Finally vegetation but also organisms has a strong influence on the top soil. Grass areas e.g.willhavea verydarkupperhorizonbecauseof thehighcontent of organicmaterial.Forestareasontheotherhand haveagreyupperhorizon becauseof thelowcontentof organicmaterial.Vegetation(plants)is important intheleachingprocess inthattheycanexchangeH+ionsforK* and then move the K* upwards where it can be flushed away by water. Furthermorevegetation will producehumus and humus increases the water-holding capacity and so theweathering process.Theimportanceof organicmatterinsoilscannotbeoveremphasizedsinceitplaysavital rolein the weathering process through theproduction of organic acids.Finally,termites are able to move fine-grained particles upwards in the soil profile, leavingbehindthemthoseparticlestoo largetocarryortodigest.The question now is how a soil profile should be described.Such a description should takeintoaccountthefollowingaspects.numberofhorizons,thickness ofthehorizons,relative arrangement of the horizons (sometimes horizons are missing),percentageorganicmatter,drainage (slope,permeability,watertableetc.),textureandstructureofthehorizons,chemical composition and mineralogy of the horizons,presenceofspecialformations,vegetation,geology of the parent material.Profiles which are similar in all matters are grouped together. They are a pedological soiltype. The soil type is subdivided in:series name, this is the most important aspect for engineering purposes; the namegiving however is very confusing sinceit doesn'tfollowa system;textureof the A-horizon.Two types of soil can havethe same series name but a different texture.Then theyareidentical in all aspects but different in the texture of the A-horizon.All series can be divided into three groups. These groups are:Zonal soils: these soils have well developed profiles, significant characteristics of which havebeen determined by climateand vegetation.Intra-zonal soils: these soils have more or less well defined profiles which are stronglyinfluencedbylocal reliefandtheparentmaterial.A-zonal soils: these soils have no well developed horizons due to the fact that the time thematerial is exposed to weathering is too short or there is no moisture or theparent materialisverystrongandtherearesteepslopes.After these three major soil groups, a division is made in great soil groups categories whichare divided in soil groups which in their turn are divided in soil series.Importantgreatsoilgroupcategoriesare:1.Podzolic soils:They dominate a broad belt in the higher latitudes of the northernhemisphere and are formed under forest vegetation. Subdivisions are made in:podzols,brownpodzolics,gray brown podzolics,graywooded podzolics.Latosolic soils:Thesesoilsarefound in the equator belt inAfrica,SouthAmerica,SE2.Asia,SEportionof NorthAmerica and thePacificIslands.Theyareformed underforest and tropical vegetation, they are strongly weathered, have a red-yellow colorandhavedeveloped inverydeepprofiles.Subdivisionsaremade in:laterites,reddishbrownlaterites,17
17 Finally vegetation but also organisms has a strong influence on the top soil. Grass areas e.g. will have a very dark upper horizon because of the high content of organic material. Forest areas on the other hand have a grey upper horizon because of the low content of organic material. Vegetation (plants) is important in the leaching process in that they can exchange H+ ions for K + and then move the K+ upwards where it can be flushed away by water. Furthermore vegetation will produce humus and humus increases the water-holding capacity and so the weathering process. The importance of organic matter in soils cannot be overemphasized since it plays a vital role in the weathering process through the production of organic acids. Finally, termites are able to move fine-grained particles upwards in the soil profile, leaving behind them those particles too large to carry or to digest. The question now is how a soil profile should be described. Such a description should take into account the following aspects. - number of horizons, - thickness of the horizons, - relative arrangement of the horizons (sometimes horizons are missing), - percentage organic matter, - drainage (slope, permeability, water table etc.), - texture and structure of the horizons, - chemical composition and mineralogy of the horizons, - presence of special formations, - vegetation, - geology