ENGINEERING CLASSIFICATION OF SOILS

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Gabriella Copeland
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ENGINEERING CLASSIFICATION OF SOILS An engineer often considers a soil as any cemented or weakly cemented accumulation of

The void space separating the particles may contain air and/or water.

products of in-situ weathering (no transport) ii) Transported soils- subdivided according to the agent of transportation into:

histories of the soil have a significant influence on the physical and engineering properties.

1 ENGINEERING CLASSIFICATION OF SOILS An engineer often considers a soil as any cemented or weakly cemented accumulation of mineral or rock particles formed by the weathering of rocks. The void space separating the particles may contain air and/or water. Sometimes soils are crudely described according to the geological processes active in their formation: i) Residual soils products of in-situ weathering (no transport) ii) Transported soils- subdivided according to the agent of transportation into: - Gravity - Wind - Water - Glaciers The range of particle sizes, the varying degree of rounding and sorting and the loading histories of the soil have a significant influence on the physical and engineering properties. Soils may be formed by physical and/or chemical weathering and biological processes. Erosive processes lead generally to coarser soils although glaciers may produce rock floor. Chemical weathering produces changes in mineral form of the parent rock and the formation of fine-grained clay soils.

2 The predominant soil types are shown in the table below, which also indicates what is referred to as their firmness or strength: Soil Type Term Field test Sands, gravels Loose Can be excavated with a spade; 50mm wooden peg can be easily driven Dense Requires a pick for excavation; 50mm wooden peg is hard to drive Slightly cemented Visual examination; pick removes soil in lumps which can be abraded Silts Soft or loose Easily moulded or crushed in the fingers. Firm or dense Can be moulded or crushed by strong pressure in the fingers Clays Very soft Exudes between the fingers when squeezed in the hand Soft Moulded by light finger pressure Firm Can be moulded by strong finger pressure Stiff Cannot be indented by the thumbnail Very stiff Can be indented by the thumbnail Organic, Peats Firm Fibres already compressed together Spongy Very compressible and open structure Plastic Can be moulded in the hand and smears the fingers Table 1. Soil types and field estimation of strength CLAY MINERALS Most clay minerals forming clay soils are of plate-like form having a high specific surface (surface area: mass ratio). Surface forces hence play an important role in the engineering behavior of clay soils. Basic clay mineral structure. Clay mineral structures are shown in Figures 1 to 3. The basic structural units consist of a silica tetrahedron and an alumina octahedron. (Silicon and Aluminium may be partially replaced these units by other elements). Basic units combine to produce characteristic sheet structures and hence the plate-like form. The most common clay minerals are kaolinite, illite and montmorillonite. They are formed by varied stacking arrangements of the tetrahedra and octahedra with different types of bonding between combined sheets.

3 Kaolinite Illite Montmorillonite Single sheet of silica tetrahedra (T) combined with single sheet of alumina octahedra (O). Sheets of alumina octahedrons between and combined with two silica tetrahedrons. (TOT:TOT) Same basic structure as illite. Very limited isomorphous substitution TO:TO sheets held fairly tightly together by hydrogen bonding (1 particle = 100+ stacks). Absorb little water. Low swelling and shrinkage potential, Table 2. Properties of Clay minerals Substitution of Al by Mg and Iron in Octahedral sheet and partial substitution of Silicon by Al in tetrahedral sheet. Combined TOT:TOT sheets held together by fairly weak bonding due to potassium ions. Absorb more water than kaolinites and have higher swelling/shrinkage potential. Partial substitution of Al by Mg in the octahedral sheet. Water molecules and (exchangeable) cations other than potassium present in space between combined TOT sheets. Very weak bond between combined TOT sheets due to these ions. Extremely high water absorption between TOT sheets, swelling and shrinkage potential. The interparticle forces between clay mineral particles influences the structural form they assume, figure 3. Dispersed net repulsion particles assume face-to-face orientation Flocculated net attraction particles tend to be edge-to-face and edge-to-edge. In natural clays - may get aggregations combining to form larger assemblages such as cardhouse/bookhouse, or turbostratic structures. Clay assemblages may form a matrix between larger particles e.g. Silt grains.

