Engineering Geology & Soil Mechanics

Engineering Geology & Soil Mechanics
Assignment 01:
Understand the common rock types, classification of soil types and establishment of the
primary design parameters for soils.

BTEC HND in Civil
Engineering
Acknowledgement
I take this opportunity to thank and appreciate Miss. Sushama De Silva, guiding
me throughout this Engineering Geology & Soil Mechanics Module and helping me with all his
experiences and knowledge by being a brilliant teacher in this second semester. Stories, working
tricks and teaching rhythm always makes me more interested and knowledgeable about this
module.
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Introduction
As Civil batch 05 students starts their second semester at ICBT campus.
Engineering Geology & Soil Mechanics module became a really important, because it contains
first few steps as a Civil Engineer should process during a construction.
This assignment includes mainly about identifying rock types, classification of
common rock types, rock forming minerals and their susceptibility to weathering and the use of
rock and uncemented sediments for construction.
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Contents
Acknowledgement...........................................................................................................................1
Introduction......................................................................................................................................2
Task 1 (P 1.1, P1.2, P1.3, M2).........................................................................................................4
1.1................................................................................................................................................4
Sedimentary rocks...................................................................................................................4
Igneous rocks...........................................................................................................................7
Metamorphic Rocks.................................................................................................................9
1.2 (P1.2)...................................................................................................................................12
Slow Weathering and Atmospherically Stable Rock forming Minerals...................................12
Quartz....................................................................................................................................12
Atmospherically Unstable Rock forming Minerals...................................................................13
Pyrite......................................................................................................................................13
Olivine...................................................................................................................................13
Amphibole.............................................................................................................................13
Pyroxene................................................................................................................................14
Rapid Weathering and Dissolve and Reprecipitate Rock forming Minerals.............................15
Gypsum..................................................................................................................................15
Calcite....................................................................................................................................15
1.3 (P1.3)...................................................................................................................................16
Common use of rock for Construction..................................................................................16
Reference.......................................................................................................................................18
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Task 1 (P 1.1, P1.2, P1.3, M2)
1.1
Sedimentary rocks
Sedimentary rocks are formed from the eroded fragments of pre-existing rocks, or
from the skeletal fragments of once-living plants or organisms. They accumulate in various
environments on the Earth's surface. Sedimentary rocks commonly have distinctive layering or
bedding. Sedimentary rocks are sub-divided into three groups, including clastic, biological and
chemical.
Clastic sedimentary rocks are made up of fragments (clasts) of pre-existing rocks.
Crystals or fragments of the pre-existing rocks are loosened by weathering, and subsequently
transported to a site where they are deposited. Clastic sedimentary rock is formed when the
sediment is buried, then compacted and cemented.
Biological sedimentary rocks form when large quantities of living plants or
organisms die and accumulate. Their remains are broken down, compressed and cemented to
form rock. Accumulations of carbon-rich plant material may form coal. Deposits composed
mainly of animal shells may form limestone or chert.
Chemical sedimentary rocks are formed by chemical precipitation from solutions.
This process begins when water passes through rock dissolving some of the minerals and
carrying them away from their source. Eventually the minerals are deposited, or precipitated,
when the water evaporates or when the water becomes over-saturated with minerals. Rock salts
are examples of chemical sedimentary rocks.
Characteristics of Sedimentary Rocks
 Clastic Sedimentary Rocks (Siltstone, Sandstone and Conglomerate) Clastic sedimentary
rocks are composed of rock and mineral grains eroded from pre-existing rocks.
 Individual grains are held together by a cement that is commonly composed of quartz or
calcite minerals.
 Clastic sedimentary rocks may contain fossils.
 Sedimentary bedding may be present, defined by variations in the texture and composition of
the constituent grains that are systematically arranged in layers.
 Colour variations reflect composition, the depositional environment of the sediments and/or
the texture and weathering state.
