Anyone who builds or maintains roads works with soils. THE VERMONT Soil is the foundation material for all roads and is the riding LOCAL ROADS PROGRAM surface for about two-thirds of the local roads in Vermont. SAINT MICHAEL'S COLLEGE Steel, concrete and wood are easy construction materials WlNOOSKl PARK to work with because they are of a "homogeneous'; unifonn composition. Their behavior can be generally predicted. Soil is COLCHESTER, VERMONT 05439 just the opposite. In its natural state soil is rarely uniform and can only be observed and worked by comparing it to a similar soil. Rock was formed in three different ways. Igneous rocks, like granite are of volcanic origin and are solidified from mol- ten masses. Sedimentary rocks, such as limestone, are formed FACT SHEET in layers as the result of minerals and other elements settling out of water solutions. Metamorphic rocks, such as marble, are transformed from materials of the first two by heat and pressure. Time, chemistry and weather have attacked these dif- ferent kinds of rocks and have worked much of their surface 'nto minute particles creating the soil. These particles have oeen well mixed by glaciers, wind, water, gravity, and man. Decaying plant and animal life have further complicated the soil composition by contributing "organic" material to the mixture. Vermont TYPES OF SOIL
Gravel is any rock-like material down to '/a inch in diameter. For simplification purposes, shot-rock boulders are included in this group also. Shot rock is a jagged material that has many angular faces (much like crushed limestone) and is usually fairly large in diameter. As the name implies, it is either "shot" out of a pit or the rock formation may be in the existing road- way. Boulders are rounded rocks larger than 10 inches in dia- meter. The rounding is usually caused by past or present water running over the rocks. The rocks from 10 inches in diameter down to the Ys inch size are referred to as stones. Generally, there is not enough smaller material present to fill the chinks St. Michael's College or "void" spaces. This material is easily recognizable at a glance. Winooski Vermont 05404 -Sands are mineral grains from '/s inch in diameter lar material can be vibrated into -a dense form down to 0.074mm in diameter. This is about the because the particles jiggle themselves about until smallest size that can normally be seen with the they find the most compacted grain arrangement, naked eye. Both rounded and angular 'materials thereby minimizing voids. Granular material does are included in this group. Sand ranges from have internal friction due to this stacking of the coarse to fine grains, but it feels gritty and its compacted particles. In the case of clean sands, strength is not affected by wetting. A close visual very little, if any, dust or crusty coatings shodd examination will reveal whether it is all one size be visible on the individual particles. If the soil is like beach sand or whether it is a mixture of large, wet, pick up a handful, knead it a few times and medium, and small-sized sand particles. In gen- then shake it off the hand. No muddy residue eral, it is called granular material because the should remain. If water is poured onto a clean grains have little to no attraction for each other. sand it will sink in immediately without making This leaves the soil with no dry strength. Cranu- any mud.
-Silt is actually a very fine sand that exhibits a thread. Clay particles have a great deal of attrac- floury appearance when dry. If pure, silt will set- tion for each other and thus clay is a cohesive ma- tle out of muddy water and leave it clear. Al- terial. Clay has a high dry strength. low erosion, though silt particle size is ,074mrn down to good workability, and it compacts very readily at O.OD5mm it is still granular material. Silt compacts the proper moisture content. However, when not very poorly, has next to no dry strength because properly compacted clay has very little internal of lack of "cohesion" between the grains, and is friction and is therefore subject to slides. Clay easily broken down and pulverized when in dry also is subject to wide "plastic" limits. It is low in lumps. All granular material permits easy permeability, when compacted, since water has passage, or flow, of groundwater and therefore is difficulty flowing through the tight pattern very "permeable." created by the individual particles, held in place -Clay is the finest size soil particle. It consists of by the surface tension bond from the natural tiny microscopic flat, scale-like particles which moisture. give clay its "plastic" properties. In a moist condi- Organic Matter is partially decomposed plant or tion, clay becomes very sticky and a small lump other previously living matter. It can exist as peat, may be rolled between the hands to form a small organic silt, or organic clay. Organic material is generally soft, emits an odor when heated, ap- There is also an amount of resistance in cohesion gears fibrous, and is usually black or very dark when the soil partides resist being pulled away brown. Organic material should not be consid- from each other. This