— —_———_——— _—*_,_—_—___.____ _ . -- SYNO-PSIS OF TYPES OF LOW-COST ROADS Thai: for the Degree of M. 5. MICHIGAN STATE COLLEGE Guy Jean-Pierre Plumail 1950 THESIS ' This is to certilt; that the thesis entitled SYNOPSIS 3:: TYPES OE" LOU—COST ILOAZ‘S presented In; GUY JELI‘I-PIFTCZT TENT-{LIL has been accepted tuwurtls fullillnu-nt ml the requiremmnts for H ”lViL YNClxhfitffiC Q . ____1_ degree 1n 'r ’ 1)?)0 I ‘ _> Date “" “‘3 _.—— —- ——— _-* .5- SYNOPSIS OF TYPES OF LOW-COST ROADS By GUY JEAN-PIERRE EEEEAIL A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Civil Engineering 1950 THESIS ACKNCWLEDGMENT The author wishes to thank Mr. Gail C. Blomquist for his helpful counsel and guidance in the preparation of this thesis. I‘ J a “I ‘ 'N .J rob TABLE CF CONTENTS Introduction .................................... Chapter I. The Soils ............................ Chapter II. The Road ........................... Chapter III. Mechanical Stabilization .......... Chapter IV. Chemical Stabilization ............. Chapter V. Soil—Bitumen Stabilization .......... Chapter VI. Soil-Cement Stabilization .......... Chapter VII. Thermic Stabilization ............. Conclusion ...................................... Appendix ........................................ Bibliograpkly .OOOOOOOOOOOOOOOOCOO0000......0.0... 59 59 67 7O 75 78 INTRODUCTION Out of approximately 3,000,000 miles of roads in the United States about 50 per cent are not surfaced, 1,000,000 miles have low.type surfacings, 305,000 miles have intermediate type surfacings and l9h,000 miles have high type surfacing. The selection of the type used depends on the volume of traffic unevenly distributed throughout the country and is generally of the low type for traffic of less than hOO vehicles per day, of the intermediate type for traffic of too to 1,000 vehicles per day, and of the high type for traffic over 1,000 vehicles per day. Hard surfaced pavements are eXpensive to build and to maintain. On the other hand, unsurfaced roads are muddy and impassable during wet periods, and although passable are dusty during dry weather and are a nuisance to both users and residents living beside the road. Various methods have been devised to process Earth Roads and render them capable of supporting light traffic under normal weather conditions with reasonable maintenance. They all take the natural soil as basic mater- ial, stabilized with or without admixtures. We shall first see what has to be known about soil as a material, what are the general features of the road, and then, what are the main types of soil stabilization to which these can be related. These main types of stabilization are: Mechanical, Chemical, Soil-Bitumen, Soil-Cement and Thermic. Mechanical, Chemical and Thermic Stabilization are low type sur- facings. Soil-Bitumen and Soil-Cement Stabilization are classified as in- termediate type. These processes utilize the soil in place to the maximum and do not 2 require highly skilled labor or Specialized equipment (except Thermic Sta- bilization), but they are well established and standardized techniques which do not emanate from the trial and error method. CHAPTER I THE SOILS Soils are composed of particles of varied size and origin. Their distribution gives to soils their properties. By their origin they can be classed as organic or inorganic. By their size they are classed: Gravel over 2.00 mm. inorganic Sand from 2.00 to 0.05 mm. inorganic Silt from 0.05 to 0.005 mm. inorganic or organic Clay below 0.005 mm. inorganic or organic Colloids below 0.001 mm. inorganic or organic They contribute differently to the behavior of soils. Gravel and sand when confined contribute mainly to the bearing power. They are cohesionless, but can develop high internal friction (eSpecially 'when of the angular type). They are pervious but show practically no capillary action. Clay and colloids contribute to cohesion