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1. SOIL COMPOSITION AND CLASSIFICATION Soil, or earth, is a substance that both supports life and structure and supplies raw materials for manufacturing and construction. Every structure constructed by humankind, that doesn’t float on the oceans or orbit the planet, must ultimately be supported by either soil or rock. The most common material used in the construction of buildings, bridges, walkways, roadways, parking facilities, hydraulic and hydroelectric structures (reservoirs, dams, etc.) is soil. Soil, in the engineering sense, is defined as all “unconsolidated material composed of discrete solid particles with gases and/or liquids between” (Sowers, 1970). Generally, soil is the “loose”, unbound material forming the top layer of the lithosphere, the outermost layer of the earth’s crust. The solid rock layer upon which the soil layer rests is referred to as the “bedrock” and the overlying soil is often called the “overburden”.

Origin of Soils In order to understand where soil comes from it is important to understand the relationship between soils and rock. The earth’s crust is composed of these two basic types of material. Beneath this crust of solid material lies a region of molten rock (lava) which flows like a liquid due to very high temperatures caused by extreme pressure at this depth. Rock, lava and soil are continually cycling from one type of material to another. For example, igneous rock forms when lava cools after volcanic events. Sedimentary rocks form when layers of different soil types, deposited and accumulated over many millennia (thousands of years) consolidate into rock under the ever-increasing weight of new soil deposits. Metamorphic rock forms from either igneous or sedimentary rock when structural, textural, chemical or mineral changes are brought about by heat, pressure and shear forces. Rocks on the other hand are continually producing soil through the processes of mechanical and chemical weathering. Mechanical weathering processes include temperature changes, frost action, rainfall, running water, wind, ice, abrasion, seismic activity and gravity. Rock masses are split apart, ground together and crushed by these forces which ultimately produce coarser soil particles such as gravel and sand and also silts which are much finer. These types of soil particles are usually three-dimensional (as opposed to flat or elongated). Chemical weathering alters the physical and chemical rock characteristics through reactions of rock minerals with oxygen, water, acids, salts, etc. or through processes such as oxidation, solution, carbonation, leaching and hydrolysis. These processes can increase the volume of material and therefore the internal stresses; or they can leave the rock honeycombed with voids and thus weaker. Chemical weathering produces tiny, twodimensional or flake shaped particles that are often in crystalline form and more commonly known as clays.

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The presence of water, minerals such as nitrogen, phosphorous, carbon and oxygen and ultraviolet radiation promotes the growth of plants and vegetable matter in the surface of the soil layer. When these plants die, their root systems decay and enrich the soil with organic nutrients. Thus, in addition to the mineral constituents from the weathering of rock, soil also contains varying degrees of organic compounds. The top layer (horizon) of soil is generally referred to as “topsoil” and is of particular interest in the landscaping portions of most engineering/architectural construction projects. Soils are usually identified or classified according to whether and how the soil particles were transported after formation. For example, a gravel deposit located at the foot of a cliff or escarpment would be called a Talus gravel deposit. Table 1.01 explains the meaning of these common soil descriptors. TABLE 1.01 – Classification of Soils by Transportation/Deposition Mode (Derucher, 1994 and Schroeder, 1996) AGENT none water ice wind gravity

DEPOSIT NAME Residual Alluvial Deltaic Marine Lacustrine Till Aeolian - Loess - Dune Sand Colluvial Talus

ENVIRONMENT OF DEPOSIT - soil remains at place of origin - flowing water (rivers, streams) - river deltas - quiet brackish water (sea floors) - quiet fresh water (lake bottoms) - glacial ice contact zones - variable - arid or coastal lands - below slope failures/landslides - base of cliffs or escarpments

Soil Properties Major Soil Types There are four major types of soil of interest in engineering: gravel, sand, silt and clay. Gravel and sand are composed of mechanically weathered rock particles that are large enough to be distinguished by eye. These are generally referred to as coarse-grained soils. Gravel, according to the ASTM, ranges in particle size between 4.76 mm (No. 4 Sieve) to 76.2 mm (3” Sieve). Sizes larger than gravel include cobbles (76.2 mm to 300 mm or 12”) and boulders (larger than 300 mm). Sand (ASTM) ranges in sizes between 0.074 mm (No. 200 Sieve) and 4.76 mm (No. 4 Sieve).

