Aenpowspeimns005-00005-00r005:002005-001005-00200-05100-00200-05100-0000-001002-00151-0001-05001-05051-0001-52883五点豆O-St00-01SEI-S$t-or11Se1-sor5510101-001OT-O1T$EI-001501-06501-080r1-06001-06511-06011-08g Paaeoenapa-iuoe aeewppppanoiaoedee10ojedaaeoojeaogsrarooedaoeop papeeapaopa-agnecoooojusopaogaeud ou oeSe pap anpapnpp2皇opedonoojensomoaraaaoAarataaeaa1004 01 od s Emood o 00d on daeo12[3joodoniniswnipuioisniuoisuousoenyuou2U0u 3041IV拍484K9AOwnas0营营福IAONASONNA ON健apompw"aREsswaisssoswnipanwniponWRISigin ou o odieuo oon0n poosrs t0nPIEEN ONienONis 10NisOaqens ToNn喜喜喜POoR 01Ppoa on dsiens sonsnains toNo建eienseNamsPoO国信福JONIF.100d kta4. 09.400poo.onapoom on a 01 400d100a.poog2poOpoogdB0SPa1a681.aXTable4RelationbetweentheUscS classificationand someengineeringproperties12
12 Table 4 Relation between the USCS classification and some engineering properties
BothclassificationsystemsshowthatmaterialswithahighLLandahighPIareratedaspoorsoils.The Atterberg limits however are related to the combined effects of two intrinsicproperties of a claybeing the particlesizeand themineralogical composition.It has beenfound that,for aparticular clay,the plasticity index depends on theclay fraction and that theratio PI/clayfraction isconstantforagiventypeof clay.TheratioPI/clayfractionistermed the colloidal activity,or simply the activity of a clay:A, = PI / clay fractionWherethe clayfractionisexpressedasthepercentage of particles<2μmpresentinthematerial tested. According to their activity, clays can be classified as shown here-after.ActivityClassification<0.75Inactive clays0.75-1.25Normal clays1.25 - 2Active clays>2 Highly active claysThehighertheactivity,themoreproblematictheclaywillbe.4. Soil forming and pedological identificationsystems4.1IntroductionThe origin of the natural soil and rock materials used by the civil engineer is shownschematically in figure 9.The origin of most of the natural gravels used in North America andNorthern Europe, where almost every river has left large deposits of durable, granularmaterial there for the digging, falls into the "transported soil" category in figure 9.Furthermore the Ice Ages and their glacial activity had a significant influence on the soilformationintheareasmentionedabove.Incontrast,inmanyotherpartsoftheworldthetransported soils are often expansive clays and not suited to the needs of the road engineerToobtainharddurablematerialsonethereforemustblastandcrushrockWhenthisisnotessentialonemayuseothermaterialssuchasweatheredrocks(residualsoilsinfigure9)andpedogenic materials such as calcrete and ferricrete. Especially in these cases, engineeringgeologyandpedology(thatisthegeologyoftheupperfewmetersoftheearthcrust)helpsin gettinga better understanding and use of these materials.In such cases more knowledgeisneededthanstraightforwardcivilengineeringknowledgePedogenicMaterialsEDETDTransported soilResidual soilWSedimentaryrockWMM,MetamorphicrockIgneousrockcRMagmaNote: C = crystallization, W = weathering, E := erosion. Transportation,D =deposition,P = pedogenesis,L=lithification, M = metamorphism, R = remelting.Figure 9 Rock -soil cycle [3].13
13 Both classification systems show that materials with a high LL and a high PI are rated as poor soils. The Atterberg limits however are related to the combined effects of two intrinsic properties of a clay being the particle size and the mineralogical composition. It has been found that, for a particular clay, the plasticity index depends on the clay fraction and that the ratio PI / clay fraction is constant for a given type of clay. The ratio PI / clay fraction is termed the colloidal activity, or simply the activity of a clay: Ac = PI / clay fraction Where the clay fraction is expressed as the percentage of particles 2 m present in the material tested. According to their activity, clays can be classified as shown here-after. Activity Classification 0.75 Inactive clays 0.75 – 1.25 Normal clays 1.25 – 2 Active clays 2 Highly active clays The higher the activity, the more problematic the clay will be. 4. Soil forming and pedological identification systems 4.1 Introduction The origin of the natural soil and rock materials used by the civil engineer is shown schematically in figure 9. The origin of most of the natural gravels used in North America and Northern Europe, where almost every river has left large deposits of durable, granular material there for the digging, falls into the “transported soil” category in figure 9. Furthermore the Ice Ages and their glacial activity had a significant influence on the soil formation in the areas mentioned above. In contrast, in many other parts of the world the transported soils are often expansive clays and not suited to the needs of the road engineer. To obtain hard, durable materials one therefore must blast and crush rock. When this is not essential one may use other materials such as weathered rocks (residual soils in figure 9) and pedogenic materials such as calcrete and ferricrete. Especially in these cases, engineering geology and pedology (that is the geology of the upper few meters of the earth crust) helps in getting a better understanding and use of these materials. In such cases more knowledge is needed than straightforward civil engineering knowledge. Pedogenic Materials P E E T D Transported soil Residual soil L W Sedimentary rock W M M Metamorphic rock Igneous rock R C Magma Note: C = crystallization, W = weathering, E = erosion, T = transportation, D = deposition, P = pedogenesis, L =lithification, M = metamorphism, R = remelting. Figure 9 Rock – soil cycle [3]
