1. Chapter II
SOIL FORMATION
frame work known as geologic cycle. This
Specific Objectives includes many processes acting
To identify the sources of soil. simultaneously. The most significant of these
To identify the rock forming being with molten magma from within the earth
minerals and types of rocks. forming into rock, then continue with the rocks
To understand the geology behind being broken down into soil, and soil being
rock weathering. converted back in to rock (fig.no.1).
To understand process of Soils are the products of weathering of
weathering and soil formation. rocks. Soil grains are disintegrated rock
To co relate the agency of crystals. As such the behaviour of soils is
weathering and properties of soil formed. dependent on the rock from which they
To co relate the properties of soil to originated and also on the process of
type of rock from which it is got. weathering and agency responsible for
deposition as we see later in this discussion.
Rocks–The Sources of Soils
Earth materials are divided into rock and
soil. Normally, the soil particles are the result of
weathering of rocks and decay of vegetation.
Some soil particles may, over a period of time,
become consolidated under the weight of
overlying material and become rock. For civil
engineers rock is a “hard, durable material that
cannot be excavated without blasting”. The
following are the important differences between
these two materials.
Rocks are generally cemented; soils
are rarely cemented.
Rocks usually have much lower
porosity than soils.
Rock masses are often discontinuous;
soil masses usually can be represented as
continuous.
Rocks have more complex and
generally unknowable stress histories. In many
rock masses, the minor principal stress is Fig.no.1. Rock–Soil formation geological cycle.
vertical; in most soils the major principal stress
is vertical. So an understanding of rocks is necessary
The geologic processes acting on earth’s to understand the behaviour of soils. Also
crust are extremely slow by human time scale, some of engineering structures are constructed
and as such no direct observation is possible. directly on or in rock. For instance gravity dams
Theories have been developed on the basis of are usually directly rested on rock because of
observations of the earth as it exists now. large masses it transfers and high bearing
Geologic theories are organized around a capacities it demands from the supporting

2. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
media. Hence an understanding of rocks and precipitates out of solution. It also precipitates
discontinuities associated with it and the in soil acting as cementing agent.
formation of rock is also necessary for a civil Mica – Translucent thin sheets or
engineer. flakes. Muscovite has silvery flakes, while
Biotite is dark grey or black, which causes
Rock Forming Minerals shear failures in certain rocks, such as Schist.
Minerals are naturally formed compounds
with specific structures and chemical Types of Rocks
compositions. As the basic constituents of Rocks are classified according to their
rocks, minerals control much of rock behaviour. place in the geologic cycle. The three major
Some minerals are very strong and resistant to categories are Igneous, Sedimentary and
deterioration, and produce rocks with similar Metamorphic rocks as discussed below.
properties, while others are much softer and Igeneous Rocks: Igneous rocks form
produce weaker rock. More than 2000 different when molten magma, from deep inside the
minerals are present in the earth crust. Most earth, moves upward toward ground surface
common minerals in majority of rocks are and gets cooled. Intrusives or Plutonic rocks
discussed below. form below ground surface, where they cool
Feldspar – This is the most abundant slowly and are coarse grained. Extrusives or
mineral. Orthoclase feldspar contains Volcanic rocks arrive at ground surface in
potassium (KaAlSi3O8) and usually range from molten state through volcano and cool rapidly
white to pink. Plagioclase feldspar contains acquiring a fine grained structure. However, a
sodium (NaAlSi3O8), Calcium (CaAl2Si2O8), or minority of igneous rocks are formed by
both, and range from white to grey to black. volcanism. Most of them however have been
They have moderate hardness. formed by the cooling of liquid crust of earth
Quartz – Also very common and major during Precambrian age. Some extrusive
ingredient in rocks. It is a silicate (SiO2), and material, such as volcanic ash, bypasses the
usually has a translucent to milky white color. rock stage and forms directly into sediment.
Quartz is harder than most minerals and
resistant to weathering. Some common igneous rocks are
Ferro magnesium minerals – A class - Granite: An intrusive and
of minerals, which contain both iron and most common and familiar rock. It contains
magnesium. This class includes Pyroxene, primarily orthoclase feldspar and Quartz,
Amphibole, Hornblende, and Olivine. These with some Biotite and Amphibole.
minerals have dark colour and moderate - Basalt: A dark, dense rock,
hardness. most abundant extrusive rock. Very hard,
Iron oxides – Another class of however possesses joints due to rapid
minerals which essentially contains iron cooling.
