Back to home page Based on part of the GeotechniCAL reference package
by Prof. John Atkinson, City University, London

Soil description and classification

It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be comprehensive (covering all but the rarest of deposits), meaningful in an engineering context (so that engineers will be able to understand and interpret) and yet relatively concise. It is important to distinguish between description and classification:

Description of soil is a statement describing the physical nature and state of the soil. It can be a description of a sample, or a soil in situ. It is arrived at using visual examination, simple tests, observation of site conditions, geological history, etc.

Soil classification is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. A classification for engineering purposes should be based mainly on mechanical properties, e.g. permeability, stiffness, strength. The class to which a soil belongs can be used in its description.

Description and classification

Basic characteristics of soils

Soils consist of grains (mineral grains, rock fragments, etc.) with water and air in the voids between grains. The water and air contents are readily changed by changes in conditions and location: soils can be perfectly dry (have no water content) or be fully saturated (have no air content) or be partly saturated (with both air and water present). Although the size and shape of the solid (granular) content rarely changes at a given point, they can vary considerably from point to point.

First of all, consider soil as a engineering material - it is not a coherent solid material like steel and concrete, but is a particulate material. It is important to understand the significance of particle size, shape and composition, and of a soil's internal structure or fabric.

Basic characteristics of soils

Soil as an engineering material

The term "soil" means different things to different people: To a geologist it represents the products of past surface processes. To a pedologist it represents currently occurring physical and chemical processes. To an engineer it is a material that can be:

built on: foundations to buildings, bridges.
built in: tunnels, culverts, basements.
built with: roads, runways, embankments, dams.
supported: retaining walls, quays.

Soils may be described in different ways by different people for their different purposes. Engineers' descriptions give engineering terms that will convey some sense of a soil's current state and probable susceptibility to future changes (e.g. in loading, drainage, structure, surface level).

Engineers are primarily interested in a soil's mechanical properties: strength, stiffness, permeability. These depend primarily on the nature of the soil grains, the current stress, the water content and unit weight.

Basic characteristics of soils

Size range of grains

The range of particle sizes encountered in soil is very large: from boulders with a controlling dimension of over 200mm down to clay particles less than 0.002mm (2mm). Some clays contain particles less than 1 mm in size which behave as colloids, i.e. do not settle in water due solely to gravity.

In theBritish Soil Classification System, soils are classified into named Basic Soil Type groups according to size, and the groups further divided into coarse, medium and fine sub-groups:

Very coarse

BOULDERS > 200 mm
COBBLES 60 - 200 mm



coarse 20 - 60 mm
medium 6 - 20 mm
fine 2 - 6 mm


coarse 0.6 - 2.0 mm
medium 0.2 - 0.6 mm
fine 0.06 - 0.2 mm



coarse 0.02 - 0.06 mm
medium 0.006 - 0.02 mm
fine 0.002 - 0.006 mm


< 0.002 mm


Size range of grains

Aids to size identification

Soils possess a number of physical characteristics which can be used as aids to size identification in the field. A handful of soil rubbed through the fingers can yield the following:

SAND (and coarser) particles are visible to the naked eye.
SILT particles become dusty when dry and are easily brushed off hands and boots.
CLAY particles are greasy and sticky when wet and hard when dry, and have to be scraped or washed off hands and boots.

Basic characteristics of soils

Shape of grains

The majority of soils may be regarded as either SANDS or CLAYS:

SANDS include gravelly sands and gravel-sands. Sand grains are generally broken rock particles that have been formed by physical weathering, or they are the resistant components of rocks broken down by chemical weathering. Sand grains generally have a rotund shape.

CLAYS include silty clays and clay-silts; there are few pure silts (e.g. areas formed by windblown Less). Clay grains are usually the product of chemical weathering or rocks and soils. Clay particles have a flaky shape.

There are major differences in engineering behaviour between SANDS and CLAYS (e.g. in permeability, compressibility, shrinking/swelling potential). The shape and size of the soil grains has an important bearing on these differences.

Shape of grains

Shape characteristics of SAND grains

SAND and larger-sized grains are rotund. Coarse soil grains (silt-sized, sand-sized and larger) have different shape characteristics and surface roughness depending on the amount of wear during transportation (by water, wind or ice), or after crushing in manufactured aggregates. They have a relatively low specific surface (surface area).

