Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1
Fig. 3.2. Using of observations, measurements and classifications in rock mechanics
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- 3.2. Rock Mechanics
- 3.1.1. 3.2.1. Uniaxial compressive strength
- Class Name Rock name Compressive strength σ c
- Table 3.1. Classification of rocks on the base of their compressive strength after Protodjakonov (Gálos-Vásárhelyi 2006)
- Fig. 3.3. Compressive strength odf several rocks (Gálos – Vásárhelyi 2006)
- Fig. 3.4., 3.5.
- Fig. 3.4. Method of tension strength examination (Gálos-Vásárhelyi 2006)
Fig. 3.2. Using of observations, measurements and classifications in rock mechanics (Gálos – Vásárhelyi 2006) 3.1. 3.1. The practice One of the most important roles of the engineering geologist is the interpretation of landforms and earth processes to identify potential geologic and related man-made hazards that may impact civil structures and human development. Nearly all engineering geologists are initially trained and educated in geology, primarily during their undergraduate education. This background in geology provides the engineering geologist with an understanding of how the earth works, which is crucial in mitigating for earth related hazards. Most engineering geologists also have graduate degrees where they have gained specialized education and training in soil mechanics, rock mechanics, geotechnics, groundwater, hydrology, and civil design. These two aspects of the engineering geologists' education provides them with a unique ability to understand and mitigate for hazards associated with earth-structure interactions. An engineering geologist or geophysicist may be called upon to evaluate the excavatability (i.e. rippability) of earth (rock) materials to assess the need for pre-blasting during earthwork construction, as well as associated impacts due to vibration during blasting on projects. 3.2. Rock Mechanics Soil mechanics is a discipline that applies principles of engineering mechanics, e.g. kinematics, dynamics, fluid mechanics, and mechanics of material, to predict the mechanical behavior of soils. Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses; it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment. The fundamental processes are all related to the behaviour of porous media. Together, soil and rock mechanics are the basis for solving many engineering geologic problems. 3.1.1. 3.2.1. Uniaxial compressive strength Compressive strength is the capacity of rocks to withstand axial loads tending to reduce size. It provides data (or a plot) of force vs deformation for the conditions of the test method. When the limit of compressive strength is reached, brittle materials are crushed. Concrete can be made to have high compressive strength, e.g. many concrete structures have compressive strengths in excess of 50 MPa, whereas a material such as soft sandstone may have a compressive strength as low as 5 or 10 MPa. By definition, the compressive strength of a material is that value of uniaxial compressive stress reached when the material fails completely. The compressive strength is usually obtained experimentally by means of a compressive test. The apparatus used for this experiment is the same as that used in a tensile test (Table 3.1.).
Table 3.1. Classification of rocks on the base of their compressive strength after Protodjakonov (Gálos-Vásárhelyi 2006) However, rather than applying a uniaxial tensile load, a uniaxial compressive load is applied. As can be imagined, the specimen (usually cylindrical) is shortened as well as spread laterally. The uniaxial compressive strength (UCS) test is an important tool used to characterize geomechanical rock behaviour. It is important because the results, ie the compressive strength of rock, allows the study of a slope’s stability for mines and road slopes, indicates the applicability of the rock for the manufacture of coated plates, can be used in the classification of rock masses, and to infer the behaviour of a substrate Rocky to support civilian structures, amongst others (Fig. 3.3.). 
Fig. 3.3. Compressive strength odf several rocks (Gálos – Vásárhelyi 2006) In engineering design practice we mostly rely on the engineering stress. In reality, the truestress is different from the engineering stress. Hence calculating the compressive strength of a material from the given equations will not yield an accurate result. The difference in values may therefore be summarized as follows: · On compression, the specimen will shorten. The material will tend to spread in the lateral direction and hence increase the cross sectional area. · In a compression test the specimen is clamped at the edges. For this reason, a frictional force arises which will oppose the lateral spread. This means that work has to be done to oppose this frictional force hence increasing the energy consumed during the process. This results in a slightly inaccurate value of stress which is obtained from the experiment (Fig. 3.4., 3.5.). 
Fig. 3.4. Method of tension strength examination (Gálos-Vásárhelyi 2006) As a final note, it should be mentioned that the frictional force mentioned in the second point is not constant for the entire cross section of the specimen. It varies from a minimum at the centre to a maximum at the edges. Due to this a phenomenon known as barrelling occurs where the specimen attains a barrel shape. 
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