Soil classification - Geotechnical Investigation and Design Tables - page 25

2.14 Consistency of cohesive soils
•

Field assessments are typically used with a tactile criterion. The pocket penetrom-
eter can also be used to quantify the values, but it has limitations due to scale
effects, conversions, sample used on and the soil type. Refer Section 5.

•

These strength terms are different for British Standards.

Table 2.14 Consistency of cohesive soil.

Term Symbol Field assessment Thumb pressure Undrained shear

penetration strength (kPa)

Very soft VS Exudes between fingers when squeezed. 25 mm 12 > <

Soft S Can be moulded by light finger pressure. 10 mm 12-25

>

Firm F Can be moulded by strong finger pressure. 10 mm 25-50

<

Stiff St Cannot be moulded by fingers.

<

5mm 50-100

Can be indented by thumb pressure.

Very stiff VSt Can be indented by thumbnail. 1 mm 100-200 <

Hard H Difficult to be indented by thumbnail. 0mm 200

∼ >

- Hard Clays can have values over 500 kPa. However above that value the
material may be referred to as a claystone or mudstone, i.e an extremely low
strength rock.

2.15 Consistency of non cohesive soils
•

The SPT value in this Table is a first approximation only using the uncorrected
SPT value.

•

The SPT values in this Table are an upper bound for coarse granular materials for
field assessment only. Correction factors are required for detailed design.

•

The SPT needs to be corrected for overburden, energy ratio and particle size. This
correction is provided in later chapters.

Table 2.15 Consistency of non-cohesive soil.

Term Symbol Field assessment SPT Density

N - value index (%)

Very loose VL 50 mm peg easily driven. Foot imprints easily.

<

4

<

15

Loose L 12 mm reinforcing bar easily Shovels easily. 4-10 15-35

pushed by hand.

Medium MD 12 mm bar needs hammer Shovelling difficult. 10-30 35-65

dense to drive 200 mm.

>

Dense D 50 mm peg hard to drive. Needs pick for 30-50 65-85

12 mm bar needs hammer to excavation.

drive

<

200 mm.

Very dense VD 12 mm bar needs hammer to Picking difficult. 50 85 > >

drive

<

60 mm.

Cemented C 12 mm bar needs hammer to Cemented, indurated 50 N/A >

drive 20 mm. or large size particles. <