Click on a link below to see the shape
Rounded: Water- or air-worn; transported sediments
Irregular: Irregular shape with round edges; glacial sediments (sometimes sub-divided into 'sub-rounded' and 'sub-angular')
Angular: Flat faces and sharp edges; residual soils, grits
Flaky: Thickness small compared to length/breadth; clays
Elongated: Length larger than breadth/thickness; scree, broken flagstone
Flaky & Elongated: Length>Breadth>Thickness; broken schists and slates

Shape of grains

Shape characteristics of CLAY grains

CLAY particles are flaky. Their thickness is very small relative to their length & breadth, in some cases as thin as 1/100th of the length. They therefore have high to very high specific surface values. These surfaces carry a small negative electrical charge, that will attract the positive end of water molecules. This charge depends on the soil mineral and may be affected by an electrolite in the pore water. This causes some additional forces between the soil grains which are proportional to the specific surface. Thus a lot of water may be held asadsorbed water within a clay mass.


Shape of grains

Specific surface

Specific surface is the ratio of surface area per unit wight.
Surface forces are proportional to surface area (i.e. to d).
Self-weight forces are proportional to volume (i.e. to d).
Therefore Surface force   
self weight forces d
Also, specific surface =        area   
r * volume d

Hence, specific surface is a measure of the relative contributions of surface forces and self-weight forces.

The specific surface of a 1mm cube of quartz (r = 2.65gm/cm) is 0.00023 m/N

SAND grains (size 2.0 - 0.06mm) are close to cubes or spheres in shape, and have specific surfaces near the minimum value.

CLAY particles are flaky and have much greater specific surface values.

Examples of specific surface

The more elongated or flaky a particle is the greater will be its specific surface.

Click on the following examples:
cubes, rods, sheets

Examples of mineral grain specific surfaces:
Mineral/Soil Grain width
d (mm)
Thickness Specific Surface
Quartz grain 100 d 0.0023
Quartz sand 2.0 - 0.06 d 0.0001 - 0.004
Kaolinite 2.0 - 0.3 0.2d 2
Illite 2.0 - 0.2 0.1d 8
Montmorillonite 1.0 - 0.01 0.01d 80

See also clay minerals

Basic characteristics of soils

Structure or fabric

Natural soils are rarely the same from one point in the ground to another. The content and nature of grains varies, but more importantly, so does the arrangement of these.

The arrangement and organisation of particles and other features within a soil mass is termed its structure or fabric. This includes bedding orientation, stratification, layer thickness, the occurrence of joints and fissures, the occurrence of voids, artefacts, tree roots and nodules, the presence of cementing or bonding agents between grains.

Structural features can have a major influence on in situ properties.

Description and classification

Origins, formation and mineralogy

Soils are the results of geological events (except for the very small amount produced by man). The nature and structure of a given soil depends on the geological processes that formed it:
breakdown of parent rock: weathering, decomposition, erosion.
transportation to site of final deposition: gravity, flowing water, ice, wind.
environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
subsequent conditions of loading and drainage - little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.

Origins, formation and mineralogy

Origins of soils from rocks

All soils originate, directly or indirectly, from solid rocks in the Earth's crust:

igneous rocks
crystalline bodies of cooled magma
e.g. granite, basalt, dolerite, gabbro, syenite, porphyry

sedimentary rocks
layers of consolidated and cemented sediments, mostly formed in bodies of water (seas, lakes, etc.)
e.g. limestone, sandstones, mudstone, shale, conglomerate

metamorphic rocks
formed by the alteration of existing rocks due to heat from igneous intrusions (e.g. marble, quartzite, hornfels) or pressure due to crustal movement (e.g. slate, schist, gneiss).

Origins, formation and mineralogy

Weathering of rocks

Physical weathering
Physical or mechanical processes taking place on the Earth's surface, including the actions of water, frost, temperature changes, wind and ice; cause disintegration and wearing. The products are mainly coarse soils (silts, sands and gravels). Physical weathering produces Very Coarse soils and Gravels consisting of broken rock particles, but Sands and Silts will be mainly consists of mineral grains.

