3.3 Field Measurement and Assessment
Four borrow pits were assessed; a situation of unsustainable engineering practice was demonstrated. Figs. 6a to 6f illustrate the present site conditions. Each of the borrow pits visited shows great potential for landslides occurrence in the area. As landslide occurrence is dependent on factors like height, steepness of the slope, density and strength (cohesion and friction) of material on the slope of the excavated area, the geometry of the borrow pit sites revealed a slope angle of between 80° and 85° which is a compromise of the standard slope of 70° and 75° during excavation process (Fig. 6a). The pits were also excavated in a vertical manner to depths as high as 15 m. Due to heavy rainfall which usually occurred around May and July, the borrow pit slopes has been weakened and if they are not quickly reclaimed, more loss of properties will be experienced. Sliding angle, overburden stress, resisting force, water saturation, pit slope angle, and pit effective depth will facilitate the occurrence of landslides in each of the borrow pits. The buildings around the area are at risk if landslides occur and this could lead to destruction of buildings, properties and even lives.
The vegetation of areas surrounding the borrow pits are characterized by thick and dense vegetation, typical of the tropical rainforest. The excavation activities carried out in the pits have turned the area to degraded land and danger zones as no meaningful agricultural practices can be carried out. Also, there is great threat on both the animal and plant species within the area and this can result in extinction of some species due to the destruction of their natural habitat occurring through unlawful and unguided excavation. Fig. 6b illustrates the soil of the area to be mostly lateritic soil. Before excavation, the topsoil (consisting of dark humus soil for agriculture) of the area is around 0.3 to 0.5 m thick. This top soil is then followed by the reddish lateritic soil which constitutes the borrow pit material. Both the active borrow pits the inactive borrow pits are currently under serious susceptibility both to water and wind erosion. The sites have all been laid bare due to the excavation activities carried out. The excavations were done in both triangular and trapezoidal manner hence creating slopes that encourage the pooling of water after rainfall (Fig. 6c). This pooled water in turn cause erosion and wash away soils to nearby settlements (Fig. 6d). Absence of functional drainage systems causes the washing off of soil from the un-tarred road surface resulting in potholes and depression. Stagnant water in the borrow pits also encourage the breeding of mosquitoes resulting in environmental quality.
Clearing of vegetation and stripping of soil from the borrow pits can cause debris flow during raining season to the downhill causing flooding of the communities, creating potholes, cutting off the road, and reducing soil fertility. The operation of heavy equipment along access roads and surrounding areas of the borrow pits leads to compaction of affected areas, thus disturbing the natural state of soils. During heavy rains, water pooled into the pits can overflow. Structural failure was noticed around the active borrow pits. There is an on-going foundation been laid just beside the excavated borrow pit (Fig. 6e). Also, some existing buildings were sighted (Fig. 6f). This failure most likely results from the use of heavy-duty equipment like bulldozers, pay loaders and trucks used for excavation and loading of the laterite soil to designated construction sites. These equipment have tendencies of producing vibration which sends lateral waves to the surrounding structures thereby weakening the foundation and can finally result into displacement of structures. Also, during rainy season, there will be creation of micro cracks which will gradually become water infiltration pathways and finally grow to mega cracks that can cause pit slope failure which can eventually lead to collapse of structures such as buildings, roads and drainage system certainly causing huge economic loss.
The borrowed area of BPI2 has the largest volume of soil removed than the other borrow pits which shows the effective and regular removal of earth materials and hence vulnerable to risk. Table 3 indicates that large volume of water has been pooled in to BPI1, Long-time effect of this large water will cause continuous digging of the pit and leads to gully erosion, scarring, siltation, changes in geochemical conditions and flooding. These four borrow pits can rarely be returned to its former state, every effort should be made to address potential residual impacts during the closure process of the borrow pit. Indiscriminate borrow pit excavation without due regard for the desired final shape of the borrow pit should not be permitted and should be rectified immediately. Undiscerning roadside excavation of borrow pits for road construction and other civil Engineering works without the intention of restoring or reclaiming the pits will lead to environmental degradation. The earth materials removed from the pits and not replaced, resulting in areas that are typically lower than their original ground levels. Some unwanted screened material will return to these borrow pits but will not complement what the original level should be. These borrow pits are usually clearly visible as low-lying areas filled with water alongside roads.