Compaction and thaw deformation of frozen soil permeability and structural effects due to freezing and thawing

University dissertation from Luleå : Luleå tekniska universitet

Abstract: This thesis is the result of the two projects:"Deformations in winter compacted soils" and "Permeability changes in a fine-grained till due to freeze/thaw". The aim of the first project was to study whether it is possible to compact frozen soils, in road embankments etc., to a satisfying result as well as to identify the most important factors influencing the compaction result. Different types of soil (fine-grained till and sand) were compacted in the laboratory at the temperatures +20, -2 and -10°C using compaction cylinders of two different sizes (1 dm³). Frozen soil cubes, having a side-length of 5 to 100 mm, were manufactured at different water contents and compacted in frozen state. The tests show that the compaction result decreases with decreasing temperature and increasing water content. However, the influence of the size of the soil cubes was of minor importance and the influence of the testing temperature was significantly smaller than what has been reported in the literature. In addition, compression tests on frozen, compacted soil samples were carried out to measure the thaw settlement. A relation ( a-value), based on the dry density of the frozen and unfrozen soil, was established from which it is possible to estimate the maximum compression taking place when the frozen soil thaws and is loaded. The aim of the second project was to investigate whether structural changes take place in a fine-grained till exposed to cyclic freezing and thawing. It was believed that freeze/thaw cycles could have a negative impact on fine-grained till, e.g. on its function as a hydraulic barrier. The permeability was chosen to be the key parameter to study whether the structure was affected of freezing and thawing or not. In addition, tests were directed to measure possible movements of small particles and stones due to freezing and thawing. In the laboratory, unfrozen soil was compacted to different void ratios into a rigid wall opermeameter and the permeability was measured. Thereafter, the sample was frozen and thawed a number of cycles. The samples were frozen one dimensionally in a closed pore water system and the permeability was measured in the thawed soil after a certain number of cycles. In addition, the particle movements were measured by granulometric tests and the stone movements by a specially developed X-ray technique. The investigations showed that the permeability in a fine-grained till is influenced by the freeze/thaw cycles. Typically, the permeability increased by 1 to 10 times in an initially dense till and decreased between 1 to 50 times in an initially loose till. The soil exhibited volume changes due to the freeze/thaw cycles and the volume typically decreased for an initially loose soil and increased for a dense soil. A residual void ratio was reached after 1-3 freeze/thaw cycles, independent of the state of the soil structure prior to freezing. The residual void ratio ranged from 0.31 to 0.40 in the studied material. Finally, no particle movements were detected, but significant stone movements in vertical as well horizontal direction were identified.

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