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Understanding the Results

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Now that we have completed an inclination test on site with the CCTV truck, let’s inspect the data and see what the graphs look like.

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• The fall of the pipe from end to end = is 121.403 − 121.626 = −0.223 m or −223 mm (minus because the pipe is dropping down in the direction of flow - this is normal for ‘happy’ pipes).

• The gradient of the pipe = is (−0.223 × 100) ÷ 23.0 = −0.97 % (minus because the pipe is ‘happy’).

So, we can calculate the overall gradient and fall of the pipe with only very basic arithmetic, but these values do not describe the shape of the pipe along its length. This is where the inclination data proves extremely useful.

What kind of values should we expect for the gradient?

There is no hard and fast answer on this, because it is dependent on the pipe size and the pipe material, but drainage design engineers try to calculate levels so that the flow of sewage and water is ‘self cleaning’ which means that it is not too fast or too slow so that the solids in the waste water are carried along with the flow and not left behind in the pipe causing blockages.

There is a ‘rule of thumb’ known as McGuire’s Rule which is very old and was born out of a time where the most common pipe material was clay, which states for small diameter foul water pipes:

4” Dia (100 mm)	6” Dia (150 mm)	9” Dia (225 mm)
1 : 40 Gradient	1 : 60 Gradient	1 : 90 Gradient
−2.5 %	−1.67 %	−1.11 %

So, the larger the size of the pipe, the flatter it can be laid in the ground because it usually carries a higher volume of water.

The above rule applies to foul water drains. Surface water pipes are always much flatter as described and are often designed at 1 : 200 gradient = −0.5 % and modern plastic pipes with very smooth internal surfaces can also be used at lower gradients for foul water because of their internal characteristics.

Shallower gradients allow for less digging depths at the installation stage which of course saves money for the land developers and increases efficiencies, and it should never be forgotten that these figures are only given as guidance and are not set in stone. Site specific conditions can have a large impact on the actual values found on site.

It should always be remembered that drains seldom have a very large gradient except in exceptional circumstances and surface water pipes usually have an even smaller gradient than foul water pipes. This is why when you see matching pairs of manholes side-by-side where one is foul and the other surface, the surface water pipes are usually large diameter and shallow, and the foul water pipes are small diameter and deep.

Because of this, a high level of accuracy is needed with the vertical confidence on GPS equipment, because even a few millimetres error can have damaging effects on the results.

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