Abstract: Non-heaving soil is widely used as cushion material for filling the canal foundation naturally made up by frost heave soil in North China. The design of the cushion is not as appropriate since non-heaving soil has not been technically defined; and thickness of the cushion relies heavily on empirical design. Frost heaving of coarse grained soil is mainly affected by the particle size, water content and groundwater level. Initial water content of frost heaving and the ability to stop capillary water from rising should be taken into consideration when choosing the right cushion material. This paper puts forward a method for choosing cushion material according to capillary water potential in layered soil based on the analysis of frost heaving resistant mechanism of cushion. The method includes two criteria, relationships between groundwater level and effective grain size and the coefficient of nonuniformity respectively. Working conditions of the design are given by assuming the most critical water content after investigation of canal working conditions in winter. After considering the allowable displacement of lining structure, the formula for thickness of canal cushion is deduced basing on thermal resistance equivalent principle. Both traditional and the proposed methods are used in the design of trapezoidal canal with curved-toe slope of Dayuzhang North main canal in Shandong province. The calculation results by the proposed approach are the same with those by the traditional calculation method which selects minimum displacement rate, but reduced 27cm of cushion compared with the traditional method which selects maximum displacement rate. Average surface temperature and frost heaving ratios of canal were measured during freezing period. Frozen soil can be considered as negative thermal expansion materials whose negative linear expansion coefficient is the ratio of frost heaving ratios and the average temperature. Elastic moduli and Poisson's ratios of frozen soil and frozen sand in different temperatures are selected which are widely used in other literatures. Meanwhile, the finite element software ANSYS was applied to simulate thermal-mechanical coupling process before and after laying cushion. The results show that the cushion can undermine frost heaving by improving temperature field and moisture field. Numerical analysis indicates that frost heaving and frost-heave force of the lining structure are apparently reduced, up to 90% at the shady slope.

Key words: canal, design method, anti-frost heave cushion, numerical simulation