and are only little pervious, but they have slight bearing power, when wet are elastic and have low capil- larity. Silt is of an intermediate type and has no cohesiveness with high campillarity. The percentages of sand, silt and clay give to the soil its particular texture. In Chart I a textural classification of soils is shown. The texture of soil can be determined by the physical characteristics of the textural groups when a small amount of moist soil is rubbed between the fingers. Chart I. Public Road Administration Textural Classification Sand: Sand is single grained. The individual grains can readily be seen or felt. Squeezed in the hand when moist, sand will form a cast which Idll crumble when gently pressed or dried. Sandy.Loam: Soil containing mainly sand but containing sufficient silt and clay to make it somewhat coherent. The individual sand grains can readily be seen and felt. If molded when moist, it'will form a cast which ‘will bear careful handling without breaking. Sands and sandy loans can be coarse, medium or fine, depending upon the proportion of the different sizes of particles that are present. ‘Eggmi Loam.is a soil having a more or less balanced mixture of the different grades of sand, silt, and clay. It is mellow with a somewhat gritty feel; yet fairly smooth and slightly plastic when moist. Silt Loam: A silt loam is a soil having a moderate amount of the fine grades of sand and a small amount of clay, with a large quantity of silt par- ticles. When in dry pulverized condition, it feels soft and floury. If the moist soil is pressed between the thumb and finger, it will not "ribbon" but will have a broken appearance. Clay Loam: Clay loam is a fine-textured soil which breaks into clods or lumps that harden upon drying. ‘When the moist soil is pressed between flue thumb and fingen it will form a thin "ribbon". The moist soil is plastic. Clay: Clay is a very fine-textured soil that forms hard lumps or clods upon drying. When the moist soil is pressed out between the thumb and finger, it will form a flexible "ribbon". The moist soil is very sticky. Texture is very important; however, the behavior of a soil cannot be predicted "a priori" by merely knowing the distribution of its particles. Presence of humus, difference in origin and formation, presence of chemical compounds which tend to floculate the fine particles (like lime and.magnesium) or defloculate them (like sodium or potassium) are factors of great importance. Identifying the soil to a definitely known type, classified in a logical system, is the first step to accomplish in any attempt toward stabilization of soils. Several systems of soil classification have been preposed and are in use throughout this Country by the various agencies dealing with soils. The Public Road Administration Classification is the most widely used in the highway field but is likely to be replaced by the Highway Research Board Classification which is a revision of the original. Since data used in this thesis is not completely covered under the Highway Research Board Classifica- tion system, it was necessary to use the Public Road Administration Classifica- tion system as well. Reference will be made to these two systems as H.R.B.C. and P.R.A.C. respectively. Chart I. Public Road Administration Textural Classification Sand: Sand is single grained. The individual grains can readily be _ seen or felt. Squeezed in the hand when moist, sand will form a cast which ‘will crumble when gently pressed or dried. Sandy Loam: Soil containing mainly sand but containing sufficient silt and clay to make it somewhat coherent. The individual sand grains can readily be seen and felt. If molded when moist, it will form a cast which ‘will bear careful handling without breaking. Sands and sandy loans can be coarse, medium or fine, depending upon the proportion of the different sizes of particles that are