type of resistance is very ered for fiil material since it will further decom- obvious in clay soils when the "elasticity" is very pose, resulting in voids. high. By comparison, the cohesion in sand and Soil can therefore be divided into two groups, gravel particles is very low. cohesive material and non-cohesive. The cohesive The measure of the shear resistance of a soil soils will have the characteristic of sticking to- is its amount of resistance to individual particles gether. Examples would be clay, coarse day sedi- sliding over each other. The rate at which the ma- ment, loamy sand, and sandy clay. The non- terial is subject to movement is dependent upon cohesive soils take on the characteristic of graded the amount of applied force, the internal friction sand, gravel, sand mixtures, sandstone, and between the individual soil particles, and the crusher run material. Generally, the soil types are cohesion of the material. Therefore, it should be found in nature in some mixed proportion. Care fairly easy to recognize material of a high shear re- should be taken in placing fill embankments to sistance versus one with a low shear resistance. A make the most advantageous use of soil proper- loosely graded non-plastic granular material, ties. which is easily compacted, would be classified as having a low shear resistance. On the other hand, SOIL PROPERTIES a clay material which is very elastic and difficult Shear Resistance is the ability of soils to resist in- to compact would be classified as having high ternal movement or slippage when subjected to an shear resistance. The point is, the more force that imposed load or to pressure from "static" or im- is required to shear the soil material from adjoin- pact compaction. This resistance comes about or ing particles, the higher the shear resistance will is the result of friction between the soil particles. be.
Elasticity, as the term implies, is a soil's ability to press down on a piece of sponge, and after remov- return approximately to its original form after the ing the pressure, the sponge will spring back to its applied load is removed. An example would be to original shape. Soils of this type are very undesirable in construction and road building. For great deal while sand and gravel shrink slightly. example, as automobiles and trucks roll over a Material which expands and contracts a great deal road surface the base material gives way to the ap- such as clay offers an undesirable base for suppor- plied load and rebounds after removal of the load, ting surfaces such as a roadbed. Soils may be fully continually flexing, which eventually causes a compacted, but as water penetrates the soil mate- breakdown of the road surface. rial, the material swells and as the water is Compressibility. As a volume of material is sub- evaporated, the material shrinks causing flex- jected to a downward force (load), the voids ibility and thus damage to the surface. This is ex- within the material are decreased, being com- tremely critical on rigid surfaces such as concrete pr&sed together, and take up less volume than the or asphalt roads or structures which are incapable original volume. As a result, the soil particles are of absorbing any flexing motion without crack- forced together after the load has been removed. ing. An example would be tamping the soil in a hole around a fence post. When the soil is first placed Table 1 provides information for each in the hole, it will occupy a given area, after being classification on the usefulness of the type soil for tamped down with a tamping pole it will even- road foundations and bases. It also gives the po- tually occupy less space than it did at the begin- tential frost action, compressibility, and drainage ning. In some cases a measurement of soil density characteristics and suggests the appropriate me- is possible by calculating the amount of the soil's chanical compaction equipment for each type of compression to a given load. soil. The divisions and classification prefixes used Capillary Action refers to a soil's ability to absorb in the chart are as follows: and disperse water. The particle surface forces, ac- ting upon the spaces between the water and the A. Coarse-grained soils. surfaces of the soil particles, produce the capillary 1. Gravels and gravelly soils, prefix G. action in the voids of the soil material. The water 2. Sands and sandy soils, prefix S. between the particles forms an elastic bond. The The gravels and sands are subdivided capillary forces acting between the soil particles into the following: can be removed by application of compression (a) Well graded soils with little or no and shear forces, driving or displacing the water fines, suffix W. out of the soil material.. (b) Medium graded soils, with little or Permeability of a soil is the rate at which water is no fines, suffix M. permitted to flow through the soil either from (c) Poorly graded soils with little or no gravitational forces from percolation or water fines, suffix P. pressure from a subsurface water table. In com- (d) Poorly graded soils with appreciable paction, this is a very important factor. The mois- fines, or