Behavior of Concrete Slabs on Grade Contains Cavities Ola Mazen Makki1,*, Majid A. Abdulhussein1 and Ali Hashim Saeed2 1Department of Civil Engineering, College of Engineering, University of Al-Qadisiyah, Al-Qadisiyah, Iraq 2Computer Technical Engineering Department, College of Technical Engineering, The Islamic University, Najaf, Iraq. Received 16 August 2024; Accepted 19 July 2025 ___________________________________________________________________________________________ Abstract Concrete foundations are the most structural member in contact with soil and exposed in the first degree to soil problems. Soil may suffer from sinkholes and cavities in dependence to soil type which surly effects on the bearing capacity of footings. Cavities may form in irregular sizes generally; it forms due to gypsum dissolving during rainfalls. The shape or cavity approximated by engineers to nearest regular possible shape. This research seeks the behavior of concrete slabs rest on soil grade under the effect of vertical void in different sizes, depths and shapes. The strength of foundation and load distribution were also investigated. It was concluded that, the slab bearing capacity affected completely by an imaginary triangular zone lies beneath the slab itself, if the void were within this triangular, the slab will deteriorate by the void, otherwise the cavity will not weaken the footing. The area of this imaginary triangular shape depends on soil type, soil compaction, soil density, footing size and shape. It was also concluded that, enlarging the cavity deteriorate the slab bearing capacity and accelerate the failure due to losing of support. The worst case in vertical cavities, occurs when the cavity is in contact with the slab bottom.