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Soil particles smaller than 0.074 mm in diameter (i.e., that pass a No. 200 sieve) are classed as fine-grained soils. Silt consists of fine-grained particles which are formed by mechanical weathering of rock (sand and gravel) and are three-dimensional in shape. Clay has very small, flat and elongated particles (2 dimensional) that are formed by chemical weathering of rocks and minerals. Clay particles are composed of crystalline minerals such as silica and alumina. Crystalline minerals have their molecules arranged in very orderly and repeated patterns, which form sheet-like structures. These sheets bond together to form the individual clay particles, which are flat or platelike in shape with particle length and width, several tens or several hundred times the thickness. The surfaces of clay mineral particles have a net negative electrical charge while the edges have positive and negative charges. These charges cause other molecules and free ions to bond or stick to the charged surfaces. Among these are cations (positively charged ions) of substances such as potassium, sodium, calcium and aluminum from the dissolving of rock minerals into water, but more significantly, water molecules. Most porous material interacts with water by absorbing it into its various cracks and crevices. Clay however adsorbs water which is to say that water molecules adhere to clay particles by this electrical attraction. It is this chemical property that gives clay its predominant physical characteristic: plasticity. There are three main types of clay found in North America: Kaolinite: very stable, low volume change when water added, particles are 75 nm thick, most common type found Illite: more plastic than Kaolinite, no volume change when water added, particles are 100 nm thick Montmorillonite: weak bond between sheets of molecules, tendency to adsorb large quantities of water, particles are 95 nm thick (varies) Bentonite, which is used in excavation for thin-walled construction is a Montmorillonite type of clay. Atterberg Limits A Swedish soil scientist, A. Atterberg, in 1911 presented a uniform method of classifying the behaviour of cohesive soils by relating the water contents at which certain physical properties in the soil changed. These critical water contents became known as the Atterberg Limits which are used to distinguish between liquid, plastic and solid states or behaviours in cohesive soils. As water is removed from a slurry of clay and water, its ability to flow as a liquid (on its own) is gradually reduced until it no longer exhibits this behaviour.

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This is the water content that defines the limit of the liquid state; otherwise know as the Liquid Limit. The clay sample can still be caused to flow, however, by being molded and reshaped by hand. This behaviour is called plasticity. If water continues to be removed, the sample will gradually lose the ability to hold together and be remolded (or its plasticity). This is the water content that defines the limit of the plastic state; otherwise know as the Plastic Limit. The clay sample will now behave as a semi-solid. As more water is removed from the sample its volume will decrease, however it will continue to hold its shape. The drying sample will eventually start to crack which defines the limit of semi-solid behaviour. This is the water content that defines the limit of the semi-solid state; otherwise know as the Shrinkage Limit. The chart below illustrates the relationships among these states and limits. Physical State

Soil-moisture Scale wL IP

water content, w

wP

{

wS

Liquid Limit, LL

Plastic Limit, PL

}

Liquid Plasticity Index

Shrinkage Limit, SL Air-dry 0

Oven-dry

}

Hygroscopic moisture

Consistency Very Soft Soft

Plastic

Semi-Solid

Solid

Stiff Very Stiff Extremely Stiff Hard

FIGURE 1.01 - Atterberg Limits and Corresponding States of Soil Consistency

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The water content of a sample of soil is determined by measuring the mass of the soil with moisture, M, then oven drying the sample for 18 hours at a temperature of 110C and measuring the mass of the dried soil, Ms. The mass of water present in the original moist sample, Mw is M – Ms. The water content, w of the original sample is then determined by dividing the mass of water by the dry mass of the soil:

Mw w  Ms Atterberg defined the Plasticity Index, PI as the range of water content over which the soil exhibits plastic behaviour. PI is calculated as follows:

PI  LL  PL The Plasticity Index, PI and Liquid Limit, LL are used by both the AASHTO and Unified Soil Classification systems to classify the fine material in a soil sample as being of high or low plasticity and whether its predominantly silt or clay. These two classification systems are the most widely used in North America. Soil classifications are useful in specifying the potential uses for soil at a site as an engineering material in construction and also to determine the treatment and/or design requirements of subgrade soil for the foundations of structures such as bridges, buildings and pavements. Gradation (Grain Size Distribution) The other useful property of coarse-grained soils in particular is the distribution of grain or particle sizes within the soil or stock aggregate. This is determined by subjecting a representative sample of the material to a sieve analysis using a standard set of sieve sizes, determining the cumulative percent of the sample that passes respectively smaller sieve openings and then plotting the percent passing versus grain size on a logarithmic scale. Grading of granular material will generally vary depending on the processes (geologic or engineering) that produced it. Samples of aggregate are generally categorized as dense graded, gap-graded, uniformly graded, well graded or open graded as illustrated in Figure 1.02. The Unified Soil Classification system also designates coarse-grained samples that have 12% or fewer fines as being either well graded or poorly graded. This is determined by producing a grading curve for the sample, determining the values of the critical particle sizes, D60, D30 and D10 that correspond to 60, 30 and 10 %.