of the parent material. Profiles which are similar in all matters are grouped together. They are a pedological soil type. The soil type is subdivided in: - series name, this is the most important aspect for engineering purposes; the name giving however is very confusing since it doesn’t follow a system; - texture of the A-horizon. Two types of soil can have the same series name but a different texture. Then they are identical in all aspects but different in the texture of the A-horizon. All series can be divided into three groups. These groups are: Zonal soils: these soils have well developed profiles, significant characteristics of which have been determined by climate and vegetation. Intra-zonal soils: these soils have more or less well defined profiles which are strongly influenced by local relief and the parent material. A-zonal soils: these soils have no well developed horizons due to the fact that the time the material is exposed to weathering is too short or there is no moisture or the parent material is very strong and there are steep slopes. After these three major soil groups, a division is made in great soil groups categories which are divided in soil groups which in their turn are divided in soil series. Important great soil group categories are: 1. Podzolic soils: They dominate a broad belt in the higher latitudes of the northern hemisphere and are formed under forest vegetation. Subdivisions are made in: - podzols, - brown podzolics, - gray brown podzolics, - gray wooded podzolics. 2. Latosolic soils: These soils are found in the equator belt in Africa, South America, SE Asia, SE portion of North America and the Pacific Islands. They are formed under forest and tropical vegetation, they are strongly weathered, have a red-yellow color and have developed in very deep profiles. Subdivisions are made in: - laterites, - reddish brown laterites
yellowishbrownlaterites,red yellow podzols, a transition soil between the podzolics and latosolics.3.Chernozemic soils:These soils areformed underprairie grass in humid to semi-aridareas intemperatetotropical climates.They canbefoundinthemiddlepartof theUSA.They have a dark A-horizon.Subdivisions are made in:chernozem(Illinois,Ohio)brunizem (South Dakota, exactly the same as the previous but only a few inches ofrain lessperyear),reddishprairie soilreddish chestnut soil,chestnut soil.4.Desertic soils:These soils are formed under mixed shrub and grass in hot or coldclimates.No weathering has taken place. Subdivisions aremade in:desert soil,red desert soil,sierozems,brown soils,reddish brown soils.Especially inthe field of agriculturea lot of effort has beendone to characterize soils in thisparticular way.This system can be of big help to engineers because if one is dealing with aparticulartypeof soil but notest resultsareavailable,one could usetest results (if available)collected on a similartype of soil which is however present at a different place of the world.In this way it is e.g.possible to use test results obtained on laterites from Ghana (Africa)forengineering purposes in Surinam (South-America)and vice versa.5.Mineralogyand soil structureAs mentioned before, the engineering properties of soil are largely determined by theminerals present, the size of the mineral particles and the geometric arrangement of theparticles (soil structure).The mineralogy of soils varies with thegrain size.For every coarsegrained soil (gravel andlarger), each particle consists of an aggregation of minerals. For sand size and smaller (lessthan 2 mm), each soil particle consists of a single mineral.The properties of the minerallargely determine the properties of the grain which in turn influences the properties of thesoil.Hence,themineralogyexertsan influenceonsoil properties.Thegrosspropertiesofthesoil are also influenced by the way the individual particles are arranged, by the density andother interrelated factors.First, consideration will be given to the properties of theminerals,then to the way these particles arearranged to form soil.These data will be used to explainengineering properties later.5.1 MineralogySomeof themore importantmineralsfound insoil include:Sand:Quartz (byfarthemostcommon,clearormilkycoloured),Feldspar (minor constituent in most sands, particles are usually white or fleshcoloured),Amphibole (minor constituent, black),Pyroxene (minor constituent, black),Calcite(dominant in beachsandsdevelopedfromdolomitescoral reefs)Magnetite (very minor constituent, black),Numerouslesscommonminerals.Silt:Quartz(verycommon),Micagroup(verycommon),Calcite and dolomite (occasionallydominant),Numerouslesscommonminerals18
