section 10 has been taken for a situation where without any protection the permafrost would thaw to a maximum depth of at least five feet during the summer season in prior art practice it normally would have taken at least a 14 foot depth of gravel to completely eliminate the thawing of the permafrost in such instance in computing the effect of insulation heat sink and gravel according to this invention the following properties were employed thermal diffusivity latent heat densitymaterial ft sup 2 hr btu lb lb ft sup 3 ______________________________________frozenpermafrost 0 0465 21 6 118 6thawedpermafrost 0 0232 21 6 118 6gravel 0 034 0 128frozen fluidin sink 0 0467 144 56 1thawed fluidin sink 0 00466 144 62 4thermal conducitivity of insulation 0 020 btu hr ft u00b0 f 2 u03c0 tmean annual air temperature 30 24 2 sin 1 98 365 used as surface temperaturewhere 34 t 34 is time in days freezing point of sink fluid 30 u00b0 f ______________________________________ considering that there may be some conduction by fluid motion in the heat sink which can be kept to a minimum by use of some congealant such as methylcellulose or some other cellulose or like material added to the liquid 24 and ignoring the added insulation effect or melted liquid 24 after it thaws so that the calculations are conservative the computed relative effects are shown in the graph of fig7 at one end it can readily be appreciated that with one foot of gravel an impractical 51 4 foot deep sink would be required with no insulation layer to prevent thawing of the permafrost layer at the other end even with seven feet of gravel and two inches of insulation some heat sink greater than 1 4 foot in depth is required.