Chemical weathering
Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or alteration. The results of chemical weathering are generally fine soils with separate mineral grains, such as Clays and Clay-Silts. The type of clay mineral depends on the parent rock and on local drainage. Some minerals, such as quartz, are resistant to the chemical weathering and remain unchanged.

A resistant and enduring mineral found in many rocks (e.g. granite, sandstone). It is the principal constituent of sands and silts, and the most abundant soil mineral. It occurs as equidimensional hard grains.
A red iron (ferric) oxide: resistant to change, results from extreme weathering. It is responsible for the widespread red or pink colouration in rocks and soils. It can form a cement in rocks, or a duricrust in soils in arid climates.
Flaky minerals present in many igneous rocks. Some are resistant, e.g. muscovite; some are broken down, e.g. biotite.
clay minerals
These result mainly from the breakdown of feldspar minerals. They are very flaky and therefore have very large surface areas. They are major constituents of clay soils, although clay soil also contains silt sized particles.

Origins, formation and mineralogy

Clay minerals

Clay minerals are produced mainly from the chemical weathering and decomposition of feldspars, such as orthoclase and plagioclase, and some micas. They are small in Electron micrograph of clay particles size and very flaky in shape.

The key to some of the properties of clay soils, e.g. plasticity, compressibility, swelling/shrinkage potential, lies in the structure of clay minerals.

There are three main groups of clay minerals:
(include kaolinite, dickite and nacrite) formed by the decomposition of orthoclase feldspar (e.g. in granite); kaolin is the principal constituent in china clay and ball clay.
(include illite and glauconite) are the commonest clay minerals; formed by the decomposition of some micas and feldspars; predominant in marine clays and shales (e.g. London clay, Oxford clay).
(also called smectites or fullers' earth minerals) (include calcium and sodium momtmorillonites, bentonite and vermiculite) formed by the alteration of basic igneous rocks containing silicates rich in Ca and Mg; weak linkage by cations (e.g. Na+, Ca++) results in high swelling/shrinking potential


For more information on mineralogy see

Origins, formation and mineralogy

Transportation and deposition

The effects of weathering and transportation largely determine the basic nature of the soil (i.e. the size, shape, composition and distribution of the grains). The environment into which deposition takes place, and subsequent geological events that take place there, largely determine the state of the soil, (i.e. density, moisture content) and the structure or fabric of the soil (i.e. bedding, stratification, occurrence of joints or fissures, tree roots, voids, etc.)

Due to combinations of gravity, flowing water or air, and moving ice. In water or air: grains become sub-rounded or rounded, grain sizes are sorted, producing poorly-graded deposits. In moving ice: grinding and crushing occur, size distribution becomes wider, deposits are well-graded, ranging from rock flour to boulders.

In flowing water, larger particles are deposited as velocity drops, e.g. gravels in river terraces, sands in floodplains and estuaries, silts and clays in lakes and seas. In still water: horizontal layers of successive sediments are formed, which may change with time, even seasonally or daily.

Origins, formation and mineralogy

Loading and drainage history

The current state (i.e. density and consistency) of a soil will have been profoundly influenced by the history of loading and unloading since it was deposited. Changes in drainage conditions may also have occurred which may have brought about changes in water content.

Loading /unloading history

Initial loading
During deposition the load applied to a layer of soil increases as more layers are deposited over it; thus, it is compressed and water is squeezed out; as deposition continues, the soil becomes stiffer and stronger.

The principal natural mechanism of unloading is erosion of overlying layers. Unloading can also occur as overlying ice-sheets and glaciers retreat, or due to large excavations made by man. Soil expands when it is unloaded, but not as much as it was initially compressed; thus it stays compressed - and is said to be overconsolidated. The degree of overconsolidation depends on the history of loading and unloading.

Drainage history

Chemical changes
Some soils initially deposited loosely in saline water and then inundated with fresh water develop weak collapsing structure. In arid climates with intermittent rainy periods, cycles of wetting and drying can bring minerals to the surface to form a cemented soil.

Climate changes
Some clays (e.g. montmorillonite clays) are prone to large volume changes due to wetting and drying; thus, seasonal changes in surface level occur, often causing foundation damage, especially after exceptionally dry summers. Trees extract water from soil in the process of evapotranspiration; The soil near to trees can therefore either shrink as trees grow larger, or expand following the removal of large trees.

