Soil and Compaction

Soil Classification

Soil Classification by Cone Penetration Testing (CPT)
Soil classification by grain size analysis is more precise than any indirect way of soil classification.

However, as CPT sounding becomes more and more popular, this test is used to predict not only the in situ relative density but also the soil type in which the sounding was made.

CPT Soil Classification Diagram after Robertson

Experience of our company with over 500 million m3 of sand and gravel compacted over the past 15 years suggests that three limit lines can be drawn that together define the zone of compactable granular soil indicated by the green box, added to the graph by Robertson et al., 1986 :

Zone Soil Behaviour


1 - sensitive fine grained
2 - organic material
3 - clay
4 - silty clay to clay
5 - clayey silt to silty clay
6 - sandy silt to clayey silt
7 - silty sand to sandy silt
8 - sand to silty sand
9 - sand
10 - gravelly sand to sand 11 - very stiff fine grained*
12 - sand to clayey sand*

* overconsolidated or cemented

Blue :
Soils above this line are already dense and need no further compaction.
Red :
Granular soils, including very loose fills, are hardly ever found to have an initial density of less than the value corresponding to the red line.
Yellow:
Experience shows that soils with a friction ratio Rf of above 0.8 % are problematic for compaction. This is believed to be mainly due to high fines content but there may also be other less well understood soil characteristics that coincide with a friction ratio above 0.8 %.