CHOICE OF GRADIENTS
Choice of gradients should be such as to maintain self-cleansing velocity under normal discharge conditions.

To achieve a satisfactory installation, diameter and gradient should be adequate for the maximum flow and competent supervision should be provided to ensure a high standard of pipe quality, laying, jointing and workmanship. This is particularly important when pipes are laid to flat gradients.

The following guidelines on gradients should be observed:

a) For flows of less than 1 L/s, pipes not exceeding 100mm nominal bore at gradients not flatter than 1:40 have proved satisfactory.
b) Where the peak flow is more than 1 L/s, a 100mm nominal bore pipe may be laid at a gradient not flatter than 1:80, provided that at least one WC is connected.
c) 150mm nominal bore pipe may be laid at a gradient not flatter than 1:150, provided that at least five WC’s are connected.
d) Experience has shown that for gradients flatter than those given in items a) and c), a high standard of design and workmanship is necessary if blockages are to be minimised. Where this has been achieved, gradients of 1:130 for 100mm nominal bore pipes and 1:200 for 150mm nominal bore pipes have been used successfully.

Where the available fall is less than that necessary to achieve the recommended gradient, increasing the pipe diameter particularly at low flows is not a satisfactory solution. It will lead to a reduction in velocity and depth of flow and an increase in the tendency for deposits to accumulate in the pipes.

Where it is expected that a drain may be affected by settlement, the selected gradient should be such as to ensure that a satisfactory fall will be maintained.

Research has shown that high velocities of sewage flow arising from steep gradients do not cause increased erosion of pipes or deposition of solids. In such situations drains should be laid at gradients, which are the most economical in excavation and cost. High velocities can, however, cause excessive turbulence at bends and manholes and lead to fouling. Where this occurs it can be mitigated for example by using long radius or sealed access fittings.

PIPE SIZING
PVCu PIPE SIZES COMPARED WITH
TRADITIONAL PIPE SIZES

The diameters of Brett Martin PVCu pipes increase in approximately 50mm increments compared with the 75mm generally for other materials, this enables pipes to be matched to design requirements more accurately and economically.

FIGURE 1 COMPARISON OF PIPE SIZES