Terrain assessment, drainage and erosion - Geotechnical Investigation and Design Tables - page 199

Table 15.23 Drainage blanket design requirements below roads.

Criteria Thickness of drainage blanket Comment

No settlement 300 mm minimum compacted

With settlement 500 mm minimum Or allowance for expected consolidation

settlement

15.24 Resistance to piping
•

Piping is the internal erosion of the embankment or dam foundation caused by
seepage.

•

Erosion starts at the downstream toe and works backwards towards the inner
reservoir forming internal channels pipes.

Table 15.24 Resistance of a soil to piping.

Resistance controlled by Suitability Property

Plasticity of the soil Suitable PI 15-20% =

Poor PI 12% ; PI 30% < >

Gradation Suitable Well graded

Poor Uniformly graded

% Stones Suitable 10% to 20%

Poor 10% or 20% < >

Compaction level Suitable Relative compaction 95% =

Poor Relative compaction 90%

<

15.25 Soil filters
•

The permeability of the filter should be greater than the soil it is filtering, while
preventing washing out of the fine material.

Table 15.25 Filter design.

Criterion Design criteria Comments

Piping D 15 (Filter)

<

5D 85 (soil) Filter must be coarser than soil yet small enough to prevent

Maximum sizing soil from passing through filter - and forming pipe

Permeability D 15 (Filter)

>

5D 15 (soil) Filter must be significantly more permeable than soil.

Minimum sizing Filter should contain 5% Fines

<

Segregation Moderately graded Avoid gap graded material, but with a low uniformity

2 U 5 coefficient U < <

D 50 (Filter) 25 D 50 (soil) For Granular filters below revetments >

•

Medium and High Plasticity clays not prone to erosion, filter criteria can be
relaxed.

•

Dispersive clays and silts prone to erosion, filter criteria should be more stringent.