well graded soils with excess ture must be permitted to flow through the soil fines, suffix F. material, to some extent, in order to reach the op- timum moisture content for satisfactory compac- B. Fine-grained soils. tion. Many times water may saturate an area with 1. Inorganic silt soils, prefix M. little or no penetration into the depth of the soil, 2. Inorganic clay soils, prefix C. as in the case of a heavy day. A soil of this nature 3. Organic silts and clays, prefix 0. is not very permeable and will probably need to Two subdivisions to the fine-grained be tilled in order to be satisfactorily compacted at soils: the correct moisture content. This is why it is (a) Soils whose liquid limits are less than recommended to add a granular material to a clay 50 percent, and which have low com- soil. The granular material allows the water to in- pressibility, suffix L. filtrate the soil material more effectively. (b) Soils whose liquid limits are greater than 50 percent, suffix H. Shrinkage is usually limited to the finer grain soils in which the water content is reduced by means of C. Fibrous soils. evaporation. Different soils swell and shrink at 1. Peat and swamp soils, prefix Pt. There different rates. Generally, clay materials shrink a are no subdivisions of the fibrous soils. Table 1 SOIL CHARACTERISTICS
M O*. yl -.rw*w-. M LOU 011 -w*-um Y- IM nnul *.La, WWY1I - *.*- an.r*.=*n*q,= m wa. "a 2- *n unw ,-tam --mu cd, " w,
SOIL MIXING slides, and make possible a wider choice of com- paction equipment. Gravel and stones alone bear Mixing soils at the barrow pit or on the job up well, but do not compact well, are unstable, is the key step that makes subsequent operations and may injure some compacting equipment. In easy or difficult. Best results are not obtained general, plastic materials are more workable but from soil of any one predominant type, but have a lower "bearing capacity," while granular rather, from good sensible mixtures of two or materials produce stability due to internal friction more types of soil if they are readily available. and inherit strength. Blending together two or more different soil types What to mix in what proportions is decided can result in a superior end product. by knowing what combinations of soil and water In a coarse grained sand, for example, fine is wanted and then using trial and error for grain sand should be added to improve the density refinements to see what combinations give the and compactability, since the smaller grains will best results. If the soils to be mixed together ap- distribute and orientate themselves among the pear in the same barrow pit in different layers, spaces between the larger grains reducing the they can often be handled efficiently by shovel or amount of voids. If possible, clay should be added belt loader. The machinery, working against the as a "binder" to make it more workable. face of the bank with the different layers of soil, In every clayey material, granular soil should mingle and blend the materials directly as they dig be added to provide internal friction, prevent and load the soil into the hauling units. More often, the different soils will come from fill in the voids among the larger material thus different sources of barrow and then they must be compacting the soil. Stabilizers such as calcium thoroughly mixed on the fill before being com- chloride, sodium chloride, lime, cements and pacted. It is a very poor practice to make alternate other materials help in the compaction process. lifts of the different materials. The different Third, it is generally agreed that the func- materials should be dumped out and thoroughly tions of these three materials in a gravel road are mixed together by use of discs, harrows, or similar to: equipment. 1. support traffic loads In earthwork, an hour's time spent in pro- 2. resist abrasion cessing is worth 3 to 5 hours of just random rol- 3. shed water ling. Dozing serves to level out and spread the 4. enhance capillarity loose material and "back-dozing" results in a Finally, there is general agreement about the pulverizing effect. Using a grader to evenly spread percentage of the three materials that should be each layer is important, for then the compaction present in good road gravel but, and here's the equipment can give the entire area the same rub, there is no certainty about the percentages. number of passes resulting in a uniform density There is no rule to follow, only a general stan- throughout the fill. In the instances where lumpy dard. For example, the New York Department of (clods) soil is encountered, other types of equip Transportation uses the following gradations for ment are generally used. Heavy discs, harrows, or surface course gravel: rotary tillers can be used to pulverize the soil before the compaction equipment is used. Seive Size % Pussing By Weight 2" (gravel) 100% %" (sand) 30-65 % HOW CAN I DECIDE IF IT'S #2M) (fines) 10-20% GOOD GRAVEL? The State of Vermont's recommended grada- tions for surface course gravel are similar to New There are two ways. Take samples from the York's for gravel and sand but Vermont's grada- pit and submit it to a