STATE OF ILLINOIS 
 
 HENRY HORNER, Governor 
 
 DEPARTMENT OF REGISTRATION AND EDUCATION 
 
 JOHN J. HALLIHAN, Director 
 
 DIVISION OF THE 
 STATE GEOLOGICAL SURVEY 
 
 M. M. LEIGHTON, Chief 
 REPORT OF INVESTIGATIONS — NO. 4 2 
 
 SOME GEOLOGICAL RELATIONS BETWEEN 
 
 THE CONSTITUTION OF SOIL MATERIALS 
 
 AND HIGHWAY CONSTRUCTION 
 
 BY 
 
 GEORGE E. EKBLAW AND RALPH E. GRIM 
 
 PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 
 
 URBANA. ILLINOIS 
 
 1936 
 
 Reprinted 193 7 
 
 
STATE OP ILLINOIS 
 
 Hox. Hexry Hornkr, Governor 
 
 DEPARTMENT OF REGISTRATION AND EDUCATION 
 
 Hox. JoHx J. Hallihan, Director 
 
 Springfield 
 
 BOARD OF 
 
 NATURAL RESOURCES AND CONSERVATION 
 
 Hox. JoHX J. Hallihan, Chairman 
 
 Edson S. Bastix, Ph.D., Geology 
 William A. Noyes, Ph.D., LL.D., 
 
 Chem.D., D.Sc, Chemistry 
 John W. Alvord, C.E., Engineering 
 William Trklease, D.Sc, LL.D., 
 
 Biology 
 
 Henry C. Cowles, Ph.D., D.Sc, 
 
 Forestry 
 Arthur Cutts Willard, D.Engr., 
 
 LL.D., President of the University of 
 
 Illinois 
 
 STATE GEOLOGICAL SURVEY DIVISION 
 
 Urhana 
 
 M. M. Leighton, Ph.D., Chief 
 
 Emd Townley, M.S., Assistant to the Chief 
 
 GEOLOGICAL RESOURCES 
 Coal 
 
 G. H. Cady, Ph.D., Senior Geologist 
 
 L. C. McCahe, M.S. 
 
 James M. Schopf, M.S. 
 
 Earle F. Taylor, M.S. 
 
 Charles C. Boley, B.S. 
 Non-Fuels 
 
 J. E. Lamar, B.S. 
 
 H. B. WlLT^MAN, Ph.D. 
 
 Oil and Gas 
 
 A. H. Bele, Ph.D. 
 
 G. V. COHEE, M.S. 
 
 A real and Engineering Geology 
 
 Georce E. Ekijlaw, Ph.D. 
 
 Victor N. Fischer, B.S. 
 Snbsnrface Geology 
 
 L. E. Workman, M.S. 
 
 J. Norman Payne, M.A. 
 
 Harold E. Vokes, Ph.D. 
 
 Donald G. Sltton, M.S. 
 Stratigraphy and Paleontology 
 
 J. Marvin Weller, Ph.D. 
 Petrography 
 
 Ralph E. Grim, Ph.D. 
 
 Richards A. Rowland, Geol. E. 
 Physics 
 
 R. J. PlERSOL, Ph.D. 
 
 M. C. Watson, Ph.D. 
 Donald 0. Holland, M.S. 
 
 GEOCHEMISTRY 
 H. Reed, Ph.D., Chief Chemist 
 
 Frank 
 
 W. F. Bradley, Ph.D. 
 
 G. C. Finger, M.S. 
 
 Fnels 
 
 Gilhert Thiessex, Ph.D. 
 
 P. E. Grotts, B.S. 
 Xon-Fiiels 
 
 J. S. Machix, Ph.D. 
 
 F. V. Tooley, M.S. 
 Analytical 
 
 O. W. Rees, Ph.D. 
 
 NoR.MAN H. Nachtrier, B.S. 
 
 L. D. McVk KER, B.S. 
 
 George W. Land, B.Ed. 
 
 MINERAL ECONOMICS ' 
 W. H. VosKiTL, Ph.D., 
 
 Min era 1 Eeon om ist 
 W. A. Newton, B.S. 
 
 EDUCATIONAL EXTENSION 
 Don L. Carroll, B.S. 
 
