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78
can be taken as 5 m of straight pipe in design calculations. Solid deposition can
also occur at a top entry into a sewer. Branch connections should be at 45 de-
grees to the horizontal.
Pipework design and pipe sizing
A pumped WC discharge unit enables the use of a 22 mm diameter cop-
per pipe to run long distances, and upwards, to connect into the soil and vent
stack at a convenient location. The intermittency of discharge from appliances
necessitates the use of discharge units that relate to the flow volume, flow time
and interval between flows from sanitary fittings in a similar way to the demand
units for water supplies to such fittings.
A 100 mm diameter stack can carry 750 discharge units, 125 mm di-
ameter stack can carry 2500 discharge units and a 150 mm diameter stack
can carry 5500 discharge units.
Disposal of surface-water
Surface-water can be removed from a site by one or more of the following
methods.
Sewer
Where the local authority agrees that there is adequate capacity, surface-
water is drained into either a combined sewer or a separate surface-water sewer.
Surface-water from garage forecourts and car parks is run in open gullies to
an interceptor chamber. Ventilation of explosive and poisonous petrol vapour
is essential, as a concentration of 2.4% in air is fatal. It is illegal to discharge
petrol, oil or explosive vapour into public sewers. The interceptor chamber is
an underground storage tank of concrete and engineering bricks, which allows
separation of the clean water from the oily scum remaining on its surface. It is
intermittently pumped out and cleaned. The discharge drain to the sewer is
turned downwards to near the bottom of the interceptor and three separate
chambers are used in series.
Soakaway
Ground permeability is established using borehole tests to measure the
rate of natural drainage within a curtilage. If running underground water is
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