This is generally true in a majority of arable topsoils. Please note, however, that the OP used the term "hardpack". In my case, below the clay topsoil, I am ripping a layer of Duripan, which is technically classified as rock, albeit soft rock in contrast to say granite. One poster above mentions ripping ever 3rd year or so to break up hardpan. Such soils that repack relatively quickly into a hardpan are technically known as Fragipan. Duripan takes centuries to millinea to reform and is much harder than Fragipan. In my conditions, I have to get the angle right AND have a lot of weight in order for the tooth to dig in. This may be the case for the OP as well-he may need a short top link length as well as BALLAST depending upon how hard his "hardpack" is.
HP depends on the number of shanks you pull. A single shank may well be able to attain a depth of 24" with several passes in the same trench.
I live in Eastern Wash. and made a sub-soiler using a spring toothed harrow. Narrow single tips, and pull 5 shanks at 16" spacing and can reach 20" in 2 passes with my Jubilee.(approx 28 hp). We have a lot of sand here.
From what I have read, A single furrow on 30" spacing or so, and getting below the compacted layer is the most effective.
My place used to be a horse ranch and the soil was really compacted to about 12". Now it drains well.
Good luck, Dave