18 - yellowish brown laterites, - red yellow podzols, a transition soil between the podzolics and latosolics. 3. Chernozemic soils: These soils are formed under prairie grass in humid to semi-arid areas in temperate to tropical climates. They can be found in the middle part of the USA. They have a dark A-horizon. Subdivisions are made in: - chernozem (Illinois, Ohio), - brunizem (South Dakota, exactly the same as the previous but only a few inches of rain less per year), - reddish prairie soil, - reddish chestnut soil, - chestnut soil. 4. Desertic soils: These soils are formed under mixed shrub and grass in hot or cold climates. No weathering has taken place. Subdivisions are made in: - desert soil, - red desert soil, - sierozems, - brown soils, - reddish brown soils. Especially in the field of agriculture a lot of effort has been done to characterize soils in this particular way. This system can be of big help to engineers because if one is dealing with a particular type of soil but no test results are available, one could use test results (if available) collected on a similar type of soil which is however present at a different place of the world. In this way it is e.g. possible to use test results obtained on laterites from Ghana (Africa) for engineering purposes in Surinam (South-America) and vice versa. 5. Mineralogy and soil structure As mentioned before, the engineering properties of soil are largely determined by the minerals present, the size of the mineral particles and the geometric arrangement of the particles (soil structure). The mineralogy of soils varies with the grain size. For every coarse grained soil (gravel and larger), each particle consists of an aggregation of minerals. For sand size and smaller (less than 2 mm), each soil particle consists of a single mineral. The properties of the mineral largely determine the properties of the grain which in turn influences the properties of the soil. Hence, the mineralogy exerts an influence on soil properties. The gross properties of the soil are also influenced by the way the individual particles are arranged, by the density and other interrelated factors. First, consideration will be given to the properties of the minerals, then to the way these particles are arranged to form soil. These data will be used to explain engineering properties later. 5.1 Mineralogy Some of the more important minerals found in soil include: Sand: Quartz (by far the most common, clear or milky coloured), Feldspar (minor constituent in most sands, particles are usually white or flesh coloured), Amphibole (minor constituent, black), Pyroxene (minor constituent, black), Calcite (dominant in beach sands developed from dolomites coral reefs), Magnetite (very minor constituent, black), Numerous less common minerals. Silt: Quartz (very common), Mica group (very common), Calcite and dolomite (occasionally dominant), Numerous less common minerals
Clay:Clay minerals (see subsequent section),Mica group (common in coarser particle size range),Quartz.Examinationshowsthatmostofthemineralsfoundinsoilsaresilicates(quartz,feldspargroup,amphibolegroup,pyroxenegroup,micagroupandtheclayminerals)withcarbonates(calciteanddolomites)dominatingwheresoilsaredevelopedfromlimestoneordolomiteparent material.The above list includes only one oxide (magnetite) and no sulphates)sulphides,phosphates,nitratesetc.Thediscussionwill thereforebelimitedtothecarbonatesand silicates.Soil is a weathering product (either mechanical or chemical) derived from a parent material.Inmanyareasthesoil isderivedfromlimestoneordolomitebedrockorreefs.Thesoilparticles will consist then of either:CalciteCaCO3orDolomiteCaMg(CO3)2Both minerals belong to the hexagonal class of crystals. Both exhibit perfect rhombohedralcleavagehencetheyreadilybreak-upintosmallersizedparticles.Theparticlesareapproximately equidimensional. Both minerals are rather soft.The perfect cleavage, softnessand solubilityin acidsgivethesemineralsa rather lowresistanceto mechanical and acidweathering.They are found in areas where the water is alkaline and where the parentmaterial is close enough to ensure the continuous supply of fresh