The Nairobi Metropolitan zone has seen enormous growth in real estate to accommodate a fast-growing population. This is due to rapid urbanisation and modern trends.
Unfortunately, some investors take shortcuts to cut down building costs and speed up construction works, leading to loss of lives and property occasioned by the collapse of buildings. This is unacceptable and must be addressed urgently.
Earth science, through geotechnical studies, equips designers and engineers with information on site properties. Testing should be conducted in in-situ and laboratory tests to understand soil physical properties such as strength, saturation levels, grain sizes, soil types and consistency, and structures of the specific site.
Studies show the NMS zone has phonolites, tuffs, trachyte rocks subdivisions, and uniquely expansive clay, occurring at varying depths. Geophysics studies, namely MASW, Resistivity, and Radar, help better understand soil/rock lithology consistency, depths, seismic property, and degree of compactions for later improvements.
Additionally, in-situ tests such as Standard Penetration Tests (SPT) and cone penetration characterise the ground strength. Core drilling is then done at selected points to determine soil strength for deep or shallow foundations.
To better understand the ground strength (bearing capacity), laboratory analysis is done for site characterisation. Collected samples are analysed in a multi-stage process to determine the direct and indirect properties of soils or rock.
Laboratory tests include the Atterberg test for liquid, plastic, and shrinkage limits. In addition, the oedometer determines consolidation on loading, while the unconfined compression, direct shear, and triaxial shear tests help understand soil's behaviour under different loading. Water content and conductivity are also determined within the pre-mentioned tests, and the studies are interpreted in the context of soil/rock mechanics.
Ground stratum layering tends to be inconsistent, and it is typical for developing estates and urban centres to have landfills. Loading resulting from construction can cause buildings to collapse. But adequate groundwork can prevent catastrophic failures. Understanding the nearby environmental condition is essential because most loading forces are distributed to the nearby soils and structures. Even with a good foundation, weak surroundings may lead to building failures.
Studies show Nairobi and Kiambu areas have clays rich in kaolinite, which demands considerable care when designing foundations due to the extensive contraction and compaction from ground wetting and drying. Proper ground treatment and the right foundation design are therefore vital. This is because even after a site is drained and buildings are put up, a rise of water during rainy seasons or drainage leaks cause the clay to absorb water and shrink.
Further, Kiambu and Nairobi are in the sub-tropical area endowed with saturated soils or rock. For instance, gravel and sand absorb water faster due to high permeability, resulting in drained strength and good bearing capacity. But clay takes in water slowly due to poor permeability resulting in soils' undrained strength and low bearing capacity. Understanding soils, therefore, will help select the correct bearing capacity value.
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Inadequate preliminary studies can give a higher bearing capacity than a site's strength capability. As water drains into clay and ground readjusts to accommodate the loading, settling or liquefaction can occur, resulting in foundation failures. This implies the investors who shun laboratory analysis to save money do it to their detriment.
The increased building failures in Nairobi and its environs can therefore be mainly attributed to the negligence of contractors and investors. The National Construction Authority (NCA) and other bodies enacted measures to verify submitted reports before issuing permits.
NCA officers should also visit construction sites regularly for quality checks to stop investors' and contractors' greed that continue to compromise lives and safety besides throwing resources down the drain.
-The writer is CEO of Geosaharaa International Limited and a graduate student at University of British Columbia.