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Sandstone
Formation of sandstone involves two principal stages. First one is a layer or layers
of sand accumulates as the result of sedimentation, either from water (as in a river, lake, or sea)
or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from
suspension. Finally, once it has accumulated, the sand becomes sandstone when it is compacted
by pressure of overlying deposits and cemented by the precipitation of minerals within the pore
spaces between sand grains. The most common cementing materials are silica and calcium
carbonate, which are often derived either from dissolution or from alteration of the sand after it
was buried. Sandstone is an arenaceous sedimentary rock composed mainly of feldspar and
quartz and varies in colour through grey, yellow, red, and white. Sandstone can be further
divided according to clast size fine (0.06-0.2mm), medium (0.2-0.6mm), coarse (0.6-2mm).
Breccia
Breccia forms where broken, angular fragments of rock or mineral debris
accumulate. The angular nature of the clasts indicates that they have not been transported very
far from their source. There are several modes of formation for breccia. Some represent
consolidated material accumulated on steep hill slopes or at the foot of cliffs. One possible
location for breccia formation is at the base of an outcrop where mechanical weathering debris
accumulates. Another would be in stream deposits near the outcrop such as an alluvial fan. Some
breccia form as debris flow deposits. The angular particle shape reveals that they have not been
transported very far. After deposition the fragments are bound together by mineral cement or by
a matrix of smaller particles that fills the spaces between the fragments.
Limestone
Limestone is a sedimentary rock consisting of more than 50% calcium carbonate.
There are many different types of limestone formed through a variety of processes. Limestone
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can be precipitated from water (chemical limestone), secreted by marine organisms such as algae
and coral (biochemical limestone), or can form from the shells of dead sea creatures (bioclastic
limestone). Some limestones form from the cementation of sand and / or mud by calcite (clastic
limestone), and these often have the appearance of sandstone or mudstone. As calcite is the
principle mineral component of limestone, it will fizz in dilute hydrochloric acid.
Conglomerate
Conglomerate is a sedimentary rock formed from rounded gravel and boulder
sized clasts cemented together in a matrix. The rounding of the clasts indicates that they have
been transported some distance from their original source, or that they have resided in a high
energy environment for some time. The cement that binds the clasts is generally one of either
calcite, silica or iron oxide. The matrix can consist solely of the cementing material, but may also
contain sand and / or silt sized clasts cemented together among the coarser clasts.
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Igneous rocks
Igneous rocks form when hot, molten rock (magma) cools and solidifies. The
magma originates deep within the Earth near active plate boundaries or hot spots, then rises
toward the surface. Igneous rocks are sub-divided into either intrusive or extrusive rocks,
depending upon where in the Earth the magma solidifies.
Various Forms of Igneous Rocks.
Intrusive, or plutonic, igneous rocks are formed when rising magma is trapped
deep within the Earth, where it cools very slowly over many thousands or millions of years until
it finally solidifies. Slow cooling allows the individual mineral grains sufficient time to grow and
form relatively large crystals. Intrusive rocks have a coarse-grained texture with interlocking
minerals. Granite is a commonly occurring intrusive rock in Hong Kong.
Extrusive, or volcanic, igneous rocks are produced when magma is erupted at, or
very near, the Earth's surface. The erupted magma cools and solidifies relatively quickly when it
is exposed to the cooler temperatures of the atmosphere. Lava and tuff are two common volcanic
rocks.
Characteristics of Intrusive Igneous Rocks
Granitic Rocks
 Granitic rocks contain predominantly feldspar and quartz minerals, with subordinate
amphiboles, and micas.
 Granitic rocks occur as plutons, dykes or sills (Figure 11).
 Individual minerals can generally be identified with the naked eye (Figure 12 & 13).
 Minerals are crystalline and show an interlocking texture (Fingure 12 & 13).
 Unweathered granite is commonly pinkish grey or light grey in colour.
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Basalt
Basalt a mafic extrusive rock, is the most widespread of all igneous rocks, and
comprises more than 90% of all volcanic rocks. Because of its relatively low silica content,
basalt lava has a comparatively low viscosity, and forms thin flows that can travel long distances.
It is also found as intrusive dikes and sills. Many moon rocks brought back by Apollo astronauts
are of basaltic composition. Basalt is the volcanic equivalent of gabbro.
