Numerical analysis of building damage caused by the exposure of a terrain threshold as a result of underground mining-induced subsidence

Underground mineral exploitation is a very important part of the economy and industry of almost every country. Despite the reduction in the extraction of fossil fuels in favour of renewable energy generation, the exploitation of others is not being reduced, e.g. metal ores, rock salts, construction minerals, etc. Consequently, further impacts of mining activities on the environment must be reckoned with, which, in terms of underground mining, are revealed in the form of land deformation, hydrological changes or ground tremors. This is due to the filling of post-mining voids by the overlying rock mass. The ways in which these deformations are revealed vary and depend on the geological situation, the method of mining and the deposit itself. As mining is carried out beneath urbanised areas, the deforming ground causes significant impacts on buildings located on the surface. For this reason, the ability to predict the behaviour of buildings, which in most cases are not designed for such additional loads, is very important in order to avoid damage or foresee the possibility of a building failure.

Although the issue of the impact of underground mining on the surface includes a whole range of different factors, it has been extensively researched and described^1,2,3,4,5. The subject of interest for the largest number of researchers was undoubtedly the forecasting of continuous deformations, which was dictated by the fact that these always occur in the case of mining, regardless of the geological and mining conditions. This contributed to the development of many methods for deformation state prediction^1,2,3,6,7. At the same time, a number of mining prevention methods were developed, consisting in monitoring the state of surface deformations^2,8,9,10,11 and minimizing their value¹². The impact of mining works on structures is characterized by many forms¹³. Attempts to classify settlement-induced damage have also recently been carried out in the Netherlands. For a selected group of buildings, the effect of the type of foundation (shallow or on piles) on the damage of masonry buildings was analysed¹⁴. Therefore, research in the field of building structures protection was carried out in parallel, resulting in the development of a number of methods for protecting buildings against emerging ground deformations, which were presented, among others, in the works^{15,16,17,18,19}. Design recommendations for the construction of buildings subjected to such influences were developed in the second half of the twentieth century and are constantly being refined. They took the form of a set of instructions issued by Poland’s Building Research Institute^20,21. Deformation of the ground is not only caused by underground mining operations, but is also influenced by the effects of open-pit mining using a case example in India. The problems of selecting a suitable foundation grid for a building on weak soil are shown in²². Therefore, one can express the view that we currently have methods of forecasting and minimizing the state of continuous deformations, as well as methods of building protection, which are sufficient for practical purposes.

A less favourable situation occurs in the case of discontinuous deformations. These often appear randomly. Discontinuous surface-type deformations (usually sinkholes) occur over shallow post-mining voids. Although there are a number of methods for predicting them^23,24, some of them also provide the probability of a sinkhole as a result of the analysis. Protection of buildings in sinkhole areas is more expensive and more difficult than in areas with continuous deformations. For this reason, it is recommended to avoid placing objects in areas with such risks^15,25. Analyses of objects that have been subjected to such influences and have not been protected are rarely conducted. Theoretical foundations were presented in the work³, while an example of numerical analysis of the object’s behaviour in such conditions was presented in the work¹⁶. In recent years, an important problem has become the more and more frequent occurrence of discontinuous linear deformations, which are most commonly represented by steps in the ground. In general, it can be stated that the occurrence of this type of deformation is related to the exploitation with caving in several seams to a common boundary and with local geological conditions²⁶. Undoubtedly, tectonic faults are a factor that increases the probability of these deformations^3,27,28,29. Peng et al.³⁰ listed among the factors causing discontinuous linear deformations: the rock mass structure, the presence of active tectonic faults, human activity and loess erosion. In some works, a relationship was noticed between the values and distribution of horizontal strain and the occurrence of discontinuous linear deformations^31,32,33. This is confirmed by the results of the research presented in the paper³⁴, which even gives a statistical relationship between the value of horizontal strains and the size of deformations. Factors determining the formation of linear deformations were also indicated. Among them, the following were mentioned: soil properties, surface topography, seam depth and thickness, method of roof control, plot dimensions and the speed of the face advance. The inclination of the terrain also has a significant impact on the nature of the deformation.

