The water main in cities must be placed at a sufficient depth below the earth’s surface to avoid freezing during extended periods of subfreezing temperatures. determine the minimum depth at which the water main must be placed at a location where the soil is initially at 15°c and the earth’s surface temperature under the worst conditions is expected to remain at -10°c for a period of 75 days. take the properties of the soil at that location to be k = 0.7 w/m°c and α = 1.4 x 10-5 m2 /s.