Due to the euphoria of people to make the impossible possible, increasingly complex construction projects are being planned and implemented. As a rule, projects are currently carried out under high cost and time pressure. This results in the need to optimize known processes in order to save additional time and costs and to continue to do justice to dwindling resources and the growing world population in the future. In the course of population growth and the shortage of space, the urbanized areas of the cities must be connected or expanded underground. Depending on the type of soil, solid rock in the mountains can pose a challenge due to its high strength. This increases the demand for pretreatment of solid rock. A possible solution is the use of microwaves.
1.1
Around 1864, the Scottish physicist James Clerk provided the first mathematical
description of electromagnetic waves, which had been partly carried out by the physicist
Ernst Lecher some years earlier through experiments
Percy Spencer developed the first microwave oven in 1950
But microwaves can also do more than just heat food: They enrich our everyday lives
with wireless communication systems such as mobile radio, Bluetooth, WLAN or
radar technology
For microwave irradiation of solid rock, first theoretical simulations exist to simplify
known drilling, blasting and digging techniques. Challenges such as high wear due to
abrasiveness, high material consumption, cost-intensive and time-consuming
processes or high dust emissions can be positively influenced or controlled. Therefore,
pretreatment with microwave radiation is an efficient component, since the microwaves
cause stresses in the rock compound and thus exceeds the critical load limit
The distinction of the rocks is made according to the type of formation. The hardest naturally occurring rocks are e.g. granites or basalts, which are called solidification rocks or magmatites (Figure 2, right). These rocks are formed by solidified magma on the earth's surface (effusive rock) or inside the earth (deep rock). Sandstones and sand-lime bricks, on the other hand, are called sedimentary or sedimentary rocks (Figure 2, left), which were deposited in the earth's crust by weathering and transport and subsequently solidified by pressure from superimposed masses. If existing solidification and sedimentary rocks are further influenced by forces such as foliation, uneven pressures, compression or rolling, the coarse structure can change and a granite, for example, becomes gneiss. These are called transformation rocks or metamorphites. For irradiation with microwaves, the properties of a coherent rock, especially discontinuities and the influence of water, have to be considered. In order to be able to represent an entire rock mass, it must be examined for intact rock, i.e. rock that does not have any continuous interfaces, and for the actually existing interfaces using laboratory and field techniques. Only then can a statement be made on the overall behaviour of the rock through the interaction between rock, parting surfaces and mountain water. An illustration of this can be found in Figure 3. Based on these findings, it becomes clear that the strength of the rock is influenced by the rock-mechanical interaction between the compact rock and the rock faces. However, the separating surfaces can have different properties depending on their type. Here the degree of separation (possibly existing material bridges and the thickness of the parting surface), parting surface distances and spatial orientation of the parting surface distance and the parting surface covering play a decisive role. Sandstone is an example of a rock with a high degree of separation and a small separation surface distance. In sandstone, the strength properties of the interface result in lower strengths compared to a rock with a low degree of separation and larger interface spacings, e.g. granite. For the example of granite, higher rock strengths can be assumed, since the rock mechanical influence of the parting surfaces is smaller. A factor not to be neglected which has not yet been mentioned in Figure 3 is the influence of water. With compact rock in the mountains it can be assumed that no water can penetrate. However, separation surfaces with a high degree of separation and a small separation surface distance offer sufficient space to be able to collect water. Existing water plays a decisive role in the irradiation of microwaves and this has to be investigated by the boundary conditions of the interfaces. 2
The technique of "pretreating" the solid rock was used until the 16th century for
"setting fire". The rock was heated by fire in order to loosen the dressing due to the
different thermal expansion of different types of rock
At the beginning of the 21st century, microwaves were used to measure moisture in
buildings. Here the relative humidity content between water and building material is
measured without affecting the condition of existing buildings. The electromagnetic
alternating field applied to the component from the outside causes the molecules of
the component to rotate. Which molecules are set in motion can be controlled by
power and frequency
The research of Hartlieb et al. (2017) investigated the forces that occur when cutting
microwave irradiated and untreated granite. The treated area of the granite showed
that pronounced crack networks run through the rock. Irradiation with microwaves at
2,450 MHz with 24 kW significantly reduced the measured peak and average forces
of the treated granite
Similar studies in Australia show that higher power and shorter irradiation times can
heat up the rock more quickly or cause it to partially melt
In previous research work, only magmatic and metamorphic rocks have been investigated, which as a rule have only a very low degree of separation and in which the interfaces play only a minor role. The influence of microwave irradiation on sedimentary rocks, for example, where the interfaces are the decisive factor, has not yet been investigated. The irradiation itself and the resulting thermal expansion is carried out similarly to Hartlieb et al. (2011).
