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ENGINEERING GEOLOGY

ENGINEERING GEOLOGY. ENGINEERING GEOLOGY Chapter 1.0 : Introduction to engineering geology Chapter 2.0 : Rock classification Igneous rocks Sedimentary rocks Metamorphic rocks Chapter 3.0: Weathering & soils Chapter 4.0: Geological structures & discontinuities in rock.

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ENGINEERING GEOLOGY

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  1. ENGINEERING GEOLOGY ENGINEERING GEOLOGY • Chapter1.0: Introduction to engineering geology • Chapter 2.0: Rock classification • Igneous rocks • Sedimentary rocks • Metamorphic rocks • Chapter 3.0: Weathering & soils • Chapter 4.0: Geological structures & discontinuities in rock. • Chapter 5.0: Ground Investigation.

  2. Table 1.5: Rock classification [Waltham, 2002].

  3. Metamorphic Rocks: • Metamorphic rocks are created by changes induced at high temperature (up to 6000C) and/or high pressures (around 500 MPa at 20 km depth). These changes (metamorphism) take place in solid state. • Pressure & temperature must act for a long time. • The type of metamorphic rock produced depends on the original rock material that was metamorphosed & the temperature & pressure condition which were imposed.

  4. Metamorphic Rocks: • Minerals in rock are stable (chemically & physically) at specific range of temperatures & pressures. When this range is exceeded metamorphism can occur in rock. • Two types of changes can occur that differentiate the metamorphosed rock from its original rock: Melting & recrystallisation of minerals that form new types of mineral. Physical change in shape & rearrangement of minerals in rock.

  5. Metamorphic Rocks: • Types of metamorphism are: Regional metamorphism – involves high pressure & temperature. Occurs in mountain chains due to continental collision on plate boundaries or, on deep seated rock body (few 10 of km’s). Extend over a large areas. Thermal or contact metamorphism – involves high temperature only. Occurs around igneous intrusions where rock has been baked. Dynamic metamorphism - less common & very localised. Occur in fault zones where there are relative movements between large rock blocks.

  6. Rock body deep in the earth’s crust is subjected to a prolong high pressure (overburden stress) & temperature, these can metamorphose the rock.

  7. Fault - relative movements (vertical or horizontal) occurs between large rock blocks (produce high T)

  8. Fold– relative movement occurs between folded beddings. Friction between folded strata produces high temperature & occasionally some pressure.

  9. Progressive regional metamorphism, from low-grade (slate) to high-grade metamorphism (gneiss)

  10. Metamorphic Rocks: • Grade of metamorphism is the overall extent of change, notably in the sequence (within regional metamorphism) from slate to schist to gneiss. • The grades are L.OW, MEDIUM & HIGH • To change shale  slate it requires low grade metamorphism; to change slate  schist it requires medium grade; to change granite gneiss it requires high grade.

  11. The typical transition in mineralogy that result from progressive grade in metamorphism

  12. Metamorphic Rocks: • New minerals are formed at the expense of less stable minerals, in the new conditions of high pressure & temperature. • Most important changes are clay minerals  micas  feldspars & mafics. Micas are the most significant minerals in metamorphic rocks & only change to feldspars at the highest grade of metamorphism. • Directional pressure within the solid state creates mineral orientation within the regionally metamorphosed rocks. New minerals grow in the line of least resistance (perpendicular to max. P).

  13. Metamorphic Rocks: • Planar weaknesses in the foliated metamorphic rocks are created by the parallel micas splitting along their minerals cleavage – causing rock cleavage (also known as slaty cleavage) & schistosity. • Non-foliated metamorphic rocks have stronger isotropic structure. These include hornfels, formed by thermal metamorphism of clay without high pressure; also marble & gneiss with little or no mica.

  14. Rearrangement of minerals in direction perperdicular to max. pressure & change of mineral shape after metamorphism

  15. Metamorphic texture – rotation of platy or elongated mineral grains (e.g. mica in granite)

  16. Metamorphic texture – flattened rock exhibiting distorted quartz grains.

  17. Minerals arrangement in metamorphic rock graphite (original rock contains high % of mica)

  18. Metamorphic rock such as mica schist, minerals arrangement after metamorphism is more distinctive, this is due to original rock (shale) contains high % of flaky mineral like mica (M). Quartz, massive mineral, does not display any change in shape.

  19. Metamorphic rock like gneiss, rearrangement of minerals after metamorphism is less distinctive, for its original rock (granite) contains high % of massive minerals (quartz, feldspar & hornblende) & less mica.

  20. Regional & contact metamorphic rock – SCHIST (originates from shale or slate)

  21. Contact metamorphic rock – SLATE (originates from shale)

  22. Contact metamorphic rock – QUARTZITE (originates from sandstone)

  23. Regional metamorphic rock – MARBLE (originates from limestone)

  24. Regional metamorphic rock – DOLOMITE (originates from limestone)

  25. Table 2.12: Types of metamorphic rock, texture & grade of metamorphism (McLean & Gribble 1980)

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