4 Mineral Structure name Particle Size ranges Between layers Approx. size (μm) Specific surface( m 2 /g) Approx. exchange capacity (me/100g) Kaolinite G G H-bond linkage l = t= Halloysite G G H 2 O tubular l = 0.5 t= Illite G K G K K + linkage l = t= Montmorillonite G G Weak crosslinkage between Mg/Al ions l = t= Vermiculite Mg Mg 2+ linkage l = t= Figure 1. Clay Mineral Structures

5 a. b. c. Figure 2. a). Kaolinite, b). Illite and c.) Montmorillonite a. b. c. Figure 3. Clay structure a) Undisturbed salt water deposit, b.) Undisturbed fresh water deposit and c.) Remoulded. SOIL TYPES Soils may be crudely classified according to grain size, Table 1, in to: i) fine grained (predominantly clay and silt size) or ii) coarse grained (sand and gravel sizes predominant or in shear strength terms i) cohesive (eg. clays, clay silt mixtures, organics) (c-soils or φ = 0 soils) ii) cohesionless (eg. sands and gravels) (C=0 or frictional soils) iii) Mixtures eg sandy clays etc (c-φ soils)

6 Size range Grain Size Term Grain Size BS/European. mm ASTM mm (in) Boulders > (12) and above F.P.C. Larger than basketball Cobbles >60< (3)-350 (12) Grapefruit Gravel Coarse >20<60 19 (0.75) 75 (3) Orange or Lemon Medium >6<20 Fine >2 < (3/16) - 19 (0.75) Sand Coarse > 0.6 <2 2.0 (3/32) (3/16) Medium > 0.2 < (0.016) (3/32) Fine > 0.06 < (#200-#40) Grape or Pea Rock Salt Sugar Powdered Sugar Silt Coarse >0.02 < Cannot be seen with naked eye at distance > 200mm Medium >0.006 <0.02 Fine >0.002 <0.006 Clay* < and below * In ASTM Silts and clays referred to as FINES further differentiation based on Index tests. Table 2. Classification of soils according to grain size. All clay size particles are not necessarily clay minerals think of an example? A clay soil may consist of a mixture of clay and silt size particles. Clays and silts possess varying properties of cohesion and plasticity Sands and gravel are cohesionless and show no plasticity unless mixed with? Most soils consist of a mixture of the various size ranges which must be determined using particle size analysis Clay size particles have major influence on engineering properties - how?

PARTICLE SIZE ANALYSIS A dried sample of soil is shaken, usually after washing, through a vertical stack of sieves, Figure 4, coarse at the top to fine at the bottom, and the mass retained on each

On this chart the percentage passing/smaller is plotted on the vertical axis (normal scale) and the particle size in mm on the horizontal axis (log scale).

8 PARTICLE SIZE ANALYSIS A dried sample of soil is shaken, usually after washing, through a vertical stack of sieves, Figure 4, coarse at the top to fine at the bottom, and the mass retained on each sieve recorded. A semi-log particle size distribution or grading curve is shown in Figure 5. On this chart the percentage passing/smaller is plotted on the vertical axis (normal scale) and the particle size in mm on the horizontal axis (log scale). The soil is thus divided into percentage values of each size range. (clay, silt, sand, gravel etc.). The flatter the curve the greater the range of particle sizes, the steeper the curve the smaller the size range. Well Graded Coarse soil where there is no excess of particles in any size range and where no intermediate sizes are lacking, Figure 6 Poorly Graded Coarse soil where there are a high proportion of particles having sizes within a narrow range (uniformly graded) or where particles of both large and small sizes exist but there is a much lower proportion of intermediate size particles (gap-graded), Figure 6. Figure 4. Particle size analysis of coarse grained soils using sieves.

