Influence of continuous deformations and tremors of rock mass on a building. Case study

. This work presents an exemplary analysis of the influence of mining exploitations on a building. Continuous deformations of the ground surface in the location of the object were considered. Analysis of the impact of tremors of rock mass on the object was performed. The results of calculations as well as the measurements of surface vibrations accelerations were taken into account. The performed analyses show the influence of a fault on increase of vibrations accelerations.


Introduction
Mining is the economy flywheel, which supplies following raw materials: energetic, chemical, metallic and rock. Despite the huge importance of mining, however, there are some negatives of it [1], taking form of its influence of environment, what is mostly associated with so-called mining damage in building facilities [2,3]. This influence shouldn't be exaggerated, because as a result of strict regulations in force in Poland, the necessity of obtaining the consent of the local government administration for the implementation of exploitation plans, damage to buildings is being limited. According to paper [4] they constitute approximately from 1% to 3% of the mining costs. The cost of mining damage removal in Germany in 1996 reached around 350 million DM while the extraction equed 39 million Mg [5]. Among negative forms of influence of mining on buildings there are [2,3]: continuous deformations, discontinuous deformations and tremors of rock mass. The predictions of mining influence on buildings performed routinely should be taking into account all geological factors which can influence the size and deformation distribution. There are many factors, the most significant of them are: exploitations performed previously, hydrogeological conditions and tectonic disturbances. Especially tectonic faults are a common reason of increased values of deformation indices, and even discontinuous deformations. They also influence the increase of values of vibrations accelerations of mining ground surface. An example of this kind of influence of the fault zone is presented in this work.

Lithology and stratigraphy
The rock mass located in the area of the building consists of layers from following periods: The overburden consists of quaternary and tertiary layers (Neogene). Carboniferous layers consist mainly of Pleistocene layers, developed as grey and yellow-grey clays stone, sands and fluvioglacial gravels.

Tectonics
The tectonic fault named Fault V is in the area of the building. It runs from north-east towards south-west direction. The throw slip in the south-eastern direction equals 70 m. The fault outcrop on the carbonic roof runs in the distance of around 150 m in the northwestern direction from the building, what is presented in the fig.1.

Mining exploitations which were performed
The table 1, shown below, presents basic information about exploitations performed since 2005. Values of deformation indices in the location of the building were calculated. The table gives information about: time of exploitation performed with particular walls, thickness of coal seam or layer, angle (slope) of the seam, depth on which the seam lay in the area of the wall, distance from the building, direction of walls location in relation to the building, value of coefficient of exploitation, the way of roof control. Figure 1 shows the location of performed exploitations in relation to the building, which is marked as number "15".

Information about the building
The object is a residential building, single-family house with two floors. It was built in 1975 in traditional brick technology. It was protected from exploitation influences by anchoring. The inclination of the building as a result of the impact of exploitations performed in previous years was visible. Many horizontal and diagonal cracks on internal and external walls have been observed since 2015. These cracks occurred mostly where the walls were weakened, so that near doors and windows ( fig. 2). No slots in walls have been observed. No linear discontinuous deformations were observed in the area of the building.

Influence of performed exploitations on the building in the light of the results of calculations
Computer programme DEFK-Win [6,7], based on W. Budryk's -S. Knothe's theory [8] was used for calculations of values of deformation indices. The accuracy of the theory has been discussed in paper [9].
The following values of parameters were adopted for calculations on the basis of experience of the coal mine:

Impact of performed exploitations on the surface in the area of the building in the light of geodetic survey.
A measuring line, consisting of observing points of numbers from 62 to 67 was set along the street in the area of the building. Observing points number 66 and 67 are located nearest the building, in the distance of around 60 m. Surveys were performed at different intervals since 2007 till 2017.
The course of subsidence of observing points in time is presented in fig. 3, and the increases of subsidence are shown in fig. 4 (1) where :

