If you dig a hole and expose hard clay then let rain or water into the hole, the top of the clay will turn to mud. Below the top 10 centimetres the clay should still be hard.
To convert hard clay to soft clay you would need to put a large amount of fill to exceed the preconsolidation pressure of the clay.
The construction site has poor temporary drainage which resulted in several instances of pooling or ponding of water on the site.
Initial soil investigations only reported hard clays whilst a second soil investigation reported soft clays.
Therefore, I was wondering whether the pooling or ponding of water caused by the poorly maintained temporary drainage could be a possible root cause of the soft clays.
The second soil investigated revealed a 3-5 m thick soft clay layer existing just below a 1-2m thick loose to medium dense silty sand/sandy silt top layer.
So you're comparing apples with oranges now- the borehole SPTs have been performed at intervals through the profile; the CPTs are continuous. The sensitivity of the clays is a factor in the SPT interpretation.
Without any information on the size of the site, spread of the field tests, I can only assume that the borehole and CPTs were indeed put down through the same soil profile- if that's the case, almost certainly a question of the data interpretation...there is no ways your 3 to 5m of hard clay became soft from short-term storm-water mismanagement. A hard clay is effectively impermeable, at least in the short term.
An interesting point though- I've also observed some big disparities between SPTs and CPTs in sensitive clay soils...half the problem comes in relying on a printout with empirical values, instead of understanding the raw data. A SPT value of xxx doesn't necessarily mean a "hard" or a "soft" clay...it depends to a degree on the properties of the clay.
A final thought, in case the borehole was put down by a contractor of ill-repute...a badly worn SPT spoon can produce erroneous SPT values.
