BACK-DROP
Where underground drain runs have to change level to go lower, it
is not possible to increase the slope downwards beyond certain limits
as anything other than clear liquid would be left behind. The way
to overcome this is to construct a manhole where the drain comes in
at the top and the sewage then free falls down a vertical pipe inside
the manhole to the new lower level. The advantage to this method is
that the manhole is easily accessible for rodding out blockages. The
manhole is called the backdrop.
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BENCHING
The channel at the bottom of the manhole is finished with smoothed
mortar render to form a self-cleaning slope. This is called benching.
Sometimes the benching may break up causing drain to block.
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BIT-MAC
Bitumen macadam coated aggregate, which is laid hot for road and drive
surfaces. It is generally laid as a two-layer make up with the base
course using 20-38mm stone and the wearing course 5-12mm of stone.
Dense bitmac (dbm) refers to graded stone where fine stone is mixed
in to fill the gaps and add strength. This is generally used for roads
or heavily trafficked drives.
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BLINDING
Sand topping approximately 30-50mm laid on the surface of the sub-base
to prevent the toppings from running into the sub-base. The blinding
also allows the toppings to move independently from the sub-base.
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BRUSHED
CONCRETE
Concrete surface that has been smoothed then finished with a lightly
dragged broom to provide grip. This is often seen on petrol garage
forecourts.
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DPM
Damp proof membrane, used immediately below the concrete ground floor
slab to prevent rising damp. This is also sometimes used on top of
the blinding on concrete drives as a slip membrane to ensure no binding
between the layers.
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FORMATION
The deepest point in an excavation for a drive or patio, which is
leveled and smoothed prior to laying the sub-base.
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HEPSLEVE
A trade name for very hard clay pipes jointed with plastic collars.
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PLASTIC
Plastic pipes are often used in domestic underground drainage, as
the pipe lengths are light and long compared to clay. The disadvantage
is that the pipes are not as strong and may have to be encased in
concrete in certain conditions, negating any flexible qualities. In
such circumstances, clay may be a better choice.
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SALT-GLAZED
Before the super baked clay drainage pipe used today was developed,
clay pipes had to be coated in a kiln-fired clear glaze to ensure
a watertight smooth surface. These pipes were jointed rigidly with
sand/cement and tarred hemp.
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SPOIL
Material dug out from an excavation, which may be referred to on site
as muck and muck-away for removing from site.
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SUB-BASE
The coarse filling such as hardcore or crushed rock that forms a frost
resistant strong base to spread and cushion the loads. This is usually
at least 100mm thick.
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TAMPED
CONCRETE
Concrete surface compaction with long heavy board leaving shallow
ridges. The ridges should be shallow to allow water to drain.
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