soil classification and description

Soil Properties and classification

Soil classification AND DESCRIPTIONSub and SuperstructureSubstructure main purposes :

distribute the load of the structure over a large area of soil or to transmit the load to hard stratum of soil below the structure.

stability of structure against tilting, overturning, due to wind earthquake or extremely asymmetric loading etc.

to prevent lateral movement of loose soils from under the building To act as a firm and level base for construction of superstructureThree Soil types

1. Agricultural soil (e.g. topsoil, subsoil) not considered as engineering soils

2. Soil is a material that can be worked without drilling or blasting. Consist of soft, loose, uncemented deposits (e.g. gravels, clays, etc). There are 6 principle soil types considering size and nature of soil particles. Boulders & cobbles Gravels Sands Silts Clays Peats3. Rock is the hard, rigid & cemented deposits (e.g. granite and sandstone) Solid material called bedrock. Rock can be classified into three types of rock Igneous rocks are those that solidify from the magma through either intrusive (Granite) or extrusive (Basalt or Obsidian) processes

Sedimentary rocks result from external forces on the Earths crust and are formed of particles deposited by rivers, glaciers, the wind, the sea or by chemical deposition from lakes or the sea (Limestone, Conglomerates & Sandstone)

Metamorphic rocks are igneous or sedimentary that have been so altered by heat and/or pressure that they have lost their original character and have often been re-crystallized into new types of rocks (Marble or Slate)

Soil Composition, Structure & Fabric

Soil is grouping of solid separate particles forming a porous structure. The pores or voids contain air and/or water. The individual particles are quite free to move. Therefore soil is regarded as a particulate system

Air = unimportant from an engineering point of view voids should be reduced as far as possible

Water = very important for the engineering properties & behaviour

Solid matter (soil skeleton) = varies widely in particle shape, size & mineral composition organic or inorganic

Groundwater Profile

Two types of Engineering Soils

1. Granular Soils

Boulder, cobbles & gravels = angular to rounded rock fragments

Sands = granular

Silts = similar to sands but smaller grains with some plasticity & cohesion

The above particles are formed by mechanical weathering shape tends to be bulky & equi-dimensional

Structure arrangement = each individual particle supported by points of contact with adjacent particles

2. Cohesive Soils

Clays = from rock weathering, mainly due to chemical weathering.

Particles are plate shaped

Possess varying degrees of plasticity & cohesion

A small amount of clay will influence the soil behaviour

Natural clay deposits contain 70% of sand/silt (sandy clay, silty clay)

Structure arrangement =

Flocculation from inter particle attraction (Van der Waals or secondary bonding) these forces increases particles closer together.

Dispersed structure repulsive forces from electrically ve nature of particle surface holds particles apartSoil classificationIn practice soils contain more than one type e.g. sandy gravel, silty sand, clayey silt, sandy clay). The British Standard classification uses symbols to group the soils in accordance with its particle size distribution or plasticity indicesPrimary letterSecondary letter

Course grained soilsGravelWell-graded

Poorly-graded

SandUniformly-graded

Gap-graded

Fine grained soilsFinesLow plasticity

Intermediate plasticity

SiltHigh plasticity

Very high plasticity

ClayExtremely high plasticity

Upper plasticity range

Organic soilsPeatOrganic (may be suffixed to any group)

For mixtures of sand silt and clay the triangular classification chart may also be used.

Figure 1 - The hypothetical XYZ soil classification scheme

Calculation of Moisture Content.

Bulk & Dry Density

Masses: Density () and Unit Weight ()

Density = Mass / Volume

Mass = no gravity

Weight = when gravity is taken into account

Unit weight = density x gravity (9.81ms-2)

Bulk density () Mg/m3 x 9.81m/s2 = Bulk unit weight () kN/m3

Dry density (d) Mg/m3 x 9.81m/s2 = Dry unit weight (d) kN/m3 Density of water (1000kg/m3 or) 1Mg/m3 x 9.81m/s2 = Unit weight of water (w) = 9.81kN/m3

Activity: A soil sample has a diameter 50mm and a length of 85mm, a mass of 325g and a moisture content of 16.8% - calculate the bulk and dry densities in Mg/m3.

Particle density

The particle density (specific gravity) of a material is the ratio of the mass or weight of volume of a material to the mass or weight of an equal volume of water. In soil mechanics the particle density of the soil grains is given the symbol s (specific gravity = Gs).