 PUBLICATIONS AND RECORDS 
 George E. Ekrlaw, Ph.D. 
 Dorothy Rose, B.S. 
 Alma R. Sweeney. A.B. 
 Meredith M. Calkins 
 
 TOPOGRAPHIC MAPPING 
 
 (In cooperation with the United States Geological Survey) 
 
 CONSULTING STAFF 
 Ceramics 
 
 Cullen Warner Parmelee, M.S., D.Sc, University of Illinois 
 Pleistocene Invertehrate Paleontology 
 
 Frank Collins Baker, B.S., University of Illinois 
 
 (26792) 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 3 3051 00005 6659 
 

 Contents 
 
 Page 
 
 Geology and engineei-ing 5 
 
 Engineering geology in Illinois 6 
 
 Present trends 7 
 
 Surficial material 8 
 
 Soil profiles 10 
 
 Relations between constitutions and properties of clays and soils 11 
 
 Determination of constitution of clays and soils 12 
 
 Composition of some common "soil materials" 14 
 
 Relation between mineral constituents and properties of soils 15 
 
 r3] 
 
Illustrations 
 
 Figure Page 
 
 1. Map of North America showing the centers of ice accumulation and the 
 
 area of glaciation 8 
 
 2. Surficial distribution of glacial drift-sheets in the upper Mississippi Valley 9 
 
 3. Generalized diagram of maturely developed soil profile and the processes 
 
 involved in its development 10 
 
 4. Schematic representation of lattice structure of the clay minerals mont- 
 
 morillonite and kaolinite (after Endell, Hofmann, and Wilm) 13 
 
 [4] 
 
SOME GEOLOGICAL RELATIONS BETWEEN THE 
 
 CONSTITUTION OF SOIL MATERIALS AND 
 
 HIGHWAY CONSTRUCTION^ 
 
 GEORGE E. EKBLAW' AND RALPH E. GRIM^ 
 
 GEOLOGY AND ENGINEERING 
 
 GEOLOGY is an important factor in practically every engineering project 
 that deals with earth materials. In some fields of engineering the 
 geological factor is so evident that it is universally recognized; in 
 others its relations to the engineering problems may be so obscure that it is 
 overlooked or ignored. However, the part that geology plays in all fields of 
 engineering is becoming ever better appreciated, as is demonstrated by the 
 increasing attention and care that is being paid to adequate exploration and 
 testing of geological materials and structures with reference to proposed 
 dams and reservoirs, canals and locks, sewer trenches, tunnels, foundations 
 for buildings and bridges, underpasses, excavations and fills, etc. 
 
 Although there has been highway engineering almost ever since man 
 found it desirable to travel from place to place, the modern profession had its 
 inception hardly more than a quarter of a century past. In that brief span 
 of time it has achieved distinction favorably comparable to that attained by 
 other older fields of engineering. When paved highways were first constructed, 
 they were designed by engineers who were experienced in constructing paved 
 city streets, gravel or stone pikes, railways, and similar developments. Since 
 then the technique of highway construction in all its phases has progressed 
 to a high degree, and searching investigations into many of its problems have 
 been undertaken and aggressively pursued. 
 
 In the demand for paved highways that increased when the benefits of 
 the first ones laid down were appreciated, one fundamental factor in their 
 construction — the geological situation — was almost universally ignored as 
 unimportant, generally from lack of knowledge of its possible consequences. 
 However, these consequences have eventually made themselves evident, and 
 
 1 Presented at the Twenty-eighth Annual Meeting- of the Mississippi Valley Conference 
 of State Highway Departments, Stevens Hotel, Chicago, Illinois, February 7, 1936, and 
 at the Twenty-third Annual Conference on Highway Engineering at the University of 
 Illinois, Urbana, February 28, 1936. 
 
 - Geologist and Head, Areal and Engineering Geology Division, Illinois State Geo- 
 logical Survey. 
 
 3 Petrographer, Illinois State Geological Survey. 
 
 r5] 
 
6 CONSTITUTION OF SOIL MATERIALS 
 
 as a result a policy of intelligent caution and investigation concerning geo- 
 logical matters is being rapidly adopted. The highway engineers in almost 
 every state, as well as in the Federal bureau and in many county and similar 
 organizations, are devoting more and more attention to the geology of the 
 materials underlying and bounding the highway and of those of which the 
 highway itself is built. 
 