present. ‘Eggm; Loam is a soil having a more or less balanced mixture of the different grades of sand, silt, and clay. It is mellow with a somewhat gritty feel; yet fairly smooth and slightly plastic when moist. Silt Loam: A silt loam is a soil having a moderate amount of the fine grades of sand and a small amount of clay, with a large quantity of silt par- ticles. When in dry pulverized condition, it feels soft and floury. If the U.S. Public Road Administration Classification Soils are classified in eight groups designated as A-l to A-8 inclusive. Group A-I. The typical material of this group is a well—graded material (sand, silt and clay) having excellent binder. A-I soils have high internal friction, high cohesion, no detrimental shrinkage, no expansion, capillarity or elasticity. They are generally found in small deposits. Group A-2. This group includes coarse and fine materials with improper gnad- ing or inferior binder. They have high internal friction and high cohesion only under certain conditions, may have detrimental shrinkage, eXpansion, capillarity, or elasticity. Group A-B. The typical material of this group is a sand without binder, with high internal friction, and without detrimental capillarity or elasticity. roup A-h. This group includes cohesionless silts and friable clays having no appreciable amount of sticky colloidal clay. They have variable internal friction, no appreciable cohesion, no elasticity and important capillarity. Group A-5. This group includes micaceous and diatomaceous silts and sands. They are similar to A-h group but in addition possess elasticity in an appre- ciable amount. Group A-6. The typical material of this group is cohesive clay in a diapersed state with low internal friction, high cohesion under low moisture content, without elasticity, but likely to eXpand and shrink in detrimental amount. Group A-7. This group includes micaceous, diatomaceous and flocculated clays. May contain lime or associated chemicals productive of flocculation in the soils. They are similar to A-6 group but in addition possess elasticity. Group A-8. Typical materials of this group are peats and mucks with low internal friction, low cohesion, apt to possess capillarity and elasticity 00 't‘ ( PUBLIC ROADS ADHIKISTRATION TABLE 1. ~ SOIL CLASSIFICATIOR A~2 ““”“‘”‘“"“'””' ‘ Group . A~l Friable . Plastic A-3 A-h _, A-iww A_6 A-7 A-8 General Stability Highly Stable Stable When Dry; Good Stable Satisfactory when Difficult to Good Stability Good Stability Incapable PrOperties at all Times May Ravel Material Ideal Support dry; LOSS Of Stabll— Compact; Stabil- When Preperly When Properly of Support When Confined ity when Wet or by ity Doubtful Compacted Compacted _-__ _ _ W Front Action Physical Constants: — o e I e e . T7 1. T : Internal Friction ........... High High High High vallable JarLable Low Low Law Cohesion High Low High None Variable LOW High High Low Shrinkage ................... Not detrimental Not significant De+rimental when poorly NOt significant Variable Variable Detrimental Detrimental Detrimental raded pransion ..................... None None Some 51}Sflt Varlaole ngh High Detrimental Detrimental Capillarity ................... None None Some Slight Detrlmental ngh High High Detrimental Elasticity . . . . . . . . . . . . . . . . . . . . None i‘lone :3 None Variable Detrimental None ' h Detrimental J L l. 1s Textural Classification: Uniformly Poor Poor coarse fine sand Micacecus and Deflocculated Drainable Peat and F1 I. .‘ w a "‘ “o e a a -1 ‘. . . 1 7'. ‘ . ' l ‘ . General Grading ............. graced; coarse“ grading; poor grading; lrferlor binder material only, coneSlonless Silt dlatomaceous COhSSlVe Clays flocculated muck o — o ‘ ‘ D, . ’1 fine, excellent binder no binder and lrlable clay clays biluiex' Approximate Limits: 4 700 5“ 5, ‘ Sandéfercent ................ 