Of naturally occurring materials and integrity of structures

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PHOTO :Allens Contracting

Sir: Common naturally occurring construction materials are lateritic soils, sands, gravels and quarry products. They are abundant in many parts of Nigeria but they are usually won without their qualitative evaluation and consideration for the environment. This write-up will focus on lateritic soils and quarry products. Like in most parts of the tropical world, lateritic soils occur abundantly in both Basement Complex and Sedimentary terrains in Nigeria. The diagnostic property is their reddish brown colour, the lay man and indeed many people that exploit and utilise the ubiquitous soils refer to them as “laterites”. The correct name is “Lateritic soils” while those that are rock like are called “laterites concretions”. A major condition for their formation is distinct wet and dry seasons typical of the tropical zone.

A typical laetrile profile (that is, layers that are observable if one digs or drills in a rocky terrain) comprises from top to bottom: the top soil, laterite horizon, mottled horizon, sarprolite, sap rock and fresh bedrock. Those that win lateritic soil for construction often scrape off the top soil and win other soils until they get to sap rock when the site has to be abandoned for another location.

Studies and experience by engineering geologists have confirmed that the only appropriate material either for foundation for structures or construction material is the soil from the laterite horizon.  The soil in the horizon possesses the best mineralogical, geochemical and geotechnical properties for construction purposes. Soil from the mottled horizon has both reddish brown and grey colours because it can be above or below water table depending on the season; it can also be won construction purpose. Lateritic soils are similar but not identical geologically and geo-technically. Their properties depend on those of the parent rocks and the prevailing environmental conditions. They must thus be thoroughly evaluated qualitatively prior to their utilisation for any construction purpose. This is a very important approach to prevent collapse of buildings and failure of roads which are rampant in Nigeria.

Blending of about 5% to 10% by volume of cement with good lateritic soils have been utilised to produce sun-cured bricks of adequate strength for construction of affordable houses. Quarry products, which are mostly, rock aggregates; rock dusts and sand-sized materials are commonly used for construction of buildings, roads bridges and dams in Nigeria. To a lay man, “all rocks are granites and they can all be used for any construction purpose.” Rocks are also similar but not identical texturally, mineralogically and geo-technically. A rock must thus be evaluated geologically and geo-technically before it can be certified good for any construction purpose. Rocks that have strong foliation are fine grained, rich in quartz and iron bearing minerals are often good for construction purposes.

It is high time construction giants and relevant government establishments in Nigeria started employing geoscientists, in order to avoid failure of our structures such as buildings and roads. This is the case in developed countries where failures of structures seldom occur.

Prof. Gabriel Oladapo Adeyemi.