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With channel rehabilitation the channel may be excavated and trimmed largely to suit the existing profile rather than to match a hard-edge trapezoidal profile of the engineer's drawing board. A rounded profile is in fact preferred and it is easier to excavate with a backactor.
It is very important that "tuck-in" terminating cut-offs be provided to prevent (a) undercutting and undermining due to sub-surface flows which may cause piping and, (b) edge damage which eads to subsequent unravelling.
The contractor must see to it that these terminating trenches are nice and straight and do not wander. Wandering will cause him to have to pull out the Hyson Cells mat "out of square" or to have unnecessary cutting and joining. Both are uneconomical. Extra concrete will also be used if terminating cutoffs are excavated carelessly.
It is important that asmoothprofile be obtained. This may undulate and have varying slopes to the wall but it must be smooth. Bridging of the cell mat must not be permitted.
CompactionThe profile must be stable in its own right. The liner must not be expected to hold up the wall but only to armour it against erosion. However, demands on compaction are not high. Use of a single drum vibrating compactor will suffice. Note that a plate compacter is just not suitable for this environment; for one thing the motor cannot operate at an angle without starving the engine of oil. Hand stamping is far too slow and uneven.
Rocky outcrops present a particular problem and each site must be studied separately. It may be possible to save having to excavate the rock. There are two basic approaches :
If the concrete is taken over the rock then it must be secured to the rock and bonded to it. The cavities in the rock must not be simply filled with loose material to level it.
If the mattress is butted to a rock face then this must be done in a secure fashion. A small terminating beam will probably have to be excavated butting the rock.
We believe that overall production of 500 m² per day with a labour team of 12 men should be possible provided the concrete can be delivered at that rate. This also depends on good access to the site from both sides.
Cost of materials
It is possible that the Hyson Cells canal could be constructed with a thinner slab than that used with conventional concrete. However this must be confirmed with design engineers and a conservative approach is to use the same thickness.
Ready Mix concrete is preferred but on-site concrete may also be used. However it must be understood that a conventional concrete mix cannot be used with Hyson Cells. Production is just not acceptable if you have to place the concrete carefully shovel by shovel to prevent the cells collapsing. A high slump concrete must be used but it must be a specially designed mix - the labourer must not be permitted to simply add water to a mix that is too stiff.
We strongly believe that the use of additives such as plasticisers and retarders gives a reward in improved production but few contractors are convinced. We would certainly use retarders at least at the beginning of the project until techniques have been learned.
We also believe that use of a concrete mixer is a limiting factor. Do your costings on the basis of using a concrete mixer but allow us to show you our unusual method of preparing on-site low slump concrete. We think that you will gain, but you will have costed using the mixer anyway.
Do not skimp. Use the quantities advised in our installation instructions. ÓSerrated pegs are better than smooth ones because the rigging does not slip.
Allow for penetration into the underlying material of 200 mm more on soft ground). Pegs must be cut on site to suit material conditions encountered.
It is important that the Hyson Cells mat is snugly in contact with the underlying material at all times and that there is no bridging.