This page has been reformatted by Knovel to provide easier navigation.

8 Geology forEngineers

P G Fookes DSc(Eng), PhD, BSc, FIMM,FIGSConsultant

Contents

8.1 Basic geology 8/38.1.1 Introduction 8/38.1.2 Principles of stratigraphy 8/38.1.3 Plate tectonics and evolution of the Earth 8/4

8.2 Geological description and classification of rock 8/58.2.1 Sedimentary rocks 8/58.2.2 Igneous rocks 8/68.2.3 Metamorphic rocks 8/98.2.4 A field identification of common rocks 8/108.2.5 Rock properties 8/10

8.3 Rock deformation in Nature – fractures and folds 8/108.3.1 Joints 8/108.3.2 Faults 8/128.3.3 Folds 8/148.3.4 Some engineering aspects of faults and

folds 8/15

8.4 Engineering geology environments 8/168.4.1 Processes acting on the Earth’s surface 8/168.4.2 Engineering significance of selected

geomorphological environments 8/168.4.3 Alluvial soils of rivers 8/24

8.5 Geological maps 8/288.5.1 General geological maps 8/288.5.2 Special geological maps 8/29

8.6 Geological information 8/308.6.1 Published data 8/308.6.2 Unpublished data 8/308.6.3 Books 8/328.6.4 Institutions 8/32

References 8/32

Bibliography 8/33

This chapter introduces civil engineers to some basic geologyand outlines the broad concepts of the subject.

Geology is concerned with the science of the Earth and thematerials comprising the Earth. This includes physical geologyor geomorphology (the surface form of the Earth), palaeontology(study of fossils), stratigraphy (the chronological sequence ofrocks), mineralogy (study of minerals), petrology (study of thecomposition of rocks) and structural geology or tectonics (thebroad structure of rocks). Together with newer and closelyrelated branches such as geochemistry, geophysics or mathema-tical geology, and applied and biological aspects, the wholesubject is rapidly developing and is now generally being calledEarth Science.

Engineering geology is the branch of geology applied to civilengineering and, in Britain particularly, is applied to all aspectsof foundation and excavation design, construction and perfor-mance. The extremes of the subject merge into the practices ofsoil mechanics, rock mechanics and some aspects of the extrac-tive industries, as sand and gravel or opencast mining (Price1).

8.1 Basic geology

8.1.1 Introduction

Rock is strictly defined in geology as any natural solid portion ofthe Earth's crust which has recognizable appearance and com-position. Some rocks are not necessarily hard, and in discussiona geologist may call peat or clay a rock as he would granite orlimestone.

There are three major classes of rocks:

(1) Sedimentary rocks formed by the deposition of material atthe Earth's crust, e.g. sandstone, clay.

(2) Igneous rocks formed from molten rock magma solidifyingeither at the Earth's surface or within the crust, e.g. basalt,granite (s.l.).

(3) Metamorphic rocks produced deep in the Earth by thetransformation of existing rocks through the action of heatand pressure, e.g. marble, slate.

The interrelation and continual recycling of rock over longperiods of geological time is illustrated in Figure 8.1.

8.1.2 Principles of stratigraphy

Sedimentary rocks cover some 75% of the Earth's land surface

but form only a discontinuous and relatively thin cover to theunderlying igneous and metamorphic rocks of the mantle.

The sedimentary layers (strata) normally lie one aboveanother in order of decreasing age, but where there has beenstructural disturbance they are faulted and folded. Study of thestrata in a particular area enables their sequence to be recorded,and this can then be compared with other local sequences. Fromsuch observations the general succession of sedimentary rocksover a wider area can be established: this has been done, forexample, for nearly the whole of the British Isles. A list of stratafor England and Wales was compiled by William Smith, 'thefather of English geology'; in 1815 he produced the first simplecoloured geological map of the country. As a result of hisstudies he stated two basic principles of stratigraphy, that 'thesame strata are always found in the same order of superposition,and contain the same peculiar fossils'. These principles are stillused to determine the relative ages of strata, i.e. in the order ofsuperposition for an undisturbed series of sedimentary beds, theoldest (i.e. the first deposited) is at the bottom, and successivelyyounger beds lie upon it. Sedimentary strata in different locali-ties can usually be correlated by the diagnostic fossil remainsthey contain. Rapidly evolving fossils act as horizon markers sothat a specimen of one of these enables the particular level of therock outcrop in which it occurs to be identified in the geologicalcolumn wherever in the world it is found.

The whole sequence of rocks comprising the geological col-umn is broadly divided into the systems and groups shown inTable 8.1; this column applies particularly to British strata. Thecolumn shows the age of each group relative to the others, andwas in use long before any of the recent radiometric methods ofdetermining the absolute age in years was developed. Names ofthe geological systems, and of the larger groups are of world-wide application; they are also used to express the periods oftime during which the rocks of the different systems wereformed, e.g. the Jurassic system and the Jurassic period, orMesozoic group and the Mesozoic era. The times of majormountain-building episodes (orogenies) and of phases of ig-neous activity in Britain are given in the third column of thetable.

There are numerous further subdivisions down to 'zones' andeven 'horizons', many of the smaller divisions being based onspecific fossils.

In any given area the deposition of sediments was notcontinuous throughout the geological periods. There are breaksin the sequence of deposits, marked by unconformities whichrepresent intervals of time during which there was no deposition

Figure 8.1 Diagrammatic representation of the long-term cyclingof rocks. (After Bradshaw, Abbot and Gelsthorpe (1978) TheEarth's changing surface. Hodder and Stoughton, London)

Mantle

Magma risingto form newocean-floorcrust

VolcanoesWind

CondensationWinds

Dust blown over ocean

EvaporationDepositionPlankton

Burial

Deepburial

Uplift Sedimentaryrock

Melting to form .̂magma and igneousrocks

Intrusions Metamorphicrock

and erosion took place. The sea floors with their sediments wereraised and became subject to erosion by wind and water. Therewere also periods of quiet sedimentation, when seas covered theland, and intervening episodes of disturbance when uplift andfolding took place. This broad pattern of events - the transgres-sion of the sea over the lands, then the regression of the sea,followed by erogenic upheaval - has been repeated many timesthroughout geological history (see Figure 8.2 which shows thetypical simplified borehole sequence of such a chain of events).

