The most suitable soil type for filling water logged lands (or lands with high water table) to make it stronger and stable enough to sustain foundations or sub-structures to be constructed over it is any highly draining soil like sharp sand. The common practice which is to fill with only Laterite (or red soils) then construct a concrete slab or foundation or interlocking pavers directly over it often yield in poor results; hence the reasons for the depressions that shows up at various portions on the surface of a poorly sand filled land which may consequently affect the overlying structure.
Unlike Laterite or red soil, highly draining soils like sharp sand barely retain surface and ground water, and do not behave abnormal across seasons (from dry to wet or vice-versa). For instance, because of the plastic nature of clay/red soils, they behave stable during the dry season and unstable during the wet (or rainy season).
Therefore if a foundation or floor is constructed directly over it in an area with high water table even with hardcore filling on it, it is most likely to start settling in-differentially or failing horizontally throughout its surface or longitudinal bearing and vertically downwards at various portions as we migrate from one season to another leading to visible cracks on the floor or from the foundation up to the roof after a period of time.
These cracks are often persistent and will continue to resurface until proper re-work is done. If left unattended to, it may degenerate into collapse of the structure over time. Sharp sands or suitable equivalents also perform well with changes in ground and surface temperatures in the day and at night giving the foundation or floor greater resistance in the face of extreme temperature or weather conditions.
|Site sand-filled with sharp sand on a bed of Laterite|
However because of the cost implication of using sharp sand or its equivalent as the only filling material for the site, the suitable alternative practice is to first fill the land from the depth of suitable sub-soil or from the existing depth to a reasonable level with Laterite or smooth sand which is relatively cheaper before introducing sharp sand to complete the fill up to the marked out level which is often above the water table and road or drainage level.
This way, the stiffening advantage of Laterite or smooth sand combines with the merits or good qualities of sharp sand to give an acceptable degree of strength, resistance and stability.
It is instructive to note that when filling a land, precaution should be taken to ensure that the filling sand or material is well leveled and compacted to allow natural flow of surface water to the drains and to prevent excess air voids or gaps respectively in order to function to full potentials. This process can be done manually or mechanically or chemically or a combination of any of the trio depending on the nature of the project. It is better to get it right at the sub-structure level before constructing the super-structure to avoid re-works and to reduce maintenance cost in the future.