Building near mining site could be catastrophic

Maduka Nweke

Despite the obvious dangers associated with setting up building near mining site, several developers often fall victim to such dilemma.

Indeed, many do not mind building their homes near mining sites. Oftentimes, they do it because they want their residential homes to be near their work places. At some times, the aim gets jettisoned when events of catastrophe happen. Those who build near mining sites also believe that the soil upon which they intend to build the house is strong eventhough it is believed that what matters in making building foundation is more than merely being strong. Making your foundation at difficult sites endangers the building not merely the foundation alone. The fact that there are different soils recommended by experts to suit your taste makes it possible that you seek the advice of professionals in that regard to know what and where to avoid while building your home.

You may also know that in the course of building your home, you may experience unstable ground,  clay soil, trees, sloping land. In all, difficult’ sites can quickly become expensive to build on. One should bear in mind that building on difficult terrain normally involves some solutions like digging down the soil deeper.

Many a time, those building near a mining site may not know that those inside the mine have burrowed far both vertically and horizontally. So when the builder fails to know that and goes ahead to build it means that he is building castle in the air while sitting on the keg of a gun powder. On ground with poor bearing capacity, such as soft sandy clays, the simplest solution is to dig down a little further. If a standard trench foundation is deep enough, its base should be supported on firm ground unaffected by seasonal changes, while beam and block floors can happily bridge across the surface. Also by digging deeper, if the foundation is within the circumference covered by the miners you will notice it before laying the pillars. However, in clay areas, the ground around the sides of the foundations will still be prone to periodic expansion and contraction as the ground becomes saturated and then dries. So to resist lateral pressure, the sides of the trenches can be lined with a flexible slip membrane which allows the clay alongside to independently shrink or swell. Also, building wider foundations can help spread the load over a larger area, but this may require steel reinforcement to prevent shearing. And, as we have seen, with deeper trench foundations it soon becomes cheaper, easier and safer to switch to either piling and or ring beam foundation.

Piles are concrete columns drilled deep into hard bedrock to securely anchor the building, aided by frictional resistance from the surrounding ground. This method has become much more common in housebuilding since the advent of cheaper short-bored systems installed using hired mini-piling rigs. It is now the most widely used foundation type after conventional trenches, thanks in no small measure to planning requirements encouraging retention of trees and the development of brownfield land.

There are good reasons why developers generally steer clear of ‘tricky’ sites. Potentially exorbitant groundwork costs, coupled with prolonged periods of uncertainty, can rapidly transform a viable project into a daring gamble. However, what might appear to be a ‘problem plot’ can sometimes turn out to be a blessing in disguise for self builders willing to take on technical challenges. The primary role of foundations is to anchor the building to good bearing ground (in other words, ground capable of supporting the building). Even relatively lightweight structures, such as timber frame houses, need to be securely ‘fixed in place’ to resist ground movement. The trouble is, conventional trench foundations start to become uneconomic below about 2m deep. So on sites where stable ground is in short supply, you are likely to need something a bit more sophisticated to protect your home from the ravages of nature. This normally means consulting a structural engineer at the design stage. The need for ‘special’ foundations can also extend the time taken for groundworks from three or four days for a conventional build, to perhaps two or three weeks more than doubling your total costs for this stage of the project.

The sinking of the ground surface due to mining starts with the removal of coal from underground. Gravity and the weight of the overlaying rock causes the layers of rock to shift and sink downward into the void left by the removal of coal. Ultimately, this process can affect the surface, causing the ground to sag and crack and holes to form, which may severely damage or destroy residences. A few inches of differential subsidence beneath a residential structure can cause millions of Naira worth of damage. Subsidence can happen suddenly and without warning. Detailed investigations of an undermined area are needed before development to resolve the magnitude of the subsidence hazard and to determine if safe construction is possible. While investigations after development can determine the extent of undermining and potential subsidence, often, existing buildings cannot be protected against subsidence hazards. This is because of the inability of available technology to predict exactly where, when and how much subsidence will take place at a given spot and the cost of remedial measures.

Where longwall mining is active and subsidence is a well-documented and predictable action, surface response to ongoing mining can be accurately estimated. However, in the case of room and pillar mines, especially where they are inaccessible and record-keeping may be inaccurate, predictions of when subsidence will happen are not possible. Several factors contribute to the timing of caving at the mine level and subsidence appearing at the surface. Pillars and timbers left in place can hold the roof of the mine up for long periods of time. Generally, the smaller the void width and the greater the number of pillars, the longer the roof can be supported.

Groundwater in the mine provides a buoyant force that helps support the ceiling. Also, pillars retain their strength because the lack of oxygen in the water-filled mine prevents the chemical breakdown of the coal. Therefore, water level changes in the mine increase the chance of pillar failure.

Changes can contribute to the initiation of subsidence 100 years or longer after the mine closes. Once block caving or sagging occurs in the mine, time and the physical characteristics of the void space and overburden will determine if subsidence reaches the surface and how much subsidence takes place. How much subsidence will occur and the features that will appear at the surface depend not only on the type of mining but on geology and several physical features of the voids left by mining.