GEOTECHNICAL PROPERTIES (CE1203) Soil Classification Ms Ikmalzatul

FIELD DESCRIPTION OF SOILS In general it would be true to say that the description of soils in the field is a task that one needs to have had a fair amount of experience at before one becomes efficient. However no mater at what level of experience the general aims behind the field description of soils remain the same. Namely :- a) to distinguish the main soil types b) to assess the strength and structure of the soil

SOIL TYPES Grain size - as a first step the soil should be split into course grained non-cohesive, fine grained cohesive and organic soils. Further subdivision into gravel, sand, silt, etc is according to the grain size and while a field estimation of this is possible it is most easily determined by sieving a sample in the laboratory. Plasticity - the fine grained cohesive soils can be split into silts and clays according to their plasticity. The simplest way to determine a soils plasticity in the field is by touch. Clays are sticky when wet and do not powder easily when dry, silts exhibit dilatancy (this is the receding of surface water when pressure is applied) and are easily powdered when dry

FINE SAND SILT CLAY SUMMARY TABLE Individual particles visible Some particles visible No particles visible exhibits dilatancy no dilatancy easy to crumble when dry easy to powder when dry very hard to powder when dry feels gritty feels “rough” feels smooth no plasticity some plasticity plastic SUMMARY TABLE

FIELD IDENTIFICATION AND DESCRIPTION OF SOIL Most soils are a mixture of the various “main” types of soil - sandy CLAY, GRAVEL with clay pockets, etc - the Soil Mechanics Ltd chart gives guidance on how they should best be described.

PARTICLE SIZE ANALYSIS BS 5930 Most systems of soil classification depend to some extent on the distribution of the various sized particles within the soil sample. For course grained soils (gravels and sands) this distribution can be determined by sieving the sample and for the fine grained soils a method of measuring the rate of settlement of the particles in water is employed (sedimentation techniques).

COURSE ANALYSIS ( Sieve Test ) For course grained soils either the wet or dry sieving method may be used. BS 1377 test 7a Standard method by wet sieving test 7b Subsidiary method by dry sieving Note :- Test method 7b shall not be used unless it has been shown that for the type of material under test it gives the same results as the method of analysis by wet sieving. In cases of doubt method 7b shall not be used. With both methods a sample of oven dried soil is passed through a batch of sieves with the dry weight of soil retained on each individual sieve recorded. For the wet sieving method this will entail oven drying the soil retained on each sieve before weighing. The dry weight of soil on each sieve is then converted to a percentage of the total sample weight and the percentage passing each sieve is then determined. The results (percentage passing and sieve size) are then plotted on a standard semi logarithmic particle size distribution (PSD) graph.

Soil passing the 63 m sieve is just recorded as silt and clay sized.

With wet sieving the soil is carefully washed through the sieves, the larger particles being brushed, with a wire or other stiff brush, to remove any fine particles which have adhered to them. If a mechanical sieve shaker is used the sieves are nested together with a receiving pan at the bottom and shaken for 2 minutes per sieve for a minimum of 10 minutes. If shaking by hand each sieve in turn, starting with the largest is placed on a receiving pan and the soil sieved using a shaking/rolling action. Large particles may be poked through the sieve by hand if they fit but must not be forced through.

Whichever method of shaking/sieving is used care must be taken not to lose any of the material when sieving. All material must be removed from the sieve for weighing using a sieve brush as necessary. If any sieve becomes overloaded with material the sample size must be reduced by riffling or the sample passed through the sieve in parts. As a general guide the mass of soil placed on the 20mm sieve should be approximately 20 kg (this will need to be reduced on finer sieves)

BRITISH STANDARD TEST SIEVE SIZES 75mm, 63mm, 50mm, 37.5mm, 28mm, 20mm, 14mm, 10mm, 6.3mm, 5.0mm, 3.35mm, 2.0mm, 1.18mm, 600m, 425m, 300m, 212m, 150m and 63m

FINE ANALYSIS (Sedimentation Test) Material smaller than 63m is too small for sieving, it is therefore separated out and graded by sedimentation. BS 1377 test 7c Standard method for fine grained soils (pipette method) test 7d Subsidiary method for fine grained soils (hydrometer method) The theory of fine grain soils analysis is based on “Stokes” law of settlement. This states that small spheres in a liquid state settle at a velocity that is a function of the diameter of the sphere. In order to avoid clumping together of the particles in the sedimentation method the sample is treated with a dispersing agent which allows the particles to settle individually. The heaviest particles will settle first, the lightest last. Measurement of the size of particles at a preset depth is taken with respect to time and from this the percentages of the different sized particles can be determined.