9 PARTICLE SIZE DISTRIBUTION CURVE Percentage finer Clay Silty Clay F M Silt SandySilt Very Sandy Silt C F M Sand Well Graded Sand C Wel l Graded Gravel mm F M Gravel C Figure 5. Particle Size Distribution Curves for various soils Figure 6. Particle Size Distribution Curves for well graded and poorly graded (uniform and gap) soil,

10 The particle sizes at which specified percentages of the soil are smaller (pass through) may be used to further characterize the soil, Figure 7 Effective size of a soil distribution, D 10 D 10 Maximum particle size of the smallest 10% of the sample. (10% passes,90% retained) Also used are: D 15 Maximum particle size of the smallest 15% of the sample. (15% passes) D 30 - maximum particle size of the smallest 30% of the sample. (30% passes) D 60 Maximum particle size of the smallest 60% of the sample. (60% passes) D 85 Maximum particle size of the smallest 85% of the sample. (85% passes) The Coefficients of uniformity, C U and Curvature, C Z reflect the general slope/shape of the grading curve where: 2 D60 D 30 C U = C Z = D D D % Percentage finer 60 Grading Curve d 10 d 30 d 60 Particle Size (mm) Figure 7. Particle Size Grading Coefficients

$Particle size analysis of silt and clay size fractions: (Hydrometer and Pipette Methods), Figure 8. The smallest sieve size opening generally used is 0.$

11 Particle size analysis of silt and clay size fractions: (Hydrometer and Pipette Methods), Figure 8. The smallest sieve size opening generally used is 0.063mm below this the distribution of silt and clay sizes is determined using sedimentation techniques (hydrometer or pipette) These methods are based on Stokes law which states that the settling velocity at which suspended spherical particles in solution is proportional to the square of the particle diameter. A sample of the fine fraction of the soil is liquefied by adding water and then allowed to settle. The specific gravity of the suspension will change with time as the particles settle (largest first). This change in specific gravity is measured using the hydrometer and correlated with the grain size present. In the pipette test samples of a suspension are taken from a fixed elevation in a measuring cylinder at times, t, and the percentage of various grain sizes determined (It should be noted that Stokes Law does not apply to particles < mm).

12 Figure 8. Particle size analysis of fine soils ATTERBERG LIMITS AND CONSISTENCY INDICES Consistency of Clays Consistency refers to the texture and firmness of a soil and is often directly related to strength. Soils may be termed Soft, Medium Stiff (Medium firm), Stiff (or firm) and Hard. With clays, the shear strength is often discussed in terms of cohesion and unconfined compressive strength. (UCS). To determine the UCS of a clay a cylindrical sample is subjected to an axial load until it fails in shear. This test can be carried out in the laboratory, Figure 9. In the field the UCS may be determined using either a pocket penetrometer or vaneshear test. (figure 10). Table shows values of strength for the various consistency terms. An important relationship which will be discussed later in the course is:

13 Unconfined Compressive Strength = 2 X Cohesion or Shear Strength of a clay soil. Figure 9. a. Laboratory Uniaxial Compression Test on Soil. a. b. Fig 10 a, Pocket Penetrometer and b. Hand shear vane tester

14 Sensitivity The strength of a clay soil is related to its structure. If the structure is altered (changes in particle arrangement (remoulding/reworking) or chemical changes) the altered strength of the clay is less than the original strength. This leads us to a type of behavior known as Sensitivity which is very important in Canada and Scandinavia. Sensitivity may be defined as the ratio of: S = UCS of undisturbed clay (at identical water contents) UCS of remoulded clay a. b. Figure 11. a. Solid to liquid State and b. Quickclay slide (Lemieux) Sensitivity in soils may be classified according to: Sensitivity Terminology 2-4 Most clays 4 8 Sensitive 8-16 Extrasensitive > 16 Quick clays

15 Consistency of remoulded soils The consistency of a soil in the remoulded soil varies in proportion to the water content. At higher water contents the soil-water mixture behaves a liquid, at lower water contents it behaves plastically and at still less water contents it behaves as a semi solid and solid. An important component of soil classification is to determine the water contents at which these phase/behavior changes occur. With reference to Figure 12 the following may be defined: Liquid Limit, (LL or w L ) is the water content at the division between liquid and plastic state.(water content, w = mass of water/mass of dry soil X 100% Plastic Limit, (PL or w P ) is the water content at the division between plastic and semi-solid states. Shrinkage Limit (SL or w S ) is the water content at the division between the semi-solid and solid states. Plasticity Index (PI) Total Soil Volume V d V a V s Natural water content Solid Semiplastic Plastic solid w s w p Liquid w L % Water Content Fig 12.Consistency indices (Atterberg Limits) Plasticity Index (PI) = w L -w P Liquidity Index, I L = w - w P = w - w p w L -w P PI If w > w L the I L > 1; remoulding turns soil to slurry. (Cu=15-50kN/m 2 ) If w <w P then I L < 0; cannot remould soil as it s outside plastic range (Cu=50-250kN/m 2 ). Most clays have I L between 0 and 1. At water contents, above the Shrinkage Limit it can be seen from Figure 12 that the total volume of the soil increases with increase in water content.

