Soil and rock terminology for description and classification for engineering purposes - Practical Engineering Geology - page 364

Table C3 Guide to soil strength terminology - based on GCO (1988) but generally compatible with

much international practice.

SOIL TYPE TERM IDENTIFICATION

Very Coarse
(COBBLES &
BOULDERS)

Loose

By inspection of voids and particles packing in the eld

Dense

' ' -

Very loose SPT N value 0 4.

Loose ' N ' value 4 - 10; can be excavated with spade, 50mm peg easily

driven.

Medium
dense

SPT 10 - 30.

Coarse
(SAND & GRAVEL)

Dense ' N ' value 30 - 50, requires pick for excavation; 50mm peg

hard to drive.

Very dense ' N ' value >50.

Very soft Undrained shear strength (S u ) <20 kPa; exudes

between

fingers when squeezed in hand.

Soft S u 20 - 40 kPa; moulded by light nger pressure.

Firm S u 40 - 75 kPa; can be moulded by strong nger pressure.

Fine
(CLAY & SILT)

Stiff S u 75 - 150 kPa; cannot be moulded by fingers; can be indented

by thumb.

Very stiff
or hard

S u >150 kPa; can be indented by thumbnail.

Organic
(ORGANIC CLAY,
SILT SAND & PEAT)

Compact
Spongy
Plastic

Fibres already compressed together.
Very compressible and open structure.
Can be moulded in hand and smears ngers.

C.5.2 Joints and discontinuities
Joints, faults and fractures are of major importance to rock engineering
and yet are rather poorly de

ned.
Norbury (2010) states that:

Discontinuities are synonymous with fractures - mechanical breaks

which intersect the soil or rock.
Joints are breaks in the continuity of a body of rock along which there
has been no visible displacement.
Fissures are exactly the same as joints but the term is reserved for soil.

Incipient fractures are natural fractures which retain some tensile
strength and so may not be readily apparent on visual inspection.

This largely tallies with ISRM (1978).