Sm Chapter II

Chapter II SOIL FORMATION

Specific Objectives To identify the sources of soil. To identify the rock forming

minerals and types of rocks. To understand the geology behind

rock weathering. To understand process of

weathering and soil formation. To co relate the agency of

weathering and properties of soil formed. To co relate the properties of soil to

type of rock from which it is got.

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 vertical; in most soils the major principal stress is vertical.

The geologic processes acting on earth’s crust are extremely slow by human time scale, and as such no direct observation is possible. Theories have been developed on the basis of observations of the earth as it exists now. Geologic theories are organized around a

frame work known as geologic cycle. This includes many processes acting simultaneously. The most significant of these being with molten magma from within the earth forming into rock, then continue with the rocks being broken down into soil, and soil being converted back in to rock (fig.no.1).

Soils are the products of weathering of rocks. Soil grains are disintegrated rock crystals. As such the behaviour of soils is dependent on the rock from which they originated and also on the process of weathering and agency responsible for deposition as we see later in this discussion.

Fig.no.1. Rock–Soil formation geological cycle. So an understanding of rocks is necessary

to understand the behaviour of soils. Also some of engineering structures are constructed directly on or in rock. For instance gravity dams are usually directly rested on rock because of large masses it transfers and high bearing capacities it demands from the supporting

HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT

----------------------------------------------------------------------------------------------------------------------------- SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA

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of rock is also necessary for a civil engineer.

Ro

in majority of rocks are disc e

grey to black. The a

most minerals and resi n

ave dark colour and moderate hard s

cks and soils and act as cem ti

prec ta

Biotite is dark grey or black, which causes

in rocks, such as Schist.

major cate

ive material, such as volcanic ash, bypasses the rock stage and forms directly into sediment.

contains prim ily or

ndant extrusive rock. Very hard, how er po

ilar to granite, with Plag clase

no Quartz.

media. Hence an understanding of rocks and discontinuities associated with it and the formation

ck Forming Minerals Minerals are naturally formed compounds

with specific structures and chemical compositions. As the basic constituents of rocks, minerals control much of rock behaviour. Some minerals are very strong and resistant to deterioration, and produce rocks with similar properties, while others are much softer and produce weaker rock. More than 2000 different minerals are present in the earth crust. Most common minerals

uss d below. Feldspar – This is the most abundant

mineral. Orthoclase feldspar contains potassium (KaAlSi3O8) and usually range from white to pink. Plagioclase feldspar contains sodium (NaAlSi3O8), Calcium (CaAl2Si2O8), or both, and range from white to

y h ve moderate hardness. Quartz – Also very common and major

ingredient in rocks. It is a silicate (SiO2), and usually has a translucent to milky white color. Quartz is harder than

sta t to weathering. Ferro magnesium minerals – A class

of minerals, which contain both iron and magnesium. This class includes Pyroxene, Amphibole, Hornblende, and Olivine. These minerals h

ne s. Iron oxides – Another class of

minerals which essentially contains iron (Fe2O3), includes Limonite and Magnetite. Though less common give a distinctive rusty colour to some ro

en ng agents. Calcite – A mineral made of Calcium

carbonate (CaCO3); usually white, pink or grey. Insoluble in water; and thus can be transported by ground water into cracks in rock where it

ipi tes out of solution. It also precipitates in soil acting as cementing agent.

Mica – Translucent thin sheets or flakes. Muscovite has silvery flakes, while

shear failures in certa

Types of Rocks Rocks are classified according to their

place in the geologic cycle. The three gories are Igneous, Sedimentary and

Metamorphic rocks as discussed below. Igeneous Rocks: Igneous rocks form

when molten magma, from deep inside the earth, moves upward toward ground surface and gets cooled. Intrusives or Plutonic rocks form below ground surface, where they cool slowly and are coarse grained. Extrusives or Volcanic rocks arrive at ground surface in molten state through volcano and cool rapidly acquiring a fine grained structure. However, a minority of igneous rocks are formed by volcanism. Most of them however have been formed by the cooling of liquid crust of earth during Precambrian age. Some extrus

Some common igneous rocks are - Granite: An intrusive and

most common and familiar rock. It ar thoclase feldspar and Quartz,

with some Biotite and Amphibole. - Basalt: A dark, dense rock,

most abuev ssesses joints due to rapid

cooling. - Diorite: Simio feldspar instead of Orthoclase

with little or- Andesite: A very hard

extrusive. - Rhyolite: Extrusive equivalent

of Granite.



