The photos shown are examples of Sand Boils found in concrete slabs.
When concrete slabs are constructed under some undesirable conditions or using various practices (or both) they can be prone to creating a condition many call "sand boils." Essentially this is a condition in which the granular (fine sandy material) overlaying the visqueen is mobilized by the speed of the moving fresh concrete during placement, leaving its original position and drawn in to the slab section. When this happens, an inclusion of sand shaped like a streak or a ball is present within the intended slab thickness. This inclusion may be spherical in shape, ranging in size from smaller than an inch in maximum section or large enough to encompass the entire slab thickness. Sand Boils are typified by the rearrangement of the granular material layer, not by defective concrete materials or the installation of an insufficient volume of concrete. Sand Boils arise specifically in the case that concrete is cast upon sand which is easily and can be drawn into the mass of the slab during concrete installation. Casting concrete within soil excavations, for example like trenched or formed footings, which by their nature are deeper members with no plastic or sand underlaying them, does not have the potential for Sand Boils in their section.
Building Forensics International has investigated several projects over the past 20 years that involve sand boils. Building Forensics International is a consulting firm that provides concrete expert witnesses who have concrete investigative experience. If you are in need of a concrete expert please consider Building Forensics International.