Granite
Granite is a felsic, generally equigranular, and relatively light coloured intrusive
rock. It comprises some of the oldest known rocks on Earth, and is the most abundant basement
rock underlying the relatively thin sedimentary rock cover of the continents. Granite is produced
in volcanic arcs, and more commonly in mountain building resulting from the collision of two
continental masses. The earliest continental masses were products of the accumulation of
volcanic arcs, and this is why granite lies in the cores of all of the continents. Granite is the
plutonic equivalent of rhyolite.
Rhyolite
Rhyolite is a relatively common volcanic rock. It is the chemical equivalent of
granite. Although the two rock types have the same chemistry, rhyolite is extrusive and granite is
intrusive. While granite has crystals that are generally easy to see, in rhyolite the crystals are
often too small to see. This is due to the more rapid cooling of the rhyolite lava compared to
granite's slower cooling magma. In general, the slower a magma cools the larger the crystal size.
Although crystals in rhyolite are usually hard to see, they are there, but as microscopic crystals
often surrounded by a glassy matrix. If the lava fails to form crystals and is essentially all glass,
then it is more correctly called obsidian.
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Metamorphic Rocks
Metamorphic rocks are formed when a pre-existing rock is subject to high
temperature, high pressure, hot and mineral-rich fluid, or a combination of these conditions. The
original rocks could be igneous, sedimentary, or earlier metamorphic rocks. In the case of
metamorphic rocks, some or all of the original minerals are replaced by new minerals, and the
original textures are commonly masked due to the deformation (such as shearing and folding)
that may accompany metamorphism. Metamorphic rocks are generally formed deep within the
Earth, or where tectonic plates meet.
Foliated metamorphic rocks exhibit a platy or sheet-like structure. Foliation
develops when platy or prismatic minerals within the rock are compressed and aligned under
extreme pressure. The foliation pattern reflects the direction in which pressure was applied.
Slate, schist, and gneiss are examples of foliated metamorphic rocks.
Non-foliated metamorphic rocks display a massive structure. Non-foliated
metamorphic rocks can be formed by contact metamorphism that occurs around intrusive
igneous rocks. The pre-existing rocks that come into contact with the intruding igneous rocks are
essentially baked by the heat. In this case, the mineral structures of the pre-existing rocks are
changed without being subjected to intense pressure. Quartzite and marble are examples of non-
foliated metamorphic rocks.
Characteristics of Metamorphic Rocks
Schist and Phyllite
 The minerals of the original rock may be replaced by new metamorphic minerals, such as
micas (platy minerals) and amphibole (a prismatic mineral).
 The rocks show a foliation, which is defined by the alignment of platy or elongate prismatic
minerals (Figure 18).
 The rocks commonly display alternating dark and light coloured bands that reflect the
concentration of dark and light coloured minerals.
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 The rocks generally display a silky or shiny appearance due to the presence of micas.
Gneiss
Gneiss is a high grade metamorphic rock, meaning that it has been subjected to
higher temperatures and pressures than schist. It is formed by the metamorphosis of granite, or
sedimentary rock. Gneiss displays distinct foliation, representing alternating layers composed of
different minerals. However, unlike slate and schist, gneiss does not preferentially break along
planes of foliation because less than 50% of the minerals formed during the metamorphism are
aligned in thin layers. Because of the coarseness of the foliation, the layers are often sub-parallel.
Gneiss is typically associated with major mountain building episodes. During these episodes,
sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated
by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during
such episodes. Gneiss is generally rough to touch. Gneiss can see crystals with the naked eye.
Gneiss generally alternating lighter and darker sub-parallel discontinuous bands
Quartzite
Quartzite is a metamorphic rock formed when quartz-rich sandstone or chert has
been exposed to high temperatures and pressures. Such conditions fuse the quartz grains together
forming a dense, hard, equigranular rock. The name quartzite implies not only a high degree of
induration, but also a high quartz content. Quartzite generally comprises greater than 90%
percent quartz, and some examples containing up to 99% quartz, and are the largest and purest
concentrations of silica in the Earth's crust. Although a quartz-rich sandstone can look similar to
quartzite, a fresh broken surface of quartzite will show breakage across quartz grains, whereas
the sandstone will break around quartz grains. Quartzite also tends to have a sugary appearance
and glassy lustre. The variety of colours displayed by quartzite is a consequence of minor
amounts of impurities being incorporated with the quartz during metamorphism. Although
quartzite can sometimes appear superficially similar to marble, a piece of quartzite will not be
able to be scratched by a metal blade, and quartzite will not fizz on contact with dilute
hydrochloric acid. And generally gritty to touch.