RECORD OF A TRANSGRESSION RECORD OF A REGRESSION(Advance of sea over the land) (Retreat of sea from the land)

(Youngest rocks) (Rocks of new land)Eolian sandstones

D e e p - w a t e r w i t h salt lenses on

marine s h a l e s n e w land sur:faceDeltaic sandstonesand shales

_. . . E s t u a r i n e o r lagoonalF ' n e - g r a m e d s h a l e s a n d marlssandstonesC?arse a n d M a r i n e sandstonesoften current-beddedConglomerate(unconformity)

Marine shales(Rocks of old land)

(Oldest rocks)

Figure 8.2 Marine transgression and regression as seen idealizedin borehole core, tens of metres long. (After Read and Watson(1971) Beginning geology, 2nd edn. Macmillan/Allen and Unwin,London)

Unconformities are often marked by beds of pebble gravel,the beach deposits of a sea which gradually inundated the landduring its submergence (see Figure 8.3). Examples of this are thepebbly quartzites at the base of the Cambrian, or the roundedflints at the base of the Eocene deposits of southeast Englandoverlying the Chalk, both marking the oncoming of marinetransgression. Boulder beds and hill or mountain screes formedon an old land surface during erosion, after uplift has takenplace, may also be preserved as the lowest members of a newerseries of rocks resting unconformably on older rocks; an ex-ample is the boulders and coarse sands at the base of theTorridonian in northwest Scotland which lie unconformably onan old land surface carved in the underlying Lewisian rocks.

An old land surface may be shown by the presence of a 'dirtbed' in which some of the old soil has been preserved, as atPurbeck, Dorset, or by other land-formed deposits. It indicatesan interval of time during which there was locally no depositionof waterborne sediments. In marine deposits a minor unconfor-mity (or nonsequence), representing a local cessation in deposi-tion, can be marked by the absence of a metre or so of beds overa relatively small area. This can be found by comparison withother areas where the sequence is complete.

8.1.3 Plate tectonics and the evolution of the Earth

The close association of volcanic and earthquake activity hasbeen known for some time but it is only during the last few yearsthat it has been more or less understood. This association,together with the coincidence of young narrow fold mountainranges on the continents, and trenches and ridges deep in theocean basins also in narrow zones, has led to a new theory ofEarth evolution known as plate tectonics. This idea was pro-posed in the late 1960s and has been received with widespread

Figure 8.3 Examples of common marine and freshwatertransgressions and regressions showing types and geometricdistribution of sediment deposited. (After Lahee (1961) Fieldgeology, 6th edn. McGraw-Hill, New York.) Lines parallel to lakeand sea floors are time lines as they join sediment depositedcontemporaneously. Lines essentially parallel to gravel, sand or claydeposits are formation lines. A, a marine transgression over the land;B, a marine regression from the land; C, a lake regression from theland; lake bottom muds are gradually covered by coarser sediments.Later transgression is shown left of a; D, an alluvial transgression bygrowth of a cone of river alluvium in mountainous area overlookinga desert plain

acceptance as more evidence has been found to fit the generalmodel.

The concept suggests that the Earth's surface layers aredivided into large segments or plates. Plates are approximately100km thick and therefore include the Earth's crust and theupper mantle, and measure several thousand kilometres across.One scheme considers there are six major plates and severalsmaller ones, covering the entire Earth. Plates slowly move overthe face of the Earth with new plate rock formed from thesolidification of slowly upwelling molten rock at the construc-tive margin as more new rock forms and travels towards thedestructive margin where it is subducted, and rock material ismoved downwards and returned to the lower mantle.

A plate may eventually accumulate a mass of lower densitysedimentary rocks on its top to form a continent. Whilst theocean-floor plate material is constantly being formed and des-troyed, the continents are not consumed downwards at thedestructive margin because their low density providesbuoyancy. The continents are subjected to changes due toerosion and deposition by surface processes, but this has theoverall effect of causing relatively light rocks to accumulate. Theoldest known continental rocks are 3900 million yr old (Table8.1) but nowhere are the ocean floor rocks known to be morethan 200 million yr old.

SEA LEVEL(Beach)

Approx. locationof borehole A ofFigure 8.2

Formation lineTime line

(Beach) SEA LEVEL

Formation lineTime line Approx. location

of borehole B ofFigure 8.2 LAKE LEVEL

Time line

Time line

8.2 Geological description andclassification of rock

Engineering classification of rock is discussed in Chapter 10,and engineering classification of soils in Chapter 9.

8.2.1 Sedimentary rocks

Sediments originate mainly from the weathering of all rocks,especially igneous rocks. Certain resistant minerals in igneousrocks such as quartz survive unchanged and are eventuallyincorporated in the new sediments; often they tend to beconcentrated in certain types of sediment (e.g. sands). Otherigneous minerals, such as the feldspars and ferromagnesianminerals, break down during weathering to give rise to newminerals and to colloidal and dissolved substances. The newminerals, chiefly clay-minerals, are concentrated in a secondgroup of sediments (e.g. clays) and the colloidal matter, usuallyiron hydroxides, in a third. The substances taken into solutioninclude calcium and magnesium salts which are precipitated bychemical and organic processes as carbonate rocks, and sodium

and potassium salts which may in certain circumstances crystal-lize out to give evaporites. Another group of sediments includ-ing coal and peat is produced by the piling up of decaying plantmatter.

The products of weathering can be related, as is showndiagrammatically in Figure 8.4, into fairly distinct chemical andgeological groups. This natural differentiation provides a simpleclassification of sediments into two broad groups:

(1) Detrital sediments made by the accumulation of fragmentedparticles of minerals or rocks, represented by (a) the pebblyrocks, and (b) the sands, made chiefly of inherited mineralsor rocks, and (c) the clays made chiefly of new minerals.

(2) Chemical-organic sediments formed by the precipitation ofmaterial from solution or by organic processes, representedmainly by the limestones, the evaporites and the coals.

The sediments produced go on changing after deposition; e.g.they may be saturated by groundwater carrying salts in solution,or deformed by the weight of new sediments laid down on top ofthem. Changes produced by such means are called diagenetic

Table 8.1 The geological column

Name of geological group or era

Quaternary

CAINOZOIC

Tertiary

MESOZOIC

(or Secondary)

PALAEOZOIC

Newer

(or Primary)

PRE-CAMBRIAN

Name of geological system orperiod (ages in millions ofyears)

[Recent ]Pleistocene I

(2)

PlioceneMioceneOligoceneEocene

(70)

CretaceousJurassicTriassic

(225)

Permian

CarboniferousDevonian(and Old Red Sandstone)

(c. 400)

SilurianOrdovicianCambrian

(c. 600)DalradianMoinian

(740 + )TorridonianUriconian

Lewisian(3500 + )

General nature of deposits, major orogenies, and igneousactivity in Britain

Alluvium, blown sand, glacial drifts, etc.At least five ice ages separated by warmer periods. TheDevensian (Weichselian or Newer Drift) is the last iceage

Sands, clays, and shell bedsAlpine orogenyIgneous activity in Scotland and Ireland

Sands, clays and chalkClays, limestones, some sandsDesert sands, sandstones and marls

Breccias, marls, dolomitic limestoneHercynian orogenyIgneous activityLimestones, shales, coals and sandstonesMarine sediments(Lacustrine sands and marls)Igneous activity

—Caledonian orogenyThick shallow-water sediments, shales and sandstones.Older Volcanic activity in the Ordovician

-SchistsSchists and granulites

Sandstones and arkosesLavas and tuffs (Shropshire)Pre-Cambrian orogeniesOrthogneisses, etc.

