SPT readings are only reliable in sandy soil profiles. Even then you get consistency ranging from very loose(N<4)to very dense(N>50), your unit weights change only by a smaller amount. Unit weight will depend on denseness or looseness, but also on the gradation of the soils and the specific gravity number which can range from 2.6 to 2.7
There are ranges of unit weights in most Soil Mechanics texts, but your most accurate would be to run an ASTM test D 1557 of a bulk sample or if the soils have enough clay content to take drive cylinder samples in place.