Figure 9 indicatesthat one can discriminate between sedimentary soils and residual soils.Thiswillbediscussedbrieflyhere-afterwhereuse ismadeof thetextofchapters7.2and7.3of reference [4].SedimentarysoilsThe formation of sedimentary soils can best be presented by considering sediment formation,sedimenttransportation and sediment deposition.SedimentformationThe most important manner of forming sediments is by the physical and chemical weatheringof rocks on the surface of the earth. Generally silt-, sand-,and gravel-sized particles areformed by the physical weathering of rocks and clay-sized particles are formed by thechemical weatheringofrocks.Theformationofclayparticlesfromrockscantakeplaceeitherby the build-up of the mineral particles from components in solution, or by the chemicalbreakdownofotherminerals.SedimenttransportationSediments can be transported by any of the following five agents: water, air, ice, gravity, andorganisms. Transportation affects sediments in two major ways: (a) it alters particle shape,size, and texture by abrasion,grinding, impact, and solution; (b)it sorts the particles.Table 5summarizessomeoftheeffectsofthefivetransportingagents.AirIceWaterGravityOrganismsSizeReduction throughConsiderableConsiderableConsiderableMinor abra-solution, littlereductiongrinding andimpactsion effectsabrasion in sus-impactfrom directpended load, someorganicabrasion and impacttranspor-tionin traction loadShapeRounding of sandHigh degreeAngular, soledAngular, nonandof roundingparticlessphericaland gravelround-nessSur-Sand:smooth,poImpact produ-Striated sur-Striated sur-facelished, shiny.cesfrostedfacesfacestextu-Silt: little effectsurfacesreSorting ConsiderableVery conside-Very little sor-No sortingLimited sor-tingtingsortingrable sorting(progressive)Table5Effectsoftransportationonsorting[4]Residual soilsResidual soils results when the products of rock weathering are not transported as sedimentsbutaccumulateinplace.If therateof rockdecompositionexceedstherateof removal of theproducts of thedecomposition,an accumulation of residual soil results.Among the factorsinfluencing the rate of weathering and the nature of the products of weathering are climate(temperature and rainfall), time, type of source rock, vegetation, drainage and bacterialactivityThe residual soil profile may be divided into three zones:the upper zone where there is a high degree of weathering and removal of material,the intermediate zone wherethere is some weathering at thetop part of the zone,but also somedeposition toward the bottom of the zone,the partially weathered zone where there is the transition from the weatheredmaterial into the unweathered parent rock.14
14 Figure 9 indicates that one can discriminate between sedimentary soils and residual soils. This will be discussed briefly here-after where use is made of the text of chapters 7.2 and 7.3 of reference [4]. Sedimentary soils The formation of sedimentary soils can best be presented by considering sediment formation, sediment transportation and sediment deposition. Sediment formation The most important manner of forming sediments is by the physical and chemical weathering of rocks on the surface of the earth. Generally silt-, sand-, and gravel-sized particles are formed by the physical weathering of rocks and clay-sized particles are formed by the chemical weathering of rocks. The formation of clay particles from rocks can take place either by the build-up of the mineral particles from components in solution, or by the chemical breakdown of other minerals. Sediment transportation Sediments can be transported by any of the following five agents: water, air, ice, gravity, and organisms. Transportation affects sediments in two major ways: (a) it alters particle shape, size, and texture by abrasion, grinding, impact, and solution; (b) it sorts the particles. Table 5 summarizes some of the effects of the five transporting agents. Water Air Ice Gravity Organisms Size Reduction through Considerable Considerable Considerable Minor abrasolution, little reduction grinding and impact sion effects abrasion in sus- impact from direct pended load, some organic abrasion and impact transporin