2.1 Assessment of borrow pits excavation Method
Field investigations covering the environs of Ado Ekiti were undertaken
to study some geo-environmental impacts as a result of borrow pits. Four
borrow pit sites were identified in some areas of Ado-Ekiti, Ekiti State
and were each investigated by direct measurements and use of
photographs. Two of the borrow pits are active and identified as
BPA1 and BPA2 while the other two are
abandoned and identified (Inactive) as BPI1 and
BPI2. A reconnaissance visit was initially made to the
sites and other features around sites for proper observations and survey
to determine the normal state of the sites during mining (for active
sites) and after mining has been done (for abandoned sites). Continual
removal of soil and the three-dimensional state of the borrow pits were
measured and modelled. Also, the longitude coordinates, latitude
coordinates and elevations of the pits were measured with E-Trex
Geographic Positioning System () and recorded. The height of the borrow
pits at different points were measured using levelling instrument and
the average height of the pit was determined. Photographs of the four
sites and areas around the sites with the infrastructures (buildings,
roads near the site) present were taken. The potential environmental
impacts and hazards such as landslides, debris flow, erosion, flood and
runoff pool of both the active and abandoned borrow pits were also
assessed through visual inspection and photographs taken.
Data generated by the GPS were used to generate 3D maps for the sites
using surfer software and AutoCAD. The data retrieved from the maps were
then used to determine the volume of overburden that has been excavated
from the borrow pits using Equation 1. The maps generated were used to
calculate for the areas and volumes of the borrow pits.
\(p\left(z\right)=p_{o}+g\int_{o}^{z}{\rho\left(Z\right)\text{dz}}\)(1)
The average depths (Z) for each of the pits were measured at different
points to get the soil overburden. Soil samples were collected from each
of the sites to determine some geotechnical properties of the
soils.
2.2 Determination of Borrow
Pit Area and Volume
The shape of the borrow pit is not a regular manner. Therefore, to
calculate their area and volume, different points around the borrow pit
were gotten and used to create a diagrammatic representation of the
surface of the pit using AutoCAD software. The area was calculated both
manually and automatically using the scissors principle (or scissors
method) and AutoCAD respectively. The scissors method is
diagrammatically represented in Fig. 1.
Area of borrow pit is thus calculated as:
\(C=\frac{A\ +\ B}{2}\) (2)
Where A is the\(\sum{Northing\times Easting\ of\ arrows\ facing\ right}\ \)
Where B is the\(\sum{Northing\times Easting\ of\ arrows\ facing\ left}\ \)
Where C is the Area of selected borrow pit.
Volume of borrow pit is thus calculated as:
V = C * \(h\) (3)
Where \(h\) is the difference of the average height at the top of the
borrow pit and the average height of the bottom of the borrow pit
Volume of water pooled into the borrow pit is calculated
\(V=A*D\) (4)
The coordinates (i.e. longitudes and latitudes), elevations and heights
of both the active and abandoned (inactive) borrow pits identified as
BPA1, BPA2, BPI1 and
BPI2 respectively are presented in Tables 1 and 2.