Figure 8.4 Sedimentary differentiation. (After Read and Watson(1971) Beginning geology, 2nd edn. Macmillan/Allen and Unwin,London)

changes and convert the sediments into consolidated or lithified(hardened) sedimentary rocks, e.g. a sand becomes a sandstone.

8.2.Ll Deposition environments and textures ofsedimentary rock

The characteristics and to a certain extent the engineeringperformance of recent sediments can be directly related to theenvironment occurring at their location of deposition, becausethe agents of deposition can still be seen in action. In the oldersedimentary rocks, the environment of deposition can be recon-structed from the characters of the rocks themselves. Theevidence for this reconstruction is provided by the compositionand texture of the rock, the type of bedding, the fossil contentand the relationship between any one bed and its neighbours.The sum of all these features decides its sedimentary fades andfrom this it is generally possible to deduce the conditions underwhich each rock was formed. This is summarized in Table 8.2.

The most obvious and characteristic feature of sedimentaryrocks is bedding, i.e. the presence of recognizably different bedsor strata in a sedimentary succession, and the presence withinany one bed of depositional surfaces which are the beddingplanes (see Figure 8.5).

Although many beds are homogeneous, some show consider-able variation, especially graded beds, in which there is a passagefrom coarser to finer particles towards the top; lateral gradationmay also be found. Thin laminae or layers, differing somewhatin colour or texture, may be present without causing a bed tolose its individuality. A bed is characterized by all of its

lithological features. These indicate that it was laid down in aparticular environment, either uniform, or varying systemati-cally. Although it may be arbitrary, some very thin strata maybest be regarded as beds rather than as laminae within a bed.For example, in glacial varves each annual deposit of summersilt and winter clay is an individual bed even though its thicknessis measured in millimetres, whereas sandy laminae in a gradedgreywacke are parts of the whole graded unit (see Figure 8.6).

In describing bedding it is necessary to distinguish firstlybetween bedding planes which are individual structures whereeach planar surface may be distinguished, and also depositionaltextures, which result from the parallel orientation of particlesthroughout a bed. Both are primary depositional features, andmay be either parallel or inclined to the separation planes,bounding individual beds (Figure 8.6). In addition, varioustextures, the parallel orientation of mica-flakes, for example,may be induced by post-depositional effects such as consolida-tion. These are post-depositional fabrics but in many instancesthey are very difficult to separate from true depositional fabrics.

8.2.2 Igneous rocks

The important characteristics of igneous rocks are the chemicalcomposition, the mineral composition and the texture.

8.2.2.1 Chemical composition

The chemical composition depends on the magma from whichthe igneous rock was derived. Some 99% of the various igneous

Original rocks Weatheringprocesses

Weathering products Sediments

Mechanicaldisintegration

Pebbles ofbroken-up rock

PSEPHlTES(gravels

etc.)

Sand grains ofresistant minerals,

mainly quartz

PSAMMlTES(sands,

etc.)

New minerals,mostly clay-

illite,montmorillorite,kaolinite

ORIGINALIGNEOUSROCKS

Mechanical andchemical break-up

of easily-alteredminerals

Colloidalsubstances

PELlTES(muds, clays,

etc.)

LESS COMMONSEDIMENTS

I V APOR/TLSmainly-rock-

saltPrecipitation

Carbonates, halidesand sulphates in

solution

LIMESTONESAND

DOLOMITESfrom solutionby animalsand plants

Extractionfrom soilby plants

Plant tissuesDecayofplants

PEATAND

COAL

DE

TR

ITA

L S

ED

IME

NT

SC

HE

MIC

AL

-OR

GA

NIC

SE

DIM

EN

TS

rocks are made up by combinations of only eight elements. Ofthese, oxygen is dominant, next is silicon and then aluminium,iron, calcium, sodium, potassium and magnesium. In terms ofoxides, silica (SiO2) is by far the most abundant, ranging from 40to 75% of the total. The silica percentage therefore forms thebasis of a fourfold chemical classification of the igneous rocks,the limits being given on Figure 8.7.

8.2.2.2 Mineral composition

Mineral composition depends largely upon the chemical compo-sition. The chief minerals present will normally be silicates of thesix common metal cations noted, together with quartz, whensilica is present in excess. The minerals which actually form willbe controlled by the silica percentage and the relative abundanceof the cations. For example, silica-poor silicates such as olivine

Figure 8.5 Idealized types of sedimentary bedding. (AfterSherbon Hills (1972) Elements of structural geology, 2nd edn.Chapman and Hall, London) A, sandstone with discrete beddingplanes parallel to separation planes. Some beds ripple-marked (r);B, sandstone with discrete bedding planes inclined to separationplanes (false or cross-bedding; an inclined deposition texture); C,conglomerate with long axes of pebbles approximately parallel toseparation planes (a parallel depositional texture); D, edgewiseconglomerate with long axes of pebbles inclined to separationplanes (an inclined depositional texture); E(a), unconsolidated mudwith random orientation of mica flakes and clay particles (a randomdepositional texture); E(b), consolidated clay or lithified mudstonewith flaky particles approximately parallel, and parallel withseparation planes (a parallel consolidation texture); F(a), mudstonewith mica flakes deposited parallel to separation planes, but lackingdiscrete bedding planes (a parallel depositional texture, cf. Cabove); F(b), mudstone with mica flakes deposited parallel toseparation planes, and showing discrete bedding planes. A thin bedof sandstone lies between the two mudstones

Table 8.2 Environments of deposition of sedimentary rocks

Environment of deposition

SEA

Shallow seas (continental shelf) Littoral (beaches, sandbanks, tidal flats)

Neritic I Shelf seas in sta^le areas

I Restricted deep basins

{ Geosynclinal seas inmobile beltsDeep seas in stable areas

Abyssal seas

LAND/SEA

DeltasEstuaries, lagoons

LANDFloodplain_ , i with outlet to sea

a es 1 in basins of interior drainageDesertsPiedmont (intermontane basins, alluvial fans)Areas of glaciation

