Use a second op amp to create a virtual ground, at 2.5V, have the piezo and resistor connected to there, instead of ground, and instead use back to back diodes to clip its amplitude to always be in the op amps common mode range
I suppose the last part comes down to what your hoping to accomplish with the circuit, if you want to measure any deviation from a set point, a window comparitor may be then better suited to trip on both positive and negative deflection.
Edit: something like this, if you want the window comparator version drawn up just ask