Sand bed and backfilling

Bed for culvertsHCMP are very tolerant to uneven bed settlement and therefore are perfect for weak or mine-troubled soil.

A number of preparation, bed arrangement, and backfilling preconditions must be respected to ensure proper performance (ground interaction) of a flexible pipe.

The quality of these works is a factor of proper facility performance and durability.

A designer must make minimum capacity calculations for intended flexible pipe bed.

Flexible pipes are compatible with weak soil thanks to their lesser base pressure compared to concrete counterparts.
Bed for culverts
If a bed fails to meet capacity requirements, design must foresee reinforcement e.g. soil replacement, geosynthetics etc.

Sand bed and backfill (gravel, sand and gravel) aggregate grain size depends on flute size.

Recommended grain size at HCMP wall interface and in vicinity thereof (0.3-0.5 m) is 31.5 mm. Larger grain is allowed in other areas.

Backfill soil must meet the following specifications:

  • anisometric factor Cu > 5.0
  • curvature factor 1 < Cc < 3
  • permeability coefficient U > 6 m/day


Sand bedding recommendations:

    • Bed for culvertscross-sectional bed width must exceed pipe circumference to half the diameter/span width or by 0.60 m, whichever is higher;


    • minimum sand bed height – 20 cm (30 cm recommended);


    • min. bed consolidation factor Is = 0.98 as per Proctor test practice;


    • compacted bed must be supplemented with appr. 5 cm sand-gravel backfill so as ensure pipe free accommodation and its complete interaction with the bed.


Installation of culverts


Backfilling recommendations:

Bed for culverts

  • backfilling must exceed pipe circumference to half the diameter/span width or by 0.60 m, whichever is higher (save for trenched lines where gap between trench and pipe wall must be 0.30 m or higher);
  • backfill layers must be placed evenly at either tube side, loose layer max. thickness 30 cm;
  • min. layer consolidation factor Is = 0.98 as per Protector test practice, save for pipe vicinity where allowable value Is = 0.95.

Installation of culverts
Pipe-adjacent backfill layers must be compacted with lightweight equipment i.e. vibration platforms or pads.

Never use heavy compaction machinery as long as complete backfill height is not obtained.

So-called supporting backfill is very important in the gap area.