traction load tion Shape Rounding of sand High degree Angular, soled Angular, non and and gravel of rounding particles spherical roundness Sur- Sand: smooth, po- Impact produ- Striated sur- Striated surface lished, shiny. ces frosted faces faces textu- Silt: little effect surfaces re Sorting Considerable Very conside- Very little sor- No sorting Limited sorsorting rable sorting ting ting (progressive) Table 5 Effects of transportation on sorting [4]. Residual soils Residual soils results when the products of rock weathering are not transported as sediments but accumulate in place. If the rate of rock decomposition exceeds the rate of removal of the products of the decomposition, an accumulation of residual soil results. Among the factors influencing the rate of weathering and the nature of the products of weathering are climate (temperature and rainfall), time, type of source rock, vegetation, drainage and bacterial activity. The residual soil profile may be divided into three zones: - the upper zone where there is a high degree of weathering and removal of material, - the intermediate zone where there is some weathering at the top part of the zone, but also some deposition toward the bottom of the zone, - the partially weathered zone where there is the transition from the weathered material into the unweathered parent rock
Temperature and other factors have been favourable to the development of significantthicknesses of residual soils in many parts of the world, particularly Southern Asia, Africa,SoutheasternNorthAmerica,CentralAmerica,theislandsoftheCaribbeanandSouthAmerica.Residualsoilstendtobemoreabundantinhumid,warmregionsthatarefavourableto chemical weathering of rock, and have sufficient vegetation to keep the weatheringproductsfrombeingeasilytransportedassediments.Since residual soils are generally located in areas of underdeveloped economies, they havereceived far less attention from engineersthan sedimentary soilswhicharepredominantinmostpopulation centres and centres of economicactivity.4.2Soil formationandpedological identification systemPedologyisthatpartofgeologythatdealswiththeupper2moftheearthcrust.Itneedsnotto be emphasized that this part of the earth crust is of very large importance to roadengineers and therefore some knowledge on soil forming,pedology and pedologicalidentificationsystemsisofimportance.One can statethat soils derived from identical parent materials and identical climatic andweathering conditions will be similar soils provided that also the vegetation, the topographyand the time of exposure to weathering are identical.In a soil we can recognize so called horizons.This is shown in figure 10.A HorizonTop soil, organic, zone of depletionB HorizonZoneofaccumulationC HorizonParent materialD HorizonSupporting materialFigure1oHorizonsthatcanbediscriminated.The number of horizons is not limited to the number shown in figure 10, quitea number ofcandevelop.Forengineering purposes especially horizons B and C areofhorizonsimportance.There are 5 major factors that determine the building up of a soil.They are:parent material,climate,time,vegetation,topography.It is quite clear that the parent material hasabig influence on soil formation.Theelementsthat arepresent in the parent material will to a large extentdetermine the properties of thesoil. The parent material can be divided in:basicigneous rocks (e.g.basalt,dolorite,gabbro),sedimentary rock (e.g.shales, marls)Also a distinction can be made in:primary materials which are divided in:intrusivematerials likegraniteandgranodiorite,extrusivematerials likebasaltandgabbro's.secondary materials which are divided in:sedimentary materials like shales,metamorphicmaterialslikeschistsandphyllites15
15 Temperature and other factors have been favourable to the development of significant thicknesses of residual soils in many parts of the world, particularly Southern Asia, Africa, Southeastern North America, Central America, the islands of the Caribbean and South America. Residual soils tend to be more abundant in humid, warm regions that are favourable to chemical weathering of rock, and have sufficient vegetation to keep the weathering products from being easily transported as sediments. Since residual soils are generally located in areas of underdeveloped economies, they have received far less attention from engineers than sedimentary soils which are predominant in most population centres and centres of economic activity. 