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Soils for Road Work

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Soils for Road Work Soils For Road Work 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.
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