Determination Of Boundary Layer Parameters And A Preliminary Research On Hydrothermal Stability Of Subgrade In Cold Region

In the past decades, great demands of earthworks raised in cold region, including the constructions of highway, railway, and foundation. Frost heave and thaw settlement are the major diseases in these geotechnical engineering works. Numerical simulation is a useful tool to investigate the frost heave and thaw settlement problems. A solid base of the numerical simulation of such problems must overcome two difficulties, i.e. an accurate estimation of boundary conditions and a method for study thermal-hydro-stress coupled problems. It is just the major goal of this dissertation.In the numerical simulation of long-term subgrade temperature fields, the daily variation of soil temperature at a certain depth h is negligible. Such phenomenon is called the "boundary layer theory". Depth h is defined as the boundary layer thickness and the soil temperature at h is approximate equal to a temperature increment plus the average atmosphere temperature. In the past, the boundary layer thickness and temperature increment were usually extracted from monitored data in the field. In this paper, a method is proposed to determinate the boundary layer thickness and temperature increment. Based on the typical designs of highway or railway, the theoretical solution of boundary layer thickness is inferred and listed. Further, the empirical equation and design chart for determining the temperature increment are given in which the following factors are addressed, including solar radiation, equivalent thermal diffusivity and convective heat-transfer coefficient. Using these equations or design charts, the boundary layer thickness and temperature increment can be easily determined and used in the simulation of long-term subgrade temperature fields. Numerical simulation is verified by comparing to the experimental data and the result demonstrates that the proposed method for determining the upper thermal boundary of subgrade is accurate and practical.To estimate the frost heave problem, which is a thermal-hydro-stress coupled problem, the following strategy can be used. First, based on the water flow theory of unsaturated soil and the heat transfer theory of soil, the differential equations for coupled water and heat transfer are established, and the coupled water and heat transfer problem of frozen soil is solved using the secondary development of COMSOL Multi-physics. Second, based on the ice content field obtained in the water-heat coupled simulation, the frost heave can further be calculated. The proposed method is used to simulate the heave problem measured in subgrade of the haerbin-dalian high speed railway. The simulation results indicate that the water content in subgrade will increase because the impermeable membrane settled under the subgrade. In winter, ice will accumulate and is transforming from soil moisture in subgrade. Subsequently, frost heave occurs in gravel subgrade with high density and impermeable membrane beneath.This research provides a solid base of the numerical simulation of frost heave problem and an applicable tool for the design of geotechnical engineering in cold region.