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