Common sedimentary rocks produced by theenvironment

Conglomerate, sandstone, shaleOrthoquartzite, current-bedded sandstone,shale, organic and chemical limestonesBlack shale

As for shelf seas with in addition greywackesand other turbidite deposits

Calcareous ooze, siliceous ooze, Red Clay

Mainly sandstone, shaleShale

Conglomerate, sandstone, shaleSandstone, shale, freshwater limestoneSandstone, shale, evaporatesSandstone, conglomerate, brecciaConglomerate, breccia, arkose, sandstoneTillite

Figure 8.6 Idealized types of sedimentary beds. Beds arebounded by separation planes (S). A, uniform, massive sandstonewith bottom structures at its base; B, simple graded bed, withuniform grading from coarse sandstone below to shale above and awashout (w); C, complex graded bed with thin sandstone laminae(I, I) in the shales; D, E, F, G, individual thin beds; H, singlesandstone bed with discrete bedding planes (b, b, etc.); I, J, twosandstone beds separated by shale parting (p); K, heterogeneousbed of sandstone containing angular shale fragments; L,heterogeneous bed of conglomerate containing lenses of sand andgravel

will be most abundant in the ultrabasic and basic rocks andabsent from the silica-rich acid rocks.

The chief minerals are quartz, orthoclase and plagioclasefeldspars, micas, amphiboles, pyroxenes and olivines. Theirdistribution in the four chemical groups - ultrabasic, basic,intermediate and acid - established by silica percentage is showndiagrammatically in Figure 8.7. Many of the names given toigneous rocks are defined according to the presence of two orthree particular minerals which are the essential minerals forthat rock type. Other accessory minerals may also be present insmall quantities, e.g. the essential minerals of granite are quartz,feldspar and mica; common accessories are zircon and ironoxide.

The predominant minerals of an igneous rock may determineits general appearance and it is usually possible to get some idea

Figure 8.7 A classification of igneous rocks based on a silicapercentage

of its composition from its colour and density. Quartz iscommonly colourless and transparent, feldspars opaque butpale coloured. Rocks made mostly of these minerals (i.e. acidand intermediate rocks) are therefore usually pale in colour andrelatively light in weight. The coloured ferromagnesian silicates,olivines, pyroxenes and amphiboles, are abundant in basic andultrabasic rocks which are usually dark and relatively heavy.Two important exceptions are very fine-grained or glassy rockswhich tend to look dark whatever their composition, andweathering or other alteration which changes the colours ofminerals. It is, therefore, usually necessary to look at fresh-broken surfaces to diagnose the parent rock type.

8.2.2.3 Texture

The texture of an igneous rock is shown by the arrangement ofthe constituent minerals and the relation of each mineral to itsneighbours. The main textural character is the grain size and ina general way this depends on the rate of cooling of the magma.Coarse-grained rocks are the result of slow cooling whichallowed time for the growth of large crystals; fine-grained rocksare produced by rapid cooling. By extremely rapid cooling, notime at all is given for crystallization and glasses are formed.Holo-crystalline rocks are entirely crystalline, hypo-crystallineare partly crystals, partly glass. A common distinctive texture isthe porphyritic texture in which crystals of two different sizesoccur: large phenocrysts are scattered through a finer-grained orglassy groundmass. The texture is an important controllingfactor in the engineering performance of the rock.

8.2.2.4 Classification

Classification of the common igneous rocks is usually made onthe basis of grain size and silica percentage as given in Table 8.3.The characteristic minerals of rocks of different compositionsare shown in Figure 8.7 which should be studied with Table 8.3.

8.2.2.5 Form

A body of magma which is under pressure in the sial may beforced upwards intruding into the upper rocks of the crust.During the process of intrusion it may incorporate some of therocks with which it comes into contact, by assimilation. In somecases it may also give off mobile fluids which penetrate andchange the rocks in its immediate neighbourhood and minerali-zation may occur. If the intrusive magma cools at some depthbelow the surface, the rocks which result are called plutonicrocks and are coarsely crystalline; a large mass of this kindconstitutes a major intrusion, e.g. a granite batholith which may

OlivineAugiteHornblende

AugiteOlivineHornblende

BiotiteHornblendeAugite Biotite ^*Hornblende

^•N^I

FERROMAGNESIAN (MAFIC) MINERALS

PLAGIOCLASE FELDSPARSORTHOCLASEFELDSPAR

QUARTZ

ULTRABASIC BASIC INTERMEDIATE ACID

Approximate percentage of silicaApprox. scale 1 m

Appro

ximate

perc

entag

eof

ferro

magn

esian

mine

rals

have an aureole of thermally altered rock. When magma risesand fills fractures or other lines of weakness in the crust, it formsminor intrusions. These include dykes, which are steep orvertical wall-like masses, with more or less parallel sides, andsills, which are sheets of igneous rock intruded between beddingplanes of sedimentary rocks and lying more or less horizontal.Dyke and sill rocks commonly have a fine-grained texture. Veinsare smaller and irregular bodies of igneous material, fillingcracks which may run in any direction.

Magma which rises to the Earth's surface and flows out as alava, is called extrusive, and under these conditions it loses mostof its gas content. These are the volcanic rocks, and since theyhave cooled comparatively quickly in the atmosphere they arefrequently glassy (i.e. noncrystalline), or very fine-grained withsome larger crystals.

These forms are summarized in Figure 8.8.

of different spatial features of the rock, both small (microscopic)and large (macroscopic). For example, gas bubble holes in anigneous rock may be characteristic of its structure. A vesicularstructure is the presence of small holes, or vesicles, throughoutthe igneous rock, such as are found in pumices and somebasalts. Holes larger than vesicles are vugs and are generallyfilled with minerals other than those forming the rock.

An important macroscopic structural feature is jointing of therock. Joints are fractures and may be open or closed and run invarious directions. They usually occur in more-or-less regularsystems and may tend to break the rock into cubes or otherregular blocks. This is an important engineering property and isdiscussed further later. Fractures or cracks are also macroscopicfeatures and may run in any direction and may intersect eachother at any angle. A fracture usually has an irregular surface incontrast to the planar or even surface of a joint.

8.2.2.7 Fabric

'Fabric' is a controversial term which sometimes is consideredas a generalization of the term 'texture'. Here, igneous fabricdenotes the spatial pattern of the rock particles which includesgrain sizes and their ratios, grain shapes, grain orientation,microfracturing, packing and interlocking of particles, thecharacter of the matrix, and so on, all of which help control theengineering performance of the rock.

