Stress below circular load, computed with Bessel's function, errorneus values

Hello

I'm trying to comptue the stress in ground below a circular loading, at differend depths, at different locations

below the load.

The formula which I use, include Bessel's function. Reference to the formula can be found here:

http://onlinepubs.trb.org/Onlinepubs/hrr/1963/39/39-004.pdf

The used formula is the equation 3c.

The formula seems to give reasonable answer with most locations and depths, but some depths

seem to give strange stress values (peaks in the graphs).

Could someone explain why these errorneous values appear and if there is a way to

overcome this problem.

Thank you for your help.