material.Soilsderivedfromigneousandmetamorphic (exceptmarble)rocksaredominantlysilicates.The basic building unit of silicate minerals is the so called silica tetrahedron (figure 11) whichconsists of a single silica ion Si4+ surrounded by four oxygen ions o2-(b)oOandO=Siliconand（i=OxygenFigure11Silicatetrahedronand sheet structure [1].Thesetetrahedral units can bearranged into numerous configurations withthemselves andwith alkaline earths (e.g. Mg) and metals (e.g. Fe). The silicates exhibit a wide range inproperties depending on the manner in which the basic building blocks are assembled.Examplesare:Space lattice minerals. In these minerals, the silicon tetrahedrons are built into anetwork in all three directions. These minerals are characterized by hardness andresistanceto chemical attack.The individualparticles are more or lesseguidimensionalinshapeSheet silicates.In sheet silicates,the tetrahedronsbuild out in two directions(seefigure 11).The individual crystal units consist of thin sheets. The chemical bondswithin a sheet arestrong but thebonds between the sheets are weak.Hence,theseminerals have excellent cleavage parallel to the sheets. They are softer than thespace lattice silicatesChain silicates. In the chain silicates, the growth of the silicate units is restricted inonedirection.Hence,crystalstendtowardelongatedshapes.19
19 Clay: Clay minerals (see subsequent section), Mica group (common in coarser particle size range), Quartz. Examination shows that most of the minerals found in soils are silicates (quartz, feldspar group, amphibole group, pyroxene group, mica group and the clay minerals) with carbonates (calcite and dolomites) dominating where soils are developed from limestone or dolomite parent material. The above list includes only one oxide (magnetite) and no sulphates, sulphides, phosphates, nitrates etc. The discussion will therefore be limited to the carbonates and silicates. Soil is a weathering product (either mechanical or chemical) derived from a parent material. In many areas the soil is derived from limestone or dolomite bedrock or reefs. The soil particles will consist then of either: Calcite CaCO3 or Dolomite CaMg(CO3)2 Both minerals belong to the hexagonal class of crystals. Both exhibit perfect rhombohedral cleavage hence they readily break-up into smaller sized particles. The particles are approximately equidimensional. Both minerals are rather soft. The perfect cleavage, softness and solubility in acids give these minerals a rather low resistance to mechanical and acid weathering. They are found in areas where the water is alkaline and where the parent material is close enough to ensure the continuous supply of fresh material. Soils derived from igneous and metamorphic (except marble) rocks are dominantly silicates. The basic building unit of silicate minerals is the so called silica tetrahedron (figure 11) which consists of a single silica ion Si4+ surrounded by four oxygen ions O2- . Figure 11 Silica tetrahedron and sheet structure [1]. These tetrahedral units can be arranged into numerous configurations with themselves and with alkaline earths (e.g. Mg) and metals (e.g. Fe). The silicates exhibit a wide range in properties depending on the manner in which the basic building blocks are assembled. Examples are: - Space lattice minerals. In these minerals, the silicon tetrahedrons are built into a network in all three directions. These minerals are characterized by hardness and resistance to chemical attack. The individual particles are more or less equidimensional in shape. - Sheet silicates. In sheet silicates, the tetrahedrons build out in two directions (see figure 11). The individual crystal units consist of thin sheets. The chemical bonds within a sheet are strong but the bonds between the sheets are weak. Hence, these minerals have excellent cleavage parallel to the sheets. They are softer than the space lattice silicates. - Chain silicates. In the chain silicates, the growth of the silicate units is restricted in one direction. Hence, crystals tend toward elongated shapes
