CHARACTERIZATION OF FREEZE-THAW EFFECTS ON SUBGRADE SOILS

The basic theoretical framework is presented for analyzing the behavior of an unsaturated, cyclically frozen soil subjected to repetitive loading. The observed effects of freezing and thawing are interpreted with respect to the basic "state variables" and a procedure is recommended for the repeated loading characterization of a subgrade soil subjected to freeze-thaw cycles. Details are given of a specific section of highway which is used as an example. The results of the study indicates that in cold regions the resilient modulus of subgrade exhibits a significant seasonal change, and that the effects of freezing and thawing should be considered with respect to the "state variable" controlling the soils behavior. It was found that matric suctions measured on undisturbed Regina clay were considerably lower than those for remoulded Regina clay. It was suggested that the difference is caused by the development of secondary structure. Field suction values appear to be between 5 and 25 pounds per square inch for the roads investigated. The psychrometer is not sufficiently accurate for the measurement of low suctions and the need is indicated for a precise measuring technique for the determination of in situ suction values. It was observed that after one freeze-thaw, the resilient modulus of the undisturbed samples decreased significantly and after 10,000 application of the load the unfrozen modulus was regained. It is suggested that this behavior reflects what happens in the roadway during spring and summer with repeated truck loading. The testing of remoulded samples does not duplicate field conditions in cold regions. For Regina clay, approximately two cycles of freeze-thaw are required to obtain moduli indicative of the spring condition.