Description and classification

Grading and composition

The recommended standard for soil classification is the British Soil Classification System, and this is detailed in BS 5930 Site Investigation.

Grading and composition

Coarse soils

Coarse soils are classified principally on the basis of particle size and grading.
Very coarse
BOULDERS> 200 mm
COBBLES60 - 200 mm
coarse20 - 60 mm
medium6 - 20 mm
fine2 - 6 mm
coarse0.6 - 2.0 mm
medium0.2 - 0.6 mm
fine0.06 - 0.2 mm

Coarse soils

Particle size tests

The aim is to measure the distribution of particle sizes in the sample. When a wide range of sizes is present, the sample will be sub-divided, and separate tests carried out on each sub-sample. Full details of tests are given in BS 1377: "Methods of test for soil for civil engineering purposes".

Particle-size tests

Wet sieving to separate fine grains from coarse grains is carried out by washing the soil specimen on a 60mm sieve mesh.
Dry sieving analyses can only be carried out on particles > 60 mm. Samples (with fines removed) are dried and shaken through a nest of sieves of descending size.
Sedimentation is used only for fine soils. Soil particles are allowed to settle from a suspension. The decreasing density of the suspension is measured at time intervals. Sizes are determined from the settling velocity and times recorded. Percentages between sizes are determined from density differences.

Particle-size analysis

The cumulative percentage quantities finer than certain sizes (e.g. passing a given size sieve mesh) are determined by weighing. Points are then plotted of % finer (passing) against log size. A smooth S-shaped curve drawn through these points is called a grading curve. The position and shape of the grading curve determines the soil class. Geometrical grading characteristics can be determined also from the grading curve.


Coarse soils

Typical grading curves

Both the position and the shape of the grading curve for a soil can aid its identity and description.
Some typical grading curves are shown in the figure:
A - a poorly-graded medium SAND (probably estuarine or flood-plain alluvium)
B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand)
C - a gap-graded COBBLES-SAND
D - a sandy SILT (perhaps a deltaic or estuarine silt)
E - a typical silty CLAY (e.g. London clay, Oxford clay)

Coarse soils

Grading characteristics

A grading curve is a useful aid to soil description. Grading curves are often included in ground investigation reports. Results of grading tests can be tabulated using geometric properties of the grading curve. These properties are called grading characteristics

First of all, three points are located on the grading curve:
d10 = the maximum size of the smallest 10% of the sample
d30 = the maximum size of the smallest 30% of the sample
d60 = the maximum size of the smallest 60% of the sample

From these the grading characteristics are calculated:
Effective size
Uniformity coefficient
Cu = d60 / d10
Coefficient of gradation
Ck = d30 / d60 d10

Both Cu and Ck will be 1 for a single-sized soil
Cu > 5 indicates a well-graded soil
Cu < 3 indicates a uniform soil
Ck between 0.5 and 2.0 indicates a well-graded soil
Ck < 0.1 indicates a possible gap-graded soil

Coarse soils

Sieve analysis example

The results of a dry-sieving test are given below, together with the grading analysis and grading curve. Note carefully how the tabulated results are set out and calculated. The grading curve has been plotted on special semi-logarithmic paper; you can also do this analysis using a spreadsheet.
Sieve mesh
size (mm)
retained (g)
finer (passing)

The soil comprises: 18% gravel, 45% coarse sand, 24% medium sand, 10% fine sand, 3% silt, and is classified therefore as: a well-graded gravelly SAND

Grading and composition

Fine soils

In the case of fine soils (e.g. CLAYS and SILTS), it is the shape of the particles rather than their size that has the greater influence on engineering properties. Clay soils have flaky particles to which water adheres, thus imparting the property of plasticity.

Fine soils

Consistency limits and plasticity

Consistency varies with the water content of the soil. The consistency of a soil can range from (dry) solid to semi-solid to plastic to liquid (wet). The water contents at which the consistency changes from one state to the next are called consistency limits (or Atterberg limits).

Two of these are utilised in the classification of fine soils:
Liquid limit (wL) - change of consistency from plastic to liquid
Plastic limit (wP) - change of consistency from brittle/crumbly to plastic

Measures of liquid and plastic limit values can be obtained from laboratory tests.

