Research On Mechanism And Calculation Method For Improving The Soft Foundation Of Highway In Cold Wetland

There lies a large amount of wetland in the alpine cold districts of the Inner Mongolia in our country. For the sake of economic development and the national defense, the expressway will extend through these areas. However, the soft clay foundation of the wetland cannot be used as the embankment only after appropriate treatment. Due to its complexity of the treatment , a series of issues need to be carefully studied, which include the local geological conditions, negative effect from the frost heave soil and environmemt destruction. New technique and method should be introduced to address these issues. Applying with the soft subgrade treatment mechanism and methodology on dynamic compacted piles in the alpine wetlands, this thesis is to provide theoretical data and guideline for soft subgrade design and construction in alpine wetlands replaced by the dynamic compaction methods. Some key technical issues have been illustrated for Erkar Wetland along Haiman Class I Highway in Inner Mogolia Autonomous Region. The study proposed in the thesis has been testified by some physical engineering works for its feasibility and efficiency. After the study, achievements have been made in the following aspects:(1) With the application of dynamic compaction, analysis is made on its functional mechanism, formation of dynamic compacted displacement piles, destructive pattern of macadam composite foundation and factors afftected the embankment stability and deformation of the macadam piles composite foundation. Soft subgrade treatment mechanism is envisaged for the alpine wetland replaced by dynamic compacted stone mixture piles.(2) Studies are made on the functional mechanism of soft subgrade treated by dynamic compaction. High-value and ultra-static pore water pressure is developed in the weak subgrade after impact load from dynamic compaction and the pore-cave expansion, which results in the hydraulic fracturing of the weak subgrade. Subgrade strength has thus been intensified with the increase of permeability of the soft soil and the instant solidation of the ultra-static pore water pressure.(3) Analysis is made to compare the calculation method of bearing on composite foundation and the analysed method of the embankment stability. Similar methodologies have been proposed by the replacement of dynamic compacted piles. Calculation method of ultimate bearing capacity of discrete material pile with variable cross-section is formulated by the application of limit equilibrium method. A new calculation method is developed for the afore-mentioned bearing capacity.(4) Construction technique, quality ensurance measures and indicators are proposed for quality control of the displaced pile composite foundation by dynamic compaction supported with the tests in-situ and an engineering works.(5) Data are analyzed and works investigated to prove that no negative effects have been shown to the permeability of the embankment by the application of dynamic compacted displacement.(6) Preventive measures for subgrade frost swelling in alpine wetlands are proposed after the test in-situ and analysis of the ground temperature. Its efficiency has been proved by an on-going engineering works.(7) The study shows that displacement by dynamic compaction is a practical and efficient measure for treatment of soft soil subgrade in alpine wetlands.