70~SS 55~80 55~80 752i) H? (max.) deSTax') 55 (max.) 55 (max.) 55 (max.) Silt-Percent ................ lfiwfiC 0~45 Ow45 lg“ “8 *dm Medium Medium Not signi— (1) L L ficant l ow or . . . . Clay~Percent ................ 5ml0 0.45 0-45 ' 30 (mln.) 30 (m1n.) Net Signi- ficant Physical Characteristics: . , , . q ' r; ' ‘. ,1, MP \2) 20-40 3 mln . _ LiO‘LLlCi. Itilllit e o a e e e e e e o a e e e e e 14:”th 55 (Hide w) 05 (“fl 1‘ 0) NT) {2) O 1 5 S ( .) 35 (10111.) 35 (mln.) 35-h00 T , . . , - _ O T. ,2 n m g: \ "*- 0-60 18 (min ) 12 ( ' ) 0-60 Big-Smmy Index 4“" MW“) .31-”: and; Not Essential 30 (max-) 30-190 ‘ ' mln‘ Field moisture Ecuivalcnt Not Essential Not 'SSenflel “CL £536“*lce *‘ 50 (max.) 30-100 30-h00 Centrifuge i‘v'ioifi'tU-I‘e n m: m; (,-,- ) 12 (max.) Not Essential Not Fssential fl . . . lemdu ev “d*~ ‘ J - Not essential Not Essential Not Essential Equivalent .................. PUBLIC ROADS ADMINISTRATION TABLE 1. u SOIL CLASSIFICATION (Cont'd.) '-~-‘ A-B Group _ _ Awl ~ ~ Friable ,ng-“Ml_".lfl:1, lQZQ __,,wgggA-5 A-6 A'7 A'8 General Stability Highly Stable Stable When Dry; Good Stable - Satisfactory when Difficult to Good Stability Good Stability Properties at all Times May Ravel Ideal Support dry; Loss of Stabil- Compact; Stabil- When Properly When PrOperly Incapable of When Confined ity when Wet or by ity Doubtful Compacted Compacted Support s-l-_ll__E{9§t Action -__l_ , Centrifuge Moisture (Cont'd.) "‘ “I“ “'“'"‘"” Shrinkage Limit ............ 14~20 ISHQU 25 (max.) Not Essential 20-30 30-120 6-1h 10—30 30-120 Shrinkage Ratio ............ 1.7ml.9 1.7wl.9 Not Essential 1.5—1.7 0.7-1.5 1.7-2.0 1.7-2.0 0o3-1oh Volume Change O»lO one None O~16 0.16 17 (mun) 17 (min.) 14-200 Lineal Shrinkage 0~5 owe None 0—1; 0-34 5 (min.) 5 (min.) 1-30 Compaction Characteristics: Maximum Dry Weight, 150 (m1n.) l20~lSO 120~13O llO~l20 80-100 80-110 80-110 90 (max.) pounds per cubic foot .... Optimum Moisture, percentage 9 9~12 ? 9“12 12-17 22-30 17-28 17-28 of dry weight (approximate) Max. field compaction required, so so 90 95 100 100 100 Waste % of max. dry weight, pounds per c.f. .................... , Rating for Kills 50 ft. or Excellent Good SOOd G003 t0 Poor to Fair to Fair to Unsatis- less in height .............. p00? very poor poor poor factory Rating for Fills more than A 50 ft. in height ............ Good Good to Fair Good to Fair J29? to Good to Very poor Very poor Very poor Unsatis- Required Total Thicfiness for Ialr fair factory Sub~base, bese and surfacing, , , inches 0—6 0—6 *0 9-3.8 9—21; 12-2!i 12-214 (1) Percentage passing No. 200 Sieve, O to 10. (2) NF — NonmPlastic. 10 GROUP A-l A-2 A-S A—4 A—5 A-6 A-7 A—B Coarse material, % (Plus No. 10 sieve) 0.65 0 O 0 O 0 .§ :a Total sandl % 70-85 55 min. 0 55 maximum 4; g Coarse sandJ 2% 45-60 0 O O O 0 '8 g Silt, % 10-20 0 o o o 0 (:5 : Clal‘Lfr’: 5-1.0 O O O O 0 «a Passing No. 200 m Sievglj'o O 0 0-10 0 O 0 Liquid limit, % 14-55 55 max. NP 20-40 55 minimum m Plasticity index 55 4—9 NP-15 NP 0—15 See chart :3 Shrinkage limit, % 14-20 0 0 20—50 See chart a 3 Field moisture c8 2 equivalent 76 O 0 O 50 max. See chart 8 Centrifuge moist— 15 25 12 0 0 ure equivalent. 16 gait. max. max. 0 Not essential NP - Non plastic Plasticity Shrinkage limit ' Field e Chart 2. U.S. Public Roads Administration soil grouping 11 in detrimental amounts. Table 1 gives a summary of the U.S.P.R.A. Classification. Highway Research Board Classification Soils are classified in seven groups designated as A-l to A-7 inclusive. When the main groups are not enough detailed to differentiate between soils belonging to the same group, use is made of subgroups and group index. Granular Materials (containing 35 per cent or less passing No. 200 sieve) Group A-l. The typical material of this group is a well-graded mixture of stone fragments or gravel, coarse sand, fine sand and a non-plastic or feebly plastic soil binder. However, this group includes also stone