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Marble
Marble is a metamorphic rock formed when limestone is exposed to high
temperatures and pressures. Marble forms under such conditions because the calcite forming the
limestone recrystallizes forming a denser rock consisting of roughly equigranular calcite crystals.
The variety of colours exhibited by marble are a consequence of minor amounts of impurities
being incorporated with the calcite during metamorphism. While marble can appear superficially
similar to quartzite, a piece of marble will be able to be scratched by a metal blade, and marble
will fizz on contact with dilute hydr ochloric acid.
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1.2 (P1.2)
Slow Weathering and Atmospherically Stable Rock forming Minerals
Quartz
It is a complex silicate in which all the oxygen atoms of the SiO4 tetrahedral are
shared between two tetrahedral, leading to complex 3-dimensional frameworks. For this reason,
quartz is referred to as a framework silicate.
Quartz is among the most common of all
rock forming minerals and is found in many metamorphic
rocks, sedimentary rocks, and those igneous rocks that are
high in silica content such as granites and rhyolites. It is a
common vein mineral and is often associated with mineral
deposits.
Cryptocrystalline varieties are used as semi-
precious stones and for ornamental purposes. These varieties
are divided more by character than by color. Chalcedony, or
agate, is divided into innumerable types that have been
named for locally common varieties. Some of the more
beautiful types have retained their names while other names have faded into obscurity. Some of
the more common are chrysoprase (a pure green agate), sard (a yellow to brown agate), sardonyx
(banded sard), onyx (black and white agate), carnelian (a yellow to orange agate), flint (a
colorful and microscopically fibrous form), jasper (a colorful impure agate) and bloodstone (a
green with red speckled agate).
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Atmospherically Unstable Rock forming Minerals
Pyrite
It is also known as "Fool's Gold" because of its
brassy-yellow metallic color, is the most common sulphide mineral
in rocks of all ages, being found in virtually every geological
environment. It is easily distinguishable from gold as it has a lower
specific gravity (specific gravity of gold is 15.9-19.3) and it is
harder (hardness of gold is 2.5-3). Pyrite is a common component
of sedimentary rocks and metamorphosed sediments, is an
accessory mineral in many igneous rocks, and forms large bodies in
hydrothermal deposits.
Olivine
It is a simple orthosilicate in which the SiO4
tetrahedra are independent of each other. It is a solid solution of
the end-members forsterite (Mg2SiO4) and fayalite (Fe2SiO4),
although most examples are closer to the forsterite end-member.
Olivine is very susceptible to alteration and often has a brownish
weathering rind of assorted clay minerals. Olivine is most
commonly found in igneous rocks of low silica content, such as
basalts and gabbros, and is occasionally found in metamorphic
rocks. Olivine is very susceptible to weathering. Bright green
olivine loses its appeal rapidly in the weathering environment. It
becomes dull, earthy, and yellowish brown.
Amphibole
The most common amphibole is hornblende. Hornblende is
quite similar to augite in that both are dark minerals, however
hornblende crystals are generally longer, thinner and shinier
than augite and the mineral cross-sections are diamond-shaped.
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Pyroxene
The most common pyroxene mineral is augite. Augite is generally dark green to
black in colour and forms short, stubby crystals which, if you look at an end-on section, have
square or rectangular cross-sections.
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Rapid Weathering and Dissolve and Reprecipitate Rock forming Minerals
Gypsum
It has a very low thermal conductivity (hence its use as
an insulating filler). A crystal of gypsum will feel noticeably warmer
than, for instance, a crystal of quartz. Gypsum is one of the more
common minerals in sedimentary environments. It is a major rock
forming mineral that produces massive beds, usually from
precipitation out of highly saline waters.
