23/9/2013 ENGINEERING GEOLOGY. Chapter 2: Rock classification:

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1 ENGINEERING GEOLOGY Chapter 2: Rock classification: 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. Chapter 5.0: Ground Investigation. ROCK & MINERALS: Rocks: mixture of minerals of variable properties. Minerals: compound of elements of fixed properties (Table 1.1). Rock properties broadly depend on: Strength & stability of constituent minerals. Interlocking or weaknesses of mineral structure. Fracture, bedding & larger rock structures. Most rock-forming minerals are silicate minerals, compound of oxygen & silicon, these are classified as dark- & light-colored minerals Table 2.2 & 2.3. Element Symbol % of earth s crust Oxygen O Silicon Si Aluminium Al 7.51 Iron Fe 4.70 Calcium Ca 3.39 Sodium Na 2.64 Potassium K 2.40 Magnesium Mg 1.94 Total % % of other elements 2.15 Mineral Colour SG Hardness Cleavages Olivine Green/dark green None (poor fracture) Pyroxene Black/brown (augite) Hornblende Black Biotite Brown (perfect) Garnet Red (variable) None Table 2.1: Composition of element in earth s crust Table 2.2: Dark-coloured mineral silicates (mafic) & their physical properties Mineral Colour SG Hardness Cleavages Feldspars White, pink, variable Clays White perfect Quartz Colourless, white, red, None variable Muscovite Colourless perfect Table 2.3: Light-coloured mineral silicates & their physical properties Cleavage (fracture) in minerals 1

$fracture) Mineral quartz: no$ $cleavage, fracture like$ quartz: no glass

$fracture planes Mineral$ $cleavage, fracture in form$ of thin sheet Mineral

2 Cleavage direction (mineral fracture) Mineral quartz: no cleavage, fracture like glass (conchoidal) Mineral quartz: no cleavage, fracture like glass (conchoidal) Microcline (feldspar): 2 set of fracture planes Mineral biotite (black mica): 1 set cleavage, fracture in form of thin sheet Mineral calcite : 3 set of cleavage, fracture in cubic form 2

By definition rocks are aggregates of one or more minerals (i.e. mineral silicates) Classification of rocks: Rock properties can show extreme variations but generalisation as Table 2.

3 By definition rocks are aggregates of one or more minerals (i.e. mineral silicates) Classification of rocks: Rock properties can show extreme variations but generalisation as Table 2.4 is useful in order to build an understanding of geology. However it must be accepted that rocks are not engineered materials and their properties vary from site to site. All rocks fall into one of three families namely: IGNEOUS ROCKS SEDIMENTARY ROCKS METAMORPHIC ROCKS Each group with broadly definable origins & properties. ROCK FAMILY Material origin Environment Rock texture Rock structure Rock strength Major types IGNEOUS SEDIMENTARY METAMORPHIC Crystalline from molten magma In earth crust; & as lava flow Mosaic of interlocking crystals Non-bedded (structureless) Erosional debris on Earth s surface Deposition basins; mainly sea Mostly granular and cemented Layered, bedding planes Uniform high strength Variable, low; planar weaknesses Granite, basalt, pumice Sandstone, limestone, shale Altered by heat and/or pressure Mostly deep inside mountain chains Mosaic of interlocking crystals Crystal orientation due to pressure Variable high; planar weaknesses Schist, slate, gneiss, quatzite Table 2.4: Rock classification [Waltham, 2002]. Igneous rocks: Magma is generated by local heating & melting of rocks within the earth s crust, mostly at depths ( km). Most composition of rock melt at temperature C. When the magma cools (as it flows up wards to the surface) it solidifies by crystallisation into a mosaic (interlocking) of minerals, to form various types of igneous rock. Volcanic eruptions may be violent & explosive if a viscous magma has a high gas pressure, or may be quiet & effusive if the magma is very fluid. There is a continuous flowing of magma into the earth s crusts (intrusive) & on the earth s surface (extrusive), in the past & at present. Extrusive igneous rocks: These form where magma extrudes onto the earth s surface to create a volcano. Lava is the name for both molten rock on the surface & also the solid rock formed when it cools. Intrusive igneous rocks: These are form when magma solidifies below the earth s surface. They may later be exposed at the surface when the cover rocks are eroded away, or when new magma flow pushes the intrusive igneous rock to the surface - uplifting (e.g. the Main Range). Batholiths are large blob-shaped intrusions, roughly equidimensional & commonly 5 50 km in diameter. Most are granite. Dykes are smaller sheet intrusions formed where magma flows into a fissure. Mostly 1 50 m wide & extends for many kms (see forms of igneous rocks). 3

4 Forms of igneous rock: rock bodies form by cooling & crystallization of magma in & on the earth s crust Granite intrusive igneous rock (acid, plutonic) Gabbro intrusive igneous rock (basic, plutonic) Basalt extrusive igneous rock (basic, volcanic) Rhyolite extrusive igneous rock (volcanic) Pumice Extrusive igneous rock (volcanic) 4

Intrusive & extrusive igneous rocks: Most common intrusive igneous rocks are granite, gabbro & granodiorite, & for extrusive igneous rocks

Granite: Acid/felsic igneous rock; coarse-grained (holocrystalline); large-scale intrusive (plutonic).

Mineral content mainly quartz 25%, feldspar 50%, micas 15% & mafic minerals 10%.