fragments, gravel, coarse sand, volcanic Cinders, etc., without soil binder. Subgroup A-l-a includes those materials consisting predominantly of stone fragments or gravel, either with or without a well-graded binder of fine material. Subgroup A-l-b includes those materials consisting predominantly of coarse sand either with or without a well-graded soil binder. Group A-2. This group includes a wide variety of "granular" material which are borderline between the materials falling in Groups A-1 and A-3 and the silt- clay materials of Groups A-h, A-S, A-6 and A-7. It includes all materials containing 35 per cent or less passing the No. 200 sieve which cannot be clas- sified as A-l or A-3 due to fines content or plasticity or both, in excess of the limitations for those groups. Subgroup A-2-h and A-2-S include various granular materials containing 35 per cent or less passing No. 200 sieve and with a.minus No. hO por- tion having the characteristics,of the A-h and A-5 groups. These groups include such materials as gravel and coarse sand with silt con- tents or plasticity indexes in excess of the limitations of Group A-1, and fine sand with nonplastic silt content in excess of the limitations of Group A-3. 12 Subgroups A—2-6 and A-2-7 include materials similar to those described under Subgroups A-2-h and A-2—S except that the fine portion contains plastic clay having the characteristics of the A-6 and A-7 group. The approximate combined effects of plasticity indexes in excess of 10 and percentage passing No. 200 sieve in excess of 15 is reflected by group index values of O to h. Group A-B. The typical material of this group is fine beach sand or fine desert-blow sand without silty or clay fines or with a very small amount of nonplastic silt. The group includes also stream—deposited mixtures of poorly graded fine sand and limited amounts of coarse sand and gravel. Silt-Clay Materials (containing more than 35 per cent passing the No. 200 sieve). Group A-h. The typical material of this group is a nonplastic or moderately plastic silty soil usually having 75 per cent more passing the No. 200 sieve. The group includes also mixtures of fine silty soil. The group index values range from 1 to 8, with increasing percentages of coarse material being re- flected by decreasing group index values. Group A-S. The typical material of this group is similar to that described under Group A-h, except that it is usually of diatomaceous or micaceous char- acter and maybe highly elastic as indicated by the high liquid limit. The group index values range from 1 to 12, with increasing values indicating the combined effect of increasing liquid limits and decreasing percentages of coarse material. Group A-6. The typical material of this group is a plastic clay soil usually having 75 per cent or more passing the No. 200 sieve. The group also includes mixtures of fine clayey soil and up to 6b per cent of sand and gravel retained on the No. 200 sieve. Materials of this group usually have high volume change between wet and dry states. The group index values range from 1 to 15, with increasing values indicating the combined effect of increasing plasticity indexes and decreasing percentages of coarse material. 13 Group A-7. The typical material of this group is similar to that described under Group A-6, except that it has the high liquid limits characteristic of the A-S group and may be elastic as well as subject to high volume change. The range of group index values is l to 20, with increasing values indicating the combined effect of increasing liquid limits and plasticity indexes and decreasing percentages of coarse material. Subgroup A-7-S includes those materials with moderate plasticity indexes in relation to liquid limit and which may be highly elastic as well as subject to considerable volume change. Subgroup A-7-6 includes those materials with high plasticity indexes in relation