16 Although the absolute values of these indices have very little direct use in design they are used in classification and correlations between the limits and engineering properties are very useful in assessing the potential engineering behavior and use of soils. The tests used for determining these limits (sometimes called Atterberg limits) are described in what are known as ASTM standards in North America. It should be recognized that the details of such tests may vary worldwide and this will be illustrated with reference to the Liquid Limit test. LIQUID LIMIT TESTS a. Using the Casagrande Device The Casagrande device is used in North America for determining the liquid limit of soils. The apparatus is shown on Figures 13 and 14. The soil is air dried and sieved through a no 40 sieve. This material is then mixed with water to form a remoulded soil paste. The paste is placed in the standard dimension Casagrande cup and level of. A groove is formed across the soil as shown in Figure 13 using a special grooving tool. The handled to the Casagrande cup is rotated at a set speed and the number of bounces of the cup on the hard base counted. The rotation is continued until the groove in the soil flows and closes over a specified length. The number of blows of the cup is noted and a sample of the soil taken to determine the moisture content. The test is repeated at increasing water contents by adding water, each time noting the number of blows required to close the groove. A graph of the log of the number of blows against water content is drawn and the moisture content requiring 25 blows to close the groove is determined the Liquid Limit of the soil. This test is subject to operator error and to increase repeatability motorized versions are available. Correlations between LL values obtained from the cone penetration test are good except at higher water contents. The Casagrande test has fallen into disuse in much of Europe having been replaced by the Cone Penetrometer. b. Using the Cone Penetration Device Air-dry and mixing the soil Sieve at least 200g of the soil through a 425μm sieve and place on a glass plate. Mix soil with distilled water into a paste Fill a 55mm diameter and 40mm deep metal cup with the paste and smooth of the surface Place cone of mass 80g level with and at center of soil surface, Figure 15. Release cone so it penetrates into soil and record amount of penetration over 5 seconds Repeat test adding a little more wet soil, until difference between two results is less than 0.5mm, Note average penetration and determine moisture content of soil. Repeat 4+ times with increasing water contents. (use enough water for penetrations in 5 secs to lie in range 15-25mm) Plot penetration against moisture content and find moisture content for 20mm penetration that is the Liquid Limit of the soil. (Figure 15) A One Point version of the Cone Penetrometer test is sometimes used. The penetration and moisture content are determined as above. The moisture content for 20mm penetration (the LL) is then found using a correction factor.

18 Figure 13. Casagrande Liquid Limit Test 40 Sieve Analysis Wc 1 #40 Log. No.Blows 3 Part of soil used for hydrometer analysis Casagrande Cup 2 Repeat test several times at different water contents Figure 14. Casagrande Test Results

Cone Penetration (mm) 550mm 30 o Cone 24 20 14 LL = 55% 50 55 60 Moisture Content (%) Figure 15.

PLASTIC LIMIT TEST Approximately 20g are prepared as for the Liquid Limit test.

ball. Roll the ball as shown in Figure 16 to form a soil thread.

At this point determine the water content of a section of the thread this value is the Plastic Limit

19 Cone Penetration (mm) 550mm 30 o Cone LL = 55% Moisture Content (%) Figure 15. Liquid Limit using Cone Penetrometer. PLASTIC LIMIT TEST Approximately 20g are prepared as for the Liquid Limit test. Mix the soil on a glass plate with just sufficient water to make it plastic enough to roll into a ball. Roll the ball as shown in Figure 16 to form a soil thread. The thread is rolled until it just starts to crumble at a thread diameter of 3mm. At this point determine the water content of a section of the thread this value is the Plastic Limit of the soil. Repeat the test at least once. It is also possible to determine the Plastic Limit using a Cone penetrometer when the water content giving a penetration of 2mm is taken as the Plastic Limit. Figure 16. Determination of Plastic Limit,

SHRINKAGE LIMIT AND LINEAR SHRINKAGE For soils with very small clay content the liquid and plastic limit tests may not produce reliable results.

