Nairobi City is founded on volcanic rocks, whose structure can be massive (e.g., trachyte), vesicular (e.g., basaltic lava), or porous (e.g., tuff). These rocks occur in flow layers, usually tens of meters thick. This mode of occurrence alternates massive and porous rock layers, leading to a seasonal perched water table at many localities.
Piezometric uplift pressure from a high perched water table is a cause of hogging in basement slabs. When this happens, it leads to structural cracks in the basement floor and a danger of weakening the foundation bases. Furthermore, the introduction of structural cracks may make pathways for harmful ground gases to be released into basements. There is evidence of leakage of the city sewer network, as seen from nitrate distribution in groundwater. Such leakage can lead to the production of methane among other gases, in the ground within the reach of basement levels.
The National Building Code, 2024 (Cap. 118), defines groundwater as subsoil water and issues guidelines only with regard to the construction of freestanding and masonry retaining walls. The Code provides guidelines on the management of surface water at a construction site, but not groundwater.
X Towers was constructed over three years between 2012 and 2015. Four years later, in 2019, the building started flooding in the lower basement during periods of heavy rain. The management noticed cracks in the floor slab, too.
The author was then approached to study and give a lasting solution to the problem, upon which the building was found to have been constructed a hundred metres downstream of a local watershed at which shallow groundwater was recharged. Excavation of the lower basement into less permeable trachyte rock led to a buildup of groundwater pressure following rainfall, leading to piezometric uplift that exerted enough force to create structural cracks in the concrete floor slab.
To solve the problem, a pressure relief well was constructed into the ground at an appropriate place, and the flowing water was directed into an existing drainage sump for wash water. The peak flow rate was estimated by a slug test during heavy flooding, and appropriate pumps were installed to provide external drainage via the stormwater drain at ground level. A groundwater management study during the design stage would have incorporated measures to prevent hogging of the basement floor slab.
This was a novel case, and the work had to contend with the fact that there were no regulations by the Water Resources Authority regarding the abstraction of groundwater for waste, and the building and city bylaws prohibit disposing of wastewater into open storm drains.