to liquid limit and which are subject to extremely high.volume changes. Group Index is given by the empirical formula Group index a 0.2a + 0.005ac + 0.01 bd in‘which a that portion of percentage passing No. 200 sieve greater than 35 and not exceeding 75, eXpressed as a positive whole number (1 to to) b s that portion of percentage passing No. 200 sieve greater than 15 and not exceeding 55, eXpressed as a positive whole number (1 to hO) c a that portion of the numerical liquid limit greater than hO and not exceed- ing 60, exPressed as a positive whole number (1 to 20) d . that portion of the numerical plasticity index greater than 10 and not exceeding 30, expressed as a positive whole number ( l to 20) Table 2 gives a summary of the H. R. B. Classification. TABLE 2. - HIGHWAY RESEARCH BOARD CLASSIFICATION OF HIGHWAY SUBGRADE MATET‘iIALS (With Suggested Subgroups) ~-_.__‘_._........-. -.,m... .. W/M "- - Silt-Clay Materials General Granular Materials M *e than ,% casing No. 200) Classification (35% or less passing No. 200) ( 01 35 p“ w “- ._..._.. ....-.. -.. ._.i, _.. .. _. --_ _.__,.-,.,_l _.i__. .r _. .. n..- ,fixwm‘r—‘w—‘F A—7 Group A'1 I O AHQ A 0 6 x 9 a t-— A—h A_S A—é A«%-§ Classification A-l—a A-l—b A-3 A-Z‘a A-e-S a—e—i earn! ‘ i_7a6 “- —_ _ “-_r-.a—a—.--_.-‘.._.~_._._.- ..»—~..~.-........._......._ ~~~~~mW~Hlm .W i. ,_ Sieve Analysis, Percent No. 10 50 max. No. hO 30 max. 50 max. 51 min. / . _. ' f . No. 200 15 max. 25 max. 10 max. 35 max. 35 max. 35 max. 35 max. 30 mlfi- 36 min. 36 min. 30 min. Characteristics of fraction passing No. hO: A ‘ . - . LiQUid Limit hO max. hl min. hO max. hl min. ho max. ul min. ho max. hl min.a Plasticity Index 6 max. I.P. 10 max. 10 max. ll min. ll min. 10 max. 10 max. ll min. ll min.- Group indexb o O h max. 8 max. 12 max. 16 max. 20 max. USual Types of significant Stone Fragments, Fine . _ n_ . , Constituent Materials" Gravel and Sand ' Sand Silty or Clayey Gravel and Sand Silty Sclls clayey Sclls General Rating as Subgrade Excellent to Good Fair to poor Classification Procedure: With required test data available, proceed from left to right on abovemchart and correct group will be found by process of elimination. the correct classification. aPlasticity index of A—7—S subgroup is equal to or less than LL minus 30. than LL minus 30. bSee group index formula for method of calculation. as: A—2-6(3), A—h(5), A-6(12), A—7—S(l7), etc. The first group from the left into which the test data will fit is Plasticity index of A-7~6 subgroup is greater Group index should be shown in parenthesis after group symbol. WI 15 Chart B Chart A Chart 3. Highway Research Board Group Index 16 CHAPTER II THE ROAD The road structure can be divided in two parts: The road surface and the foundation. The road surface consists essentially of a base course whose structure must be such that it can support without failure the combined detrimental action of weather and traffic. The road surfaces we are concerned with in this study are of the flexible type, i.e. they are not designed to support the pressure of the applied traffic load, but to transmit it to the foundation. The road surface is subject to complex efforts: vertical pressure of the vehicles load, horizontal shears in all directions, impact due to accel- erating, decelerating, braking, or skidding. Rain by splash and runoff action tends to wash away the fine soil fraction, dryness with wind blow causes dust and granular fractions to loosen. By the impact, stamping, suction, tangential efforts, friction, vibration of the wheels, the surface fails in several spots, grains are projected and pot holes are formed. As the surface becomes rough vibrations of the vehicles' Springs assembly loosen material in longitudinal stripes like waves whose distance is regular and correSponds to the period of the Spring vibration for the average Speed of the bulk of the vehicles. In order to prevent this deterioration a "wearing course" is usually placed which provides a surface to resist abrasion of tires and water, The foundation or subgrade must carry without failure the stresses transmitted through the base course. Therefore it is necessary to have def- inite information regarding its bearing capacity. Methods used for that purpose are the California Bearing Ratio Test, the triaxial compression test, 17 the plate bearing test and the cone bearing test.