Soil strength parameters from classification and testing - Geotechnical Investigation and Design Tables - page 54

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Some considerations in using this tool are:
- It does not consider scale effects
- Caution on use of results when used in gravelly clays. This is not an
appropriate test in granular materials.
- Do not use PP on an SPT sample, which are disturbed from the effects of
driving (Table 4.1). Soft to firm samples are compressed and often provide
stiff to very stiff results and hard samples are shattered and also provide stiff
to very stiff results.

Table 5.2 Evaluating strength from PP values (Look, 2004).

Material Unconfined compressive strength q u

In general 0.8 PP

Fills 1.15 PP

Fissured clays 0.6 PP

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For Soils: Three Pocket Penetrometer (PP) Readings on Undisturbed tube sample
(base of tube): Report the PP value - do not convert to a C u on the borelog.

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Some field supervisors are known to use the PP on SPT samples - this practice is
to be avoided as the PP value is meaningless on a disturbed sample.

5.3 Clay strength from SPT data
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As a first approximation C u = 5 SPT is commonly used. However this correlation

is known to vary from 2 to 8.

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The overburden correction is not required for SPT values in clays.

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Sensitivity of clay affects the results.

Table 5.3 Clay strength from SPT data.

Material Description SPT - N (blows/300 mm) Strength

Clay V. Soft 2 0-12 kPa ≤

Soft 2-5 12-25 kPa

Firm 5-10 25-50 kPa

Stiff 10-20 50-100 kPa

V. Stiff 20-40 100-200 kPa

Hard 40 200 kPa > >

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An indication of the variability of the correlation in the literature is as follows
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4N for high plasticity clays and increasing to about
15N for low plasticity clays.
- Contrast with Stroud and Butler's (1975) graph which shows Cu

Sower's graphs uses C u =

= 4.5N for

PI > 30%, and increasing to Cu = 8N for low plasticity clays (PI = 15%).

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Therefore use with caution, and with some local correlations.