8.2.3 Metamorphic rocks

Rocks formed by the complete or incomplete recrystallization,i.e. the change in crystal shape or in composition, of igneous orsedimentary rocks by high temperatures, high pressures, and/orhigh shearing stresses, are metamorphic rocks. A platy orfoliated structure in such rocks indicates that high shearingstresses have been the principal agency in their formation.

Foliation is not always visible to the naked eye, but individualgrains may exhibit strain lines when seen under the microscope.

Figure 8.8 Idealized forms of intrusive plutonic rocks

8.2.2.6 Structure

The use of the term structure is reserved for more pronouncedfeatures of a rock than those described by the term 'texture'. Inigneous rocks the structure may indicate a relative arrangement

Aureole

Extrusiveflows(lava)

Volcano

Table 8.3 A classification of igneous rocks on silica percentage and grain

Basic Intermediate Acid

Coarse-grained (plutonic) rocks. Grain size larger than about 5 mm. Liable to be brittle owing to presence of large crystals

GabbroNorite(Not very common in the British Isles)

SyeniteDiorite(Comparatively rare in the British Isles)

GraniteGranodiorite(Widely distributed in the British Isles)

Medium-grained (hypabyssal) rocks. Grain size between about 1 and 5 mm. Very frequently possess intergrown texture: includesome of the best roadstones

DoleriteDiabase(Widely distributed in the British Isles)

PorphyryPorphyrite(Fairly common in the British Isles)

MicrograniteGranophyre(Fairly common in the British Isles)

Fine-grained (volcanic) rocks. Grain size below about 1 mm, i.e. below the limit of visible recognition. Similar to medium-grainedrocks, but sometimes liable to be brittle and splintery

BasaltSpilite(Widely distributed in the British Isles)

«_Dark colour *

High specific gravity-*(2.9)

TrachyteAndesite(Not very common in the British Isles)

— Continuous variation in properties

(Due to increase in ferromagnesianminerals)

(Due to increase in ferromagnesianminerals)

RhyoliteFelsite(Not very common in the British Isles)

-^ Light colour

>Low specific gravity

(2.6)

NeckSill

Dyke

Tor

LaccolithBatholith

Metamorphic rocks formed without intense shear action have amassive structure. In Table 8.4 the most common metamorphicrocks are subdivided into two basic classes according to theirstructure. Foliated rocks usually have directional engineeringproperties.

8.2.4 A field identification of common rocks

Table 8.5 gives a simple field guide to the identification of themore common rocks. It is after the scheme by Krynine andJudd2 for engineers with little training in geology and has beendevised to present those features first seen when picking up ahand specimen. It is based primarily on texture and structure.They consider the scheme fits the most common occurrences ofthe rock but some variations will occur.

Textbooks such as the one by Lahee3 give more specializedfield identification techniques. Difficult or contentious identifi-cation should be carried out by a geologist who may requirethin-section examination of a slice of the rock or even geochemi-cal methods for complete identification.

8.2.5 Rock properties

Engineering characteristics of rocks are given more fully inChapter 10 on rock mechanics and of soils in Chapter 9 on soilmechanics. Geological characteristics are given in the engineer-ing geology and mineralogy and petrology textbooks listed inthe bibliography.

Table 8.6 (from Shergold4) gives some general properties ofcommon rocks and Table 8.7 (in part from Attewell andFarmer5) gives a range of mechanical properties of rocksidentified by their British Standard (BS) 812:1951 trade groupclassification. This classification should be used with caution asthe rocks listed in each group do not necessarily have closemechanical affinities. The results listed are probably on fairlyfresh rock types, i.e. not weathered in the manner following.

8.3 Rock deformation in Nature -fractures and folds

When rocks of the Earth's upper mantle are subject to largestresses, they either break or bend with the production offractures or folds. The kind of structure formed depends on thecondition of the rocks and the rate at which deformation takesplace. Most rocks are brittle at surface conditions and tend tofracture under stress though they may yield slowly by bending.At deeper levels where temperatures and pressures are high themajority of rocks become ductile and deform without breaking.Many special conditions at the Earth's surface cause minorfractures and folds, e.g. cooling of igneous lava, thermal stressby daily temperature changes, ground ice movement, and soildesiccation.

Figure 8.9 Block diagram of simple joint systems in an igneousrock. Systematic S joints are more commonly called longitudinaljoints, and Q joints are more commonly called cross-jointsperpendicular to the flow lines of the original molten rock

In sedimentary rocks, there are often two systems of mutuallyperpendicular joints, both perpendicular to the bedding plane.

Joints also may be grouped into strike joints and dip joints.Figure 8.10 illustrates the terms 'strike' and 'dip' where the rockbed is assumed to be an oblique plane. Strike is the direction ofcontour lines or lines of equal elevation on the surface of therock mass, and the dip is the maximum slope of its surface. InFigure 8.10 the dip is the angle a made by the line AB with thehorizontal. In measurements of dip, it is important to measurethe 'true' dip, i.e. the angle located in a plane perpendicular tothe strike; otherwise, a misleading apparent dip, ft in Figure 8.10,is recorded. These terms also apply to beds, faults and othergeometric features.

Joints and their orientation with respect to other structureshave been widely studied in the field and it has been establishedthat systematic joints usually show well-defined relationships tofolds and faults which develop during the same tectonic episode.

The spacing of joints varies considerably and is of importancein engineering. Some rocks, such as sandstones and limestonesin which the joints may be widely spaced, yield large blocks and

"Lines showing the flow of the originally liquid magma and indicatedby the long axes of crystals

8.3.1 JointsJoints are fractures without any displacement. They may appearto be somewhat random in direction, but a careful field exami-nation will usually show that they have-some relation to the hostrock, e.g. with the bedding in sedimentary rock or with flowlines in igneous rock.*

In igneous rocks there are often three regular sets of joints(Figure 8.9). In an ideal situation one set lies approximatelyhorizontal and parallel to the flow lines and is termed flat-lying.Another set, the cross-joints, is roughly perpendicular to theflow lines. The third set, the longitudinal joints, dips steeply andstrikes parallel to the flow lines if the latter are projected to aplane surface such as a map.