5.2ClaymineralogyIt has already been pointed out that the bulk of the serious soil problems in road engineeringare associated with fine grained materials. It has been shown that if fine grained materialshaveahigh LL and ahighPI, theywill show excessive swelling and shrinkageas well asalow bearing capacity when wet.Thequestion now is why fine grained, cohesive soils are sodifferent from sands and why is there such a large variation in behaviour when differenttypes of fine grained cohesive soils are compared with each other.The reason for this is thatcohesivesoilsaremainlybuiltupfromclayminerals.Clay minerals are distinguished by both their mineralogy and their particle size.Most particlesare less than 2 microns (0.002 mm) in diameter.Mineralogical, they are all silicates built oftwo basic building blocks, the silicate tetrahedron and the aluminium or magnesiumoctahedron.The tetrahedron and the tetrahedron sheet are already shown in figure 11. The sharing ofthe oxygen ions in the tetrahedron sheet is done in such a way that the tetrahedrons all siton a triangular base with their points in the same direction. The centres of the tetrahedronsgenerally contain a silicon ion (Si4+) but occasionally this is replaced by an aluminium ion(Ai3+)which, although it has not the same electrical value, has almost the same size as thesilicate ion. This phenomenon is called isomorphous substitution and will be discussed ingreater detail later on.Each oxygen ion at the base of the tetrahedron belongs to two tetrahedrons.The oxygens atthe tips however are often linked witha hydrogen ion (H*)toform a hydroxyl ion (OH).Thetetrahedronsheetmaythereforebeconsideredasalayerofsiliconionsbetweenalayerofoxygenanda layerof hydroxyl ions.Theprinciple of theoctahedron is shown infigure 12(a)(6)Oand=Hydroxyl=Aluminum,magnesium,etc.Figure12Magnesium octahedron [1]As can be seen from figure 12, the octahedron consists of a central cation and six oxygenions. The octahedrons are all arranged to lie on their triangular faces and shear edges.Hence,two octahedronsarelinkedtogetherbytwo common oxygen ions.Theoctahedrallayer is just one octahedron thick but many octahedrons wide.The centre positions in the octahedrons may all be filled with magnesium ions (Mg2t) or maybe replaced by aluminium (A/3+).Because of the extra charge on the aluminium and therequirement of electrical neutrality,only two-thirds of the octahedrons are flled withaluminium ions.This replacement isalso basedon theprincipleof isomorphoussubstitution.The octahedrons may also be partially filled with Mg2+ and partially with A/3+ as long as thetotal electrical charge isneutral (total positivevalue isequal to thetotal negativevalue).Invarious clay minerals, the octahedral layer may also contain iron ions (Fe2+ or Fe3+) andcertainotherlesscommonions.Theclaymineralsareformed by sandwichingtetrahedral and octahedral layerstogethertoformsheetsand sheetstogethertoformparticles.Thesymbolsshown infigure13areusedto indicate the tetrahedrons and octahedrons.20
20 5.2 Clay mineralogy It has already been pointed out that the bulk of the serious soil problems in road engineering are associated with fine grained materials. It has been shown that if fine grained materials have a high LL and a high PI, they will show excessive swelling and shrinkage as well as a low bearing capacity when wet. The question now is why fine grained, cohesive soils are so different from sands and why is there such a large variation in behaviour when different types of fine grained cohesive soils are compared with each other. The reason for this is that cohesive soils are mainly built up from clay minerals. Clay minerals are distinguished by both their mineralogy and their particle size. Most particles are less than 2 microns (0.002 mm) in diameter. Mineralogical, they are all silicates built of two basic building blocks, the silicate tetrahedron and the aluminium or magnesium octahedron. The tetrahedron and the tetrahedron sheet are already shown in figure 11. The sharing of the oxygen ions in the tetrahedron sheet is done in such a way that the tetrahedrons all sit on a triangular base with their points in the same direction. The centres of the tetrahedrons generally contain a silicon ion (Si4+) but occasionally this is replaced by an aluminium ion (Al3+) which, although it has not the same electrical value, has almost the same size as the silicate ion. This phenomenon is called isomorphous substitution and will be discussed in greater detail later on. Each oxygen ion at the base of the tetrahedron belongs to two tetrahedrons. The oxygens at the tips however are often linked with a hydrogen ion (H+ ) to form a hydroxyl ion (OH- ). The tetrahedron