(Fe2O3), includes Limonite and Magnetite. - Diorite: Similar to granite, with
Though less common give a distinctive rusty Plagioclase feldspar instead of Orthoclase
colour to some rocks and soils and act as with little or no Quartz.
cementing agents. - Andesite: A very hard
Calcite – A mineral made of Calcium extrusive.
carbonate (CaCO3); usually white, pink or grey. - Rhyolite: Extrusive equivalent
Insoluble in water; and thus can be transported of Granite.
by ground water into cracks in rock where it
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- Gabbaro: The intrusive organisms on the bottom of the ocean, and
equivalent of Basalt. Darker in colour than usually extend over large areas. Some of
Granite or Diorite. these deposits were later uplifted by
Unweathered igneous rocks generally have tectonic forces of the earth and now exist
excellent engineering properties and are good below land areas.
materials to build on. Intrusive rocks are Chalk- these are similar to limestone
especially good. However fractures form but much softer and porous.
planes of weakness. Dolomite- similar in grain structure
Sedimentory Rocks: These form the and color to limestone and are in fact, lime
second major category of rocks. They are stones in which the calcite (CaCO3)
formed due to induration or lithification of soil interbonded with magnesium. Hence, the
deposits. These are of two types, viz.,Clastic principal ingredient of dolomite is calcium
and Carbonate. magnesium carbonate [CaMg(CO3)2]
• Clastic rocks – Clastic rocks are formed
when deep soil deposits become hardened Metamorphic Rocks: Metamorphic
as a result of pressure from overlying rocks are much less common at the earth’s
strata and cementation through surface than sedimentary rocks are. They are
precipitation of water soluble minerals such produced when Igneous and Sedimentary
as calcium carbonate or iron oxide. rocks literally change their texture and structure
Because of their mode of deposition, many as well as mineral and chemical composition,
clastic rocks are layered or stratified, which as a result of heat, pressure and shear.
make them quite different from massive Some metamorphic rocks are foliated,
formations. which means they have oriented grains similar
Most Conglomerate, Breccias, Sand to bedding planes in sedimentary rocks. These
stone and Arkose rocks generally have foliations are important because the shear
favorable engineering properties. Those strength is less along these foliations.
cemented with silica or iron oxide are Common foliated rocks:
especially durable and are difficult to Slate- derived from shale, dense.
excavate. Some fine and very fine grained Schist-with large mica content, foliation is
Clastic rocks are subject to slaking, which called schistosity.
is a deterioration after excavation and Gneiss- derived from granite, coarse
exposure to the atmosphere and wetting grained banded rock.
and drying cycles. Rocks that exhibit Common non foliated rocks:
strong slaking will rapidly degenerate to Quartzite-composed mainly or entirely of
soil, and thus can create problems for quartz, derived from limestone, very strong
engineering structures built on them. and hard.
• Carbonate rocks – A different type of Marble-derived from limestone or dolomite,
sedimentary rock forms when organic used for decorative purposes and for
materials accumulate and become statues.
indurated. Because of their organic origin,
they are called carbonates. Common Structural Geology
carbonate rocks are, Structural geology is the study of the
Lime stone- common type of configuration and orientation of rock
carbonate rock, is composed primarily of formations. This is an important part of
calcite (CaCO3). Most lime stones are engineering geology because it gives important
formed from the accumulation of marine insights on how a rock mass will behave.
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Bedding planes and schistocity: mass, so they form potential failure surfaces.
Sedimentary rocks are formed in horizontal or There are three types of fractures.
near horizontal layers and these layers reflect - Joints: are fractures that have not
successive layers of deposition. This process experienced any shear movement. They may
produces parallel bedding planes. When these be due to cooling (in case of igneous rocks)
rocks are uplifted by tectonic forces in the tensile tectonic stresses etc. Joints usually
earth, the bedding planes usually were rotated occur at fairly regular spacing and a group of
to a different angle, as shown in fig.no.2. such joints is called a set.
- Shear Jones: are fractures that have
experienced a small shear displacement, of the
order of few cms. Serve as water conduits.
- Faults: similar to shear zones, except
they have experienced much greater shear
displacements (>1m). These displacements
are normally associated with earth quakes.
Faults are classified according to their
geometry and direction of movement, If
overhanging block is moving downward –
normal fault, if it is moving upward – reverse
fault. A fault with small dip angle is called thrust
fault. In the above Dip slip fault movement is
primarily along dip. In strike slip fault
movement is primarily along strike. The fault
trace is the intersection of the fault and the
ground surface. The term discontinuity is
often used to include bedding planes,
Fig.No.2. Orientation of bedding planes.
schistocity, joints, shear zones, faults and all
other defects in rock.