Non-systematicjoints

Systematicjoints

S jointsQ joints

Flow linesQ joints

Flat joints

Table 8.4 Metamorphic rock classification

Structure and texture

FOLIATED OR PLATY

MASSIVE:Banded, consisting of alternatinglensesGranular, consisting mostly ofequidimensional grains

Composition

Various tabular and/or prismaticminerals (generally elongated)

Various tabular, prismatic, and granularminerals (frequently elongated)

Calcite, dolomite, quartz, in smallparticles

Rock name

Schist, some serpentines, slate, phyllite

Gneiss

Marble or quartzite

Table 8.5 Field identification of rocks (specimens should be unweathered and not altered in any way)

GRAINS OR CRYSTALS VISIBLE TO NAKED EYE

Angular particles Rounded particles Erratic large grains

LargeFine tomedium Very fine

Foliated orbanded Large

Fine tomedium Very fine Rounded Angular

Pegmatite

Granite (+ Q,+ F)*Granodiorite( + Q, +F)*Monzonite(-Q, +F)*Syenite (-Q,+ F)*Marble(reacts withHCl)Arkose(usuallybedded)

Tuff(containsglasslikefragments)

Felsite*(rhyolite+ Q andtrachyte-Q)

Schist(shiny)Gneiss (mayhavesub-angularparticles)

Conglomerate(+10% ofgrains over2mmdiameter)Sandstone(bedded) (if itreacts to HCl= calcareoussandstone; if itgets slickwhen wet =argillaceoussandstone)

Quartzite(not friableand veryhard)

Siltstone Depositionalbreccia

Volcanicbreccia andagglomeratefault breccia(may haveclay)

NO GRAINS OR SPARSE CRYSTALS VISIBLE TO NAKED EYE

Glassylustre Dull lustre Shiny lustre Earthy appearance Laminated

Quartzite Felsites*(rhyolite,trachyte)

Schist(foliated)

Spongy,light wt

PumiceVolcanicash

Porous,moderate

wtSlickwhen wet

Chalk(HClreaction)

Shale

Not slick

ShaleSlate(dull)Phyllite(shiny)

Slick whenwet

ClaystoneMudstoneSerpentine(usuallygreasy andmay bebanded)

Not slick

Reactionto HCl

LimestoneChalk(earthy)

Noreactionto cold

HCl

Dolomite

GRAINS OR CRYSTALS VISIBLE TO NAKED EYE

Angular particles Rounded to subangular particles

Fine to medium

Peridotite(-Q, -B)*Gabbro(-Q, -B)*Diorite(-Q, +B)*Dolerite(-Q, +B)*

Very fine to glassy Graywacke (fine- to medium-grained)Dark sandstones

Andesite*Basalt (usually vesicular)*

NO GRAINS OR SPARSE CRYSTALS VISIBLE TO NAKED EYE

Glassy lustre Dull lustre - laminated Dull lustre - not laminated

Slick when wet

Obsidian Shale

Not slick

Shale (flexible)Slate (brittle)(dull)Phyllite (shiny)

Basalt*Serpentine (usually greasy and may bebanded)

* Rocks may contain occasional large crystals embedded in a very fine-grained matrix or occasional very large crystals in a medium-grained matrix - in either case the term'porphyry' is appended to the rock name, e.g. syenite porphyry.(+ Q) = contains numerous white or colourless quartz crystals. (+ B) = contains numerous flakes of black mica (biotite).(- Q) = contains little or no quartz. (- B) = contains little or no black mica.(-I- F) = contains numerous white to pink feldspar crystals.

[LIG

IIT-C

OLO

URE

D]

[DAR

K CO

LOUR

ED (

DARK

GRE

Y OR

GRE

EN T

O BL

ACK)

]

Table 8.6 Summary of means and range of values for mechanical tests in each trade rock-group

Trade Group classification (BS 812:1951)

Artificial

Basalt

Flint

Granite

Gritstone

Hornfels

Limestone

Porphyry

Quartzite

All groupsf

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

MeanRangeNumber of samples

Aggregate*crushingvalue

28(15-39)55

14(7-25)123

18(7-25)63

20(9-35)41

17(7-29)81

13(5-15)28

24(11-37)164

14(9-29)62

16(9-25)57

19(5-39)724

Aggregate*impactvalue

27(17-33)18

15(7-25)79

23(19-27)32

19(9-35)32

19(9-35)45

12(9-17)24

23(17-33)61

14(9-23)29

21(11-33)37

19(7-35)370

Aggregate*abrasionvalue

8.3(3-15)18

6.1(2-12)65

1.1(1-2)45

4.8(3-9)28

7.0(2-6)31

2.2(1-4)13

13.7(7-26)34

3.7(2-9)23

3.0(2-6)29

5.7(1-26)311

Water*absorption(per cent)

0.7(0.2-1.8)19

1.1(0.0-2.3)68

1.0(0.3-2.4)24

0.4(0.2-0.9)16

0.6(0.1-1.6)33

0.4(0.2-0.8)15

1.0(0.2-2.9)42

0.6(0.4-1.1)30

0.7(0.3-1.3)21

0.7(0.0-3.7)313

Specificgravity

2.71(2.6-3.4)19

2.80(2.6-3.0)68

2.54(2.4-2.6)24

2.69(2.6-3.0)16

2.69(2.6-2.9)33

2.82(2.7-3.0)15

2.66(2.5-2.8)42

2.73(2.6-2.9)30

2.62(2.6-2.7)21

2.68(2.3-3.4)313

Polished-stonecoefficient

0.50(0.35-0.60)9

0.56(0.45-0.70)25

0.35(0.30-0.40)4

0.56(0.45-0.70)13

0.69(0.60-0.80)18

0.45(0.40-0.50)4

0.43(0.30-0.75)33

0.51(0.45-0.60)13

0.57(0.45-0.65)8

0.53(0.30-0.80)134

"In these tests a numerically lower result indicates a better performance in the test. flncluding results from unclassified samples.

Figure 8.10 Idealized block diagram to show dip and strikerelationships

may be suitable for masonry, for example, whereas other rocksmay be so closely jointed as to break up into small pieces andmay be suitable for aggregate or other purposes. Some joints insedimentary rocks run only from one bedding plane to the next,but others may cross several bedding planes, and are calledmaster joints.

The ease of quarrying, excavating or tunnelling in hard rockslargely depends on the regular or irregular nature of the joints

and their surface characteristics, e.g. attitude, size, frequency,openness and spacing. Joints and other fractures controlgroundwater and air flow in otherwise intact rock and help topromote rock weathering.

8.3.2 FaultsFaults are fractures in the crust along which there has beendisplacement of the rocks on one side relative to those on theother.

The surface on which movement takes place during faulting isthe fault plane. It may be vertical, steeply inclined or gentlyinclined as with thrust faults. The intersection of a fault with theground surface is known as the fault line or fault trace. Theupper side of an inclined fault, and the rock which lies above it,is referred to as the hanging wall. Rock below it is the foot wall;dip faults strike parallel to the local direction of dip of the beds,strike faults are parallel to the strike and oblique faults cutacross both strike and dip directions.