Fine soils

Plasticity index

The consistency of most soils in the ground will be plastic or semi-solid. Soil strength and stiffness behaviour are related to the range of plastic consistency. The range of water content over which a soil has a plastic consistency is termed the Plasticity Index (IP or PI).
IP = liquid limit - plastic limit
= wL - wP

Fine soils

The plasticity chart and classification

In the BSCS fine soils are divided into ten classes based on their measured plasticity index and liquid limit values: CLAYS are distinguished from SILTS, and five divisions of plasticity are defined:

Low plasticitywL = < 35%
Intermediate plasticitywL = 35 - 50%
High plasticitywL = 50 - 70%
Very high plasticitywL = 70 - 90%
Extremely high plasticitywL = > 90%

A plasticity chart is provided to aid classification.

Fine soils


So-called 'clay' soils are not 100% clay. The proportion of clay mineral flakes (< 2 mm size) in a fine soil affects its current state, particularly its tendency to swell and shrink with changes in water content. The degree of plasticity related to the clay content is called the activity of the soil.

Activity = IP / (% clay particles)

Some typical values are:
Mineral Activity Soil Activity
Muscovite 0.25 Kaolin clay 0.4-0.5
Kaolinite 0.40 Glacial clay and loess 0.5-0.75
Illite 0.90 Most British clays 0.75-1.25
Montmorillonite > 1.25 Organic estuarine clay > 1.25


Grading and composition

Specific gravity

Specific gravity (Gs) is a property of the mineral or rock material forming soil grains.
It is defined as

Method of measurement
For fine soils a 50 ml density bottle may be used; for coarse soils a 500 ml or 1000 ml jar. The jar is weighed empty (M1). A quantity of dry soil is placed in the jar and the jar weighed (M2). The jar is filled with water, air removed by stirring, and weighed again (M3). The jar is emptied, cleaned and refilled with water - and weighed again (M4).

[The range of Gs for common soils is 2.64 to 2.72]

Description and classification

Volume-weight properties

The volume-weight properties of a soil define its state. Measures of the amount of void space, amount of water and the weight of a unit volume of soil are required in engineering analysis and design.

Soil comprises three constituent phases:
Solid: rock fragments, mineral grains or flakes, organic matter.
Liquid: water, with some dissolved compounds (e.g. salts).
Gas: air or water vapour.

In natural soils the three phases are intermixed. To aid analysis it is convenient to consider a soil model in which the three phases are seen as separate, but still in their correct proportions.

Volume-weight properties

Volumes of solid, water and air: the soil model

The soil model is given dimensional values for the solid, water and air components: Total volume,
V = Vs + Vw + Va

Since the amounts of both water and air are variable, the volume of solids present is taken as the reference quantity. Thus, the following relational volumetric quantities may be defined:

Note also that:
n = e / (1 + e)
e = n / (1 - n)
v = 1 / (1 - n)

Typical void ratios might be 0.3 (e.g. for a dense, well graded granular soil) or 1.5 (e.g. for a soft clay).

Volumes of solid, water and air: the soil model

Degree of saturation

The volume of water in a soil can only vary between zero (i.e. a dry soil) and the volume of voids; this can be expressed as a ratio:

For a perfectly dry soil:
Sr = 0
For a saturated soil:
Sr = 1

Note: In clay soils as the amount water increases the volume and therefore the volume of voids will also increase, and so the degree of saturation may remain at Sr = 1 while the actual volume of water is increasing.

Volumes of solid, water and air: the soil model

Air-voids content

The air-voids volume, Va , is that part of the void space not occupied by water.

Va = Vv - Vw
= e - e.Sr
= e.(1 - Sr)

Air-voids content, Av
Av = (air-voids volume) / (total volume)
= Va / V
= e.(1 - Sr) / (1+e)
= n.(1 - Sr)

For a perfectly dry soil:
Av = n
For a saturated soil:
Av = 0

Volume-weight properties

Masses of solid and water: water content

The mass of air may be ignored. The mass of solid particles is usually expressed in terms of their particle density or grain specific gravity.

Grain specific gravity

Hence the mass of solid particles in a soil
Ms = Vs .Gs .rw
(rw = density of water = 1.00Mg/m)
[Range of Gs for common soils: 2.64-2.72]

Particle density
rs = mass per unit volume of particles
= Gs .rw

The ratio of the mass of water present to the mass of solid particles is called the water content, or sometimes the moisture content.