A slope as suggested in fig.no.2.(a) is
more likely to fail than that in fig.no.2.(b) as it
undermines the bedding planes. Many land
slides have occurred on slopes with
unfavourable bedding orientations.
Folds: Tectonic forces also distort rock
masses. When horizontal compressive forces
are present, the rock distorts into a wavy
pattern called folds. Sometimes these are
gradual, other times very abrupt. When folds
are oriented concave downward they are called
anticlines; when concave upward they are
called synclines.
Fractures: Fractures are cracks in rock
mass. Their orientation is very important
because the shear strength along these
fractures is less than that of the intact rock
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5. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
Strike and Dip: When developing
geologic maps, we are interested in both the
presence of certain geologic structures and
their orientation in space. A rock mass may be Fig.no.3. Strike and dip
unstable if it has joints oriented in one The Dip (fig.no.3) is the angle between the
direction, but much more stable if they are geologic surface and the horizontal, and is
oriented in other direction, refer fig. no.2. For measured in a vertical plane oriented
similar reasons we are interested in their perpendicular to the strike. The Dip also needs
presence and orientation. Many of these a direction, when expressed together; this data
structures are roughly planar, at least for short is called an altitude.
distances, and therefore may be described by Sometimes we also need to know the dip
defining the orientation of this plane in space. as it would appear in a vertical plane other than
The strike (fig.no.3) is the compass one perpendicular to strike. This Dip is called
direction of the intersection of the plane and apparent dip and computed as (refer fig.no.4)
the horizontal, and is expressed as a bearing tan δa = tan δ sinα
from true north.
Fig.no.4. Apparent Dip
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6. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
Rock Weathering and Soil produce, transport and deposit will facilitate the
important function of engineering judgement.
Formation Residual soils: when rock weathering
Rocks exposed to the atmosphere are process is faster than the transport processes
immediately subjected to physical, chemical, resulting soil remains in place. It is known as
and biological break down through weathering. residual soil. It retains many characteristics of
There are many weathering processes parent rock. The transition with depth from soil
including, to weathered rock to intact rock is typically
Erosive action of water, ice and wind. gradual with no distinct boundaries.
Chemical reactions induced by In tropical regions, residual soil layers can
exposure to O2, H2O and other chemicals. be very thick, sometimes extending hundreds
Loosening through the growth of of meters before reaching unweathered rock.
plants. Cooler and more arid regions normally have
Growth of minerals in cracks thinner layers and often no residual soil at all.
Thermal expansion and contraction.
Land slides and rock falls.
Abrasion from down hill movement of
nearby rock and soil.
Rock passes through various stages of
weathering, eventually being broken down into
small particles, soil.
Weathering processes continue even after
the rock becomes a soil. As soil become older,
they change due to continued weathering. The
rate of change depends on many factors
including
The general climate, especially precipitation and
temperature.
The physical and chemical make up of soil.
The elevation and slope of ground surface.
The depth of ground water table. saprolite
The type and extent of flora and fauna.
The presence of micro organisms. Decomposed granite (DG) is a sandy
The drainage characteristics of the soil. residual soil.
Shales, consisting largely clay minerals
Soil Formation, Transport and weather to form clayey residual soils.
Deposition Saprolite, used to refer for residual soils
that are not extensively weathered and still
Geotechnical engineers need to focus on retain much of the structure of the parent rock.
soil than rock for two reasons. Laterite, residual soil formed in tropical
I. More civil engineering projects are built regions. Soil is cemented with iron oxides,
on soil. which gives it a high dry strength.
II. Soil being generally weaker and more
compressible than rock, is more often a
source of problems.
Glacial soils: Much of the earth’s
Along with engineering properties of soils northern region was covered with huge masses
and understanding of geologic cycle that of ice called Glaciers. They were extending up
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7. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
to Ohio River in N. America and Germany in
Europe. Now these areas are heavily
populated and number of civil engineering
activities happening in this area necessitates
study of soil deposited by Glaciers.
Glaciers grind down the rock and soil, and
transport these materials over hundreds of
Kms. As such resulting deposits often contain a
mixture from many different sources. These
deposits can have a wide range of hardness
and particle size and are among the most Glacial deposit
complex and heterogeneous soils.
