SOIL AND ITS ORIGINSOILDefined as the accumulation of unconsolidated sediments and deposits of soil particles as a result of integration of rocks.ROCKDefined as the accumulation of unconsolidated sediments and deposits of soil particles as a result of integration of rocks.Rock types Igneous Rock Sedimentary Rock Metamorphic RockIgneous RockIgneous rock resulted from cooling and hardening of molten rock called magma that has originated within the earth.Sedimentary RocksSedimentary Rocks are formed from accumulated deposits of soil particles or remains of certain organism that have hardedned by pressure or cemented by materials.

Metamorphic RocksMetamorphic Rocks are rocks resulted from metamorphism due to changes in temperature, pressure and plastic flow changing the original rock structure and mineral composition of rock.

Categories of SoilResidual Soil or Sedimentary SoilFormed from weathering of rocks or accumulation of organic materials remained at the location of origin.Mechanical WeatheringPhysical disintegration due to effects of wind, rain, running water or tectonic forces (earthquake).Chemical and Solution WeatheringDecomposition of rock due to chemical reactions that occur as a result of exposure to atmosphere, temperature changes, reaction with water or other materials.Transported SoilThose materials transported from their place of origin.SOIL TYPES Boulder Cobble Gravel Sand Silt Clay Colloid Loam Loess Peat Silty Sand Silty ClayCHARACTERISTICS OF SOIL1. Grain size2. Grain shape3. Surface texture and electrical surface charges.General Characteristics and classification of soil particles1. Gravel2. Coarse sand3. Fine Sand4. Silt grain5. Clay6. Colloidal Clay7. GravelConsist of rock fragments more or less rounded by water action or abrasion.Quartz, well rounded pebbles and boulders, slightly worm gravel.Fine SandHas particles that are more angular than coarse and fine sand particles.Coarse SandUsually rounded like gravel with which it is found and generally contains the same materials.Silt SandSimilar to fine sand with the same mineral composition. They are found in rock flour in glacier moraines.Contains:1. Pumice2. Loess3. Materials foreign to associated sand.ClayPlate like, scale like, or rod like in shape as a result of chemical weathering. Performance is influenced by moisture and surface chemistry.Colloidal ClayA finer clay particle that remains suspended in water and does not settle by gravity.Important Grain Shape Characteristics1. Rounded particles are considered strong materials.2. Flat and flaky particles are weak and variable.3. Angular or roughly cubical shape particles increases resistance of soil mass to deformation when subjected to load.4. Rounded particle has tendency to roll over each other when subjected to load.

SOIL CLASSIFICATIONSWhy soil classification?The particles above gravels are identified as rocks and are generally not considered as a soil. Soil classification is primarily based on the size of particles. The purpose of soil classification is to arrange soil into groups and label them according to certain characteristics and engineering behavior. The name of the group gives useful information about the behavior of soil as a construction material and foundation support.Although soil classification apparently gives very useful information about soil behavior and about some of its properties but it doesnt eliminate the requirement of in-detail study of soil behavior and study of properties like shear strength, unit weight, compaction characteristics, degree of saturation, susceptibility to frost action etc.Soil Classification SystemsDifferent organizations from around the world have made systems to classify the soil according to their requirements like some agricultural department, highway department, building department, aviation department etc. There are two soil classification systems in common use for engineering purposes. The Unified Soil Classification System is used for virtually all geotechnical engineering work except highway and road construction, where the AASHTO soil classification system is used. Both systems use the results of grain size analysis and determinations of Atterberg limits to determine soils classification. Soil components may be described as gravel, sand, silt, or clay. A soil comprising one or more of these components is given a descriptive name and a designation consisting of letters or letters and numbers which depend on the relative proportions of the components and the plasticity characteristics of the soil.Two of the most commonly used soil classification systems are(a) USCS Unified Soil Classification system which is mostly used for buildings and for structures where soil will be used to support foundations or footings(b) AASHTO (American Association for State Highway and Transportation officials) soil classification systemTest Required for Classification of SoilTo classify the soil in the laboratory thus some tests results were used to classify the soil;Tests include;(a) Liquid limit and Plastic limit tests Atterberg limit results(b) Plastic size analysis testUnified soil classification system (USCS):The Unified Soil Classification System is based on the airfield classification system developed by Casa Grande during World War II. With some modification it was jointly adopted by several U.S. government agencies in 1952. Additional refinements were made and it is currently standardized as ASTM D 2487-93. It is used in the U.S. and much of the world for geotechnical work other than roads and highways. In the unified system soils are designated by a two-letter symbol: the first identifies the primary component of the soil, and the second describes its grain size or plasticity characteristics. For example, poorly graded sand is designated SP and low plasticity clay is CL. Five first-letter symbols are used:

G for gravelS for sandM for siltC for clayO for organic soil

Clean sands and gravels (having less than 5% passing the No. 200 sieve) are given a second letter P if poorly graded or W if well graded. Sands and gravels with more than 12% by weight passing the No. 200 sieve are given a second letter M if the fines are silty or C if fines are clayey. Sands and gravels having between 5 and 12% are given dual classifications such as SP-SM. Silts, clays, and organic soils are given the second letter H or L to designate high or low plasticity. The specific rules for classification are summarized as follows and described in detail in ASTM D 2487.

