Subgrades and pavements - Geotechnical Investigation and Design Tables - page 155

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An approximate comparative classification is provided in this table.

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Swell is based on sample compacted to MDD (Standard Proctor) at its OMC and
using a 4 day soak.

Table 13.4 Subgrade volume change classification for embankments.

Weighted Soaked Subgrade volume Comments

Plasticity index % CBR swell change classification

1200 1% Very Low Generally acceptable for base sub - base

< <

1200-2200 1%-2% Low Applicable for capping layers

2200-3200 2%-3% Moderate Design for some movements

3200-5000 3%-5% High Unsuitable directly below pavements

5000 5% Very High Should be removed and replaced or stabilised > >

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Materials with a very low volume change potential tends to be high CBR material
(strong to very strong).

•

>

Clayey materials may still have swell after 4 days. Any WPI 3200 should use a

7 day soaked test.

13.5 Minimising subgrade volume change
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Providing a suitable non volumetrically active capping layer is the most cost
effective way to minimise volume change.

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If sufficient non reactive materials are unavailable then stabilisation of the
subgrade may be required, for the thickness indicated.

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Indicative thickness only. Depends also on climatic environment, which influences
active zone.

Table 13.5 Typical improved subgrade to minimise volume change.

Subgrade volume change Thickness of non reactive overlying layer

classification

Fills Cuts

Very Low Subgrade strength governs pavement design

Low Subgrade strength governs pavement design

Moderate 0.5 m to 1.0 m 0.25 m to 0.5 m

High 1.0 m-2.0 m 0.5 m to 1.0 m

Very High

>

2.0 m

>

1.0 m

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Thickness of overlying layer includes pavement in addition to improved subgrade
layer.

•

Pavement thickness (based on strength design) may be sufficient for no improved
subgrade layer.

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Remoulded clays (fills) have a higher potential for movement (in its first few years
of wet/dry cycles) than undisturbed clay subgrades (cuts).

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However the potential for rebound must also be checked for deep cuttings.
Rebound is not a cyclic movement.

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Non Reactive material has WPI < 1200.