The aim of the article is to present the possibilities of an expert approach to the analysis of buildings that have been subjected to significant loads caused by discontinuous deformations of the terrain—a terrain threshold—which were not taken into account in the design. An important aspect is the fact that the analysed building was protected against the impact of continuous terrain deformations, which significantly reduced the amount of damage.

These observations are confirmed by the results of the research contained in the works^35,36. In these case studies, similar reasons for the occurrence of steps in the ground were indicated, at the same time indicating the threats posed by these deformations to buildings. It should also be noted that the presented cases of deformation are not isolated. In the work³⁷ a case of a building was presented under which a discontinuous linear deformation with a height of h = 15 cm occurred. There was numerous damage to the building, and the width of the cracks in the walls was 2 cm. This damage was assessed as serious, making it difficult to use the building. Elsewhere^38,39 cases of damage to buildings located in the vicinity of terrain thresholds (steps) were shown. The observed damage was much smaller than in the case of a threshold directly under the building, as in the publication³⁷.

Taking into account the relevance of the topic related to the formation of discontinuous linear deformations and their impact on buildings, this work presents an analysis of the case of the formation of a step in the ground and its impact on a building.

The numerical approach presented in the article to the problem of building damage as a result of, for example, a terrain threshold revealing itself is one of the few possibilities of interpreting the damage that occurred, as well as predicting the behaviour of the structure under the influence of similar deformations. However, this is not an easy method, because the number of variables influencing both the size, intensity, and place of occurrence of discontinuous deformations is significant, and buildings are erected using different technologies, quality of workmanship, maintenance of the facility and may be subjected to other catastrophic influences (hurricanes, floods, etc.) is also not small. This means that proper consideration of the initial and material conditions of the analysis is only possible when the analysis is performed by people with significant experience in this field. The experience of the authors allowed them to take into account all these elements, which resulted in an analysis that well reflects the actual behaviour of the facility—overlapping places of cracks, deformations of the basement floor, the size of the inclination of the facility. Therefore, the presented approach can be used to analyse similar cases by people with knowledge in this field.

The problem of damage to buildings caused by deforming soil due to underground exploitation of raw materials is already known and has been given guidelines for securing buildings against these effects. This is relatively simple with continuous deformations of the terrain (gentle subsidence basin) at the time of designing buildings and there are appropriate design recommendations for this. Discontinuous deformations, which are much less predictable and often occur in places where they are not expected, where buildings are not prepared for them. Additionally, the intensity of discontinuous deformations is significant and their growth time is short, which causes significant damage to the building and lack of time to react. Therefore, it is recommended to avoid locating objects in places where such deformations may occur (recommendations of mining companies in Poland), and if it is necessary, the approach to the problem must be individual. For this reason, there are no guidelines for designing buildings in such conditions and the approach must be individual each time. The presented case shows the possibility of conducting an analysis of an existing building and drawing conclusions as to the extent of damage, which is essential in making decisions about the possibility of further safe use of the building.

It should therefore be concluded that under the above-mentioned geological-mining conditions, the occurrence of discontinuous linear deformations can be expected. The locations of these deformations can be roughly predicted, but it is advisable to carry out studies aimed at determining the magnitude of deformations under the conditions of a given basin. Having comprehensive information about the expected deformations, it is possible to carry out an analysis of their impact on building structures using numerical methods and indicate the most vulnerable areas. The results of such analyses may allow the development of methods for strengthening building structures in areas affected by discontinuous linear deformations. This will make it possible to fill an important gap in the protection of buildings exposed to damage from this type of deformation.

The aim of the article is to present a numerical analysis of the complex problem of soil-structure interaction, which is subjected to mining-related impacts. Appropriate, precise definition of the structure and deformation of the ground allows for a good reproduction of the numerical model in relation to the behavior of the real object. The presented calculations can be an indication and guidelines for the analysis of other facilities and for experts to determine their opinions regarding the safety of facilities. However, it is not possible to speak of direct copying of solutions, but of a scheme of procedure, due to the significant diversity of structural and material solutions of the structure and the intensity of mining impacts.