Organic soils are distinguished by a dark-brown to black color, an organic odor, and visible fibrous matter.For soils that are not notably organic the first step in classification is to consider the percentage passing the No. 200 sieve.If less than 50% of the soil passes the No. 200 sieve, the soil is coarse grained, and the first letter will be G or S;if more than 50% passes the No. 200 sieve, the soil is fine grained and the first letter will be M or C.For coarse-grained soils, the proportions of sand and gravel in the coarse fraction (not the total sample) determine the first letter of the classification symbol. The coarse fraction is that portion of the total sample retained on a No. 200 sieve. If more than half of the coarse fraction is gravel (retained on the No. 4 sieve), the soil is gravel and the first letter symbol is G. If more than half of the coarse fraction is sand, the soil is sand and the first letter symbol is S. For sands and gravels the second letter of the classification is based on gradation for clean sands and gravels and plasticity of the fines for sands and gravels with fines.

For clean sands (less than 5% passing the No. 200 sieve), the classification is well-graded sand (SW) if C 6 and 1 Cc 3. Both of these criteria must be met for the soil to be SW, otherwise the classification is poorly graded sand (SP). Clean gravels (less than 5% passing the No. 200 sieve) are classified as well-graded gravel (GW) if Cu 4 and 1 Cc 3. If both criteria are not met, the soil is poorly graded gravel (GP). For sands and gravels where more than 12% of the total sample passes the No. 200 sieve, the soil is a clayey sand (SC), clayey gravel (GC), silty sand (SM), or silty gravel (GM).

The second letter is assigned based on whether the fines classify as clay (C) or silt (M) as described for fine-grained soils below. For sands and gravels having between 5 and 12% of the total sample passing the No. 200 sieve, both the gradation and plasticity characteristics must be evaluated and the soil is given a dual classification such as SP-SM, SP-SC, GW-GC, etc. The first symbol is always based on gradation, whereas the second is always based on plasticity. For fine-grained soils and organic soils, classification in the unified system is based on Atterberg limits determined by the fraction passing the No. 40 sieve. The liquid limit and plasticity index are determined and plotted on the plasticity chart. The vertical line at LL = 50 separates high-plasticity soils from low-plasticity soils. The A-line separates clay from silt. The equation of the A-line is

PI = 0.73 (LL 20)The U-line is not used in classification but is an upper boundary of expected results for natural soils. Values plotting above the U-line should be checked for errors. Inorganic soils with liquid limits below 50 that plot above the A-line and have PI values greater than 7 are lean clays and are designated CL; those with liquid limits above 50 that plot above the A-line are fat clays and are designated CH. Inorganic soils with liquid limits below 50 that plot below the A-line are silt and are designated ML; those with liquid limits above 50 that plot below the A-line are elastic silts and are designated MH.

The plasticity chart has a shaded area; soils that plot in this area (above the A-line with PI values between 4 and 7) are silty clay and are given the dual symbol CL-ML. If the soil under consideration is the fines component of a dually classified sand or gravel, the soil is classified as SM-SC or GM-GC. Soils with sufficient organic contents to influence properties that have liquid limits below 50 are classified as OL; those with liquid limits above 50 are classified as OH. Soils that are predominantly organic, with visible vegetable tissue, are termed peat and given the designation Pt.

Explanation of GI the Group IndexGI = (F - 35)[ 0.2 + 0.005 (LL-40) ] + 0.01 (F - 15) (PI - 10) F = Percent of soil passing the 0.075 mm sieve LL = Liquid Limit PI = Plastic Index

a) Soils which are non-plastic and for which the Liquid Limit cannot be found have GI = 0.b) A-2-6 and A-2-7 soils use only the second term of GI equation.c) If the final GI has a negative number, it is reported as zero (0).d) Although not explicitly expressed in either the AASHTO or ASTM specifications, GI for A-2-4 and A-2-5 are zero.Introduction to AASHTO Classification System:This system of classification is based upon the observed field performance of soils under highway pavements (flexible and rigid) and is widely used around the world for highway and airfield construction.Soils which have same general load carrying capacity i.e. bearing capacity and serviceability characteristics are grouped together and classified into eight groups from A1 to A8A-1, A-2 and A-3 soils are coarse-grained soils While A-4, A-5, A-6 and A-7 represent the fine grained soils. A-8 includes Peat, muck, and other highly organic soils.A-1 soil is best to use as sub-grade then a-2 and so on. And poorest is A-7 soil.Group IndexTo check the quality of the soil, a term group index is defined. Higher the G.I weaker will be the soil, a soil having G.I of 20 is considered as poorest soil overall.Following is the formula for Calculating G.IGI = (F200 35) [0.2 + 0.005(LL-40)] + 0.01 (F200 15)(PI-10)WhereF200 = %age passing for sieve no. 200 (0.075 mm) expressed as whole numberPI is plasticity Index where PI = LL PLAnd LL is liquid limit and PL is plastic limitSome rules:a. If Group Index for your soil comes out to be negative take it as zerob. Group index is expressed as rounded whole numberc. No upper limit can be any integerd. For soils A-2-6 and A-2-7 following Group Index formula should be used;G.I = 0.01(F200-15)(PI-10)Soil is represented as A-4(5) where A-4 is soil type and Group Index is 5A group index of (0) indicates the best construction material for sub-base, or subgrade.A group index of more than (20) indicates a very poor material for sub-base or subgrade.ExampleClassify the following soil by the AASHTO classification system:Percent passing no. 4 sieve = 82Percent passing no. 10 sieve = 71Percent passing no. 40 sieve = 64Percent passing no. 200 sieve = 41Liquid limit = 31Plasticity index = 12SolutionRefer to table 8. More than 35% passes through a no. 200 sieve, so it is a silt-clay material. It could be A-4, A-5, A-6, or A-7. Because LL = 31 (that is, less than 40) and PI = 12 (that is, greater than 11) this soil falls in group A-6. From equation (23),GI = (F35) [0.02+0.005(LL40)] + 0.01(F15) (PI 10)\soGI = (4135) [0.02+0.005(3140)] + 0.01(4115)(1210)=0.370Thus the soil is A-6(0).