Soil Description & Classification

8/13/2019 Soil Description & Classification

1/8

June 2004

The purpose of the sheets in this section is to provide quick reference guides to the description ofsoils and rocks in sufficient detail to cover most situations. The sheets follow BS 5930

recommendations except where otherwise stated. However, to avoid including too much detail, they

are not comprehensive, and are not intended to replace the BS or other source documents, which

should be referred to for special situations or in case of doubt.

CHAPTER 1

SOIL DESCRIPTION AND CLASSIFICATION


2/8

1-1June 2004 Sheet 1 of 1

Residual

Alluvial

Colluvial

Glacial,

periglacial,

fluvio-glacial

Aeolian or

loessian

Organic

Volcanic

Evaporites Materials precipitated

or evaporated from

solutions of high salt

content.

Forms cemented soils or soft sedimentary rocks. Includes oolites

precipitated from calcium in sea water and gypsum precipitated from

sulphate-rich playa lakes in deserts. Evaporites may form as a hard

crust just below the surface in arid regions.

Highly uniform gradation with indistinct or no stratification. Typically silt

or fine sand sized but sometimes the surface is covered by a single

layer of pebbles. Loess typically has a secondary structure of vertical

cracks, joints and root holes.

Formed in place bygrowth and decay of

plants.

Peats are dark coloured, fibrous or amorphous and highlycompressible. Mixtures of fine sediment and organic matter produce

organic silts and clays.

Materials transported

and deposited by

wind.

Ash and pumice

deposited in volcanic

eruptions.

Silt-sized particles along with larger volcanic debris. Particles are

highly angular and often vesicular. Weathering produces a highly

plastic, sometimes expansive, clay. The weathered consolidated

deposits sometimes form a light, easily-worked stone.


and deposited by

water action.

Vary from finest clays to very coarse gravel and boulders. Soils usually

show pronounced stratification. River gravels are usually rounded.


by gravity.

Includes screes, avalanches, landslips, hillside creep, downwash

material and solifluxion deposits. Varies from clays to boulders.

Material is usually heterogeneous with a side range of particle sizes.

Often termed hill wash or head deposits.


and deposited by

glacial ice or by melt

waters from glaciers.

Glacial Till (previously called Boulder Clay) and moraine deposits

usually have broad gradings ranging from clay to boulders. Grain size

in the outwash material decreases with distance from the source of

melt water. Stratification in moraines and till is usually heterogeneous

but outwash deposits give rise to laminated (varved) silt and clay in

glacial lakes. Grains are typically angular or sub-angular.

BROAD CLASSIFICATION OF SOIL ACCORDING TO ORIGIN

Classification Nature of depositsProcess of

formation

Chemical weathering

of parent rock with

little or no movement

of particles.

Product of complete weathering is typically a clay whose type depends

mainly on the weathering process. Products of partial weathering are

more stony and depend more on rock type. Soil becomes more

compact, more stony and less weathered with increasing depth.


3/8

DESCRIBING A SOIL EXAMPLES

Loose brown very sandy subangular fine to coarse sandstone GRAVE

Soft laminated dark blue sandy CLAY.

MADE GROUND (Stiff orange-brown CLAY with scattered brick fragm

Shear strength / relative density Colour

SAND AND GRAVEL Discontinuities Spacing (mm) Angular PARTICLE S

SPT N Grey Subangular

Very loose < 4 Very widely > 2000 Subrounded Boulders

Widely 2000 - 600 Brown Slightly sandy < 5% Rounded

Loose 4-10 Medium 600 - 200 Flat Cobbles

Closely 200 - 60 Blue-grey Sandy 5 - 20% Elongated

Medium dense 10-30 Very closely 60 - 20 Irregular

Extremely closely < 20 Mottled Very sandy > 20% coarse

Dense 30-50 yellow Rough Gravel medium

and brown SAND and Smooth fine

Very dense > 50 Fissured GRAVEL Polished

Dark green coarse

Sandstone Sand medium

etc. Limestone fine

Sheared Granite

SILT Brick

Uncompact

Bedding Spacing (mm) Calcareous coarse 0

Compact Slightly sandy < 35% shaley Silt medium 0

Very thickly bedded > 2000 Glauconitic fine 0.

CLAYS Thickly bedded 2000 - 600 Sandy 35 - 65% Micaceous

cu (kPa) Medium bedded 600 - 200 etc.

Thinly bedded 200 - 60 Very sandy > 65% Silt/clay 0.0

Very thinly bedded 60 - 20

Thickly laminated 20 - 6

Thinly laminated < 6

Clay

Stratified

Heterogeneous

Intact

ORGANIC SOILS

Firm Fibres easily compressed together. Fibrous Plant remains recognisable, some strength. Grey,

Spongy Very compressible, open texture. Pseudo-fibrous Plant remains recognisable, strength lost. black,

Plastic Mouldable. Smears fingers. Amorphous Recognisable plant remains absent. etc.

Note: this table is based on Table 13 of BS 5930 : 1999. For full details see the BS.

Composite soil types

Can be indented by

thumb.

Discontinuities and bedding

Breaks into blocks

along unpolished

discontinuities.

