Apart from the disruption that water can cause in open cast mining operations, other consequences could lead to generation of acid water drainage, enlargement of cracks, instability of slope and initiation of landslide that can severely pose threat on lives and properties around the mine. In Bo et al (2019) study, it was discovered that rainfall infiltration was a major factor affecting the slope stability in open pit coal mining operations. There is a significant change in the distribution of pore water pressure and a noticeable variation in the value of safety factor under different rainfall conditions at Haizhou open pit coal mine in the southeastern Fuxin city of Liaoning province in China. The effects of rainy season in mineral extraction process cannot be overemphasized as the impact varies from loss of production to increase in health and safety of mineworkers. These become a leading problem when different climate changes were not considered at the mine planning and design stages.
However, there is formation of pit-lake in open cast mining operation during raining season. That is, water filling the mined-out area during or upon completion of mining activities. Gammon et al (2000 categorised formation and filling into two, namely; natural and artificial flooding. Natural flooding is when the pits are filled hydrologically and a pit filled by rain is regarded as a natural pit lake. Rainfall is a natural phenomenon which is impossible to prevent and therefore causes water to settle at the mine pit bottom. According to Bargawa et al. (2019), excessive mine water from rainfall and mud has the potential to create mud accumulation. A combination of the wet mining area, water accumulation, and mud sediments result in a major problem which complicates mining activities in coal mines leading to failure in reaching production targets as shown in Fig. 1. This makes it impossible for the day to day activities in the mine to continue until the water accumulation issue is resolved.
According to Mason et al (2013) study, the impact of the flooded pit can be categorised as directly or indirectly which can be further classified into immediate and long-term impacts. Under immediate impacts, the affected areas include production, overtopping of levy banks and damage of coal conveyors while the long-term impacts include delay in coal production and strict monitoring by the officials.
Furthermore, heavy and consistence rainfall during rainy season also affects the haulage of materials from mining face to the muckpile. Material handling in surface mining involves transporting the fragmented rocks from the pits to the dumping site using excavators, fronts end loaders, draglines and trucks (Adam & Bansa, 2016). The haulage road system is one of the most vital and critical components in open pit production process, most especially the truck-based system (Thompson & Visser, 2000). During the rainy seasons, rain tends to cause underperformance of haul roads and this has an impact on mining productivity and road-related costs. Productivity, the safety of operations and equipment life all depend on a properly designed and well-maintained road.
One of the disadvantages of opencast mining is the regular maintenance of haul roads, this problem worsens during rainfalls. Eventually, all roads in an opencast mine will deform after some time due to the frequency of their use, the weather conditions such as rainy periods, and the nature of equipment operating on them (Nuric, 2011). In short, surface mines do experience delays in production as a result of bad haul road design and maintenance. Having appropriate haul roads management and maintenance strategy can limit the impact rain has on roads while increasing production and saving on costs. During these periods, the haul roads become muddy and slippery and this requires frequent maintenance. Most time is used focusing on maintaining haul roads than using them for hauling. This means that even though the equipment is available they are not being used and this affect the planned production. Tannant & Regensburg (2001) stated that there are two major concerns in terms of haul road integrity during extreme weather conditions (typically heavy rainfall). These are tyres adhesion to the road (traction) and rolling resistance.
In addition, during rainy season, there possibility of experiencing lightning as a result of heavy downpour. According to Kithil and Alzamora (2013), the lightning also cause havoc in mining operations as this can affect mine communication systems, explosive storage, load and haul stations, drilling and fuel farm which contributes massively to extraction process. The impact of the lightning on various areas that contribute to production losses in mining is presented in Table 1.
Table 1
Factors affecting production as a result of lightening (Kithil & Alzamora, 2013).
S/No
|
Sources
|
Percentage Contribution
|
1.
|
Load and haul
|
35
|
2.
|
Drilling
|
25
|
3.
|
Radio communications
|
15
|
4.
|
Explosive storage
|
13
|
5.
|
Fuel farm
|
10
|
6.
|
Others
|
2
|
Similarly, lightning has resulted in many fatalities and injuries in the mines and this can lead to mine closure which affects the overall production. According to Kithil & Alzamora (2013), education is an important tool to be given to outdoor employees and contractors. To avoid having to halt mining operations as a result of lightning-related injuries or fatalities, safety measures should be taken. Using lightning detectors, avoiding water and metal contact, avoiding trees, staying of telephones, are some of the measures to reduce the impact on employees. To reduce the damage that rain has on the haul roads, the design and management of the roads must be properly done. The three basic steps to properly design haul roads that can withstand operating conditions in mining are shown in Fig. 2.
Significantly, construction of drainage could also help in reducing the impacts of heavy rainfall on haulage road. According to Adams (2019), V- drains are more often located next to the majority of the haul roads and are a critical component of a proper drainage system. The main function of these drains is to maintain the haul roads integrity and thus should always be in good conditions to ensure maximum protection of the roads. During rainy season, the V-drains tends to have a huge amount of water to accommodate and even though sometimes the water flow can be very much considerable. This can result in the loss of drain lining condition and can even cause scouring, hence the frequent maintenance of the drains.
One of the most common techniques used in managing water in the pit during rainy season is dewatering method. This process entails pumping out of water from the pit at regular intervals. This is important as it helps to remove mine water from accessibility of production (Sahoo, et al., 2014). It is essential for the open pit mine to have pumps that are ready to run at any moment as water must be pumped out of the pit to allow mining operations to continue immediately after rain. Pit dewatering is an essential operation for safety and efficiency purposes in quarries, open pit and opencast mines that extend below the groundwater level. The diagram in Fig. 3 shows the approach to which mine water can be managed in an open pit mining environment.
Over the years, mining companies have realized that heavy rains, flooding, and lightning and high temperature pose threat to the safety of personnel, equipment and structures. The most reliable tool that is often under-leveraged by sites and operations managers is accurate weather forecasting. Having access to consistent reliable weather reports provide a better criterion in assessing when to remobilise certain operations and how certain resources can be allocated away from heavily affected and dangerous site areas.