 ENGINEERING GEOLOGY IN ILLINOIS 
 
 Kot many years after the program of improved highway construction in 
 Illinois was begun, there appeared certain difficulties which the engineers of 
 the State Division of Highways recognized as probably due to geologic con- 
 ditions. These problems were referred to the State Geological Survey for 
 counsel and to them Dr. M. M. Leighton, Chief^ gave his personal attention 
 for several years. Through these services, as well as similar services provided 
 for problems concerning dams and reservoirs and other engineering projects 
 presented to the Survey, the field of engineering geology in the State was 
 established. It grew ra])idly so that in 1927 the Division of Engineering 
 Geology of tlie State (Jeological SuiTcy was organized, with Dr. Ekblaw in 
 charge. 
 
 At first, the ])roblems presented by the highway engineers concerned only 
 landslides and miidflows, with consequent dislocation of alignment and frac- 
 tured oi' wavy paveiiHMits, and estimates of earth and rock to be excavated in 
 highway construction, but in the nearly nine years that Dr. Ekblaw has 
 continued the cooperation with them, the highway engineers have presented 
 scores of situations, no two of which have ever been identical although they 
 can be grouped into a relatively few classes of problems. Moreover, as an 
 undoubted result of the engineers' better ac(iuaintance with geology and 
 consecpient better appreciation of its application in highway construction, the 
 field included by the ])ioblems they have presented has steadily broadened. 
 Landslides of all types and sizes, seepage and drainage, peat bogs, location 
 of fills, materials for fills, character of aggregate materials, selection of pro- 
 posed highway routes, heaving and cracking of pavements, bridge foundations, 
 and grade separations — either underpass or overhead — include most of the 
 specific problems. Also, in 1929 and 1930, at the request of the State Division 
 of Highways, the State Geological Survey carried on over most of the State 
 a detailed reconnaissance of limestone and sand and gravel deposits suitable 
 for local sources of materials for secondary roads. Within the last two years 
 the highway engineers have asked for particular information as to deposits 
 of plastic clay suitable for use in stabilized gravel roads. 
 
RELATED TO HIGHWAY CONSTRUCTION 7 
 
 PRESENT TRENDS 
 
 An account of the <iooloov concerned with some of these problems, its 
 significance, and the measures consecjueutly ado})ted would be interesting but 
 would probably only duplicate experiences of every highway engineer. There- 
 fore, the present discussion is restricted to some considerations concerning 
 soil materials that have probably never occurred to engineers as having any 
 special bearing on highway construction but which we hope to show are 
 actually of fundamental importance, especially in view of the present emphasis 
 on better subgrades, better subgrade drainage, and stabilized gravel roads. 
 The same considerations also more or less directly play a part in many of 
 the other geological highway construction problems. 
 
 The two most recent problems on which the Illinois State highway 
 engineers have sought onr geologic advice have emphasized these very con- 
 siderations. The problems have concerned failure of pavements at localities 
 and in situations where it would hardly be expected. One case occurred in 
 Lake County, where in situations that appear to have at least reasonably good 
 drainage it is found that at times of heavy rain, water collects between the 
 pavement and the subgrade. softens the surface of the subgrade, and thereby 
 destroys its supporting power; the pavement vibrates under impact of traffic 
 and pumps muddy water out through joints, cracks, and along the sides of 
 the slab ; and eventually so much of the subgrade is thus removed along 
 transverse joints that the pavement settles, cracks, and is ruined. The other 
 case occurred in Greene County, where in similar situations of apparently 
 reasonably good drainage a ^'black-top^^ pavement has repeatedly failed. Again 
 examinations revealed that during rainy periods water accumulates between 
 the bearing course and subgrade, the pavement vibrates under traffic and 
 pumps out muddy water through cracks, thus removing the supporting base, 
 and eventually the pavement fails. An added factor in this case is the fact 
 that because the pavement is not rigid, traffic traveling in relatively narrow 
 lanes tends to press the pavement in these lanes down into the subgrade 
 when it is softened by the water, and thus creates ruts in the subgrade in 
 which more water accumulates, aggravates the situation, and hastens the 
 disruption of the surface. It is obvious that these undesirable conditions result 
 solely because the soil material on the subgrade and in the shoulders is 
 relatively so impervious that the water cannot escape either downward or 
 laterally but accumulates between the pavement and the subgrade; it is also 
 obvious that the remedy consists of altering or amending the construction 
 design to provide means either of preventing excess water, especially that 
 drained off the pavement surfaces themselves, from getting under the slab 
 or of removing it as rapidly as it accumulates. However, these conclusions 
 do not answer the question raised by the engineers as to why such conditions 
 occur at some places and not universally in the State. The studies of clay 
 