Breaks into blocks

along polished

discontinuities.

Other terms

Example descriptions

Coarse-grained soils

Note: Pure silt is rarely encountered, and silty soils are usually more accurately desc

though this is not recommended by BS 5930), especially where a consistency is givHardWeathered (See 1-5 for a fuller description

of weathering.)

*Sands/gravels can be classed as Loose orCompact on the basis of these characteristics but

this is no longer recommended by BS5930.

Soft 20-40

Can be indented by

thumb nail.

75-150

> 300

50%

Very soft < 20

Easily moulded or crushed in the

fingers.

Can be moulded or crushed by

strong pressure in the fingers.

Fine-grained soils

Moulded by light finger

pressure.

Can be squeezed out

between fingers.

Can be scratched by

thumb nail.

Moulded by strongfinger pressure.

Descriptions should include all or some of the items listed in the headings below. By working from left to right

across the columns below and noting the properties of a soil sample, a useful description of the soil on a

standardised format will be obtained.

Needs pick for

excavation. 50mm

square peg hard to

drive.*

Very stiff

40-75Firm

Stiff

150-300

Can be dug by spade.

50mm square peg easily

driven.*

Particle shape,

composition

Type of partic

Slightly organic / organic /

very organic, etc

Organic soils

discrete part

Example descriptions


4/8


THIS TABLE TO BE READ IN CONJUNCTION WITH THE NOTES AND EXPLANATIONS GIVEN ON SHEET 2.

SUB-GROUPS

Well graded GRAVEL GW -Uniformly graded GPuGap graded GPgWell graded GWM

Poorly graded GPM

Well graded GWCPoorly graded GPC

Low plasticity fines GCL 90

Well graded SAND SW - -Uniformly graded SPu 0-5Gap graded SPgWell graded SWMPoorly graded SPM 5-15Well graded SWCPoorly graded SPC

Low plasticity fines SCL 90

As for CG As CG

Low plasticity CLG 90

Low plasticity CL 90

ORGANIC SOILS Letter 'O' suffixed to any group or subgroup symbol. e.g. MHO - organic si lt of high plastici ty.

PEAT Pt Peat soils consist predominantly of plant remains (fibrous or amorphous)

* Gravelly if >50% coarse material is gravel sized: sandy if >50% coarse material is sand sized.

GPPoorly graded GRAVEL

-

C

M

CLAY

SILT

S-CClayey SAND

GC

As CAs for C

FSILT or CLAY

CS

MS

Sandy CLAY

Sandy SILT

As CG

As CG

As for CG

As for CGFS

Sandy SILT or

sandy CLAY*

S-F

SFVery clayey SAND

Very silty SAND As SC

CG

MG

Gravelly CLAY

Gravelly SILT

Subdivisions as for SC

GC

GW

Very silty or clayey

SAND

Silty or clayey

SAND

Slightly silty orclayey SAND

S-MSilty SAND

SPPoorly graded SAND

S

Very silty GRAVEL

Silty GRAVEL G-M

Clayey GRAVEL G-C

As GC

FINESOILS(>35%

fines)

B.S. SOIL CLASSIFICATION

GML

etc.

MLG

etc.

SILTSandCLAYS(>65%

fines)

SANDS(>50%

ofcoarsematerialisof

sandsize-

0.0

6mmt

o2.0

0mm)

ML

etc.

MLS

etc.

Gravellyorsandy

SILTSorCLAYS(

35-

65%

fines)

CLS

etc.

GRAVELS(>50%

ofcoarsematerialisof

gravelsize->2mm)

SOIL GROUPSDESCRIPTION AND

IDENTIFICATION

COARSESO

ILS( 2000 Very widely Spacing Give description on left or detailed fracture log.

Thickly 600 - 2000 Widely Persistence Give measurement.Medium 200 - 600 Medium Termination Describe if visible.

Thinly 60 - 200 Closely Type Bedding, joint, cleavage, foliation, veining.

Very thinly 20 - 60 Very closely Shape (Roughness large scale) Planar, curviplanar, undulose, irregular.

Thickly Texture (Roughness small scale) Smooth, rough, stepped, slickensided.

laminated 6 - 20 Openness Tight, open (give measurement).

(narrow) Extremely Surface Stained, solution weathered.

Thinly closely Infill Sand, clay, spar fill (give description).

laminated < 6 Wall strength Use index test, e.g. Schmidt rebound hammer.

very narrow) Dip Give dip degrees and dip/strike direction if known.

CORE MECHANICAL MEASUREMENTS

Total core recovery (TCR) Total core recovered as a percentage of core run length.

Rock quality designation (RQD) Solid core > 100mm recovered expressed as percentage of core run length.

Fracture index (FI) Number of fractures per metre or over an arbitrary length. May also be reported as fracture spacing (I f).

GRAIN SIZE

Fine Coarse Refers to the average dimension of the dominant minerals or rock fragments.

< 60mm > 2mm

Solid core recovery (SCR)

60mm - 2mm

Medium

On thin slabs, corners or edges can be broken off by heavy hand pressure.

Solid core recovered as a percentage of core run length. (Solid core is core with at least one full

diameter (not necessarily full circumference), measured along core axis.)

