I have read the continuous discussions on amending "hardpan" described as
clay soils. I live in PNW where hardpan is described as glacial till. It is
second only to asphalt in ease of digging. I have known it as clay my entire
life. I recently attended a seminar on landslide prevention put on by the
engineering dept. of the City of Seattle. Several speakers were making a
distinction between hardpan and clay layers in the soil. I got to speak with
a soil engineer and he described hardpan as a matrix of silt and rocks left
from the glacial period. He said it is even harder than clay when dry. Yet
with enough moisture it can soften and even wash away. This reminded me of a
neighbor who bragged about getting a load of river silt that had washed
under a friends house in a flood. It looked just like gray hardpan. All
discussions I have seen describe hardpan as clay and warn about creating
cisterns in holes dug and amended with something more permeable. I think
clay is impervious to water, I have seen clay on beaches that does not
dissolve, hardpan on the other hand, will absorb water and soften. To test
if you will get a cistern, fill the hole to the top of the hardpan layer
with water. If it drains 15" of water in 12 hours an inch of rain is not
going to create a sump. To aid in digging you can add a foot of water to
the hole and return the next day to find 2-3" have softened and are easily
removed. It is slow but easier than using a 6' 14# digging bar. My personal
experience is limited to PNW glacial till if you really have clay and not
silt, the test above should disclose it.