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properties and are good mat

ategory of rocks. They are

depand•

heir mode of deposition, many clas

xhibit

•

te and become indu

r large areas. Some of thes

nd now exist belo

CO3) interbonded with magnesium. Hence, the

entary rock

s similar to bedding planes in sedimentary rocks. These foliations are important because the shear stre

rocks:

content, foliation is

on foliated rocks:

Marble-derived from limestone or dolomite,

urposes and for

rock

insights on how a rock mass will behave.

- Gabbaro: The intrusive equivalent of Basalt. Darker in colour than Granite or Diorite. Unweathered igneous rocks generally have

excellent engineeringerials to build on. Intrusive rocks are

especially good. However fractures form planes of weakness.

Sedimentory Rocks: These form the second major cformed due to induration or lithification of soil

osits. These are of two types, viz.,Clastic Carbonate. Clastic rocks – Clastic rocks are formed when deep soil deposits become hardened as a result of pressure from overlying strata and cementation through precipitation of water soluble minerals such as calcium carbonate or iron oxide. Because of t

tic rocks are layered or stratified, which make them quite different from massive formations.

Most Conglomerate, Breccias, Sand stone and Arkose rocks generally have favorable engineering properties. Those cemented with silica or iron oxide are especially durable and are difficult to excavate. Some fine and very fine grained Clastic rocks are subject to slaking, which is a deterioration after excavation and exposure to the atmosphere and wetting and drying cycles. Rocks that estrong slaking will rapidly degenerate to soil, and thus can create problems for engineering structures built on them. Carbonate rocks – A different type of sedimentary rock forms when organic materials accumula

rated. Because of their organic origin, they are called carbonates. Common carbonate rocks are,

Lime stone- common type of carbonate rock, is composed primarily of calcite (CaCO3). Most lime stones are formed from the accumulation of marine

organisms on the bottom of the ocean, and usually extend ove

e deposits were later uplifted by tectonic forces of the earth a

w land areas. Chalk- these are similar to limestone

but much softer and porous. Dolomite- similar in grain structure

and color to limestone and are in fact, lime stones in which the calcite (Ca

principal ingredient of dolomite is calcium magnesium carbonate [CaMg(CO3)2]

Metamorphic Rocks: Metamorphic

rocks are much less common at the earth’s surface than sedimentary rocks are. They are produced when Igneous and Sedim

s literally change their texture and structure as well as mineral and chemical composition, as a result of heat, pressure and shear.

Some metamorphic rocks are foliated, which means they have oriented grain

ngth is less along these foliations. Common foliated Slate- derived from shale, dense. Schist-with large micacalled schistosity. Gneiss- derived from granite, coarse grained banded rock. Common nQuartzite-composed mainly or entirely of quartz, derived from limestone, very strong and hard.

used for decorative pstatues.

Structural Geology Structural geology is the study of the

configuration and orientation of formations. This is an important part of engineering geology because it gives important



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n the earth, the bedding planes usually were rotated to a different angle, as shown in fig.no.2.

Bedding planes and schistocity: Sedimentary rocks are formed in horizontal or near horizontal layers and these layers reflect successive layers of deposition. This process produces parallel bedding planes. When these rocks are uplifted by tectonic forces i

Fig.No.2. Orientation of bedding planes.

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

ave downward they are called anticlines; when concave upward they are

intact rock

mas s

etc. Joints usually occ a

ave exp n

ear

lude bedding planes, schistocity, joints, shear zones, faults and all other defects in rock.

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 conc

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

s, o they form potential failure surfaces. There are three types of fractures.

- Joints: are fractures that have not experienced any shear movement. They may be due to cooling (in case of igneous rocks) tensile tectonic stresses

ur t fairly regular spacing and a group of such joints is called a set.

- Shear Jones: are fractures that herie ced 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 shdisplacements (>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 inc



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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 unstable if it has joints oriented in one direction, but much more stable if they are oriented in other direction, refer fig. no.2. For similar reasons we are interested in their presence and orientation. Many of these structures are roughly planar, at least for short distances, and therefore may be described by defining the orientation of this plane in space.

The strike (fig.no.3) is the compass direction of the intersection of the plane and the horizontal, and is expressed as a bearing from true north.

Fig.no.3. Strike and dip

The Dip (fig.no.3) is the angle between the geologic surface and the horizontal, and is measured in a vertical plane oriented perpendicular to the strike. The Dip also needs a direction, when expressed together; this data is called an altitude.

