Erosion control blocks (page 7)

Feasibility Study (abr.) into the Hydraulic Applications of Terraforce "G" 150 & "HD" blocks (cont.)

FILTER LAYER DESIGN

Erosion of the subsoil, leading to the collapse of the revetment, can occur either by turbulence or by ground water flowing out through the armour layer. The latter case may occur after a period where the water in the canal has been elevated for some time and has dropped quickly, for example after the peak of a flood. The ground water level will have increased during the high flow condition and will need time to adjust to the lower level. This loss of subsoil has to be prevented to ensure the stability of the structure as a whole. This is achieved by the construction of a "filter" behind the protection layer, consisting of graded layers of granular material and/or geofabrics

Geotextiles are favoured over granular filters due to their ease of placement and general convenience. A two-dimensional woven fabric is preferred with a lower-limit opening size greater than 0,5 mm. This ensures that, after the root growth has taken place through the geotextile, the fabric permeability is still at least ten times that of the subsoil.

PROTECTION AGAINST INFREQUENT FLOW

Grasscovered Block Revetments

Sometimes, surfaces above the normal water level are also exposed to flow conditions and may need protection. Terraforce blocks may be used in conjunction with grass for the protection of such areas subject to occasional extreme flow, floods or infrequent usage. If grass is submerged for several days, it starts to die, reducing the effective bonding that its root structure gave to the soil and thus reducing its protective function. Areas where protection against infrequent flow may be required are :

• the downstream face of embankments that are at risk of overtopping during flood events, such as earth dams, flood storage embankments and flood embankments along low lying rivers; and
• purpose-made overflow channels that convey occasional high-velocity flow, such as auxiliary spillways on dams, flood relief channels and bypass channels.