Till is soil deposited directly by Glacier, soil
particles vary from clay to gravel. Soil
deposited in ridges or mounds is called
ablation till. These ridges and mounds are
called moraines and are loose and easy to
excavate. Soil caught beneath the Glacier,
called lodgement till, has been consolidated
heavily under the weight of ice. It has a very
high unit weight and often is nearly as strong
as concrete. They are sometimes also called
as hard pan. Varved clays
When Glaciers melted, they generate large Alluvial soils: Alluvial soils or fluvial soils
quantities of runoff. This water erodes much of or alluvium are those transported to their
the till and deposits it downstream forming present position by the rivers and streams.
glaciofluvial soils (outwash) (fig.no.5). They are These soils are very common.
more uniform and sources of sand and gravel. When river or stream is flowing rapidly silts
The fine grained portion of till often remain and clays remain in suspension and carried
suspended in run off water until reaching a lake downstream, while sands, gravels and
or ocean, where it finally settles to the bottom. boulders are deposited. When they loose their
These are called glacio-lacustrine soils and velocity more of the finer soils also are
glaciomarine soils. Sometimes silts and clays deposited. Because of rapid flow in heavy
were deposited in alternating layers according rainfall time and slow flow in draught alluvial
to the seasons, thus forming a banded soil soils often contain alternating horizontal layers
called varved clay. These soils are soft and of different soil types.
compressible and pose problems. When a stream reaches the foot of a
canyon, looses its velocity considerably and
deposit much of its soil load. This process
forms alluvial fans, most obvious type of
alluvium.
Fig.no.5. Glacial soils
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8. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
Alluvial fan
Braided streams are high gradient rapidly
flowing streams. They are highly erosive and
carry large range and quantity of sediment.
Minor changes in velocity cause deposition of
sediments. The deposits from braided streams
are very irregular in stratification and have wide
range of grain sizes. The grain sizes usually
vary from gravel to silt. (fig.no.6) Clay size
Braided stream deposits
particles are generally not found. Soil in a given
The valley floor in which a river meanders
pocket or lense is uniform. Void ratio and unit
is referred to as meander belt. As water moves
weight may vary over a wide range at given
through a channel bend, velocity along the
depth within a lateral distance of few mts.
inside edge decreases, while that along the
outer edge increases. Therefore soil at outer
edge is eroded and carried further while its
sediment is deposited along inner edge called
point bar deposits. This action, over a period of
time, may increase the bend significantly,
eventually leading the river to cut across a
large bend to form Oxbow lakes (fig.no.7).
Fig.no.6.Alluvim of braided streams.
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formed due to action of water. They have
respectively similar properties.
Aeolian soils: Soils deposited by wind
are known as Aeolian soils. This mode of
transport generally produces very poorly
graded soils because of the strong sorting
power of wind. These soils are generally very
loose and thus have only fair engineering
properties. Wind causes soil transportation in
the following ways.
Suspension – wind lifts
individual silt and clay particles to high
altitudes and transports them great
distances.
Fig.no.7. Oxbow lakes
Fig.no.8.Flood plains
During floods, rivers overflow low lying
areas. The sand and silt size particles carried
by the river are deposited along the banks to
form ridges known as natural levees. Finer soil Fig.no.9.Suspension, saltation and creep.
particles are carried beyond levees on to flood
plains (fig.no.8). These particles settle at Saltation – (saltacio – to
different rates to form back swamp deposits. dance) soil particles become temporarily
Oxbow lake may be filled during floods and the airborne, and then fall back to earth. Upon
clayey particles settle to the bottom to form landing, the particle either bounces or
highly plastic and compressible layers. dislodges another particle, initiating
Lacustine and marine soils similar to another flight. Particles rise to 1m altitude
glaciolacustrine and glaciomarine soils may be and move horizontal distances around 4m.
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10. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
Creep: medium to coarse sands roll and
slide along the ground surface.
Aeolian sands can form irregular hills
called sand dunes which migrate in wind
direction between 2-3m per year. Aeolian silts
often form deep deposits called loess. Because
of its deposition mode, loess typically has a
very high porosity. It is fairly strong when dry
but collapses if wetted. These soils are very
much prone to erosion and often have deep
gullies.
.
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Note: ASSIGNMENT
Give detailed information about a major
soil failure from engineering history which
is not mentioned in the handout.
Colluvial soils: a colluvial soil is one Give two examples for different type of
transported down slope by gravity. The cause rocks and soils other than those mentioned
of movement may be creep or landslide. Creep in the handout.
occurs due to gravity induced down slope Answer neatly, legibly and within three to
shear stresses (fig.no10) four pages and submit it within one week
from the day of completing the
discussion on this chapter.
Note: Hand outs are not exhaustive.
Students are advised to refer numerous
Fig.no.10.Colluvial soils references available in the library and
suggested in the course outline.
Creep may extend to depth 0.3 to 3m,
maximum being at surface. Rapid down slope
movement may be due to landslide and mud
flow.
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DEPARTMENT
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