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FIGURE 1.02 – Grading Designations passing, calculating the values of the coefficients of uniformity, Cu and curvature, Cz and finally comparing these values to critical ranges to determine whether the material is well graded or poorly graded. These coefficients are calculated as shown below:

D60 Cu  D10

(D 30 ) 2 Cz  D 60 D 10

The values of D60, D30 and D10 can be scaled from the gradation graph or they can be calculated from the gradation data using semi-linear interpolation with the following equation:

 D high  D P i  D low    D low 

 P i  P low     P high  P low 

The term D10 is also known as the effective grain size and is related to soil classification as well as the soil’s permeability (the speed that water can seep through the soil) and its capillarity (the rise of water through a soil above the water table). Another important parameter, especially for evaluating asphalt concrete paving mixtures, is the Nominal Maximum Particle Size which is one sieve size larger than the first sieve to retain more than a cumulative 10% of a sample by weight.

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To illustrate these concepts, consider the following example of sieve results for a sample of fine aggregate (concrete sand). TABLE 1.02 – Example Sieve Analysis Data for Fine Aggregate SIEVE Designation Size (mm) 3/4” 19.1 1/2” 12.7 3/8” 9.52 No. 4 4.75 No. 8 2.38 No. 16 1.19 No. 30 0.59 No. 50 0.297 No. 100 0.149 No. 200 0.074

% PASSING 100.0 100.0 100.0 98.6 87.9 71.1 44.5 18.1 7.2 5.3

The gradation curve for the sample data above is presented in Figure 1.03 below.

FIGURE 1.03 – Example Gradation Chart The critical grain sizes, D10, D30 and D60 can be determined graphically as illustrated in Figure 1.03 or they can be calculated as illustrated below:

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 0.297  D 10  0.149   0.149 

 10  7.2     18.1  7.2 

 0.59  D 30  0.297   0.297 

 1.19  D 60  0.59   0.59  Cu 

 0.1779mm

 30  18.1     44.5  18.1 

 60  44.5     71.1  44.5 

, D30  0.4047mm

, D60  0.8880mm

D60 0.8880   4.992 D10 0.1779

0.4047  (D 30 ) 2 Cz    1.037 D 60 D 10 0.8880 0.1779  2

In cases where the percent passing the No. 200 sieve is greater than 10% the No. 100 sieve would be used for the Dhigh and Phigh and the No. 200 sieve used for Dlow and Plow. This uses the interpolation equation to extrapolate a D10 value. For example, if the P200 = 11.2% and P100 = 15.3%, then D10 is extrapolated as follows:

 0.149  D 10  0.074   0.074 

 10  11.2     15.3  11.2 

 0.0603mm

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The final evaluation of grading is based on the Unified Soil Classification system’s grading criteria as presented in Table 1.03 below. In order to be classed as well graded, both criteria must be met. For this example the curvature criterion is met but the Cu value is less than the minimum required value of 6 for sands. Therefore this material is classified as poorly graded. TABLE 1.03 – Unified Soil Classification Grading Criteria CRITERION Uniformity Curvature

MATERIAL GRAVEL SAND Cu > 4 Cu > 6 1  Cz  3 1  Cz  3

The effective grain size for this material is 0.1779 mm and the Nominal Maximum Particle Size is a No. 4 or 4.76 mm as less than 10% of the sample is retained on the No. 4 but more than 10% is retained on the No. 8 sieve.

