Provision must be made in the scheme for the leakage from the
water fittings, and for the subsoil water, which will
inevitably find its way into the sewers. The quantity will vary
very considerably, and is difficult of estimation. If the water
is cheap, and the supply plentiful, the water authority may not
seriously attempt to curtail the leakage; but in other cases it
will be reduced to a minimum by frequent house to house
inspection; some authorities going so far as to gratuitously
fix new washers to taps when they are required. Theoretically,
there should be no infiltration of subsoil water, as in nearly
all modern sewerage schemes the pipes are tested and proved to be
watertight before the trenches are filled in; but in practice this
happy state is not obtainable. The pipes may not all be bedded as
solidly as they should be, and when the pressure of the earth comes
upon them settlement takes place and the joints are broken. Joints
may also be broken by careless filling of trenches, or by men
walking upon the pipes before they are sufficiently covered.
Some engineers specify that all sewers shall be tested and
proved to be absolutely water-tight before they are "passed"
and covered in, but make a proviso that if, after the
completion of the works, the leakage into any section exceeds
1/2 cubic foot per minute per mile of sewer, that length shall
be taken up and relaid. Even if the greatest vigilance is
exercised to obtain water-tight sewers, the numerous house
connections are each potential sources of leakage, and when the
scheme is complete there may be a large quantity of
infiltration water to be dealt with. Where there are existing
systems of old sewers the quantity of infiltration water can be
ascertained by gauging the night flow; and if it is proved to