From the soil model it can be seen that
w = (Sr .e .rw) / (Gs .rw)

Giving the useful relationship:
w .Gs = Sr .e

Volume-weight properties

Densities and unit weights

Density is a measure of the quantity of mass in a unit volume of material.
Unit weight is a measure of the weight of a unit volume of material.

There are two basic measures of density or unit weight applied to soils: Dry density is a measure of the amount of solid particles per unit volume. Bulk density is a measure of the amount of solid + water per unit volume.

The preferred units of density are:
Mg/m, kg/m or g/ml.

The corresponding unit weights are:

Also, it can be shown that
r = rd(1 + w) and
g = gd(1 + w)

Volume-weight properties

Laboratory measurements

It is important to quantify the state of a soil immediately it is received in the testing laboratory and just prior to commencing other tests (e.g. shear tests, compression tests, etc.).

The water content and unit weight are particularly important, since these could change during transportation and storage.

Some physical state properties are calculated following the practical measurement of others; e.g. void ratio from porosity, dry unit weight from unit weight & water content.

Laboratory measurements

Water content

The most usual method of determining the water content of soil is to weigh a small representative specimen, drying it to constant weight and then weighing it again. Drying can be carried out using an electric oven set at 104-105 Celsius or using a microwave oven.

Example: A sample of soil was placed in a tin container and weighed, after which it was dried in an oven and then weighed again. Calculate the water content of the soil.

Weight of tin empty = 16.16 g
Weight of tin + moist soil = 37.82 g
Weight of tin + dry soil = 34.68 g
Water content, w = (mass of water) / (mass of dry soil)
= (37.82 - 34.68) / (34.68 - 16.16)
= 0.169
Percentage water content= 16.9 %

Laboratory measurements

Unit weight

Clay soils: Specimens are usually prepared in the form of regular geometric shapes, (e.g. prisms, cylinders) of which the volume is easily computed.
Sands and gravels: Specimens have to be placed in a container to determine volume (e.g. a cylindrical can).

A soil specimen had a volume of 89.13 ml, a mass before drying of 174.45 g and after drying of 158.73 g; the water content was 9.9 %. Determine the bulk and dry densities and unit weights.

Bulk density
r = (mass of specimen) / (volume of specimen)
= 174.45 / 89.13 g/ml
= 1.957 Mg/m
[1 g/ml = 1 Mg/m]
Unit weight
g = 9.81m/s x r Mg/m
= 19.20 kN/m
Dry density
rd = (mass after drying) / (volume)
= 158.73 / 89.13
= 1.781 Mg/m
rd = r / (1 + w)
= 1.957 / (1+0.099)
= 1.781 Mg/m
Dry unit weight
gd = g / (1 + w)
= 19.20 / (1+0.099)
= 17.47 kN/m


Volume-weight properties

Field measurements

Measurements taken in the field are mostly to determine density/unit weight. The most common application is the determination of the density of rolled and compacted fill, e.g. in road bases, embankments, etc.

Note: These methods are covered in detail by BS1377. You should understand the general principle that density is calculated from the mass and volume of a sample. How a sample of known volume is obtained depends on the nature of the soil. You are not expected to remember the details of each method.

The core cutter method
This method is suitable for soft fine grained soils.

A steel cylinder is driven into the ground, dug out and the soil shaved off level. The mass of soil is found by weighing and deducting the mass of the cylinder. Small samples are taken from both ends and the water content determined.

The sand-pouring cylinder method
This method is suitable for stony soils

Using a special tray with a hole in the centre, a hole is formed in the soil and the mass of soil removed is weighed.

The volume of the hole is calculated from the mass of clean dry running sand required to fill the hole.

The sand-pouring cylinder is used to fill the hole in a controlled manner. The mass of sand required to fill the hole is equal to the difference in the weight of the cylinder before and after filling the hole, less an allowance for the sand left in the cone above the hole.