4.2 Soil formation and pedological identification system Pedology is that part of geology that deals with the upper 2 m of the earth crust. It needs not to be emphasized that this part of the earth crust is of very large importance to road engineers and therefore some knowledge on soil forming, pedology and pedological identification systems is of importance. One can state that soils derived from identical parent materials and identical climatic and weathering conditions will be similar soils provided that also the vegetation, the topography and the time of exposure to weathering are identical. In a soil we can recognize so called horizons. This is shown in figure 10. A Horizon Top soil, organic, zone of depletion B Horizon Zone of accumulation C Horizon Parent material D Horizon Supporting material Figure 10 Horizons that can be discriminated. The number of horizons is not limited to the number shown in figure 10, quite a number of horizons can develop. For engineering purposes especially horizons B and C are of importance. There are 5 major factors that determine the building up of a soil. They are: - parent material, - climate, - time, - vegetation, - topography. It is quite clear that the parent material has a big influence on soil formation. The elements that are present in the parent material will to a large extent determine the properties of the soil. The parent material can be divided in: - basic igneous rocks (e.g. basalt, dolorite, gabbro), - sedimentary rock (e.g. shales, marls). Also a distinction can be made in: - primary materials which are divided in: intrusive materials like granite and granodiorite, extrusive materials like basalt and gabbro’s. - secondary materials which are divided in: sedimentary materials like shales, metamorphic materials like schists and phyllites
Theorderof resistanceto weathering is asfollows:quartzite>graniteandbasalt>sandstone>dolomiteandlimestone.Themost importantfactorof thecompositionof theparentrocks isthecontent ofalkaliesand alkalines.Alkalies are oxidesof NatandK+whilealkalines are oxidesof Mg2+,Ca2+,etc.Rocks e.g.that contain no alkalies, can yield only kaolinite (kaolinite is a clay mineral thatshowslimitedshrinkageandswelling,seealsothesectiononcayminerals)orJateriteweatheringproductsunlessgroundwatermovementbringsalkaliestotheenvironment.Ingeneousrocks,shales,slates,schistsand argillaceouscarbonates canyield avariety ofweathering products, at least in the initial stages of weathering, because of their content ofalkaliesandalkalines inadditiontoalumina,silicaetc.Chemical characteristics of a soil, such as high pH and a rather abundant amount of calciumand magnesium provide conditions favourable for the formation of montmorillonite(montmorillonite is a clay mineral that exhibits severe swelling and shrinkage).Highalkalineearthcontent(especiallyMg oxides)favourstheformationof expanding claymineralsparticularlywherelowpermeabilityofclaypreventsstrongleaching.Acid rocks will produce kaolinite while basic rocks will produce either illite (another claymineral, not as good in quality as kaolinite but far better than montmorillonite) andmontmorillonite.The only soils consistently high in montmorillonite (high swelling andshrinkage)aretheblacksoilsformedfrombasicigneousrocks.AnexampleofsuchasoilistheblackcottonclaywhichisabundantlyavailableinEthiopia.Characteristicsofthismaterialwill be discussed later on. In other soils the occurrence of montmorllonite as a majorconstituent usually reflectslocal parent material or poordrainageconditions.Furthermore one can say that K+ is an essential constituent of ilite, hence rocks low in K+(somebasicingneousrocks)weathertoclaysthatarelowinorhaveno illite.Timeisanotherimportantfactor.If amaterialhas beenexposedtoweatheringfor arelativelyshortperiod of time,thenthe soil willbe similartotheparent material.Howeverasweathering continues, the influence of the parent material is masked and other factors maydominate.Thespeedatwhichchemicalandphysicalalterationprocessestakeplaceisstronglyrelatedto moistureand temperature which immediatelymeans that climate is an important factor insoil formation.Expansive soils e.g.(containing montmorillonite)form predominantly wherethere is a distinct seasonal variation in rainfall so in cases where there are dry and wetseasons. Kaolinites on the other hand are developed where the precipitation is higher thantheevaporationandthepHoftheparentmaterial islow.In locations where theannual precipitation is 500mm or less,only the surfaceof the groundwill be wetted and this water will soon evaporate. The soluble constituents are retained onthe surface and the soil becomes alkaline.Furthermore water moves upwards in the dryseasonbringingsalt;whenthewater evaporatessaltsareleft intheupperprofile.Hot dry summers with high annual rainfall together with slightly alkaline conditions will resultintheformation.ofillite.In some parts of the world, e.g. South Africa, materials like calcretes and ferricretes arerathercommonand often usedforroadengineeringpurposes.Calcretesgenerallyformonflattish land orin depressions where moisture,whichmay already process carbonate insolution, is able to accumulate, weather the underlying material, release calcium andmagnesium and, eventually,be evaporated in situ.Later in these lecture notes moreattention will be paid to thesetypes of materials.Solution and transportation of soil materials depend on moisture flowing downwards. This inturn means thattopography has a strong influence on soil formation.Well drained soils willfavour the formation of kaolinite because leaching is an important prerequisite for thedevelopment of kaolinite. If the slope orientation is such that it results in poor drainage,expansive soils may develop if all other conditions are also satisfied. With a rollingtopography,theclay content increasesdownslope.16
