Design of Honeycomb Retaining Walls


Basic Design

The basic design procedure is similar to that required for all retaining structures and consists of:

  • Determining the geotechnical properties of both the backfill and foundation materials using soil mechanical methods
  • Calculating the coefficients of KA and KP of earth pressure.
  • Checking the factors of safety against rotation, sliding and bearing of the wall.

The following factors of safety are recommended:

  • Rotation:     1.5
  • Sliding:        1.3
  • Bearing:       2.5

In most walls the factor against sliding is critical but it can be accommodated by keying the wall and foundation.

Each block weighs 30kg and is increased by 33kg when filled by well compacted material, having a density 1800 kg/m3 to give a total of 63 kg. The number of blocks required per square metre of wall face is 10, thus giving a unit mass of 6.3 KN/m height/m length.

The soil properties required to calculate the forces acting on the wall are:

  • Density of backfill
  • Angle of shear of backfill
  • Density of foundation material
  • Angle of shear of foundation material

Drainage

Where problems with water are anticipated behind the wall, a filter drain should be installed at the base of the wall in order to prevent any build up of pore pressure behind the wall.

In reinforced backfill or clayey conditions the filter drain should be extended up the back of the wall with an outlet at the base and so secure free drainage.

The filter drain is best constructed from a mattress of 13mm concrete stone some 20mm wide/thick surrounded in geofabric.

Foundations

Since Honeycomb structures are built up in interlocking blocks with no cement agents they can accommodate much differential movement without endangering the overall stability of the structure. Thus it has been found that only light foundations are required, provided they give adequate protection against sliding. The normal foundation being a layer of concrete some 150mm thick laid to line and level, into which the first row of blocks is embedded while the concrete is setting. The depth of the foundation excavation will depend on the bearing capacity of the soil and depth to afford protection against sliding. In practice this is usually between 300mm and 500mm in good conditions. The usual measures should be taken by the designer in cases of unsuitable founding conditions such as recent fill or collapsing sands.

If the bearing and sliding properties of foundations give difficulties these can be accommodated by increasing the width and depth of the mass concrete together with down stand keys or in extreme cases by a full reinforced concrete solution.

Vertical Walls

Where sections of vertical walls may be required, it is recommended that the hollow in the blocks be filled with concrete and reinforce with steel bars if necessary.


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