A large quantity of wet, soft, silty clay was modified with a low (1.5 to 2 percent) mixture of hydrated high-calcium lime in conjunction with freeway cut and fill grading operations in the Ottawa area. The natural clay derives from a marine saline deposit known locally as Leda Clay. In the project area the in situ moisture and sensitivity of the clay were such that it was unsuitable for conventional roadway fill construction could be maintained. The modified clay was used as fill up to the subgrade level. Subsequent frost action in the fill areas created distortions so severe that the driving lanes had to be closed. An outline is presented of the design-site and laboratory-investigation analysis, the construction procedures and test data, the performance of the roadway including related climatic data, the site investigation during and after the frost heaving, remedial measures to restore the roadway, and follow-up laboratory testing and evaluation to determine causes of the unexpected frost action. Detailed test procedures and data are provided. Design and construction procedures to be used on projects with similar soft clay problems where lime modification may be an advantage are also discussed. Direct frost-heave-measurement tests on prepared specimens of the modified soil proposed for fill construction are recommended.