CONSTITUTION OF SOIL MATERIALS 
 
 minerals as carried on by Dr. Grim during the past four years and as applied 
 to soil materials provide some interesting data pertinent to the question. 
 
 SURFICIAL MATERIAL 
 
 In Illinois, as in the other states of the Mississippi Valley, most of the 
 subgrade on \\hieh highways are constructed is unconsolidated material, and 
 in the states in the upper Mississippi Valley a part of the unconsolidated 
 
 Fig. 1 
 
 -Map of North Amlrica Showing thl Ckntkks of Ici: 
 Accumulation and thl Ark a of Glac iation 
 
 material is glacial drift, that is, material that was picked up by the continental 
 glaciers which spread from Canada over northern United States, was carried 
 by them to localities far from their source, and was dropped when the glaciers 
 melted away (Fig. 1). Much of the surficial material is loess, a deposit of 
 silt derived from various sources and carried and laid down by the wind, 
 and the rest is alluvium deposited either by glacial or present streams. 
 
RELATED TO HIGHWAY CONSTRUCTION 9 
 
 Four distinct glaciers successively invaded northern United States and 
 are named, in order of age, Nebraskan, Kansan, Ulinoian, and Wisconsin 
 (Table 1). Between each two glacial stages there was an interval of time 
 
 Table 1. — The Glacial Stages of North America 
 
 Glacial invasions 
 
 Interglacial epochs 
 
 Wisconsin (fourth) 
 
 Ulinoian (third) . 
 Kansan (second). 
 Nebraskan (first) 
 
 Sangamon (third) 
 Yarmouth (second) 
 Aftonian (first) 
 
 LEGEND 
 
 nzi] ^ Dim xzz^ 
 
 WISCONSIN ILL INDIAN KANSAN NEBRASKAN 
 
 ^ 
 
 SCALE 
 
 200 300 
 
 Fig. 2. — Surficial Distribution of Glacial Drift-Sheets in the 
 Upper Mississippi Valley 
 
 during which the climate was much the same as at present, so that on the 
 preceding glacial drifts operated the same processes that are now operating, 
 and one of these is the formation of soils. Conse(|uently at some places we 
 
10 
 
 CONSTITUTION OF SOIL MATERIALS 
 
 find two or more deposits of glacial drift, each separated by a soil horizon. 
 The Nebraskan and Kansan drifts are found in Nebraska, Kansas, southern 
 Iowa, and Missouri as far south as Missouri River; the Illinoian drift occurs 
 in southern and western Illinois, southeastern Iowa, southern Indiana, and 
 southern Ohio; the Wisconsin drift covers northern Ohio, northern Indiana, 
 northeastern Illinois, northern Iowa, and the other northern states (Fig. 2). 
 
 SOIL PROFILES 
 
 Geologists recognize that in the formations of soils there are four major 
 chemical reactions which, in the order of their relative rate of progress, are 
 respectively (1) formation of humus organic material, (2) oxidation, (3) 
 leaching or solution of lime and magnesium carbonates, etc., and (4) decom- 
 position of silicate minerals with consequent concentration of colloidal material. 
 
 PHYSICAL PROCESSES ZONES 
 
 eluviation: accumulation 
 
 OF ORGANIC HUMUS 
 
 ACCUMULATION OF 
 COLLOIDAL MATERIAL 
 
 CHEMICAL PROCESSES 
 
 FORMATION OF 
 ORGANIC HUMUS 
 
 DECOMPOSITION OF 
 SILICATE MINERALS 
 
 LEACHING OF 
 CARBONATES, ETC. 
 