The aim of the dissertation is to analyse and evaluate the effects of microwave irradiation on solid rock or rock that is difficult to dissolve in accordance with Class 7 of DIN EN ISO 14688-1, both qualitatively and quantitatively. The interaction of irradiation duration and intensity and the differentiated investigation of material and composition of the solid rock are the key points of this dissertation. Furthermore, further findings on the properties of heated solid rock with the effect on the bond and the influence of interfaces occurring in the rock can be investigated. The result of this work in a scientific sense is the knowledge regarding altered rock mechanical properties of solid rock. Based on this, a recommendation for a more effective mining method can be developed.
The assessment of the changed rock mechanical properties, in particular in connection with interfaces of solid rock, is in the foreground here. Since in tunnelling and mining the compressive strength is particularly relevant for the mining and loosening of rock material, special attention is paid to the reduction of compressive strength by microwave irradiation. The dimensioning of tunnel boring machines or conventional excavation methods is determined to a large extent by the compressive strength of the rock, whereby better results can be achieved in the execution phase in terms of cost savings and environmental compatibility. 4 4.1
The work includes a theoretical and an experimental focus. The theoretical part contains the basics of electromagnetic waves and how they can be applied in building practice and how they can be refined in the future. Separated from this, an introduction to rock mechanics is given, whereby rock and rock properties, existing interfaces, stresses in the rock and the influence of water are dealt with in more detail. In the experimental section own model experiments are carried out and solid rocks in their input and output state are analytically investigated, i.e. before, during and after irradiation with consideration of heat development, change of strength and stress development.
The first step is to prepare the state of the art and research. The basis for this are publications on the basics of microwave technology and the definition and differentiation of solid rocks within their soil class. In particular, the problems of foundation engineering and tunnel construction will be dealt with. A further focus will be the representation of natural interfaces in solid rock. Furthermore the consideration follows, to what extent laboratory tests with microwaves were accomplished by third parties and these play a role for own methods. The publications are compiled, evaluated and general questions concerning the mining of hard and solid rock are clarified. 4.3
At the beginning of the laboratory experiments an analytical recording of different rock types and the differentiation or problem of the laboratory conditions in comparison to the natural occurrence is carried out. The central task here is to what extent the experiments can be investigated or reproduced as realistically as possible. In order to limit the deviations, a large number of specimens and tests are required. In order to distinguish themselves from works such as Satish et al. (2006) and Hartlieb et al. (2011), sedimentary rocks and metamorphites are increasingly used as rocks. First, the specimens are obtained in standardised sizes from regional quarries, adapted to the irradiation equipment and the subsequent laboratory tests. A microwave with up to 3.2 kW power is used as the irradiation device. The selection of solid rocks with uniform and different microstructures is determined in advance. The samples are analysed according to their grain and large structure in order to record the characteristic values and fluctuation ranges within the rock groups. Since the electric field in the microwave will react particularly to water components in the rock structure, the water content must be determined in advance. Some selected samples are subjected to a rock analysis by thin section or X-ray diffraction. This allows the qualitative and quantitative evaluation of the mineral content to be verified.
In the case of solid rocks with pronounced interface structures, e.g. sedimentary rocks, standardized specimens, such as those required for uniaxial compression tests, can only be obtained to a limited extent. More often only specimens with nonstandardized dimensions are available. Therefore, in the next step, non-standardized samples of different geometries are collected and evaluated accordingly. It can be assumed that the heat propagation will be distributed unevenly. Here the question is clarified to what extent the geometry and composition of e.g. sedimentary rocks with high discontinuity behave in contrast to magmatic or metamorphic rocks with a low degree of separation.
In order to measure the heat development on the surface and in the core of the sample, the specimen is cut along the core, reassembled for irradiation in the microwave and after irradiation the temperature is measured at various points, in particular the surface and core temperature. The procedure is analogous to Hartlieb et al. (2011). Infrared cameras are used to measure the temperature. After irradiation, they illuminate the interior and exterior of the rock and thus provide information about the temperature distribution. Since the temperature distribution will behave differently depending on the type of rock, this process must be repeated several times. Therefore, an alternative measuring method is to be developed within the framework of the experiments, so that the temperature measurement can be carried out without affecting the sample beforehand, e.g. by cutting or drilling. Conventional temperature sensors will influence the electric field in the microwave, which is why, for example, fiber-optic temperature measurement can be used. Optical sensors can be used to measure intensity, spectral distribution of the wavelength, time dependence due to frequency, pulse duration and decay and, if modified accordingly, strain changes. These can be analytically investigated in the following process.
After irradiation, the specimens are examined in the laboratory. Special attention is paid to a possible decrease of the compressive strength. The tests are carried out according to Mutschler (2004) in order to implement the requirements of the test equipment, test specimens, procedure for the execution and evaluation of uniaxial compression tests in standardized procedures. Less interesting for the construction industry, but relevant for the general scientific consideration of changes in the rock mechanical properties of solid rock, is the point load test according to Thuro (2010) and the splitting tensile test according to Lepique (2008).
Subsequently, the results are evaluated and the tests carried out with different input values are compiled. It can be assumed that the different variants of the temperature measurement and the choice of geometries will influence the output values accordingly.