Movements on a fault may be in any direction. The displace-

Table 8.7 Typical rock strengths, porosity and bulk densities of rock materials

Rock

GraniteDioriteDoleriteGabbroBasaltSandstoneShaleLimestoneDolomiteCoalQuartziteGneissMarbleSlateRhyoliteAndesite

StrengthN/mm-2

Compressive Tensile Shear

100-250150-300100-350150-300150-30020-1705-100

30-25030-2505-50

150-30050-200

100-250100-200

—50-200

7-2515-3015-3515-3010-304^252-105-25

15-252-5

10-305-207-207-20—~

14^50—

25-60—

20-608̂ 103-30

10-50——

20-60——

15-30——

Bulk density(Mg/nr3)

2.6-2.92.7-3.05?.7-3.052.8-3.12.8-2.92.0-2.62.0-2.42.2-2.62.5-2.6

—2.6-2.72.8-3.02.6-2.72.6-2.72.4-2.62.2-2.3

Porosity(«%)

0.5-1.50.1-1.00.1-0.50.1-0.20.1-1.0

5-2510-305-201-5—

0.1-0.50.5-1.50.5-2.00.1-0.5

4-610-15

ment or slip is the sum of all the previous effects of movementand is shown by the relative positions on either side of the faultof two originally contiguous features as a bedding plane. Thevertical component of the slip, taken by itself, is called the throwof the fault (see Figure 8.11).

Faults can be classified, according to the direction of move-ment that has taken place on them, into normal faults, reversefaults and transcurrent or strike-slip faults.

Maximum principal stress

fault plane. Small normal faults are extremely common inalmost all geological situations and may even occur in Quater-nary sediments. Large normal faults, occurring in groups,produce a considerable effect of lengthening and are especiallycommon in the more stable areas of the Earth's crust. Groups offaults are arranged so that alternate dislocations dip in oppositedirections and produce the effect of block faulting illustrated inFigure 8.12; the crust is separated into high blocks or horstsbetween outward-dipping faults and low blocks, troughs orgraben between inward-dipping faults.

(b ) Reverse fau

Figure 8.11

Normal faults. Normal faults (originally so-called becausethey are the normal type found in coalfields in the UK) are thosein which the hanging-wall rocks have moved down the dip of the

Figure 8.12 Idealized block diagram of some common faultgroups. Note there is little effect on topography here as the surfacebed is the same in all locations, but where difficult beds areexposed by the faulting, scarp topography may be found

Reverse faults. Reverse faults are those on which the rocks ofthe hanging wall move up the dip of the fault plane. They resultin shortening across the fault and in duplication of strata;reverse faults with low dips are thrusts.

Transcurrent faults. These are wrench faults, tear faults orstrike-slip faults on which horizontal movement takes place.The fault planes are almost vertical and the effect of faultingwhen seen on a map is to shift rocks laterally, even for manytens of kilometres. Examples of block diagrams to illustratemapped outcrop patterns of faults are shown in Figure 8.13.

An example of the relationship between faulting and jointingin one complete episode is shown in Figure 8.14 from thetextbook by Price.6 Techniques and the use of stereographicprojection in geology is given in Phillips.7

Maximumprincipalstress

•Foot wallNormal fault

Hanging wall

Figure 8.13 Idealized outcrop patterns of faulted beds. A, dipfault, i.e. movement in the dip direction; B, strike fault withdownthrow in dip direction; C, strike fault with downthrow againstthe dip angle. (After Read and Watson (1971) Beginning geology,2nd edn. Macmillan/Allen and Unwin, London)

8.3.3 FoldsIn geology weak rocks which deform under stress are termedincompetent whereas strong rocks that buckle and fracture aretermed competent. These terms should not be confused, how-ever, with similar terms describing the bearing capacity offoundation rocks.

A complete fold is composed of an arched portion, oranticline, and a depressed trough or syncline (Figure 8.15a). Thehighest point of an anticline is called the crest, and the inclinedparts of the strata where anticline and syncline merge are thelimbs of the fold. The youngest beds outcrop in the middle of asyncline and the oldest in the middle of an anticline.

The plane bisecting the vertical angle between equal slopes oneither side of the crest line is the axial plane. Where this isvertical, as in Figure 8.15a, the fold is upright and symmetrical;where it is inclined the fold is asymmetrical (Figure 8.15b).Sometimes the middle limb has been brought into a verticalposition by the compression which buckled the strata, andunder still more severe conditions an overturned fold, or over-fold, is produced (Figure 8.15c). Here the middle limb is inclinedin the same attitude as the axial plane, and the beds of which it iscomposed have a reversed dip, i.e. upper beds are now broughtto dip steeply beneath lower beds, an inversion of the truesequence.

If the compression is so extreme as to pack a series of foldstogether so that their limbs are all virtually parallel and steeplydipping, the structure is referred to as isoclinal folding, i.e. alllimbs have the same slope (Figure 8.15c).

Where the axial plane is inclined at a low or zero angle, thefold is said to be recumbent (Figure 8.15d), a type which is oftenfound in intensely folded mountain regions such as the Alps.

The term monocline is for the kind of flexure which has twoparallel gently dipping limbs with a steeper middle limb betweenthem: it is in effect a local steepening of the dip in gently dipping(or horizontal) beds.

Figure 8.14 (a) Block diagram showing orientation of faults andjoints in unfolded rocks which may result from various phases ofcompression and tension related to one complete tectonic episode;(b) stereogram of fault orientations shown in (a); (c) stereogram ofjoint orientation shown in (a); (d)-(g) orientation of stress fieldswhen the various groups of faults were initiated. (Redrawn fromPrice (1966) Fault and joint development in brittle and semi-brittlerock. Pergamon Press, Oxford)

Recumbent Overturned Isoclinal

After faultingand erosionBefore faulting

Axial planeAxis

Figure 8.15 Idealized fold types, (a) Simple or gentlesymmetrical; (b) simple or gentle asymmetrical; (c) tightassymetrical, recumbent, overturned and isoclinical; (d) recumbentpassing into a thrust fault

The dimensions of anticlines and synclines vary between wideextremes, from small puckers millimetres across in sharplyfolded sediments, to broad archings of strata whose extent ismeasured in kilometres. The growth of such structures is, ingeneral, a process which goes on slowly as stresses develop inany particular part of the Earth's crust; but superficial folds maydevelop in a comparatively short space of time, e.g. earthquakeripples forming quickly, in weak sediments or some types ofhillcreep. Simple land topography largely controlled by foldingis illustrated in Figure 8.16.

Figure 8.16 Simple fold forms and related topography. A, steptopography; B, unconformity; C, normal fault; D, anticline; E, hog'sback ridge; F, syncline; G, dip slope; H, scarp slope

8.3.4 Some engineering aspects of faults and folds

Any geological structure that influences one of the mass proper-ties of the in situ rock, such as the strength, modulus ofdeformation or permeability, is highly significant. The mostcommon structural features of significance are joints, beddingplanes and foliation surfaces and 'shears' or faults. These are allplanar or near-planar discontinuities, and have a strong aniso-tropic effect on the mass properties.