(1)* The foundation must be uniform. Weak Spots are likely to settle irregularly with traffic, deflection and cracks may appear to the surface allowing water to enter the structure. It must be stable. Volume changes due to climatic variation and moisture fluctuation must be eliminated. By shrinkage or swelling they provoke breakdown of the soil structure and the lowering of the bearing capacity. (lehe values given by these different tests are only comparative and have not yet been related one to another. * See Appendix I 18 CHAPTER III MECHANICAL STABILIZATION Mechanical stabilization is the title given to those methods of con- struction in which soils or soil-aggregate materials, are mechanically manipulated to provide foundation, base or surface courses, able to carry with only slight degradation traffic loads under all normal moisture and traffic conditions. The purpose of this manipulation is either to increase the density of, to produce a greater uniformity in, or to reduce the moisture content of the soil. This is to be obtained because of the following facts: Effect of Density on Compressive Strength '1 "T l -1- s .i‘i kg . l_ H.- “l," L‘ Figure l emphasizes the following relationzasdensity is lowered, bearing capacity falls off rapidly. Effect of Moisture Content on Bearing Capacity Figure 2 shows that as the moisture content of a mechanically stabilized mix- ture is increased, its compressive strength decreases rapidly. In effect the values corresponding to cohesion, adhesion, compression, shearing resistance are plotted on the same graph. As the moisture content 19 is increased it is readily seen that maximum reactions occur in the plastic .—.— __ -— range 0 l_'._ . -l14.: ' .'\ O -.V -r._'._l1_ .‘fi' 3‘; . . These facts stress the importance of P.I. and L.L. in the selection of -i_1_' materials. In mechanical stabilization soils having P.I. of less than 6 are generally Specified. Once the prOper material is selected moisture contrdl is of utmost impor- tance. It is accomplished by good drainage, proper gradation and adequate .compaction. .Drainage is obtained by shoulders and gutters for surface water and by sub-drains and eventually subbases for ground water. Gradation generally adopted falls in the grading band shown in Fig. h 20 Compaction is made at 95%-lOO% of Optimum moisture content given by the Moisture Density Test*. Mechanical stabilization is obtained by the utilization of the natural properties of the soil components: Gravel and sand make the skeleton. Clay acts as a binder. Silt is a filler. Materials Gravel and sand must be sound and durable. The angular shaped grains are most desirable for they give better interlocking action. Clay must be free from organic or deleterous material. Two types of mixtures are considered:* ' Type A Sand clay mortar Type B Coarse graded aggregate (gravel, crushed stone, slag) Surface Material (1) Gradation Type A Sand Clay Mortar Passing Percentage by Weight 1 inch sieve 100 No. 10 sieve 65 - lOO Material passing No. 10 sieve No. 10 sieve 100 No. 20 sieve 55 - 90 No. 40 sieve 55 - 70 No. 200 sieve 8 — 25 *See Appendix *Ref. 44 .. 21 _Type B Coarse Graded Aggregate Passi Percentage bygfieight 1 inch sieve 100 5/4 inch sieve 85 — 100 5/8 inch sieve 55 — 100 No. 4 sieve 55 - 85 No. 10 sieve 4O — 70 No. 40 sieve 25 - 45 No. 200 sieve 10 — 25 (2) Characteristics of material passing the No. 40 sieve for surface courses 4