13 X LS 150g of soil is prepared and made into a paste in a similar manner as for the LL and PL tests. The paste is then placed into a brass mould, Figure 17 and the surface leveled off.

20 SHRINKAGE LIMIT AND LINEAR SHRINKAGE For soils with very small clay content the liquid and plastic limit tests may not produce reliable results. An estimate of the plasticity index can then be found by measuring the linear shrinkage and using: PL = 2.13 X LS 150g of soil is prepared and made into a paste in a similar manner as for the LL and PL tests. The paste is then placed into a brass mould, Figure 17 and the surface leveled off. The soil is then air-dried at 60-65oC until it has shrunk clear of the mould and then placed in an oven and drying completed at oC. After cooling the sample length is measured and the LINEAR SHRINKAGE found from: % Linear Shrinkage,LS = {1- Length after drying/initial length} X 100. The Shrinkage Limit may be determined using a 76mm long and 38mm diameter soil core. The cylindrical core is slow dried and frequent measurements of mass and volume taken. The volume measurements may be made by immersing the soil in a known volume of mercury in a mercury displacement vessel. A volume/water content graph can be constructed and the Shrinkage Limit as shown in Figure 12 determined. This test is not very common as the SL value has little direct use in soil classification. Figure 17. Linear Shrinkage apparatus.

21 UNIFIED SOIL CLASSIFICATION Various engineering classifications of soils have been adopted throughout the world. The most common classification, the Unified Soil Classification is presented in Figure 18. Working inwards from the LHS: Coarse grained soils are distinguished from fine grained soils by the percentage (> or < 50% respectively) retained on the No. 200 sieve (0.074mm mesh) o Coarse grained soils are further subdivided into gravels and sands based on the percentage (> or < 50% respectively) retained on the No. 4 sieve (4.75mm (3/16 ) mesh) Gravels are sub-divided into clean (Well graded, GW, or Poorly Graded, GP) and with fines (silty (GM) or clayey (GC)) Sands are sub-divided into clean (Well graded, SW, or Poorly Graded, SP) and with fines (silty (SM) or clayey (SC)) (The grading curve coefficients, Cu and Cz are used to distinguish between well and poorly graded coarse soils. Fine grained soils are subdivided according to their Liquid Limits into low plasticity clays, CL, silts ML and organic clays/silts OL (LL < 50%) and high plasticity clays, CH, silts, MH and organic clays/silts, OH (LL > 50%) Highly organics soils are recognized as Peat (Pt) Coarse-grained soils Fine-grained soils Organic soils Pt = PEAT Primary letter G = GRAVEL S = SAND F = FINES M = SILT C = CLAY O = organic Secondary letter W = well graded P = poorly graded Pu = uniform Pg = gap graded L = low plasticity (undifferentiated) I = intermediate plasticity H = high plasticity V = very high plasticity E = extremely high plasticity

22 Figure 18. UNIFIED SOIL CLASSIFICATION SYSTEM

23 FIELD IDENTIFICATION OF SOILS Figure 19 shows commonly used criteria in field identification of soils including: Grain size Crushing strength Dilatency Consistency Forest Practice Road Guidebook (Soil Grain Size) Forest Practice Road Guidebook (Soil Description)

24 Forest Practice Road Guidebook (Soil Density Non Cohesive Soils) Forest Practice Road Guidebook (Field Consistency Test for Cohesive Soils)

25 ENGINEERING USE OF SOILS The engineering classification of soils can be used as a preliminary estimate of the potential engineering use of soils. Will the soils be freely draining or impermeable, will they be highly compressible etc. These factors are obviously important in choosing a suitable material for the core material of an earth dam or for the foundation of a building. Figure 20 shows a chart for assessing the engineering use of soils. Some soils are liable to collapse, others to liquefaction. Some clays containing high montmorillonite contents are particularly susceptible to shrinkage and swelling with seasonal changes in water content.

Classification of Soils

Classification of Soils Soils - What are they? Particulate materials - Sedimentary origins (usually) - Residual Wide range of particle sizes - larger particles: quartz, feldspar - very small particles: