Subgrades and pavements - Geotechnical Investigation and Design Tables - page 154

13.2 Subgrade strength classification
•

The subgrade strength is here defined in terms of the soaked CBR.

•

The soaked CBR may not be necessarily applicable at a given site.

Table 13.2 Subgrade strength classification.

Soaked CBR Strength Comments

classification

1% Extremely weak Geotextile reinforcement and separation layer

with a working platform typically required.

<

1%-2% Very weak Geotextile reinforcement and/or separation

layer and/or a working platform typically
required.

2%-3% Weak Geotextile separation layer and/or a working

platform typically required.

3%-10% Medium

10%-30% Strong Good subgrade to Sub - base quality material.

30% Extremely strong Sub - base to base quality material.

>

•

Extremely weak to weak layers need a capping layer.

•

Capping layer also referred to as a working platform.

•

Design subgrade CBR values above 20% seldom used irrespective of test results.

13.3 Damage from volumetrically active clays
•

Volumetrically active materials are also called shrinkage clays, expansive clays,
reactive clays, and plastic clays.

Table 13.3 Damage to roadways resulting from volumetrically active clays.

Mechanism Effect on roadway

Swelling due to wetting/ Longitudinal cracks on pavements and/or

Shrinkage due to drying Unevenness of riding surface

Culverts can rise out of ground

Swelling pressures where Cracking of culverts

movement is prevented High Pressures of retaining walls greater

than at rest earth pressure coefficient

Loss of strength due to Localised failure of subgrade

swelling or shrinkage Slope failures of embankments

13.4 Subgrade volume change classification
•

A subgrade strength criteria may be satisfied, but may not be adequate for volume
change criteria, which must be assessed separately.

•

The Weighted Plasticity Index (WPI) can be used for an initial assessment although
the soaked CBR swell provides a better indicator of movement potential for design
purposes.