Geology is the branch of science that deals with the study of earth, its history, structures and the processes that act upon it.
Engineering Geology on the statutes of IAEG is defined as the science devoted to investigation, study, and solution of environmental and engineering problems which may arise due to the interaction between the geology and human activities and also the prediction of or development of measures to control such hazards.
Scope of Engineering Geology
- Nature of soil materials can be found.
- Useful for river control and construction of dams, highways.
- Provides knowledge about materials used in construction.
Engineering Geological System (EGS)
The lithology of the intact rock, structural geological information, tectonics of the area, mechanical properties of the rock mass, mechanical behaviour of the ground mass, earthquake and seismicity and hydrogeological condition of the area are the important parameters for the evaluation of EGS.
Lithology provides information on composition, texture, structure and origin of rocks.
- Geological Structures
Geological structures are classified as primary structure and secondary structure. Primary structures are those structures that are formed during the formation of rocks. Secondary structures are those structures that are formed after the formation of rocks.
(Detail on primary and secondary structures are discussed in Geology I).
Hydrogeology is the branch of geology that refers to the study of groundwater and the geologic process of surface water.
Geomorphology deals with the form and morphology of the earth surface.
Weathering is defined as the phenomenon in which breaking of Earth's rocks, soils and minerals occur through direct contact with the atmosphere.
It is defined as the shaking or trembling caused by the sudden release of energy.
EGS occurs in four different phases as Planning, Design, Construction, and Maintenance phases.
Important Rock Forming Minerals and Their Engineering Significance
- Important Rock Forming Minerals
- Quartz - SiO2
- ORTHOCLASE - KAlSi3O8
- PLAGIOCLASE - NaAlSi3O -- CaAl3Si2O
- MUSCOVITE - KAl3Si3O10(OH)2
- BIOTITE - K(Mg,Fe)3AlSi3O10(OH)2
- Ferromagnesian Minerals
- HORNBLENDE - Ca2Na(Mg,Fe)4(Al,Fe,Ti)3Si6O22(O,H)2
- PYROXENE - Ca(Mg,Fe,Al)(Si,Al)2O6
- OLIVINE - (Fe,Mg)2SiO4
- Calcite - CaCO3
- Gypsum - CaSO4.2H2O
- Halite - NaCl
- Engineering Significance
- The response of rocks or minerals to weathering, erosion, and tectonic processes are useful in making estimates of site condition.
- Different minerals have different weathering stability. i.e. clay minerals swell after saturation with water.
- Carbonate minerals dissolve completely in water which contains dissolved CO2 .
- The minerals which are crystallized at greater depth are chemically unstable at surface conditions.
- The chemical weathering changes the certain silicate minerals like feldspar into insoluble clay minerals.
- Orientation of platy micas and foliated structure reduce the strength of the rock.
- Deformed and fractured minerals found in metamorphic rocks may reduce the strength of the rock.
Engineering Geological Maps: Their Classification and Preparation
The graphical representation of purpose specific geological information or data obtained from the field which can be quantified with reference to the north at the certain scale in engineering geological map. Maps used for engineering geologic applications includes:
- site or project specific soil and rock units.
- drainage characteristics, and slope and other conditions related to development, such as existing cut and fill slopes, roads, proposed building footprints, etc.
Mapping should be done on a suitable plan metric or topographic base map or aerial photograph, at an appropriate scale with satisfactory horizontal and vertical control. Each map or photo should include the date and source of the base.
They can be classified as analytical and comprehensive. The analytical map provides information on individual components of the geological environment (deals about seismic hazard). The comprehensive map provides information on geological zoning.
Dhar, M.S., and Ghimire, P.C., 2006, Engineering Geology, Swargadwari Offset Press, Sankhamul, Kathmandu.