For coarse-grained soils, a 500-1000 ml density bottle (gas jar) may be used, but for fine- grained soils, a special conical topped glass jar, called a pyknometer, should be used.

Should be noted that the range of particle density of common soil particles is very narrow: between 2.60 and 2.70 Mg/m3.

Guessing a value within this range will produce an error of no more than 3%.

In order to make the laboratory test worthwhile for standard soils therefore, a high level of accuracy in the procedures and weighing is necessary, i.e. better than 1%.

Particle Size Distribution by Sieve Analysis The analysis involves passing soil through a series of sieves of decreasing mesh size and recording the proportion of soil retained on each.

Grading The shape of the Particle Size Distribution curve indicates the range of particle sizes within a soil. A well graded soil has approximately equal proportions of particle sizes and the curve is usually smooth. A poorly graded soil may contain a high proportion of material within a narrow band.

Activity: A grading test undertaken on a sample of sand gave the following results tabulated below. Draw the particle size distribution curves for the sample, determine the effective size, uniformity coefficient of the sample and state the type of curve.

BS sieve size

(mm)Mass retained (grams)% retainedCumulative %

retainedCumulative %

passing

6.30-

2.3676

1.1870

0.6075

0.30200

0.1575

0.0754

500

Using the particle size distribution curve describe the soil using the following classification tablesClassification tables

Consistency Limits (Atterberg Limits)

These stages describe the consistency of the soil

Which in turn relate to its engineering properties

The moisture content at which the soil becomes plastic is defined as the Plastic Limit (P.L.)The moisture content at which the soil changes from plastic to liquid is defined as the Liquid Limit (L.L.).The range of moisture content over which the soil remains in a plastic condition is defined as the Plasticity Index (P.I.).

TESTS TO DETERMINE LIQUID AND PLASTIC LIMITS PLASTIC LIMITThe moisture content is determined at which a thread of soil can be rolled without breaking until it is only 3mm in diameter.LIQUID LIMIT

By cone penetrometer.

The classification of fine soils is based on soil plasticity.

Plasticity is a function of a fine soils capacity to absorb water and remain in a cohesive state. If water is added to a coarse soil, the water will fill the voids and saturate the sample, and further water will simply drain off and not be absorbed by the soil. If water is added to a fine soil, water will initially fill the voids.

When the soil is saturated it will continue to absorb water due to the properties of the clay minerals and an increase in volume of the soil mass will occur.

At the same time the soil is progressively softened by the water which increases the distance between the clay minerals and decreases the attracting forces between them, thus decreasing the cohesion. As the soil gets weaker it becomes pliable and when it is sufficiently pliable to be rolled out into a thread, it is said to be in a plastic state.

Eventually the soil loses all its strength and starts to flow under its own weight, when it is said to be in a liquid state.Use of the A Line Classification chart The classification chart for fine soils divides the soil according to ranges of liquid limit, & plasticity index.

Generally the higher the Liquid Limit the higher the plasticity is said to be. Silts and organic soils have a low Plasticity Index (i.e. a small range of moisture content over which they are plastic) compared to their Liquid Limit. Clays have a high Plasticity Index in relation to their Liquid Limit.

The clay minerals have the capacity to take in moisture and still retain some cohesion.To classify a fine soil according to the chart:i) Determine Liquid and Plastic Limits, and therefore Plasticity Index.ii) Using values of PI and LL, plot the soil on the chart.iii) Observe which segment the soil comes into (CL, CI, ML etc)iv) Write down the soil name, CLAY for C soils, SILT for M soils.v) Follow the name with the plasticity i.e. CI = CLAY of intermediate plasticity.

Example: Use the A Line Classification chart to classify the following soilCone Penetration Test

Agricultural soil (topsoil overlying subsoil)

Ground Level

?m

solid particles

water

air

Idealised Model of Soil Structure

Typical Soil Structure

Agricultural soil (topsoil overlying subsoil)

At any stage the water table could be encountered upper zone of the soil which is fully saturated

Bedrock

?m

Bedrock

Engineering soil

?m

Engineering soil

Pyknometer

Boring made from ground level

The upper level of the water table is the level at which water would stand

Excavation

Gas jar

Plastic

Liquid

WATER

mass

EMBED Equation.DSMT4

s =

SUPERSTRUCTURE

SUBSTRUCTURE



325g

where : w = moisture content (expressed as decimal)

w = moisture content (expressed as percentage)



V

SOLID

AIR





M

V




SAND

mass


WATER

w1= mass of container (g)

w2 = mass of container & wet soil (g)

w3 = mass of container & dry soil (g)