When held in the hand, rock can be broken by hammer blows.

Rock rings on hammer blows. Broken only by sledge hammer.

0.6 - 2.5

Rock mass characteristics*

ROCK DESCRIPTIONS

BS 5930 requires a full factual description of the rock material and rock mass characteristics. The state of weathering must be described

but not necessarily the weathering grade. However, weathering grades are useful for assessing engineering properties and giving

foundation recommendations and should usually be included.*

State of weather ing*, e.g. changes in colour, strength and fracture state, degree of penetration of

weathering from discontinuities (give measurement if possible) and information on weathering products

(kaolinised, iron pans, calcretes, etc.).Distribution and condition of discontinuities described individually for each set, e.g. spacing, persistence,

termination, dip, strike and dip directions, type, roughness (shape and texture), openness/aperture,

surface infill, wall strength, etc..

Description Field estimation

0.25 - 0.6

> 10 > 200

Uniaxial

compressive

strength (UCS)

Is50^

(MPa)

12.5 - 50

5 - 12.5


7/8

1-5August 2007 Sheet 1 of 2

WEATHERING CLASSIFICATION FOR ROCKS - GENERAL

BS 5930 grade Eurocode 7 grade

VI RESIDUALSOIL 5

V COMPLETELY WEATHERED 4

IV HIGHLY WEATHERED 3

III MODERATELY WEATHERED 2

II SLIGHTLY WEATHERED 1

I FRESH ROCK 0

1.

2.

3. BS 5930:1999 Code of practice for site investigations, Figure 19.

(50-90% rock). Considerably weathered throughout.

Possessing some strength - large pieces cannot be broken

by hand. Reasonable core recovery. Often stained. Difficult to

rip. Fairly good foundation material.

(>90% rock). Distinctly weathered through much of the rock

fabric, with slight joint staining. Strength approaches that of

fresh rock. Requires explosives for excavation. Highly

permeable through open joints.

The Engineering Classification of Residual Tropical Soils, A. L. Little, Proceedings of the 7th International Conference on Soil Mechanics

and Foundation Engineering (Mexica 1965)

The Logging of Rock Cores for Engineering Purposes British Geotechnical Society Engineering Group Working Party Group, Quarterly

Journal of Engineering Geology, vol. 3(1970)

Unchanged from original rock, but may have some staining of

joints, indicating water percolation.

Rock completely decomposed by weathering but texture still

recognisable. Can be excavated by hand. Slakes in water.

Unsuitable for foundations of large structures.

(


8/8


CLASSIFICATION AND PRESUMED BEARING VALUES FOR HIGH POROSITY CHALK

From Table 2 of BS 8004 : 1986 - "British Standard Code of practice for Foundations".

Grade Brief description

VI Extremely soft structureless chalk containing small lumps of intact chalk.

Structureless remoulded chalk containing lumps of intact chalk.

Dry chalk above the water table.

WEATHERING SCHEME AND PRESUMED BEARING VALUES FOR MERCIA MUDSTONE (KEUPER MARL)

From Table 3 of BS 8004 : 1986 - "British Standard Code of practice for Foundations".

Zone Description Notes

II

See BS8004 pp 12-16Unweathered

II Blocky, medium-hard weak chalk. Joints more than 200mm apart and closed. 750 to 1000

I As for grade II but hard (moderately weak) and brittle. 1000 to 1500

Water content varies owing to depositionalvariations.

IVb

750 to 1000Mudstone (often fissured).I

Matrix with frequent lithorelicts up to

25mm. As weathering progresses

lithorelicts become less angular.

Water content of matrix greater than that

of lithorelicts.

Unweathered blocks of weathered marl

Spheroidal weathering. Matrix starting to

encroach along joints: first indications of

chemical weathering.

III

500 to 750

Determine by

appropriate insitu andlaboratory testing.

Partially

weathered

Little or no trace of original (zone I)

structure, though may be fissured. Lower

permeability than underlying layers.

Matrix with occasional clay-stone pellets

less than 3mm in diameter but more

usually coarse sand size.

IVa 125 to 250

Can be confused with solifluction or drift

deposits, but contains no pebbles. Plasticslightly silty clay. May be fissured.Fully weathered Matrix only

CLASSIFICATION OF WEATHERED ROCK

Presumed bearing

value* - kPa (kN/m2)

Determine by

appropriate insitu and

lab testing.

125 to 250V

IV 250 to 500

500 to 750

For more details on high porosity chalk and weathered Mercia Mudstone, especially foundation pressures, see BS 8004.

*Bearing capacity related to settlement can be obtained from a correlation between chalk grades, SPT N values

and settlement - see the Bearing Capacity section for details.

Presumed bearing

value* - kPa (kN/m2)

Rubbly, partly weathered chalk with bedding and jointing. Joints 10mm to 60mm apart, open to

20mm, and often infilled with soft remoulded chalk and fragments.

Rubbly to blocky unweathered chalk. Joints 60mm to 200mm apart, open to 3mm, and sometimes

infilled with fragments.

Degree of

weathering

250 to 500

III

Documents

Soil Description & Classification