Rock fill dams are the most common kind of dams at hydro power stations in Sweden. The dams consist of different zones like core, fine filter, coarse filter, rock fill and rip rap. One of the most important parts is the core of the dam, which serves as embankment. The core consists at best of fine grained moraine. Most often the core connects directly against the spillway construction of concrete, which means that the concrete is the only frost protection against the core in the longitudinal direction. The connection between the core and the concrete structure is by experience a weakness zone when it comes to rock fill dams. Geotechnical investigations close to the spillway construction has shown a very low degree of compaction of the soil, which has been explained by hang up effects and difficulties to compact the moraine closest to the concrete structure at the construction stage. Eventually a source to the low degree of compaction might be effects caused by freezing and thawing since freezing take place through the spillway construction into the core of moraine. The purpose of this thesis was to investigate if freezing take place through the concrete construction into the core, and to study effects by possible freezing. The process of freezing requires three basic conditions. The basic conditions required for frost action to occur include a frost-susceptible soil, a supply of water and soil temperatures sufficiently low to cause some of the soil water to freeze. Depending on the frost index, the amount of days with negative temperature, the frost depth will be more or less extensive. The three basic conditions required for frost action to take place are more or less fulfilled at the connection between the core and the spillway structure. Air temperature below zero degrees Celsius will create a difference of temperature per length which will cause a heat flux from the ground surface that is called a thermal gradient. Due to the 0 °C isotherm is getting deeper into the soil, the thermal gradient changes and the frost depth reaches its maximum. When the soil thaws the ice is converted to water. Since the process of freezing has agglomerated more water in the shape of ice than the volume of the pores in the soil may contain, there will be a lot of excess water in the soil. This excess water must be drained away in order to keep the balance in consideration of the water content of the soil. If the excess water is not drained the strength of the soil will be lowered. When soil freezes the geotechnical properties will be changed. Changes in density, water content and porosity are some examples of differences that might occur. When these properties of the soil are changed changes in micro structure, formation of cracks and changes due to permeability may take place. Mentioned changes will create a zone of weakness and the risk of inner erosion with potential piping will increase. By modeling the frost penetration with Temp/W, a finite element program used to calculate heat flow in geotechnical constructions, effects caused by freezing and thawing in the connection between the core and the spillway construction has been investigated. The analysis has been performed with climate data equal to a winter occurring every period of 50 and 100 year in Luleå respectively Jokkmokk. The calculations did show that freezing will occur through the concrete construction into the core of the dam. Depending on the freezing index and thermal bridges the depth of the frost penetration will differ. Freezing will already occur at a climate data equal to a normal winter for analysis of both Lulea and Jokkmokk. In Lulea the frost depth will differ between 2.7- 4.0 meters depending on the climate data and the distribution of the reinforcement in the concrete structure. Regarding analysis with input corresponding to the conditions in Jokkmokk the frost depth will diversify between 3.0- 4.6 meters depending on the climate data and the distribution of reinforcement at the concrete construction. To reduce the effects of freezing and thawing cut-off walls might be installed at the connection between the core and the concrete structure. In order to prevent freezing heating coils can be installed connected to the spillway structure. Since the problem of freezing and thawing is complexly some generalizations has been made to be able to investigate the three-dimensional problem with a software which handle with two-dimensional analysis, Temp/W. Further studies should be performed based of more careful investigations of the soil properties and the impact of the freezing process regarded to the amplitude of regulation and the ground water level of the dam. Even external circumstances like snow and wind relation should be considered in further analysis. A recommendation is that these investigations should be performed with software where it is possible to create a three-dimensional model in order to get a more realistic analysis.