laboratory. Your district tions for silts are 0-8%. highway office can arrange for tests. The lab will Where does that leave us? The important do a sieve analysis which determines the grada- thing to remember is that good gravel requires tions of stone and sand and their percentages. gradation of materials which means some percent- Tests can also be conducted for the wearability age of fines and silts. The percentage to use comes and soundness of the stone itself. with experience and in consulting with your State Tests can be done visibly but this takes some District Transportation Administrator. experience. Look at the gravel. Are the stones Table 2, "Gravel Selection", shows the range uniformly graded from large cobbles to ?hinch or of particle sizes necessary for good road gravel. A are they all one sue? Does there seem to be an good bank run gravel would lie in the shaded equal number of stones of one sue that would area. give stability? Are there lumps of clay, silt or other contaminants? In handling the gravel do the GRAVELlBlTUMlNOUS SURFACE fines stick to your hands? If so, it's "duty" gravel and not useful for roads. "Clean" gravel will give A gravel base which is to be topped with a bi- good stability and good drainage. tuminous surface should have far fewer fines than There are some generalizations to make when a road that is to remain a gravel road. Why7 With- we talk about the composition of gravel. First, the out a bituminous surface the moisture in the road three materials-gravel, sand, and silts- should is drawn up due to the capillary action of the fines be present. It is generally agreed that stones above and is free to evaporate. Once the road is surfaced, ?h inch or so are gravel. Particles smaller than ?A the moisture will continue to be drawn up but its inch are sands. Real small particles- those that evaporation will be blocked. This means that pass through a -200 seive-are called fines or silts. your gravel road must not have much clay or silt Fines have the consistency of flour. All three are or it will make a poor base, will retain water, and necessary to make good road gravel. you will be wasting your money if you cover it Second, it is agreed that the smaller materials with a bituminous top. Table 2 GRAVEL SELECTION Sieve Sizes - U.S. Standard
,005 psnlcie sire rn.rn. 7 .I49 .m .42 .84 clay sill line coarse llns medium coarss 5111 11 clay -and gml
Summary % gravel 5070 Upper limit lor Surface gravel % nand 25-40 Lawe, lirnlt lor Base gravel % rill 6 clay 010 lor Base A gnod bank run gravel should run Y. dll 6 clay &15 lor Sudace In ths shadedarea.
SOURCES OF GRAVEL Poor gravel can be improved by mixtures of Good natural gravel sources are shrinking in calcium or sodium chloride, lime, fly ash, and Vermont. Only a few towns own their own gravel cements. pits. Gravel used to cost five or ten cents a cubic yard a few years ago and is now costing towns SUMMARY closer to two dollars. It is possible to obtain gravel from riverbeds Not all gravel is good road gravel. It needs under certain conditions. Be sure to contact the certain features to make it work well for roads. Agency of Environmental conservation to obtain Good gravel is hard enough so it doesn't form dust a permit. (a sign of disintegration) yet loose enough to A town can make its own gravel. Mix '/2 inch drain. It supports the weight of traffic and or ?4 inch crushed stone with sand using a 50/50 distributes traffic loads sufficiently so it doesn't ratio. destroy the subgrade. When buying gravel remember the "bulking" Good road gravel contains a uniform mixture process. Gravel "swells" or "bulks" from its natural of stones with a mixture of sand and fines to bind setting to the truck. Gravel in the truck will shrink the stones together. up to 15% when compacted. Road gravel should contain 40-80% stone, uniformly graded from l/r inch in diameter to 4-6 percentage of Fines in a road that is to be paved inches in diameter. The 20-60% portion that is will make a poor road base. not stone should consist of sand particles (smaller than i/p inch) with not over 8-10% of the sand be- RESOURCES ing fines (finer than flour). The stones should be hard and not easily "Design and Upgrading of Surfacing and Other fractured or disintegrated. Stone you can aumble Aspects of Low-Volume Roads," Transportation with your hands is weak stone. The gravel should Research Record 875, Transportation Research be free of contaminants such as loam and vegeta- Board, 1982. tion. "Road Maintenance Techniques" Center For Loal In summary, the gradation of soils in road Government Technology" Oklahoma State Uni- gravel is important; silts or fines are necessary to versity, 1983. bind larger partides together; stabilization of materials can be added by the use of chlorides, 'Xoad Gravels," Transportation Technology Sup- cements and others: while there is no hard and port For Developing Countries, Compendium 7, fast rule for the percentage of fines or silts in road Tmnsportation Research Board, 1979. gravel, it is important to remember that a high Idby the Tmmponotimn br@nnnlioa hhmpnl Saint Mirbwli Collqr. Wi80oasL.i Vermo8ll OSM 119841