sheet may therefore be considered as a layer of silicon ions between a layer of oxygen and a layer of hydroxyl ions. The principle of the octahedron is shown in figure 12. Figure 12 Magnesium octahedron [1]. As can be seen from figure 12, the octahedron consists of a central cation and six oxygen ions. The octahedrons are all arranged to lie on their triangular faces and shear edges. Hence, two octahedrons are linked together by two common oxygen ions. The octahedral layer is just one octahedron thick but many octahedrons wide. The centre positions in the octahedrons may all be filled with magnesium ions (Mg2+) or may be replaced by aluminium (Al3+). Because of the extra charge on the aluminium and the requirement of electrical neutrality, only two-thirds of the octahedrons are filled with aluminium ions. This replacement is also based on the principle of isomorphous substitution. The octahedrons may also be partially filled with Mg2+ and partially with Al3+ as long as the total electrical charge is neutral (total positive value is equal to the total negative value). In various clay minerals, the octahedral layer may also contain iron ions (Fe2+ or Fe3+) and certain other less common ions. The clay minerals are formed by sandwiching tetrahedral and octahedral layers together to form sheets and sheets together to form particles. The symbols shown in figure 13 are used to indicate the tetrahedrons and octahedrons
Silica sheettipsdowntips upOctahedralsheetvariouscationsinoctahedralcoordinationGibbsite sheetoctahedral sheet cationsaremainlyaluminumGBrucite sheetoctahedral sheet cations aremainlymagnesiumBFigure 13Basicbuildingblocksofclayminerals.There are three common groups of clay minerals of interest to soil and road engineers beingkaolinite,illiteand montmorillonite.Thesegroupswill bebrieflydiscussedhere-after.KaoliniteKaolinite consists of a silica sheet and a gibbsite sheet.These sheets are tightly bondtogetherbycommonoxygenions.Thisisschematicallyshown infigure14.6-OH-6+124A140-102-HO+164-Si5-012(a)(6)Figure14The structure of kaolinite.(a)Atomic structure.(b)Symbolic structure [4]The thickness of one kaolinite sheet is 7.2 A. The chemical bonds inside of a sheet ofkaolinite are covalent bonds and are very strong. The kaolinite sheets are locked together byweaker hydrogen bonds between the oxygens of the silica sheet and the hydroxyls of thealumina sheet.Hencetheparticles have well developed cleavageparallel to the sheets.Thehydrogenbondsarestrongenough,however,toprovideconsiderablereinforcementbetweenthesheetsandasaresult,thekaoliniteflakesgrowfairlylarge in nature(often1oo ormoresheets in thickness).In addition, water cannot enter between sheets to expand (or shrink)the particles.Thus, the mineral is relatively stable.The width of the platy particle is about 0.3 to 3 μm, while the thickness of the plates is 1/3 to1/10ofthewidth.21
21 Silica sheet or tips down tips up Octahedral sheet various cations in octahedral coordination Gibbsite sheet octahedral sheet cations are mainly aluminum G Brucite sheet octahedral sheet cations are mainly magnesium B Figure 13 Basic building blocks of clay minerals. There are three common groups of clay minerals of interest to soil and road engineers being kaolinite, illite and montmorillonite. These groups will be briefly discussed here-after. Kaolinite Kaolinite consists of a silica sheet and a gibbsite sheet. These sheets are tightly bond together by common oxygen ions. This is schematically shown in figure 14. Figure 14 The structure of kaolinite. (a) Atomic structure. (b) Symbolic structure [4]. The thickness of one kaolinite sheet is 7.2 Å. The chemical bonds inside of a sheet of kaolinite are covalent bonds and are very strong. The kaolinite sheets are locked together by weaker hydrogen bonds between the oxygens of the silica sheet and the hydroxyls of the alumina sheet. Hence the particles have well developed cleavage parallel to the sheets. The hydrogen bonds are strong enough, however, to provide considerable reinforcement between the sheets and as a result, the kaolinite flakes grow fairly large in nature (often 100 or more sheets in thickness). In addition, water cannot enter between sheets to expand (or shrink) the particles. Thus, the mineral is relatively stable. The width of the platy particle is about 0.3 to 3 m, while the thickness of the plates is 1/3 to 1/10 of the width