Erosion control blocks (page 8)
Feasibility Study (abr.) into the Hydraulic Applications
of Terraforce "G" 150 & "HD" blocks
(cont.)
General Requirements
Interlinked concrete blocks covered with an established grass
layer have been shown to withstand a water flow velocity of
8 m/s (Hewlett et al., 1987), provided that :
- there is minimal downslope seepage below the concrete
blocks that could undermine the blocks' foundation;
- surface irregularities are avoided;
- lateral restraint is provided between adjacent blocks,
there is more than 75 % face contact between blocks; alternatively,
the lateral restraint is provided by grass root structures
between the blocks; and
- substantial anchorage exists between the block and the
subsoil, as provided by the root structure of the grass
or, where necessary, mechanical anchors.
Hewlett et al (1987) also recommend a number of restrictions
to maintain hydraulic stability of the blocks.
From a comparison of the characteristics of the Terraforce blocks,
it appears that, except for the minimum block width, they meet
the requirements in the table.
Suitability of Terraforce Blocks
The anchorage of the concrete blocks to the subsoil is dependent
on the root structure of the grass. It has been found that in
general only 10 % of the roots exceed a depth of 200 mm (Hewlett
et al, 1987). This means that the Terraforce "HD"
block will have reduced anchorage to the subsoil, possibly even
inadequate anchorage.
The Terraforce "G" and "HD" blocks do
not mechanically interlock and can rely only on interblock
contact for lateral and rotational restraints. Hewlett et
al., 1987 maintain that reduced face-to-face contact down
to 40 % reduced the maximum permitted flow velocity to 6 m/s.
Hence the Terraforce "G" can be used for grassed
waterway banks with a flow velocity not exceeding 6 m/s.
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