TYPICAL GRADING CURVES A flat portion of the curve indicates that little of that particle size is present A steep portion of the curve indicates that a lot of that particle size is present A well graded soil gives a smooth concave curve

Curve A :- poorly graded medium SAND - narrow range of sizes therefore poorly graded Curve B :- well graded material GRAVEL SAND - wide range of sizes Curve C :- very silty SAND - significant silt fraction Curve D :- sandy SILT Curve E :- silty CLAY - typical London Clay

Curve 1 :- well graded with excess fines, clay tends to keep the larger particles apart. Fines begin to exert a controlling influence over the soil.

Curve 2 :- well graded with clay present, can be closely compacted, clay provides cohesion which acts as a binder and increases shear strength. When compacted gives a dense, tough, high shear strength material with low permeability. Ideal fill material.

Curve 3 :-well graded this type of soil would exhibit high shear strength and density and low permeability

Curve 4 :- poorly graded material or uniformly close graded material Curve 4 :- poorly graded material or uniformly close graded material. Grains are nearly all one size thus it cannot be tightly packed and will therefore have low shear strength.

Curve 5 :- poorly or gap graded little or no sand or fine gravel present, poor compactability, not usely employed by itself

Using the grading curves further quantitative analysis is possible by using what are known as the grading characteristics which are obtained from the characteristic sizes. These grading characteristics are the effective size, the uniformity coefficient and the coefficient of graduation. To determine these grading characteristics the characteristic sizes are determined, these are :- d10 - the size such that 10% of the sample consists of particles having a smaller nominal diameter d30 - as d10 but 30% d60 - as d10 but 60% The d10 , d30 , d60 sizes can be read directly off the grading curve.

the effective size = d10 uniformity coefficient (Cu) = coefficient of gradation (Cg ) = Uniformly graded soils will tend to have low Cu values (< 3.0 ) with well graded soils having Cu value of > 5.0. A single sized soil would have a Cu value of 1.0. Cg values of about 2 are ideal with values between 0.5 and 2.0 indicating a well graded soil.

GEOTECHNICAL PROPERTIES (CE1203) Soil Classification Ms Ikmalzatul

TUTORIAL EXAMPLE 1 Original Weight: 147.2g Total 144.2 Sieve Size (mm) Mass Retained (g) % Retained % Passing 20 0.00 100 14.4 1.7 1.15 98.85 10 2.3 1.56 97.28 6.3 8.4 5.71 91.58 5 5.7 3.87 87.70 3.35 12.9 8.76 78.94 2 3.5 2.38 76.56 1.18 1.1 0.75 75.82 0.6 30.5 20.72 55.10 0.425 45.3 30.77 24.32 0.212 25.4 17.26 7.07 0.063 7.4 5.03 2.04 Total 144.2

TUTORIAL EXAMPLE 1 D60 D30 D10 Gravel Sand Silt & Clay 60 20 2 0.6 0.2 0.06 D60 D30 D10 Gravel Sand Silt & Clay

TUTORIAL EXAMPLE 1 Effective Size Uniformity Coefficient, Cu Coefficient of Gradation, Cg

TUTORIAL EXAMPLE 2 Sieve size (mm) Mass retained (g) % Retained % Passing 50 0.00 100 37.5 15.5 5.07 94.93 20 17 5.56 89.38 14 10 3.27 86.11 11 3.59 82.52 6.3 33 10.78 71.73 5 33.5 10.95 60.78 3.35 81 26.47 34.31 2 18 5.88 28.43 1.18 31 10.13 18.30 0.6 32.5 10.62 7.68 0.212 9 2.94 4.74 0.15 8 2.61 2.12 0.063 5.5 1.80 0.33

TUTORIAL EXAMPLE 2 D60 D30 D10 Gravel Sand Silt & Clay 60 20 2 0.6 0.2 0.06 D60 D30 D10 Gravel Sand Silt & Clay