A search for discontinuities and other faults is not always

Figure 8.17 Representation of structural geology data concerning four possible slope failure modes, plotted on equatorial equal-areanets as poles and great circles. (After Hoek and Bray (1974) Rock slope engineering, 2nd edn. Institute of Mining and Metallurgy,London)

Slope face Poles ofindividualjoint planes

Circular failure inheavily jointed rockwith no identifiablestructural pattern

Great circlerelevant topole concen-tration

Plane failure in highlyordered structure suchas slate

Wedge failure on twointersecting sets ofjoints

Toppling failure causedby steeply dipping joints

Poleconcentration

effective during site investigation, and significant faults, forexample, are sometimes not discovered until construction oreven afterwards. Stability of hillsides, cut slopes, quarry facesand so on may often be controlled by the geometric arrange-ment of joints and faults. (For examples see Figure 8.17.) Alsothe groundwater pattern may be controlled by the condition ofthe joints and faults whether they are open or closed or filledwith debris or gouge and the persistence or continuity of suchfractures may be important.

On large works the determination of whether a fault is active,inactive or passive may be important. Active faults are those inwhich movements have occurred during the recorded historyand along which further movements can be expected any time(such as the San Andreas and some other faults in California).Inactive faults have no recorded history of movement and areassumed to be and probably will remain in a static condition.Unfortunately, it is not possible yet to state definitely if anapparently inactive fault will remain so. The fault may reopen,either because of a new stress accumulation in the locality orfrom the effect of earthquake vibrations.

From the alteration products of faulting, gouge is probably ofthe most concern in foundation problems. This is usually arelatively impervious clay-grade material and may hinder orstop the movement of groundwater from one side of the fault tothe other and so create hydrostatic heads, e.g. if encountered ina tunnel. It may also reduce sliding friction along the faultplane. The presence of soft fault breccia or gouge may causesudden squeezes in a tunnel that intersects the fault. Arch actionof rocks in tunnels may be reduced by the presence of joints andfaults. Rock falls on cuts and in tunnels, patterns of rock bolts,grout holes and so on are all controlled to a large extent by thejoint and fault pattern.

In foundations, folds are generally not so critical as faultsthough they may give stability problems if their geometry isunfavourable. Occasionally, folds may influence the selection ofa dam site; e.g. when the reservoir is located over a monoclinecontaining pervious strata, there may be excessive seepage if themonocline dips downstream. If the monocline were to dipupstream, the reservoir might have little seepage providing themonocline contained some impervious layers such as shalewhich were not fractured in the folding. Serious water problemsmay arise in the construction and maintenance of tunnelsintersecting synclines containing water-bearing strata. In deepcuts, analogous water problems arise that may create conti-nuous maintenance problems.

Dipping beds, which must be part of a fold system, may causestability problems if the dip is unfavourable into a cut face(Figure 8. ITb).

8.4 Engineering geology environments

A geological environment is the sum total of the externalconditions which may act upon the situation. For example, a'shallow marine environment' is all the conditions acting off-shore which control the formation of deposits on the sea bed:the water tenjperature, light, current action, biological agen-cies, source of sediment, sea bed chemistry and so on.

The concept of geological environment forms a suitable basisto study systematically the engineering geology of the depositsformed in or influenced by the various environments, as theycondition the in situ engineering behaviour of the variousdeposits. A knowledge of the parameters of the environmentenables predictions and explanations of the engineering be-haviour to be attempted. Geomorphology is the study of thegeology of the Earth's surface (see Fookes and Vaughan8).

8.4.1 Processes acting on the Earth's surface

A landform may be defined as an area of the Earth's surfacediffering by its form and other features from the neighbouringareas. Mountains, valleys, plains and even swamps are land-forms.

The principal processes that are continually acting on theEarth's surface are gradation, diastrophism and vulcanism.

(1) Gradation is the building up or wearing down of existinglandforms (including mountains), formation of soil andvarious deposits. Erosion is a particular case of gradation bythe action of water, wind or ice.

(2) Diastrophism is the process where solid, and usually therelatively large, portions of the Earth move with respect toone another as in faulting or folding.

(3) Vulcanism is the action of magma, both on the Earth'ssurface and within the Earth.

With the exception of vulcanism and sometimes erosion, theseprocesses may take hundreds and even millions of years tochange the face of the Earth significantly. The sudden eruptionof a volcano, for example, with the ensuing flow of lava ordeposition of volcanic ash, can abruptly change land overnight.

Origin of soils. The majority of the soils are formed by thedestruction of rocks. The destructive process may be physical, asthe disintegration of rock by alternate freezing and thawing orday-night temperature changes. It may also be by chemicaldecomposition, resulting in changes in the mineral constituentsof the parent rock and the formation of new ones.

Soils formed by disintegration and chemical decompositionmay be subsequently transported by the water, wind or icebefore deposition. In this case they are classified as alluvial,aeolian, or glacial soils and are generally called transported soils.However, in many parts of the world, the newly formed soilsremain in place. These are called residual soils.

In addition to the two major categories of transported andresidual soils, there exist a number of soils that are not derivedfrom the destruction of rocks. For example, peat is formed bythe decomposition of vegetation in swamps; some marly soilsare the result of precipitation of dissolved calcium carbonate.

Soil-forming processes. There are very many and varied pro-cesses that take place in weathered rock and soils that affect theformation of soil profiles to varying degrees, but the major soil-forming processes are: (1) organic accumulation; (2) eluviation;(3) leaching; (4) illuviation; (5) precipitation; (6) cheluviation;and (7) organic sorting.

The soil-forming processes produce an assemblage of soillayers at horizons, called the soil profile. In its simplest it iscategorized as three layers A, B and C but numerous varieties ofthis and many other soil classifications exist. Probably the mostgenerally accepted one is that based on a geographical ap-proach. This is the zonal scheme thought to reflect zones ofclimate, vegetation and other factors of the local environment.

8.4.2 Engineering significance of selectedgeomorphological environments

Much of what can be called 'classical' geotechnical engineeringhas developed in temperate climate regions of the Earth. As aresult many of the concepts of soil and rock behaviour and theirproperties have been conditioned by the soil and rock foundthere. The climate and local geology play a major role indetermining the local geotechnical characteristics of the soilsand rocks. Figure 8.18 shows the generalized distribution of thefour principal climatic engineering soil zones after Sanders andFookes.9