Effect, detection, and mitigation of voids beneath slabs-on-ground

A slab that is in solid, uniform contact with the subgrade does not usually exhibit a significant (high-Q) resonance, and the stiffness is relatively constant in frequency. There tends to be very little resonance response to impact loading, such as footfall. However, if there is a void of significant horizontal dimension beneath the slab, it will typically result in a resonant system, the frequency of which depends upon the horizontal extent and support conditions around the void. Footfall excitation of a slab over a void will be much more severe than without the void, and may cause problems if the floor is to support vibration-sensitive equipment or processes. The resonance frequency is observable using an impulse response test. The void may be corrected using several methods of injection. The paper examines several case studies in which voids were identified, characterized, and repaired, presenting data and methodologies.