Soil Classification Systems The driving force behind the development of the soil classification systems used today has been to provide the engineering and construction professions with meaningful ways of assessing a soil's behavioural characteristics when it is required for use as a building material or structural foundation or other engineering use. These behavioural characteristics include but are not limited to the soil's handling characteristics, shear strength, volume change and permeability. In North America, three systems are commonly used to classify soils for engineering applications. These are the AASHTO (American Association of State Highway and Transportation Officials), USC (Unified Soil Classification) and ASTM (American Society for Testing and Materials) systems. The AASHTO and USC classification systems require sampling and lab analyses in order to quantify the plasticity and grain size characteristics upon which these classifications are based. AASHTO SYSTEM The AASHTO System was originally developed by the U.S. Bureau of Public Roads in 1928 for use in highway design. It divided all soils into eight groups with symbols A-1 through A8, with A-1 representing the most desirable soil to use for a highway subgrade (well-graded gravel and sand with slight cohesion), A-8 representing the least desirable soil (organic) and the progression between (A-2, A-3, … A-7) representing gradually decreasingly desirable soils. The Highway Research Board tightened up the definitions of these classifications in 1945 by specifying numerical gradation and plasticity criteria. The American Association of State Highway Officials put the finishing touches on the system in 1966 (Table 1.04), adding subclasses to A-2 and A-7 and Group Indices for soils containing significant fine material.

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TABLE 1.04 – AASHTO Soil Classification System General Classification

Granular Materials 35% or less passing No. 200 A-1

Group Classification Sieve Analysis % Passing

No. 10 No. 40

Characteristics of fraction passing No. 40 Sieve

No. 200

A-1-a

A-2

A-1-b

50 max 30 max 15 max

50 max 25 max

Liquid Limit

-

Plasticity Index

6 max

Usual Constituents General Rating for Subgrade

Silt-clay materials More than 35% passing No. 200

A-3

A-5

A-6

A-7 A-7-5, A-7-6

A-2-4

A-2-5

A-2-6

A-2-7

-

-

-

-

-

-

-

-

-

51 min

-

-

-

-

-

-

-

-

40 max

41 min

10 max

nonplastic

Crushed stone gravel and sand

A-4

Fine sand

35 max 40 max

41 min

10 max

36 min

40 max

41 min

40 max

11 min

Silty or clayey gravel and sand

Excellent to good

41 min

10 max

11 min

Silty soils

Clayey soils

Fair to poor

For A - 7 soils : subclassification is A - 7 - 5 if PI  LL - 30 and A - 7 - 6 if PI  LL - 30 For A-4, A-5, A-6 and A-7 soils, report Group Index, GI: GI = (F – 35)[0.2 + 0.005(LL-40)] + 0.01(F – 15)(PI – 10) For A-2-6 and A-2-7 soils, report Partial Group Index, PGI: PGI = 0.01(F – 15)(PI – 10) where F = percent passing the No. 200 sieve, LL = Liquid Limit and PI = Plasticity Index The GI and PGI are always reported to the nearest whole number, negative values are reported as 0; the higher the Group Index, the less suitable the soil. For example, a clay soil with a group index of 25 might be classified as A-7-6(25). The classification chart is read from left to right, eliminating options until the first match with the criteria is met.

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Example 1.1 Classify the following five soils by the AASHTO system.

a) b) c) d) e)

No 10 76 100 45 79 100

% Passing No. 40 42 85 18 62 68

No. 200 28 62 3 18 45

LL 48 53 22 36

PL 18 32 19 18

Solution: a) With 28% passing the No. 200 sieve (P200 = 28%), the soil is granular in nature (P200 < 35%). Starting with A-1-a: P10 =76% > 50%, therefore it’s NOT A-1-a. Next, try A-1-b: P40 = 42% < 50% (O.K.), but P200 = 28% > 25%, hence A-1-b is out. Then, A-3: P40 = 42% < 51%, thus eliminating A-3. The only gradation criterion for A-2 has already been met (P200 = 28% < 35%), therefore the plasticity criteria must be checked. The Plasticity Index, PI = LL – PL = 48% – 18% = 30%. Starting with A-2-4: LL = 48% > 40%, eliminating A-2-4 (and A-2-6). Then A-2-5: LL = 48% > 41% (O.K.), but PI = 30% > 10%, A-2-5 is out. Then A-2-7: LL = 48% > 41% (O.K.) and PI = 30% > 11%, A-2-7 is a match. For A-2-7 soils, calculate a Partial Group Index, PGI as follows: F = 28%, PI = 30%, therefore PGI = 0.01(28-15)(30-10) = 2.6  3 The soil therefore has an AASHTO classification code of A-2-7(3). b) With P200 = 62% > 35%, this soil is a clay-silt. The PI = 53% – 32% = 21% which eliminates A-4 and A-5 (maximum PI is 10%). A-6 is eliminated since LL = 53% > 40%. Therefore it falls under the A-7 category, (LL = 53% > 41% and PI = 21% > 11%).