Calcite
It is the only common non-silicate rock forming
mineral, being instead calcium carbonate. It has two refractive
indices causing a significant double refraction effect - when a
clear calcite crystal is placed on an image, a double image is
observed; See the sample below. Calcite will fizz when dilute
hydrochloric acid is placed on it. It may be fluorescent,
phosphorescent; thermo luminescent and triboluminescent (see
fluorite for definitions of these properties).
Calcite is one of the most ubiquitous minerals,
being an important rock forming mineral in sedimentary
environments. It is an essential component of limestone’s, and
occurs in other sedimentary rocks. It also occurs in metamorphic
and igneous rocks, and is common in hydrothermal environments.
Calcite is a common vein filling mineral in many rock types.
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1.3 (P1.3)
Common use of rock for Construction
Rocks normally consist of several minerals, some essential and some accessory.
Each rock type was formed under certain specific conditions, resulting in the formation of a
fairly predictable group of minerals. Rocks fall into three classes according to their origin as
Igneous, Sedimentary and Metamorphic
 LIMESTONE
A sedimentary rock, it is used mainly in the manufacture of Portland cement, the
production of lime, manufacture of fiberglass, glass.
 SHALE
A sedimentary rock well stratified in thin beds. It splits unevenly more or less
parallel to bedding plane and may contain fossils. It can be a component of bricks and
cement.
 CONGLOMERATE
A sedimentary rock with a variable hardness consisted of rounded or angular rock
or mineral fragments cemented by silica, lime, iron oxide, etc. Usually found in mostly thick,
crudely stratified layers. Used in the construction industry.
 GRANITE
An igneous-plutonic rock, medium to coarse-grained that is high in silica,
potassium, sodium and quartz but low in calcium, iron and magnesium. It is widely used for
architectural construction, ornamental stone and monuments.
 GABBRO
An igneous-plutonic rock, generally massive, but may exhibit a layered structure
produced by successive layers of different mineral composition. It is widely used as crushed
stone for concrete aggregate, road metal, railroad ballast, etc.
 SCHIST: A metamorphic uneven-granular, medium to coarse grained, crystalline with
prominent parallel mineral orientation. Goes from silvery white to all shades of gray with
yellow to brown tones depending on the mineral concentration. Some schists have graphite
and some are used as building stones.
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 GNEISS
A metamorphic uneven granular medium to coarse grained crystalline with more
or less parallel mineral orientation. Colors are too variable to be of diagnostic value. Due to
physical and chemical similarity between many gneisses and plutonic igneous rocks some are
used as building stones and other structural purposes.
 MARBLE
A metamorphic even-granular grain to medium grained and may be uneven
granular and coarse grained in calc-silicate rock. The normal color is white but accessory
minerals act as coloring agents and may produce a variety of colors. Depending upon its
purity, texture, color and marbled pattern it is quarried for use as dimension stone for
statuary, architectural and ornamental purposes.
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Reference
[1] -. 2012. Rocks and Their Uses. [ONLINE] Available at:
http://www.coaleducation.org/lessons/wim/20.htm. [Accessed 06 August 14].
[2] University Of Auckland New Zealand. 2005. Geology Rocks and Minerals. [ONLINE]
Available at: http://flexiblelearning.auckland.ac.nz/rocks_minerals/minerals/olivine.html.
[Accessed 06 August 14].
[3] Ministry of Energy, Mines and Petroleum Resources. 2010. Common Rock-Forming
Minerals. [ONLINE] Available at:
http://www.empr.gov.bc.ca/Mining/Geoscience/PublicationsCatalogue/InformationCirculars/
IC1987-05/Pages/contain.aspx. [Accessed 05 August 14].
[4] Author, Unknown, 2000. Weathering: the decay of rocks and the source of sediments in and
the source of sediments in sedimentary rocks. Weathering: the decay of rocks and the source of
sediments in and the source of sediments in sedimentary rocks Geology 200 Geology 200
Geology for Environmental Scientists, [Online]. 200, 22,23,24. Available at:
http://pages.geo.wvu.edu/~kammer/g100/Weathering.pdf [Accessed 05 August 2014].
[5] Lecture notes
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