Effects Of Straw Mulch On Subgrade Under Construction In Seasonally Frozen Soil Area

The subgrade preparation for high speed railway construction in deep seasonally frozen soil area, if under given design, the maximum frost-free depth can be ensured by not reaching the water- rich and frost-susceptible natural soil situated below.However, due to various reasons, the subgrade construction may not be completed before winter comes. In the case without any preventive measures, the said frost-susceptible soil will be in frozen status that may cause those completed subgrade portion in deformation or lowered compact level and finally affects the quality of construction work. To prevent the soil beneath the subgrade from frozen, considering local available resources, paving corn stover onto subgrade is an economical and practical measures. This article is to provide engineering parameters on stover coverage, by studying the thermal state of the subgrade and use of commercial software.A direct relationship between the thermal conductivity and water content. Fistly,a flat ground model that can simulate rainfall process is built, obtained the data of underground water mark. Compared with the measured water level, we can draw the conclusion that: The simulated waterline is matched very well with the measured waterline in summer, however in winter time it is slightly higher than the measured values. So, the selected parameters and model can simulate the actual situation well.Using the same method, calculated the water content distribution of slope after cutting excavation, to prepare for determining the soil thermal parameters. A completed subgrade model is built by Geo-Studio software, then determined the thermal parameters according to the water content. As a consequence, the feasibility of the model and selected parameters will be verified by comparison with the data obtained.The results proved the temperature from the center line of the subgrade agreed to the data obtained but with deviation for both side edges of the subgrade. It proves that the selected model and the corresponding parameters can be used to analyze the temperature field of the subgrade under straw mulching condition.Using the selected model and the corresponding parameters, the thermal conditions of the stover covered subgrade are calculated. It found that, for the subgrade filled with less than 2/3 of design thickness in absence of straw mulch, the maximum frozen depth are likely to enter the natural soil with lower high water content, high rate of frost heave. When the roadbed filling height is a constant value,to calculate the embankment temperature field with two different straws(wheat and rice) covering, the results shows that the heat preservation effect of wheat straw is better than rice. To calculation the embankment temperature field with different straw thickness, the results shows that the ground frost level could move up with the increase of covering thickness, there is a minimum covering thickness existed. In a case of different water content and compaction density of straw, but the same covering thickness, It Shows that, the change of moisture content and compaction density have great influence on the cover effect. The next step is to calculate the temperature conditions in the case of the subgrade filled with different filler thickness taking maximum frozen depth not entering the bottom of the subgrade as the criterion, it offers the minimum thickness of straw mulch corresponding to different filler thickness under average temperature, minimum temperature and extreme minimum temperature respectively. This result has a certain reference value to the practical engineering design.