Sometimes we also need to know the dip as it would appear in a vertical plane other than one perpendicular to strike. This Dip is called apparent dip and computed as (refer fig.no.4)

tan δa = tan δ sinα

Fig.no.4. Apparent Dip



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Rock Weathering and Soil Formation

Rocks exposed to the atmosphere are immediately subjected to physical, chemical, and biological break down through weathering.

There are many weathering processes including,

Erosive action of water, ice and wind. Chemical reactions induced by

exposure to O2, H2O and other chemicals. Loosening through the growth of

plants. Growth of minerals in cracks 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. The type and extent of flora and fauna. The presence of micro organisms. The drainage characteristics of the soil.

Soil Formation, Transport and Deposition

Geotechnical engineers need to focus on soil than rock for two reasons.

I. More civil engineering projects are built on soil.

II. Soil being generally weaker and more compressible than rock, is more often a source of problems.

Along with engineering properties of soils and understanding of geologic cycle that

produce, transport and deposit will facilitate the important function of engineering judgement.

Residual soils: when rock weathering process is faster than the transport processes resulting soil remains in place. It is known as residual soil. It retains many characteristics of parent rock. The transition with depth from soil to weathered rock to intact rock is typically gradual with no distinct boundaries.

In tropical regions, residual soil layers can be very thick, sometimes extending hundreds of meters before reaching unweathered rock. Cooler and more arid regions normally have thinner layers and often no residual soil at all.

saprolite

Decomposed granite (DG) is a sandy

residual soil. Shales, consisting largely clay minerals

weather to form clayey residual soils. Saprolite, used to refer for residual soils

that are not extensively weathered and still retain much of the structure of the parent rock.

Laterite, residual soil formed in tropical regions. Soil is cemented with iron oxides, which gives it a high dry strength.

Glacial soils: Much of the earth’s

northern region was covered with huge masses of ice called Glaciers. They were extending up



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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 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.

When Glaciers melted, they generate large quantities of runoff. This water erodes much of the till and deposits it downstream forming glaciofluvial soils (outwash) (fig.no.5). They are more uniform and sources of sand and gravel.

The fine grained portion of till often remain suspended in run off water until reaching a lake or ocean, where it finally settles to the bottom. These are called glacio-lacustrine soils and glaciomarine soils. Sometimes silts and clays were deposited in alternating layers according to the seasons, thus forming a banded soil called varved clay. These soils are soft and compressible and pose problems.

Fig.no.5. Glacial soils

Glacial deposit

Varved clays

Alluvial soils: Alluvial soils or fluvial soils or alluvium are those transported to their present position by the rivers and streams. These soils are very common.

When river or stream is flowing rapidly silts and clays remain in suspension and carried downstream, while sands, gravels and boulders are deposited. When they loose their velocity more of the finer soils also are deposited. Because of rapid flow in heavy rainfall time and slow flow in draught alluvial soils often contain alternating horizontal layers of different soil types.

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.



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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 particles are generally not found. Soil in a given pocket or lense is uniform. Void ratio and unit weight may vary over a wide range at given depth within a lateral distance of few mts.

Fig.no.6.Alluvim of braided streams.

Braided stream deposits

The valley floor in which a river meanders is referred to as meander belt. As water moves through a channel bend, velocity along the 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).



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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 particles are carried beyond levees on to flood plains (fig.no.8). These particles settle at different rates to form back swamp deposits. Oxbow lake may be filled during floods and the clayey particles settle to the bottom to form highly plastic and compressible layers.

Lacustine and marine soils similar to glaciolacustrine and glaciomarine soils may be

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.9.Suspension, saltation and creep.

Saltation – (saltacio – to

dance) soil particles become temporarily airborne, and then fall back to earth. Upon landing, the particle either bounces or dislodges another particle, initiating another flight. Particles rise to 1m altitude and move horizontal distances around 4m.



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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.

Colluvial soils: a colluvial soil is one

transported down slope by gravity. The cause of movement may be creep or landslide. Creep occurs due to gravity induced down slope shear stresses (fig.no10)

Fig.no.10.Colluvial soils

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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Note: ASSIGNMENT Give detailed information about a major

soil failure from engineering history which is not mentioned in the handout.

Give two examples for different type of rocks and soils other than those mentioned in the handout.

Answer neatly, legibly and within three to 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 references available in the library and suggested in the course outline.



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SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA


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SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA

Sm Chapter II

Technology

Transcript of Sm Chapter II