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Backfill material classification (NG5.1)

The assumed limiting performance of the five classes of backfill material defined in A1 is shown in Table NG5.1.

Table NG5.1 Backfill material performance
Backfill material class Material performance % CBR
A Over 15
B 7 to 15
C 4 to 7
D 2 to 4
E Less than 2

Modified Type 1F Unbound Mixture is the preferred option in narrow trenches and small openings. It has a lower nominal aggregate size compared with Type 1 Unbound Mixtures, making it more resistant to segregation during transport and placement. Performance within a trench will be comparable with Type 1 Unbound Mixtures.