Influence of tremors on the object
a -acceleration of vibrations, mm/s 2  table 3. This table also shows values of calculated maximal accelerations and the duration of an intense phase of accelerations of vibrations t Ha . As it can be noticed, the highest registered value was 629 mm/s 2 . Values of calculated accelerations for all tremors were lower than measured ones. It is interesting that the tremor which occurred on 02.01.2017 had similar epicentre coordinates and energy to the tremor which occurred on 27.01.2017, and these tremors caused significantly different accelerations of vibrations. The mine rejects the possibility of having made a measurement mistake. These results, so different from expected, were most probably caused by occurrence of the fault zone in the area of the building. The influence of tectonic faults on seismic activity of the rock mass was a subject of many studies [10,11,12,13,14]. A wide overview can be found in the work [15]. The author shows two types of seismic phenomenon. First of them are the occurrences directly related to mining works, which characterise with low epicentral energies. Epicentres of these tremors occur in the area of mining extraction. The second type is related to impact of fault zone by running an exploitation in its neighbourhood. Thus, the combined effect of mining and tectonic factors takes place. Tremors like that characterise with higher energies and their epicentres occur in the area of fault zone. Works like [10] confirm this. Similar cases can be found in foreign literature for example the work [11] shows the case where two tremors have occurred in the gold ore mine in South Africa. The epicentres were located at the depth of around 2 km in the distance of 700m from each other. These tremors have been caused by the mining exploitation and the influence of the fault zone. The influence of tectonics of the deposit on damage of buildings as a result of highly energetic tremor is presented in work [12]. The work presents correlations between accelerations and speeds of vibrations in the location of damaged buildings and distance from fault zone.
It is confirmed by the results of analyses presented in the paper [14], which describes the occurrence of very high amplitudes of accelerations of vibrations in the measuring station in the area of the fault zone. Authors expressed their opinion, that a significant reason for this, was directionality of radiation. It was concluded that tremors characterising with similar energy and distance from measuring station, can cause different amplitudes of vibrations accelerations of the ground surface.
GSI scales are being constantly developed and actualized [16]. Regarding the scale GSI GZWKW -2012-A these tremors can be classified to extents shown in the table. The tremor which caused the highest value of accelerations can be classified to extent II. Due to small distance from the measuring station to the object it can be assumed that the values of vibrations accelerations could have been higher than the calculated ones, also in the location of the building. Therefore there are justified assumptions that the building could have been exposed to maximal accelerations of around 600 mm/s 2 , which can be classified to extent II in GSI GZWKW -2012-A scale.
The locations of measuring station and the object have been marked in the figure 5 (small red marks). Also tremors epicentres, data about which can be found in table 4, have been marked. Diameter of the circle represents energy of the tremor. Additionally, for easier distinction, the tremor of energy 4.00E+09 J has been marked with yellow. Epicentres of other tremors were marked with blue colour.

Summary and conclusions
Analyses of: mining and geological conditions, results of calculations of values of deformation indices and results of surveys and also impact of the tremors on the surface, performed in this work allow us to conclude: 1. Continuous deformations of the mining area, which developed as a result of exploitations performed in recent years (since 2005) were minor and fit in category I of the mining area. Results of calculations broadly correlate with results of geodetic surveys, which confirm that the surface was not recently influenced by any subsidence. In some cases, however, this compatibility does not occur in tectonically disturbed zone. This is why having access to results of geodetic surveys, which can verify and complete results of calculations, is very important. 2. Analysis of results of calculations and surveys of vibrations accelerations, caused by rock mass tremors, implies as influence of the fault on increase of values of vibrations accelerations of mining ground surface. Formula, which was used, refers to tectonically undisturbed rock mass, which is the reason for observing higher values of vibrations accelerations than calculated ones and even lack of their negative influence on the object. So that also results of surveys of vibrations accelerations proved to be essential for an objective assessment of the impact of mining exploitations on the surface. 3. Mining and geological conditions of the area, including tectonical disturbances, should be considered while assessing the impact of mining works on buildings, since they can influence the size and distribution of deformations, but also they can increase the values of vibrations accelerations on the surface.