16 The order of resistance to weathering is as follows: quartzite granite and basalt sandstone dolomite and limestone. The most important factor of the composition of the parent rocks is the content of alkalies and alkalines. Alkalies are oxides of Na+ and K+ while alkalines are oxides of Mg2+, Ca2+, etc. Rocks e.g. that contain no alkalies, can yield only kaolinite (kaolinite is a clay mineral that shows limited shrinkage and swelling, see also the section on clay minerals) or laterite weathering products unless ground water movement brings alkalies to the environment. Ingeneous rocks, shales, slates, schists and argillaceous carbonates can yield a variety of weathering products, at least in the initial stages of weathering, because of their content of alkalies and alkalines in addition to alumina, silica etc. Chemical characteristics of a soil, such as high pH and a rather abundant amount of calcium and magnesium provide conditions favourable for the formation of montmorillonite (montmorillonite is a clay mineral that exhibits severe swelling and shrinkage). High alkaline earth content (especially Mg oxides) favours the formation of expanding clay minerals particularly where low permeability of clay prevents strong leaching. Acid rocks will produce kaolinite while basic rocks will produce either illite (another clay mineral, not as good in quality as kaolinite but far better than montmorillonite) and montmorillonite. The only soils consistently high in montmorillonite (high swelling and shrinkage) are the black soils formed from basic igneous rocks. An example of such a soil is the black cotton clay which is abundantly available in Ethiopia. Characteristics of this material will be discussed later on. In other soils the occurrence of montmorillonite as a major constituent usually reflects local parent material or poor drainage conditions. Furthermore one can say that K+ is an essential constituent of illite, hence rocks low in K+ (some basic ingneous rocks) weather to clays that are low in or have no illite. Time is another important factor. If a material has been exposed to weathering for a relatively short period of time, then the soil will be similar to the parent material. However as weathering continues, the influence of the parent material is masked and other factors may dominate. The speed at which chemical and physical alteration processes take place is strongly related to moisture and temperature which immediately means that climate is an important factor in soil formation. Expansive soils e.g. (containing montmorillonite) form predominantly where there is a distinct seasonal variation in rainfall so in cases where there are dry and wet seasons. Kaolinites on the other hand are developed where the precipitation is higher than the evaporation and the pH of the parent material is low. In locations where the annual precipitation is 500 mm or less, only the surface of the ground will be wetted and this water will soon evaporate. The soluble constituents are retained on the surface and the soil becomes alkaline. Furthermore water moves upwards in the dry season bringing salt; when the water evaporates salts are left in the upper profile. Hot dry summers with high annual rainfall together with slightly alkaline conditions will result in the formation of illite. In some parts of the world, e.g. South Africa, materials like calcretes and ferricretes are rather common and often used for road engineering purposes. Calcretes generally form on flattish land or in depressions where moisture, which may already process carbonate in solution, is able to accumulate, weather the underlying material, release calcium and magnesium and, eventually, be evaporated in situ. Later in these lecture notes more attention will be paid to these types of materials. Solution and transportation of soil materials depend on moisture flowing downwards. This in turn means that topography has a strong influence on soil formation. Well drained soils will favour the formation of kaolinite because leaching is an important prerequisite for the development of kaolinite. If the slope orientation is such that it results in poor drainage, expansive soils may develop if all other conditions are also satisfied. With a rolling topography, the clay content increases downslope