Bulk density
r = (mass of soil) / (volume of core cutter or hole)

Description and classification

Current state of soil

The state of soil is essentially the closeness of packing of the grains in the range:
Closely packed   Loosely packed
Dense Loose
Low water content High water content
Strong and stiff Weak and soft

The important indicators of the current state of a soil are:
current stresses: vertical and horizontal effective stresses
current water content: effecting strength and stiffness in fine soils
liquidity index: indicates state in fine soils
density index: indicates state of compaction in coarse soils
history of loading and unloading: degree of overconsolidation

Engineering operations (e.g. excavation, loading, unloading, compaction, etc.) on soil bring about changes in its state. Its initial state is the result of processes of erosion and deposition. It is possible for the engineer to predict changes that could result from a proposed engineering operation: changes from the soil's current state to a new future state.

Current state of soil

Soil history: deposition and erosion

Original deposition
Most soils are formed in layers or lenses by deposition from moving water, ice or wind.

One-dimensional compression occurs as overlying layers are added. Vertical and horizontal stresses increase with deposition.

Erosion causes unloading; stresses decrease; some vertical expansion occurs.

Plastic strain has occurred; the soil remains compressed, i.e. overconsolidated.

Subsequent changes
Subsequent changes may occur in the depositional environment: further loading/unloading due to glaciation, land movement, engineering; and ageing processes.

Current state of soil

Soil history: ageing

The term ageing includes processes that occur with time, except loading and unloading. Ageing processes are independent of changes in loading.

Vibration and compaction
Coarse soils can be made more dense by vibration or compaction at essentially constant effective stress

Fine soils creep and continue to compress and distort at constant effective stress after primary consolidation is complete.

Cementing and bonding
Intergranular cementing and bonding occurs due to deposition of minerals from groundwater, e.g. calcium carbonate; disturbance due to excavation fractures the bonding and reduces strength.

Physical and chemical changes take place in soils near the ground surface due to the influence of changes in rainfall and temperature.

Changes in salinity
Changes in the salinity of groundwater are due to changes in relative sea and land levels, thus soil originally deposited in sea water may later have fresh water in its pores, such soils may be prone to sudden collapse.

Current state of soil

Density index (relative density)

The void ratio of coarse soils (sands and gravels) varies with the state of packing between the loosest practical state in which it can exist and the densest. Some engineering properties are affected by this, e.g.shear strength, compressibility, permeability.

It is therefore useful to measure the in situ state and this can be done by comparing the in situ void ratio (e) with the minimum and maximum practical values (emin and emax) to give a density index ID

emin is determined with soil compacted densely in a metal mould
emax is determined with soil poured loosely into a metal mould

Density index is also known as relative density
Relative states of compaction are defined:
Density index State of compaction
0-15% Very loose
15-35 Loose
35-65 Medium
65-85 Dense
85-100% Very dense


Current state of soil

Liquidity index

In fine soils, especially clays, the current state is dependent on the water content with respect to the consistency limits (or Atterberg limits). The liquidity index (IL or LI) provides a quantitative measure of the current state:

wP = plastic limit and
wL = liquid limit

Significant values of IL indicating the consistency of the soil are:
IL < 0 semi-plastic solid or solid
0 < IL < 1  plastic
1 < IL  liquid

Current state of soil

Predicting stiffness and strength from index properties

Preliminary estimates of strength and stiffness can provide a useful basis for early design and feasibility studies, and also the planning of more detailed testing programmes. The following suggestions have been made; they are simple, but not necessarily reliable, and should be not be used in final design calculations.

Undrained shear strength

su = 170 exp(-4.6 IL) kN/m
[Schofield and Wroth (1968)]
su = (0.11 + 0.37 IP) s'vo kN/m
where s'vo = vertical effective stress in situ
[Skempton and Bjerrum (1957)]

The slope of the critical state line may be estimated from:
l = IP .Gs / 461
[After Skempton and Northey (1953)]

The compressibility index may be estimated from:
Cc = l ln10 = IP Gs / 200
(where IP is in percentage units)

Description and classification

BS system for description and classification

BS 5930 Site Investigation recommends the terminology and a system for describing and classifying soils for engineering purposes. Without the use of a satisfactory system of description and classification, the description of materials found on a site would be meaningless or even misleading, and it would be difficult to apply experience to future projects.

