Fill specifications - Geotechnical Investigation and Design Tables - page 215

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A reinforced soil slope can tolerate greater fines. This limits water intruding into
the sloping face.

Table 17.3 Backfill requirements (Holtz et al. 1995).

Property Specification requirement

Reinforced soil structure Reinforced soil slope

Sieve size Percent passing

100 mm 100 100

20 mm 100 100-75

4.75 mm 100-20 100-20

0.425 mm 60-0 60-0

0.075 mm 15-0 50-0

Plasticity index PI 12% PI 22%

< <

17.4 Typical grading of granular drainage material
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Granular drainage materials should be uniformly graded and be more permeable
than the surrounding soil, as well as prevent washing of fines from the material
being drained.

Table 17.4 Grading of filter material (Department
of transport, 1991).

Sieve size Percentage by mass passing

63 mm 100%

37.5 mm 85-100

20 mm 0-25

10 mm 0-5

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When used as a drainage layer below sloping faces such as revetments or chimney
drains, angular material should be used.

17.5 Pipe bedding materials
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A well-graded envelope provides the optimum strength and support for the
pipes. However, this requires compaction to be adequate. Pipes in trenches may
not have a large operating area and obtaining a high compaction is usually
difficult.

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A reduced level of compaction is therefore usually specified and with a single size
granular material which would be self compacting.

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The larger size provides a better pipe support, but is unsuitable for small size
pipes.