 RESULTING MATERIAL 
 
 DARK SILTY SOIL 
 
 DENSE PLASTIC CLAY 
 CUMBOTIL 
 
 LEACHED AND 
 OXIDIZED MATERIAL 
 
 OXIDIZED, BUT 
 UNLEACHED MATERIAL 
 
 UNALTERED PARENT 
 MATERIAL 
 
 Fig. 3. — Generalized Diagram of a Maturely Developed Soil Profile 
 AND THE Processes Involved in its Development (After Leighton and Mac- 
 Clintock) 
 
 A typical well-developed soil profile on glacial material consists of four major 
 horizons (Fig. 3) which, from top to bottom, are (1) the humus horizon, 
 a black or gray silty zone generally not more than several inches thick, (2) 
 a zone of dense oxidized leached clay, typically gray but locally reddish, hard 
 when dry and generally plastic when wet, and practically impervious to all 
 
RELATED TO HIGHWAY CONSTRUCTION 11 
 
 drainaoe, (3) an oxidized, leached zone, and (4) an oxidized but calcareous 
 zone, beneath which occurs the unaltered parent material (5). The thickness 
 of these zones depends on the length of time the soil-forming processes have 
 operated. All of them are well developed in the soil profiles formed on all 
 the glacial drifts except the last one (Wisconsin). Because zone 2, when 
 typically developed from glacial till over broad flat areas, is extremely plastic 
 and gumbo-like, it has been termed gumbotil. It is a reliable source of plastic 
 clay in almost all of the territory covered by the three older glacial drifts. 
 Zone 2, as developed on deposits of fine loess, is also a possible source of 
 plastic clay but is not so satisfactory as the gumbotil. The density and 
 general imperviousness of this zone are the factors that contribute so greatly 
 to the problem of satisfactorily draining subgrades in so many places in the 
 Mississippi Valley. 
 
 With these general relationships of glacial drifts and soil-profile zones in 
 mind, we shall proceed to a consideration of the constitution of clays and soils. 
 
 RELATIONS BETWEEN CONSTITUTIONS AND PROPERTIES 
 OF CLAYS AND SOILS 
 
 It is obvious that the character and properties of the materials on which 
 the highways are directly laid will exert a tremendous influence on their 
 durability and permanence. Every engineer knows that not all soils possess 
 the same properties and that different soils yield different results when high- 
 ways are built over them or when they are used in fills. Further, it is obvious 
 that the most satisfactory material for fills cannot be selected from that avail- 
 able in a given area and the most satisfactory subgrade drainage cannot be 
 designed until it is known why different soils possess different physical 
 properties. This knowledge is necessary before an examination and conse- 
 quent determination that certain specific materials will have certain properties 
 and other material will have other specific properties can be intelligently made. 
 
 Soils, clays, tills, loess, and shale are all. argillaceous materials — that is, 
 they contain clayey material and possess properties, such as plasticity when 
 wet, which are associated with the term clay. The physical properties of 
 argillaceous materials are dependent upon their constitution. The important 
 factors of constitution influencing physical properties are texture, chemical 
 composition, mineral composition, and base-exchange characteristics. The 
 literature contains the results of much research on the influence of the first 
 two factors. It is known, for example, that in general the finer the grain 
 size of such material, the greater the plasticity, water adsorptive ability, 
 shrinkage, etc., and that electrolytes exert a great infiuence on the degree to 
 which clays can be placed in suspension in water. More recent research has 
 emphasized the importance of the mineral composition and base-exchange 
 characteristics as factors determining the properties of argillaceous materials. 
 
12 CONSTITUTIOX OF SOIL MATERIALS 
 
 It has shown that two clays may have the same grade size distribution and 
 yet possess different physical properties if their mineral compositions are 
 different. Likewise, two clays may yield the same ultimate chemical composi- 
 tion but possess different physical properties if their mineral compositions and 
 base-exchange characteristics are different. Eesearch into the mineral composi- 
 tion and base-exchange properties of argillaceous materials and the influence 
 of these factors of constitution on physical properties has been Dr. Grimes 
 major interest in recent years. 
 
