Abstract
Geo-environmental hazards associated with abandoned borrow pits in Nigeria are on the rise and a major concern to citizens, environmentalists and governments. Several highway failure spots are directly linked to the action of erosion initiated by active or abandoned (inactive) borrow pits situated close to the roads. This study examines the negative environmental impacts of the continuous removal of soil from borrow pits in some areas of Ado Ekiti, Nigeria. Four borrow pits were selected; two active sites and two abandoned sites. At inception, topographical and 3-Dimensional maps of the borrow pits were drawn and modelled. The area of the borrow pits and the volume of overburden excavated soils were calculated. The soil over burden pressure at the average height of the borrow pits were measured. The active borrow pit sites were checked again after two months to know the difference in the volume of overburden removed in the pit for that period. The volume of soil removed from borrow pit 1 (Active site), 2 (Active site), 3(Abandoned) and 4 (Abandoned) are 37000 m3, 34000 m3, 114000 m3 and 81000 m3 respectively. Environmental assessment of the study area through photographs showed prevalence of landslides, erosion, flooding, vegetation removal and structural failure. The volume of soil overburden removed from the abandoned borrow pit is more than the volume extracted from the active sites, this significant change in the value of overburden removed causes significant change to the terrain of the borrow pit. Some measures were then suggested to curb the problem occurring from the uncontrolled and indiscriminate borrow pits excavation thereby improving environmental sustainability. This study serves as a basis for government to put in place laws that help protect the environment from indiscriminate mining of borrow pits.
Keywords: Borrow pits; Excavation; Landslides; Erosion; Vegetation; Environmental sustainability
Introduction
In ancient time, the process of excavation of sand and gravel started. Boxam (2011) reported that the concept of excavation from borrow pit started in year 1800 when slaves were used by their masters to burrow the soils for small construction purposes, although, this activity were done using non-sophisticated tools like hand diggers, shovel and head pan. However, increase in population and advancement in technology have necessitated the use of sophisticated equipment for ground excavation as there must be equilibrium between the constant construction of infrastructure and the demand for earth materials (which are mostly lateritic sand, river sand and gravels). The need for land development (cut and fill) in road construction, sports field development, and other groundwork often demand supplementary earth materials. In most developing countries where construction work like roads construction, erosion control projects, building construction projects and so on, is always ongoing, several acres of land are degraded by abandoned borrow pits that turn to ponds or waste dumping pits. Most lateritic soil obtained is used as sub-base and base course for construction of highway embankments and foundation filling for building construction (Head, 2010). Soils can prove problematic because they expand collapse, disperse, and undergo excessive settlement with a distinct lack of strength due to large volume changes that are directly related to changes in water content (Owolabi and Ola, 2014).
Undiscerning excavation of borrow pits for road construction and other Civil Engineering works, without the intention of restoring or reclaiming the pits will lead to serious environmental degradation. Some of which include erosion, formation of sinkholes, loss of biodiversity, contamination of soil, groundwater and surface water pollution by chemicals from mining activities. Lawal (2011) reported the disturbance of landscape and distortion of topography as a result of excessive soil excavation in Nigeria. The borrowpit soils excavated are mostly used for sub-grade and sub-base course after thorough geotechnical investigation has been done (Owolabi and Aderinola, 2014). In some cases, additional forest logging is done in the vicinity of mines to increase the available room for the storage of the created debris and soil. Besides creating environmental damage, the contamination resulting from leakage of chemicals also affects the health of the local population (Nuss and Eckelman, 2014). There are several abandoned borrow pits scattered mostly in South-eastern part of Nigeria and all over other developing countries of the world where the clearing of vegetation for borrow pits were not restored to its original states has caused distortion of ecosystem and has led to the extinction of some fauna and flora species (Nwachukwu and Osoro, 2013). Minimization of the negative effects of sand and gravel mining requires a detailed understanding of the reaction of site to these disturbances (Goddard, 2007). Mining operations involve deforestation, habitat destruction and biodiversity erosion (Saviour, 2012).
Public safety has been threatened by some abandoned borrow pits due to the dangerous deep vertical walls created during excavation that are highly prone to landslides. In some other places, abandoned borrow pits were filled with stormwater and become ponds. Some without water serve as dump sites for end of live vehicles, illegal dumping of wastes and as hide-out for armed robbers (Nwachukwu et al. 2017). When all these situations arise close to residential areas, major socio-environmental problems confront residents but if these borrow pits are progressively reclaimed during its active excavation; the aforementioned problems can then be minimized. Slope failure in clay occurs on a very deep-seated surface, while it occurs on the surface for a sandy slope. Instability of a slope is linked with a change in cohesion or shear strength of the soil (Ola, 2013). As a way of fostering meaningful professionalism, sustainable engineering practice in the excavation of borrow pit must come to bear. This study therefore assessed some geo-environmental hazards caused by borrow pits in our immediate environment using four borrow pit sites in Ado Ekiti Nigeria as case study.
Materials and Methods