BS system for description and classification

BS description system

A recommended protocol for describing a soil deposit uses ninecharacteristics; these should be written in the following order:

e.g. loose, dense, slightly cemented
bedding structure
e.g. homogeneous or stratified; dip, orientation
spacing of beds, joints, fissures
weathered state
degree of weathering
main body colour, mottling
grading or consistency
e.g. well-graded, poorly-graded; soft, firm, hard
e.g. GRAVEL, SAND, SILT, CLAY; (upper case letters) plus silty-, gravelly-, with-fines, etc. as appropriate
soil class
(BSCS) designation (for roads & airfields) e.g. SW = well-graded sand
geological stratigraphic name
(when known) e.g. London clay

Not all characteristics are necessarily applicable in every case.

(i) Loose homogeneous reddish-yellow poorly-graded medium SAND (SP), Flood plain alluvium
(ii) Dense fissured unweathered greyish-blue firm CLAY. Oxford clay.

BS system for description and classification

Definitions of terms used in description

A table is given in BS 5930 Site Investigation setting out a recommended field indentification and description system. The following are some of the terms listed for use in soil descriptions:

Particle shape
angular, sub-angular, sub-rounded, rounded, flat, elongate
loose, medium dense, dense (use a pick or driven peg, or density index )
Bedding structure
homogeneous, stratified, inter-stratified
Bedding spacing
massive(>2m), thickly bedded (2000-600 mm), medium bedded (600-200 mm), thinly bedded (200-60 mm), very thinly bedded (60-20 mm), laminated (20-6 mm), thinly laminated (<6 mm).
i.e. spacing of joints and fissure: very widely spaced(>2m), widely spaced (2000-600 mm), medium spaced (600-200 mm), closely spaced (200-60 mm), very closely spaced (60-20 mm), extremely closely spaced (<20 mm).
red, pink, yellow, brown, olive, green, blue, white, grey, black
very soft (exudes between fingers), soft (easily mouldable), firm (strong finger pressure required), stiff (can be indented with fingers, but not moulded) very stiff (indented by sharp object), hard (difficult to indent).
well graded (wide size range), uniform (very narrow size range), poorly graded (narrow or uneven size range).
Composite soils
In SANDS and GRAVELS: slightly clayey or silty (<5%), clayey or silty (5-15%), very clayey or silty(>15%)
In CLAYS and SILTS: sandy or gravelly (35-65%)

BS system for description and classification

British Soil Classification System

The recommended standard for soil classification is the British Soil Classification System, and this is detailed in BS 5930 Site Investigation. Its essential structure is as follows:
Soil group   Symbol   Recommended name
Coarse soils     Fines %  
GRAVEL GW 0 - 5 Well-graded GRAVEL
  GPu/GPg 0 - 5 Uniform/poorly-graded GRAVEL
G-F GWM/GWC 5 - 15 Well-graded silty/clayey GRAVEL
  GPM/GPC 5 - 15 Poorly graded silty/clayey GRAVEL
GF GML, GMI... 15 - 35 Very silty GRAVEL [plasticity sub-group...]
  GCL, GCI... 15 - 35 Very clayey GRAVEL [..symbols as below]
SAND S SW 0 - 5 Well-graded SAND
  SPu/SPg 0 - 5 Uniform/poorly-graded SAND
S-F SWM/SWC 5 - 15 Well-graded silty/clayey SAND
  GPM/GPC 5 - 15 Poorly graded silty/clayey SAND
SF SML, SMI... 15 - 35 Very silty SAND [plasticity sub-group...]
  SCL, SCI... 15 - 35 Very clayey SAND [..symbols as below]
Fine soils   >35% fines Liquid limit%  
SILT M MG   Gravelly SILT
MS   Sandy SILT
ML, MI...   [Plasticity subdivisions as for CLAY]
CLAY C CG   Gravelly CLAY
CS   Sandy CLAY
CL <35 CLAY of low plasticity
CI 35 - 50 CLAY of intermediate plasticity
CH 50 - 70 CLAY of high plasticity
CV 70 - 90 CLAY of very high plasticity
CE >90 CLAY of extremely high plasticity
Organic soils O     [Add letter 'O' to group symbol]
Peat Pt     [Soil predominantly fibrous and organic]


Produced by Dr. Leslie Davison, University of the West of England, Bristol, May 2000
in association with Prof. Sarah Springman, Swiss Federal Technical Institute, Zurich