 DETERMINATION OF CONSTITUTION OF CLAYS AND SOILS 
 
 Complete research into the mineral com])osition of these uuiterials is of 
 relatively recent date for the reason that satisfactory research tools and 
 methods for studying the finest constituents have been only recently developed. 
 It is comparatively easy to identify tlie mineral constituents in the coarser 
 grade sizes, but special technique is necessary to identify those in the finest 
 grade sizes, particularly those of colloidal size, that is, those which have to 
 be measured in fractions of a micron. In the laboratory of the Illinois State 
 Geological Survey a reseaich procedure has l)een devised whereby complete 
 thoroughgoing data on the mineralogy of clays can be obtained by special 
 microscopic techiii(|ue using magnifications u)) to 1-500 diamet(M-s, l)y improved 
 X-i'ay teclmicpie, and by cluMnical analysis following scpai-ation of the matei'ial 
 into mineral concentrat(\< by means of a sui)(M'centrifuge. The clays are liter- 
 ally taken apart into ihciv component mineral constituents. It is these con- 
 stituents in the finest fractions which are most inii)()rtant in influencing 
 physical properties. 
 
 This work has shown (1) that the concc^pt that all clays contain a 
 inyst(M-i()us universal clay substance, sometimes called clayite, to which they 
 owe their properties, is fallacious; (2) that the concept that all clay materials 
 are composed essentially of the mineral kaolinite is erroneous; (3) that an 
 alternative conce})t that these materials are simply heterogeneous assemblages 
 of almost any species of minerals existing in very small particles and that 
 therefore their properties are entirely dependent u])on the size-grade distri- 
 bution of the component particles is also erroneous; and (4) that they contain 
 little or no amorphous material. 
 
 On the contrary, this work has shown that clays actually are composed 
 of crystalline constituents throughout — even the colloidal fraction is com- 
 posed of crystalline particles, the colloidal properties depending on smallness 
 ot particle size and also on the shape of the individual particles. It has also 
 substantiated the concept, which has become increasingly prevalent in recent 
 years, that argillaceous materials are composed essentially of one or more of 
 a comparatively small group of minerals known as the clay minerals (Table 2). 
 
RELATED TO HIGHWAY CONSTRUCTION 
 
 13 
 
 Table 2. — Clay Minerals 
 
 Name 
 
 Chemical composition 
 
 Remarks 
 
 Kaolinite 
 
 Anauxite 
 
 Halloysite 
 
 Beidellite 
 
 Al,0..2Si02.2H20 [ 
 Al,033SiOo.2H,0 \ 
 A1.032Si0..xH,0 
 Al.Os.SSiO^.xH.O } 
 Fe.Os.SSiO^.xH.O f 
 AL.03.4SiO..H.O 
 
 2K,0.3M0.8R.03.24Si02.12H,0 
 
 Anauxite and kaolinite form 
 an isomorphous series 
 
 Beidellite and nontronite 
 
 Nontronite 
 
 form an isomorphous series 
 
 Montmorillonite .... 
 Sericite-like mineral 
 
 Montmorillonite, beidellite, 
 and nontronite probably 
 contain essential alkalies or 
 alkaline earths. 
 
 cal4A 
 
 C - AXIS 
 
 LEGEND 
 
 o = Si a = OH 
 0=0 
 
 O = A) or F e 
 
 AA^A^ 
 
 ^ A A f J> ,A, 
 
 K^ 
 'i 
 
 :9(:ff. 
 
 ia£X 
 
 I4.3A 
 
 f>x A A i. A 40+2(0H) 
 
 ■'^^i^Y' 9 "i 40 + 2(oh) 
 
 4Si 
 60 
 
 ^ J^ 6 (oh) 
 "^i^ 4 + 2(oh) 
 
 4 Si 
 60 
 
 b - AXIS 
 
 b -AXIS 
 
 montmorillonite kaolinite 
 
 Hj O • (Alj O3 • Fcj, 03)-4 Si Oj 2 H^O • Al^ 0_, • 2 Si 0^ 
 
 Fig. 4. — Schematic Representation of Lattice Structure of the Clay 
 Minerals Montmorillonite and Kaolinite (after Endell, Hofmann, and 
 Wilm) showing the layered arrangement that accounts for their flake-like 
 crystals. Note the great and variable interval between the successive layers 
 in montmorillonite, as compared with the limited interval in kaolinite, 
 which accounts for its greater adsorptive ability and other properties. 
 