AIR

Activity: Calculate the moisture content for the following:

EMBED Excel.Sheet.8

Ms


SOLID

Flocculated structure

WATER

AIRR

SOLID

Dispersed structure

Mw


85mm

50mm

Shrinkage limit, SL

Liquid limit, LL

Plastic limit, PL

Boundaries between the states are known as the consistency limits


Moisture content below which no further reduction in volume will occur

Measured by slowly drying a sample out & periodically measuring its volume & mass

Graph of moisture content vs volume

Empirical boundary defined as the moisture content at which a 3mm diameter thread of soil can be rolled by hand without breaking up

Empirical boundary defined as the moisture content at which the soil is assumed to flow under its own weight measured using a cone penetrometer or Cassegrande cup

Plasticity index, PI is the range of moisture content PL & LL

PI=LL-PL

Semi-Solid

Solid

Moisture content increases

Cohesive soil

0 10 20 30 40 50 60 70 80 90 100 110 120

70

60

50

40

30

20

10

6

0

Liquid limit (%)

ML

MV

MH

MI

ME

CE

CH

CV

CI

CL

Plasticity index (%)

Upper plasticity range (U)

CI

Extremely high plasticity (E)

Very high (V)

High (H)

Intermediate(I)

Low (L)

A-line

CI

0 10 20 30 40 50 60 70 80 90 100 110 120

SILT

CLAY

70

60

50

40

30

20

10

6

0

Liquid limit (%)

ML

EMBED Excel.Sheet.8

EMBED Excel.Sheet.8

To classify a fine soil according to the chart:

i) Determine Liquid and Plastic Limits, and therefore Plasticity Index.

w1= mass of container (g)

w2 = mass of container & wet soil (g)

w3 = mass of container & dry soil (g)


MV

Calculate the moisture content


PI= 40 13.5 = 26.5%

= 13+14/2 = 13.5%

EMBED Excel.Sheet.8

Plastic Limit Test

PI=LL-PL

Plasticity index

Plastic limit = average of results


MH

MI

ME

CE

CH

CV

CI

CL

Plasticity index (%)

Upper plasticity range (U)

Extremely high plasticity (E)

Very high (V)

High (H)

Intermediate(I)

Low (L)

A-line

SILT

CLAY

CI = CLAY of intermediate plasticity.

PI= 26.5%

LL=40%

ii) Using values of PI and LL, plot the soil on the chart.

iii) Observe which segment the soil comes into (CL, CI, ML etc)

iv) Write down the soil name, CLAY for C soils, SILT for M soils.

v) Follow the name with the plasticity

_1263829410.unknown

_1263906760.unknown

_1263961112.xlsSheet1

Testw1w2w3Cone penetration (mm)moisture content %

110.320.11817.227

211.21815.821.248

3101915.723.858

412.221.817.530.181

Sheet1

0

0

0

0

moisture content %

Moisture content (%)

Cone Penetration (mm)

Sheet2

Testw1w2w3moisture content %

A10.226.324.413

B13.225.323.814

Sheet3

_1263961765.xlsSheet1


110.320.11817.227

211.21815.821.248

3101915.723.858

412.221.817.530.181

Sheet1

0

0

0

0

moisture content %



Sheet2


A10.226.324.413

B13.225.323.814

Sheet3

_1263961766.unknown

_1263961114.unknown

_1263961764.unknown

_1263961111.xlsSheet1


110.320.118.017.227

211.218.015.821.248

310.019.015.723.858

412.221.817.530.181

Sheet1

0

0

0

0

moisture content %



Sheet2


A10.226.324.413

B13.225.323.814

Sheet3

_1263961110.unknown

_1263830096.unknown

_1263830098.unknown

_1263829519.unknown

_1263829406.unknown

_1263829408.unknown

_1263829409.unknown

_1263829407.unknown

_1263829404.unknown

_1263829405.unknown

_1263829402.unknown

_1263829403.unknown

_1263829122.xlsSheet1


18.118.215.517.236

27.316.213.721.239

37.717.714.823.841

46.916.113.230.146

Sheet1

0

0

0

0

moisture content %



Sheet2


A16.526.624.329

Sheet3

_1263829401.unknown

_1263829121.unknown

soil classification and description

Documents

nature of soil particles

soil properties

structure fabric soil

agricultural soil

principle soil types

slate soil composition

hard stratum of soil

individual particles