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To determine the subclass, LL – 30% = 53% – 30% = 23% which is > the PI = 21%. This means the soil has a classification code of A-7-5. The Group Index, GI is found as follows: GI = (62 - 35)[0.2 + 0.005(53 - 40)] + 0.01(62 - 15)(21 - 10) = 12.325  12 The soil therefore has an AASHTO classification code of A-7-5(12). c) With P200 = 3% < 35%, this is a granular soil. Since there were no LL or PL values given, the soil must be non-plastic. Starting with A-1-a: P10 = 45% < 50% (O.K.), P40 = 18% < 30% (O.K.), and P200 = 3% < 15% (O.K.), therefore the soil is A-1-a. d) With P200 = 18% < 35%, this is a granular soil. The PI is 22% – 19% = 3%. Starting with A-1-a: P10 = 79% > 50%, therefore it’s NOT A-1-a. Next, try A-1-b: P40 = 62% > 50%, hence A-1-b is out. Then, A-3: PI = 3% so the soil has plasticity thus eliminating A-3. The only gradation criterion for A-2 has already been met (P200 = 18% < 35%), therefore the plasticity criteria must be checked. Starting with A-2-4: LL = 22% < 40% and PI = 3% < 10% Thus the soil is classified as A-2-4. e) With P200 = 45% > 35%, this soil is a clay-silt. The PI = 36% – 18% = 18% which eliminates A-4 and A-5 (maximum PI is 10%). Since LL = 36% < 40%, the soil meets the criteria for an A-6 classification. Therefore it falls under the A-7 category, (LL = 53% > 41% and PI = 21% > 11%). The Group Index, GI is found as follows: GI = (45 - 35)[0.2 + 0.005(36 - 40)] + 0.01(45 - 15)(18 - 10) = 4.2  4 The soil therefore has an AASHTO classification code of A-6(4).

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UNIFIED SOIL CLASSIFICATION (USC) SYSTEM The Unified System is the most widely used soil classification system by North American geotechnical specialists. It was originally developed by Professor Arthur Casagrande for the Corps of Engineers of the U.S. Army. It was develop to assist in the design and construction of airfields during the Second World War. Soils are first divided into coarse-grained and fine-grained categories. Two types of coarsegrained soils are specified: sand and gravel. Three types of fine-grained soils are specified: silt, clay and organic soils. If the course-grained soils have less than 5% fines then the grading of the particle sizes is evaluated using the criteria presented in Table 1.03. If the course-grained soils have more than 12% fines then the fine material is classified as silty or clayey based on the plasticity criteria used for fine-grained soils. Fine-grained soils are further distinguished as having High or Low plasticity. Table 1.05 below shows the primary and secondary symbols used in the USC system. TABLE 1.05 – Unified Soils Classification System Symbology Grain

Coarse

Fine Peat

Primary Symbol Interpretation G Gravel S Sand

C M O PT

Secondary Symbol Interpretation W Well Graded P Poorly Graded C Clayey M Silty H High Plasticity L Low Plasticity

Clay Silt Organic fibrous to amorphous decomposing vegetable tissue

The primary symbols are simply the first letter of the material’s name except for silt. Since we can’t represent both sand and silt with the same letter, M is used for silt. This comes from the Swedish word for silt which is “mo”. Material coarser than 75 mm in diameter that is contained within a sample of soil have the words “with cobbles or boulders or both” added to the classification code. These sizes are discussed in the ASTM Classification System to follow. The primary and secondary symbols that make up the USC classification code shown in Table 1.05 are determined by performing tests on samples of the soil. These tests include the liquid and plastic limits and grain size analysis. For soils suspected of being Organic, liquid limit tests are performed on samples that have been oven dried in addition to those that are undried or in their “natural” moisture state. Table 1.06 shows the logic applied to the lab tests mentioned above to arrive at a USC classification code.

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TABLE 1.06a – USC Classification Chart for Coarse-Grained Soils (P200 < 50%) Gravel or Sand?

Fines Range

Cu ≥4 &1≤Cz≤3

P200 < 5%

100  P4  50% 100  P200

Fines (Figure 1.4Plasticity Chart)

Grading Cu