 They are hydrous ahiminum silicates and some of them also contain alkalies 
 or alkaline earths. In most of them the aluminum may be isomorphously re- 
 placed by ferric iron. At the present stage of the researches, it is not possible 
 to write exact formulae for all of them; the formulae of beidellite, nontronite, 
 and the unnamed sericite-like mineral particularly are to be regarded as 
 
14 CONSTITUTION OF SOIL MATERIALS 
 
 tentative. The sericite-like mineral is so named because it resembles the 
 sericite form of white mica. It is possible that future research will show 
 that there are a few other clay minerals. All the clay minerals have a 
 general micaceous crystalline habit, as a consequence of which their individual 
 particles are flat and flake-shaped (Fig. 4). It is important to note that 
 the flake-shaped characteristic is not developed to the same degree in all clay 
 minerals. 
 
 COMPOSITION OF SOME COMMON ''SOIL MATERIALS" 
 
 The essential features of the mineral composition of the several argil- 
 laceous materials commonly encountered in highway construction are as 
 follows : 
 
 GumhotiJ is composed essentially of beidellite in particles .00006 mm. in 
 diameter or in slightly larger particles, .001-. 0001 mm. in diameter, which 
 are easily reduced by slight working of the material to the finer grade size. 
 In addition to beidellite, silty gumbotil contains the sericite-like mineral and 
 also quartz varying in amounts according to the degree of siltiness. Coarser 
 grains of quartz and possibly large grains of other minerals are present if the 
 gumbotil is sandy. 
 
 The finest grade sizes, that is clay-size grades, of //*// are composed of 
 a mixture of beidellite and the sericite-like mineral. If the till is calcareous, 
 calcile is present in particles which may attain a minimum size of .0001 mm. 
 The coarser fractions of till are mixtures of quartz and fragments of a variety 
 of dillferent minerals and rocks. Different tills vary from each other in the 
 relative abundance of these constitutions, but the relative abundance of 
 beidellite and the sericite-like mineral are of prime importance in influencing 
 physical properties. 
 
 Loess is composed primarily of quartz and mica with a smaller quantity 
 of a variety of other minerals, such as feldspar, pyroxene, and amphibole, all 
 existing in particles ranging in size from about .06 to .001 mm. A small 
 amount of finer material, which is a mixture of beidellite and the sericite-like 
 mineral, is present. Calcite is present if the material is calcareous. 
 
 The finest grade sizes of soil developed on loess in Illinois are a mixture 
 of beidellite, the sericite-like mineral, organic material, limonite (hydrated 
 ferric oxide), and quartz. The coarser fractions have compositions similar to 
 those of the coarser fraction of the loess from which the soil was derived. 
 These constituents are not present in uniform amounts in all soils nor are 
 they uniformly distributed vertically from the top of the soil downward, 
 but are to a large degree dependent on the maturity of the soil and the 
 drainage conditions under which it has developed. Thus, organic material 
 tends to occur mainly in the surface horizon (Fig. 3, zone 1). Beidellite is a 
 secondary product derived from the alteration of many of the primary con- 
 stituents. It forms primarily in the upper horizon and as the soil profile 
 
RKLATKI) TO HIGHWAY CONSTRUCTION 15 
 
 develops it is carried downward and redeposited a short distance beneath the 
 surface. At a result, zone 2 of the soil profile is formed as an impervious 
 liorizon. whose thickness and depth vary with the age of the soil, and at the 
 same time the top horizon consequently becomes increasingly silty. When the 
 soil profile has reached a fairly advanced stage of weathering, the beidellite 
 is decomposed into its constituent oxides and rounded limonitic aluminous 
 pellets tend to develop at or near the base of zone 2. The process of leaching, 
 by which downward seeping groundwater removes from the upper horizon 
 not only the calcite but also any alkalies that may be present, causes the upper 
 horizons of the soil profile to become acid. 
 
