Back to Working with Inclination Data
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Understanding the Results
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.
In this example, we will first consider the basic data about the pipe that has been entered into the section header. The inspection direction is downstream and the pipe full length from end to end is 23.00m. Then there is the altitude data, which looks like this:
Altitude data for the worked example.
Here, we can see:
The ID of the upstream and downstream node
The depth of the pipe at the upstream and downstream ends of the pipe. Notice that the value of depth at the downstream end is is less than that at the upstream end. Does this mean that the pipe is going uphill? No, read on.
The upstream and downstream end invert levels which is the height above sea level for the ends of the pipe. Notice that the height above sea level is lower at the downstream end than it is at the upstream end, so this is how we know that the pipe is flowing downhill.
So, how is the depth of the pipe at the downstream end so much less than the depth of the pipe at the upstream end? This is because the ground is not level, and demonstrates the point that you can never use the pipe depths to establish if the pipe is flowing the correct way. The only way to prove this is by considering and calculating the altitudes of the ends of the pipe above sea level.
Understanding altitudes and depths.
This would normally be done by measuring the altitude of the cover level above seas level using GPS equipment, then measuring the depth of the pipe at each end with a tape measure, entering these values into WinCan VX, and then using the ‘Calculate Invert Levels’ tool in the ‘Tools’ ribbon to do the subtraction without user error.
From this data, and even before we have looked at any charts or graphs, based on the GPS and manhole measurements, we can calculate:
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, 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.