Spheroidal weathering leaves rounded corestones in soil matrix.

5 Intrusive & extrusive igneous rocks: Most common intrusive igneous rocks are granite, gabbro & granodiorite, & for extrusive igneous rocks are basalt, rhyolite & pumice. Texture of igneous rocks due to the locations where magma (lava) crystallizes. Granite: Acid/felsic igneous rock; coarse-grained (holocrystalline); large-scale intrusive (plutonic). Texture coarse interlocking crystal mosaic with random arrangement of minerals (aphanitic texture). Mineral content mainly quartz 25%, feldspar 50%, micas 15% & mafic minerals 10%. Granite: Weathering is mainly slow decomposition of feldspar to clay minerals & leaving quatrz to form sandy soils. Spheroidal weathering leaves rounded corestones in soil matrix. Strength is high with all physical properties good; UCS 200 MPa, Safe Bearing Press. (SBP) 10M Pa. As foundations it is a very strong rock, except where partially decomposed to clayey soils near the surface or along deep joint zone. Spheroidal weathering (onion peeling) of granite Spheroidal weathering (onion peeling) of granite Corestones in soil matrix in construction this creates problem in excavation 5

Basalt: Exposed corestones due to excavation Basic/mafic igneous rock fine-grained (holohyaline), extrusive (volcanic).

Mineral content mainly feldspar 50%, & mafic minerals 50%. Weathering is rusts (iron staining) & decompose to clayey soils, may be spheroidal weathering.

Classification of igneous rocks: Depending on the composition (%) of light- & darkcoloured mineral silicates, igneous rock can be classified as Ultrabasic/Ultramafic, Basic/Mafic,

6 Weathering profile various zones (1 to 6). Corestones is found in zone 4 & 5. Zone 6 is residual soils. Exposed corestones due to excavation. Corestones is characteristic of weathering zone in granite rock. Basalt: Exposed corestones due to excavation Basic/mafic igneous rock fine-grained (holohyaline), extrusive (volcanic). Texture is fine (phaneritic or glassy texture) interlocking crystal mosaic with no textural orientation, may have vesicles (old gas bubbles). Mineral content mainly feldspar 50%, & mafic minerals 50%. Weathering is rusts (iron staining) & decompose to clayey soils, may be spheroidal weathering. Compact basalt are very strong UCS 250 MPa & SBP 10 MPa (less on young lava). Good aggregates & valuable roadstone. Classification of igneous rocks: Depending on the composition (%) of light- & darkcoloured mineral silicates, igneous rock can be classified as Ultrabasic/Ultramafic, Basic/Mafic, Intermediate/Andesitic & Acid/Felsic ( ph). This simple classification are based on 2 basis; colour & size of mineral grains. When a magma body cools down (as it rises to the surface), each mineral crystallises at different temperature (Bowen s reaction series - order of crystallisation). This process leads to the formation of different types of igneous rock (each with different mineralogy, colour & grain size), although they originate from the same magma body. Bowen s reaction series & composition of igneous rocks. 6

Order of crystallisation: Crystallisation (solidify) of part of magma body at early stage (deep in the earth s crust & high temperature) forms intrusive igneous rocks with large mineral crystals due

Classification of igneous rock is as Table 1.5 & 1.6. Crystallisation at early stage (temp ) produces ultrabasic igneous with minerals composition of olivine, augite & plagioclase (Ca-rich).

7 Order of crystallisation: Crystallisation (solidify) of part of magma body at early stage (deep in the earth s crust & high temperature) forms intrusive igneous rocks with large mineral crystals due to slow cooling process. Cooling of the remaining magma body at later stage (close to & on earth s surface & low temperature) forms extrusive igneous rocks, with fine-grained due to fast cooling. Classification of igneous rock is as Table 1.5 & 1.6. Crystallisation at early stage (temp ) produces ultrabasic igneous with minerals composition of olivine, augite & plagioclase (Ca-rich). Crystallisation at later stage (temp ) produces acid igneous rocks with minerals composition quartz, orthoclase, plagioclase (Na-rich) & mica (biotite & muscovite). In general acid igneous rock is light colour (e.g. granite) & basic igneous rock is dark colour (e.g. basalt). Boundary for these 4 classes is not that clear, but it can be generalised that when igneous rocks do not contain quartz (Si O 2 ) or olivine, they can be classified as basic or acid, respectively. Table 2.5: Minerals composition of igneous rocks resulting from differential crystallisation Table 2.6: Minerals composition (approx. %) of igneous rocks resulting from differential crystallisation Mineralogy of common igneous rocks & the magmas from which they form. Classification of igneous rocks: The form of occurrence determines the structure of the igneous rock; also, lavas may cool in hours or days while batholith may take million years to cool (crystallise). The cooling rate determines the grain size of the rock. Chemical composition is determined by what rocks had melted to form the original magma; silica rich magmas are referred to as acidic & are generally low in black iron minerals, & therefore lighter in colour than basic rocks. In fine-grained (holohyaline) rocks grains cannot be seen with the naked eye, the limit is 0.1 mm (similar to 0.06 mm in soils). 7

9/4/2015. Feldspars White, pink, variable Clays White perfect Quartz Colourless, white, red, None

9/4/2015. Feldspars White, pink, variable Clays White perfect Quartz Colourless, white, red, None 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