 RELATION BETWEEN MINERAL CONSTITUENTS AND 
 PROPERTIES OF SOILS 
 
 The next step of our research has been to determine the influence that 
 specific mineral constituents exert on the physical properties of the materials 
 containing them. It has been found, for example, that beidellite and mont- 
 morillonite differ from kaolinite and the sericite-like mineral in possessing 
 far higher adsorptive ability for water; in the ability to break down easily 
 to extremely small particles, e. g., beidellite commonly occurs in particles less 
 than .00006 mm. or can easily be reduced to this size whereas the sericite-like 
 mineral and kaolinite rarely are in particles smaller than .0001 mm.; and 
 in their ability to possess exchangeal)le bases. It follows from these and other 
 considerations that increasing amounts of beidellite in a soil will increase its 
 plasticity, its stickiness, its impervious character, and the swelling and shrink- 
 age on the addition or removal of moisture. Stated another way, two soils, 
 one of which is largely composed of beidellite and the other of the sericite-like 
 mineral, may have about the same size grade distribution in the natural state 
 and about the same ultimate chemical analysis, but w^hen these soils are 
 utilized, the one containing the most beidellite will tend to be most impervious, 
 stickiest, most plastic, and most subject to volume variation. 
 
 The addition of electrolytes has been known for some time to cause 
 changes in the physical properties of clays. In the last few years the relation 
 of this phenomenon to base-exchange has been recognized and its importance 
 has been realized. Clays possess the ability to carry bases which, under certain 
 conditions, can be exchanged for other bases. Thus, if a clay containing ex- 
 changeable calcium is treated with a strong solution of a sodium salt, the 
 sodium will go into the clay replacing the calcium which will come out in the 
 solution. 0])inions diff'er as to whether the exchangeable bases are held on 
 the surface of the colloidal particles or within their lattice structure. Like- 
 wise, opinions differ as to whether it is a stoichiometrical chemical reaction or 
 simply adsorption. 
 
 The seat of the exchange capacity is in the clay mineral content, and dif- 
 ferent clay minerals possess different exchange capacities. Thus, the exchange 
 
16 CONSTITUTION OF SOIL MATERIALS 
 
 capacity for montmorillonite is very high, for beidellite it is moderately high, 
 and for kaolinite and the sericite-like mineral it is low. Obviously soils 
 composed of beidellite will have higher exchange capacity than those composed 
 of the sericite-like mineral. Therefore, whatever influence base-exchange phe- 
 nomena exert on physical properties will be reflected to a higher degree by 
 those composed of beidellite than those composed of the sericite-like mineral. 
 
 Certain properties of soils vary with the identity of the exchangeable base 
 they contain. For example, the plasticity of a given clay or soil differs accord- 
 ing to whether sodium or calcium is the exchangeable base. The shrinkage 
 can be changed by substituting calcium for sodium. A soil saturated with 
 sodium is far more impervious to water than an acid soil or one in which 
 calcium is the exchangeable base. 
 
 The reason for this influence may be conceived in part by considering the 
 flake-shaped clay mineral particles to be encased in a hull of basic ions which 
 in turn is enclosed by a hydration sphere. Probably because different ions 
 have different dissociation abilities, the hydration sphere varies with the iden- 
 tity of the exchangeable base and the physical properties show a correlative 
 variation. 
 
 In the present state of infornuition it is impossible to give much specific 
 data and many detailed conclusions on this subject. The Illinois State 
 Geological Survey has under way extensive I'esearches to determine the con- 
 stitution, particularly the mineral constitution, and base-exchange character- 
 istics of the argillaceous nuiterials occurring within the State. Our object 
 is further to determine exactly how these factors influence the properties of 
 the material as a whole, which, in turn, determine its utilizations, and thereby 
 to extend and to im})rove their utilization in the field of ceramics and in 
 other fields in which such materials are used. Our researches have progressed 
 so far that a comparatively rapid petrographic microscopic analysis of the 
 mineral composition of these materials commonly enables a prediction of their 
 physical properties. However, this procedure is not intended as a substitute 
 for actual testing, but its value lies primarily (1) in selecting from n\aterial 
 available that which is most satisfactory for a given purpose, and (2) in the 
 search for the seat of trouble in material which has already been used