact. The comparison of the calculation result and actual measured data shows that the appraisal method of stability of soft soil foundation in the cold marsh area under the effect of embankment fill loading by steps is reasonable.